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Thermodynamic feasibility of double effect generation absorption system using water-salt and alcohol-salt mixtures as working fluids
Heat Recovery Systems Vol. 5, No. 1, pp. 19-26, 1985.
0198-7593/85 $3.00 + 0.00 Pergamon Press Ltd
Printed in Great Britain.
THERMODYNAMIC FEASIBILITY OF DOUBLE EFFECT GENERATION ABSORPTION SYSTEM USING WATER-SALT AND ALCOHOL-SALT MIXTURES AS WORKING FLUIDS S. C. KAUSmK Centre of Energy Studies, I.I.T., New Delhi l l0016, India S. CHANDRA Mechanical Engng Dept, Melbourne University, Melbourne, Australia
and S. M. B. GADHI Mechanical Engng Dept, I.I.T. New Delhi, India Abstr~t--This communication presents a comparative thermodynamic assessment of a double effect generation absorption cooling cycle using water-LiBr and methanol-LiBr.ZnBr2as working fluids. The proposed systemconsists of a second effectgenerator between the first effectgenerator and the condenser along with two heat recuperatorsbetweenthesegeneratorsand the absorber. A numericalcomputermodel based on the simultaneous solution of heat, mass and material balance equations for various components of the system has been carried out. The influenceof component temperature on the cooling capacity and COP of the system has been investigated to obtain the optimum operating conditions. It is seen that for both the combinations the system has optimum generator temperatures for which COWs are maximum.The COP valuesof double effectabsorption cyclesapproaches twiceto that of single effectones for both the working fluids. It is further seen that a water-LiBr combination has more cooling capacity than that of methanol-salt mixture; however, the latter may have the advantage of working at lower evaporator temperatures.
INTRODUCTION Cold storage of food products is an important application of refrigeration. An increase in the number and capacity of cold storage systems is liable to make a significant contribution to the alleviation of food storage problems by arresting the spoilage. Another application of the refrigeration cycle is the space-conditioning o f buildings which leads to the improvement of human comfort, increase in industrial production and the feasibility o f establishing high technology industries. Both these applications are o f great relevance to food and energy problems. The conventional space-conditioning devices need electrical energy and/or fossil fuels for their operation. With the advent of the energy crisis and the realization o f the depleting nature of fossil fuels, the use o f alternative sources of energy has become essential. Among the alternative sources, solar energy has a very high potential since it is the most gentle source o f energy having no pollution or hazards associated with it and is readily and freely available everywhere. Absorption refrigeration is one of the oldest methods of producing cold and has the potential advantage of being operated with low grade heat (like solar energy) or waste heat. Many solar operated absorption refrigeration and air-conditioning units have been developed and tested extensively but there is insufficient experience for their cost effectiveness. Some experimental results on absorption chillers have also been reported [1-5] in the literature. However, the overall actual efficiency o f presently available absorption refrigeration or heat pump systems is low when we relate their power consumption to the theoretical needs at the given temperature limits. Various modifications to absorption cycles have also been proposed in the literature [2-4]. One possible way to increase the COP of an absorption refrigeration cycle greatly is by incorporating a second effect (or stage a concept not dissimilar from multiple effect desalination). The double effect generation absorption cycle, due to its large COP, has a high potentiality for 19
20
S.C. KAUSHIKet al.
