In be pr c~:clllL' < :1L the J11t c r11:1t101 1:.1 l So J;u Ene r gy Socie t y ~ 1cct i_ n g , C;1iro, I: gy pt , M:1rc h 198(1.

INTRODUCTION

LEAVE TH IS SPACE BLA f\;!:

SOLAR COOLING OF BUILDINGS WITH ,ABSORPTION AND DESICCANT SYSTEMS

PROGRESS AND PROSPECTS George O. G. Lof

Solar Energy Applications Laboratory Colorado State University

Fort Co I I Ins, Co I or ado, l:J. S. A. 80523

Although the cooling of buildings with solar energy has not reached general use, several types of systems have been developed and demonstrated. These systems are ;

I of two main classes, mechanical and thermal. tvlechanical systems provide cool Ing , by use of vapor compression refrigeration equipment similar or Identical to con- i ventlonal electrically driven air-conditioning machinery. Solar may be the primary energy source through the use of electricity or mechanical work generated i by some type of engine powered by steam or other vapor produced In a solar heated boiler, Another option Is the generation of the electrical requirements by use of , photovoltaic eel ls. It Is evident that the solar energy aspect of this type of 1 cool Ing Is baslcal ly the generation of power from solar energy.

I Thermal energy from the sun can be used more directly to provide cool _I ng by · several processes. A conventional air-conditioner or water chi 1 ler employing an I absorption refrigeration cycle can be adapted to a solar heat source rather than a

1

1

conventional fuel source. A few thousand such systems have been lnstal led in residential and commercial bull dings, but this appl !cation cannot yet be con-sidered fully commercial.

Another system which has been adapted to solar heat supply is a combination desic- : cant, heat exchange, and evaporative cool Ing process In which air ls dehumidified and evaporatively cooled, solar heat being used for regeneration of the desiccant medium. A few solar driven residential systems of this type have been sold.

Several variations of the absorption and desiccant processes have been experimen-tally Investigated. The absorption refrigeration system has been operated In an open-eye I e mode, I nvol v Ing reconcentrat I on of the absorbent so I ut I on by ev apora-t l on Into solar heated air or by direct exposure of the solution to solar radiation on a roof across wh ich th e absorbent solution Is flowing. Experi ments have also been conducted with I iquld desiccants reconcentrated by solar energy In a closed cycle (employing solar heated water In a closed loop) and in open cycles with solar heated air or direct solar radiation on a slop ing roof.

l\9SORPT ION REFRIGERATION

The first use of a complete solar cool Ing system for an entire building was at . Colorado State University (CSU) In 1974, when a I ithlum bromide absorption machine was lnstal led in a residential structure. Performance data on this system were obtained and improvements were made and tested. Solar cool Ing experiments in

[ ·- - - --

- --------·

~+ner - res-(dent I a, - bu I f(j I ngs a nd--J n a few commerc 1-al"buTTciTngs were-s -ubse-q-uent I y l ~onducted at CSU and In other solar devel opment centers.

In 1977, absorption cool Ing units designed expressly for operation with solar heat supply became available in the United States and Japan. These machines employed the I ithlum bran Ide cycle for production of chi I led water which was then employed for space cooling at rates of 3 to 10 kW (l to 3 refrigeration tons) In residen-tial lnstal latlons. They were designed to use a hot water energy source at about 80 C, obtainable with flat plate solar collectors of good qual Jty.

,Many hundreds of solar cooling systems of this type were lnstal led in residential buildings Jn the United States and Japan during the next five years. Many of these lnstal lat ions were partially or completely. subsidized under government solar development programs. A few larger Jnstal lat ions ranging In size up to 100 to 500 kW, some of which Invo lved the use of evacuated tube solar collectors, were also made, primarily Jn association with the solar heating and cooli ng demonstration program In the United States •

. On a few of the residential and commercial systems, performance monitoring was undertaken and operating results were compiled and published. In most Instances, system performance was l<;>wer than expected. Overal I performance factor, that is cool Ing output per unit of solar radiation, has been low, rarely over 10 percent. Operating problems have been frequently encountered.

' In nearly all solar cooling lnstallatlons, auxll lary cooling Is provided In some form, usually as a supplementary heat supply for driving the chi Iler when neither direct solar heat nor stored solar heat is available. In order for solar cool Ing to cont I nue Into the even Ing hours, energy storage must be prov I ded. Storage of chi I led water Jn smal I systems (3 tons or less) has been found less effective than hot water storage, particularly in systems that are designed for solar heating as wel I as solar cool Ing. Heat storage in water at temperatures above 100 degrees requires a tank suitable for moderate Internal pressure, In the range of two at-mospheres. If the system Is al so used for heating In a freezing cl !mate, the collector must be dralnable (ff water Is used), or a non-freezing I iquld (such as an aqueous glycol solution) must be used, with a suitable exchanger for transfer of heat to storage.

