Hindawi Publishing CorporationISRN Renewable EnergyVolume 2013 Article ID 102947 10 pageshttpdxdoiorg1011552013102947

Research ArticleA New Low-Cost Plastic Solar Collector

Luis E Juanicoacute and Nicolaacutes Di Lalla

Bariloche Atomic Center Rio Negro 8400 Bariloche Argentina

Correspondence should be addressed to Luis E Juanico juanicocabcneagovar

Received 4 July 2013 Accepted 24 July 2013

Academic Editors C Koroneos and P Poggi

Copyright copy 2013 L E Juanico and N Di LallaThis is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

An innovative solar collector based on a long plastic hose that is connected directly in series from the district water grid toconsumption is presented It takes advantage of plastic tubing to develop a simple self-construction collector that costs about 70dollars for a one-family unit In addition due to its solar-pond configuration it could achieve a good thermal performance as itwas demonstrated by modeling

1 Introduction

The photovoltaic panel is the preferred renewable technologyto obtain electricity from homes in developed countriesbut regarding new high-efficiency home appliances thisconsumption is expected to decrease in the future On theother hand energy consumption for heatingwater will remainunchanged considering that domestic heaters have reachedtheir ideal efficiency Although this represents only 15 of thetotal household energy demand in cold-climate developedcountries it increases up to 40 in developing countries withtemperate climate and evenmore in poorer tropical onesThisis contradictory whereas solar collectors have a large marketin developed countries this is minimal in developing onesThis discrepancy has a correlation regarding the evolution ofsolar technology which should be studied considering thatsolar collectors produce energy five times cheaper than PVs[1]

It is well known that a small solar collector based onmodern vacuum tubes (2-3m2) can satisfy the one-familydemand anywhere This design is more suitable for coldhigh-latitude locations than flat collectors since its cylindricalshape receives almost twice as much solar irradiation [2] andminimizes heat losses as well Nowadays more than 90 ofthe worldwide market is supplied by Chinese manufacturedvacuum-tube collectors with final prices as low as 500USDin developed countries with large solar markets On the otherhand the scenario is quite different in developing countriesDue to the small size of the market in these countries prices

generally exceed the previous level by far Actually pricesare double or more in origin due to the low scale of tradinginvolved and the final price paid by users can be even higherbecause of the following

(1) Freight cost within the country adds up to 200U$DMany developing countries are large with low-population density and a weak transportation infras-tructure so freight costs for fragile devices are usuallyhigh

(2) Costs of installation add up to 400U$D A largecountry with small market and weak dealerrsquos networkis a big drawback in this sense the complexity of theheat-pipe technology does not help

(3) Perception of risk by users is usually high due toseveral reasons

(i) Due to lack of national regulations and certifica-tions userrsquos uncertainties regarding the annualyield of solar collectors are very high

(ii) Since dealers are small businessmenwithout thesupport of a large enterprise userrsquos doubts aboutwarranty postsell services and so forth are oftenvery high

(iii) The risk of theft or vandalism is not negligible

Certainly there are barriers for the development of solar mar-kets in developing countries (beyond economic issues) not

2 ISRN Renewable Energy

Figure 1 Schematic drawing of a conventional flat collector

solved for the vacuum-tube technology since its complexityimplies that these collectors must be built by a faraway largemanufacturer and therefore with high freight costs needingprofessional installation and so forth Leading technologiesare going in the opposite direction than what developingcountries need

On the other hand the demand of sanitary hot water(down to 45∘C) in temperate and tropical countries is agood niche market for local manufacturers [3] The lowtemperature difference involved allows them to build cheapercollectors avoiding expensive solutions like selective paintsinner-gas arc welding vacuum tubes and so forth but thesmall size of local markets does not help Governments usu-ally hesitate between supporting small local manufacturersand supporting massive importation of Chinese collectors(which is often frowned upon by their scientific community)and ultimately do not make any decision Still up to nowmost people in developing countries have neither used a solarcollector nor known anybody who did and thus the wheelnever begins to turn

Actually this problem exceeds the scientific approach thatusually focuses onmaximizing a collectorrsquos efficiency Insteadand by taking a holistic approach these problems could besolved by using low-cost materials universally available thatcould be easily assembled in a simple home-made collectorAt first glance plastic appears as an excellent choice butpresent designs do not support this choice as will be discussedlater This paper presents a new design that could solve thisdeficiency

2 Thermal-Hydraulic Analysis ofFlat Solar Collectors

21 Conceptual Analysis of Conventional Flat Collectors Flatsolar collectors have followed the same conceptual designillustrated in Figure 1 from their beginnings in the sixties Itconsists of a black metal plate absorbing solar radiation and

Figure 2 Schematic drawing of a conventional collector built witha single hose

transferring it to an assembly of parallel tubes connected tothe upper storage tank through the hot leg of a hydraulic loopthat is closed by means of its cold leg and is driven by naturalconvection Heat losses are minimized by mounting glazingcovers onto the collector and insulating its back side as well

The use of many parallel tubes short in length is manda-tory for minimizing hydraulic losses The difference of den-sities between both legs can barely pump the coolant flowthrough this grid of tubes connected to two large flow collec-tors For example for a tank 2meters above the collector and atemperature difference of 40∘C the buoyancy pressure is only3 cm of water column This design emerges as a compromisesolution between hydraulic and thermal behaviors Ideallythe heat absorbed is maximized by using a homogeneousmesh of infinite thin cooling channels (so that transversalthermal gradients in the absorbing plate are minimized andthe absorbing area is not reduced) meanwhile the hydraulicflow is maximized by using a few channels of a large diameterand short in length In addition the efficiency is improved byusing tubes of high thermal conductivity like copper

The copper collector was generalized in the seventies andcontinues to be used today for home-made collectors sinceit is relatively easy to weld the thin tubes to both large flowcollectors However in the last two decades the increasingcopper costs together with new welding technologies havesubstituted copper with aluminum or stainless steel butkeeping the same conceptual design perhaps the vacuumtube (specially the heat pipe) has been the only conceptualinnovation observed

The natural-convection grid-type collector continues tobe used today because it apparently leads to the simplestsolution but actually it implies strong limitations regardingopportunities created by new plastic materials From here itis clear that the simplest solution would be achieved by usinga single long hose (illustrated in Figure 2) but this choicedoes not work within the natural-convection approach dueto its (highest) hydraulic restriction Herein for example a

ISRN Renewable Energy 3

Figure 3 View of the polycarbonate collector

collector based on one 100m hose has a hydraulic restriction10000 times larger than a grid assembled with one hundred1m long parallel tubes On the other hand this last designrequires numerous sealed fittings that imply major concernsthat we will discuss now

Despite enormous advances in plastic tubing during thelast two decades all fittings can be classified into three kinds

(1) Barb fittings These fittings are used with more elastictubing materials like LDPE It provided at best amedium-quality fitting and so it is not suitablefor assembling a grid with many connections Inaddition it requires numerous tee-type connectionsin order to assemble each thin tube to both flowcollectors

(2) Threaded pipe joints This kind of fittings is used onmore rigid materials like PVC It provides a good-quality fitting but considering the grid scheme itimplies many overlapping joints (double-threadedjoints and change-of-section joints) So the increasein the number of fittings increases the risk of failureas well as costs

(3) Thermofusion joints The HDPE pipes can be joinedby thermal fusion to form a joint that is as strongas the pipe itself and is almost leak free but itsapplication to the grid layout is too cumbersomesince it would require welding simultaneously allparallel tubes to each flow collector or using toomanyoverlapping joints

Summarizing the application of new plastic tubing tostandard collectors has strong limitations that annul theirapparent advantages The next design must be created on anew thermal-hydraulic paradigm that allows changing thegrid-type design

22 Analysis of Previous Original Designs of Plastic Collec-tors Several designs have already been developed speciallyfor plastic collectors Let us analyze now three innovativedesigns (1) the polycarbonate-integrated collector (2) theroof-integrated collector and (3) the tube-integrated collec-tor

221 The Polycarbonate-Integrated Collector The polycar-bonate collector shown in Figure 3 has been in use for more

than a decade in Brazil having hundreds of units successfullybuilt [4]Modern polycarbonate sheets havingUVfilters withgood mechanical resistance carry a 10-year lifetime warrantyThis collector integrates the absorbing plate and the hydraulicgrid in one sheet of transparent alveolar polycarbonate inwhich its bottom is painted black and both headers areattached to PVC large tubeswith a long slot drilled along theirlength This way this design solves in an elegant manner thecumbersome manufacture of the grid by using just one lowcost (20U$Dm2) standard pieceHowever to drill a long andnarrow slot with exigent dimensional tolerance requires theuse of specialized tools and so this technology is prohibitiveas a home-made solution

On the other hand regarding the thermal-hydraulic con-figuration this collector belongs in the conventional categoryThus its performance is poor since noupper glazing selectivepaint or back insulation is used This concern is minor intropical countries but since these are the poorest ones in themanufacturing process there is a major barrier on the otherhand for middle-income countries with temperate climatesits low efficiency is a major drawback The next design solvesthis problem by means of a new approach

222 The Water-Pond Collector The roof-integrated solarroof has been recently presented at a conceptual level forhorizontal roofs [5] and for inclined roofs [6 7] This is anactive system that integrates a large water pond in the roofbuilt with long water cushions hydraulically interconnectedmounted on the roof It uses water redistribution to becomea configurable roof workingwith the household space heatingsystem (see Figure 4) This way four working configurationsare obtained

(1) During winter days the water pond is filled in orderto heat the water with solar irradiation which in turnheats the indoor space by infrared radiation

(2) During winter nights the heated water is drained toan insulated tank from where it is used for the infloorspace heating system and hot water consumption

(3) During summer days the water pond is filled in orderto limit the increase in temperature This way thiswater roof is used as a ldquothermal bufferrdquo while aconcrete roof could easily reach 80∘C in a sunny day[8] this water roof keeps the temperature down to40∘C [6 7] This feature has been extensively used inother previous designs like the Skyterm [9]

(4) During summer nights the previous behavior can beenhanced by draining the water pond into a gardenreflecting pool in order to be cooled

Despite its high configurability maybe the major contri-bution of this system is that it includes a water pondinstead of the natural-convection scheme Hence its thermalperformance is noticeably improved since the whole waterinventory is heated simultaneously instead of only a smallpart overheated in the collector To understand this key factorlet us compare the solar efficiency of the solar collector with

4 ISRN Renewable Energy

Winter day

Water chamberUpper classLower classAir chamber

Slope 025

Infrared radiation

Longitudinal cut Col

d w

ater

inle

t

Hot

wat

er o

utle

t

Scrollable curtain

Metallic ceiling oftunned emissivity

Figure 4 Schematic drawing of water-pond roof on winter-day configuration

the water pond The thermal efficiency (120583) for any collectorcan be described by a linear function as

120583 = 1198860minus 1198861

(119879119898minus 119879119886)

119868119899

(1)

where 1198860is the efficiency obtained for the non-heat-losses

condition (if the mean temperature 119879119898 is equal to ambient

temperature119879119886) 1198861(Wm2∘C) is the linear coefficient related

to heat losses and 119868119899is the normal flux of solar irradiance

(Wm2) The mean temperature 119879119898in the grid collector is

determined by the average between the inlet cold temperature(119879119888) and the exit (119879

ℎ) hot temperature Since the cooling flow

is driven just by this temperature difference Δ119879 it couldeasily rise to 40∘C [10] So let us consider for illustrativepurposes a standard collector in which 119879

119886= 10∘C 119879

119888=

30∘C and Δ119879 = 40∘C Hence 119879ℎ= 70∘C and thus 119879

119898=

50∘C obtaining 119879119898minus 119879119886= 40∘C On the other hand the

mean temperature of the equivalent water pond is just 30∘Csince 119879

119888= 119879ℎ= 119879119898 and thus 119879

119898minus 119879119886= 20∘C half

of the previous case Therefore according to (1) the solarefficiency of a water pond is noticeably higher than a standardcollector of similar quality Although this comparison canonly be quantified by setting the right (119886

0 1198861) parameters

it is clear that this trend will be greater for low-qualitycollectors (ie having higher 119886

1values) like the ones used

in developing countries Tables 1 and 2 show bibliographicvalues [10 11] of (119886

0 1198861) for different collectors and water

ponds respectively in which the efficiency is themerit figureThe use of water ponds shows an improvement of up to 200for the poorest-quality collectors against 38 for the bestones However note that the cheaper (unglazed) collectorcannotwarmwater up to 30∘C in this cold ambient (10∘C) andneither can the equivalent water pond (120583 = 25) Thereforethe middle-quality collector emerges as the best compromise

Table 1 Characteristic average parameters and results for standardflat solar collectors Efficiency calculated for 119879

119886= 10∘C 119879

119888= 30∘C

119879119898= 50∘C and 119868

119899= 600Wm2 ∘C

Collector type 1198860() 119886

1(Wm2 ∘C) Efficiency ()

Unglazed 085 18 minus035Single glazing 080 8 027Double glazing 075 5 042

solution between improvement and performance while itsimprovement is remarkable (+96) its efficiency is good(53) too

This comparison is even better when the full heatingprocess is analyzed Let us consider both single-glazingcollectors starting in the morning with their water inventoryat ambient temperature (119879tank = 119879119886 = 10

∘C) that is desired toheat up to 45∘CThe standard collector always works at Δ119879 =40∘C but its 119879

119898is increased throughout the day following

the 119879tank increases and so its efficiency is decreased from aninitial 53 (119879

119888= 10∘C 119879

ℎ= 50∘C and 119879

119898= 30∘C) to a final

6 (119879119888= 45∘C 119879

ℎ= 85∘C and 119879

119898= 65∘C) and hence this

temperature level could hardly be achieved even for the highnormal flux (119868

119899= 600Wm2) considered For this reason

the cheapest flat collectors cannot collect solar energy duringevening hours as it is an usual observation

On the other hand the water pond starts in the morningwith an impressive efficiency of 80 (119879

119888= 119879ℎ= 119879119898=

10∘C) that decreases down to 33 which is five times greaterthan the collectorrsquos efficiency at that moment So the waterpond collects energy throughout the day in a much moreconvenient manner than the conventional collector

This promising design has been developed only up to aconceptual level although it has a good chance of successHowever the manufacture of large plastic waterproof bags

ISRN Renewable Energy 5

Table 2 Characteristic parameters for water-pond collectors Efficiency calculated for 119879119886= 10∘C 119879

119888= 119879119898= 30∘C and 119868

119899= 600Wm2 ∘C

improvement related to Table 1

Collector type 1198860() 119886

1(Wm2 ∘C) Efficiency () Improvement ()

Unglazed 085 18 025 200Single glazing 080 8 053 96Double glazing 075 5 058 38

Figure 5 View of the tubing collector wrapped with recycled PETbottles

by thermal sealing (which is the last design proposed forany roof) is prohibitive for a do-it-yourself project Thus itis necessary to use simpler construction techniques to fit thisrequirement as the next design shows

223 The Tubing Collector This collector uses many LDPEblack tubing wrapped with PET glazing in order to integratethe hydraulic grid with the absorbing plate Taking advan-tage of very low costs of LDPE tubes this collector usesmany thin (0510158401015840) hoses in order to maximize the absorbingarea (see Figure 5) Many collectors of this type have beenconstructed in Brazil but the experience has shown thatits construction is cumbersome due to the extensive fittingrequired In addition its thermal efficiency is low since it isa conventional natural-convection collector Therefore it hasnot been successful although it provides a low-cost solutionavailable for a do-it-yourself project Up to now there is a lackof a feasible collector that achieves all the desired goals

3 The New Design forUltra-Low-Cost Collectors

Here a new design is presented of solar collectors speciallydeveloped for exploiting the new features of plastic tubingtechnologiesThis collector has all the advantages of previousdesigns and more due to a new paradigm regarding itsthermal-hydraulic behavior It is a single long LDPE hose ofa large diameter connected in series between district gridand consumption The full collector is completed with atransparent plastic layer mounted around this tube (selectedamong several ultra-low-cost options like PET thin filmsPET recycled soda bottles or air-packed LDPE films) and

District watergrid

Consumption

Figure 6 Schematic drawing of the hose collector mounted on theroof

simply resting on the roof (see Figure 6) This way manyadvantages are simultaneously obtained

First a third option is proposed for the dilemma betweennatural and forced convection by using district grid asdriven force which is noticeably higher than the buoyancyforce involved (about 500 times) and so a highly restrictedhydraulic configuration is feasible The consumption flow iscirculated through the whole collector but the pressure dropcan be managed by choosing a large diameter hose

Second the storage tank is eliminated Selecting a largediameter hose the whole water inventory is stored insideBesides many valves and connections are eliminated

Third according to its water-pond design the efficiency isvery good and can collect the solar irradiance throughout theday

Fourth the selection of an LDPE hose allows it tomanageits volumetric variations due to thermal dilatations and iceformation LDPE provides flexible long hoses of very lowcosts (U$D 60 for a 1510158401015840 diameter 100 meter hose) that canwithstand pressures of 6 bars or more and that still can beeasily bent by heating it

Fifth the absorbing plate is removedThe solar irradiationis absorbed directly by the hose This way concerns aboutlow thermal conductivity and high thermal gradients into theabsorbing plate are avoided

Sixth the transparent glazing and back insulation areboth provided by a single (or double for colder locations)transparent layer at a very low cost In addition the sup-porting structure is eliminated enhancing the mechanicalresistance of the collectorThe black hose and the PET glazingboth have excellent durability

Seventh the number of hydraulic joints is reduced tojust one pair this reduction is a key to guarantee the systemreliability considering that barb fittings are used

6 ISRN Renewable Energy

Table 3 Equivalent water inventory (106 liter) obtained by different hose diameters Flows calculated for 06 bar district pressure and pricesfor 25-bar hoses

Diameter (inches) Length (m) Max flow (litersmin) Normal surface (m2) Price (U$D)210158401015840 56 212 34 601510158401015840 100 84 46 5012510158401015840 144 45 56 80110158401015840 224 20 68 10007510158401015840 400 66 98 130

Table 4 Parallel configurations based on many 100-meter sections for 06 bar district pressure Prices include hydraulic joints

Diameter (inches) Number of lines () Max flow (litersmin) Inventory (liters) Solar area (m2) Price (U$D)0510158401015840 9 43 106 460 20007510158401015840 4 56 106 307 150110158401015840 2 60 95 199 100

This design is developed intending to get the simplest col-lector rather than the most efficient one This way both low-cost and home-made construction objectives are achievedBesides its efficiency is expected to be better than averageregarding the solar pond and cylindrical shape featuresThesefeatures allow it to be mounted at a nonoptimal angle acondition desirable in order to eliminate the costs of technicalinstallation and mechanical supporting structures

4 Thermal-Hydraulic Analysis

41 Hydraulic Modeling Optimizing the hydraulic behavioris key in providing the desired hot-water flow for consump-tion meanwhile the large surface of the hose is excellent tocollect solar energy The maximum flow is obtained whenthe driving force (district-grid pressure) is balanced by allpressure drops along the line The total pressure drop canbe calculated as the sum of concentrated and distributedlosses relating to the mean flow velocity (119881) the length (119871)and diameter (119863) of hose and the concentrated (119870

119888) and

frictional (119891) coefficients of losses as [12]

Δ119901 =1

2(119870119888+ 119891

119871

119863)1205881198812 (2)

The frictional Darcyrsquos coefficient 119891 is related to the flowReynolds number (Re = 119881119863120588]) based on dynamic viscosity(]) and density (120588) of fluid for turbulent flows into smoothpipes it can be approximated by [12]

119891 = 031Reminus025 (3)

On the other hand the 119870119888value is related to the geometry of

restriction considered for example considering a 100m hosemounted on a roof of a 10 meter slope which is bent witha long radius curve (119870

119888= 2) along the edge of the roof a

total119870119888= 20 is obtained since this value is much lower than

the total distributed coefficient (119891119871119863) the results are almostindependent of the hose layout

Table 3 shows the flow calculated for different hosediameters It is observed that a 100m 1510158401015840 hose is maybe thesimplest choice balancing hydraulic solar normal area (119863 lowast

119871) and cost behaviors and provides enough (106 liters) hotwater The thinnest hose of equivalent volume is heated veryquickly but provides low flow meanwhile a larger diameterhose (210158401015840) has a small solar area In addition a larger diameterimplies a thicker wall (5mm for a 210158401015840 hose against 3mm fora 1510158401015840 one considering 4-bar hoses) in order to withstandthe inner pressure which in turn implies a more expensivehose and that is harder to bend As a general rule diametersbetween 110158401015840 and 1510158401015840 are right for a simple system providingenough flow on a modest 06 bar district pressure Of coursethere is no single solution depending on many factors forexample district pressure solar resource and climate patternas well as practical conditions like hose diameters availablein local hardware stores and the pattern of use For examplefor application at a day school it is impractical to keepwater warm during the night using a large-diameter hose (apractical technique that will be studied in the next section)and on the other hand it can be desirable to use thin hoses forquick heating in the morning

Let us remark on the flexibility and open configuration ofthis design For example if a too thin 07510158401015840 hose is selectedand the flow is too low the user could always improve itby dividing the 400m long hose in two parallel 200m linesthis way and according to the previous case shown in Table 3the flow is almost tripled obtaining a very good (194 Lmin)flow Table 4 illustrates different configurations based on rollsof a 100m hose connected in parallel intending to achievehigh flow and high solar area This configuration allows theuse of the thinnest hose possible to maximize the surfaceor conversely to use larger diameter hoses in order to lowercosts

We also could think about a ldquomixed assemblyrdquo intendingto balance the maximum solar area and maximum flowcriteria This can be achieved by just adding an onoff valveon each parallel line to obtain water heated quickly duringthe morning using a thin hose together with another fractionkept warm during the evening in the large-diameter hoseThe users could use water from the most convenient linein any case supported by temperature signals Furthermoreeven smarter designs could be achieved for largest systemsby adding a microcontroller This apparently sophisticated

ISRN Renewable Energy 7

feature is possible using a standard garden irrigation systemthat costs fifty dollars

All these features illustrate the open configuration andsimplicity of this design a key in solving userrsquos barrierspreviously analyzed after all there is nothing more accessiblethan an LDPE hose Moreover the normal surface noticeablylarger than on a standard flat solar collector is useful forachieving good heating efficiencies a characteristic rein-forced by the solar-pond concept as will be studied now

42 Thermal Modeling Together with hydraulics the ther-mal analysis is a key in developing an efficient home-madecollector The thermal behavior along the daily transient isdescribed by its dynamic modeling in which the evolutionof the normal solar flux (119868

119899) must be considered For a

cylinder whose generatrix has a north-south orientation itsnormal surface 119878

119899exposed to sun rays is independent of the

azimuthal (120595) solar angle and is only related to the altitudesolar angle (120572) and the roof title angle (120573) according to

119878119899= 119863119871 sin (120572 + 120573) if 120572 gt 0 (during day)

or 119878119899= 0 if 120572 lt 0 (during night)

(4)

Where for any hour (119905) of a given day (119889) the solar altitudeat a certain latitude (120579) location having a 120575 declination anglecan be calculated going through (5)

120575 = 2345 sin(360 (119889 minus 81)365

) for 119889 = 1 2 365

120595 =360∘119905

24 hminus 180

∘ for 0 lt 119905 lt 24 h

1198621= sin (120579) sin (120575)

1198622= cos (120579) cos (120575)

1198781= 1198621+ 1198622cos (120595)

1198782= radic1 minus 1198622

1

120572 = arctan(11987811198782

)

(5)

From here we can calculate the ldquoidealrdquo (on a sunny day) solarpower 119875irrad received in any instant as

119875irrad = 119878119899119868119899 if 120572 gt 0 (6)

The energy absorbed by the collector and the heat losses areboth considered within the efficiency equation (1) So the netpower gained in any instant is

119875net = 119875irrad 120583 = 119878119899119868119899 (1198860 minus 1198861(119879 minus 119879

119886)

119868119899

) (7)

Hence the dynamic energy-balance equation for a hose-based collector can be numerically calculated by approximat-ing the temperature rate by its differential increment (119879

119899minus

119879119899minus1

) on the n-sime time step (Δ119905) as

119898119888119901

119889119879

119889119905asymp 119898119888119901

119879119899minus 119879119899minus1

Δ119905= 119875net (8)

010203040506070

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 7 Daily temperature evolution for middle season (119889 = 81)at a temperate location

where 119898 and 119888119901

are mass and heat capacity of waterrespectively From here the temperature of collector in then-sime time step can be derived as

119879119899=1198781198991198860119868 + 119863119871119886

1119879119886+ (119898119888

119901119879119899minus1Δ119905)

(119898119888119901Δ119905) + 119863119871119886

1

(9)

So the water temperature can be numerically simulated byusing this last equation during the day or modified for nightuse (by setting 119878