applications in solar cooling systems. The first prototype double effect unit was developed by South West Research Institute in 1956--58 under funding from the American Gas Association [5]. Ritcher [6] and Philips [3] have investigated this cycle qualitatively. Wilbur and Mitchell [7] gave a preliminary assessment of the double effect water-LiBr absorption cooling systems. Although the double effect concept has been known for several years, but surprisingly there has not been significant interest in its exploitation for solar cooling applications. The double effect absorption cycle could not get its due importance possibly because it was designed during the availability of cheap energy. Recently there is a renewed interest in the double effect absorption cycle [2,9]. The water-LiBr system has mostly been chosen as it is suitable for solar air-conditioning. The double-effect absorption cycle, because of its expected enhanced COP, has the potential of favourably competing with other cooling and heat pump cycles. Since this cycle operates at higher generator temperature, so it has additional constraints on the working fluid to be used. A thermodynamic assessment of double-effect cooling system using water-LiBr as working fluid was presented by Vliet and Saidi [9] and Kaushik and Chandra [8]. It has been reported that for a water-LiBr pair, the crystalisation problem hinders the wide range application of the system; moreover, temperatures below freezing point cannot be achieved with this combination. They also ruled out the possibility of using NH3-H20 mixtures for this cycle as operating pressures in the system at desired temperatures exceed the practically feasible value. Thus it warrants the identification and evaluation of potential working fluids to be used in double effect absorption system which may be free from these major problems. Recently Aker et al. [10] proposed alcohol salt mixture as absorption refrigeration solutions. A thermodynamic properties evaluation of alcohol-salt mixture (viz. LiBr/LiCI-ZnBr2 in methanol or ethanol) was carried out in order to determine potentially useful working fluid for absorption cooling systems. In this communication the authors have presented the thermodynamic evaluation and feasibility of a double effect absorption systems using water-LiBr and methanoI-LiBr-ZnBr2 mixture as the working fluids. The analysis is based on the simultaneous solution of mass, material and energy balance equations for each component of the system to evaluate various heat transfer rates and COP of the cycle for feasible range of operating temperatures. It is found that for both the working fluid combinations, COPs of double effect generation absorption system are almost twice to their respective single stage cycle; however, COP is little lower for methanol-LiBr-ZnBr2 mixture and the cooling capacity is higher for water-LiBr case. A detailed parametric study of the new working fluid for double effect operation has also been carried out. BASIC P R I N C I P L E OF S I N G L E / D O U B L E EFFECT G E N E R A T I O N ABSORPTION CYCLES The single effect absorption cycle (Fig. la) consists of a generator, condenser, evaporator, absorber, expansion devices and heat exchanger. Low grade heat may be supplied to the generator which drives the system to extract heat from the space to be cooled via the evaporator, while heat is rejected from the condenser and absorber to the atmosphere. Refinements of the cycle can be made by adding liquid heat exchanger which is used to recover heat energy from the weak solution recirculated from the generator to the absorber which is transferred to the strong solution entering the generator. An expansion valve is used after the condenser to expand liquid refrigerant from the condenser pressure to the evaporator pressure. Another value is used after the generator to reduce the high pressure of the weak solution to the absorber pressure. A schematic diagram of the double effect generation absorption cycle is shown in Fig. l(b). The major modification from a single stage absorption cycle is that a second effect generator is introduced between the first generator and the condenser. The input heat for the second effect generator is taken from the heat of condensation of the refrigerant vapour generated in the first effect generator and condensed in the second effect generator. For condensing the refrigerant vapour in second effect generator, it is necessary to increase the pressure to a high value which causes an increase in both the temperature and pressure of the first effect generator. This necessitates a mechanical pump rather than the vapour lift pump. There are three pressure zones in the system: (i) The low pressure, prevailing in the evaporator and absorber is determined by the
Double effect generation absorption system
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Fig. 1. (a) A single effect generation absorption cycle. (b) A double effect generation absorption cycle.
evaporator temperature, (ii) The medium pressure in the condenser and second effect generator is determined by the condenser temperature and (iii) The high pressure in the first effect generator is determined by the temperature in the second effect generator. Because of the utilization of the heat of condensation of the refrigerant vapour of the first effect generation in the second effect generation, the heat per ton of refrigeration rejected to the cooling water is thus reduced by approximately 20-40~ as compared to single effect unit. It must be mentioned here that the double effect generation cycle yields the maximum COP if all the refrigerant from the first effect generator condenser in the second effect generator and its heat of condensation just matches with the heat required for the evaporation of the refrigerant in the second effect generator. In the absence of this matching degradation in performance results significantly with the increase of generator temperature. The cooling coefficient of performance is defined as COP = Refrigerating capacity =--Qe Input generator heat supplied Q~" T H E R M O D Y N A M I C EVALUATION OF DOUBLE EFFECT G E N E R A T I O N CYCLE In order to study the thermodynamic performance of the absorption cooling cycle, a number of basic assumptions are made following Kaushik [2] for single effect cycle. Similar assumptions are used for the double effect generation cycle except that the high pressure in the system is the
22
s.c. KAUSHIKet al.