The temperatures required for the heat supply to the I ithium bromide absorption cool Ing cycle are low enough to accommodate the use of flat plate solar collectors. Temperatures of 70 to 90 degrees are adequate, with 80 degrees being typical. At that supply temperature, a flat plate collector of good quality can operate at summer efficiencies of about 40 percent, and the chi Iler at a coeffi-cient of performance (COP) of about 0.6. The overal I ratio of cool Ing del lvered per unit of solar radiation (the solar cool Ing performance factor [SCPF]> can therefore be, under Ideal conditions , about 0.24. Thus, 4 square meters of solar

I co I I ector s can theoret I ca I I y prov I de coo I Ing at a rate of about 1 kW at noon on a sunny day. On a clear summer day, about 1.5 kW hours of cool Ing can theoretically be del Jvered per square meter of solar col I ector.

In practice, however, the! output of sola r absorption cool Ing systems has usually been considerably lower than the Ideal Indicated above. Seasonal solar collection efficiencies have rarely averaged more than 30 percent, and the typical COP of chll lers being operated with solar heat at variable supply temperatures has been about 0.5. The resulting SCPF has seldom exceeded 0.15. On a seasonal basis, Intermittent cloudy weather and system faults have more commonly I lmlted the SCPF

,ratio to 0.10. Accordingly, the dally average del Ivery of 100 kW hours of cool Ing \ C~pproxlmately 30 ton hours) has typlcally required collector areas of about 140 im. Operation Jn the evening hours and during cloudy periods requires either the

~a-of-storage tanks for so I ar heated water or aux 11 I ary ener gy sources--torcoo 1.:1 11ng during those periods.

Improved performance and reduced col I ector areas have been achieved by use of evacuated tube solar col I ectors rather than the f I at pl ate type. Higher col I ec-t I on efficiencies, particularly at lower solar Intensity levels, and less dependence of collector eff l~lency on operating temperatures are distinct ad-vantages for absorption cool Ing operation. Under favorable conditions, col lectlon efficiencies above 50 percent are possible. Average seasonal eff iclencies of 40

,Percent have been obtained, and with Improved chll ler design and control, average COP levels of 0.7 can be achieved. Solar cool Ing performance factors of nearly 0.30 should therefore bezobtalnable In favorable circumstances. Collector area requirements of about 50 m per 100 kW hours of ·cool Ing can thus be expected in the future.

The absorption chi! ler Itself may also be subject to addltlonal development which cou Id I ead to Improved pert ormance. Such changes are beyond the scope of this paper, but steps to Improve heat transfer rates In the chlller components, In-creased heat transfer areas, and control refinements can lead to reductions In supply temperature requirements. Solar collector operation can then be at lower temperature and higher col lectlon eff iclency.

I The development of solar absorption cooling has fol lowed a path of coup I Ing solar collectors to cooling machines designed for conventional energy supply. The cool-ing machines have then been mod If led to .accommodate the different conditions

; Imposed. The results have not been fully successful, and poor performance has frequently been the result of a mismatch of these major components. Highly vari-able and fluctuating heat supply temperatures force the chi Iler to operate at off-design conditions most of the time. Decreased capacity and efficiency result. Inadequate control and large thermal losses, particularly fran hot water storage

1tanks, are chronic sources of degraded system performance which need to be lm-·proved by development of better designs and operating methods. I 1There have been several modifications In conventional absorption cool Ing machines :to make them more comp at Ible w I th so I ar operation. The first . changes In small machines, up to 10 kW capacity, Involved generator redesign to permit heat supply In the form of hot water rather than as steam or fuel. In a U.S. product, a mechanical pump was substituted for the bubble pump for recirculating absorbent solution, and chi! led water replaced cooled air as the del lvered output of the system. These and other changes permitted reduced temperatures of the heating source. In Japan two models are produced, one for fuel operation, the other for

.solar operation.

Another development, not :~dlrectly solar related, was the elimination of the water cool Ing tower by combining Its function with the chlller Itself. The absorber and condenser are evaporatively cooled by a flow of water and a stream of forced air In direct contact with the heat transfer surfaces. A second technique for el lmlnatlng the troublesome cool Ing tower was developed by use of an air-cooled chiller In which the absorber and condens er are cooled by atmospheric air. In this machine, heat supp Ii temperatures are necessarily higher , Ideally about 90 C. Achieving that temperature at acc~ptable collector eff iclencles requires the use of evacuated tube collectors.