119899= 0) and by setting 119879 = 119879

119886during the

earliest hours before sunrise For this preliminary study theirradiance flux from the sun reaching the Earthrsquos surface (119868)can be considered as constant along the day despite changesinduced by atmospheric conditions and for simple field testswithout local measurements of 119868

119899 this constant 119868 value could

be estimated from average data of the daily total irradiance11986610158401015840 on a level surface available from local solar mapsLet us calculate the performance of a 1510158401015840-diameter 100m

hose (119898 = 106Kg 119863 = 00368m and 119871 = 100m) withone PET glazing (119886

0= 08 119886

1= 8Wm2 ∘C) mounted over

an inclined (120573 = 40∘) north-oriented roof located in BuenosAires (120579 = minus38∘) on 21 March (119889 = 81 120575 = 0) From theArgentinean solar map [13] the monthly averaged daily solarirradiance is 11986610158401015840 = 45 kWhm2 and hence 119868 = 440Wm2in addition from climatic data 119879

119886can be approximated by

a sinusoidal function of a half-day period of 20∘C plusmn 5∘CUsing this set of parameter data the temperature evolutioncan be easily simulated by using a spreadsheetThe numericalconvergence shows that a time step of 360 seconds providesaccuracy results using this value the temperature evolution isillustrated in Figure 7 Here different trends are observed

(a) The temperature increases sharply from sunrise (at6 am) and useful levels (above 35∘C) are achievedfrom 830 am to 10 pm which comprises the rangeof utilization for average users

(b) A noticeable peak (62∘C) is obtained at 4 pm andvery high levels (above 45∘C) are kept between 10 amand 8 pm which provides a long range for exigentusers

(c) The temperature decreases sharply after sunset reach-ing 28∘C at midnight

It is interesting to study the evolution of efficiency alongthe day illustrated in Figure 8 The collector starts in the

8 ISRN Renewable Energy

6 8 10 12 14 16 18 200

20

40

60

80

100

minus20

minus40

minus60

()

Figure 8 Daily evolution of efficiency for the previous case (119889 =

81)

0

10

20

30

40

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 9 Daily temperature evolution for winter (119889 = 182)conditions

Table 5 Sensitivity analysis of roof inclination for winter (119889 = 182)conditions

120573 119879peak 119879 at 6 pm 119879 at 8 pm0∘ 23∘C 21∘C 19∘C20∘ 32∘C 29∘C 24∘C40∘ 38∘C 34∘C 28∘C60∘ 42∘C 37∘C 30∘C80∘ 42∘C 37∘C 31∘C90∘ 41∘C 37∘C 30∘C

morning with an impressive efficiency (80) and reaches auseful temperature of 35∘C at 830 am having an averageefficiency of 64 up to this moment This fact together withthe large solar area provided by the hose (enhanced by itscylindrical shape) explains this excellent performance Onthe other hand the negative efficiency observed at night is itsmajor drawback related to conventional collectors in whichthe heated water is stored into the insulated tank

The collector shows excellent performance during thelast-summer day but the sharp decrease during the nightopens a doubt about winter performanceThis case is studiedon 1st July (119889 = 182) for the same location (11986610158401015840 = 2 kWhm2so 119868 = 250Wm2 and119879

119886= 10∘Cplusmn8∘C) in Figure 9 Here it is

observed that the collector becomes a useful preheater duringthe day but never provides hot water This performance canbe improved by adding a second glazing layer (119886

0= 075

1198861= 5Wm2 ∘C)This way aminor improvement is obtained

with a peak temperature (38∘C) higher than the previousvalue (34∘C)

Figure 10 A simple test facility example

0102030405060708090

100

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 11 Temperatures in summer (119889 = 1) for simple and doubleglazing

Table 6 Sensitivity analysis of hose diameter for winter conditionsand 120573 = 60∘

Diameter 119879peak 119879 at 6 pm 119879 at 8 pm0510158401015840 53∘C 36∘C 22∘C07510158401015840 50∘C 39∘C 27∘C110158401015840 47∘C 39∘C 29∘C12510158401015840 44∘C 38∘C 30∘C1510158401015840 41∘C 37∘C 30∘C210158401015840 37∘C 34∘C 30∘C

Several sensitivity analyses can be performed for thislast condition Table 5 shows that collectors mounted onroofs with a steep incline get higher temperatures that thosemounted on barely inclined roofs as those use better theevening sun rays This way the peak temperature obtainedfor a level-surface roof is just 23∘C against 42∘C for anotherinclined 60∘ The influence of hose diameter is shown inTable 6 showing that thinner hoses get higher temperaturesbut their cooling at night is sharper too Again here the bestchoice is found between 110158401015840 and 1510158401015840 hoses Figure 10 shows asimple test facility that comprises three ldquonakedrdquo hoses (0510158401015840110158401015840 and 1510158401015840) and similarly the same hoses but wrapped with

ISRN Renewable Energy 9

Water grid Consumption

Mechanical thermostat12

998400998400 hose

1 5998400998400 hose

6998400998400 hose

Figure 12 Schematic drawing of the three-line mixed system

Table 7 Performance of the three-line system by using previous conditions (120573 = 40∘)

Diameter Summer (119889 = 1) Autumn (119889 = 81) Winter (119889 = 182)119879peak 119879 at 10 pm 119879peak 119879 at 10 pm 119879peak 119879 at 10 pm

610158401015840 53∘C 50∘C 42∘C 39∘C 23∘C 20∘C1 510158401015840 76∘C 46∘C 65∘C 33∘C 35∘C 14∘C0510158401015840 82∘C 25∘C 73∘C 16∘C 40∘C 6∘C

double layer of air-packed transparent polyethylene film avery low-cost glazing (03USDm2)

Finally it is interesting to study the performance on hotsummer days (1st January 119889 = 1) at this temperate location(119879119886= 25∘C plusmn 10∘C 11986610158401015840 = 65 kWhm2 so 119868 = 500Wm2)

on a 60∘ inclined roof Figure 11 compares temperatures forsingle and double glazing for this case Here we observethat both systems provide enough hot water throughoutthe day but the double glazing reaches a dangerous peakof 90∘C a temperature too high for LDPE Of course theflow of consumption is a simple way to limit this effect butthe system must withstand extreme cases like for exampleduring the holidays Thus this behavior should be checkedbefore choosing reinforced insulation

Regarding this concern about overheating it is interestingto study the thermal performance of a ldquomixed systemrdquo builtwith several hoses of different diameters assembled on theparallel layout illustrated in Figure 12 In this example weconsider the following

(a) A 610158401015840 PVC tube of 8-meter length (106 liters) withdouble glazing

(b) A 100m 1510158401015840 hose (106 liters) with single glazing

(c) A 100m 0510158401015840 hose (12 liters) with single glazing

This way we simultaneously obtain the following

(i) A small volume of quickly heated water in themorning (by the 0510158401015840 hose) that can be quicklyldquoreplenishedrdquo every hour

(ii) A large volume (106 liters) available throughout theday and evening

(iii) A large volume (106 liters) kept warm overnight

The user could choose the most convenient line at anytime or this choice could be automatized by using low-cost microcontrolled onoff valves Another simple solutionconsists in adding a bimetallic thermostat on the exit of eachline This option is less flexible than the previous one but thethermostat is a very reliable and low-cost device

The use of a very large diameter (610158401015840) is useful to providehot water throughout the night and besides it supports thechoice of using single glazing on the thinner hoses this wayavoiding dangerous overheating during hot summer daysTable 7 shows the results of the thermal simulation of thissystem by using the same cases previously studied Here isobserved that all objectives are successfully achieved

5 Conclusions

This paper presents a new design of a low-cost and robustsolar collector It was developed intending to fit the lack ofa simple home-made and self-installation device a key forsuccess in developing countries To achieve simultaneously allthese goals a new thermal-hydraulic paradigmwas proposedfrom which we have created a new design for a solarcollector that takes full advantage of new plastic and tubingtechnologies Although rather similar designs are well knownfrom the open literature the comprehensive thermal andhydraulic discussion (for both stationary and daily transientstates) developed here is a worthy contribution of this workOn this base it is easy to ldquotune uprdquo the collector regardingthe specific solar and climate resources among other issuesIn this way different hose diameters and glazing can be usedin order to obtain the best solution for any case The use ofcontrolling systems and mixing configurations are also cost-effective options to enhance the performance

In this paper the deeply rooted thermal-hydraulicparadigm of conventional collectors is discussed In addition

10 ISRN Renewable Energy

this work presents the advantages of the water-pond designwhich has long been known but its benefits have never beenacknowledged [14] The improved efficiency of water pondsregarding conventional solar collectors is a key to achieveefficient low-cost collectors even in temperate climates

Acknowledgments

Argentinean Du Pont enterprise is acknowledged for itssupport by funds given through the National Prize Du Pontto Clean Energies 2009 to this project Grace De Haro isacknowledged for her careful translation

References

[1] S Ozlu I Dincer and G F Naterer ldquoComparative assessmentof residential energy options in Ontario Canadardquo Energy andBuildings vol 55 pp 674ndash684 2012

[2] L T Kostic and Z T Pavlovic ldquoOptimal position of flat platereflectors of solar thermal collectorrdquo Energy and Buildings vol45 pp 161ndash168 2012

[3] M Khalaji Assadi A Khalesi Doost A A Hamidi and MMizani ldquoDesign construction and performance testing of anew system for energy saving in rural buildingsrdquo Energy andBuildings vol 43 no 12 pp 3303ndash3310 2011

[4] Sociedade Do Sol httpwwwsociedadedosolorgbr[5] L Juanico ldquoA new design of roof-integrated water solar collec-

tor for domestic heating and coolingrdquo Solar Energy vol 82 no6 pp 481ndash492 2008

[6] L E Juanico ldquoA new design of configurable solar awning formanaging cooling and heating loadsrdquo Energy and Buildings vol41 no 12 pp 1381ndash1385 2009

[7] L E Juanico ldquoNew design of solar roof for household heatingand coolingrdquo International Journal of Hydrogen Energy vol 35no 11 pp 5823ndash5826 2010

[8] R Sarachitti C Chotetanorm C Lertsatitthanakorn and MRungsiyopas ldquoThermal performance analysis and economicevaluation of roof-integrated solar concrete collectorrdquo Energyand Buildings vol 43 no 6 pp 1403ndash1408 2011

[9] H R Hay and J I Yellott ldquoInternational aspects of air condi-tioning with movable insulationrdquo Solar Energy vol 12 no 4pp 427ndash430 1969

[10] E Aranovitch ldquoHeat transfer processes in solar collectorsrdquoEnergy and Buildings vol 3 no 1 pp 31ndash47 1981

[11] P T Tsilingiris ldquoTowards making solar water heating tech-nology feasiblemdashthe polymer solar collector approachrdquo EnergyConversion andManagement vol 40 no 12 pp 1237ndash1250 1999

[12] F White Fluid Mechanics McGraw-Hill New York NY USA6th edition 2006

[13] R Righini H G Gallegos and C Raichijk ldquoApproach to draw-ing new global solar irradiation contour maps for ArgentinardquoRenewable Energy vol 30 no 8 pp 1241ndash1255 2005

[14] M N A Hawlader and B J Brinkworth ldquoAn analysis of thenon-convecting solar pondrdquo Solar Energy vol 27 no 3 pp 195ndash204 1981

TribologyAdvances in

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International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

2 ISRN Renewable Energy

Figure 1 Schematic drawing of a conventional flat collector

solved for the vacuum-tube technology since its complexityimplies that these collectors must be built by a faraway largemanufacturer and therefore with high freight costs needingprofessional installation and so forth Leading technologiesare going in the opposite direction than what developingcountries need

On the other hand the demand of sanitary hot water(down to 45∘C) in temperate and tropical countries is agood niche market for local manufacturers [3] The lowtemperature difference involved allows them to build cheapercollectors avoiding expensive solutions like selective paintsinner-gas arc welding vacuum tubes and so forth but thesmall size of local markets does not help Governments usu-ally hesitate between supporting small local manufacturersand supporting massive importation of Chinese collectors(which is often frowned upon by their scientific community)and ultimately do not make any decision Still up to nowmost people in developing countries have neither used a solarcollector nor known anybody who did and thus the wheelnever begins to turn

Actually this problem exceeds the scientific approach thatusually focuses onmaximizing a collectorrsquos efficiency Insteadand by taking a holistic approach these problems could besolved by using low-cost materials universally available thatcould be easily assembled in a simple home-made collectorAt first glance plastic appears as an excellent choice butpresent designs do not support this choice as will be discussedlater This paper presents a new design that could solve thisdeficiency

2 Thermal-Hydraulic Analysis ofFlat Solar Collectors

21 Conceptual Analysis of Conventional Flat Collectors Flatsolar collectors have followed the same conceptual designillustrated in Figure 1 from their beginnings in the sixties Itconsists of a black metal plate absorbing solar radiation and

Figure 2 Schematic drawing of a conventional collector built witha single hose

transferring it to an assembly of parallel tubes connected tothe upper storage tank through the hot leg of a hydraulic loopthat is closed by means of its cold leg and is driven by naturalconvection Heat losses are minimized by mounting glazingcovers onto the collector and insulating its back side as well

The use of many parallel tubes short in length is manda-tory for minimizing hydraulic losses The difference of den-sities between both legs can barely pump the coolant flowthrough this grid of tubes connected to two large flow collec-tors For example for a tank 2meters above the collector and atemperature difference of 40∘C the buoyancy pressure is only3 cm of water column This design emerges as a compromisesolution between hydraulic and thermal behaviors Ideallythe heat absorbed is maximized by using a homogeneousmesh of infinite thin cooling channels (so that transversalthermal gradients in the absorbing plate are minimized andthe absorbing area is not reduced) meanwhile the hydraulicflow is maximized by using a few channels of a large diameterand short in length In addition the efficiency is improved byusing tubes of high thermal conductivity like copper

The copper collector was generalized in the seventies andcontinues to be used today for home-made collectors sinceit is relatively easy to weld the thin tubes to both large flowcollectors However in the last two decades the increasingcopper costs together with new welding technologies havesubstituted copper with aluminum or stainless steel butkeeping the same conceptual design perhaps the vacuumtube (specially the heat pipe) has been the only conceptualinnovation observed

The natural-convection grid-type collector continues tobe used today because it apparently leads to the simplestsolution but actually it implies strong limitations regardingopportunities created by new plastic materials From here itis clear that the simplest solution would be achieved by usinga single long hose (illustrated in Figure 2) but this choicedoes not work within the natural-convection approach dueto its (highest) hydraulic restriction Herein for example a

ISRN Renewable Energy 3

Figure 3 View of the polycarbonate collector

collector based on one 100m hose has a hydraulic restriction10000 times larger than a grid assembled with one hundred1m long parallel tubes On the other hand this last designrequires numerous sealed fittings that imply major concernsthat we will discuss now

Despite enormous advances in plastic tubing during thelast two decades all fittings can be classified into three kinds

(1) Barb fittings These fittings are used with more elastictubing materials like LDPE It provided at best amedium-quality fitting and so it is not suitablefor assembling a grid with many connections Inaddition it requires numerous tee-type connectionsin order to assemble each thin tube to both flowcollectors

(2) Threaded pipe joints This kind of fittings is used onmore rigid materials like PVC It provides a good-quality fitting but considering the grid scheme itimplies many overlapping joints (double-threadedjoints and change-of-section joints) So the increasein the number of fittings increases the risk of failureas well as costs

(3) Thermofusion joints The HDPE pipes can be joinedby thermal fusion to form a joint that is as strongas the pipe itself and is almost leak free but itsapplication to the grid layout is too cumbersomesince it would require welding simultaneously allparallel tubes to each flow collector or using toomanyoverlapping joints

Summarizing the application of new plastic tubing tostandard collectors has strong limitations that annul theirapparent advantages The next design must be created on anew thermal-hydraulic paradigm that allows changing thegrid-type design

22 Analysis of Previous Original Designs of Plastic Collec-tors Several designs have already been developed speciallyfor plastic collectors Let us analyze now three innovativedesigns (1) the polycarbonate-integrated collector (2) theroof-integrated collector and (3) the tube-integrated collec-tor

221 The Polycarbonate-Integrated Collector The polycar-bonate collector shown in Figure 3 has been in use for more

than a decade in Brazil having hundreds of units successfullybuilt [4]Modern polycarbonate sheets havingUVfilters withgood mechanical resistance carry a 10-year lifetime warrantyThis collector integrates the absorbing plate and the hydraulicgrid in one sheet of transparent alveolar polycarbonate inwhich its bottom is painted black and both headers areattached to PVC large tubeswith a long slot drilled along theirlength This way this design solves in an elegant manner thecumbersome manufacture of the grid by using just one lowcost (20U$Dm2) standard pieceHowever to drill a long andnarrow slot with exigent dimensional tolerance requires theuse of specialized tools and so this technology is prohibitiveas a home-made solution

On the other hand regarding the thermal-hydraulic con-figuration this collector belongs in the conventional categoryThus its performance is poor since noupper glazing selectivepaint or back insulation is used This concern is minor intropical countries but since these are the poorest ones in themanufacturing process there is a major barrier on the otherhand for middle-income countries with temperate climatesits low efficiency is a major drawback The next design solvesthis problem by means of a new approach

222 The Water-Pond Collector The roof-integrated solarroof has been recently presented at a conceptual level forhorizontal roofs [5] and for inclined roofs [6 7] This is anactive system that integrates a large water pond in the roofbuilt with long water cushions hydraulically interconnectedmounted on the roof It uses water redistribution to becomea configurable roof workingwith the household space heatingsystem (see Figure 4) This way four working configurationsare obtained

(1) During winter days the water pond is filled in orderto heat the water with solar irradiation which in turnheats the indoor space by infrared radiation

(2) During winter nights the heated water is drained toan insulated tank from where it is used for the infloorspace heating system and hot water consumption

(3) During summer days the water pond is filled in orderto limit the increase in temperature This way thiswater roof is used as a ldquothermal bufferrdquo while aconcrete roof could easily reach 80∘C in a sunny day[8] this water roof keeps the temperature down to40∘C [6 7] This feature has been extensively used inother previous designs like the Skyterm [9]

(4) During summer nights the previous behavior can beenhanced by draining the water pond into a gardenreflecting pool in order to be cooled

Despite its high configurability maybe the major contri-bution of this system is that it includes a water pondinstead of the natural-convection scheme Hence its thermalperformance is noticeably improved since the whole waterinventory is heated simultaneously instead of only a smallpart overheated in the collector To understand this key factorlet us compare the solar efficiency of the solar collector with

4 ISRN Renewable Energy

Winter day

Water chamberUpper classLower classAir chamber

Slope 025

Infrared radiation

Longitudinal cut Col

d w

ater

inle

t

Hot

wat

er o

utle

t

Scrollable curtain

Metallic ceiling oftunned emissivity

Figure 4 Schematic drawing of water-pond roof on winter-day configuration

the water pond The thermal efficiency (120583) for any collectorcan be described by a linear function as

120583 = 1198860minus 1198861

(119879119898minus 119879119886)

119868119899

(1)

where 1198860is the efficiency obtained for the non-heat-losses

condition (if the mean temperature 119879119898 is equal to ambient

temperature119879119886) 1198861(Wm2∘C) is the linear coefficient related

to heat losses and 119868119899is the normal flux of solar irradiance

(Wm2) The mean temperature 119879119898in the grid collector is

determined by the average between the inlet cold temperature(119879119888) and the exit (119879

ℎ) hot temperature Since the cooling flow

is driven just by this temperature difference Δ119879 it couldeasily rise to 40∘C [10] So let us consider for illustrativepurposes a standard collector in which 119879

119886= 10∘C 119879

119888=

30∘C and Δ119879 = 40∘C Hence 119879ℎ= 70∘C and thus 119879

119898=

50∘C obtaining 119879119898minus 119879119886= 40∘C On the other hand the

mean temperature of the equivalent water pond is just 30∘Csince 119879

119888= 119879ℎ= 119879119898 and thus 119879

119898minus 119879119886= 20∘C half

of the previous case Therefore according to (1) the solarefficiency of a water pond is noticeably higher than a standardcollector of similar quality Although this comparison canonly be quantified by setting the right (119886

0 1198861) parameters

it is clear that this trend will be greater for low-qualitycollectors (ie having higher 119886

1values) like the ones used

in developing countries Tables 1 and 2 show bibliographicvalues [10 11] of (119886

0 1198861) for different collectors and water

ponds respectively in which the efficiency is themerit figureThe use of water ponds shows an improvement of up to 200for the poorest-quality collectors against 38 for the bestones However note that the cheaper (unglazed) collectorcannotwarmwater up to 30∘C in this cold ambient (10∘C) andneither can the equivalent water pond (120583 = 25) Thereforethe middle-quality collector emerges as the best compromise

Table 1 Characteristic average parameters and results for standardflat solar collectors Efficiency calculated for 119879

119886= 10∘C 119879

119888= 30∘C

119879119898= 50∘C and 119868

119899= 600Wm2 ∘C

Collector type 1198860() 119886

1(Wm2 ∘C) Efficiency ()

Unglazed 085 18 minus035Single glazing 080 8 027Double glazing 075 5 042

solution between improvement and performance while itsimprovement is remarkable (+96) its efficiency is good(53) too

This comparison is even better when the full heatingprocess is analyzed Let us consider both single-glazingcollectors starting in the morning with their water inventoryat ambient temperature (119879tank = 119879119886 = 10

∘C) that is desired toheat up to 45∘CThe standard collector always works at Δ119879 =40∘C but its 119879

119898is increased throughout the day following

the 119879tank increases and so its efficiency is decreased from aninitial 53 (119879

119888= 10∘C 119879

ℎ= 50∘C and 119879

119898= 30∘C) to a final

6 (119879119888= 45∘C 119879

ℎ= 85∘C and 119879

119898= 65∘C) and hence this

temperature level could hardly be achieved even for the highnormal flux (119868

119899= 600Wm2) considered For this reason

the cheapest flat collectors cannot collect solar energy duringevening hours as it is an usual observation

On the other hand the water pond starts in the morningwith an impressive efficiency of 80 (119879

119888= 119879ℎ= 119879119898=

10∘C) that decreases down to 33 which is five times greaterthan the collectorrsquos efficiency at that moment So the waterpond collects energy throughout the day in a much moreconvenient manner than the conventional collector

This promising design has been developed only up to aconceptual level although it has a good chance of successHowever the manufacture of large plastic waterproof bags

ISRN Renewable Energy 5

Table 2 Characteristic parameters for water-pond collectors Efficiency calculated for 119879119886= 10∘C 119879

119888= 119879119898= 30∘C and 119868

119899= 600Wm2 ∘C

improvement related to Table 1

Collector type 1198860() 119886

1(Wm2 ∘C) Efficiency () Improvement ()

Unglazed 085 18 025 200Single glazing 080 8 053 96Double glazing 075 5 058 38

Figure 5 View of the tubing collector wrapped with recycled PETbottles

by thermal sealing (which is the last design proposed forany roof) is prohibitive for a do-it-yourself project Thus itis necessary to use simpler construction techniques to fit thisrequirement as the next design shows

223 The Tubing Collector This collector uses many LDPEblack tubing wrapped with PET glazing in order to integratethe hydraulic grid with the absorbing plate Taking advan-tage of very low costs of LDPE tubes this collector usesmany thin (0510158401015840) hoses in order to maximize the absorbingarea (see Figure 5) Many collectors of this type have beenconstructed in Brazil but the experience has shown thatits construction is cumbersome due to the extensive fittingrequired In addition its thermal efficiency is low since it isa conventional natural-convection collector Therefore it hasnot been successful although it provides a low-cost solutionavailable for a do-it-yourself project Up to now there is a lackof a feasible collector that achieves all the desired goals

3 The New Design forUltra-Low-Cost Collectors

Here a new design is presented of solar collectors speciallydeveloped for exploiting the new features of plastic tubingtechnologiesThis collector has all the advantages of previousdesigns and more due to a new paradigm regarding itsthermal-hydraulic behavior It is a single long LDPE hose ofa large diameter connected in series between district gridand consumption The full collector is completed with atransparent plastic layer mounted around this tube (selectedamong several ultra-low-cost options like PET thin filmsPET recycled soda bottles or air-packed LDPE films) and

District watergrid

Consumption

Figure 6 Schematic drawing of the hose collector mounted on theroof

simply resting on the roof (see Figure 6) This way manyadvantages are simultaneously obtained

First a third option is proposed for the dilemma betweennatural and forced convection by using district grid asdriven force which is noticeably higher than the buoyancyforce involved (about 500 times) and so a highly restrictedhydraulic configuration is feasible The consumption flow iscirculated through the whole collector but the pressure dropcan be managed by choosing a large diameter hose

