Analysis of solar absorption cooling systems with low generator temperatures

Analysis of solar absorption cooling systems with low generator temperatures P. K u m a r * and S. D e v o t t a i "

Department of Chemical and Gas Engineering, University of Salford, Salford M5 4WT, U K Received 19 August 1985

Closed cycle absorption cooling systems have been analysed using water-lithium bromide and ammoniawater as working fluid pairs. An open cycle system using water-lithium bromide has also been analysed and compared with the closed cycle system. A flat plate collector has been considered for heat supply to the generator of the closed cycle systems. The effect of high flow ratios on the overall solar collector area required has been studied. It has been shown that the daily operating time of a cooling system can be increased by employing relatively high flow ratios. (Keywords: solar absorption cooling systems; low generator temperatures)

Analyse des systSmes frigorifiques solaires fi absorption avec gbnbrateur basse temperature Les auteurs ont analys~ des syst~mes frigorifiques ~ absorption ?tcycle ferm~ utilisant les couples de fluides actifs eau-bromure de lithium et ammoniac~eau. Un syst~me ~ cycle ouvert utilisant le couple eau-bromure de lithium a aussi ~t~ analys~ et compar~ au syst~me ~ cycle fermi. Un capteur plan a dt~ examin~ pour le chauffage du g~n~rateur des syst~mes ~ cycle fermi. Einfluence du rapport ~fun d~bit ~lev~ ~ la surface globale n~cessaire du capteur solaire a ~t~ ~tudi~e. On montre que le temps de fonctionnement journalier d'un syst~me fri#orifique peut ~tre augment~ en employant des d~bits relativement importants.

(Mots clrs: syst/~mefrigorifiquesolaire fi absorption; basse temprrature du generateur)

Nomenclature

A COP Cp

H I

Q T

Wp X

Area (m 2) Coefficient of performance Heat capacity (kJ k g - 1 K - 1) Enthalpy (kW k g - 1) Solar insolation (kW m - 2) Heat load (kW) Temperature (°C or K) Energy input by pump (kW kg-1) Weight fraction of absorbent LiBr or water

This study considers the use of a solar division, water-lithium bromide absorption system for air conditioning duties. The flow diagram of a closed cycle absorption cooling system using water-lithium bromide as a working fluid pair is shown in Figure 1. To take advantage of flat plate collectors the temperature in the generator should be reduced, and this can be achieved in two ways. The first is to operate with a high ratio of mass flow of absorbent to the refrigerant 1'2. The second way is to use an open cycle which is only feasible when water is used as the refrigerant. Some experimental investigations using open cycle solar absorption cooling systems for Present addresses: * Centre for Energy Studies, Indian Institute of Technology, New Delhi-110 016, India. "1"Chemical Engineering Division, National Chemical Laboratory, Pune-411 008, India 0140-7007/85/060356~4503.00 ~:~ Butterworth & Co (Publishers) Ltd and IIR

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Int. J. Refrig. 1985 Vol 8 November

Subscripts

AB AM CO EV EX GE IN SC 1-10

Absorber Ambient Condenser Evaporator Economizer heat exchanger Generator Inlet of solar collector Solar collector State points corresponding to Figure I

relatively dry and high temperature ambient conditions have been reported by Kakabayev and co-workers 3'4. Theoretical analyses by Prasad et al. and Collier have shown the feasibility of such systems for a wide range of conditions 5 7. Theoretical considerations

For the purpose of analysis a number of assumptions have been made: 1.0 kg h - 1 of refrigerant is vaporized in the evaporator; the streams at state points l, 3, 6, 8 and 9 are in equilibrium; the evaporating temperature, TEV, the ambient temperature, TAM, and the temperature at state point 3 are 4, 35 and 35°C, respectively. These conditions have been chosen for tropical conditions where the

Solar absorption cooling systems: P. Kumar and S. Devotta (from low temperature heatsource) QEV=/"

I v"'°r'l 1t

•

!tomed,um

QGE=(Hs--Hs)+FR(H6 -Hs)+R(Hs-H9)

I

2 Expansion ~ v a l v e

GAS 3 t

water the heat balance equation is:

