An Introduction to Solar Collectors for Heating & Cooling of Buildings & Domestic Hot Water Heating

8/13/2019 An Introduction to Solar Collectors for Heating & Cooling of Buildings & Domestic Hot Water Heating

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J. PAUL GUYER, P.E., R.A.

Paul Guyer is a registered civil engineer,mechanical engineer, fire protectionengineer and architect with over 35 yearsexperience designing all types of buildingsand related infrastructure. For an additional9 years he was a public policy advisor onthe staff of the alifornia !egislaturedealing with infrastructure issues. "e is agraduate of #tanford $niversity and hasheld numerous local, state and nationaloffices with the %merican #ociety of ivil&ngineers and 'ational #ociety ofProfessional &ngineers. "e is a Fellow ofthe %merican #ociety of ivil &ngineers andthe %rchitectural &ngineering (nstitute.

) *. Paul Guyer +-+ -

An Introduction toSolar Collectors forHeating and Cooling

of Buildings andDomestic Hot WaterHeating

G U Y E R P A R T N E R S + l u b h o u s e / r i v e

& l 0 a c e r o , % 9 5 1 - 2 5 3 4 5 2 6 1 1 3

7 p g u y e r 8 p a c b e l l . n e t


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CONTENTS

1. INTRODUCTION

1.1 SCOPE

1.2 RELATED CRITERIA

1.3 SOLAR ENERGY

2. FLAT PLATE SOLAR COLLECTORS

2.1 COLLECTORS

2.2 ENERGY STORAGE AND AUXILIARY HEAT

2.3 DOMESTIC HOT WATER SYSTEMS (DHW)

2.4 THERMOSYPHON, BATCH AND INTEGRAL

COLLECTOR SYSTEMS

2. SPACE HEATING AND DHW SYSTEMS

2.! PASSI"E SYSTEMS

2.# SOLAR COOLING SYSTEMS

2.$ SYSTEM CONTROLS

) *. Paul Guyer +-+ +

This course is adapted from the Unified Facilities Criteria of the United Statesgovernment,which is in the pulic domain, has unlimited distriution and is not

The #igures, Tales and S!mols in this document are in some cases a little difficult toread, ut the! are the est availale" DO NOT PURCHASE THIS COURSE IF THEFIGURES, TABLES AND SYMBOLS ARE NOT ACCEPTABLE TO YOU.


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1. INTRODUCTION

1.1 SCOPE.his course presents design criteria and cost analysis methods for the

si:ing and 7ustification of solar heat collectors for potable water and space heaters.

(nformation is presented to enable engineers to understand solar space conditioning

and water heating systems or conduct feasibility studies based on solar collector

performance, site location, and economics. ;oth retrofit and new installations are

considered.

1.2 RELATED CRITERIA. #tandards and performance criteria relating to solar

heating systems have been evolved by government agencies and various associations

and institutes. he most widely used are listed below. ;ecause solar technology is a

continuously evolving field, be aware that publications listed below may have been

revised or superseded.

#ub7ect /ocument

#olar ollector (nstantaneous %#"


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esting racAing oncentrator #olar &nergy (ndustries %ssociation,ollectors =0ethodology for /etermining the

hermal Performance


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=(nstallation Guidelines for #olar /"?#ystems in Cne and wo6Family /wellings=Dand'ational ;ureau of #tandards, ';#( li@uid and air. !i@uids may be water, an antifree:e mixture, or various

hydrocarbon and silicone heat transfer oils. %ir6type collectors use air as the collector

fluid. he absorber plate is that part of the collector which absorbs the solar energy and

converts it to thermal energy. % portion of the thermal energy is carried to the building or

thermal storage unit by the fluid which circulates through passages in the absorber

plate. he absorber plates can be made of metal, plastic, or rubber compounds. he

metals commonly used in order of decreasing thermal conductivity are copper,

aluminum, and steel. Plastics polyolefins4 and rubbers ethylene propylene

compounds4 are relatively inexpensive, but due to their low thermal conductivity and

their temperature limitations, they are suitable only for low temperature applications,

such as heating swimming pool water or for use with water source heat pumps. ypical



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cross sections of solar collector types are shown in Figure +6-. Cther ma7or components

of a solar collector include>

%bsorber plate coating 6 o enhance the heat transfer and protect the absorber

plate.

Cne or more transparent covers 6 o reduce thermal losses by radiation using

the =greenhouse effect=4 and by convection wind, etc.4. #pacings are nominally

-B+ inch or more.

(nsulation 6 Cne to three inches are used to reduce heat loss through the side

and bacA of the absorber plate.

ollector box or housing 6 o provide a rigid mounting to hold the components.

0ust be weatherproofed.

GasAets and seals 6 o insure a weathertight seal between components while

allowing thermal expansion of the components. 'ormally these seals remain

ductile to accomplish their purpose.

Flat6plate collectors are most suitable for low temperature applications such as

domestic hot water and space heating. hey collect both direct and diffuse radiation. (t

is not re@uired that they tracA the sun, thus initial cost and maintenance are minimi:ed.

% properly designed flat6plate collector has a life expectancy of - to +5 years, or

sometimes longer. %ll copper and glass systems currently exhibit the longest lives.

$sing softened water will help. ubes should be -B+ inch in diameter or greater for low

pressure drop and longer life. he better the attachment of tube6to6 plate such as by

soldering4, the better the heat transfer, but the greater the manufacturing cost.

%dvances in collector cost reduction will probably be made in the direction of cheaper

manufacturing processes. #ome collectors not made from tube and sheet may nottolerate /"? line pressures. #pecifications for pressuri:ed collector circuits should

re@uire collectors which will taAe proof test pressure e@ual to -5 of expected circuit

pressure. (n hot climates, it is important to reduce roof heat load due to collector heat

) *. Paul Guyer +-+ -2


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gain in summerD this can be accomplished by venting the space between collector plate

and gla:es with dampers or by covering the collectors. % normal amount of dirt and dust

Figure +6-

ypes of #olar "eat ollectors

) *. Paul Guyer +-+ -9


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on the glass cover will reduce heat collected by about 5. 'ormal rainfall is usually

sufficient to relieve this problem. &xcept for warm climates with high insolation ( K B 6

- ;tuBft+6day4, two cover glasses may be optimum. (n warm climates, one glass is

optimum. 0any plastics have an undesirable transparency to infrared radiation, to which

glass is nearly opa@ue, so the desired =greenhouse effect= is not so pronounced with

plastic materials as with glass. "owever, losses by radiation from the collector are small

compared with convective losses due to windD thus plastics can be employed to reduce

breaAage and cost, but with some loss in collector performance. Plastics with maximum

opa@ueness to infrared and maximum transparency to ultraviolet $L4 and visible

radiation and with high resistance to $L degradation should be specified. he following

sections give more detailed information on collector designs and components.

2.1.1 LI%UID AND AIR&TYPE COLLECTORS.!i@uid and air type collectors each have

some advantages which are summari:ed in able +6-. !i@uid types are more suited to

/"?, the collector area is usually smaller, and more information is available about

li@uid systems. ollectors for heating air do not re@uire protection from free:ing and

have minimal corrosion problems, leaAs do not cause serious damage, they may cost

less per unit area, and are better suited to direct space heating for residences where

duct6worA is already present. "owever, since leaAs in air systems are less easilydetected, they can degrade system performance if not corrected. ?herever this manual

discusses li@uid collectors, air collectors are included, and cost analyses apply e@ually

to both. he design procedure for air collectors differs, however. "eat transfer oils used

in li@uid systems offer free:e protection and some corrosion protection, but they also

re@uire heat exchangers for heating domestic hot water, as do antifree:e6water

mixtures.