Second the storage tank is eliminated Selecting a largediameter hose the whole water inventory is stored insideBesides many valves and connections are eliminated

Third according to its water-pond design the efficiency isvery good and can collect the solar irradiance throughout theday

Fourth the selection of an LDPE hose allows it tomanageits volumetric variations due to thermal dilatations and iceformation LDPE provides flexible long hoses of very lowcosts (U$D 60 for a 1510158401015840 diameter 100 meter hose) that canwithstand pressures of 6 bars or more and that still can beeasily bent by heating it

Fifth the absorbing plate is removedThe solar irradiationis absorbed directly by the hose This way concerns aboutlow thermal conductivity and high thermal gradients into theabsorbing plate are avoided

Sixth the transparent glazing and back insulation areboth provided by a single (or double for colder locations)transparent layer at a very low cost In addition the sup-porting structure is eliminated enhancing the mechanicalresistance of the collectorThe black hose and the PET glazingboth have excellent durability

Seventh the number of hydraulic joints is reduced tojust one pair this reduction is a key to guarantee the systemreliability considering that barb fittings are used

6 ISRN Renewable Energy

Table 3 Equivalent water inventory (106 liter) obtained by different hose diameters Flows calculated for 06 bar district pressure and pricesfor 25-bar hoses

Diameter (inches) Length (m) Max flow (litersmin) Normal surface (m2) Price (U$D)210158401015840 56 212 34 601510158401015840 100 84 46 5012510158401015840 144 45 56 80110158401015840 224 20 68 10007510158401015840 400 66 98 130

Table 4 Parallel configurations based on many 100-meter sections for 06 bar district pressure Prices include hydraulic joints

Diameter (inches) Number of lines () Max flow (litersmin) Inventory (liters) Solar area (m2) Price (U$D)0510158401015840 9 43 106 460 20007510158401015840 4 56 106 307 150110158401015840 2 60 95 199 100

This design is developed intending to get the simplest col-lector rather than the most efficient one This way both low-cost and home-made construction objectives are achievedBesides its efficiency is expected to be better than averageregarding the solar pond and cylindrical shape featuresThesefeatures allow it to be mounted at a nonoptimal angle acondition desirable in order to eliminate the costs of technicalinstallation and mechanical supporting structures

4 Thermal-Hydraulic Analysis

41 Hydraulic Modeling Optimizing the hydraulic behavioris key in providing the desired hot-water flow for consump-tion meanwhile the large surface of the hose is excellent tocollect solar energy The maximum flow is obtained whenthe driving force (district-grid pressure) is balanced by allpressure drops along the line The total pressure drop canbe calculated as the sum of concentrated and distributedlosses relating to the mean flow velocity (119881) the length (119871)and diameter (119863) of hose and the concentrated (119870

119888) and

frictional (119891) coefficients of losses as [12]

Δ119901 =1

2(119870119888+ 119891

119871

119863)1205881198812 (2)

The frictional Darcyrsquos coefficient 119891 is related to the flowReynolds number (Re = 119881119863120588]) based on dynamic viscosity(]) and density (120588) of fluid for turbulent flows into smoothpipes it can be approximated by [12]

119891 = 031Reminus025 (3)

On the other hand the 119870119888value is related to the geometry of

restriction considered for example considering a 100m hosemounted on a roof of a 10 meter slope which is bent witha long radius curve (119870

119888= 2) along the edge of the roof a

total119870119888= 20 is obtained since this value is much lower than

the total distributed coefficient (119891119871119863) the results are almostindependent of the hose layout

Table 3 shows the flow calculated for different hosediameters It is observed that a 100m 1510158401015840 hose is maybe thesimplest choice balancing hydraulic solar normal area (119863 lowast

119871) and cost behaviors and provides enough (106 liters) hotwater The thinnest hose of equivalent volume is heated veryquickly but provides low flow meanwhile a larger diameterhose (210158401015840) has a small solar area In addition a larger diameterimplies a thicker wall (5mm for a 210158401015840 hose against 3mm fora 1510158401015840 one considering 4-bar hoses) in order to withstandthe inner pressure which in turn implies a more expensivehose and that is harder to bend As a general rule diametersbetween 110158401015840 and 1510158401015840 are right for a simple system providingenough flow on a modest 06 bar district pressure Of coursethere is no single solution depending on many factors forexample district pressure solar resource and climate patternas well as practical conditions like hose diameters availablein local hardware stores and the pattern of use For examplefor application at a day school it is impractical to keepwater warm during the night using a large-diameter hose (apractical technique that will be studied in the next section)and on the other hand it can be desirable to use thin hoses forquick heating in the morning

Let us remark on the flexibility and open configuration ofthis design For example if a too thin 07510158401015840 hose is selectedand the flow is too low the user could always improve itby dividing the 400m long hose in two parallel 200m linesthis way and according to the previous case shown in Table 3the flow is almost tripled obtaining a very good (194 Lmin)flow Table 4 illustrates different configurations based on rollsof a 100m hose connected in parallel intending to achievehigh flow and high solar area This configuration allows theuse of the thinnest hose possible to maximize the surfaceor conversely to use larger diameter hoses in order to lowercosts

We also could think about a ldquomixed assemblyrdquo intendingto balance the maximum solar area and maximum flowcriteria This can be achieved by just adding an onoff valveon each parallel line to obtain water heated quickly duringthe morning using a thin hose together with another fractionkept warm during the evening in the large-diameter hoseThe users could use water from the most convenient linein any case supported by temperature signals Furthermoreeven smarter designs could be achieved for largest systemsby adding a microcontroller This apparently sophisticated

ISRN Renewable Energy 7

feature is possible using a standard garden irrigation systemthat costs fifty dollars

All these features illustrate the open configuration andsimplicity of this design a key in solving userrsquos barrierspreviously analyzed after all there is nothing more accessiblethan an LDPE hose Moreover the normal surface noticeablylarger than on a standard flat solar collector is useful forachieving good heating efficiencies a characteristic rein-forced by the solar-pond concept as will be studied now

42 Thermal Modeling Together with hydraulics the ther-mal analysis is a key in developing an efficient home-madecollector The thermal behavior along the daily transient isdescribed by its dynamic modeling in which the evolutionof the normal solar flux (119868

119899) must be considered For a

cylinder whose generatrix has a north-south orientation itsnormal surface 119878

119899exposed to sun rays is independent of the

azimuthal (120595) solar angle and is only related to the altitudesolar angle (120572) and the roof title angle (120573) according to

119878119899= 119863119871 sin (120572 + 120573) if 120572 gt 0 (during day)

or 119878119899= 0 if 120572 lt 0 (during night)

(4)

Where for any hour (119905) of a given day (119889) the solar altitudeat a certain latitude (120579) location having a 120575 declination anglecan be calculated going through (5)

120575 = 2345 sin(360 (119889 minus 81)365

) for 119889 = 1 2 365

120595 =360∘119905

24 hminus 180

∘ for 0 lt 119905 lt 24 h

1198621= sin (120579) sin (120575)

1198622= cos (120579) cos (120575)

1198781= 1198621+ 1198622cos (120595)

1198782= radic1 minus 1198622

1

120572 = arctan(11987811198782

)

(5)

From here we can calculate the ldquoidealrdquo (on a sunny day) solarpower 119875irrad received in any instant as

119875irrad = 119878119899119868119899 if 120572 gt 0 (6)

The energy absorbed by the collector and the heat losses areboth considered within the efficiency equation (1) So the netpower gained in any instant is

119875net = 119875irrad 120583 = 119878119899119868119899 (1198860 minus 1198861(119879 minus 119879

119886)

119868119899

) (7)

Hence the dynamic energy-balance equation for a hose-based collector can be numerically calculated by approximat-ing the temperature rate by its differential increment (119879

119899minus

119879119899minus1

) on the n-sime time step (Δ119905) as

119898119888119901

119889119879

119889119905asymp 119898119888119901

119879119899minus 119879119899minus1

Δ119905= 119875net (8)

010203040506070

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 7 Daily temperature evolution for middle season (119889 = 81)at a temperate location

where 119898 and 119888119901

are mass and heat capacity of waterrespectively From here the temperature of collector in then-sime time step can be derived as

119879119899=1198781198991198860119868 + 119863119871119886

1119879119886+ (119898119888

119901119879119899minus1Δ119905)

(119898119888119901Δ119905) + 119863119871119886

1

(9)

So the water temperature can be numerically simulated byusing this last equation during the day or modified for nightuse (by setting 119878

119899= 0) and by setting 119879 = 119879

119886during the

earliest hours before sunrise For this preliminary study theirradiance flux from the sun reaching the Earthrsquos surface (119868)can be considered as constant along the day despite changesinduced by atmospheric conditions and for simple field testswithout local measurements of 119868

119899 this constant 119868 value could

be estimated from average data of the daily total irradiance11986610158401015840 on a level surface available from local solar mapsLet us calculate the performance of a 1510158401015840-diameter 100m

hose (119898 = 106Kg 119863 = 00368m and 119871 = 100m) withone PET glazing (119886

0= 08 119886

1= 8Wm2 ∘C) mounted over

an inclined (120573 = 40∘) north-oriented roof located in BuenosAires (120579 = minus38∘) on 21 March (119889 = 81 120575 = 0) From theArgentinean solar map [13] the monthly averaged daily solarirradiance is 11986610158401015840 = 45 kWhm2 and hence 119868 = 440Wm2in addition from climatic data 119879

119886can be approximated by

a sinusoidal function of a half-day period of 20∘C plusmn 5∘CUsing this set of parameter data the temperature evolutioncan be easily simulated by using a spreadsheetThe numericalconvergence shows that a time step of 360 seconds providesaccuracy results using this value the temperature evolution isillustrated in Figure 7 Here different trends are observed

(a) The temperature increases sharply from sunrise (at6 am) and useful levels (above 35∘C) are achievedfrom 830 am to 10 pm which comprises the rangeof utilization for average users

(b) A noticeable peak (62∘C) is obtained at 4 pm andvery high levels (above 45∘C) are kept between 10 amand 8 pm which provides a long range for exigentusers

(c) The temperature decreases sharply after sunset reach-ing 28∘C at midnight

It is interesting to study the evolution of efficiency alongthe day illustrated in Figure 8 The collector starts in the

8 ISRN Renewable Energy

6 8 10 12 14 16 18 200

20

40

60

80

100

minus20

minus40

minus60

()

Figure 8 Daily evolution of efficiency for the previous case (119889 =

81)

0

10

20

30

40

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 9 Daily temperature evolution for winter (119889 = 182)conditions

Table 5 Sensitivity analysis of roof inclination for winter (119889 = 182)conditions

120573 119879peak 119879 at 6 pm 119879 at 8 pm0∘ 23∘C 21∘C 19∘C20∘ 32∘C 29∘C 24∘C40∘ 38∘C 34∘C 28∘C60∘ 42∘C 37∘C 30∘C80∘ 42∘C 37∘C 31∘C90∘ 41∘C 37∘C 30∘C

morning with an impressive efficiency (80) and reaches auseful temperature of 35∘C at 830 am having an averageefficiency of 64 up to this moment This fact together withthe large solar area provided by the hose (enhanced by itscylindrical shape) explains this excellent performance Onthe other hand the negative efficiency observed at night is itsmajor drawback related to conventional collectors in whichthe heated water is stored into the insulated tank

The collector shows excellent performance during thelast-summer day but the sharp decrease during the nightopens a doubt about winter performanceThis case is studiedon 1st July (119889 = 182) for the same location (11986610158401015840 = 2 kWhm2so 119868 = 250Wm2 and119879

119886= 10∘Cplusmn8∘C) in Figure 9 Here it is

observed that the collector becomes a useful preheater duringthe day but never provides hot water This performance canbe improved by adding a second glazing layer (119886

0= 075

1198861= 5Wm2 ∘C)This way aminor improvement is obtained

with a peak temperature (38∘C) higher than the previousvalue (34∘C)

Figure 10 A simple test facility example

0102030405060708090

100

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 11 Temperatures in summer (119889 = 1) for simple and doubleglazing

Table 6 Sensitivity analysis of hose diameter for winter conditionsand 120573 = 60∘

Diameter 119879peak 119879 at 6 pm 119879 at 8 pm0510158401015840 53∘C 36∘C 22∘C07510158401015840 50∘C 39∘C 27∘C110158401015840 47∘C 39∘C 29∘C12510158401015840 44∘C 38∘C 30∘C1510158401015840 41∘C 37∘C 30∘C210158401015840 37∘C 34∘C 30∘C

Several sensitivity analyses can be performed for thislast condition Table 5 shows that collectors mounted onroofs with a steep incline get higher temperatures that thosemounted on barely inclined roofs as those use better theevening sun rays This way the peak temperature obtainedfor a level-surface roof is just 23∘C against 42∘C for anotherinclined 60∘ The influence of hose diameter is shown inTable 6 showing that thinner hoses get higher temperaturesbut their cooling at night is sharper too Again here the bestchoice is found between 110158401015840 and 1510158401015840 hoses Figure 10 shows asimple test facility that comprises three ldquonakedrdquo hoses (0510158401015840110158401015840 and 1510158401015840) and similarly the same hoses but wrapped with

ISRN Renewable Energy 9

Water grid Consumption

Mechanical thermostat12

998400998400 hose

1 5998400998400 hose

6998400998400 hose

Figure 12 Schematic drawing of the three-line mixed system

Table 7 Performance of the three-line system by using previous conditions (120573 = 40∘)

Diameter Summer (119889 = 1) Autumn (119889 = 81) Winter (119889 = 182)119879peak 119879 at 10 pm 119879peak 119879 at 10 pm 119879peak 119879 at 10 pm

610158401015840 53∘C 50∘C 42∘C 39∘C 23∘C 20∘C1 510158401015840 76∘C 46∘C 65∘C 33∘C 35∘C 14∘C0510158401015840 82∘C 25∘C 73∘C 16∘C 40∘C 6∘C

double layer of air-packed transparent polyethylene film avery low-cost glazing (03USDm2)

Finally it is interesting to study the performance on hotsummer days (1st January 119889 = 1) at this temperate location(119879119886= 25∘C plusmn 10∘C 11986610158401015840 = 65 kWhm2 so 119868 = 500Wm2)

on a 60∘ inclined roof Figure 11 compares temperatures forsingle and double glazing for this case Here we observethat both systems provide enough hot water throughoutthe day but the double glazing reaches a dangerous peakof 90∘C a temperature too high for LDPE Of course theflow of consumption is a simple way to limit this effect butthe system must withstand extreme cases like for exampleduring the holidays Thus this behavior should be checkedbefore choosing reinforced insulation

Regarding this concern about overheating it is interestingto study the thermal performance of a ldquomixed systemrdquo builtwith several hoses of different diameters assembled on theparallel layout illustrated in Figure 12 In this example weconsider the following

(a) A 610158401015840 PVC tube of 8-meter length (106 liters) withdouble glazing

(b) A 100m 1510158401015840 hose (106 liters) with single glazing

(c) A 100m 0510158401015840 hose (12 liters) with single glazing

This way we simultaneously obtain the following

(i) A small volume of quickly heated water in themorning (by the 0510158401015840 hose) that can be quicklyldquoreplenishedrdquo every hour

(ii) A large volume (106 liters) available throughout theday and evening

(iii) A large volume (106 liters) kept warm overnight

The user could choose the most convenient line at anytime or this choice could be automatized by using low-cost microcontrolled onoff valves Another simple solutionconsists in adding a bimetallic thermostat on the exit of eachline This option is less flexible than the previous one but thethermostat is a very reliable and low-cost device

The use of a very large diameter (610158401015840) is useful to providehot water throughout the night and besides it supports thechoice of using single glazing on the thinner hoses this wayavoiding dangerous overheating during hot summer daysTable 7 shows the results of the thermal simulation of thissystem by using the same cases previously studied Here isobserved that all objectives are successfully achieved

5 Conclusions

This paper presents a new design of a low-cost and robustsolar collector It was developed intending to fit the lack ofa simple home-made and self-installation device a key forsuccess in developing countries To achieve simultaneously allthese goals a new thermal-hydraulic paradigmwas proposedfrom which we have created a new design for a solarcollector that takes full advantage of new plastic and tubingtechnologies Although rather similar designs are well knownfrom the open literature the comprehensive thermal andhydraulic discussion (for both stationary and daily transientstates) developed here is a worthy contribution of this workOn this base it is easy to ldquotune uprdquo the collector regardingthe specific solar and climate resources among other issuesIn this way different hose diameters and glazing can be usedin order to obtain the best solution for any case The use ofcontrolling systems and mixing configurations are also cost-effective options to enhance the performance

In this paper the deeply rooted thermal-hydraulicparadigm of conventional collectors is discussed In addition

10 ISRN Renewable Energy

this work presents the advantages of the water-pond designwhich has long been known but its benefits have never beenacknowledged [14] The improved efficiency of water pondsregarding conventional solar collectors is a key to achieveefficient low-cost collectors even in temperate climates

Acknowledgments

Argentinean Du Pont enterprise is acknowledged for itssupport by funds given through the National Prize Du Pontto Clean Energies 2009 to this project Grace De Haro isacknowledged for her careful translation

References

[1] S Ozlu I Dincer and G F Naterer ldquoComparative assessmentof residential energy options in Ontario Canadardquo Energy andBuildings vol 55 pp 674ndash684 2012

[2] L T Kostic and Z T Pavlovic ldquoOptimal position of flat platereflectors of solar thermal collectorrdquo Energy and Buildings vol45 pp 161ndash168 2012

[3] M Khalaji Assadi A Khalesi Doost A A Hamidi and MMizani ldquoDesign construction and performance testing of anew system for energy saving in rural buildingsrdquo Energy andBuildings vol 43 no 12 pp 3303ndash3310 2011

[4] Sociedade Do Sol httpwwwsociedadedosolorgbr[5] L Juanico ldquoA new design of roof-integrated water solar collec-

tor for domestic heating and coolingrdquo Solar Energy vol 82 no6 pp 481ndash492 2008

[6] L E Juanico ldquoA new design of configurable solar awning formanaging cooling and heating loadsrdquo Energy and Buildings vol41 no 12 pp 1381ndash1385 2009

[7] L E Juanico ldquoNew design of solar roof for household heatingand coolingrdquo International Journal of Hydrogen Energy vol 35no 11 pp 5823ndash5826 2010

[8] R Sarachitti C Chotetanorm C Lertsatitthanakorn and MRungsiyopas ldquoThermal performance analysis and economicevaluation of roof-integrated solar concrete collectorrdquo Energyand Buildings vol 43 no 6 pp 1403ndash1408 2011

[9] H R Hay and J I Yellott ldquoInternational aspects of air condi-tioning with movable insulationrdquo Solar Energy vol 12 no 4pp 427ndash430 1969

[10] E Aranovitch ldquoHeat transfer processes in solar collectorsrdquoEnergy and Buildings vol 3 no 1 pp 31ndash47 1981

[11] P T Tsilingiris ldquoTowards making solar water heating tech-nology feasiblemdashthe polymer solar collector approachrdquo EnergyConversion andManagement vol 40 no 12 pp 1237ndash1250 1999

[12] F White Fluid Mechanics McGraw-Hill New York NY USA6th edition 2006

[13] R Righini H G Gallegos and C Raichijk ldquoApproach to draw-ing new global solar irradiation contour maps for ArgentinardquoRenewable Energy vol 30 no 8 pp 1241ndash1255 2005

[14] M N A Hawlader and B J Brinkworth ldquoAn analysis of thenon-convecting solar pondrdquo Solar Energy vol 27 no 3 pp 195ndash204 1981

TribologyAdvances in

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International Journal of

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FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

ISRN Renewable Energy 3

Figure 3 View of the polycarbonate collector

collector based on one 100m hose has a hydraulic restriction10000 times larger than a grid assembled with one hundred1m long parallel tubes On the other hand this last designrequires numerous sealed fittings that imply major concernsthat we will discuss now

Despite enormous advances in plastic tubing during thelast two decades all fittings can be classified into three kinds

(1) Barb fittings These fittings are used with more elastictubing materials like LDPE It provided at best amedium-quality fitting and so it is not suitablefor assembling a grid with many connections Inaddition it requires numerous tee-type connectionsin order to assemble each thin tube to both flowcollectors

(2) Threaded pipe joints This kind of fittings is used onmore rigid materials like PVC It provides a good-quality fitting but considering the grid scheme itimplies many overlapping joints (double-threadedjoints and change-of-section joints) So the increasein the number of fittings increases the risk of failureas well as costs

(3) Thermofusion joints The HDPE pipes can be joinedby thermal fusion to form a joint that is as strongas the pipe itself and is almost leak free but itsapplication to the grid layout is too cumbersomesince it would require welding simultaneously allparallel tubes to each flow collector or using toomanyoverlapping joints

Summarizing the application of new plastic tubing tostandard collectors has strong limitations that annul theirapparent advantages The next design must be created on anew thermal-hydraulic paradigm that allows changing thegrid-type design

22 Analysis of Previous Original Designs of Plastic Collec-tors Several designs have already been developed speciallyfor plastic collectors Let us analyze now three innovativedesigns (1) the polycarbonate-integrated collector (2) theroof-integrated collector and (3) the tube-integrated collec-tor

221 The Polycarbonate-Integrated Collector The polycar-bonate collector shown in Figure 3 has been in use for more

than a decade in Brazil having hundreds of units successfullybuilt [4]Modern polycarbonate sheets havingUVfilters withgood mechanical resistance carry a 10-year lifetime warrantyThis collector integrates the absorbing plate and the hydraulicgrid in one sheet of transparent alveolar polycarbonate inwhich its bottom is painted black and both headers areattached to PVC large tubeswith a long slot drilled along theirlength This way this design solves in an elegant manner thecumbersome manufacture of the grid by using just one lowcost (20U$Dm2) standard pieceHowever to drill a long andnarrow slot with exigent dimensional tolerance requires theuse of specialized tools and so this technology is prohibitiveas a home-made solution

On the other hand regarding the thermal-hydraulic con-figuration this collector belongs in the conventional categoryThus its performance is poor since noupper glazing selectivepaint or back insulation is used This concern is minor intropical countries but since these are the poorest ones in themanufacturing process there is a major barrier on the otherhand for middle-income countries with temperate climatesits low efficiency is a major drawback The next design solvesthis problem by means of a new approach

222 The Water-Pond Collector The roof-integrated solarroof has been recently presented at a conceptual level forhorizontal roofs [5] and for inclined roofs [6 7] This is anactive system that integrates a large water pond in the roofbuilt with long water cushions hydraulically interconnectedmounted on the roof It uses water redistribution to becomea configurable roof workingwith the household space heatingsystem (see Figure 4) This way four working configurationsare obtained

(1) During winter days the water pond is filled in orderto heat the water with solar irradiation which in turnheats the indoor space by infrared radiation

(2) During winter nights the heated water is drained toan insulated tank from where it is used for the infloorspace heating system and hot water consumption

(3) During summer days the water pond is filled in orderto limit the increase in temperature This way thiswater roof is used as a ldquothermal bufferrdquo while aconcrete roof could easily reach 80∘C in a sunny day[8] this water roof keeps the temperature down to40∘C [6 7] This feature has been extensively used inother previous designs like the Skyterm [9]

(4) During summer nights the previous behavior can beenhanced by draining the water pond into a gardenreflecting pool in order to be cooled

Despite its high configurability maybe the major contri-bution of this system is that it includes a water pondinstead of the natural-convection scheme Hence its thermalperformance is noticeably improved since the whole waterinventory is heated simultaneously instead of only a smallpart overheated in the collector To understand this key factorlet us compare the solar efficiency of the solar collector with

4 ISRN Renewable Energy

Winter day

Water chamberUpper classLower classAir chamber

Slope 025

Infrared radiation

Longitudinal cut Col

d w

ater

inle

t

Hot

wat

er o

utle

t

Scrollable curtain

Metallic ceiling oftunned emissivity

Figure 4 Schematic drawing of water-pond roof on winter-day configuration

the water pond The thermal efficiency (120583) for any collectorcan be described by a linear function as

120583 = 1198860minus 1198861

(119879119898minus 119879119886)

119868119899

(1)

where 1198860is the efficiency obtained for the non-heat-losses

condition (if the mean temperature 119879119898 is equal to ambient

temperature119879119886) 1198861(Wm2∘C) is the linear coefficient related

to heat losses and 119868119899is the normal flux of solar irradiance

(Wm2) The mean temperature 119879119898in the grid collector is

determined by the average between the inlet cold temperature(119879119888) and the exit (119879

ℎ) hot temperature Since the cooling flow

is driven just by this temperature difference Δ119879 it couldeasily rise to 40∘C [10] So let us consider for illustrativepurposes a standard collector in which 119879