(to mediumtemperature heatsink)~ QCO

J_JEconomizerl 6 " heat

exchan

,rl

t~ePtg ~n~e

~ " " Solution circulating pump Figure I Closed cycle absorption cooling system

Equation (14) is different from Equation (13) because the system operating with ammonia-water is provided with a distillation column. In the present analysis, the value of reflux ratio, R, for the distillation column is taken to be 0.2 (Reference 9). The performance of a flat plate solar collector is preducted by Equation (15):

~8

1

(14)

tO J GE

5 (,romb¢, temperature heat source)

~/sc= 0.77 - 7.0(TIN -- TAM)/I

(15)

where I is the solar insolation. The temperature, TIN,at the generator outlet or the solar, collector inlet is given by:

Figure 1 Syst~me frigorifique fi absorption ~ cycle ferm~ TIN = TGE + ATGE

cooling requirement is high. The condensing temperature, Tco, for the closed cycle systems is 35°C. The generator temperature, TGE,in all cases, is taken to be the temperature of a strong solution at the generator outlet. In closed cycle systems, TGEwill be very close to the bulk temperature in the generator since the liquid is boiling. It has been reported that in the case of the open cycle, the temperature across the collector-generator unit remains almost constant s . While using water-lithium bromide in the system, the energy required by the liquid pump is considered to be insignificant. Pressure drops and the heat losses in the systems are negligible. The coefficient of performance of a cooling system is defined as: COP = QEV/QGE

(16)

The temperature at the generator outlet, ATGE, is taken to be 5°C in the present analysis. The solar collector area required is calculated from: A = QGE(r/scl)

(17)

The equilibrium properties for water-lithium bromide

(1)

Other equations defining the heat and mass balances are as follows: ii

Flow ratio (FR)=(X 1 -Xa)/(X 3-X2)

(2)

QAB = H1 + H 7 - H a [ F R + 1]

(3)

r/EX = ( T6 -- T2)/(T6

(4)

-

-

T3)

Q~x = FR x Cp2tlEx(T6 -- 7"3) = (Pco - PEV)I/3

70

2

~E 6 0

(5) (6)

H 4 = n 3 + Wp

(7)

H s = H 4 + QEx(FR + 1)

(8)

H7 = H6 - QEX/FR

(9)

H9 - - H l o

(10)

HT=H2

(11)

QEv=Ht --Hlo

(12)

QGE=

(13)

40-5

I

I0

I

15

I

I

I

20 25 30 Flow ratio (dimensionless)

I

35

40

Figure 2 Generator temperature versus flow ratio for closed and open

(Hs

- H s ) + FR(H6 - H s )

In the case of the closed cycle system using ammonia-

cooling systems using water-lithium bromide. A, Closed cycle. Open cycle: B, 0.034 bar; C, 0.027 bar, D, 0.020 bar; E, 0.013 bar Figure 2 Temperature du g~n~rateur en fonction du d~bit pour des syst ~mes frigorifiques ferm~s et ouverts utilisant l'eau-bromure de lithium. A, cycle ferra~. Cycle ouvert : B, 0,034 bar; C, 0,027 bar; D, 0,020 bar; E, 0,013 bar

Rev. Int. Froid 1985 Vol 8 Novembre

357

Solar absorption cooling systems: P. Kumar and S. Devotta 0.90

air. The C O P values have been plotted with and without an economizer heat exchanger. The effectiveness of this heat exchanger is included. These plots show that the open cycle system is better than a closed cycle system. Figure 4 is a similar plot for the closed cycle system using ammonia-water. Both TGE and COP curves show the same trend. In this case the TGEcurves tend to become flat after a flow ratio value of ,~ 15.0 This plot shows that systems using water-lithium bromide give higher COP values than the system using ammonia-water. But the limitation of systems using water-lithium bromide is that the evaporator cannot be operated at subzero temperatures. In both cases, the inclusion of an economizer heat exchanger is advantageous.