Solar cool Ing systems of large capacity, 100 kW and upwards, have also been demonstrated in commercial lnstal latlons. Conventional I lthium branlde absorption systems with only minor modlf icatlons have usually been operated w Ith evacuated tube collectors. As with smal !er systems, performance has often been considerably

1below design objectives. Operating problems with the collectors have frequently

1 been responsible for low cooling del Ivery. In a few Instances satisfactory opera-tion has been routinely achieved, but at high system cost. 1

An example of a large solar absorption cooling system may be found2at the Solar Energy Research Centre Jn Baghdad. A multi-story building of 6,361 m floor area Is provided with two 60-ton (210 kW each) I lthium branlde absorption chillers and an auxll lary system of two 40-ton heat pumps. Solar heated water Is supplied by 1 ,557 evacuated he~t pipe collector panels (fluorocarbon fluid) wit~ a total ac-t Ive area of 2, 076 m • Ma In therma I storage Is prov I ded by two 150 m water tanks Jn which chilled . water is stored at a design temperature of 8 C and heated water Jn winter at 45 C. Heat Is transferred from the collector circuit to water In two accumulator-heat exchanger units for winter heating or to absorption chi I lers for summer use. Heat pumps are used In al I seasons when there Is not enough solar to meet requirements. Auxll Jary boilers provide additional heat when necessary.

Another large solar lnstal latlon Jn the Middle East ls used for cool Ing 293 luxury apartments l~ Baghdad. Energy Is provided to 1 lthium branlde absorption chll lers from 10,000 m of flat plate solar collector.s operating in a drain-down mode to prevent bol I Ing and freez Ing. It has been reported that about half of the cooling requirements have been met by solar, the balance by auxi I lary heat, and that electrlcal use has been only 10 percent of conventional vapor compression requ I rements.

In a 1983 review, Lot and Karaki reported the performance of about 25 solar ab-sorption cool Ing systems and 2 compression systems driven by solar powered Rankine engines. Figure 1 summarizes the results of monitoring those systems for periods

, of a few months to more than a ful I year. The design capacities cover a range from 2 tons (7 kW) to 174 tons.

It Is observed that about half of the systems achieved a SCPF of about O. 1, com-pared with possible maximums (based on average measured COP and collector

!' efficiency) typically at the 0.15 level. In well-designed and supervised instal-

l at Ions (at CSU and Los Alamos), performance was usually higher than In others, thereby Indicating that the outputs of most systems could be Improved.

To JI lustrate the performance characteristics of solar absorption cool Ing, operat-ing results from well Instrumented and closely monitored residential systems at

°' 0 t; 0 .3 ~

Cl z 0 . 1

§

0

:c "' :;)

"' u

SOLAR COOLING PERFORMANCE FACTOR• INCIO~~~~~ATION lol EVACUATED TWE COLLECTOR ANO AIR COOLED CHILLER lb l DIRECT EXPANSION AIR CONDITIONER l cl RANKINE CYCU

LEGEND c:::::J MAX POSSIBLE ~ACHIEVED

11ZZZZZi1 ESTIMATED

Fig. Maximum possible and measured solar cool Ing perfor-mance factor.

11-he Solar Energy Appl lcatJons Laboratory of Colorado State University are ' presented. Two systems In adjacent i dent I ca J houses are compared. One Involves a fl at plate col tector supplying heat to an experimental evaporatJveJy cooled 3-ton J lthium bromide absorption chJJ Jer, and the other Is an air cooled 3-ton 1 lthium bromide absorption chlJ fer to which heat Is suppl led from a pressurized water storage tank and an evacuated tube col Jector. Table 1 contains the essential characteristics of these systems and the bull dings in which they are used. Schematic diagrams are shown in Figures 2 and 3.

TABLE 1 Specifications of two solar cooling systems at Colorado State University.