119886= 10∘C 119879

119888=

30∘C and Δ119879 = 40∘C Hence 119879ℎ= 70∘C and thus 119879

119898=

50∘C obtaining 119879119898minus 119879119886= 40∘C On the other hand the

mean temperature of the equivalent water pond is just 30∘Csince 119879

119888= 119879ℎ= 119879119898 and thus 119879

119898minus 119879119886= 20∘C half

of the previous case Therefore according to (1) the solarefficiency of a water pond is noticeably higher than a standardcollector of similar quality Although this comparison canonly be quantified by setting the right (119886

0 1198861) parameters

it is clear that this trend will be greater for low-qualitycollectors (ie having higher 119886

1values) like the ones used

in developing countries Tables 1 and 2 show bibliographicvalues [10 11] of (119886

0 1198861) for different collectors and water

ponds respectively in which the efficiency is themerit figureThe use of water ponds shows an improvement of up to 200for the poorest-quality collectors against 38 for the bestones However note that the cheaper (unglazed) collectorcannotwarmwater up to 30∘C in this cold ambient (10∘C) andneither can the equivalent water pond (120583 = 25) Thereforethe middle-quality collector emerges as the best compromise

Table 1 Characteristic average parameters and results for standardflat solar collectors Efficiency calculated for 119879

119886= 10∘C 119879

119888= 30∘C

119879119898= 50∘C and 119868

119899= 600Wm2 ∘C

Collector type 1198860() 119886

1(Wm2 ∘C) Efficiency ()

Unglazed 085 18 minus035Single glazing 080 8 027Double glazing 075 5 042

solution between improvement and performance while itsimprovement is remarkable (+96) its efficiency is good(53) too

This comparison is even better when the full heatingprocess is analyzed Let us consider both single-glazingcollectors starting in the morning with their water inventoryat ambient temperature (119879tank = 119879119886 = 10

∘C) that is desired toheat up to 45∘CThe standard collector always works at Δ119879 =40∘C but its 119879

119898is increased throughout the day following

the 119879tank increases and so its efficiency is decreased from aninitial 53 (119879

119888= 10∘C 119879

ℎ= 50∘C and 119879

119898= 30∘C) to a final

6 (119879119888= 45∘C 119879

ℎ= 85∘C and 119879

119898= 65∘C) and hence this

temperature level could hardly be achieved even for the highnormal flux (119868

119899= 600Wm2) considered For this reason

the cheapest flat collectors cannot collect solar energy duringevening hours as it is an usual observation

On the other hand the water pond starts in the morningwith an impressive efficiency of 80 (119879

119888= 119879ℎ= 119879119898=

10∘C) that decreases down to 33 which is five times greaterthan the collectorrsquos efficiency at that moment So the waterpond collects energy throughout the day in a much moreconvenient manner than the conventional collector

This promising design has been developed only up to aconceptual level although it has a good chance of successHowever the manufacture of large plastic waterproof bags

ISRN Renewable Energy 5

Table 2 Characteristic parameters for water-pond collectors Efficiency calculated for 119879119886= 10∘C 119879

119888= 119879119898= 30∘C and 119868

119899= 600Wm2 ∘C

improvement related to Table 1

Collector type 1198860() 119886

1(Wm2 ∘C) Efficiency () Improvement ()

Unglazed 085 18 025 200Single glazing 080 8 053 96Double glazing 075 5 058 38

Figure 5 View of the tubing collector wrapped with recycled PETbottles

by thermal sealing (which is the last design proposed forany roof) is prohibitive for a do-it-yourself project Thus itis necessary to use simpler construction techniques to fit thisrequirement as the next design shows

223 The Tubing Collector This collector uses many LDPEblack tubing wrapped with PET glazing in order to integratethe hydraulic grid with the absorbing plate Taking advan-tage of very low costs of LDPE tubes this collector usesmany thin (0510158401015840) hoses in order to maximize the absorbingarea (see Figure 5) Many collectors of this type have beenconstructed in Brazil but the experience has shown thatits construction is cumbersome due to the extensive fittingrequired In addition its thermal efficiency is low since it isa conventional natural-convection collector Therefore it hasnot been successful although it provides a low-cost solutionavailable for a do-it-yourself project Up to now there is a lackof a feasible collector that achieves all the desired goals

3 The New Design forUltra-Low-Cost Collectors

Here a new design is presented of solar collectors speciallydeveloped for exploiting the new features of plastic tubingtechnologiesThis collector has all the advantages of previousdesigns and more due to a new paradigm regarding itsthermal-hydraulic behavior It is a single long LDPE hose ofa large diameter connected in series between district gridand consumption The full collector is completed with atransparent plastic layer mounted around this tube (selectedamong several ultra-low-cost options like PET thin filmsPET recycled soda bottles or air-packed LDPE films) and

District watergrid

Consumption

Figure 6 Schematic drawing of the hose collector mounted on theroof

simply resting on the roof (see Figure 6) This way manyadvantages are simultaneously obtained

First a third option is proposed for the dilemma betweennatural and forced convection by using district grid asdriven force which is noticeably higher than the buoyancyforce involved (about 500 times) and so a highly restrictedhydraulic configuration is feasible The consumption flow iscirculated through the whole collector but the pressure dropcan be managed by choosing a large diameter hose

Second the storage tank is eliminated Selecting a largediameter hose the whole water inventory is stored insideBesides many valves and connections are eliminated

Third according to its water-pond design the efficiency isvery good and can collect the solar irradiance throughout theday

Fourth the selection of an LDPE hose allows it tomanageits volumetric variations due to thermal dilatations and iceformation LDPE provides flexible long hoses of very lowcosts (U$D 60 for a 1510158401015840 diameter 100 meter hose) that canwithstand pressures of 6 bars or more and that still can beeasily bent by heating it

Fifth the absorbing plate is removedThe solar irradiationis absorbed directly by the hose This way concerns aboutlow thermal conductivity and high thermal gradients into theabsorbing plate are avoided

Sixth the transparent glazing and back insulation areboth provided by a single (or double for colder locations)transparent layer at a very low cost In addition the sup-porting structure is eliminated enhancing the mechanicalresistance of the collectorThe black hose and the PET glazingboth have excellent durability

Seventh the number of hydraulic joints is reduced tojust one pair this reduction is a key to guarantee the systemreliability considering that barb fittings are used

6 ISRN Renewable Energy

Table 3 Equivalent water inventory (106 liter) obtained by different hose diameters Flows calculated for 06 bar district pressure and pricesfor 25-bar hoses

Diameter (inches) Length (m) Max flow (litersmin) Normal surface (m2) Price (U$D)210158401015840 56 212 34 601510158401015840 100 84 46 5012510158401015840 144 45 56 80110158401015840 224 20 68 10007510158401015840 400 66 98 130

Table 4 Parallel configurations based on many 100-meter sections for 06 bar district pressure Prices include hydraulic joints

Diameter (inches) Number of lines () Max flow (litersmin) Inventory (liters) Solar area (m2) Price (U$D)0510158401015840 9 43 106 460 20007510158401015840 4 56 106 307 150110158401015840 2 60 95 199 100

This design is developed intending to get the simplest col-lector rather than the most efficient one This way both low-cost and home-made construction objectives are achievedBesides its efficiency is expected to be better than averageregarding the solar pond and cylindrical shape featuresThesefeatures allow it to be mounted at a nonoptimal angle acondition desirable in order to eliminate the costs of technicalinstallation and mechanical supporting structures

4 Thermal-Hydraulic Analysis

41 Hydraulic Modeling Optimizing the hydraulic behavioris key in providing the desired hot-water flow for consump-tion meanwhile the large surface of the hose is excellent tocollect solar energy The maximum flow is obtained whenthe driving force (district-grid pressure) is balanced by allpressure drops along the line The total pressure drop canbe calculated as the sum of concentrated and distributedlosses relating to the mean flow velocity (119881) the length (119871)and diameter (119863) of hose and the concentrated (119870

119888) and

frictional (119891) coefficients of losses as [12]

Δ119901 =1

2(119870119888+ 119891

119871

119863)1205881198812 (2)

The frictional Darcyrsquos coefficient 119891 is related to the flowReynolds number (Re = 119881119863120588]) based on dynamic viscosity(]) and density (120588) of fluid for turbulent flows into smoothpipes it can be approximated by [12]

119891 = 031Reminus025 (3)

On the other hand the 119870119888value is related to the geometry of

restriction considered for example considering a 100m hosemounted on a roof of a 10 meter slope which is bent witha long radius curve (119870

119888= 2) along the edge of the roof a

total119870119888= 20 is obtained since this value is much lower than

the total distributed coefficient (119891119871119863) the results are almostindependent of the hose layout

Table 3 shows the flow calculated for different hosediameters It is observed that a 100m 1510158401015840 hose is maybe thesimplest choice balancing hydraulic solar normal area (119863 lowast

119871) and cost behaviors and provides enough (106 liters) hotwater The thinnest hose of equivalent volume is heated veryquickly but provides low flow meanwhile a larger diameterhose (210158401015840) has a small solar area In addition a larger diameterimplies a thicker wall (5mm for a 210158401015840 hose against 3mm fora 1510158401015840 one considering 4-bar hoses) in order to withstandthe inner pressure which in turn implies a more expensivehose and that is harder to bend As a general rule diametersbetween 110158401015840 and 1510158401015840 are right for a simple system providingenough flow on a modest 06 bar district pressure Of coursethere is no single solution depending on many factors forexample district pressure solar resource and climate patternas well as practical conditions like hose diameters availablein local hardware stores and the pattern of use For examplefor application at a day school it is impractical to keepwater warm during the night using a large-diameter hose (apractical technique that will be studied in the next section)and on the other hand it can be desirable to use thin hoses forquick heating in the morning

Let us remark on the flexibility and open configuration ofthis design For example if a too thin 07510158401015840 hose is selectedand the flow is too low the user could always improve itby dividing the 400m long hose in two parallel 200m linesthis way and according to the previous case shown in Table 3the flow is almost tripled obtaining a very good (194 Lmin)flow Table 4 illustrates different configurations based on rollsof a 100m hose connected in parallel intending to achievehigh flow and high solar area This configuration allows theuse of the thinnest hose possible to maximize the surfaceor conversely to use larger diameter hoses in order to lowercosts

We also could think about a ldquomixed assemblyrdquo intendingto balance the maximum solar area and maximum flowcriteria This can be achieved by just adding an onoff valveon each parallel line to obtain water heated quickly duringthe morning using a thin hose together with another fractionkept warm during the evening in the large-diameter hoseThe users could use water from the most convenient linein any case supported by temperature signals Furthermoreeven smarter designs could be achieved for largest systemsby adding a microcontroller This apparently sophisticated

ISRN Renewable Energy 7

feature is possible using a standard garden irrigation systemthat costs fifty dollars

All these features illustrate the open configuration andsimplicity of this design a key in solving userrsquos barrierspreviously analyzed after all there is nothing more accessiblethan an LDPE hose Moreover the normal surface noticeablylarger than on a standard flat solar collector is useful forachieving good heating efficiencies a characteristic rein-forced by the solar-pond concept as will be studied now

42 Thermal Modeling Together with hydraulics the ther-mal analysis is a key in developing an efficient home-madecollector The thermal behavior along the daily transient isdescribed by its dynamic modeling in which the evolutionof the normal solar flux (119868

119899) must be considered For a

cylinder whose generatrix has a north-south orientation itsnormal surface 119878

119899exposed to sun rays is independent of the

azimuthal (120595) solar angle and is only related to the altitudesolar angle (120572) and the roof title angle (120573) according to

119878119899= 119863119871 sin (120572 + 120573) if 120572 gt 0 (during day)

or 119878119899= 0 if 120572 lt 0 (during night)

(4)

Where for any hour (119905) of a given day (119889) the solar altitudeat a certain latitude (120579) location having a 120575 declination anglecan be calculated going through (5)

120575 = 2345 sin(360 (119889 minus 81)365

) for 119889 = 1 2 365

120595 =360∘119905

24 hminus 180

∘ for 0 lt 119905 lt 24 h

1198621= sin (120579) sin (120575)

1198622= cos (120579) cos (120575)

1198781= 1198621+ 1198622cos (120595)

1198782= radic1 minus 1198622

1

120572 = arctan(11987811198782

)

(5)

From here we can calculate the ldquoidealrdquo (on a sunny day) solarpower 119875irrad received in any instant as

119875irrad = 119878119899119868119899 if 120572 gt 0 (6)

The energy absorbed by the collector and the heat losses areboth considered within the efficiency equation (1) So the netpower gained in any instant is

119875net = 119875irrad 120583 = 119878119899119868119899 (1198860 minus 1198861(119879 minus 119879

119886)

119868119899

) (7)

Hence the dynamic energy-balance equation for a hose-based collector can be numerically calculated by approximat-ing the temperature rate by its differential increment (119879

119899minus

119879119899minus1

) on the n-sime time step (Δ119905) as

119898119888119901

119889119879

119889119905asymp 119898119888119901

119879119899minus 119879119899minus1

Δ119905= 119875net (8)

010203040506070

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 7 Daily temperature evolution for middle season (119889 = 81)at a temperate location

where 119898 and 119888119901

are mass and heat capacity of waterrespectively From here the temperature of collector in then-sime time step can be derived as

119879119899=1198781198991198860119868 + 119863119871119886

1119879119886+ (119898119888

119901119879119899minus1Δ119905)

(119898119888119901Δ119905) + 119863119871119886

1

(9)

So the water temperature can be numerically simulated byusing this last equation during the day or modified for nightuse (by setting 119878

119899= 0) and by setting 119879 = 119879

119886during the

earliest hours before sunrise For this preliminary study theirradiance flux from the sun reaching the Earthrsquos surface (119868)can be considered as constant along the day despite changesinduced by atmospheric conditions and for simple field testswithout local measurements of 119868

119899 this constant 119868 value could

be estimated from average data of the daily total irradiance11986610158401015840 on a level surface available from local solar mapsLet us calculate the performance of a 1510158401015840-diameter 100m

hose (119898 = 106Kg 119863 = 00368m and 119871 = 100m) withone PET glazing (119886

0= 08 119886

1= 8Wm2 ∘C) mounted over

an inclined (120573 = 40∘) north-oriented roof located in BuenosAires (120579 = minus38∘) on 21 March (119889 = 81 120575 = 0) From theArgentinean solar map [13] the monthly averaged daily solarirradiance is 11986610158401015840 = 45 kWhm2 and hence 119868 = 440Wm2in addition from climatic data 119879

119886can be approximated by

a sinusoidal function of a half-day period of 20∘C plusmn 5∘CUsing this set of parameter data the temperature evolutioncan be easily simulated by using a spreadsheetThe numericalconvergence shows that a time step of 360 seconds providesaccuracy results using this value the temperature evolution isillustrated in Figure 7 Here different trends are observed

(a) The temperature increases sharply from sunrise (at6 am) and useful levels (above 35∘C) are achievedfrom 830 am to 10 pm which comprises the rangeof utilization for average users

(b) A noticeable peak (62∘C) is obtained at 4 pm andvery high levels (above 45∘C) are kept between 10 amand 8 pm which provides a long range for exigentusers

(c) The temperature decreases sharply after sunset reach-ing 28∘C at midnight

It is interesting to study the evolution of efficiency alongthe day illustrated in Figure 8 The collector starts in the

8 ISRN Renewable Energy

6 8 10 12 14 16 18 200

20

40

60

80

100

minus20

minus40

minus60

()

Figure 8 Daily evolution of efficiency for the previous case (119889 =

81)

0

10

20

30

40

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 9 Daily temperature evolution for winter (119889 = 182)conditions

Table 5 Sensitivity analysis of roof inclination for winter (119889 = 182)conditions

120573 119879peak 119879 at 6 pm 119879 at 8 pm0∘ 23∘C 21∘C 19∘C20∘ 32∘C 29∘C 24∘C40∘ 38∘C 34∘C 28∘C60∘ 42∘C 37∘C 30∘C80∘ 42∘C 37∘C 31∘C90∘ 41∘C 37∘C 30∘C

morning with an impressive efficiency (80) and reaches auseful temperature of 35∘C at 830 am having an averageefficiency of 64 up to this moment This fact together withthe large solar area provided by the hose (enhanced by itscylindrical shape) explains this excellent performance Onthe other hand the negative efficiency observed at night is itsmajor drawback related to conventional collectors in whichthe heated water is stored into the insulated tank

The collector shows excellent performance during thelast-summer day but the sharp decrease during the nightopens a doubt about winter performanceThis case is studiedon 1st July (119889 = 182) for the same location (11986610158401015840 = 2 kWhm2so 119868 = 250Wm2 and119879

119886= 10∘Cplusmn8∘C) in Figure 9 Here it is

observed that the collector becomes a useful preheater duringthe day but never provides hot water This performance canbe improved by adding a second glazing layer (119886

0= 075

1198861= 5Wm2 ∘C)This way aminor improvement is obtained

with a peak temperature (38∘C) higher than the previousvalue (34∘C)

Figure 10 A simple test facility example

0102030405060708090

100

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 11 Temperatures in summer (119889 = 1) for simple and doubleglazing

Table 6 Sensitivity analysis of hose diameter for winter conditionsand 120573 = 60∘

Diameter 119879peak 119879 at 6 pm 119879 at 8 pm0510158401015840 53∘C 36∘C 22∘C07510158401015840 50∘C 39∘C 27∘C110158401015840 47∘C 39∘C 29∘C12510158401015840 44∘C 38∘C 30∘C1510158401015840 41∘C 37∘C 30∘C210158401015840 37∘C 34∘C 30∘C

Several sensitivity analyses can be performed for thislast condition Table 5 shows that collectors mounted onroofs with a steep incline get higher temperatures that thosemounted on barely inclined roofs as those use better theevening sun rays This way the peak temperature obtainedfor a level-surface roof is just 23∘C against 42∘C for anotherinclined 60∘ The influence of hose diameter is shown inTable 6 showing that thinner hoses get higher temperaturesbut their cooling at night is sharper too Again here the bestchoice is found between 110158401015840 and 1510158401015840 hoses Figure 10 shows asimple test facility that comprises three ldquonakedrdquo hoses (0510158401015840110158401015840 and 1510158401015840) and similarly the same hoses but wrapped with

ISRN Renewable Energy 9

Water grid Consumption

Mechanical thermostat12

998400998400 hose

1 5998400998400 hose

6998400998400 hose

Figure 12 Schematic drawing of the three-line mixed system

Table 7 Performance of the three-line system by using previous conditions (120573 = 40∘)

Diameter Summer (119889 = 1) Autumn (119889 = 81) Winter (119889 = 182)119879peak 119879 at 10 pm 119879peak 119879 at 10 pm 119879peak 119879 at 10 pm

610158401015840 53∘C 50∘C 42∘C 39∘C 23∘C 20∘C1 510158401015840 76∘C 46∘C 65∘C 33∘C 35∘C 14∘C0510158401015840 82∘C 25∘C 73∘C 16∘C 40∘C 6∘C

double layer of air-packed transparent polyethylene film avery low-cost glazing (03USDm2)

Finally it is interesting to study the performance on hotsummer days (1st January 119889 = 1) at this temperate location(119879119886= 25∘C plusmn 10∘C 11986610158401015840 = 65 kWhm2 so 119868 = 500Wm2)

on a 60∘ inclined roof Figure 11 compares temperatures forsingle and double glazing for this case Here we observethat both systems provide enough hot water throughoutthe day but the double glazing reaches a dangerous peakof 90∘C a temperature too high for LDPE Of course theflow of consumption is a simple way to limit this effect butthe system must withstand extreme cases like for exampleduring the holidays Thus this behavior should be checkedbefore choosing reinforced insulation

Regarding this concern about overheating it is interestingto study the thermal performance of a ldquomixed systemrdquo builtwith several hoses of different diameters assembled on theparallel layout illustrated in Figure 12 In this example weconsider the following

(a) A 610158401015840 PVC tube of 8-meter length (106 liters) withdouble glazing

(b) A 100m 1510158401015840 hose (106 liters) with single glazing

(c) A 100m 0510158401015840 hose (12 liters) with single glazing

This way we simultaneously obtain the following

(i) A small volume of quickly heated water in themorning (by the 0510158401015840 hose) that can be quicklyldquoreplenishedrdquo every hour

(ii) A large volume (106 liters) available throughout theday and evening

(iii) A large volume (106 liters) kept warm overnight

The user could choose the most convenient line at anytime or this choice could be automatized by using low-cost microcontrolled onoff valves Another simple solutionconsists in adding a bimetallic thermostat on the exit of eachline This option is less flexible than the previous one but thethermostat is a very reliable and low-cost device

The use of a very large diameter (610158401015840) is useful to providehot water throughout the night and besides it supports thechoice of using single glazing on the thinner hoses this wayavoiding dangerous overheating during hot summer daysTable 7 shows the results of the thermal simulation of thissystem by using the same cases previously studied Here isobserved that all objectives are successfully achieved

5 Conclusions

This paper presents a new design of a low-cost and robustsolar collector It was developed intending to fit the lack ofa simple home-made and self-installation device a key forsuccess in developing countries To achieve simultaneously allthese goals a new thermal-hydraulic paradigmwas proposedfrom which we have created a new design for a solarcollector that takes full advantage of new plastic and tubingtechnologies Although rather similar designs are well knownfrom the open literature the comprehensive thermal andhydraulic discussion (for both stationary and daily transientstates) developed here is a worthy contribution of this workOn this base it is easy to ldquotune uprdquo the collector regardingthe specific solar and climate resources among other issuesIn this way different hose diameters and glazing can be usedin order to obtain the best solution for any case The use ofcontrolling systems and mixing configurations are also cost-effective options to enhance the performance

In this paper the deeply rooted thermal-hydraulicparadigm of conventional collectors is discussed In addition

10 ISRN Renewable Energy

this work presents the advantages of the water-pond designwhich has long been known but its benefits have never beenacknowledged [14] The improved efficiency of water pondsregarding conventional solar collectors is a key to achieveefficient low-cost collectors even in temperate climates

Acknowledgments

Argentinean Du Pont enterprise is acknowledged for itssupport by funds given through the National Prize Du Pontto Clean Energies 2009 to this project Grace De Haro isacknowledged for her careful translation

References

[1] S Ozlu I Dincer and G F Naterer ldquoComparative assessmentof residential energy options in Ontario Canadardquo Energy andBuildings vol 55 pp 674ndash684 2012

[2] L T Kostic and Z T Pavlovic ldquoOptimal position of flat platereflectors of solar thermal collectorrdquo Energy and Buildings vol45 pp 161ndash168 2012

[3] M Khalaji Assadi A Khalesi Doost A A Hamidi and MMizani ldquoDesign construction and performance testing of anew system for energy saving in rural buildingsrdquo Energy andBuildings vol 43 no 12 pp 3303ndash3310 2011

[4] Sociedade Do Sol httpwwwsociedadedosolorgbr[5] L Juanico ldquoA new design of roof-integrated water solar collec-

tor for domestic heating and coolingrdquo Solar Energy vol 82 no6 pp 481ndash492 2008

[6] L E Juanico ldquoA new design of configurable solar awning formanaging cooling and heating loadsrdquo Energy and Buildings vol41 no 12 pp 1381ndash1385 2009

[7] L E Juanico ldquoNew design of solar roof for household heatingand coolingrdquo International Journal of Hydrogen Energy vol 35no 11 pp 5823ndash5826 2010

[8] R Sarachitti C Chotetanorm C Lertsatitthanakorn and MRungsiyopas ldquoThermal performance analysis and economicevaluation of roof-integrated solar concrete collectorrdquo Energyand Buildings vol 43 no 6 pp 1403ndash1408 2011

[9] H R Hay and J I Yellott ldquoInternational aspects of air condi-tioning with movable insulationrdquo Solar Energy vol 12 no 4pp 427ndash430 1969

[10] E Aranovitch ldquoHeat transfer processes in solar collectorsrdquoEnergy and Buildings vol 3 no 1 pp 31ndash47 1981

[11] P T Tsilingiris ldquoTowards making solar water heating tech-nology feasiblemdashthe polymer solar collector approachrdquo EnergyConversion andManagement vol 40 no 12 pp 1237ndash1250 1999

[12] F White Fluid Mechanics McGraw-Hill New York NY USA6th edition 2006

[13] R Righini H G Gallegos and C Raichijk ldquoApproach to draw-ing new global solar irradiation contour maps for ArgentinardquoRenewable Energy vol 30 no 8 pp 1241ndash1255 2005

[14] M N A Hawlader and B J Brinkworth ldquoAn analysis of thenon-convecting solar pondrdquo Solar Energy vol 27 no 3 pp 195ndash204 1981

TribologyAdvances in

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International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