2.1.2 SELECTI"E SURFACES. #ome collectors are manufactured with a blacA coating

which absorbs the high fre@uency incoming solar radiation very well and which emits

low fre@uency infrared radiation poorly. his is a highly desirable combination of

properties for a collector. he absorptance should be .9 or higher and emittance may

be .- or lower. #uch coatings are approximately e@ual in effect to one cover glass.



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able +6-

%dvantages and /isadvantages of %ir and !i@uid "eating #ystems

) *. Paul Guyer +-+ +-


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hus, a selective coating plus one cover glass may be expected to be about e@ual in

efficiency to a collector with two cover glasses and a flat blacA painted surface.

&lectroplated blacA nicAel, blacA chrome, copper oxide or anodi:ed aluminum are

common types of selective coatings. ost of selective surface coatings may be greater

than an extra sheet of glass, but much research is being done to produce low cost,

easily applied coatings. he stability of blacA nicAel, chrome and aluminum in the

presence of moisture has not yet been proven. !ong6term stability in the presence of

moisture or other expected environmental factors salt air, etc.4 must be included in

specifications for selective surfaces. able +6+ is a summary of absorber coatings both

selective and nonselective.

2.1.3 COLLECTOR CO"ERS (GLA'ES).he transparent covers serve to admit solar

radiation to the absorber while reducing convection and radiation heat losses from the

collector. he covers also protect the absorber from dirt, rain, and other environmental

contaminants. he material used for covers include glass andBor plastic sheets. Glass

is most commonly used because of its superior optical properties and durability.

#tandard plate glass reflects about 2 and absorbs about 1 of normal incident solar

radiation, resulting in a transmissivity of about 21. Met it is essentially opa@ue to long6wave thermal radiation from the absorber. ransmission of solar radiation into the

collector can be increased by minimi:ing the reflectance and the absorptance of the

glass covers. %bsorptance of solar radiation by the collector can be increased with the

use of thinner tempered glass and by using glass that has a low iron content. %lthough

glass is sub7ect to impact damage and is more expensive than plastic, it does not

degrade in sunlight or at high collector temperatures, and is generally considered to be

more durable than plastic. (mpact damage may be reduced with the use of tempered

glass and small collector widths. %lso -B+6inch wire mesh may be hung over glass

covers for protection, but the effective absorber area will be reduced by approximately

-5. (n general, screens are not recommended. 0ost plastic covers transmit the solar

spectrum as well or better than glass gla:ing. $nfortunately, they transmit infrared

) *. Paul Guyer +-+ ++


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able +6+haracteristics of %bsorber oatings

selective coatings alphaBepsilon K +D non6selective coatings alphaBepsilon N -4

) *. Paul Guyer +-+ +3


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able +6+ continued4haracteristics of %bsorber oatings

selective coatings alphaBepsilon K +D non6selective coatings alphaBepsilon N -4

able +63% omparison of Larious 0aterials $sed for ollector overs.



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radiation well also, increasing radiation losses from the collector. able +63 compares

the different characteristics of glass and plastic covers. %lthough resistant to impact

damage, plastics generally degrade in sunlight and are limited as to the temperatures

they can sustain without undergoing serious deformation. Cften they do not lie flat,

resulting in a wavy appearance. (n general, acrylic is the most $L resistant and F


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able +6

Guide to #election of 'umber of ransparent over Plates.

separated from the absorber plate by-B+ to 3B inch and have a reflective foil facing the

absorber plate. (f fiberglass insulation is used, it should not be typical construction grade

which contains phenolic binders that may =outgas= at the stagnation temperature of the

collector. (n all cases, specifications should call for insulations that are not flammable,

have a low thermal expansion coefficient, do not melt or outgas at collector stagnation



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temperatures 3 deg. N deg. F4, and whenever possible4 contain reflective foil to

reflect thermal radiation bacA to the absorber.

2.1. COLLECTOR HOUSINGS. he housing or collector box serves to>

#upport the collector components.

Protect the absorber and insulation from the environment.


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performance of the collector. wo suitable sealing methods are shown in Figures +6+

and +63. he gasAets provide flexible support and the primary weather sealant insures

able +65

"eat ransfer Fluids



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able +65 continued4

"eat ransfer Fluids



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Figure +6+

#ingle gasAet seal for double gla:ing

Figure +63

ypical sealing method for single or double gla:ing

) *. Paul Guyer +-+ 3


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against moisture leaAage. /esiccants are sometimes placed between the two gla:ings

to absorb any moisture that may remain after cover installation. ?hen selecting

collector gasAets and sealants, certain material re@uirements must be Aept in mind. he

gasAets and seals must>

?ithstand significant expansion and contraction without destruction.

%dhere effectively to all surfaces.


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"eating 0aterials "andbooA, has proposed the following criteria to reduce the risA of

fire in the use of solar heat transfer fluids> he flash point of the li@uid heat transfer fluid

shall e@ual or exceed the highest temperature determined from a4, b4 and c4 below>

a4 % temperature of 5 deg. above the design maximum flow temperature of the

fluid in the solar systemD or

b4 -4 % temperature + deg. F below the design maximum no6flow

temperature of the fluid attained in the collector provided the collector

manifold assembly is located outside of the building and exposed to the

weather and provided that relief valves located ad7acent to the collector or

collector manifold do not discharge directly or indirectly into the building

and such discharge is directed away from flames and ignition sourcesD or,

+4 he design maximum no6flow temperature of the fluid in all

other manifold and relief valve configurationsD

c4 - deg. F

(f there is no danger of free:ing and the collector loop consists of all copper flow

passages, then ordinary water would be the choice for collector fluid. (f free:ing

conditions are encountered, there are a number of designs that should be consideredbefore it is decided to use a heat transfer oil or antifree:e mixture. hese free:e

protection schemes are summari:ed here using Figure +6 as the basic open loop type

collector circuit.

2.1.#.1 DRAIN DOWN OR DRAIN BAC METHOD6 he water in the collector is

drained out of the system, or into a tanA near the collector, or into the main storage tanA

when temperatures in the collector approach free:ing. his scheme re@uires automatic

valves to dump the water and purge air from the system. Cften a larger pump will be

re@uired to overcome the system head and re6prime the collectors. % way to avoid

automatic solenoid4 valves is to drain the collectors whenever the pump shuts off. his

still re@uires a larger pump. hree6way valves exist that can use city water pressure to

reprime the systemD otherwise pumps must be used. #ome drainbacA systems only

) *. Paul Guyer +-+ 3+


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drain the water to a small tanA near the collectors thus re@uiring only a small additional

pump. "eat exchangers may be re@uired to separate potable water from

nonpotable water.

2.1.#.2 HEAT TAPES6 &lectric resistance heat tapes are thermostatically activated to

heat the water. his scheme re@uires extra energy and is not completely reliable.

(nsertion of heat tapes into preconstructed collectors may be difficult.

2.1.#.3 RECIRCULATION METHOD6 (n this method the control system of Figure +6

merely turns on the pump if free:ing approaches. (n this way, warm water from storage

circulates through the collectors until the free:ing condition is over. he only extra

component needed is a free:e sensor on the collector which is a minimum cost item.

"owever, by circulating heated water, the capacity of storage decreases and less is

available the following day. his method is probably the most reliable of the three since

it does not depend on additional electrical valves or heating tape, provided that bacA up

power is available to operate pumps in the event of power failure. (f the preceding

methods are not acceptable or if the choice of water is not acceptable due to concern

about corrosion, then a heat transfer fluid must be used. he heat transfer fluid must be

used with a heat exchanger in a =closed66loop= configuration as shown in Figure +6.

he configuration shown in Figure +6 will be from -6+5 less efficient due to the

temperature penalty associated with the heat exchanger and the low specific heat of theheat transfer fluid as compared to water. 'ote an additional pump is also re@uired. (f the

heat transfer fluid is toxic or non6potable such as antifree:e4 then a double6walled heat

exchanger must be used for protection. he different types of heat exchangers are

explained in Figure +65. (t is difficult to estimate the most cost effective free:e

protection method. #ome studies have shown that for many areas in the $.#., the

recirculation method is best particularly where free:ing days are few in number. (t tends

to have the lowest capital cost and energy use cost. "owever, all the methods except

heat transfer fluids rely on the presence of electricity to operate. % simultaneous

electrical failure and free:ing condition would result in potential failure of the systems.