House I House 111

Building Floor Area 260 m2 260 m2

Solar Collector Type Heat Pipe Double Glazed Evacuated Tube Flat Plate

Gross Collector Area 66 m2 49 m2

Collector Slope 45 degrees 45 degrees

Heat Transfer Fluld ·Water Water

Collector Flow Rate 36 I lters/mln 45 I lters/mln

Heat Storage 3600 I lters 4000 I lters Pressurized Water Water

Storage Stratification Vertical Vertical Diffusers Di ffusers

Absorption Chi Iler Type Air-Cooled Evaporatively !Carrier) Coo led (Arklal

Nomlnal Chiller Capacity 3 tons 3 tons

Generator Supply Temperature 80.2-102.3 C 74.4-90.0 C

Average Generator Supply T 84.9 C 84 C

Solar Water Heater Co 11 In None DHW Tank

4e4or WATER THERMAL

ITOllAOE

( ___________ :. 1'11 IS l.lXlfl P£MITTS CJ:)l(T1'0L Of ~ WATUI TtMl'£RATURE nl EVAPORATOR (SPECIAL TUT!HG °'4LY )

..:>OM AIR DELIVERY

t D

Fig. 2 Solar absorption cool Ing system, CSU Solar House I JI.

L' --------------· -··-----

I

I

PIT VALV{

PllESSU,.IZED WATER TANK

l'HA!l£ - C>IAHG[ COLO STORA()[

Fig. 3 Solar absorption cooling system, CSU Solar House I.

The average monthly eff lclency of the flat plate col iectors and average storage and ambient temperatures are shONn In Figure 4. Performance of the entire system w Ith the evaporatlvely cooled chi Iler is shONn In Figure 5. The product of col-lect Ion eff lclency and the coefficient of performance Is the SCPF shONn in Figure 5 as a seasonal average of 0.10. During an 8-day period in July, the average SCPF was 0.17. On the best operating days t h is factor was as h igh as 0.20, but the averages ore reduced by various and fluctuating operating factors.

It should be pointed out that the solar installations at CSU are for combined heating and cool Ing purposes. Collector slope Is, therefore, selected for maximum yearly energy col iectlon and utll lzatlon rather than for ma x imum cool Ing capab i I lty.

lj a ~ ,.. ... = ~ ! i w c c ,.. c E ~ i

22 1111

20 .. ., .. 18

II

14

12

10

I

• 4

t o,__..,,..,....._.~~~~~~~

DAYS

i 'TOTAL IOLA,. ,.ADIAT l()Oj

UOIATK* -ILE COLLECTING

ENCllOY COLLECTED

Fig. 4 Flat plate col lectlon efficiency, CSU Solar House I II.

1200

,.. ~ ~ 1000

i! ~~ 800

"'"' ~ ffi ~ :?

..J 800

:n: =~ ~. 400 c

FI g. 5

ilOTAC .,..,.,.""""TIOOI (lrrCWQ T CD_L[CT'[O

t<AT TO ICH loof(.AT "°'°V?:D «T ~

'""" QX)UNG

Evaporatively-cooled chi Iler performance, CSU Solar House I 11.

L _______ _

I

An alternative to a water-cooled absorption chi I fer operated by solar heat from a ' flat plate collector Is an air-cooled absorption chiller supplied with heat at higher temperature from evacuated tube collectors. A schematic diagram of such a system Is shown In Figure 3, and the results of Its operation In a solar cooling system at Colorado State University are shown In Figures 6, 7, and· 8. The el imlnatlon of the water cool Ing tower Is a cost advantage and also a convenience, because smal I cooling towers (discharging up to 25 kW) involve conslderabl e cap I- ; tal and maintenance costs, and the use of slgnif lcant quantities of water. Higher

1 condenser and absorber temperatures require an Increase In generator temperatures from 7 5-85 degrees to 85-95 degrees. It Is seen In FI gure 6 that even at this :

I elevated temperature, efficiencies of the evacuated tube collectors are above 40 percent.

Figure 7 shows an average dally solar cooling supply of about 18 ton-hours during the ful I summer season. A smal I portion was obtained by use of a eutectic salt cold storage un It, but I ts energy transfer rates were too I ow for pract I ca I use. ' The average seasonal thermal COP was 0.79. Chi I led water was del lvered at an average temperature of 10.7 C.

100 "' ~87. 6 86. 5 85.3

~; 80 ~TT-TT

;; '.: 01111

i

.. 0

22 i Total ln1olallon

Solar while Collecting

Energy Collected

i 20 111.8 ..... ~ 18 c 0

t .! 0 u ~ c 0 .. l ., .. ~ • c

11.J .. 0 0 en • .,. ~ • .. C(

Fig. 6

JUL AUG SEP AVG

Monthly average energy col lec-tlon, CSU Solar House I. Heat pipe evacuated tube collectors.

300

{ ., :IE '1:1 Cl 0 _, p ~ 0

100 0 u

FI g. 7

( 18.4) Ton-Hours/cloy (221)

~:::::. 267 Supplied

Direct Cooling .

Solar space cool Ing load, CSU Solar House I.