4 ISRN Renewable Energy

Winter day

Water chamberUpper classLower classAir chamber

Slope 025

Infrared radiation

Longitudinal cut Col

d w

ater

inle

t

Hot

wat

er o

utle

t

Scrollable curtain

Metallic ceiling oftunned emissivity

Figure 4 Schematic drawing of water-pond roof on winter-day configuration

the water pond The thermal efficiency (120583) for any collectorcan be described by a linear function as

120583 = 1198860minus 1198861

(119879119898minus 119879119886)

119868119899

(1)

where 1198860is the efficiency obtained for the non-heat-losses

condition (if the mean temperature 119879119898 is equal to ambient

temperature119879119886) 1198861(Wm2∘C) is the linear coefficient related

to heat losses and 119868119899is the normal flux of solar irradiance

(Wm2) The mean temperature 119879119898in the grid collector is

determined by the average between the inlet cold temperature(119879119888) and the exit (119879

ℎ) hot temperature Since the cooling flow

is driven just by this temperature difference Δ119879 it couldeasily rise to 40∘C [10] So let us consider for illustrativepurposes a standard collector in which 119879

119886= 10∘C 119879

119888=

30∘C and Δ119879 = 40∘C Hence 119879ℎ= 70∘C and thus 119879

119898=

50∘C obtaining 119879119898minus 119879119886= 40∘C On the other hand the

mean temperature of the equivalent water pond is just 30∘Csince 119879

119888= 119879ℎ= 119879119898 and thus 119879

119898minus 119879119886= 20∘C half

of the previous case Therefore according to (1) the solarefficiency of a water pond is noticeably higher than a standardcollector of similar quality Although this comparison canonly be quantified by setting the right (119886

0 1198861) parameters

it is clear that this trend will be greater for low-qualitycollectors (ie having higher 119886

1values) like the ones used

in developing countries Tables 1 and 2 show bibliographicvalues [10 11] of (119886

0 1198861) for different collectors and water

ponds respectively in which the efficiency is themerit figureThe use of water ponds shows an improvement of up to 200for the poorest-quality collectors against 38 for the bestones However note that the cheaper (unglazed) collectorcannotwarmwater up to 30∘C in this cold ambient (10∘C) andneither can the equivalent water pond (120583 = 25) Thereforethe middle-quality collector emerges as the best compromise

Table 1 Characteristic average parameters and results for standardflat solar collectors Efficiency calculated for 119879

119886= 10∘C 119879

119888= 30∘C

119879119898= 50∘C and 119868

119899= 600Wm2 ∘C

Collector type 1198860() 119886

1(Wm2 ∘C) Efficiency ()

Unglazed 085 18 minus035Single glazing 080 8 027Double glazing 075 5 042

solution between improvement and performance while itsimprovement is remarkable (+96) its efficiency is good(53) too

This comparison is even better when the full heatingprocess is analyzed Let us consider both single-glazingcollectors starting in the morning with their water inventoryat ambient temperature (119879tank = 119879119886 = 10

∘C) that is desired toheat up to 45∘CThe standard collector always works at Δ119879 =40∘C but its 119879

119898is increased throughout the day following

the 119879tank increases and so its efficiency is decreased from aninitial 53 (119879

119888= 10∘C 119879

ℎ= 50∘C and 119879

119898= 30∘C) to a final

6 (119879119888= 45∘C 119879

ℎ= 85∘C and 119879

119898= 65∘C) and hence this

temperature level could hardly be achieved even for the highnormal flux (119868

119899= 600Wm2) considered For this reason

the cheapest flat collectors cannot collect solar energy duringevening hours as it is an usual observation

On the other hand the water pond starts in the morningwith an impressive efficiency of 80 (119879

119888= 119879ℎ= 119879119898=

10∘C) that decreases down to 33 which is five times greaterthan the collectorrsquos efficiency at that moment So the waterpond collects energy throughout the day in a much moreconvenient manner than the conventional collector

This promising design has been developed only up to aconceptual level although it has a good chance of successHowever the manufacture of large plastic waterproof bags

ISRN Renewable Energy 5

Table 2 Characteristic parameters for water-pond collectors Efficiency calculated for 119879119886= 10∘C 119879

119888= 119879119898= 30∘C and 119868

119899= 600Wm2 ∘C

improvement related to Table 1

Collector type 1198860() 119886

1(Wm2 ∘C) Efficiency () Improvement ()

Unglazed 085 18 025 200Single glazing 080 8 053 96Double glazing 075 5 058 38

Figure 5 View of the tubing collector wrapped with recycled PETbottles

by thermal sealing (which is the last design proposed forany roof) is prohibitive for a do-it-yourself project Thus itis necessary to use simpler construction techniques to fit thisrequirement as the next design shows

223 The Tubing Collector This collector uses many LDPEblack tubing wrapped with PET glazing in order to integratethe hydraulic grid with the absorbing plate Taking advan-tage of very low costs of LDPE tubes this collector usesmany thin (0510158401015840) hoses in order to maximize the absorbingarea (see Figure 5) Many collectors of this type have beenconstructed in Brazil but the experience has shown thatits construction is cumbersome due to the extensive fittingrequired In addition its thermal efficiency is low since it isa conventional natural-convection collector Therefore it hasnot been successful although it provides a low-cost solutionavailable for a do-it-yourself project Up to now there is a lackof a feasible collector that achieves all the desired goals

3 The New Design forUltra-Low-Cost Collectors

Here a new design is presented of solar collectors speciallydeveloped for exploiting the new features of plastic tubingtechnologiesThis collector has all the advantages of previousdesigns and more due to a new paradigm regarding itsthermal-hydraulic behavior It is a single long LDPE hose ofa large diameter connected in series between district gridand consumption The full collector is completed with atransparent plastic layer mounted around this tube (selectedamong several ultra-low-cost options like PET thin filmsPET recycled soda bottles or air-packed LDPE films) and

District watergrid

Consumption

Figure 6 Schematic drawing of the hose collector mounted on theroof

simply resting on the roof (see Figure 6) This way manyadvantages are simultaneously obtained

First a third option is proposed for the dilemma betweennatural and forced convection by using district grid asdriven force which is noticeably higher than the buoyancyforce involved (about 500 times) and so a highly restrictedhydraulic configuration is feasible The consumption flow iscirculated through the whole collector but the pressure dropcan be managed by choosing a large diameter hose

Second the storage tank is eliminated Selecting a largediameter hose the whole water inventory is stored insideBesides many valves and connections are eliminated

Third according to its water-pond design the efficiency isvery good and can collect the solar irradiance throughout theday

Fourth the selection of an LDPE hose allows it tomanageits volumetric variations due to thermal dilatations and iceformation LDPE provides flexible long hoses of very lowcosts (U$D 60 for a 1510158401015840 diameter 100 meter hose) that canwithstand pressures of 6 bars or more and that still can beeasily bent by heating it

Fifth the absorbing plate is removedThe solar irradiationis absorbed directly by the hose This way concerns aboutlow thermal conductivity and high thermal gradients into theabsorbing plate are avoided

Sixth the transparent glazing and back insulation areboth provided by a single (or double for colder locations)transparent layer at a very low cost In addition the sup-porting structure is eliminated enhancing the mechanicalresistance of the collectorThe black hose and the PET glazingboth have excellent durability

Seventh the number of hydraulic joints is reduced tojust one pair this reduction is a key to guarantee the systemreliability considering that barb fittings are used

6 ISRN Renewable Energy

Table 3 Equivalent water inventory (106 liter) obtained by different hose diameters Flows calculated for 06 bar district pressure and pricesfor 25-bar hoses

Diameter (inches) Length (m) Max flow (litersmin) Normal surface (m2) Price (U$D)210158401015840 56 212 34 601510158401015840 100 84 46 5012510158401015840 144 45 56 80110158401015840 224 20 68 10007510158401015840 400 66 98 130

Table 4 Parallel configurations based on many 100-meter sections for 06 bar district pressure Prices include hydraulic joints

Diameter (inches) Number of lines () Max flow (litersmin) Inventory (liters) Solar area (m2) Price (U$D)0510158401015840 9 43 106 460 20007510158401015840 4 56 106 307 150110158401015840 2 60 95 199 100

This design is developed intending to get the simplest col-lector rather than the most efficient one This way both low-cost and home-made construction objectives are achievedBesides its efficiency is expected to be better than averageregarding the solar pond and cylindrical shape featuresThesefeatures allow it to be mounted at a nonoptimal angle acondition desirable in order to eliminate the costs of technicalinstallation and mechanical supporting structures

4 Thermal-Hydraulic Analysis

41 Hydraulic Modeling Optimizing the hydraulic behavioris key in providing the desired hot-water flow for consump-tion meanwhile the large surface of the hose is excellent tocollect solar energy The maximum flow is obtained whenthe driving force (district-grid pressure) is balanced by allpressure drops along the line The total pressure drop canbe calculated as the sum of concentrated and distributedlosses relating to the mean flow velocity (119881) the length (119871)and diameter (119863) of hose and the concentrated (119870

119888) and

frictional (119891) coefficients of losses as [12]

Δ119901 =1

2(119870119888+ 119891

119871

119863)1205881198812 (2)

The frictional Darcyrsquos coefficient 119891 is related to the flowReynolds number (Re = 119881119863120588]) based on dynamic viscosity(]) and density (120588) of fluid for turbulent flows into smoothpipes it can be approximated by [12]

119891 = 031Reminus025 (3)

On the other hand the 119870119888value is related to the geometry of

restriction considered for example considering a 100m hosemounted on a roof of a 10 meter slope which is bent witha long radius curve (119870

119888= 2) along the edge of the roof a

total119870119888= 20 is obtained since this value is much lower than

the total distributed coefficient (119891119871119863) the results are almostindependent of the hose layout

Table 3 shows the flow calculated for different hosediameters It is observed that a 100m 1510158401015840 hose is maybe thesimplest choice balancing hydraulic solar normal area (119863 lowast

119871) and cost behaviors and provides enough (106 liters) hotwater The thinnest hose of equivalent volume is heated veryquickly but provides low flow meanwhile a larger diameterhose (210158401015840) has a small solar area In addition a larger diameterimplies a thicker wall (5mm for a 210158401015840 hose against 3mm fora 1510158401015840 one considering 4-bar hoses) in order to withstandthe inner pressure which in turn implies a more expensivehose and that is harder to bend As a general rule diametersbetween 110158401015840 and 1510158401015840 are right for a simple system providingenough flow on a modest 06 bar district pressure Of coursethere is no single solution depending on many factors forexample district pressure solar resource and climate patternas well as practical conditions like hose diameters availablein local hardware stores and the pattern of use For examplefor application at a day school it is impractical to keepwater warm during the night using a large-diameter hose (apractical technique that will be studied in the next section)and on the other hand it can be desirable to use thin hoses forquick heating in the morning

Let us remark on the flexibility and open configuration ofthis design For example if a too thin 07510158401015840 hose is selectedand the flow is too low the user could always improve itby dividing the 400m long hose in two parallel 200m linesthis way and according to the previous case shown in Table 3the flow is almost tripled obtaining a very good (194 Lmin)flow Table 4 illustrates different configurations based on rollsof a 100m hose connected in parallel intending to achievehigh flow and high solar area This configuration allows theuse of the thinnest hose possible to maximize the surfaceor conversely to use larger diameter hoses in order to lowercosts

We also could think about a ldquomixed assemblyrdquo intendingto balance the maximum solar area and maximum flowcriteria This can be achieved by just adding an onoff valveon each parallel line to obtain water heated quickly duringthe morning using a thin hose together with another fractionkept warm during the evening in the large-diameter hoseThe users could use water from the most convenient linein any case supported by temperature signals Furthermoreeven smarter designs could be achieved for largest systemsby adding a microcontroller This apparently sophisticated

ISRN Renewable Energy 7

feature is possible using a standard garden irrigation systemthat costs fifty dollars

All these features illustrate the open configuration andsimplicity of this design a key in solving userrsquos barrierspreviously analyzed after all there is nothing more accessiblethan an LDPE hose Moreover the normal surface noticeablylarger than on a standard flat solar collector is useful forachieving good heating efficiencies a characteristic rein-forced by the solar-pond concept as will be studied now

42 Thermal Modeling Together with hydraulics the ther-mal analysis is a key in developing an efficient home-madecollector The thermal behavior along the daily transient isdescribed by its dynamic modeling in which the evolutionof the normal solar flux (119868

119899) must be considered For a

cylinder whose generatrix has a north-south orientation itsnormal surface 119878

119899exposed to sun rays is independent of the

azimuthal (120595) solar angle and is only related to the altitudesolar angle (120572) and the roof title angle (120573) according to

119878119899= 119863119871 sin (120572 + 120573) if 120572 gt 0 (during day)

or 119878119899= 0 if 120572 lt 0 (during night)

(4)

Where for any hour (119905) of a given day (119889) the solar altitudeat a certain latitude (120579) location having a 120575 declination anglecan be calculated going through (5)

120575 = 2345 sin(360 (119889 minus 81)365

) for 119889 = 1 2 365

120595 =360∘119905

24 hminus 180

∘ for 0 lt 119905 lt 24 h

1198621= sin (120579) sin (120575)

1198622= cos (120579) cos (120575)

1198781= 1198621+ 1198622cos (120595)

1198782= radic1 minus 1198622

1

120572 = arctan(11987811198782

)

(5)

From here we can calculate the ldquoidealrdquo (on a sunny day) solarpower 119875irrad received in any instant as

119875irrad = 119878119899119868119899 if 120572 gt 0 (6)

The energy absorbed by the collector and the heat losses areboth considered within the efficiency equation (1) So the netpower gained in any instant is

119875net = 119875irrad 120583 = 119878119899119868119899 (1198860 minus 1198861(119879 minus 119879

119886)

119868119899

) (7)

Hence the dynamic energy-balance equation for a hose-based collector can be numerically calculated by approximat-ing the temperature rate by its differential increment (119879

119899minus

119879119899minus1

) on the n-sime time step (Δ119905) as

119898119888119901

119889119879

119889119905asymp 119898119888119901

119879119899minus 119879119899minus1

Δ119905= 119875net (8)

010203040506070

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 7 Daily temperature evolution for middle season (119889 = 81)at a temperate location

where 119898 and 119888119901

are mass and heat capacity of waterrespectively From here the temperature of collector in then-sime time step can be derived as

119879119899=1198781198991198860119868 + 119863119871119886

1119879119886+ (119898119888

119901119879119899minus1Δ119905)

(119898119888119901Δ119905) + 119863119871119886

1

(9)

So the water temperature can be numerically simulated byusing this last equation during the day or modified for nightuse (by setting 119878

119899= 0) and by setting 119879 = 119879

119886during the

earliest hours before sunrise For this preliminary study theirradiance flux from the sun reaching the Earthrsquos surface (119868)can be considered as constant along the day despite changesinduced by atmospheric conditions and for simple field testswithout local measurements of 119868

119899 this constant 119868 value could

be estimated from average data of the daily total irradiance11986610158401015840 on a level surface available from local solar mapsLet us calculate the performance of a 1510158401015840-diameter 100m

hose (119898 = 106Kg 119863 = 00368m and 119871 = 100m) withone PET glazing (119886

0= 08 119886

1= 8Wm2 ∘C) mounted over

an inclined (120573 = 40∘) north-oriented roof located in BuenosAires (120579 = minus38∘) on 21 March (119889 = 81 120575 = 0) From theArgentinean solar map [13] the monthly averaged daily solarirradiance is 11986610158401015840 = 45 kWhm2 and hence 119868 = 440Wm2in addition from climatic data 119879

119886can be approximated by

a sinusoidal function of a half-day period of 20∘C plusmn 5∘CUsing this set of parameter data the temperature evolutioncan be easily simulated by using a spreadsheetThe numericalconvergence shows that a time step of 360 seconds providesaccuracy results using this value the temperature evolution isillustrated in Figure 7 Here different trends are observed

(a) The temperature increases sharply from sunrise (at6 am) and useful levels (above 35∘C) are achievedfrom 830 am to 10 pm which comprises the rangeof utilization for average users

(b) A noticeable peak (62∘C) is obtained at 4 pm andvery high levels (above 45∘C) are kept between 10 amand 8 pm which provides a long range for exigentusers

(c) The temperature decreases sharply after sunset reach-ing 28∘C at midnight

It is interesting to study the evolution of efficiency alongthe day illustrated in Figure 8 The collector starts in the

8 ISRN Renewable Energy

6 8 10 12 14 16 18 200

20

40

60

80

100

minus20

minus40

minus60

()

Figure 8 Daily evolution of efficiency for the previous case (119889 =

81)

0

10

20

30

40

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 9 Daily temperature evolution for winter (119889 = 182)conditions

Table 5 Sensitivity analysis of roof inclination for winter (119889 = 182)conditions

120573 119879peak 119879 at 6 pm 119879 at 8 pm0∘ 23∘C 21∘C 19∘C20∘ 32∘C 29∘C 24∘C40∘ 38∘C 34∘C 28∘C60∘ 42∘C 37∘C 30∘C80∘ 42∘C 37∘C 31∘C90∘ 41∘C 37∘C 30∘C

morning with an impressive efficiency (80) and reaches auseful temperature of 35∘C at 830 am having an averageefficiency of 64 up to this moment This fact together withthe large solar area provided by the hose (enhanced by itscylindrical shape) explains this excellent performance Onthe other hand the negative efficiency observed at night is itsmajor drawback related to conventional collectors in whichthe heated water is stored into the insulated tank

The collector shows excellent performance during thelast-summer day but the sharp decrease during the nightopens a doubt about winter performanceThis case is studiedon 1st July (119889 = 182) for the same location (11986610158401015840 = 2 kWhm2so 119868 = 250Wm2 and119879

119886= 10∘Cplusmn8∘C) in Figure 9 Here it is

observed that the collector becomes a useful preheater duringthe day but never provides hot water This performance canbe improved by adding a second glazing layer (119886

0= 075

1198861= 5Wm2 ∘C)This way aminor improvement is obtained

with a peak temperature (38∘C) higher than the previousvalue (34∘C)

Figure 10 A simple test facility example

0102030405060708090

100

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 11 Temperatures in summer (119889 = 1) for simple and doubleglazing

Table 6 Sensitivity analysis of hose diameter for winter conditionsand 120573 = 60∘

Diameter 119879peak 119879 at 6 pm 119879 at 8 pm0510158401015840 53∘C 36∘C 22∘C07510158401015840 50∘C 39∘C 27∘C110158401015840 47∘C 39∘C 29∘C12510158401015840 44∘C 38∘C 30∘C1510158401015840 41∘C 37∘C 30∘C210158401015840 37∘C 34∘C 30∘C

Several sensitivity analyses can be performed for thislast condition Table 5 shows that collectors mounted onroofs with a steep incline get higher temperatures that thosemounted on barely inclined roofs as those use better theevening sun rays This way the peak temperature obtainedfor a level-surface roof is just 23∘C against 42∘C for anotherinclined 60∘ The influence of hose diameter is shown inTable 6 showing that thinner hoses get higher temperaturesbut their cooling at night is sharper too Again here the bestchoice is found between 110158401015840 and 1510158401015840 hoses Figure 10 shows asimple test facility that comprises three ldquonakedrdquo hoses (0510158401015840110158401015840 and 1510158401015840) and similarly the same hoses but wrapped with

ISRN Renewable Energy 9

Water grid Consumption

Mechanical thermostat12

998400998400 hose

1 5998400998400 hose

6998400998400 hose

Figure 12 Schematic drawing of the three-line mixed system

Table 7 Performance of the three-line system by using previous conditions (120573 = 40∘)

Diameter Summer (119889 = 1) Autumn (119889 = 81) Winter (119889 = 182)119879peak 119879 at 10 pm 119879peak 119879 at 10 pm 119879peak 119879 at 10 pm

610158401015840 53∘C 50∘C 42∘C 39∘C 23∘C 20∘C1 510158401015840 76∘C 46∘C 65∘C 33∘C 35∘C 14∘C0510158401015840 82∘C 25∘C 73∘C 16∘C 40∘C 6∘C

double layer of air-packed transparent polyethylene film avery low-cost glazing (03USDm2)

Finally it is interesting to study the performance on hotsummer days (1st January 119889 = 1) at this temperate location(119879119886= 25∘C plusmn 10∘C 11986610158401015840 = 65 kWhm2 so 119868 = 500Wm2)

on a 60∘ inclined roof Figure 11 compares temperatures forsingle and double glazing for this case Here we observethat both systems provide enough hot water throughoutthe day but the double glazing reaches a dangerous peakof 90∘C a temperature too high for LDPE Of course theflow of consumption is a simple way to limit this effect butthe system must withstand extreme cases like for exampleduring the holidays Thus this behavior should be checkedbefore choosing reinforced insulation

Regarding this concern about overheating it is interestingto study the thermal performance of a ldquomixed systemrdquo builtwith several hoses of different diameters assembled on theparallel layout illustrated in Figure 12 In this example weconsider the following

(a) A 610158401015840 PVC tube of 8-meter length (106 liters) withdouble glazing

(b) A 100m 1510158401015840 hose (106 liters) with single glazing

(c) A 100m 0510158401015840 hose (12 liters) with single glazing

This way we simultaneously obtain the following

(i) A small volume of quickly heated water in themorning (by the 0510158401015840 hose) that can be quicklyldquoreplenishedrdquo every hour

(ii) A large volume (106 liters) available throughout theday and evening

(iii) A large volume (106 liters) kept warm overnight

The user could choose the most convenient line at anytime or this choice could be automatized by using low-cost microcontrolled onoff valves Another simple solutionconsists in adding a bimetallic thermostat on the exit of eachline This option is less flexible than the previous one but thethermostat is a very reliable and low-cost device

The use of a very large diameter (610158401015840) is useful to providehot water throughout the night and besides it supports thechoice of using single glazing on the thinner hoses this wayavoiding dangerous overheating during hot summer daysTable 7 shows the results of the thermal simulation of thissystem by using the same cases previously studied Here isobserved that all objectives are successfully achieved

5 Conclusions

This paper presents a new design of a low-cost and robustsolar collector It was developed intending to fit the lack ofa simple home-made and self-installation device a key forsuccess in developing countries To achieve simultaneously allthese goals a new thermal-hydraulic paradigmwas proposedfrom which we have created a new design for a solarcollector that takes full advantage of new plastic and tubingtechnologies Although rather similar designs are well knownfrom the open literature the comprehensive thermal andhydraulic discussion (for both stationary and daily transientstates) developed here is a worthy contribution of this workOn this base it is easy to ldquotune uprdquo the collector regardingthe specific solar and climate resources among other issuesIn this way different hose diameters and glazing can be usedin order to obtain the best solution for any case The use ofcontrolling systems and mixing configurations are also cost-effective options to enhance the performance

In this paper the deeply rooted thermal-hydraulicparadigm of conventional collectors is discussed In addition

10 ISRN Renewable Energy

this work presents the advantages of the water-pond designwhich has long been known but its benefits have never beenacknowledged [14] The improved efficiency of water pondsregarding conventional solar collectors is a key to achieveefficient low-cost collectors even in temperate climates

Acknowledgments

Argentinean Du Pont enterprise is acknowledged for itssupport by funds given through the National Prize Du Pontto Clean Energies 2009 to this project Grace De Haro isacknowledged for her careful translation

References

[1] S Ozlu I Dincer and G F Naterer ldquoComparative assessmentof residential energy options in Ontario Canadardquo Energy andBuildings vol 55 pp 674ndash684 2012

[2] L T Kostic and Z T Pavlovic ldquoOptimal position of flat platereflectors of solar thermal collectorrdquo Energy and Buildings vol45 pp 161ndash168 2012

[3] M Khalaji Assadi A Khalesi Doost A A Hamidi and MMizani ldquoDesign construction and performance testing of anew system for energy saving in rural buildingsrdquo Energy andBuildings vol 43 no 12 pp 3303ndash3310 2011

[4] Sociedade Do Sol httpwwwsociedadedosolorgbr[5] L Juanico ldquoA new design of roof-integrated water solar collec-

tor for domestic heating and coolingrdquo Solar Energy vol 82 no6 pp 481ndash492 2008

[6] L E Juanico ldquoA new design of configurable solar awning formanaging cooling and heating loadsrdquo Energy and Buildings vol41 no 12 pp 1381ndash1385 2009

[7] L E Juanico ldquoNew design of solar roof for household heatingand coolingrdquo International Journal of Hydrogen Energy vol 35no 11 pp 5823ndash5826 2010

[8] R Sarachitti C Chotetanorm C Lertsatitthanakorn and MRungsiyopas ldquoThermal performance analysis and economicevaluation of roof-integrated solar concrete collectorrdquo Energyand Buildings vol 43 no 6 pp 1403ndash1408 2011

[9] H R Hay and J I Yellott ldquoInternational aspects of air condi-tioning with movable insulationrdquo Solar Energy vol 12 no 4pp 427ndash430 1969