%n exception is that new thermally actuated draindown valves are becoming available to

replace the sometimes troublesome solenoid valves. herefore, the absolute safest

system would be the nonfree:ing heat transfer fluids and these might be considered for



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the very cold parts of the country ;oston, hicago, etc.4. &ach potential pro7ect should

be considered individually using local weather criteria, free:e protection capital costs,



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Figure +6

ypical configurations for solar water heater systems

additional energy to run the system, reliability, maintenance, and type of system as the

criteria. Cften a detailed computer simulation would be re@uired to choose. "owever,

any of the methods will provide some degree of protection. (f heat transfer fluids are

selected for corrosion or free:e protection, the following paragraphs discuss pertinent

criteria. 0ost heat transfer fluids contain some degree of toxicity. o minimi:e the

probability of contamination of potable water systems the following items should be

addressed in any specification or bid>

%ssurances to preclude the possibility of cross connection of potable water piping

with heat transfer fluid piping. he use of tags, color coding, different pipe

connections, etc, are suggestions.

"ydrostatic testing of system to find leaAs.

olor indicators in heat transfer fluid to find leaAs.

#afe designs for heat exchangers as given in Figure +65.

/etermine toxicity classification of heat transfer fluids. #uggested categories as

a minimum are>

Cral toxicity C


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Ccular irritant eye4.

/ermal irritant sAin4.

;efore heat transfer fluids are discussed, a review of basic corrosion theory is in order.

he two types of corrosion which cause the most damage in solar systems are galvanicand pitting corrosion &yre, -924. Galvanic corrosion is a type of corrosion which is

caused by an electrochemical reaction between two or more different metals in contact

with each other. % chemical reaction between the metals causes a small electrical

current which erodes material from one of the metals. #olar energy systems generally

contain a number of different metals such as aluminum, copper, brass, tin, and steel.

his maAes the solar system a prime candidate for galvanic corrosion. (f the dissimilar

metals are physically 7oined or if they are contacted by a common storage or heat6

transfer fluid, the possibility of galvanic corrosion becomes much greater. Pitting

corrosion is a highly locali:ed form of corrosion resulting in deep penetration at only a

few spots. (t is one of the most destructive forms of corrosion because it causes

e@uipment to fail by perforation with only a very small weight loss. ?hen heavy metal

ions such as iron or copper plate on a more anodic metal such as aluminum, a small

local galvanic cell can be formed. his corrosion spot or =pit= usually grows downward in

the direction of gravity. Pits can occur on vertical surfaces, although this is not as

fre@uent. he corrosion pits may re@uire an extended period months to years4 to form,

but once started they may penetrate the metal @uite rapidly. "eavy metal ions can

either come as a natural impurity in a water mixture heat transfer fluid or from corrosion

of other metal parts of the solar system. Pitting corrosion has the same mechanism

concentration cell4 as crevice corrosion thus it can also be aggravated by the presence

of chloride or other chemicals which can be part of the water mixture or a contaminant

from solder fluxes. %luminum is very susceptible to pitting corrosion, while copper

generally is not. here are several preventive measures which will eliminate or at least

minimi:e galvanic and pitting corrosion in collector systems which use an a@ueous

collector fluid. he best method to prevent galvanic corrosion is to avoid using dissimilar

metals. ?here this is not possible or practical, the corrosion can be greatly reduced by

using nonmetallic connections between the dissimilar metals, thus isolating them.

Galvanic protection in the form of a sacrificial anode is another method of protecting the



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Figure +65

"eat exchangers for solar water heating systems



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parent metals. %lso, use of similar metals reduces the problems of fatigue failure

caused by thermal expansion. Pitting corrosion is essentially eliminated if copper

absorber plates are used. orrosion inhibitors can minimi:e pitting corrosion in

aluminum absorbers. he types of heat transfer fluids available may be divided into two

categories, nona@ueous and a@ueous. #ilicones and hydrocarbon oils maAe up the

nona@ueous group, while the a@ueous heat transfer fluids include untreated potable

tap4 water, inhibited6distilled water, and inhibited glycolBwater mixtures. he potable tap

water and inhibited distilled water do not, of course, offer free:e protection. able +65

shows characteristics of some of the most common heat transfer fluids.

2.1.#.1 SILICONE FLUIDS.#ilicone heat transfer fluids have many favorable properties

which maAe them prime candidates for collector fluids. hey do not free:e, boil, or

degrade. hey do not corrode common metals, including aluminum. hey have

excellent stability in solar systems stagnating under deg. F. #ilicone fluids are also

virtually nontoxic and have high flash and fire points. urrent evidence indicates that

silicone fluids should last the life of a closed6loop collector system with stagnation

temperatures under 35 deg. 6 deg. F. he flash point is fairly high, 5 deg. F, but

since the "$/ standards state that heat transfer fluids must not be used in systems

whose maximum stagnation temperature is less than - deg. F lower than the fluids

flash point, this limits most silicone oils to systems with a maximum temperature of 35deg. F or less. %lso silicones do not form sludge or scale, so system performance does

not decrease with time. he main drawbacA of silicone fluids is their cost. hus the

cost of the + to 3 gallons of collector fluid re@uired for a typical 5 ft +collector

system becomes considerable. %s with hydrocarbon oils, the lower heat capacity and

higher viscosity of silicone fluid re@uires larger diameter and more expensive piping.

/ue to the higher viscosity, larger pumps will be re@uired and subse@uent higher

pumping costs. Cne other problem with silicone fluids is the seepage of fluid at pipe

7oints. his problem can be prevented by proper piping installation and by pressuri:ing

the system with air to test for leaAs. here have also been reports of seepage past the

mechanical seals of circulating pumps. he use of magnetic drive or canned wet rotor

pumps when available in the proper si:e is a method of avoiding mechanical seal

leaAage. #ilicones have the advantage of lasting the life of the system with little



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maintenance. ?hile this helps minimi:e operating expenses, the initial cost of silicones

is marAedly higher than that of other available heat transfer fluids. "owever, the high

initial cost of silicone heat transfer fluid may be less than the savings that result from

minimum maintenance and no replacement of collector fluid. he use of silicone fluid

allows absorbers with aluminum fluid passages to be used without fear of corrosion.

he savings gained from the use of aluminum absorbers as opposed to copper

absorbers could be substantial.

2.1.#.2 HYDROCARBONS."ydrocarbon oils, liAe silicones, also give a long service

life, but cost less. hey are relatively noncorrosive, nonvolatile, environmentally safe,

and most are nontoxic. hey are designed for use in systems with lower operating

temperatures, since some brands breaA down at higher temperatures to form sludge

and corrosive organic acids. ypical closed6cup flashpoints run from 3 deg. F to +

deg. F, but the fluids with higher flashpoints have a higher viscosity. he "$/ bulletin

on minimum property standards for solar heating systems recommends a closed6cup

flashpoint - deg. F higher than maximum expected collector temperatures.

$nsaturated hydrocarbons are also sub7ect to rapid oxidation if exposed to air,

necessitating the use of oxygen scavengers. #ome hydrocarbons thicAen at low

temperatures and the resultant higher viscosity can cause pumping problems. 'ewer

hydrocarbons are being developed which do not harm rubber or materials ofconstruction, since this has been a problem with hydrocarbons. (n general, they cannot

be used with copper, as it serves as a catalyst to fluid decomposition. he thermal

conductivity of hydrocarbons is lower than that of water, although the performance of

some brands is much better than others. he cost of typical hydrocarbon and other

synthetic heat transfer oils vary. % typical li@uid collector of 5 ft+plus the piping to and

from storage will re@uire from + to 3 gallons of collector fluid. he lower heat capacity

and higher viscosity of these oils will also re@uire larger diameter pipe, increasing

materials costs further. (f hydrocarbon fluids are used, the additional capital cost should

be compared with expected savings due to lower maintenance costs. he use of

aluminum absorbers rather than copper absorbers will also result in substantial savings.