118,!114 MJ Total ln1olollon Ourlno 84 doy1 Ourino Cool ino S101on

PHILIPS VTR- 361

4!1 Yo °"tocol and TMrmol Coll1ctor

Lo1111

!\% Tron1port L.01111 ond Hoot Rejection

'"" 41"'

Chiller

Fig. 8 Energy flow for cool fng season, CSU Solar House I.

The energy flow pattern over the ful I cool fng season Is depicted In Figure 8. Al I of the hot water requirements were met by solar heat representlng 4 p8rcent of the total lnsolatlon, and the heat supply tc the generator of the chll ler was 41 per-cent of Incident solar radiation. The average solar cooling performance factor was 0.22. Under favorable conditions, the SCPF ratio occasionally reached 0.3.

The major significance of the results obtained In the two experimental programs described above Is the evidence that the performance of solar absorption cool fng systems can be substantially higher than usually obtained. Flat plate collectors or evacuated tube collectors can be advantageously used, depending on the tempera-ture requirements of the chfller cycle. Systems designed for combined heating, hot water, and cool fng supply have economic advantages stemming from nearly com-plete utll lzatlon of their solar col lectfon and storage capabll lty.

In a cooperative project between the U.S. and Saudi Arabia, four commercial size solar cool Ing systems were constructed and tested fn Phoenix, Arizona, and four other active solar cool fng systems are planned for fnvestfgatfon fn unfversfty laboratories In Saudi Arabia. Table 2 shows the principal features of the systems that were evaluated In the U.S. Two vapor compression systems operated by solar Rankine engines and two 1 ithfum bromide absorption systems were used. Three of the solar col I ectors were of the parabol re trough focusing type, and one was an evacuated tube system. Although al I four systems provided solar cool Ing, none of . them operated at ful I design capacity or eff lcfency, and numerous design and operating problems were encountered. Afr leakage fnto the vacuum chambers of the absorption systems was a recurring problem, Imperfect matching of collector and chll ler capacftfes was observed, and collector performcnce degradation occurred. It was concluded, however, that most of the problems could be corrected, but that considerable additional development of these systems would be needed. The ex-perimental nature of these projects precluded rel icble assessment of cost ~ffectlveness, but It Is clear that under present economic conditions, none of the

L __ - ·- . -, ------ ---- - TABLE 2 Characteristics of four solar cooling systems In

Soleras testing program, Phoenix.

I '

Cycle Type

Cycle Fluld

Cool r ng Rat Ing

Heat DI scharge

Col I ector Type

Col I ector Area

Typical Work Output Solar Radiation

Typical SCPF*

*Ratio Cool Ing Del lyered Solar Radiation

Rank l ne

R-11

63 kW 18 tons

Air Cooled

Parabol le Trough

122 m2

.13

Rankine

R-113

49 kW 14 tons

Arr Cooled

Evacuated Tube

219 ~2

.025

Absorpt Ion Absorption

L lBr-H20 L 1Br-H2o 53 kW 35 kW

15 tons 10 tons

Water Water Cooled Cooled

Parabol le Parabol le Trough Trough

134 m 2 89 m 2

.18 .09

systems Is competitive with conventional types operated with fuel or central sta-tion electricity.

The principal features of the active solar cool Ing projects planned in Saudi Arabia are shown In Table 3. Solar collectors Include parabol lc troughs and evacuated tubes. Heat fran the collectors wll I be used to produce motive power In Rankine engines employing a fluorocarbon In one boiler, steam In another, and to drive I ithium bromide absorption chll lers In the single cycle and dual cycles. Electricity produced In a photovoltaic generator Is also planned for test opera-tion of a thermoelectric type of refrigerating unit.

TABLE 3 Proposed solar cooling systems In Saudi Arabia-Soleras Program.

Cycle Type Rank I ne Absorpt Ion Rankine

Cycle Fluld R-113 L IBr-H 0 CDual Cy~lel

Steam

Cool Ing Rat l ng 26.3 kW 1-3 ton 77 kW 7 .5 ton 2-10 ton 22 ton

Heat Discharge Afr Water Air Cooled Cooled Cooled

Afr Cooled

Col I ector Type Parabol le Evacuated Parabol le Trough and Tube Trough Evacuated

Tube

Collector Area 333 m2 400 m2

Notes: Gas Ff red sr ngl e Cycle Fuel-Bor I er to and Dual Cycle Assisted

Increase Both to be cor 1 > FI u Id Temp. Tested

r=:- --JSOLAR DESICCANT COOL ING

/systems for dehumidifying air by use of sol Id and I Jquid desiccant materials regenerated by heat are commercially aval I able. Moisture removed from air being dehumldlf led Is subsequently stripped from the desiccant material by use of hot air, hot water, or steam, produced by combustion of fuel. Desiccants ~hich can be 1regenerated at temperatures as low as 60 Care candidates for solar operation by substitution of flat plate solar collectors for the conventional source of heat.