[10] E Aranovitch ldquoHeat transfer processes in solar collectorsrdquoEnergy and Buildings vol 3 no 1 pp 31ndash47 1981

[11] P T Tsilingiris ldquoTowards making solar water heating tech-nology feasiblemdashthe polymer solar collector approachrdquo EnergyConversion andManagement vol 40 no 12 pp 1237ndash1250 1999

[12] F White Fluid Mechanics McGraw-Hill New York NY USA6th edition 2006

[13] R Righini H G Gallegos and C Raichijk ldquoApproach to draw-ing new global solar irradiation contour maps for ArgentinardquoRenewable Energy vol 30 no 8 pp 1241ndash1255 2005

[14] M N A Hawlader and B J Brinkworth ldquoAn analysis of thenon-convecting solar pondrdquo Solar Energy vol 27 no 3 pp 195ndash204 1981

TribologyAdvances in

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International Journal of

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FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

ISRN Renewable Energy 5

Table 2 Characteristic parameters for water-pond collectors Efficiency calculated for 119879119886= 10∘C 119879

119888= 119879119898= 30∘C and 119868

119899= 600Wm2 ∘C

improvement related to Table 1

Collector type 1198860() 119886

1(Wm2 ∘C) Efficiency () Improvement ()

Unglazed 085 18 025 200Single glazing 080 8 053 96Double glazing 075 5 058 38

Figure 5 View of the tubing collector wrapped with recycled PETbottles

by thermal sealing (which is the last design proposed forany roof) is prohibitive for a do-it-yourself project Thus itis necessary to use simpler construction techniques to fit thisrequirement as the next design shows

223 The Tubing Collector This collector uses many LDPEblack tubing wrapped with PET glazing in order to integratethe hydraulic grid with the absorbing plate Taking advan-tage of very low costs of LDPE tubes this collector usesmany thin (0510158401015840) hoses in order to maximize the absorbingarea (see Figure 5) Many collectors of this type have beenconstructed in Brazil but the experience has shown thatits construction is cumbersome due to the extensive fittingrequired In addition its thermal efficiency is low since it isa conventional natural-convection collector Therefore it hasnot been successful although it provides a low-cost solutionavailable for a do-it-yourself project Up to now there is a lackof a feasible collector that achieves all the desired goals

3 The New Design forUltra-Low-Cost Collectors

Here a new design is presented of solar collectors speciallydeveloped for exploiting the new features of plastic tubingtechnologiesThis collector has all the advantages of previousdesigns and more due to a new paradigm regarding itsthermal-hydraulic behavior It is a single long LDPE hose ofa large diameter connected in series between district gridand consumption The full collector is completed with atransparent plastic layer mounted around this tube (selectedamong several ultra-low-cost options like PET thin filmsPET recycled soda bottles or air-packed LDPE films) and

District watergrid

Consumption

Figure 6 Schematic drawing of the hose collector mounted on theroof

simply resting on the roof (see Figure 6) This way manyadvantages are simultaneously obtained

First a third option is proposed for the dilemma betweennatural and forced convection by using district grid asdriven force which is noticeably higher than the buoyancyforce involved (about 500 times) and so a highly restrictedhydraulic configuration is feasible The consumption flow iscirculated through the whole collector but the pressure dropcan be managed by choosing a large diameter hose

Second the storage tank is eliminated Selecting a largediameter hose the whole water inventory is stored insideBesides many valves and connections are eliminated

Third according to its water-pond design the efficiency isvery good and can collect the solar irradiance throughout theday

Fourth the selection of an LDPE hose allows it tomanageits volumetric variations due to thermal dilatations and iceformation LDPE provides flexible long hoses of very lowcosts (U$D 60 for a 1510158401015840 diameter 100 meter hose) that canwithstand pressures of 6 bars or more and that still can beeasily bent by heating it

Fifth the absorbing plate is removedThe solar irradiationis absorbed directly by the hose This way concerns aboutlow thermal conductivity and high thermal gradients into theabsorbing plate are avoided

Sixth the transparent glazing and back insulation areboth provided by a single (or double for colder locations)transparent layer at a very low cost In addition the sup-porting structure is eliminated enhancing the mechanicalresistance of the collectorThe black hose and the PET glazingboth have excellent durability

Seventh the number of hydraulic joints is reduced tojust one pair this reduction is a key to guarantee the systemreliability considering that barb fittings are used

6 ISRN Renewable Energy

Table 3 Equivalent water inventory (106 liter) obtained by different hose diameters Flows calculated for 06 bar district pressure and pricesfor 25-bar hoses

Diameter (inches) Length (m) Max flow (litersmin) Normal surface (m2) Price (U$D)210158401015840 56 212 34 601510158401015840 100 84 46 5012510158401015840 144 45 56 80110158401015840 224 20 68 10007510158401015840 400 66 98 130

Table 4 Parallel configurations based on many 100-meter sections for 06 bar district pressure Prices include hydraulic joints

Diameter (inches) Number of lines () Max flow (litersmin) Inventory (liters) Solar area (m2) Price (U$D)0510158401015840 9 43 106 460 20007510158401015840 4 56 106 307 150110158401015840 2 60 95 199 100

This design is developed intending to get the simplest col-lector rather than the most efficient one This way both low-cost and home-made construction objectives are achievedBesides its efficiency is expected to be better than averageregarding the solar pond and cylindrical shape featuresThesefeatures allow it to be mounted at a nonoptimal angle acondition desirable in order to eliminate the costs of technicalinstallation and mechanical supporting structures

4 Thermal-Hydraulic Analysis

41 Hydraulic Modeling Optimizing the hydraulic behavioris key in providing the desired hot-water flow for consump-tion meanwhile the large surface of the hose is excellent tocollect solar energy The maximum flow is obtained whenthe driving force (district-grid pressure) is balanced by allpressure drops along the line The total pressure drop canbe calculated as the sum of concentrated and distributedlosses relating to the mean flow velocity (119881) the length (119871)and diameter (119863) of hose and the concentrated (119870

119888) and

frictional (119891) coefficients of losses as [12]

Δ119901 =1

2(119870119888+ 119891

119871

119863)1205881198812 (2)

The frictional Darcyrsquos coefficient 119891 is related to the flowReynolds number (Re = 119881119863120588]) based on dynamic viscosity(]) and density (120588) of fluid for turbulent flows into smoothpipes it can be approximated by [12]

119891 = 031Reminus025 (3)

On the other hand the 119870119888value is related to the geometry of

restriction considered for example considering a 100m hosemounted on a roof of a 10 meter slope which is bent witha long radius curve (119870

119888= 2) along the edge of the roof a

total119870119888= 20 is obtained since this value is much lower than

the total distributed coefficient (119891119871119863) the results are almostindependent of the hose layout

Table 3 shows the flow calculated for different hosediameters It is observed that a 100m 1510158401015840 hose is maybe thesimplest choice balancing hydraulic solar normal area (119863 lowast

119871) and cost behaviors and provides enough (106 liters) hotwater The thinnest hose of equivalent volume is heated veryquickly but provides low flow meanwhile a larger diameterhose (210158401015840) has a small solar area In addition a larger diameterimplies a thicker wall (5mm for a 210158401015840 hose against 3mm fora 1510158401015840 one considering 4-bar hoses) in order to withstandthe inner pressure which in turn implies a more expensivehose and that is harder to bend As a general rule diametersbetween 110158401015840 and 1510158401015840 are right for a simple system providingenough flow on a modest 06 bar district pressure Of coursethere is no single solution depending on many factors forexample district pressure solar resource and climate patternas well as practical conditions like hose diameters availablein local hardware stores and the pattern of use For examplefor application at a day school it is impractical to keepwater warm during the night using a large-diameter hose (apractical technique that will be studied in the next section)and on the other hand it can be desirable to use thin hoses forquick heating in the morning

Let us remark on the flexibility and open configuration ofthis design For example if a too thin 07510158401015840 hose is selectedand the flow is too low the user could always improve itby dividing the 400m long hose in two parallel 200m linesthis way and according to the previous case shown in Table 3the flow is almost tripled obtaining a very good (194 Lmin)flow Table 4 illustrates different configurations based on rollsof a 100m hose connected in parallel intending to achievehigh flow and high solar area This configuration allows theuse of the thinnest hose possible to maximize the surfaceor conversely to use larger diameter hoses in order to lowercosts

We also could think about a ldquomixed assemblyrdquo intendingto balance the maximum solar area and maximum flowcriteria This can be achieved by just adding an onoff valveon each parallel line to obtain water heated quickly duringthe morning using a thin hose together with another fractionkept warm during the evening in the large-diameter hoseThe users could use water from the most convenient linein any case supported by temperature signals Furthermoreeven smarter designs could be achieved for largest systemsby adding a microcontroller This apparently sophisticated

ISRN Renewable Energy 7

feature is possible using a standard garden irrigation systemthat costs fifty dollars

All these features illustrate the open configuration andsimplicity of this design a key in solving userrsquos barrierspreviously analyzed after all there is nothing more accessiblethan an LDPE hose Moreover the normal surface noticeablylarger than on a standard flat solar collector is useful forachieving good heating efficiencies a characteristic rein-forced by the solar-pond concept as will be studied now

42 Thermal Modeling Together with hydraulics the ther-mal analysis is a key in developing an efficient home-madecollector The thermal behavior along the daily transient isdescribed by its dynamic modeling in which the evolutionof the normal solar flux (119868

119899) must be considered For a

cylinder whose generatrix has a north-south orientation itsnormal surface 119878

119899exposed to sun rays is independent of the

azimuthal (120595) solar angle and is only related to the altitudesolar angle (120572) and the roof title angle (120573) according to

119878119899= 119863119871 sin (120572 + 120573) if 120572 gt 0 (during day)

or 119878119899= 0 if 120572 lt 0 (during night)

(4)

Where for any hour (119905) of a given day (119889) the solar altitudeat a certain latitude (120579) location having a 120575 declination anglecan be calculated going through (5)

120575 = 2345 sin(360 (119889 minus 81)365

) for 119889 = 1 2 365

120595 =360∘119905

24 hminus 180

∘ for 0 lt 119905 lt 24 h

1198621= sin (120579) sin (120575)

1198622= cos (120579) cos (120575)

1198781= 1198621+ 1198622cos (120595)

1198782= radic1 minus 1198622

1

120572 = arctan(11987811198782

)

(5)

From here we can calculate the ldquoidealrdquo (on a sunny day) solarpower 119875irrad received in any instant as

119875irrad = 119878119899119868119899 if 120572 gt 0 (6)

The energy absorbed by the collector and the heat losses areboth considered within the efficiency equation (1) So the netpower gained in any instant is

119875net = 119875irrad 120583 = 119878119899119868119899 (1198860 minus 1198861(119879 minus 119879

119886)

119868119899

) (7)

Hence the dynamic energy-balance equation for a hose-based collector can be numerically calculated by approximat-ing the temperature rate by its differential increment (119879

119899minus

119879119899minus1

) on the n-sime time step (Δ119905) as

119898119888119901

119889119879

119889119905asymp 119898119888119901

119879119899minus 119879119899minus1

Δ119905= 119875net (8)

010203040506070

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 7 Daily temperature evolution for middle season (119889 = 81)at a temperate location

where 119898 and 119888119901

are mass and heat capacity of waterrespectively From here the temperature of collector in then-sime time step can be derived as

119879119899=1198781198991198860119868 + 119863119871119886

1119879119886+ (119898119888

119901119879119899minus1Δ119905)

(119898119888119901Δ119905) + 119863119871119886

1

(9)

So the water temperature can be numerically simulated byusing this last equation during the day or modified for nightuse (by setting 119878

119899= 0) and by setting 119879 = 119879

119886during the

earliest hours before sunrise For this preliminary study theirradiance flux from the sun reaching the Earthrsquos surface (119868)can be considered as constant along the day despite changesinduced by atmospheric conditions and for simple field testswithout local measurements of 119868

119899 this constant 119868 value could

be estimated from average data of the daily total irradiance11986610158401015840 on a level surface available from local solar mapsLet us calculate the performance of a 1510158401015840-diameter 100m

hose (119898 = 106Kg 119863 = 00368m and 119871 = 100m) withone PET glazing (119886

0= 08 119886

1= 8Wm2 ∘C) mounted over

an inclined (120573 = 40∘) north-oriented roof located in BuenosAires (120579 = minus38∘) on 21 March (119889 = 81 120575 = 0) From theArgentinean solar map [13] the monthly averaged daily solarirradiance is 11986610158401015840 = 45 kWhm2 and hence 119868 = 440Wm2in addition from climatic data 119879

119886can be approximated by

a sinusoidal function of a half-day period of 20∘C plusmn 5∘CUsing this set of parameter data the temperature evolutioncan be easily simulated by using a spreadsheetThe numericalconvergence shows that a time step of 360 seconds providesaccuracy results using this value the temperature evolution isillustrated in Figure 7 Here different trends are observed

(a) The temperature increases sharply from sunrise (at6 am) and useful levels (above 35∘C) are achievedfrom 830 am to 10 pm which comprises the rangeof utilization for average users

(b) A noticeable peak (62∘C) is obtained at 4 pm andvery high levels (above 45∘C) are kept between 10 amand 8 pm which provides a long range for exigentusers

(c) The temperature decreases sharply after sunset reach-ing 28∘C at midnight

It is interesting to study the evolution of efficiency alongthe day illustrated in Figure 8 The collector starts in the

8 ISRN Renewable Energy

6 8 10 12 14 16 18 200

20

40

60

80

100

minus20

minus40

minus60

()

Figure 8 Daily evolution of efficiency for the previous case (119889 =

81)

0

10

20

30

40

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 9 Daily temperature evolution for winter (119889 = 182)conditions

Table 5 Sensitivity analysis of roof inclination for winter (119889 = 182)conditions

120573 119879peak 119879 at 6 pm 119879 at 8 pm0∘ 23∘C 21∘C 19∘C20∘ 32∘C 29∘C 24∘C40∘ 38∘C 34∘C 28∘C60∘ 42∘C 37∘C 30∘C80∘ 42∘C 37∘C 31∘C90∘ 41∘C 37∘C 30∘C

morning with an impressive efficiency (80) and reaches auseful temperature of 35∘C at 830 am having an averageefficiency of 64 up to this moment This fact together withthe large solar area provided by the hose (enhanced by itscylindrical shape) explains this excellent performance Onthe other hand the negative efficiency observed at night is itsmajor drawback related to conventional collectors in whichthe heated water is stored into the insulated tank

The collector shows excellent performance during thelast-summer day but the sharp decrease during the nightopens a doubt about winter performanceThis case is studiedon 1st July (119889 = 182) for the same location (11986610158401015840 = 2 kWhm2so 119868 = 250Wm2 and119879

119886= 10∘Cplusmn8∘C) in Figure 9 Here it is

observed that the collector becomes a useful preheater duringthe day but never provides hot water This performance canbe improved by adding a second glazing layer (119886

0= 075

1198861= 5Wm2 ∘C)This way aminor improvement is obtained

with a peak temperature (38∘C) higher than the previousvalue (34∘C)

Figure 10 A simple test facility example

0102030405060708090

100

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 11 Temperatures in summer (119889 = 1) for simple and doubleglazing

Table 6 Sensitivity analysis of hose diameter for winter conditionsand 120573 = 60∘

Diameter 119879peak 119879 at 6 pm 119879 at 8 pm0510158401015840 53∘C 36∘C 22∘C07510158401015840 50∘C 39∘C 27∘C110158401015840 47∘C 39∘C 29∘C12510158401015840 44∘C 38∘C 30∘C1510158401015840 41∘C 37∘C 30∘C210158401015840 37∘C 34∘C 30∘C

Several sensitivity analyses can be performed for thislast condition Table 5 shows that collectors mounted onroofs with a steep incline get higher temperatures that thosemounted on barely inclined roofs as those use better theevening sun rays This way the peak temperature obtainedfor a level-surface roof is just 23∘C against 42∘C for anotherinclined 60∘ The influence of hose diameter is shown inTable 6 showing that thinner hoses get higher temperaturesbut their cooling at night is sharper too Again here the bestchoice is found between 110158401015840 and 1510158401015840 hoses Figure 10 shows asimple test facility that comprises three ldquonakedrdquo hoses (0510158401015840110158401015840 and 1510158401015840) and similarly the same hoses but wrapped with

ISRN Renewable Energy 9

Water grid Consumption

Mechanical thermostat12

998400998400 hose

1 5998400998400 hose

6998400998400 hose

Figure 12 Schematic drawing of the three-line mixed system

Table 7 Performance of the three-line system by using previous conditions (120573 = 40∘)

Diameter Summer (119889 = 1) Autumn (119889 = 81) Winter (119889 = 182)119879peak 119879 at 10 pm 119879peak 119879 at 10 pm 119879peak 119879 at 10 pm

610158401015840 53∘C 50∘C 42∘C 39∘C 23∘C 20∘C1 510158401015840 76∘C 46∘C 65∘C 33∘C 35∘C 14∘C0510158401015840 82∘C 25∘C 73∘C 16∘C 40∘C 6∘C

double layer of air-packed transparent polyethylene film avery low-cost glazing (03USDm2)

Finally it is interesting to study the performance on hotsummer days (1st January 119889 = 1) at this temperate location(119879119886= 25∘C plusmn 10∘C 11986610158401015840 = 65 kWhm2 so 119868 = 500Wm2)

on a 60∘ inclined roof Figure 11 compares temperatures forsingle and double glazing for this case Here we observethat both systems provide enough hot water throughoutthe day but the double glazing reaches a dangerous peakof 90∘C a temperature too high for LDPE Of course theflow of consumption is a simple way to limit this effect butthe system must withstand extreme cases like for exampleduring the holidays Thus this behavior should be checkedbefore choosing reinforced insulation

Regarding this concern about overheating it is interestingto study the thermal performance of a ldquomixed systemrdquo builtwith several hoses of different diameters assembled on theparallel layout illustrated in Figure 12 In this example weconsider the following

(a) A 610158401015840 PVC tube of 8-meter length (106 liters) withdouble glazing

(b) A 100m 1510158401015840 hose (106 liters) with single glazing

(c) A 100m 0510158401015840 hose (12 liters) with single glazing

This way we simultaneously obtain the following

(i) A small volume of quickly heated water in themorning (by the 0510158401015840 hose) that can be quicklyldquoreplenishedrdquo every hour

(ii) A large volume (106 liters) available throughout theday and evening

(iii) A large volume (106 liters) kept warm overnight

The user could choose the most convenient line at anytime or this choice could be automatized by using low-cost microcontrolled onoff valves Another simple solutionconsists in adding a bimetallic thermostat on the exit of eachline This option is less flexible than the previous one but thethermostat is a very reliable and low-cost device

The use of a very large diameter (610158401015840) is useful to providehot water throughout the night and besides it supports thechoice of using single glazing on the thinner hoses this wayavoiding dangerous overheating during hot summer daysTable 7 shows the results of the thermal simulation of thissystem by using the same cases previously studied Here isobserved that all objectives are successfully achieved

5 Conclusions

This paper presents a new design of a low-cost and robustsolar collector It was developed intending to fit the lack ofa simple home-made and self-installation device a key forsuccess in developing countries To achieve simultaneously allthese goals a new thermal-hydraulic paradigmwas proposedfrom which we have created a new design for a solarcollector that takes full advantage of new plastic and tubingtechnologies Although rather similar designs are well knownfrom the open literature the comprehensive thermal andhydraulic discussion (for both stationary and daily transientstates) developed here is a worthy contribution of this workOn this base it is easy to ldquotune uprdquo the collector regardingthe specific solar and climate resources among other issuesIn this way different hose diameters and glazing can be usedin order to obtain the best solution for any case The use ofcontrolling systems and mixing configurations are also cost-effective options to enhance the performance

In this paper the deeply rooted thermal-hydraulicparadigm of conventional collectors is discussed In addition

10 ISRN Renewable Energy

this work presents the advantages of the water-pond designwhich has long been known but its benefits have never beenacknowledged [14] The improved efficiency of water pondsregarding conventional solar collectors is a key to achieveefficient low-cost collectors even in temperate climates

Acknowledgments

Argentinean Du Pont enterprise is acknowledged for itssupport by funds given through the National Prize Du Pontto Clean Energies 2009 to this project Grace De Haro isacknowledged for her careful translation

References

[1] S Ozlu I Dincer and G F Naterer ldquoComparative assessmentof residential energy options in Ontario Canadardquo Energy andBuildings vol 55 pp 674ndash684 2012

[2] L T Kostic and Z T Pavlovic ldquoOptimal position of flat platereflectors of solar thermal collectorrdquo Energy and Buildings vol45 pp 161ndash168 2012

[3] M Khalaji Assadi A Khalesi Doost A A Hamidi and MMizani ldquoDesign construction and performance testing of anew system for energy saving in rural buildingsrdquo Energy andBuildings vol 43 no 12 pp 3303ndash3310 2011

[4] Sociedade Do Sol httpwwwsociedadedosolorgbr[5] L Juanico ldquoA new design of roof-integrated water solar collec-

tor for domestic heating and coolingrdquo Solar Energy vol 82 no6 pp 481ndash492 2008

[6] L E Juanico ldquoA new design of configurable solar awning formanaging cooling and heating loadsrdquo Energy and Buildings vol41 no 12 pp 1381ndash1385 2009

[7] L E Juanico ldquoNew design of solar roof for household heatingand coolingrdquo International Journal of Hydrogen Energy vol 35no 11 pp 5823ndash5826 2010

[8] R Sarachitti C Chotetanorm C Lertsatitthanakorn and MRungsiyopas ldquoThermal performance analysis and economicevaluation of roof-integrated solar concrete collectorrdquo Energyand Buildings vol 43 no 6 pp 1403ndash1408 2011

[9] H R Hay and J I Yellott ldquoInternational aspects of air condi-tioning with movable insulationrdquo Solar Energy vol 12 no 4pp 427ndash430 1969

[10] E Aranovitch ldquoHeat transfer processes in solar collectorsrdquoEnergy and Buildings vol 3 no 1 pp 31ndash47 1981

[11] P T Tsilingiris ldquoTowards making solar water heating tech-nology feasiblemdashthe polymer solar collector approachrdquo EnergyConversion andManagement vol 40 no 12 pp 1237ndash1250 1999

[12] F White Fluid Mechanics McGraw-Hill New York NY USA6th edition 2006

[13] R Righini H G Gallegos and C Raichijk ldquoApproach to draw-ing new global solar irradiation contour maps for ArgentinardquoRenewable Energy vol 30 no 8 pp 1241ndash1255 2005

[14] M N A Hawlader and B J Brinkworth ldquoAn analysis of thenon-convecting solar pondrdquo Solar Energy vol 27 no 3 pp 195ndash204 1981

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

6 ISRN Renewable Energy

Table 3 Equivalent water inventory (106 liter) obtained by different hose diameters Flows calculated for 06 bar district pressure and pricesfor 25-bar hoses

Diameter (inches) Length (m) Max flow (litersmin) Normal surface (m2) Price (U$D)210158401015840 56 212 34 601510158401015840 100 84 46 5012510158401015840 144 45 56 80110158401015840 224 20 68 10007510158401015840 400 66 98 130

Table 4 Parallel configurations based on many 100-meter sections for 06 bar district pressure Prices include hydraulic joints

Diameter (inches) Number of lines () Max flow (litersmin) Inventory (liters) Solar area (m2) Price (U$D)0510158401015840 9 43 106 460 20007510158401015840 4 56 106 307 150110158401015840 2 60 95 199 100

This design is developed intending to get the simplest col-lector rather than the most efficient one This way both low-cost and home-made construction objectives are achievedBesides its efficiency is expected to be better than averageregarding the solar pond and cylindrical shape featuresThesefeatures allow it to be mounted at a nonoptimal angle acondition desirable in order to eliminate the costs of technicalinstallation and mechanical supporting structures

4 Thermal-Hydraulic Analysis

41 Hydraulic Modeling Optimizing the hydraulic behavioris key in providing the desired hot-water flow for consump-tion meanwhile the large surface of the hose is excellent tocollect solar energy The maximum flow is obtained whenthe driving force (district-grid pressure) is balanced by allpressure drops along the line The total pressure drop canbe calculated as the sum of concentrated and distributedlosses relating to the mean flow velocity (119881) the length (119871)and diameter (119863) of hose and the concentrated (119870

119888) and

frictional (119891) coefficients of losses as [12]

Δ119901 =1

2(119870119888+ 119891

119871

119863)1205881198812 (2)

The frictional Darcyrsquos coefficient 119891 is related to the flowReynolds number (Re = 119881119863120588]) based on dynamic viscosity(]) and density (120588) of fluid for turbulent flows into smoothpipes it can be approximated by [12]

119891 = 031Reminus025 (3)

On the other hand the 119870119888value is related to the geometry of

restriction considered for example considering a 100m hosemounted on a roof of a 10 meter slope which is bent witha long radius curve (119870