equilibrium pressure corresponding to the second stage generator temperature and the intermediate pressure is the equilibrium pressure corresponding to the temperature in the condenser while the low pressure in the system is corresponding to the equilibrium evaporator temperature. These assumptions along with the following mass, material and energy conservation equations and the thermodynamic state equations lead to the complete thermodynamic analysis of the absorption cooling cycle. The system component equations are Em= 0 (mass balance) ~-mx= 0 (material balance) E,,~ = + Q (heat balance). The thermodynamic properties state equations for water-LiBr mixture are taken from Grassie [11] while that of Methanol-LiBr-ZnBr2 mixture are taken following Gadhi et al. [12]. These thermodynamic state equations for both the working fluids are nonlinear in nature and where the unknown property was not explicit, an iterative technique viz. the Newton Rapson method was used for the solution of the specific state equation. A computer model for predicting the performance of a double effect generation cycle is developed which can use different subroutines for these working fluids. The input variables to the programme are varied in the operating range which is admissible from practical point of view. T~ = 340-380 K
TA = 300--315 K Tc = 300-320 K Te = 273-293 K r/l.l, = 0.6--0.9 and the refrigerant mass flow rate leaving the condenser is assumed to be fixed ( ~ 0.01 kg s-~). The program is run to study the effect of each of the component temperature on the cooling COP and cooling capacity of the system for both the working fluids. First the effect of generator temperature is considered. Figure 2 shows the variation of the cooling coefficient of performance as a function of the generator temperature for both the working fluids for the same set of absorber/condenser/evaporator temperatures. Both the solution heat exchangers are assumed to have the effectiveness 0.9. For the sake of comparison of the COP of single effect absorption cycle are also shown in the same figure. It is seen that the COP of both the working fluids is more or less the same (being little higher for water-LiBr case) for both the single effect and double effect absorption cycles. The COP of the double effect cycle approaches twice to that of thesingle effect cycle although the starting generator temperature is higher for the double effect cycle as compared to single effect cycle. Figure 3 shows the comparison of COP values for a double effect absorption cycle using water-LiBr and Methanol-LiBr. ZnBr2 at various condenser/absorber temperatures. COP of the double effect cycle decreases slowly with the increase of the absorber/condenser temperature. These results of Figs. 3 and 4 are corresponding to T o = 3 7 0 K , T e = 2 8 3 K and r/i.,, =0.9, while TA = 300 K when Tc is varied and Tc = 305 K when TA is varied. Figure 4 depicts the variations of the specific input generator heat and refrigerating capacity per unit mass of the strong solution at various condenser and absorber temperatures. It is seen that for both the cases of TA = 300 K and Tc being varied or Tc = 305 K and TA being varied, the specific heat transfer rate decreases with the increase of the sink (absorber/condenser) temperature. The rate of decrease is faster for qe as compared to qG for both the working fluids viz. water-LiBr and methanol-LiBr.ZnBr2 mixture. The other parameters are similar to Fig. 3. To have an idea of effect of operating temperatures on COP of a double effect cycle, detailed parametric study using methanol-LiBr. ZnBr2 mixture is also being presented in the form of Tables 1 and 2. It is seen that for fixed TA and Tc the effect of T~ and Te can be directly ascertained. Increase of Te causes enhanced COP value (maximum being 1.63 for Te = 283 K at TA = Tc = 293 K) and
23
Double effect generation absorption system FIXED 1.50
CONDITIONS:-
EVAP. TEMP. TE : Z83 K COND. TEMP. Tc - 303 K ABSORBER
L 0 u
.
.