If used for comfort air-conditioning, the dehumldif lcatlon or desiccation process must be associated with another process for air temperature reduction. Not only Is It necessary to cool the air below Its original temperature, but heat which results from moisture absorption must also be removed. One method for reducing the temperature of the dried air Is cooling by means of a vapor compression or an absorption type of refrigeration machine. Another method is direct or Indirect evaporative cooling of the air. Although the desiccant process Is separate from the subsequent cool i ng step, the two may be c I ose I y interconnected and mutua I I y

0

dependent, particularly If the final step Is evaporative cool Ing.

IThe relationship of the desiccant process to a solar heat supply results from the /requirement for regeneration of the desiccant material after rt has absorbed or .adsorbed moisture from the air being dried. By heating the desiccant materlal to temperatures of 50 to 100 C, part of the moisture rt contains rs evaporated Into the atmosphere so that the material can then be reused. The temperature to wh ich the desiccant must be heated depends on Its properties, particularly its equi-1 lbrlum moisture content at various temperatures.

Several sol id compounds may be used for dehumidifying air, sf I lea gel, activated alumina, molecular sieve, and the chloride salts of I lthlum and calcium.

Figure 9 shows the relationship between air relative humidity and equil lbrlum moisture content In typical desiccant materials. At a given percent relative humidity, the equil Jbr!um moisture content of the desiccant Is not appreciably I 4

... ... .... ... ... 1-z

0.4

~ o.z z 0 u a: w

i

LIQUIDS

Lithium Chloride

z

20 40 60 80 ()0 RELATIVE HUMIDITY

Fig. 9 Equll lbrlum Isotherms for sol id and I iquld desiccants.

dependent on temperature. It Is evident that at low re latlve humidity, sll lea gel has a low equll lbrlum moisture content and that It has high moisture at high humidity. In cycl Ing between these conditions, the mater ial can therefore pick up moisture and subsequently discharge It. The path of humidity and temperature fol-lowed by the air being dried Is shown on a psychranetrlc chart from point ( 1) to point (2) (Figure 10).

To provide comfort cooling, dehumidified air may be partially cooled by heat ex-change w Ith air returning fran the rooms of the building and then further cooled by spray evaporation for supply to building areas. After passing through the heat

0 0.0 10 ;::

<( a: >-... Ci

l-'7""c_;hr"'°-1--7""i~--l---+-"7""~~--l-::7'"C-.t----i0 . 005 ~

~:f;::d:~±::=::d::::±:::=t=:I._l__Jo 15 20 25 30 35 40 45 50 55 60

DRY BULB TEMPERATURE (°C)

CD - ® DESICCANT DRYING @ - @ SENSIBLE COOLING

Fig. 10 De-slccant cooling process.

:i::

exchanger, the return air Is heated by solar or au x iliary energy and passed through the desiccant unit to vaporize water absorbed fran the Incoming air. The desiccant unit may be a rotating drum in which a sol id desiccant is In the form of granules or as a coating on some type of Inert mesh or packing. The drum slowly revolves between the two air streams, one side of the drum being regenerated with hot air at the same time the other half of the drum Is absorbing moisture fran the Incoming air. A I iquid or seml-1 lquid medium may also be used In this conf lguratlon.

A schematic diagram of one of the practical systems for this type of solar cooling Is shown In Figure 11, with the corresponding conditions at each point depicted on the accompanying psychrometrlc chart. Figure 12 depicts a desiccant cool Ing machine now being sold In the United States, for either solar or fuel operation.

In the ventilation cycle, Figure 11, outdoor air at condition (1) Is first dehumldlf led In the desiccant whee l, a process which al so r e sults In an Increase In air temperature. Hot dry air at (2) Is then cooled by heat exchange In a rotary type regenerator from which It leaves at (3). Air te mperature Is further reduced by evaporative cooling to condition (4) at wh ich It enters the rooms of the bulldlng. The tempetature and humidity of the air Increase In the rooms, and this air Is suppl led to the regeneration cycle at condition (5).

Air for regenerating the desiccant Is first cooled by evaporative cooling to con-dition (6) so that It can be used In the rotary heat exchanger to cool air for the building. After Its temperature rises In the heat exchanger to condition (7), the air Is further heated by solar energy or by fuel or by a combination of the two sources, to condition (8). This hot air then passes through the moisture-laden desiccant, evaporates most of the absorbed water, and is discharged to the atmos-phere at (9).