119888= 2) along the edge of the roof a

total119870119888= 20 is obtained since this value is much lower than

the total distributed coefficient (119891119871119863) the results are almostindependent of the hose layout

Table 3 shows the flow calculated for different hosediameters It is observed that a 100m 1510158401015840 hose is maybe thesimplest choice balancing hydraulic solar normal area (119863 lowast

119871) and cost behaviors and provides enough (106 liters) hotwater The thinnest hose of equivalent volume is heated veryquickly but provides low flow meanwhile a larger diameterhose (210158401015840) has a small solar area In addition a larger diameterimplies a thicker wall (5mm for a 210158401015840 hose against 3mm fora 1510158401015840 one considering 4-bar hoses) in order to withstandthe inner pressure which in turn implies a more expensivehose and that is harder to bend As a general rule diametersbetween 110158401015840 and 1510158401015840 are right for a simple system providingenough flow on a modest 06 bar district pressure Of coursethere is no single solution depending on many factors forexample district pressure solar resource and climate patternas well as practical conditions like hose diameters availablein local hardware stores and the pattern of use For examplefor application at a day school it is impractical to keepwater warm during the night using a large-diameter hose (apractical technique that will be studied in the next section)and on the other hand it can be desirable to use thin hoses forquick heating in the morning

Let us remark on the flexibility and open configuration ofthis design For example if a too thin 07510158401015840 hose is selectedand the flow is too low the user could always improve itby dividing the 400m long hose in two parallel 200m linesthis way and according to the previous case shown in Table 3the flow is almost tripled obtaining a very good (194 Lmin)flow Table 4 illustrates different configurations based on rollsof a 100m hose connected in parallel intending to achievehigh flow and high solar area This configuration allows theuse of the thinnest hose possible to maximize the surfaceor conversely to use larger diameter hoses in order to lowercosts

We also could think about a ldquomixed assemblyrdquo intendingto balance the maximum solar area and maximum flowcriteria This can be achieved by just adding an onoff valveon each parallel line to obtain water heated quickly duringthe morning using a thin hose together with another fractionkept warm during the evening in the large-diameter hoseThe users could use water from the most convenient linein any case supported by temperature signals Furthermoreeven smarter designs could be achieved for largest systemsby adding a microcontroller This apparently sophisticated

ISRN Renewable Energy 7

feature is possible using a standard garden irrigation systemthat costs fifty dollars

All these features illustrate the open configuration andsimplicity of this design a key in solving userrsquos barrierspreviously analyzed after all there is nothing more accessiblethan an LDPE hose Moreover the normal surface noticeablylarger than on a standard flat solar collector is useful forachieving good heating efficiencies a characteristic rein-forced by the solar-pond concept as will be studied now

42 Thermal Modeling Together with hydraulics the ther-mal analysis is a key in developing an efficient home-madecollector The thermal behavior along the daily transient isdescribed by its dynamic modeling in which the evolutionof the normal solar flux (119868

119899) must be considered For a

cylinder whose generatrix has a north-south orientation itsnormal surface 119878

119899exposed to sun rays is independent of the

azimuthal (120595) solar angle and is only related to the altitudesolar angle (120572) and the roof title angle (120573) according to

119878119899= 119863119871 sin (120572 + 120573) if 120572 gt 0 (during day)

or 119878119899= 0 if 120572 lt 0 (during night)

(4)

Where for any hour (119905) of a given day (119889) the solar altitudeat a certain latitude (120579) location having a 120575 declination anglecan be calculated going through (5)

120575 = 2345 sin(360 (119889 minus 81)365

) for 119889 = 1 2 365

120595 =360∘119905

24 hminus 180

∘ for 0 lt 119905 lt 24 h

1198621= sin (120579) sin (120575)

1198622= cos (120579) cos (120575)

1198781= 1198621+ 1198622cos (120595)

1198782= radic1 minus 1198622

1

120572 = arctan(11987811198782

)

(5)

From here we can calculate the ldquoidealrdquo (on a sunny day) solarpower 119875irrad received in any instant as

119875irrad = 119878119899119868119899 if 120572 gt 0 (6)

The energy absorbed by the collector and the heat losses areboth considered within the efficiency equation (1) So the netpower gained in any instant is

119875net = 119875irrad 120583 = 119878119899119868119899 (1198860 minus 1198861(119879 minus 119879

119886)

119868119899

) (7)

Hence the dynamic energy-balance equation for a hose-based collector can be numerically calculated by approximat-ing the temperature rate by its differential increment (119879

119899minus

119879119899minus1

) on the n-sime time step (Δ119905) as

119898119888119901

119889119879

119889119905asymp 119898119888119901

119879119899minus 119879119899minus1

Δ119905= 119875net (8)

010203040506070

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 7 Daily temperature evolution for middle season (119889 = 81)at a temperate location

where 119898 and 119888119901

are mass and heat capacity of waterrespectively From here the temperature of collector in then-sime time step can be derived as

119879119899=1198781198991198860119868 + 119863119871119886

1119879119886+ (119898119888

119901119879119899minus1Δ119905)

(119898119888119901Δ119905) + 119863119871119886

1

(9)

So the water temperature can be numerically simulated byusing this last equation during the day or modified for nightuse (by setting 119878

119899= 0) and by setting 119879 = 119879

119886during the

earliest hours before sunrise For this preliminary study theirradiance flux from the sun reaching the Earthrsquos surface (119868)can be considered as constant along the day despite changesinduced by atmospheric conditions and for simple field testswithout local measurements of 119868

119899 this constant 119868 value could

be estimated from average data of the daily total irradiance11986610158401015840 on a level surface available from local solar mapsLet us calculate the performance of a 1510158401015840-diameter 100m

hose (119898 = 106Kg 119863 = 00368m and 119871 = 100m) withone PET glazing (119886

0= 08 119886

1= 8Wm2 ∘C) mounted over

an inclined (120573 = 40∘) north-oriented roof located in BuenosAires (120579 = minus38∘) on 21 March (119889 = 81 120575 = 0) From theArgentinean solar map [13] the monthly averaged daily solarirradiance is 11986610158401015840 = 45 kWhm2 and hence 119868 = 440Wm2in addition from climatic data 119879

119886can be approximated by

a sinusoidal function of a half-day period of 20∘C plusmn 5∘CUsing this set of parameter data the temperature evolutioncan be easily simulated by using a spreadsheetThe numericalconvergence shows that a time step of 360 seconds providesaccuracy results using this value the temperature evolution isillustrated in Figure 7 Here different trends are observed

(a) The temperature increases sharply from sunrise (at6 am) and useful levels (above 35∘C) are achievedfrom 830 am to 10 pm which comprises the rangeof utilization for average users

(b) A noticeable peak (62∘C) is obtained at 4 pm andvery high levels (above 45∘C) are kept between 10 amand 8 pm which provides a long range for exigentusers

(c) The temperature decreases sharply after sunset reach-ing 28∘C at midnight

It is interesting to study the evolution of efficiency alongthe day illustrated in Figure 8 The collector starts in the

8 ISRN Renewable Energy

6 8 10 12 14 16 18 200

20

40

60

80

100

minus20

minus40

minus60

()

Figure 8 Daily evolution of efficiency for the previous case (119889 =

81)

0

10

20

30

40

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 9 Daily temperature evolution for winter (119889 = 182)conditions

Table 5 Sensitivity analysis of roof inclination for winter (119889 = 182)conditions

120573 119879peak 119879 at 6 pm 119879 at 8 pm0∘ 23∘C 21∘C 19∘C20∘ 32∘C 29∘C 24∘C40∘ 38∘C 34∘C 28∘C60∘ 42∘C 37∘C 30∘C80∘ 42∘C 37∘C 31∘C90∘ 41∘C 37∘C 30∘C

morning with an impressive efficiency (80) and reaches auseful temperature of 35∘C at 830 am having an averageefficiency of 64 up to this moment This fact together withthe large solar area provided by the hose (enhanced by itscylindrical shape) explains this excellent performance Onthe other hand the negative efficiency observed at night is itsmajor drawback related to conventional collectors in whichthe heated water is stored into the insulated tank

The collector shows excellent performance during thelast-summer day but the sharp decrease during the nightopens a doubt about winter performanceThis case is studiedon 1st July (119889 = 182) for the same location (11986610158401015840 = 2 kWhm2so 119868 = 250Wm2 and119879

119886= 10∘Cplusmn8∘C) in Figure 9 Here it is

observed that the collector becomes a useful preheater duringthe day but never provides hot water This performance canbe improved by adding a second glazing layer (119886

0= 075

1198861= 5Wm2 ∘C)This way aminor improvement is obtained

with a peak temperature (38∘C) higher than the previousvalue (34∘C)

Figure 10 A simple test facility example

0102030405060708090

100

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 11 Temperatures in summer (119889 = 1) for simple and doubleglazing

Table 6 Sensitivity analysis of hose diameter for winter conditionsand 120573 = 60∘

Diameter 119879peak 119879 at 6 pm 119879 at 8 pm0510158401015840 53∘C 36∘C 22∘C07510158401015840 50∘C 39∘C 27∘C110158401015840 47∘C 39∘C 29∘C12510158401015840 44∘C 38∘C 30∘C1510158401015840 41∘C 37∘C 30∘C210158401015840 37∘C 34∘C 30∘C

Several sensitivity analyses can be performed for thislast condition Table 5 shows that collectors mounted onroofs with a steep incline get higher temperatures that thosemounted on barely inclined roofs as those use better theevening sun rays This way the peak temperature obtainedfor a level-surface roof is just 23∘C against 42∘C for anotherinclined 60∘ The influence of hose diameter is shown inTable 6 showing that thinner hoses get higher temperaturesbut their cooling at night is sharper too Again here the bestchoice is found between 110158401015840 and 1510158401015840 hoses Figure 10 shows asimple test facility that comprises three ldquonakedrdquo hoses (0510158401015840110158401015840 and 1510158401015840) and similarly the same hoses but wrapped with

ISRN Renewable Energy 9

Water grid Consumption

Mechanical thermostat12

998400998400 hose

1 5998400998400 hose

6998400998400 hose

Figure 12 Schematic drawing of the three-line mixed system

Table 7 Performance of the three-line system by using previous conditions (120573 = 40∘)

Diameter Summer (119889 = 1) Autumn (119889 = 81) Winter (119889 = 182)119879peak 119879 at 10 pm 119879peak 119879 at 10 pm 119879peak 119879 at 10 pm

610158401015840 53∘C 50∘C 42∘C 39∘C 23∘C 20∘C1 510158401015840 76∘C 46∘C 65∘C 33∘C 35∘C 14∘C0510158401015840 82∘C 25∘C 73∘C 16∘C 40∘C 6∘C

double layer of air-packed transparent polyethylene film avery low-cost glazing (03USDm2)

Finally it is interesting to study the performance on hotsummer days (1st January 119889 = 1) at this temperate location(119879119886= 25∘C plusmn 10∘C 11986610158401015840 = 65 kWhm2 so 119868 = 500Wm2)

on a 60∘ inclined roof Figure 11 compares temperatures forsingle and double glazing for this case Here we observethat both systems provide enough hot water throughoutthe day but the double glazing reaches a dangerous peakof 90∘C a temperature too high for LDPE Of course theflow of consumption is a simple way to limit this effect butthe system must withstand extreme cases like for exampleduring the holidays Thus this behavior should be checkedbefore choosing reinforced insulation

Regarding this concern about overheating it is interestingto study the thermal performance of a ldquomixed systemrdquo builtwith several hoses of different diameters assembled on theparallel layout illustrated in Figure 12 In this example weconsider the following

(a) A 610158401015840 PVC tube of 8-meter length (106 liters) withdouble glazing

(b) A 100m 1510158401015840 hose (106 liters) with single glazing

(c) A 100m 0510158401015840 hose (12 liters) with single glazing

This way we simultaneously obtain the following

(i) A small volume of quickly heated water in themorning (by the 0510158401015840 hose) that can be quicklyldquoreplenishedrdquo every hour

(ii) A large volume (106 liters) available throughout theday and evening

(iii) A large volume (106 liters) kept warm overnight

The user could choose the most convenient line at anytime or this choice could be automatized by using low-cost microcontrolled onoff valves Another simple solutionconsists in adding a bimetallic thermostat on the exit of eachline This option is less flexible than the previous one but thethermostat is a very reliable and low-cost device

The use of a very large diameter (610158401015840) is useful to providehot water throughout the night and besides it supports thechoice of using single glazing on the thinner hoses this wayavoiding dangerous overheating during hot summer daysTable 7 shows the results of the thermal simulation of thissystem by using the same cases previously studied Here isobserved that all objectives are successfully achieved

5 Conclusions

This paper presents a new design of a low-cost and robustsolar collector It was developed intending to fit the lack ofa simple home-made and self-installation device a key forsuccess in developing countries To achieve simultaneously allthese goals a new thermal-hydraulic paradigmwas proposedfrom which we have created a new design for a solarcollector that takes full advantage of new plastic and tubingtechnologies Although rather similar designs are well knownfrom the open literature the comprehensive thermal andhydraulic discussion (for both stationary and daily transientstates) developed here is a worthy contribution of this workOn this base it is easy to ldquotune uprdquo the collector regardingthe specific solar and climate resources among other issuesIn this way different hose diameters and glazing can be usedin order to obtain the best solution for any case The use ofcontrolling systems and mixing configurations are also cost-effective options to enhance the performance

In this paper the deeply rooted thermal-hydraulicparadigm of conventional collectors is discussed In addition

10 ISRN Renewable Energy

this work presents the advantages of the water-pond designwhich has long been known but its benefits have never beenacknowledged [14] The improved efficiency of water pondsregarding conventional solar collectors is a key to achieveefficient low-cost collectors even in temperate climates

Acknowledgments

Argentinean Du Pont enterprise is acknowledged for itssupport by funds given through the National Prize Du Pontto Clean Energies 2009 to this project Grace De Haro isacknowledged for her careful translation

References

[1] S Ozlu I Dincer and G F Naterer ldquoComparative assessmentof residential energy options in Ontario Canadardquo Energy andBuildings vol 55 pp 674ndash684 2012

[2] L T Kostic and Z T Pavlovic ldquoOptimal position of flat platereflectors of solar thermal collectorrdquo Energy and Buildings vol45 pp 161ndash168 2012

[3] M Khalaji Assadi A Khalesi Doost A A Hamidi and MMizani ldquoDesign construction and performance testing of anew system for energy saving in rural buildingsrdquo Energy andBuildings vol 43 no 12 pp 3303ndash3310 2011

[4] Sociedade Do Sol httpwwwsociedadedosolorgbr[5] L Juanico ldquoA new design of roof-integrated water solar collec-

tor for domestic heating and coolingrdquo Solar Energy vol 82 no6 pp 481ndash492 2008

[6] L E Juanico ldquoA new design of configurable solar awning formanaging cooling and heating loadsrdquo Energy and Buildings vol41 no 12 pp 1381ndash1385 2009

[7] L E Juanico ldquoNew design of solar roof for household heatingand coolingrdquo International Journal of Hydrogen Energy vol 35no 11 pp 5823ndash5826 2010

[8] R Sarachitti C Chotetanorm C Lertsatitthanakorn and MRungsiyopas ldquoThermal performance analysis and economicevaluation of roof-integrated solar concrete collectorrdquo Energyand Buildings vol 43 no 6 pp 1403ndash1408 2011

[9] H R Hay and J I Yellott ldquoInternational aspects of air condi-tioning with movable insulationrdquo Solar Energy vol 12 no 4pp 427ndash430 1969

[10] E Aranovitch ldquoHeat transfer processes in solar collectorsrdquoEnergy and Buildings vol 3 no 1 pp 31ndash47 1981

[11] P T Tsilingiris ldquoTowards making solar water heating tech-nology feasiblemdashthe polymer solar collector approachrdquo EnergyConversion andManagement vol 40 no 12 pp 1237ndash1250 1999

[12] F White Fluid Mechanics McGraw-Hill New York NY USA6th edition 2006

[13] R Righini H G Gallegos and C Raichijk ldquoApproach to draw-ing new global solar irradiation contour maps for ArgentinardquoRenewable Energy vol 30 no 8 pp 1241ndash1255 2005

[14] M N A Hawlader and B J Brinkworth ldquoAn analysis of thenon-convecting solar pondrdquo Solar Energy vol 27 no 3 pp 195ndash204 1981

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

ISRN Renewable Energy 7

feature is possible using a standard garden irrigation systemthat costs fifty dollars

All these features illustrate the open configuration andsimplicity of this design a key in solving userrsquos barrierspreviously analyzed after all there is nothing more accessiblethan an LDPE hose Moreover the normal surface noticeablylarger than on a standard flat solar collector is useful forachieving good heating efficiencies a characteristic rein-forced by the solar-pond concept as will be studied now

42 Thermal Modeling Together with hydraulics the ther-mal analysis is a key in developing an efficient home-madecollector The thermal behavior along the daily transient isdescribed by its dynamic modeling in which the evolutionof the normal solar flux (119868

119899) must be considered For a

cylinder whose generatrix has a north-south orientation itsnormal surface 119878

119899exposed to sun rays is independent of the

azimuthal (120595) solar angle and is only related to the altitudesolar angle (120572) and the roof title angle (120573) according to

119878119899= 119863119871 sin (120572 + 120573) if 120572 gt 0 (during day)

or 119878119899= 0 if 120572 lt 0 (during night)

(4)

Where for any hour (119905) of a given day (119889) the solar altitudeat a certain latitude (120579) location having a 120575 declination anglecan be calculated going through (5)

120575 = 2345 sin(360 (119889 minus 81)365

) for 119889 = 1 2 365

120595 =360∘119905

24 hminus 180

∘ for 0 lt 119905 lt 24 h

1198621= sin (120579) sin (120575)

1198622= cos (120579) cos (120575)

1198781= 1198621+ 1198622cos (120595)

1198782= radic1 minus 1198622

1

120572 = arctan(11987811198782

)

(5)

From here we can calculate the ldquoidealrdquo (on a sunny day) solarpower 119875irrad received in any instant as

119875irrad = 119878119899119868119899 if 120572 gt 0 (6)

The energy absorbed by the collector and the heat losses areboth considered within the efficiency equation (1) So the netpower gained in any instant is

119875net = 119875irrad 120583 = 119878119899119868119899 (1198860 minus 1198861(119879 minus 119879

119886)

119868119899

) (7)

Hence the dynamic energy-balance equation for a hose-based collector can be numerically calculated by approximat-ing the temperature rate by its differential increment (119879

119899minus

119879119899minus1

) on the n-sime time step (Δ119905) as

119898119888119901

119889119879

119889119905asymp 119898119888119901

119879119899minus 119879119899minus1

Δ119905= 119875net (8)

010203040506070

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 7 Daily temperature evolution for middle season (119889 = 81)at a temperate location

where 119898 and 119888119901

are mass and heat capacity of waterrespectively From here the temperature of collector in then-sime time step can be derived as

119879119899=1198781198991198860119868 + 119863119871119886

1119879119886+ (119898119888

119901119879119899minus1Δ119905)

(119898119888119901Δ119905) + 119863119871119886

1

(9)

So the water temperature can be numerically simulated byusing this last equation during the day or modified for nightuse (by setting 119878

119899= 0) and by setting 119879 = 119879

119886during the

earliest hours before sunrise For this preliminary study theirradiance flux from the sun reaching the Earthrsquos surface (119868)can be considered as constant along the day despite changesinduced by atmospheric conditions and for simple field testswithout local measurements of 119868

119899 this constant 119868 value could

be estimated from average data of the daily total irradiance11986610158401015840 on a level surface available from local solar mapsLet us calculate the performance of a 1510158401015840-diameter 100m

hose (119898 = 106Kg 119863 = 00368m and 119871 = 100m) withone PET glazing (119886

0= 08 119886

1= 8Wm2 ∘C) mounted over

an inclined (120573 = 40∘) north-oriented roof located in BuenosAires (120579 = minus38∘) on 21 March (119889 = 81 120575 = 0) From theArgentinean solar map [13] the monthly averaged daily solarirradiance is 11986610158401015840 = 45 kWhm2 and hence 119868 = 440Wm2in addition from climatic data 119879

119886can be approximated by

a sinusoidal function of a half-day period of 20∘C plusmn 5∘CUsing this set of parameter data the temperature evolutioncan be easily simulated by using a spreadsheetThe numericalconvergence shows that a time step of 360 seconds providesaccuracy results using this value the temperature evolution isillustrated in Figure 7 Here different trends are observed

(a) The temperature increases sharply from sunrise (at6 am) and useful levels (above 35∘C) are achievedfrom 830 am to 10 pm which comprises the rangeof utilization for average users

(b) A noticeable peak (62∘C) is obtained at 4 pm andvery high levels (above 45∘C) are kept between 10 amand 8 pm which provides a long range for exigentusers

(c) The temperature decreases sharply after sunset reach-ing 28∘C at midnight

It is interesting to study the evolution of efficiency alongthe day illustrated in Figure 8 The collector starts in the

8 ISRN Renewable Energy

6 8 10 12 14 16 18 200

20

40

60

80

100

minus20

minus40

minus60

()

Figure 8 Daily evolution of efficiency for the previous case (119889 =

81)

0

10

20

30

40

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 9 Daily temperature evolution for winter (119889 = 182)conditions

Table 5 Sensitivity analysis of roof inclination for winter (119889 = 182)conditions

120573 119879peak 119879 at 6 pm 119879 at 8 pm0∘ 23∘C 21∘C 19∘C20∘ 32∘C 29∘C 24∘C40∘ 38∘C 34∘C 28∘C60∘ 42∘C 37∘C 30∘C80∘ 42∘C 37∘C 31∘C90∘ 41∘C 37∘C 30∘C

morning with an impressive efficiency (80) and reaches auseful temperature of 35∘C at 830 am having an averageefficiency of 64 up to this moment This fact together withthe large solar area provided by the hose (enhanced by itscylindrical shape) explains this excellent performance Onthe other hand the negative efficiency observed at night is itsmajor drawback related to conventional collectors in whichthe heated water is stored into the insulated tank

The collector shows excellent performance during thelast-summer day but the sharp decrease during the nightopens a doubt about winter performanceThis case is studiedon 1st July (119889 = 182) for the same location (11986610158401015840 = 2 kWhm2so 119868 = 250Wm2 and119879

119886= 10∘Cplusmn8∘C) in Figure 9 Here it is

observed that the collector becomes a useful preheater duringthe day but never provides hot water This performance canbe improved by adding a second glazing layer (119886

0= 075

1198861= 5Wm2 ∘C)This way aminor improvement is obtained

with a peak temperature (38∘C) higher than the previousvalue (34∘C)

Figure 10 A simple test facility example

0102030405060708090

100

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 11 Temperatures in summer (119889 = 1) for simple and doubleglazing

Table 6 Sensitivity analysis of hose diameter for winter conditionsand 120573 = 60∘

Diameter 119879peak 119879 at 6 pm 119879 at 8 pm0510158401015840 53∘C 36∘C 22∘C07510158401015840 50∘C 39∘C 27∘C110158401015840 47∘C 39∘C 29∘C12510158401015840 44∘C 38∘C 30∘C1510158401015840 41∘C 37∘C 30∘C210158401015840 37∘C 34∘C 30∘C

Several sensitivity analyses can be performed for thislast condition Table 5 shows that collectors mounted onroofs with a steep incline get higher temperatures that thosemounted on barely inclined roofs as those use better theevening sun rays This way the peak temperature obtainedfor a level-surface roof is just 23∘C against 42∘C for anotherinclined 60∘ The influence of hose diameter is shown inTable 6 showing that thinner hoses get higher temperaturesbut their cooling at night is sharper too Again here the bestchoice is found between 110158401015840 and 1510158401015840 hoses Figure 10 shows asimple test facility that comprises three ldquonakedrdquo hoses (0510158401015840110158401015840 and 1510158401015840) and similarly the same hoses but wrapped with

ISRN Renewable Energy 9

Water grid Consumption

Mechanical thermostat12

998400998400 hose

1 5998400998400 hose

6998400998400 hose

Figure 12 Schematic drawing of the three-line mixed system

Table 7 Performance of the three-line system by using previous conditions (120573 = 40∘)

Diameter Summer (119889 = 1) Autumn (119889 = 81) Winter (119889 = 182)119879peak 119879 at 10 pm 119879peak 119879 at 10 pm 119879peak 119879 at 10 pm

610158401015840 53∘C 50∘C 42∘C 39∘C 23∘C 20∘C1 510158401015840 76∘C 46∘C 65∘C 33∘C 35∘C 14∘C0510158401015840 82∘C 25∘C 73∘C 16∘C 40∘C 6∘C

double layer of air-packed transparent polyethylene film avery low-cost glazing (03USDm2)