-
~ . e ~ D O U E I L E E FFECT CYCLE
TEMP. TA~,,~O~K
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i
0.20 320
530
I 540
t
I 350
I , ~$60
I 570
,
I
i
Ta
K
~
6ENERATOR TEMPERATURE
J 590
i
I I 400
I 410
J
I 4ZO
Fig. 2. Variations of the cooling coefficients of performance as a function of the generator temperature for single effect and double effect generation absorption cycles. - - A - - methanoI-LiBr. ZnBr2; - - Q - water-LiBr.
the system works for a wide range of generator temperatures. As we go on increasing TA or Tc, the COP value is reduced as well as the range of operation is restricted. There is an optimum generator temperature corresponding to maximum COP value and then COP decreases slowly with the increase of the generator temperature. The variation of intermediate parameters viz. solution concentrations, flow rates, pressures and component heat transfer rates as a function of operating temperatures are shown in Table 3. FIXED
CONDITIONS:-
GENERATOR T E M P .
TQ-
E V A P O R A T O R TEMP. T i m
370
K
285 K
HEAT E X C H A N G E R S EFFECTIVENESS
]~HE m
1.8
0.90
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to 0 U
1.5
290
; 295 CONDENSER ( T e ) / A B S O R B E R
i 500 (Ta )
TEMPERATURE
I 305 K
Fig. 3. Variations of the generator heat input and refrigerating capacity per unit mass of the strong solution as a function of condenser/absorber temperatures for a double effect generation cycle. - - Q - water-LiBr; - - & - - methanol-LiBr-ZnBr2.
24
KAUSrllK e t
S.C.
FIXED
al.
CONDITIONS:-
GENERATOR TEMR TG==370 K EVAPORATOR TEMP. TE = 283 K HEAT EXCHA N6ERS
700
EFFECTIVENESS
-\
T~ " 0 . 9 0 HI[ 1,2
\ - - - - O - - - Water- tiBr
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200
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-
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I 505
I 510
CONOENSER (T¢)/ABSORBER (TA) TEMPERATURE K Fig. 4. V a r i a t i o n s o f the cooling C O P as a function o f condenser/absorber temperatures for a d o u b l e effect generation cycle. - - Q - - w a t e r - L i B r ; - - A - - methanol-LiBr.ZnBr,.
It can be seen that the operating low pressures are little higher than that for water-LiBr case and hence the methanol-LiBr.ZnBr2 mixture can be used to produce lower evaporating temperatures. Thus the methanol-LiBr.ZnBr2 mixture has the advantage of working at low temperatures which may have the scope for refrigeration and airconditioning applications. In addition, Table I. Parametric thermodynamic analysis for methanol LiBr.ZnBr, double effect cycle Gen. 268
= TA= 293 K TE(K) 273 278 283
1.46 1.47 1,47 1.47 1.46 1.46
1.51 1.53 1.53 1.52 1.51 1.50 1.49 1.48
Tc
temp. TG(K) 345 350 360 370 380 390 400 410 415
1.59 1.59 1.58 1.57 1.55 1.54 1.52 1.50 1.49
1.63 1.62 1.60 1.59 1.57 1.55 1.53
Tc
268
1.39 1.41 1.41 1.41
= T, = 298 K TE(K) 273 278 283
1.44 1.46 1.47 1.47
1.46 1.52 1.52 1.52 1,51 1.50
273
303 K 278 283
1.40 1.42
1.50 1.51 1.51 1.51 1.49 1.48
Tc = T A =
268
1.58 1.58 1.57 1.56 1.55 1.53 1,52
1.45 1.46 1.46 1.45
345 350 360 370 380 390 400 410 415
268
1.46 1.47 1.47 1.47 1.46 1.46
T~ = 293 K 273 278 1.51 1,53 1.53 1.52 1.51 1.50 1.49 1.48
1.59 1.59 1,58 1.57 1.55 1,54 1.52 1.51
283 1.63 1.62 1.61 1.59 1.57 1.55 1.53
268
1.42 1.43 1.43
= TA= 308 K 273 278 283
1.38 1.40 1.40
1.43 1.45 1.45 1.45 1.44
T4 = 308 K 273 278
283
1,42 1.44 1,45 1.45 1.39
1,44 1.49 1.50 1.50 1.50 1.49 1.45
1.33
Table 2. Parametric thermodynamic analysis for methanoI-LiBr.ZnBr 2 doable effect cycle Gen. Temp. TG (K)
Tc
268
T~ = 298 K 273 278
283
1.52 1.53 1.53 1.53 1.52 1.51
1.59 1.59 1.59 1.57 1.56 1.55 1.53
1.41 1.47 1.49 1.49 1.48
TA=
268
1.38
273
1.40 1.43 1.44 1.44
303 K 278
283
1.43 1.48 1.49 1.49 1.49
1.53 1.54 1.54 1.54 1.53 1.52
(T c
= 293 fixed)
268
1.37 1.39
Double elT~t generation absorption system
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26
S.C. KAUSHIKet al.