,.

"

ROOM T{M•[AATVA( • CX)MFO~ABU:

MUMIOrTY CS) COMFOAT&IL[

WATER

COLO

MOIS T

EXHAUST fX 4 WA~W I

• I AMB IENT

TEMPERATURE

~· HEAT E XHA UST

0£HUMl01F IE~ WARM W( T

HOT

FI g. 11 Vent i I at I on cycle desiccant cool i ng system, schematic.

SOLAR -HEATI NG COIL

Fig. 12 Solar heating and sol id desiccant cooling system, schematic.

Another type of unit comprises two stationary desiccant beds, through one of which Incoming air passes as exhaust air passes through the ot her for regeneration. Dampers reverse the functions of t he two beds Intermittently by switching air flows every few minutes.

The dehumldlf lcatlon process results In simultaneous heating of the supply air as moisture Is removed, the heat of co~densatlon and adsorption of water vapor being

I !berated. In the process previously described, the elevated temperature of the resultlng air Is reduced by heat exchange and evaporative cooling In separate units. Another strategy Involves cooling of the desiccant bed Itself, thereby reducing or el iminatlng the rise In temperature of the air passing through it. Cool Ing can be accomp I i shed by use of water co 11 s or a separate a Ir stream passing through channels around which the desiccant material is packed and through which a rr Is being passed for dehumidlf lcatlon.

Performance studies on the commercial desiccant unit now avai I able in the United States have recently been Initiated at Colorado State University. Results of preliminary tests show that air can be cooled and dehumidified at conditions sultable for residentlal use, with heat supply at 50 to 75 C. The range of operating conditions for about 90 percent of the tests are:

Inlet air temperature 18 c - 29 c Inlet air humidity ratio .004 - • 011

Temperature of air supply to rooms 10 c - 12 c Humidity ratio of room air supply • 008 - • 011

Temperature In I Iv Ing space 22 c - 24 c Humidity ratio In I iv Ing space .008 - • 011

Hot water supply temperature 50 c - 75 c Effective cooling rate 6 kW - 10 kW Effective thermal COP o. 7 - 1.0 Effective electrlcal COP 5 - 8

In one run, for example, ambient air was cooled fran 26 C to a room supply tem-perature of 11 C, by use of hot water at 75 C. Cooling at a rate of 9 kW was provided at a thermal COP of 0.8 and electric COP of 7.8.

Further development and optimization of this solar desiccant cool Ing system are proceeding.

Fundamental research on desiccant dehumidification and cooling Is also being carried out at the U.S. Solar Energy Research Institute and elsewhere. The design and operation of rotary desiccant beds employing sil lea gel particles supported on a splrally wound web or film are In progress, analytlcal studies are being made, and Internally cooled stationary beds employing supported sf I lea gel particles are being developed.

The practical prospects for this technology, In combination with solar heat supply in w Inter, are promising. Temperature requirements are wel I within the capabil !ties of good flat plate solar collectors, and the cost of the desiccant-cool Ing equipment appears reasonable. Further evaluation of the I imits of appl lcabll lty, the cl lmatlc zones and conditions suitable for use, and practical size I Imitations are needed for future developments.

OPEN-CYCLE ABSORPTION REFRIGERATION

Several experimental studies of absorption cooling Involving solar regeneration of the absorbent In an open evaporator are being carried out. The conventional I lthium bromide cycle Is modified by el lminatlng the condensor and substituting a small fresh water supply, and by using some type of external evaporation unit rather than the closed cycle generator to reconcentrate the absorbent solution. In one study, based on a similar cycle demonstrated In the USSR, a I ithium chloride solution Is pumped out of the absorber and reconcentrated by trick! Ing It across a sloping roof exposed directly to the sun. In another Investigation, solutions of I ithlum chloride or I ithium broolde are reconcentrated In a short

packed bed by means of hot air from a solar air collector. A schematic diagram of the latter system ls shown in Figure 13.

HEAT EXCHANGER

EVAPORATOR

FROM ROOMS

Fig. 13 Open-cycle cooling system using absorption chll ler.

In both systems, the absorber-evaporator operates at the typical low pressure of a conventional I ithium bromide absorption cycle, 7-10 mm Hg. Chilled water ls produced at temperatures of 8-12 C. As in the closed cycle, the cool Ing output of the machine ls essentially equal to the heat required to vaporize water in the evaporator, about two-thirds of a kilowatt-hour per kilogram of water evaporated. Similarly, water vaporization in the reconcentrator has an equivalent refrigera-tion effect, so one ton of cooling capacity (3.5 kW) requires the vaporization of approximately 5 kl lograms of water per hour in the reconcentrator and an equal addition of fresh water In the evaporator.