2.1.#.3 DISTILLED WATER./istilled water has been suggested for use in solar

collectors since it avoids some of the problems of untreated potable water. First, since



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the distillation process removes contaminants such as chlorides and heavy metal ions,

the problem of galvanic corrosion, though not completely eliminated, should be

alleviated. "owever, distilled water is still sub7ect to free:ing and boiling. For this

reason, an anti6free:eBanti6boil agent such as ethylene glycol is often added.

2.1.#.4 WATER&ANTI&FREE'E.'onfree:ing li@uids can also be used to provide free:e

protection. hese fluids are circulated in a closed loop with a double wall heat

exchanger between the collector loop and the storage tanA see Figure +654.

?aterBantifree:e solutions are most commonly used because they are not overly

expensive. &thylene and propylene glycol are the two most commonly used

antifree:es. % 565 waterBglycol solution will provide free:e protection down to about

63 deg. F, and will also raise the boiling point to about +3 deg. F. he use of

waterBglycol solution presents an additional corrosion problem. ?ater glycol systems

will corrode galvani:ed pipe. %t high temperatures glycols may breaA down to form

glycolic acid. his breaAdown may occur as low as -2 deg. F and accelerate at +

deg. F. his acid corrodes most all metals including copper, aluminum, and steel. he

rate of glycol decomposition at different temperatures is still a sub7ect of uncertainty.

he decomposition rate of glycol varies according to the degree of aeration and the

service life of the solution. 0ost waterBglycol solutions re@uire periodic monitoring of the

p" level and the corrosion inhibitors. he p" should be maintained between 1.5 and2..


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water inlet to the collector at the bottom, and outlet at the top. are must be taAen so

that e@ual flow goes to all collectors. (f the pipe manifold pressure drop is large, then

end collectors will get little flow. he design most usually used is one in which the

collectors are connected in parallel. his results in low pressure drop and high efficiency

of each collector. % series hooAup results in the highest temperature and the highest

pressure drop but lowest collector efficiency. "igher temperatures than in the parallel

arrangement may be obtained with parallel6series connections, but at the expense of

reduced efficiency and greater cost. hese high temperatures are not usually re@uired

for hot water and space heating. Figure +61 shows different connection configurations.

%ll collector systems should be installed using a reverse6return O flow4 piping layout as

shown in figure +61a. $p to about -+ collectors in a row can be accommodated. Lery

large installations may merit computer simulations to optimi:e the flow balance of each

stage.

2.1. COLLECTOR EFFICIENCY AND HEAT LOSSES.(n the preceding sections,

many details as to the construction and choice of components of a solar collector have

been given. %ll of these features contribute to how well a collector will perform or how

efficient it will be. #olar collectors, depending on their construction and materials, suffer

from several Ainds of heat losses. hey can lose heat by convection of wind blowingover their top and bottom surfaces. %s the collector temperature increases above the

temperature of the surrounding air, the radiation heat losses increase. his results in

lower heat collected lower efficiency4 at higher collector temperatures. "eat can be lost

by conduction from the bacA and sides of a collector. o evaluate the effects of all these

parameters individually would involve detailed and difficult calculations. Fortunately,

collector performance can be compared much more easily by a single graph depicting

collector efficiency versus the parameter B(. collector efficiency is defined as the ratio

of the heat collected to the insolation (4 falling on the surface of the collector. %lso>

I i6 a



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Figure +61onnection #chemes for #olar "eating #ystems

Figure +61a

ollector Piping

where



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iI temperature of fluid entering collector inlet4.

aI ambient air temperature.

Figure +6 gives the efficiency of some typical flat plate solar collectors. he most

efficient solar collector would convert - of the suns energy falling on it to usable

heat. %s shown in Figure +6, this is impossible so the designer looAs for a collector that

converts the greatest percentage of solar energy to heat, at the re@uired temperature,

and at the lowest cost. (t is important that each collector be tested according to an

exacting standard. he early standard for testing solar collectors, was ';#(< 6135

published by the 'ational ;ureau of #tandards. his is the standard the previous

edition of this report used to report collector efficiencies. #ubse@uently, the %merican

#ociety of "eating,


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Figure +6

ypical #olar ollector &fficiencies

) *. Paul Guyer +-+


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% large amount of test data on solar collectors is becoming available through the

national certification program run by #&(%, the '&! tests, and individual laboratories

testing for the manufacturers. he 'ational ertification Program managed by #&(% is

now the primary source of solar collector test data. able +61 represents a random

sampling of the many solar collectors available. (t is not a comprehensive list nor is it an

endorsement of any particular collector. hese data were excerpted from the #olar


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able +61

#olar ollector est


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able +61 continued4

#olar ollector est


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able +61 continued4

#olar ollector est


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able +61 continued4

#olar ollector est


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able +61 continued4

#olar ollector est


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able +61 continued4

#olar ollector est


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able +61 continued4

#olar ollector est


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Figure +6

ypical #olar ollector &fficiencies



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7udgments, while able +61 should be used for typical slope and intercept values. his

avoids the errors associated with trying to =read off= numbers on Figure +6.

Figure +62

&vacuated tube solar heat collector.

2.1.1* OTHER TYPES OF SOLAR COLLECTORS. he three most common types of

solar collectors are flat plate collectors, evacuated tube collectors, and concentrating

collectors. /ue to certain cost and performance advantages, flat plate collectors have

been used extensively for residential /"? and space heating applications. &vacuated

tube and concentrating collectors are used mostly in solar applications re@uiring very

high temperatures. #ome applications re@uiring large solar arrays are using evacuated

and concentrating collectors. % brief description follows.

2.1.1*.1 E"ACUATED&TUBE COLLECTORS. Figure +62 shows an evacuated6tube

collector. his type of collector uses a vacuum between the absorber and the



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transfer li@uid is circulated. his type of collector also re@uires a tracAing mechanism

and can collect only direct radiation. Figure +69c4 shows a compound parabolic mirror

collector. he design of the mirrors allows the collector to collect and focus both direct

and diffuse radiation without tracAing the sun. Periodic changes in the tilt angle are the

only ad7ustments necessary. /irect radiation is intercepted by only a portion of the

mirror at a time, thus this collector does not collect as much solar energy as a focusing

collector which tracAs the sun. (t is, however, less expensive to install and maintain. he

absorber tube is encased within an evacuated tube to reduce heat losses. 0any other

types of concentrating collectors have been developed which produce high

temperatures at good efficiencies. "owever, the potentially higher cost of installing and

maintaining tracAing collectors may limit their use in some applications. hese points

should be addressed early in pro7ect development when tracAing collectors are

considered. (n addition, concentrating collectors must be used only in those locations

where clear6sAy direct radiation is abundant.

2.2 ENERGY STORAGE AND AUXILIARY HEAT. #ince effective sunshine occurs

only about 5 to 1 hours per day in temperate latitudes4, and since heating and hot

water loads occur up to + hours a day, some type of energy storage system is needed

when using solar energy. he design of the storage tanA is an integral part of the totalsystem design. %lthough numerous storage materials have been proposed, the most

common are water for li@uid collectors and rocA for air. hese have the advantages of

low cost, ready availability and well Anown thermal properties. Precise heat storage

si:ing is not necessary, but economics and system design to determine the optimum

range of si:es. he temperature range wherein useful heat is stored is important in

determining optimum system si:e. (f the volume of storage is too large, the temperature

of the storage medium will not be high enough to provide useful heat to the building.