A 0.85

0.80

i_ c

o 0.75

== ._E

--.. 0.70

g 0.55 g E ~Q. 0.60 .-~ 0.55

\ \

\ \

\

\

N \

N

~.,B

\ \\

x \\

\\

_

x

\ \ \

0.45 -

5

10

,

1.0 ~ "

~1o0

c

~.D\

\

,

1.2

\

0.50 -

o4o

o~120 i~110

-,

,

,

\

| 70

\\\

~

15 20 25 30 Flow ratio (dimen$ionlem)

60

I

10

,

,

35

40

Figure 3 Coefficientof performanceversus flow ratio for closed cycle and open cycle cooling systems using water-lithium bromide. ---~/EX=0.8; - - -, r/EX=0.0. Open cycle:A,0.013bar; B,0.020bar; C, 0.02"] bar; D, 0.034 bar. E, Closed cycle Figure 3 Coefficient de performance en fonction du ddbit pour les systimes frigorifiques d cycle fermk et d cycle ouvert utilisant reaubromure de lithium. - - - , ~IEX=0,8 ; - - - , flEX=0,0. Cycle ouvert : A, 0,013 bar; B, 0,020 bar; C, 0,027 bar; D, 0,034 bar. E, Cycle ferme

and ammonia-water systems are taken from previously published papers 1Lt 2

•-~ O 0.2~U

A I

I

I

!

I

I

I B

0.0 ~

20 30 40 Flow ratio (dim(msionles)

Figure 4 Generator temperature versus flow ratio for closed cycle cooling system using ammonia-water. - - , IGE; . . . . , COPA. A, ~/EX=0.8; B, r/EX=0.0 Figure 4 Temp&ature du generateur en fonction du debit pour le systeme frigorifique d cycle fermk utilisant I'ammoniac-eau. , IGE; COPA. ,4, r/EX =0,8; B, r/EX =0,0

0.95

~ 0.90

.=

-

~3 0.85

A

m

Results and discussion Figure 2 is a plot of generator temperature, TGE,against

flow ratio (FR) for the closed and open cycle systems using water-lithium bromide. This plot shows that increasing the flow ratio to ~ 30 leads to a significant reduction in the generator temperature, TGE. A higher flow ratio at a constant absorber concentration implies lower concentration in the generator and therefore lower generator temperature. It can also be seen from F i g u r e 2 that in an open cycle system, generator temperatures are considerably lower than in a closed cycle system and TGE is progressively reduced with increasing dryness of the ambient air. In the case of an open cycle system, lower generator temperature leads to higher efficiency of the solar collector. The coefficient of performance values for the systems using water-lithium bromide have been plotted against the flow ratio in Figure 3. The C O P values for closed cycle systems decrease as the flow ratio increases. Figure 3 further shows that COP values for open cycle systems decrease with increase in flow ratio except with relatively dry ambient air. For a given flow ratio, the C O P values progressively increase with increasing dryness of ambient

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Int. J. Refrig. 1 9 8 5 Vol 8 N o v e m b e r

~ 0.~ ~ 0.75

~ 0.70

0.65

I

I

0.013 0.020 0.027 0.034 Partial preuure of water vapour in ambient air, p (bar)

Figure 5 Coefficient of performance versus partial pressure of water vapour for an open cycle cooling system with heat exchangers of different efficiences and a flow ratio of 14. ~/EXvalues: A, 1.0; B,0.9; C,0.8; D,O.7; E, 0.6; F, 0.5; G, 0.0 Figure 5 Coefficient de performance en fonction de la pression partielle de la vapeur d'eau pour un systdme frigorifique d cycle ouvert avec des dchangeurs de chaleur de diffirents rendements et un d~bit de 14. r/Ex valeurs : ,4, 1,0; B, 0,9; C, 0,8; D, 0,7: E, 0,6; F, 0,5: G, 0,0

Solar absorption cooling systems." P. Kumar and S. Devotta doubles with the decrease in the generator temperature from 95 to 75°C. F i g u r e 6 is the plot of the solar collector required against flow ratio for closed cycle cooling systems and the same cooling duty, with various solar insolation levels, for water-lithium bromide. It can be seen that as the solar insolation decreases the maxima of the curves shift towards a higher value of flow ratio. The economizer heat exchanger in the system reduces the area requirement and makes the system independent of flow ratio for a range of values. Therefore, it can be concluded that a closed cycle system operating with a given solar collector area will work for a longer time when a relatively high flow ratio is used. In the present particular case if 0.6 k W is considered as the m i n i m u m solar insolation, the o p t i m u m flow ratio value would be ~ 25.0 for the system using water-lithium bromide. F o r the a m m o n i a - w a t e r system the corresponding value is ~ 10.0.