Recent experiments on the open cycle absorption cooling system at CSU have in-volved aqueous I ithium bromide s'21utions and a short packed bed to which solar h3ated air ls supplied. A 58 m collector supplies hot air to a bed in which 0.2 m of 2 cm hollow plastic shapes are packed and into which the bromide solution ls pumped from the absorber. Ch 11 I ed water Is produced In an evaporator-absorber unit which had previously been part of a conventional closed cycle absorption chll fer. Selected operating results are shown in Table 4.

TABLE 4 Performance data on open-cycle solar absorption coo I i ng system.

Absorber-Evaporator

Ch II led Water Del Ivery Temp, C Cooling Water Inlet Temp, C Pressure, mm Hg Solution Concentration,% Cool Ing Rate, kW

Packed Bed

Hot Air Entering Temp, C Air Flow Rate, kg/sec Air Inlet humidity, g/kg Solution temperature to bed, C Solution flow rate, kg/sec Inlet solution concentration,% Equivalent bed cooling rate, kW

System Ratios

Cool Ing output/regenerator heat Input Cool Ing output/heat to air supply Cool Ing output/solar radiation CSCPF>

9.9 29.3

8.3 59. 1 5.2

75.7 0.66 3.5

52.8 o. 73

57.6 6.33

0.46 0.13 0.07

10.0 29.6 8.2

59.6 5.7

75.2 0.66 3.5

51.1 0. 73

58.1 7.63

0.46 0.15 0.08

9.9 29.5 8.2

59.9 6.6

74.4 0.66 3.5

51.8 0.73

58.4 7.88

0.51 0.18 0.09

I '

These results and those Involving reconcentratlon of 1 lthlum hal Ide sol utlons by direct exposure to solar radiation on a sloping roof show that the open cycle ab-sorption process can be used to produce chi I led wat er for the cool Ing of buildings. Very prel imlnary findings Indicate performance levels approaching those In closed cycle systems. Elimination of the condenser and the conventlonal generator, and a reduction in cooling tower capacity requirements, are potential benefits, but non-condensables and other atmospheric contaminants entering the evaporator may Impose difficult design and operating problems. Further research and development are needed before the prospects for the open cycle systems can be appra I sed.

CONQUS IONS

An exhaustive analysis of the numerous applications and research and development activities In solar cooling Is much beyond the scope of this paper. Brief descriptions of the methods receiving most development activity are given, and some examples of the more promising systems have been presented.

Omitted from this review Is discussion of cool Ing by conventional vapor compres-sion machines driven by solar operated heat engines. Although some experimental systems of this type have shown satisfactory performance, their practical use would be fully dependent on the technology and economics of the solar engine and the product I on of so I ar generated e I ectr I city or mechan i ca I work. The cool Ing machine Itself can be a fully commercial type, with I ittle or no modifications. A discussion of the technology and experience in solar power production Is, hOl'l'ever, an extensive subject Itself, considerably beyond the I imitations of this review. It Is cl ear that the electricity or machanical power generated In a solar engine, If economical, can be used to meet any electrical requirement, lncludlng cooling.

The most w ldely used solar cooling systems Involve single effect I ithium bromide absorption chillers and either flat plate or evacuated tube collectors in small systems (up to 10 kW), and evacuated tubes or parabol lc troughs In large systems (up to 100 kW). Their performance has generally been less than fully satisfac-tory, and Improved designs and operating procedures are needed. Varlabil ity In heat supply rate and temperature appears to be a chronic disadvantage which needs to be dealt with. Although high qual lty components, such as the chillers and the col fectors, are available, combination of these components Into effective systems and adequate control of the operation of those systems are the usual problems with securing good performance.

It Is not yet clear whether open cycle absorption may have an economic advantage over closed cycle absorption, but prospects for reduced hardware requirements are Interest Ing.

Use of solar heat to regenerate desiccant materials In a combination desiccant-evaporative cool Ing process, now In very I lmited practical use In the U.S., may become Important. Moderate temperature requirements permit the use of flat plate collectors, the technology Is simple, convenient combination with solar heat sup-ply and solar hot water Is possible, collector area requirements are as low or I ow er than other solar cooling systems, and the cost of the cool Ing unit appears to be considerably lower than that of an absorption type. It Is evident that the potential of this process, In Its several forms and with various materials, should be fully Investigated.