%lso, overdesigned storage re@uires excess floor space. (f the storage is too

small, the storage medium temperature will be too high, resulting in low collector

efficiency. Practical experience in the industry as well as computer simulations and



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experiments have resulted in general rules of thumb for storage si:ing. hese

guidelines give storage si:es for which the performance and cost of active solar

systems are optimi:ed and relatively insensitive to changes within the range indicated.

he optimum si:e of storage for active solar systems is -5 ;tuBdeg. FBft +of collector

area. he range is -6+ ;tuBdeg. FBft++6 Q*Bdeg. Bm+4. For water or air

systems application of the rule gives the following.

WATER SYSTEMS. #ince water has a specific heat of - ;tuBlb6deg.

F, then -5 lb of water storage are needed per s@uare foot of collector or

considering the density of water, 2.33 lbBgal or 1+. lbBft3, then -.2 gal of storage

are needed for each s@uare foot of collector range -.+ to +. galBft +4. he range

in #( units is 56- litersBm+.

AIR SYSTEMS. #ince rocA has a specific heat of .+- ;tuBlb6deg. F, and rocA

densities - lbBft34 typically contain +6 voids, then the optimum storage

si:e is .2 ft3per s@uare foot of collector range .5 to -.-5 ft3per s@uare foot of

collector4. he range in #( units is .-5 to .35 m3Bm+.

(n general, for e@ual storage capacity, the rocA pebble bed would have to occupy avolume +6-B+ to 3 times larger than a water tanA.


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Figure +69

oncentrating collectors for solar energy

second tanA up to temperature. #ingle tanA arrangements, while possible and

economical, are not recommended due to the fact that they tend to activate the heating

element every time there is a draw of water rather than wait for the solar collectors to

provide additional heated water.


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Figure +6-

#chematic of potable hot water heating system using solar storage tempering4 tanA

ahead of conventional fueled or electric service water heater

re@uired for latent heat storage than for heat storage in rocA beds. "owever, problems

of slow solidification and low heat conductivity retards effective heat transfer to and from

the material. %s a result, a large surface area6to6volume ratio is re@uired, which

significantly increases the effective volume of latent storage. solar storage tempering4

tanA ahead of conventional fueled or electric service water heater.R !atent storage

materials are often expensive when compared to rocA. (n addition, they must be

pacAaged in individual containers to allow ade@uate heat transfer area. 0any latent heat

materials cannot withstand fre@uent recycling and must be replaced periodically.


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which can withstand extended recycling. '&! is investigating a dissolved salt storage

unit that uses immiscible li@uids for the heat exchange surface which greatly reduces

the problem of crystalli:ation during recycling. (nitial tests have been encouraging.

%nother ma7or drawbacA of latent heat storage is that heat is stored at an average

temperature with essentially no thermal stratification occurring in the storage unit. %

high level of thermal stratification maximi:es thermal performance because low

temperature fluid can be delivered to the collectors and high temperature fluid can be

delivered to the heat load. For example, the high degree of thermal stratification in rocA6

beds results in the delivery of 9 deg. F air to the collector and -+ deg. F to -5 deg. F

air to the heat load. (n comparison, latent heat storage in Glaubers salt occurs near an

average temperature of 9 deg. FD thus air at 9 deg. F is delivered to both the

collectors and the heat load. /ue to the problems discussed, latent heat storage has not

received widespread use. #ince it is not economically 7ustifiable to store huge

@uantities of heat, most solar systems cannot be depended on to provide - of the

buildings needs. /epending on the geographical area and si:e of the system, about

to 2 of the heat re@uirement is the average to design for. herefore auxiliary

heaters are necessary. hey should be si:ed to provide all the energy re@uirements,

although in some cases, again depending on location, it may be possible to increase

storage volume and provide less than - bacAup auxiliary heat. his is especiallytrue if the use of passive solar designs can be incorporated with active systems. he

auxiliary heater should operate automatically as needed, use the most economical fuel,

and share a common heat delivery system with the solar system. Cften a heat pump is

a good choice in that it can serve both as an auxiliary heater and worA together with the

solar system. (n retrofit situations, the existing heater would be the choice.

2.2.1 STORAGE TANS.?ater may be stored in a variety of containers usually made

of steel, concrete, plastics, fiberglass, or other suitable materials. #teel tanAs are

commercially available and have been used for water storage. hey are available in

many si:es and are relatively easy to install. "owever, steel tanAs are susceptible to

corrosion and should be lined or galvani:ed. /issimilar metal at pipe connections

should be separated by high temperature rubber connections or galvanic corrosion will



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occur. #teel tanAs must be well insulated to minimi:e heat losses. oncrete tanAs are

durable, but may be difficult to install. oncrete tanAs cast in place, prefabricated septic

tanAs, or large diameter pipes may be used for water storage. % high temperature

sealant or lining should be applied to the interior of the tanA to prevent seepage of water

through the tanA. %lthough concrete is less conductive than steel, concrete tanAs should

also be insulated to reduce thermal losses. !eaAs are difficult to repair. Fiberglass and

plastic tanAs are corrosion resistant and easily installed. hey are available in many

shapes and si:es. %lthough many commonly fabricated tanAs will begin to soften at

temperatures above - deg.6-1 deg. F, there are more expensive, specially

fabricated tanAs available that can withstand temperatures up to +5 deg. F. he types

of plastics needed to store large @uantities of water at high temperatures can be more

expensive than steel. ?hen storage tanAs are to be custom made, a calculation of heat

loss against expected fuel cost inflation will almost always 7ustify increasing insulation

around the tanA to


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used. #ince it is possible for solar collectors to reach very hot temperatures, a

tempering or mixing valve should be used. % typical two6tanA installation with proper

valves and connections would be as shown in Figure +6--. o si:e the collectors and

storage tanA it is necessary to estimate or measure the hot water consumption of the

facility or building. For typical family residences, + galBdayBperson of hot water is

normally consumed. (f it is estimated the hot water consumption is larger than average,

use 3 galBdayBperson. #o, 2 to -+ galBday should serve a typical four6person family.

able +69 gives water consumption data for different types of conventional facilities and

may be used to supplement over data.

2.4 THERMOSYPHON, BATCH, AND INTEGRAL STORAGE COLLECTOR

SYSTEMS. % variation of the /"? system is the thermosyphon system which uses the

principle of natural convection of fluid between a collector and an elevated storage tanA.

%s water is heated in the collector it rises naturally to the tanA above. he bottom of the

tanA should be mounted about + feet higher than the highest point of the collector. his

is the main disadvantage in that structural re@uirements will often prohibit the weight of

a water tanA on a high point of the structure. %lso, since the thermosyphon system is

connected directly to the potable water supply it is difficult to protect from free:ing."owever, new models are coming on the marAet that use Freon as the heat transfer

fluid, solving the free:ing problem. he advantages of thermosyphon units are that they

do not re@uire pumps or electronic control systems. "ence the costs to purchase and

operate these components are eliminated. %lso these systems save by virtue of

eliminating these components as a source of reliability or maintenance problems. % last



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able +6

%dvantages and disadvantages of tanA types



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able +62

#torage anA osts

advantage is that they are completely independent of electrical grid power. ;atch and

integral storage collector (#4 systems are similar in that they also do not have pumps

or controllers. ;atch systems often called =breadbox= also4 are simply a blacA painted

storage tanA or several4 installed in a weathertight box and gla:ed with glass or plastic.

hey depend on their heat transfer by flow of water through the system initiated

whenever there is demand for water by the occupants. (ntegral storage collectors put

the tanA and collector together to form a large mass of fluid to be heated by the sun.

he intent is to have a large enough mass of water that free:ing will not be a problem

except in the severest of climate. #urprisingly only about 36 gallons of water are

needed to accomplish this over most of the $nited #tates. (# systems also depend on

system demand for their flow, but some models have also been configured to use the



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Figure +6--

ypical /"? (nstallations

thermosyphon principle. he testing of these units is different than regular solar

collectors since the %#"


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able +69

"ot ?ater /emands and $se for Larious ypes of ;uildings



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Figure +6--a

hermosyphon #ystem ests



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is not enough to rule out the use of these systems especially when their advantages of

improved reliability and maintenance are considered. he important conclusion of these

tests is that the performance is similar enough that the choice of which to use can be

made by considering other pertinent factors of the installation. he results of system

tests on these models are reported in the /irectory of #


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advantages of air versus li@uid4. he heat storage tanA is replaced by a rocA bed

nominally -63 inch diameter4.