Finally it is interesting to study the performance on hotsummer days (1st January 119889 = 1) at this temperate location(119879119886= 25∘C plusmn 10∘C 11986610158401015840 = 65 kWhm2 so 119868 = 500Wm2)

on a 60∘ inclined roof Figure 11 compares temperatures forsingle and double glazing for this case Here we observethat both systems provide enough hot water throughoutthe day but the double glazing reaches a dangerous peakof 90∘C a temperature too high for LDPE Of course theflow of consumption is a simple way to limit this effect butthe system must withstand extreme cases like for exampleduring the holidays Thus this behavior should be checkedbefore choosing reinforced insulation

Regarding this concern about overheating it is interestingto study the thermal performance of a ldquomixed systemrdquo builtwith several hoses of different diameters assembled on theparallel layout illustrated in Figure 12 In this example weconsider the following

(a) A 610158401015840 PVC tube of 8-meter length (106 liters) withdouble glazing

(b) A 100m 1510158401015840 hose (106 liters) with single glazing

(c) A 100m 0510158401015840 hose (12 liters) with single glazing

This way we simultaneously obtain the following

(i) A small volume of quickly heated water in themorning (by the 0510158401015840 hose) that can be quicklyldquoreplenishedrdquo every hour

(ii) A large volume (106 liters) available throughout theday and evening

(iii) A large volume (106 liters) kept warm overnight

The user could choose the most convenient line at anytime or this choice could be automatized by using low-cost microcontrolled onoff valves Another simple solutionconsists in adding a bimetallic thermostat on the exit of eachline This option is less flexible than the previous one but thethermostat is a very reliable and low-cost device

The use of a very large diameter (610158401015840) is useful to providehot water throughout the night and besides it supports thechoice of using single glazing on the thinner hoses this wayavoiding dangerous overheating during hot summer daysTable 7 shows the results of the thermal simulation of thissystem by using the same cases previously studied Here isobserved that all objectives are successfully achieved

5 Conclusions

This paper presents a new design of a low-cost and robustsolar collector It was developed intending to fit the lack ofa simple home-made and self-installation device a key forsuccess in developing countries To achieve simultaneously allthese goals a new thermal-hydraulic paradigmwas proposedfrom which we have created a new design for a solarcollector that takes full advantage of new plastic and tubingtechnologies Although rather similar designs are well knownfrom the open literature the comprehensive thermal andhydraulic discussion (for both stationary and daily transientstates) developed here is a worthy contribution of this workOn this base it is easy to ldquotune uprdquo the collector regardingthe specific solar and climate resources among other issuesIn this way different hose diameters and glazing can be usedin order to obtain the best solution for any case The use ofcontrolling systems and mixing configurations are also cost-effective options to enhance the performance

In this paper the deeply rooted thermal-hydraulicparadigm of conventional collectors is discussed In addition

10 ISRN Renewable Energy

this work presents the advantages of the water-pond designwhich has long been known but its benefits have never beenacknowledged [14] The improved efficiency of water pondsregarding conventional solar collectors is a key to achieveefficient low-cost collectors even in temperate climates

Acknowledgments

Argentinean Du Pont enterprise is acknowledged for itssupport by funds given through the National Prize Du Pontto Clean Energies 2009 to this project Grace De Haro isacknowledged for her careful translation

References

[1] S Ozlu I Dincer and G F Naterer ldquoComparative assessmentof residential energy options in Ontario Canadardquo Energy andBuildings vol 55 pp 674ndash684 2012

[2] L T Kostic and Z T Pavlovic ldquoOptimal position of flat platereflectors of solar thermal collectorrdquo Energy and Buildings vol45 pp 161ndash168 2012

[3] M Khalaji Assadi A Khalesi Doost A A Hamidi and MMizani ldquoDesign construction and performance testing of anew system for energy saving in rural buildingsrdquo Energy andBuildings vol 43 no 12 pp 3303ndash3310 2011

[4] Sociedade Do Sol httpwwwsociedadedosolorgbr[5] L Juanico ldquoA new design of roof-integrated water solar collec-

tor for domestic heating and coolingrdquo Solar Energy vol 82 no6 pp 481ndash492 2008

[6] L E Juanico ldquoA new design of configurable solar awning formanaging cooling and heating loadsrdquo Energy and Buildings vol41 no 12 pp 1381ndash1385 2009

[7] L E Juanico ldquoNew design of solar roof for household heatingand coolingrdquo International Journal of Hydrogen Energy vol 35no 11 pp 5823ndash5826 2010

[8] R Sarachitti C Chotetanorm C Lertsatitthanakorn and MRungsiyopas ldquoThermal performance analysis and economicevaluation of roof-integrated solar concrete collectorrdquo Energyand Buildings vol 43 no 6 pp 1403ndash1408 2011

[9] H R Hay and J I Yellott ldquoInternational aspects of air condi-tioning with movable insulationrdquo Solar Energy vol 12 no 4pp 427ndash430 1969

[10] E Aranovitch ldquoHeat transfer processes in solar collectorsrdquoEnergy and Buildings vol 3 no 1 pp 31ndash47 1981

[11] P T Tsilingiris ldquoTowards making solar water heating tech-nology feasiblemdashthe polymer solar collector approachrdquo EnergyConversion andManagement vol 40 no 12 pp 1237ndash1250 1999

[12] F White Fluid Mechanics McGraw-Hill New York NY USA6th edition 2006

[13] R Righini H G Gallegos and C Raichijk ldquoApproach to draw-ing new global solar irradiation contour maps for ArgentinardquoRenewable Energy vol 30 no 8 pp 1241ndash1255 2005

[14] M N A Hawlader and B J Brinkworth ldquoAn analysis of thenon-convecting solar pondrdquo Solar Energy vol 27 no 3 pp 195ndash204 1981

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

8 ISRN Renewable Energy

6 8 10 12 14 16 18 200

20

40

60

80

100

minus20

minus40

minus60

()

Figure 8 Daily evolution of efficiency for the previous case (119889 =

81)

0

10

20

30

40

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 9 Daily temperature evolution for winter (119889 = 182)conditions

Table 5 Sensitivity analysis of roof inclination for winter (119889 = 182)conditions

120573 119879peak 119879 at 6 pm 119879 at 8 pm0∘ 23∘C 21∘C 19∘C20∘ 32∘C 29∘C 24∘C40∘ 38∘C 34∘C 28∘C60∘ 42∘C 37∘C 30∘C80∘ 42∘C 37∘C 31∘C90∘ 41∘C 37∘C 30∘C

morning with an impressive efficiency (80) and reaches auseful temperature of 35∘C at 830 am having an averageefficiency of 64 up to this moment This fact together withthe large solar area provided by the hose (enhanced by itscylindrical shape) explains this excellent performance Onthe other hand the negative efficiency observed at night is itsmajor drawback related to conventional collectors in whichthe heated water is stored into the insulated tank

The collector shows excellent performance during thelast-summer day but the sharp decrease during the nightopens a doubt about winter performanceThis case is studiedon 1st July (119889 = 182) for the same location (11986610158401015840 = 2 kWhm2so 119868 = 250Wm2 and119879

119886= 10∘Cplusmn8∘C) in Figure 9 Here it is

observed that the collector becomes a useful preheater duringthe day but never provides hot water This performance canbe improved by adding a second glazing layer (119886

0= 075

1198861= 5Wm2 ∘C)This way aminor improvement is obtained

with a peak temperature (38∘C) higher than the previousvalue (34∘C)

Figure 10 A simple test facility example

0102030405060708090

100

0 2 4 6 8 10 12 14 16 18 20 22 24

Figure 11 Temperatures in summer (119889 = 1) for simple and doubleglazing

Table 6 Sensitivity analysis of hose diameter for winter conditionsand 120573 = 60∘

Diameter 119879peak 119879 at 6 pm 119879 at 8 pm0510158401015840 53∘C 36∘C 22∘C07510158401015840 50∘C 39∘C 27∘C110158401015840 47∘C 39∘C 29∘C12510158401015840 44∘C 38∘C 30∘C1510158401015840 41∘C 37∘C 30∘C210158401015840 37∘C 34∘C 30∘C

Several sensitivity analyses can be performed for thislast condition Table 5 shows that collectors mounted onroofs with a steep incline get higher temperatures that thosemounted on barely inclined roofs as those use better theevening sun rays This way the peak temperature obtainedfor a level-surface roof is just 23∘C against 42∘C for anotherinclined 60∘ The influence of hose diameter is shown inTable 6 showing that thinner hoses get higher temperaturesbut their cooling at night is sharper too Again here the bestchoice is found between 110158401015840 and 1510158401015840 hoses Figure 10 shows asimple test facility that comprises three ldquonakedrdquo hoses (0510158401015840110158401015840 and 1510158401015840) and similarly the same hoses but wrapped with

ISRN Renewable Energy 9

Water grid Consumption

Mechanical thermostat12

998400998400 hose

1 5998400998400 hose

6998400998400 hose

Figure 12 Schematic drawing of the three-line mixed system

Table 7 Performance of the three-line system by using previous conditions (120573 = 40∘)

Diameter Summer (119889 = 1) Autumn (119889 = 81) Winter (119889 = 182)119879peak 119879 at 10 pm 119879peak 119879 at 10 pm 119879peak 119879 at 10 pm

610158401015840 53∘C 50∘C 42∘C 39∘C 23∘C 20∘C1 510158401015840 76∘C 46∘C 65∘C 33∘C 35∘C 14∘C0510158401015840 82∘C 25∘C 73∘C 16∘C 40∘C 6∘C

double layer of air-packed transparent polyethylene film avery low-cost glazing (03USDm2)

Finally it is interesting to study the performance on hotsummer days (1st January 119889 = 1) at this temperate location(119879119886= 25∘C plusmn 10∘C 11986610158401015840 = 65 kWhm2 so 119868 = 500Wm2)

on a 60∘ inclined roof Figure 11 compares temperatures forsingle and double glazing for this case Here we observethat both systems provide enough hot water throughoutthe day but the double glazing reaches a dangerous peakof 90∘C a temperature too high for LDPE Of course theflow of consumption is a simple way to limit this effect butthe system must withstand extreme cases like for exampleduring the holidays Thus this behavior should be checkedbefore choosing reinforced insulation

Regarding this concern about overheating it is interestingto study the thermal performance of a ldquomixed systemrdquo builtwith several hoses of different diameters assembled on theparallel layout illustrated in Figure 12 In this example weconsider the following

(a) A 610158401015840 PVC tube of 8-meter length (106 liters) withdouble glazing

(b) A 100m 1510158401015840 hose (106 liters) with single glazing

(c) A 100m 0510158401015840 hose (12 liters) with single glazing

This way we simultaneously obtain the following

(i) A small volume of quickly heated water in themorning (by the 0510158401015840 hose) that can be quicklyldquoreplenishedrdquo every hour

(ii) A large volume (106 liters) available throughout theday and evening

(iii) A large volume (106 liters) kept warm overnight

The user could choose the most convenient line at anytime or this choice could be automatized by using low-cost microcontrolled onoff valves Another simple solutionconsists in adding a bimetallic thermostat on the exit of eachline This option is less flexible than the previous one but thethermostat is a very reliable and low-cost device

The use of a very large diameter (610158401015840) is useful to providehot water throughout the night and besides it supports thechoice of using single glazing on the thinner hoses this wayavoiding dangerous overheating during hot summer daysTable 7 shows the results of the thermal simulation of thissystem by using the same cases previously studied Here isobserved that all objectives are successfully achieved

5 Conclusions

This paper presents a new design of a low-cost and robustsolar collector It was developed intending to fit the lack ofa simple home-made and self-installation device a key forsuccess in developing countries To achieve simultaneously allthese goals a new thermal-hydraulic paradigmwas proposedfrom which we have created a new design for a solarcollector that takes full advantage of new plastic and tubingtechnologies Although rather similar designs are well knownfrom the open literature the comprehensive thermal andhydraulic discussion (for both stationary and daily transientstates) developed here is a worthy contribution of this workOn this base it is easy to ldquotune uprdquo the collector regardingthe specific solar and climate resources among other issuesIn this way different hose diameters and glazing can be usedin order to obtain the best solution for any case The use ofcontrolling systems and mixing configurations are also cost-effective options to enhance the performance

In this paper the deeply rooted thermal-hydraulicparadigm of conventional collectors is discussed In addition

10 ISRN Renewable Energy

this work presents the advantages of the water-pond designwhich has long been known but its benefits have never beenacknowledged [14] The improved efficiency of water pondsregarding conventional solar collectors is a key to achieveefficient low-cost collectors even in temperate climates

Acknowledgments

Argentinean Du Pont enterprise is acknowledged for itssupport by funds given through the National Prize Du Pontto Clean Energies 2009 to this project Grace De Haro isacknowledged for her careful translation

References

[1] S Ozlu I Dincer and G F Naterer ldquoComparative assessmentof residential energy options in Ontario Canadardquo Energy andBuildings vol 55 pp 674ndash684 2012

[2] L T Kostic and Z T Pavlovic ldquoOptimal position of flat platereflectors of solar thermal collectorrdquo Energy and Buildings vol45 pp 161ndash168 2012

[3] M Khalaji Assadi A Khalesi Doost A A Hamidi and MMizani ldquoDesign construction and performance testing of anew system for energy saving in rural buildingsrdquo Energy andBuildings vol 43 no 12 pp 3303ndash3310 2011

[4] Sociedade Do Sol httpwwwsociedadedosolorgbr[5] L Juanico ldquoA new design of roof-integrated water solar collec-

tor for domestic heating and coolingrdquo Solar Energy vol 82 no6 pp 481ndash492 2008

[6] L E Juanico ldquoA new design of configurable solar awning formanaging cooling and heating loadsrdquo Energy and Buildings vol41 no 12 pp 1381ndash1385 2009

[7] L E Juanico ldquoNew design of solar roof for household heatingand coolingrdquo International Journal of Hydrogen Energy vol 35no 11 pp 5823ndash5826 2010

[8] R Sarachitti C Chotetanorm C Lertsatitthanakorn and MRungsiyopas ldquoThermal performance analysis and economicevaluation of roof-integrated solar concrete collectorrdquo Energyand Buildings vol 43 no 6 pp 1403ndash1408 2011

[9] H R Hay and J I Yellott ldquoInternational aspects of air condi-tioning with movable insulationrdquo Solar Energy vol 12 no 4pp 427ndash430 1969

[10] E Aranovitch ldquoHeat transfer processes in solar collectorsrdquoEnergy and Buildings vol 3 no 1 pp 31ndash47 1981

[11] P T Tsilingiris ldquoTowards making solar water heating tech-nology feasiblemdashthe polymer solar collector approachrdquo EnergyConversion andManagement vol 40 no 12 pp 1237ndash1250 1999

[12] F White Fluid Mechanics McGraw-Hill New York NY USA6th edition 2006

[13] R Righini H G Gallegos and C Raichijk ldquoApproach to draw-ing new global solar irradiation contour maps for ArgentinardquoRenewable Energy vol 30 no 8 pp 1241ndash1255 2005

[14] M N A Hawlader and B J Brinkworth ldquoAn analysis of thenon-convecting solar pondrdquo Solar Energy vol 27 no 3 pp 195ndash204 1981

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

ISRN Renewable Energy 9

Water grid Consumption

Mechanical thermostat12

998400998400 hose

1 5998400998400 hose

6998400998400 hose

Figure 12 Schematic drawing of the three-line mixed system

Table 7 Performance of the three-line system by using previous conditions (120573 = 40∘)

Diameter Summer (119889 = 1) Autumn (119889 = 81) Winter (119889 = 182)119879peak 119879 at 10 pm 119879peak 119879 at 10 pm 119879peak 119879 at 10 pm

610158401015840 53∘C 50∘C 42∘C 39∘C 23∘C 20∘C1 510158401015840 76∘C 46∘C 65∘C 33∘C 35∘C 14∘C0510158401015840 82∘C 25∘C 73∘C 16∘C 40∘C 6∘C

double layer of air-packed transparent polyethylene film avery low-cost glazing (03USDm2)

Finally it is interesting to study the performance on hotsummer days (1st January 119889 = 1) at this temperate location(119879119886= 25∘C plusmn 10∘C 11986610158401015840 = 65 kWhm2 so 119868 = 500Wm2)

on a 60∘ inclined roof Figure 11 compares temperatures forsingle and double glazing for this case Here we observethat both systems provide enough hot water throughoutthe day but the double glazing reaches a dangerous peakof 90∘C a temperature too high for LDPE Of course theflow of consumption is a simple way to limit this effect butthe system must withstand extreme cases like for exampleduring the holidays Thus this behavior should be checkedbefore choosing reinforced insulation

Regarding this concern about overheating it is interestingto study the thermal performance of a ldquomixed systemrdquo builtwith several hoses of different diameters assembled on theparallel layout illustrated in Figure 12 In this example weconsider the following

(a) A 610158401015840 PVC tube of 8-meter length (106 liters) withdouble glazing

(b) A 100m 1510158401015840 hose (106 liters) with single glazing

(c) A 100m 0510158401015840 hose (12 liters) with single glazing

This way we simultaneously obtain the following

(i) A small volume of quickly heated water in themorning (by the 0510158401015840 hose) that can be quicklyldquoreplenishedrdquo every hour

(ii) A large volume (106 liters) available throughout theday and evening

(iii) A large volume (106 liters) kept warm overnight

The user could choose the most convenient line at anytime or this choice could be automatized by using low-cost microcontrolled onoff valves Another simple solutionconsists in adding a bimetallic thermostat on the exit of eachline This option is less flexible than the previous one but thethermostat is a very reliable and low-cost device

The use of a very large diameter (610158401015840) is useful to providehot water throughout the night and besides it supports thechoice of using single glazing on the thinner hoses this wayavoiding dangerous overheating during hot summer daysTable 7 shows the results of the thermal simulation of thissystem by using the same cases previously studied Here isobserved that all objectives are successfully achieved

5 Conclusions

This paper presents a new design of a low-cost and robustsolar collector It was developed intending to fit the lack ofa simple home-made and self-installation device a key forsuccess in developing countries To achieve simultaneously allthese goals a new thermal-hydraulic paradigmwas proposedfrom which we have created a new design for a solarcollector that takes full advantage of new plastic and tubingtechnologies Although rather similar designs are well knownfrom the open literature the comprehensive thermal andhydraulic discussion (for both stationary and daily transientstates) developed here is a worthy contribution of this workOn this base it is easy to ldquotune uprdquo the collector regardingthe specific solar and climate resources among other issuesIn this way different hose diameters and glazing can be usedin order to obtain the best solution for any case The use ofcontrolling systems and mixing configurations are also cost-effective options to enhance the performance

In this paper the deeply rooted thermal-hydraulicparadigm of conventional collectors is discussed In addition

10 ISRN Renewable Energy

this work presents the advantages of the water-pond designwhich has long been known but its benefits have never beenacknowledged [14] The improved efficiency of water pondsregarding conventional solar collectors is a key to achieveefficient low-cost collectors even in temperate climates

Acknowledgments

Argentinean Du Pont enterprise is acknowledged for itssupport by funds given through the National Prize Du Pontto Clean Energies 2009 to this project Grace De Haro isacknowledged for her careful translation

References

[1] S Ozlu I Dincer and G F Naterer ldquoComparative assessmentof residential energy options in Ontario Canadardquo Energy andBuildings vol 55 pp 674ndash684 2012

[2] L T Kostic and Z T Pavlovic ldquoOptimal position of flat platereflectors of solar thermal collectorrdquo Energy and Buildings vol45 pp 161ndash168 2012

[3] M Khalaji Assadi A Khalesi Doost A A Hamidi and MMizani ldquoDesign construction and performance testing of anew system for energy saving in rural buildingsrdquo Energy andBuildings vol 43 no 12 pp 3303ndash3310 2011

[4] Sociedade Do Sol httpwwwsociedadedosolorgbr[5] L Juanico ldquoA new design of roof-integrated water solar collec-

tor for domestic heating and coolingrdquo Solar Energy vol 82 no6 pp 481ndash492 2008

[6] L E Juanico ldquoA new design of configurable solar awning formanaging cooling and heating loadsrdquo Energy and Buildings vol41 no 12 pp 1381ndash1385 2009

[7] L E Juanico ldquoNew design of solar roof for household heatingand coolingrdquo International Journal of Hydrogen Energy vol 35no 11 pp 5823ndash5826 2010

[8] R Sarachitti C Chotetanorm C Lertsatitthanakorn and MRungsiyopas ldquoThermal performance analysis and economicevaluation of roof-integrated solar concrete collectorrdquo Energyand Buildings vol 43 no 6 pp 1403ndash1408 2011

[9] H R Hay and J I Yellott ldquoInternational aspects of air condi-tioning with movable insulationrdquo Solar Energy vol 12 no 4pp 427ndash430 1969

[10] E Aranovitch ldquoHeat transfer processes in solar collectorsrdquoEnergy and Buildings vol 3 no 1 pp 31ndash47 1981

[11] P T Tsilingiris ldquoTowards making solar water heating tech-nology feasiblemdashthe polymer solar collector approachrdquo EnergyConversion andManagement vol 40 no 12 pp 1237ndash1250 1999

[12] F White Fluid Mechanics McGraw-Hill New York NY USA6th edition 2006

[13] R Righini H G Gallegos and C Raichijk ldquoApproach to draw-ing new global solar irradiation contour maps for ArgentinardquoRenewable Energy vol 30 no 8 pp 1241ndash1255 2005

[14] M N A Hawlader and B J Brinkworth ldquoAn analysis of thenon-convecting solar pondrdquo Solar Energy vol 27 no 3 pp 195ndash204 1981

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

10 ISRN Renewable Energy

this work presents the advantages of the water-pond designwhich has long been known but its benefits have never beenacknowledged [14] The improved efficiency of water pondsregarding conventional solar collectors is a key to achieveefficient low-cost collectors even in temperate climates

Acknowledgments

Argentinean Du Pont enterprise is acknowledged for itssupport by funds given through the National Prize Du Pontto Clean Energies 2009 to this project Grace De Haro isacknowledged for her careful translation

References

[1] S Ozlu I Dincer and G F Naterer ldquoComparative assessmentof residential energy options in Ontario Canadardquo Energy andBuildings vol 55 pp 674ndash684 2012

[2] L T Kostic and Z T Pavlovic ldquoOptimal position of flat platereflectors of solar thermal collectorrdquo Energy and Buildings vol45 pp 161ndash168 2012

[3] M Khalaji Assadi A Khalesi Doost A A Hamidi and MMizani ldquoDesign construction and performance testing of anew system for energy saving in rural buildingsrdquo Energy andBuildings vol 43 no 12 pp 3303ndash3310 2011

[4] Sociedade Do Sol httpwwwsociedadedosolorgbr[5] L Juanico ldquoA new design of roof-integrated water solar collec-

tor for domestic heating and coolingrdquo Solar Energy vol 82 no6 pp 481ndash492 2008

[6] L E Juanico ldquoA new design of configurable solar awning formanaging cooling and heating loadsrdquo Energy and Buildings vol41 no 12 pp 1381ndash1385 2009

[7] L E Juanico ldquoNew design of solar roof for household heatingand coolingrdquo International Journal of Hydrogen Energy vol 35no 11 pp 5823ndash5826 2010

[8] R Sarachitti C Chotetanorm C Lertsatitthanakorn and MRungsiyopas ldquoThermal performance analysis and economicevaluation of roof-integrated solar concrete collectorrdquo Energyand Buildings vol 43 no 6 pp 1403ndash1408 2011

[9] H R Hay and J I Yellott ldquoInternational aspects of air condi-tioning with movable insulationrdquo Solar Energy vol 12 no 4pp 427ndash430 1969

[10] E Aranovitch ldquoHeat transfer processes in solar collectorsrdquoEnergy and Buildings vol 3 no 1 pp 31ndash47 1981

[11] P T Tsilingiris ldquoTowards making solar water heating tech-nology feasiblemdashthe polymer solar collector approachrdquo EnergyConversion andManagement vol 40 no 12 pp 1237ndash1250 1999

[12] F White Fluid Mechanics McGraw-Hill New York NY USA6th edition 2006

[13] R Righini H G Gallegos and C Raichijk ldquoApproach to draw-ing new global solar irradiation contour maps for ArgentinardquoRenewable Energy vol 30 no 8 pp 1241ndash1255 2005

[14] M N A Hawlader and B J Brinkworth ldquoAn analysis of thenon-convecting solar pondrdquo Solar Energy vol 27 no 3 pp 195ndash204 1981

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

TribologyAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

FuelsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Industrial EngineeringJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Power ElectronicsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

CombustionJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Renewable Energy

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

StructuresJournal of

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear InstallationsScience and Technology of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Solar EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Wind EnergyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nuclear EnergyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014