the prevailing high pressure in this system reduces the pumping cost for methanol-LiBr'ZnBr2 working fluid and will also reduce the size of the equipment for a given capacity. Further more, less evacuation cost and little chances of leakage into the system are also the possible advantage in case of methanoi-LiBr-ZnBr 2 mixture. Although the solution circulation ratio may be of the same order of magnitude. CONCLUSIONS In conclusion, a double effect absorption cycle is a viable available option in the development of cooling cycles for solar applications. Although there is a loss in collector efficiency at the temperatures required for the operation of a double effect absorption cycle but the improvement in the COP is expected to override the loss by a reasonable margin. The major limitation is the choice of the working fluids in the development of these double effect generation systems. A water-LiBr system is restricted to limited range of operating conditions due to the onset of crystallization problem while methanol-LiBr.ZnBr: has the potential advantage of producing lower evaporator temperatures due to low freezing point of methanol and reduced crystallization problem due to the addition of ZnBr2 in LiBr-methanol mixture. Moreover, lower evaporator temperatures ( < 0°C) cannot be achieved with water-LiBr mixtures since water is the refrigerant. If water-NH3 mixture is used as the working fluid, the pressure in first effect generator is excessively high and hence not suitable from a practical point of view. On the other hand the NH3-H_,O system has the advantage of being operated efficiently at higher sink temperatures, allowing the possible use of air cooling. Acknowledgements--The authors gratefully acknowledge the financial support from Tata Energy Research Institute, India.
REFERENCES 1. Sofrata eta/., Solar absorption air conditioning: a critical review, Research report, King Saud University, No. AM-5199, Riyadh (1979). 2. S. C. Kaushik, Solar absorption refrigeration and air-conditioning. Chapter-IV in Reviews in Renewable Sources of Energy, Wiley Eastern Ltd., New Delhi (1982). 3. B. A. Phillips, Absorption cycles for aircooled solar air-conditioning, presented at ASHRAE semi-annual meeting, Dallas (1976). 4. S. C. Kaushik, Advanced absorption cooling systems: Thermal modelling and designing aspects, paper presented at SOLARAS meeting on Solar Building Workshop at K.S.U., Riyadh (1984). 5. E. P. Whitlow and J. S. Swearingen, An improved absorption refrigeration cycle, Gas Age 122(9), 19-22 (1958). 6. K. H. Ritcher, Multistage absorption refrigeration systems, J. Refrigeration, 5, 105 (1962). 7. P. J. Wilbure and C. E. Mitchell, Solar absorption air-conditioning alternatives, Solar Energy 17, 193 (1975). 8. S. C. Kaushik and S. Chandras, Thermal modelling and parametric study of a double effect generation absorption cycle for solar air conditioning, Energy Conserv. Manag (In Press) (1984). 9. G. C. Vliet et aL, A water-LiBr double effect absorption cooling cycle analysis, ASHRAE Trans., 88(1), 811-823 (1982). 10. J. E. Aker and L. F. Albright, An evaluation of alcohol-salt mixtures as absorption refrigeration solution, ASHRAE Trans. Jan 0968). II. S. L. Grassie, Solar operated absorption air-conditioner, PhD. thesis, Queensland University, Brisbane (1976). 12. S. M. B. Gadhi. R. S. Agarwal and S. C. Kaushik, Thermodynamic properties state equations for the alcohol-salt mixtures (methanol-LiBr. ZnBr2) used in absorption air conditioning, paper presented in SOLERAS meeting on Solar Building Workshop at King Saud University, Riyadh (1984).