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he air is usually heated in a central location and ducted to the individual rooms. his

method is used particularly in residential buildings. "ydronic heating is another

common heat distribution method. (n hydronic heating systems hot water or steam is

circulated through pipes to =convectors= located in the individual rooms of a building.

0odern hot water convectors are comprised of one or more finned tubes located on the

wall near the floor. hese baseboard heaters deliver heat to the room mainly by

convection as air moves through the fins. % less common heating system consists of

lengths of tubing embedded in the floors, walls, or ceilings of the living space. ?arm

water is supplied to the tubes by a boiler and the heat is transferred to the room by

convection and radiation.

2..1 HEAT DISTRIBUTION FOR LI%UID&TYPE SOLAR SYSTEMS.he temperature

re@uirements of a hydronic heating system are dependent on the amount of heat

exchanger surface. 0ost baseboard heaters have comparatively small surface areas,

so they re@uire higher temperatures, typically about -2 deg, F. (f larger heat transfer

areas are available as in older or modified hot water systems, temperatures of -+ deg.

F may be sufficient. emperatures of - deg. F are ade@uate for the system which

uses entire floors, walls, and ceilings as radiator surfaces. /uring the winter, typical

li@uid6type solar systems are seldom operated at delivery temperatures above -5 deg.F. hus it is evident that the use of solar heated water in standard baseboard heaters is

impractical. Cnly modified baseboard heaters of ade@uate si:e or radiant panels are

suitable for use in hydronic systems which use solar heated water. Cne of the most

economical means of auxiliary heat supply and heat distribution for li@uid6type solar

systems involves the use of a warm air system. % typical system is illustrated in Figure

+6-1. (n this system the warm air furnace is located downstream from a li@uid6to6air heat

exchanger which is supplied with solar6heated water. he furnace can then serve to

boost air temperature when insufficient heat is available from the solar heated water, or

it can meet the full heat load if no heat is available in solar storage. %uxiliary heat can

be supplied by a gas, oil, or electric furnace, or by the condenser of an air6to6air heat

pump. %nother method of heat distribution involves the use of a water6to6air heat pump

) *. Paul Guyer +-+


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Figure +6-+

0inimum heating system, showing relationship of

collector, storage, and room unit heater



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which draws heat from the solar storage tanA and pumps it to a condenser coil which is

placed in a central air duct. he advantage of this system is that it can effectively use

heat from solar storage at temperatures down to 5 deg. F, thus more of the stored heat

is available. %lso, average storage temperatures are lower, resulting in significantly

increased collector efficiency.

2..2 HEAT DISTRIBUTION FOR AIR&TYPE SOLAR SYSTEMS. he pipes and

pumps of the li@uid6type system are replaced by air ducts and fans. he warm air

system is obviously the best heat distribution system for use with an air6type solar

system. he ability to circulate building air directly through the collectors is one of the

ma7or advantages of an air6type solar system. he rocA bed storage also worAs best

with a warm air system. %lthough warm air as low as - deg. F can be used to heat

an occupied building, most existing warm air systems are si:ed assuming warm air

temperatures of -+ deg. F to -5 deg. F. ypical mid6day collection temperatures

usually range from -3 deg. F to - deg. F. 0aximum storage temperatures are

typically around - deg. F at the end of the collection period. hus the heating load

can be met by the temperature of the solar heated air a large portion of the day. ?hen

storage temperatures are insufficient to maintain the desired temperature in the

building, heat from an auxiliary source must be added to supplement the solar heatedair. he auxiliary furnace is located downstream from the rocA bed so that the rocA bed

serves as a pre6heater for the furnace. his arrangement allows the rocA bed to deliver

useful heat until all of the rocAs are at room temperature. %n air handler unit provides

the dampers and blowers necessary to direct air circulation between the solar

collectors, rocA6bed, and building as needed. %n air handler unit may be more

expensive than the combined cost of individual dampers and blowers, but it will

probably be less expensive to install. (t is also more compact.

2..3 HEAT PUMPS. "eat pumps have been mentioned in previous sections as a

possible choice for auxiliary heaters. #ome manufacturers are combining solar systems

with heat pumps for the purpose of reducing auxiliary energy costs. ?hen a heat pump

and a solar system are combined in this manner, the system is usually called solar



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assisted or solar augmented heat pump #%"P4 system. #olar assisted heat pump

systems can be configured in many different ways. For example, the solar collectors

can be either water or air types, the heat storage medium can be water or a solid

material such as rocA or bricA, and the heat pump can be of either the air6to6air design

or the water6to6air design. ;ut heat pumps have a characteristic which can limit their

effectiveness> the efficiency and capacity of a heat pump decreases as the temperature

of the heat source usually outdoor air4 decreases. his deficiency can be overcome,

however, by using solar collectors to gather the suns energy for the purpose of Aeeping

the heat source in the temperature range re@uired for efficient heat pump operation.

2..3.1 AIR&TO&AIR HEAT PUMPS.#ome air6to6air heat pumps function very well as

an auxiliary heater at temperatures down to + deg. F. ;elow these temperatures, they

suffer in efficiency and performance. ?hen solar assisted by heat from a rocA6pebble

storage bed and air collectors, the heat pump adds much to the performance of the

solar energy system. ?ithout such a solar assist, air6to6air heat pumps have limited

utility in cold climates. heir use should be carefully checAed with the local utility and

pump manufacturer. he heat pump also provides cooling during the summer. (t thus

has year6round utility. "eat pumps should be comparison6shopped. he purchaser

should looA at the cost, performance, service, and expected life. $nits differ

considerably from manufacturer to manufacturer.2..3.2 LI%UID&TO&AIR HEAT PUMPS. he li@uid6to6air heat pump is an ideal

auxiliary heater when coupled with li@uid solar storage. (t operates at very low cost. %nd

it greatly enhances solar energy collection by drawing down the temperature of the solar

storage water to as low as 5 deg. F. (t should be considered for all installations, except

those with existing fossil fuel furnaces and no need for summer cooling. Cut of the

many #%"P configurations which could be used, the two most in use are called the

=series= and =parallel= configurations. Figure +6- is a series #%"P system. ?hen the

system is used for heating, water from the storage tanA is circulated through water6

cooled collectors where it is heated before returning to the storage tanA. ?arm water

from the storage tanA is also circulated through a water6to6air heat pump. "eat is

removed from the water and transported to the indoor air by the heat pump and the

water returns to the storage tanA at a lower temperature. (f heat is added to the water in



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the tanA faster than it is removed by the heat pump, the temperature of the water will

rise. ?hen the water temperature is high enough about - deg. F4, heat can be

extracted directly from the water by means of water6to6air heat exchanger. (n this mode

of operation, the heat pump is shut off. %uxiliary electrical resistance heaters are

provided to maAe up the balance of the heat load if the heat from the heat pump or

water air heat exchanger is not sufficient to meet the demand. 'ormally this could be

=off6peaA= power for the auxiliary heater. ?hen used for cooling, the heat pump

transports heat from the building to the water in the storage tanA thereby causing the

temperature of the water in the tanA to rise. /uring spring and fall, when it is not unusual

to have a light cooling load during the day and a light heating load at night, the heat in

the storage system is simply shuttled from the building to storage during the day and

from storage to the building at night, and the solar collectors are used only to maAe up

for lost heat. /uring periods of prolonged cooling demand, the heat pumped into the

storage tanA might be sufficient to cause the temperature of the water to rise to where

the heat pump will no longer operate. hus, provision must be made for re7ecting

excess heat. Cne method is to add a cooling tower to the system to cool the water.