20.0 18.0 16.0

\

\

&-14.0

4'

12.0

1~ 10.0

"i

8.0

A

o

~_ 8.o

Acknowledgement

e..

it

4o -

. . . . .C -

- 7 -_

2.0

The authors wish to thank Professor F. A. Holland and the late M r F. A. W a t s o n for their valuable c o m m e n t s and discussion on this Paper.

.

B

C

References 0.0/ 5

I I0

I 15

I I I i 20 25 30 35 Flow ratio (dimensionless)

i 40

Figure 6 Flat plate solar collector area against flow ratio for a closed cyclecooling system using water-lithium bromide. , r/EX= 0.8; - - -, r/EX=0.0. I values (kW): A, 0.6; B, 0.8; C, 1.0 Figure 6 Surface de capteur solaire plan en fonction du d~bit pour le

1 2

syst~me frigor~que ~tcycle fermb utilisant r eau-bromure de lithium. flEX=0.8; - - - , r/EX=0,0. I valeurs (kW): A, 0,6; B, 0,8; C, 1,0

3

In the case of the closed cycle system using a m m o n i a water the power requirement of the solution p u m p at a flow ratio of 10.0, as given by Equation (6), is 0.0034 kW. It becomes an important constraint for higher flow ratios. In the case of water-lithium b r o m i d e this p o w e r requirement is negligible because of relatively low pressure differential across the generator and the absorber. Fi#ure 5 is a plot for open cycle systems operating with a flow ratio of 14.0 using water-lithiurri bromide. It can be seen that a high efficiency heat exchanger is required to make the performance of the open cycle system almost independent of the ambient humidity. It has also been found that b o t h absorber and generator heat loads are relatively independent of flow ratio for the open cycle system operating in a relatively dry environment. In the case of the closed cycle system the economizer heat exchanger reduces the absorber and generator heat loads more than in open cycle. F o r closed cycle systems using a m m o n i a - w a t e r , the economizer heat exchanger provides a large advantage. In the case of closed cycle systems with fiat plate solar collectors, the solar energy collection efficiency, ~/sc, almost

4 5 6

7 8 9 10 11

12

Kumar,P., Devotta, S., Holland, F. A. The effect of flow ratio in the performance of an experimental absorption cooling system, Chem Eng Res Des (in press) Dao, K., Simmons, M., Wolgust, R., WagUg, M. Performance of an air cooled NH3-H20 absorption airconditioner at lower generator temperatures, in Sharin# the Sun - Solar Technology in the Seventies Vol. 3 (Boer, K. W., Ed.), Pergamon Press, New Yor, USA (1976) 46-52 Kakabayev,A., Golaev, M. Glazed flat surface as a solution regenerator for use in absorption solar cooling system, Applied Solar Energy (1979) 7(4) 357-366 Baum, V., Kakabayev, A. The use of solar radiation on creating comfortable temperature conditions in SSR, Proc lnt Solar Energy Congress New Delhi, India (1978) 1556 Prasad, M., Kumar, P. Open solar regeneration of Licl brine, Much Eng Bulletin Central Mechanical Engineering Research Institute, Durgapur, West Bengal, India (1981) 12(2) 47-52 Prasad, M., Kumar, P., Raychaudhuri, B. C. Solar still regenerator for brine, Mech En# Bulletin Central Mechnaical Engineering Research Institute, Durgapur, West Bengal, India (1981) 12(4) 99-103 Collier,R. K. The analysis and simulation of an open cycle absorption refrigeration system, Solar Energy (1979) 23(4) 357-366 Mullick,S. C., Gapla, M. C. Solar desorption of absorbent solutions, Solar Energy 0974) 16(1) 19-24 Stoecker,W. F., Reed, L. D. Effect of operating temperatures on the coefficient of performance of aqua ammonia refrigerant systems ASHRAE Trans (1971) 77(I) 163-170 Beckmann,W. A., Klein, S. A., Duffle, J. Solar Heating Design by F Chart Method John Wiley & Sons, New York, USA (1977) 18 Bessler,W. P, Shen, C. M. Study of parameter variation for a solar powered LiBr absorption cooling system, Proceedings of a forum on solar heating and cooling convened by ERDA at the School of Continuing Studies, University of Miami, Florida, USA (1976) Jain, P. C., Gable, G. K. Equilibrium property data for aquaammonia mixture, ASHRAE Trans (1971) 77(1) 149-151

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