0198-7593/85 $3.00 + 0.00 Pergamon Press Ltd
Printed in Great Britain.
THERMODYNAMIC FEASIBILITY OF DOUBLE EFFECT GENERATION ABSORPTION SYSTEM USING WATER-SALT AND ALCOHOL-SALT MIXTURES AS WORKING FLUIDS S. C. KAUSmK Centre of Energy Studies, I.I.T., New Delhi l l0016, India S. CHANDRA Mechanical Engng Dept, Melbourne University, Melbourne, Australia
and S. M. B. GADHI Mechanical Engng Dept, I.I.T. New Delhi, India Abstr~t--This communication presents a comparative thermodynamic assessment of a double effect generation absorption cooling cycle using water-LiBr and methanol-LiBr.ZnBr2as working fluids. The proposed systemconsists of a second effectgenerator between the first effectgenerator and the condenser along with two heat recuperatorsbetweenthesegeneratorsand the absorber. A numericalcomputermodel based on the simultaneous solution of heat, mass and material balance equations for various components of the system has been carried out. The influenceof component temperature on the cooling capacity and COP of the system has been investigated to obtain the optimum operating conditions. It is seen that for both the combinations the system has optimum generator temperatures for which COWs are maximum.The COP valuesof double effectabsorption cyclesapproaches twiceto that of single effectones for both the working fluids. It is further seen that a water-LiBr combination has more cooling capacity than that of methanol-salt mixture; however, the latter may have the advantage of working at lower evaporator temperatures.
INTRODUCTION Cold storage of food products is an important application of refrigeration. An increase in the number and capacity of cold storage systems is liable to make a significant contribution to the alleviation of food storage problems by arresting the spoilage. Another application of the refrigeration cycle is the space-conditioning o f buildings which leads to the improvement of human comfort, increase in industrial production and the feasibility o f establishing high technology industries. Both these applications are o f great relevance to food and energy problems. The conventional space-conditioning devices need electrical energy and/or fossil fuels for their operation. With the advent of the energy crisis and the realization o f the depleting nature of fossil fuels, the use o f alternative sources of energy has become essential. Among the alternative sources, solar energy has a very high potential since it is the most gentle source o f energy having no pollution or hazards associated with it and is readily and freely available everywhere. Absorption refrigeration is one of the oldest methods of producing cold and has the potential advantage of being operated with low grade heat (like solar energy) or waste heat. Many solar operated absorption refrigeration and air-conditioning units have been developed and tested extensively but there is insufficient experience for their cost effectiveness. Some experimental results on absorption chillers have also been reported [1-5] in the literature. However, the overall actual efficiency o f presently available absorption refrigeration or heat pump systems is low when we relate their power consumption to the theoretical needs at the given temperature limits. Various modifications to absorption cycles have also been proposed in the literature [2-4]. One possible way to increase the COP of an absorption refrigeration cycle greatly is by incorporating a second effect (or stage a concept not dissimilar from multiple effect desalination). The double effect generation absorption cycle, due to its large COP, has a high potentiality for 19
20
S.C. KAUSHIKet al.