%nother method is to circulate water through the solar collectors at night and re7ect heat

by radiation to the night sAy. /uring periods of high cooling load it is not desirable to

also add heat to the storage tanA by circulating water through the solar collectors.herefore, when the system is in the cooling mode the solar collector circuit can be

used to heat /"?. he =parallel= #%"P system is shown in Figure +6-2. he solar

heating system and the heat pump operate in parallel. #olar heat is used directly rather

than being transferred to a storage medium and then transported into the building with a

heat pump. his system is essentially a direct solar heating system with an air6to6air

heat pump as a bacAup heating system. he choice of a =best= system is difficult to

maAe due to the many variables involved. For example, in addition to the two

configurations shown in Figures +6- and +6-2, one could examine a series system with

low cost ungla:ed4 collectors, or a series system with air6collectors and rocA storage, or

a parallel system with low cost collectors, etc. &ach system would be highly dependent

on geographical location, type of construction, etc. Cne such analysis done at '&!

comparing several systems to a standalone air source heat pump, showed the =parallel=

) *. Paul Guyer +-+


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system to have the best comparative performance. &ach heat pump configuration

should be considered on a case6by6case basis. he analysis of these systems is

beyond the scope of the worAsheets given in this course, and the reader is directed to

more sophisticated computer programs.

2.! PASSI"E SYSTEMS. '&! has published a contract report, =Passive #olar

/esign Procedures for 'aval (nstallations= that is a reference on this sub7ect. (t contains

data and worAsheets to si:e passive solar designs at + geographical locations. Cver

- different passive designs can be considered and the method is applicable for single

family residences, family townhouses, dormitories i.e. ;&Es4, small offices, and other

concrete blocA buildings. % =passive= solar energy system is one which uses the

building structure as a collector, storage and transfer mechanism with a minimum

amount of mechanical e@uipment. #ome would include a thermosyphon, batch, and (#

systems in this definition. %s a rule, passive systems are generally difficult to retrofit

%nother disadvantage is that the owner or occupant may be re@uired to perform daily

tasAs, such as covering a south facing window at night, opening and closing shutters,

etc. %lthough the specific arrangements vary, all of these systems rely on direct solar

heating of storage. he storage then heats the house. % few examples are shown in

Figure +6-9. Given the solar gain available on a vertical surface, the simplest and mostobvious means of solar heating is 7ust to let the sun shine in through large, south6facing

windows. (n fact, in a house with any south6facing windows, that is what is already

happening to some degree. ;ut the sunshine through the windows seldom heats the

whole house. here are two reasons for this. First, most houses do not have enough

south6facing glass. #econd, houses lacA enough storage to soaA up the heat and Aeep

it until night. &ven rooms that overheat during the day cool off all too rapidly in the

evening. Cn many buildings it is possible to add south6facing windows or sAylights to

increase direct solar heating. "owever, the extra window area can cause a =fry or

free:e= situation unless storage and night window insulation is added as well. here

must also be provisions for getting heat from the rooms receiving sunlight to the rest of

the house. Providing such storage and delivery of solar heat gained through windows is



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Figure +6-3

#pace heating system with closed collector loop

Figure +6-

#pace heating and domestic water system



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the basis of passive solar heating systems. %s shown in Figure +6-9 the type of storage

used and where it is located with respect to the windows varies for different passive

systems. all metal or fiberglass tubes can be used to hold water instead of drums.

&ntire walls of solid concrete or grout6filled masonry store solar heat well. #lab floors

can absorb solar heat coming in through windows, sAylights, or greenhouse glass. (n

each of these systems, the sunlight coming in through the glass must shine directly on

the storage. (f it does not the storage cannot absorb enough solar heat to provide much

warmth for the house. 0ost passive systems deliver heat to the rest of the house

=naturally= 6 that is, the heat moves by itself without use of pumps or fans. here is

some natural regulation of how fast heat moves from the storage into the house 6 the

colder the house gets, the faster the heat is drawn out of the storage. hat is how the

drum wall worAs. (n other passive systems, solar heat is =trapped= between the glass

and storage in the air space between the glass and a concrete wall, or in an entire

greenhouse4, and the amount of heat allowed into the house is controlled by opening

and closing vents, either manually or automatically. he performance of passive

systems depends not 7ust on how much solar heat they can collect, but also on how

much of that heat is lost through the glass at night. he most common solution to the

problem of heat loss is to install movable insulation such as insulating curtains4

between the glass and the storage. he curtains or other devices are moved during theday to let the sunshine in, and closed at night to reduce heat loss. ertain conditions

must be present to do a simple passive retrofit. #ince the basis for passive heating is to

=let the sun shine in,= the building must have extensive south6facing windows or

sAylights or places where they can be added. (n addition, there must be a place close to

the windows where storage can be located. he storage must receive midday sun. he

problem here is that drums of water and masonry walls are so heavy that most existing

floors cant support them. (f the floor is not strong enough, there are at least two

possible alternatives. Cne is to put the water or masonry wall on its own foundation on

the exterior of the south wall. %nother is the techni@ue of turning a room addition into a

solar heater that provides warmth for the rest of the house as well. %s with active solar

systems and heat pumps, there are endless variations of the passive techni@ue, limited

only by ones imagination. here are systems that use water on the roof to absorb heat

) *. Paul Guyer +-+


78/99

directly, and there are clever ways to insulate glass at night by blowing #tyrofoam

beads between two glass panes ;&%/?%!! of #teve ;aer4. %lso natural ob7ects

such as earth berms to protect from winds and trees which shade in summer and

let light pass in winter should be considered. Figures +6+ through +6+3 show various

representations of some of these passive techni@ues used either by itself or in

con7unction with air collectors and thermosyphon systems. %lthough passive systems

are rather simple in construction and design, their performance analysis is often

complicated by a vast interplay of many components. "ere are some =rules of thumb=

that should be useful for passive designs>

#outh6facing passive storage walls in direct sunlight should have a minimum of

36lb water storage or -56lb masonry concrete4 storage per s@uare foot of

south vertical gla:ing. (f the storage media is not located in direct sunlight, four

times this amount will be needed. %t least 561 gallons water storage about 5

lb4 per s@uare foot of south glass is recommended.

#hading of south windows should be used to reduce summer and fall

overheating. Cne effective geometry is a roof overhang which will 7ust shade the

top of the window at noon solar time4 sun elevation of 5 deg. and will fully

shade the window at noon sun elevation of 2 deg. F.

he best thicAness of a rombe wall is from -+ to -1 inches. he masonry should

have a high density 6 at least - lbBft3. hermocirculation vents can be used to

increase daytime heating but will not increase nighttime minimums. Lents should

have lightweight passive bacAdraft dampers or other means of preventing

reverse flow at night.

wo to three s@uare feet of south6facing double gla:ing should be used for each

;tuBdeg. F6hr of additional thermal load i.e., exclusive of the gla:ing4. his will

give to 2 solar heating in northern 'ew 0exico !os %lamos4 for a

building Aept within the range of 15 deg. F to 5 deg. F.

%n easier to use rule is that for a well6insulated space in deg. ' latitude in

cold climates outdoor temperature I + deg. F to 3 deg. F4 the ratio of south

gla:ing to floor area is in range .+ to .+5 to maintain an average space



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temperature of 12 deg. F over + hours e.g., a + ft+floor space needs 65

ft+of south gla:ing4. (n temperate climates 35 deg. F to 5 deg. F outdoor

temperature4 use ratios in the range .--6.-.

For greenhouses> o determine solar gain> # I -+ ;tuBft +of gla:ing per clear

day, # I ;tuBft+per average day. /ouble gla:e only south wall. (nsulate all

opa@ue surfaces to


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Figure +6-5

ypical air6type space heating system

bromide and water, and ammonia and water. here have been a number of proposed

solid material absorption systems also. Figure +6+ shows a typical lithium bromide

!i;r4 absorption cooler. (n the absorption cooler, heat is supplied to the generator in

which a refrigerant is driven from a strong solution. he refrigerant is cooled in the

condenser and allowed to expand through the throttling valve. he cooled, expanded

refrigerant receives heat in the evaporator to provide the desired cooling, after which the

refrigerant is reabsorbed into the cool, weaA solution in the absorber. he pressure of

the resulting strong solution is increased by pumping and the solution is available to

repeat the process. he performance of the system is governed largely by the

temperature difference between the generator and the condenser and absorber units.



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Figure +6-1

% li@uid6to6air heat delivery system

#ince the generator temperatures in solar driven systems are only moderate, it isimportant to Aeep the condenser and absorber temperatures as low as possible. he

!i;r system is preferred over ammonia systems for solar energy applications because

of the lower generator temperatures re@uired. Permissible generator temperatures for a

water6cooled !i;r system range from - deg. F to +- deg. F 1 deg. 699 deg. 4

compared to the +5 deg. F to +2 deg. F 95 deg. 6-+ deg. 4 temperatures

re@uired for a water6cooled ammonia absorption system. 0ost, if not all, of the

commercially available absorption units use !i;r and water as the absorbent6refrigerant

fluid pair. ;ecause the !i;r will crystalli:e at the higher absorber temperatures

associated with air cooling, these units must be water cooled. % prototype ammonia6

water unit, amenable to direct air cooling, has been built by !awrence ;erAeley

!aboratories. % number of e@uipment re@uirements and limitations must be considered

) *. Paul Guyer +-+ 2-


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Figure +6-

#eries6connected, solar6assisted heat pump system

in the analysis and design of solar powered absorption systems. he first consideration

involves the type of collector used. he temperatures re@uired by absorption coolers are

obtainable with flat plate collectors but at low collection efficiencies. ollection efficiency

is improved with an increased number of gla:ings and with a selective surface,

therefore, it may be cost effective to improve the collector rather than to simply oversi:e.

oncentrating or evacuated tube collectors are usually used in these applications. (f

concentrating collectors are used, the associated higher costs and potentially increasedmaintenance for the tracAing mechanism must be considered. (n general, concentrating

collectors operate at higher efficiency at these higher temperatures. "owever, the

higher temperatures are usually not re@uired to operate the space heating system.



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Figure +6-9

Passive solar energy systems

state performance. his problem has been overcome in at least one installation by the

use of a cold storage unit. he cold storage unit permits continuous operation of the

absorption cooler and thus allows some reduction in the system and cooler si:e. %

fourth consideration is the need for some means of cooling the absorber and the

condenser. % cooling tower or some other low temperature cooling system must be

used to obtain reasonable performance. %ll of the commercially available units re@uire a

cooling tower which is another maintenance item. urrent research is underway to

develop units that do not have a separate cooling tower.

2.#.2 RANINE CYCLE HEAT ENGINE COOLING.


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by a heat engine drives the compressor in a conventional vapor compression6type

cooling machine. he thermal energy input to the heat engine can be from a solar

collector or from a solar collector and a fossil fuel combustor. he fossil fuel can

supplement solar energy, or it can be used alone as the auxiliary energy supply when

no solar energy is available. %lternatively, electricity can be used as the auxiliary energy

supply by coupling an electric motor directly to the compressor shaft. %nother option is a

motor6generator using a heat engine for generating electricity when solar energy is

available and there is little or no cooling load. From state6of6the6art considerations, two

types of fluid heat engines are primarily feasible in solar cooling units. (n one type of

engine, the worAing fluid cyclically changes phase from li@uid to gas and bacA to li@uid.

he most widely used engine of this type operates on the


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Figure +6+

'ew construction office4 passive solar energy system

Figure +6+-

Lertical wall solar collector



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Figure +6++

#outh wall solar collector with combined storage



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Figure +6+3


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Figure +6+

#chematic of lithium bromide absorption cooler

the cooling e@uipment. #ince most compressors are designed for certain speed and

tor@ue inputs, the varying operation of a solar heat engine will probably reduce the

overall CP of the unit. %lso the solar heat engine is at high efficiency at high storage

tanA temperatures whereas the solar collectors are at low efficiency which will also

affect the CP of the system. hese systems are designed for large cooling load

applications.

2.#.3 DESICCANT COOLING. he


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cycle arrangements are feasible> the ventilation mode and the recirculation mode. (n the

ventilation mode, fresh air is continually introduced into the conditioned space. (n the

recirculation mode, exhaust air from the conditioned space is reconditioned and

returned to the space. Figure +6+5 illustrates a ventilation system in which a solid

desiccant material mounted on a slowly rotating wheel provides the basis for obtaining

a cooling effect.

Figure +6+5

#chematic of solar desiccant cooling

he hot desiccant material absorbs moisture from incoming ventilation air and increases

the dry6bulb temperature. his dry air stream is cooled in two steps. First, it is sensibly

cooled by heat exchange with the building exhaust air. hen it is evaporatively cooled

and partially rehumidified by contact with a water spray. he exhaust air from the

building is evaporatively cooled to improve the performance of the heat exchanger. %fter

being heated by heat exchange with the incoming air, the exhaust air is further heated

by energy from the solar system andBor from an auxiliary energy source. he hot

exhaust air passes through the desiccant material and desorbs moisture from it, thereby



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regenerating it for continuation of the process. /esiccant systems have faced problems

of high parasitic power and large space re@uirements relative to capacity. ;ecause of

their bulAiness, the systems may have primary application in the low capacity range

i.e., residential systems4 if and when ways can be found to reduce parasitic power

re@uirements to acceptable levels. he (nstitute of Gas echnology (G4 has been

investigating design modifications in a prototype 36ton system. %i


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storage unit. /uring the day, warm air from the building can be cooled by passing it

through the cool pebble bed. his method is not very effective in humid geographical

areas. he storage volume can also be cooled using a small refrigeration compressor.

0ost through6the6wall air conditioners use such compressors to cool the indoor air. his

unit acts as the bacAup or auxiliary cooling system 6 analogous to the bacAup heating

system. (f operated only at night, its capacity can be as small as half that of an

independently functioning unit and still meet peaA cooling demands. 'ighttime operation

will be particularly wise if electric companies charge more for electricity during times of

peaA loads on hot summer afternoons. %n even smaller compressor can be used if it

operates continuously night and day 6 cooling the storage when not needed by the

house.

2.#. ESTIMATING SYSTEM SI'E.he si:ing of cooling system components is

dependent on hardware, climate, and economic constraints. he cooling unit must be

si:ed so as to provide the maximum cooling load under conceivable adverse conditions

of high humidity and low or erratic solar insolation. he collection area re@uired is

dependent on the fraction of the cooling load to be provided by solar. Lery large

collector areas may be re@uired for - solar cooling under adverse conditions of high

humidity and low insolation. %lthough a detailed calculation method, as provided in theworAsheets in the following sections for heating systems, is not available for solar

cooling, an estimate of the re@uired collector area can be made by the e@uation>

% I ooling loadBCP4B (x SetaRcollect x SetaRdelivery4

where> ooling load I the portion of the total cooling load provided by

solar calculated using %#"

8/13/2019 An Introduction to Solar Collectors for Heating & Cooling of Buildings & Domestic Hot

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An Introduction to Solar Collectors for Heating & Cooling of Buildings & Domestic Hot Water Heating