applications in solar cooling systems. The first prototype double effect unit was developed by South West Research Institute in 1956--58 under funding from the American Gas Association [5]. Ritcher [6] and Philips [3] have investigated this cycle qualitatively. Wilbur and Mitchell [7] gave a preliminary assessment of the double effect water-LiBr absorption cooling systems. Although the double effect concept has been known for several years, but surprisingly there has not been significant interest in its exploitation for solar cooling applications. The double effect absorption cycle could not get its due importance possibly because it was designed during the availability of cheap energy. Recently there is a renewed interest in the double effect absorption cycle [2,9]. The water-LiBr system has mostly been chosen as it is suitable for solar air-conditioning. The double-effect absorption cycle, because of its expected enhanced COP, has the potential of favourably competing with other cooling and heat pump cycles. Since this cycle operates at higher generator temperature, so it has additional constraints on the working fluid to be used. A thermodynamic assessment of double-effect cooling system using water-LiBr as working fluid was presented by Vliet and Saidi [9] and Kaushik and Chandra [8]. It has been reported that for a water-LiBr pair, the crystalisation problem hinders the wide range application of the system; moreover, temperatures below freezing point cannot be achieved with this combination. They also ruled out the possibility of using NH3-H20 mixtures for this cycle as operating pressures in the system at desired temperatures exceed the practically feasible value. Thus it warrants the identification and evaluation of potential working fluids to be used in double effect absorption system which may be free from these major problems. Recently Aker et al. [10] proposed alcohol salt mixture as absorption refrigeration solutions. A thermodynamic properties evaluation of alcohol-salt mixture (viz. LiBr/LiCI-ZnBr2 in methanol or ethanol) was carried out in order to determine potentially useful working fluid for absorption cooling systems. In this communication the authors have presented the thermodynamic evaluation and feasibility of a double effect absorption systems using water-LiBr and methanoI-LiBr-ZnBr2 mixture as the working fluids. The analysis is based on the simultaneous solution of mass, material and energy balance equations for each component of the system to evaluate various heat transfer rates and COP of the cycle for feasible range of operating temperatures. It is found that for both the working fluid combinations, COPs of double effect generation absorption system are almost twice to their respective single stage cycle; however, COP is little lower for methanol-LiBr-ZnBr2 mixture and the cooling capacity is higher for water-LiBr case. A detailed parametric study of the new working fluid for double effect operation has also been carried out. BASIC P R I N C I P L E OF S I N G L E / D O U B L E EFFECT G E N E R A T I O N ABSORPTION CYCLES The single effect absorption cycle (Fig. la) consists of a generator, condenser, evaporator, absorber, expansion devices and heat exchanger. Low grade heat may be supplied to the generator which drives the system to extract heat from the space to be cooled via the evaporator, while heat is rejected from the condenser and absorber to the atmosphere. Refinements of the cycle can be made by adding liquid heat exchanger which is used to recover heat energy from the weak solution recirculated from the generator to the absorber which is transferred to the strong solution entering the generator. An expansion valve is used after the condenser to expand liquid refrigerant from the condenser pressure to the evaporator pressure. Another value is used after the generator to reduce the high pressure of the weak solution to the absorber pressure. A schematic diagram of the double effect generation absorption cycle is shown in Fig. l(b). The major modification from a single stage absorption cycle is that a second effect generator is introduced between the first generator and the condenser. The input heat for the second effect generator is taken from the heat of condensation of the refrigerant vapour generated in the first effect generator and condensed in the second effect generator. For condensing the refrigerant vapour in second effect generator, it is necessary to increase the pressure to a high value which causes an increase in both the temperature and pressure of the first effect generator. This necessitates a mechanical pump rather than the vapour lift pump. There are three pressure zones in the system: (i) The low pressure, prevailing in the evaporator and absorber is determined by the
Double effect generation absorption system
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SolHeat ution [ Exchong*r Tr
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I
Fig. 1. (a) A single effect generation absorption cycle. (b) A double effect generation absorption cycle.
evaporator temperature, (ii) The medium pressure in the condenser and second effect generator is determined by the condenser temperature and (iii) The high pressure in the first effect generator is determined by the temperature in the second effect generator. Because of the utilization of the heat of condensation of the refrigerant vapour of the first effect generation in the second effect generation, the heat per ton of refrigeration rejected to the cooling water is thus reduced by approximately 20-40~ as compared to single effect unit. It must be mentioned here that the double effect generation cycle yields the maximum COP if all the refrigerant from the first effect generator condenser in the second effect generator and its heat of condensation just matches with the heat required for the evaporation of the refrigerant in the second effect generator. In the absence of this matching degradation in performance results significantly with the increase of generator temperature. The cooling coefficient of performance is defined as COP = Refrigerating capacity =--Qe Input generator heat supplied Q~" T H E R M O D Y N A M I C EVALUATION OF DOUBLE EFFECT G E N E R A T I O N CYCLE In order to study the thermodynamic performance of the absorption cooling cycle, a number of basic assumptions are made following Kaushik [2] for single effect cycle. Similar assumptions are used for the double effect generation cycle except that the high pressure in the system is the
22
s.c. KAUSHIKet al.
equilibrium pressure corresponding to the second stage generator temperature and the intermediate pressure is the equilibrium pressure corresponding to the temperature in the condenser while the low pressure in the system is corresponding to the equilibrium evaporator temperature. These assumptions along with the following mass, material and energy conservation equations and the thermodynamic state equations lead to the complete thermodynamic analysis of the absorption cooling cycle. The system component equations are Em= 0 (mass balance) ~-mx= 0 (material balance) E,,~ = + Q (heat balance). The thermodynamic properties state equations for water-LiBr mixture are taken from Grassie [11] while that of Methanol-LiBr-ZnBr2 mixture are taken following Gadhi et al. [12]. These thermodynamic state equations for both the working fluids are nonlinear in nature and where the unknown property was not explicit, an iterative technique viz. the Newton Rapson method was used for the solution of the specific state equation. A computer model for predicting the performance of a double effect generation cycle is developed which can use different subroutines for these working fluids. The input variables to the programme are varied in the operating range which is admissible from practical point of view. T~ = 340-380 K
TA = 300--315 K Tc = 300-320 K Te = 273-293 K r/l.l, = 0.6--0.9 and the refrigerant mass flow rate leaving the condenser is assumed to be fixed ( ~ 0.01 kg s-~). The program is run to study the effect of each of the component temperature on the cooling COP and cooling capacity of the system for both the working fluids. First the effect of generator temperature is considered. Figure 2 shows the variation of the cooling coefficient of performance as a function of the generator temperature for both the working fluids for the same set of absorber/condenser/evaporator temperatures. Both the solution heat exchangers are assumed to have the effectiveness 0.9. For the sake of comparison of the COP of single effect absorption cycle are also shown in the same figure. It is seen that the COP of both the working fluids is more or less the same (being little higher for water-LiBr case) for both the single effect and double effect absorption cycles. The COP of the double effect cycle approaches twice to that of thesingle effect cycle although the starting generator temperature is higher for the double effect cycle as compared to single effect cycle. Figure 3 shows the comparison of COP values for a double effect absorption cycle using water-LiBr and Methanol-LiBr. ZnBr2 at various condenser/absorber temperatures. COP of the double effect cycle decreases slowly with the increase of the absorber/condenser temperature. These results of Figs. 3 and 4 are corresponding to T o = 3 7 0 K , T e = 2 8 3 K and r/i.,, =0.9, while TA = 300 K when Tc is varied and Tc = 305 K when TA is varied. Figure 4 depicts the variations of the specific input generator heat and refrigerating capacity per unit mass of the strong solution at various condenser and absorber temperatures. It is seen that for both the cases of TA = 300 K and Tc being varied or Tc = 305 K and TA being varied, the specific heat transfer rate decreases with the increase of the sink (absorber/condenser) temperature. The rate of decrease is faster for qe as compared to qG for both the working fluids viz. water-LiBr and methanol-LiBr.ZnBr2 mixture. The other parameters are similar to Fig. 3. To have an idea of effect of operating temperatures on COP of a double effect cycle, detailed parametric study using methanol-LiBr. ZnBr2 mixture is also being presented in the form of Tables 1 and 2. It is seen that for fixed TA and Tc the effect of T~ and Te can be directly ascertained. Increase of Te causes enhanced COP value (maximum being 1.63 for Te = 283 K at TA = Tc = 293 K) and
23
Double effect generation absorption system FIXED 1.50
CONDITIONS:-
EVAP. TEMP. TE : Z83 K COND. TEMP. Tc - 303 K ABSORBER
L 0 u
.
.
-
~ . e ~ D O U E I L E E FFECT CYCLE
TEMP. TA~,,~O~K
,T. EXCH. EFF. r/m= O. SO I,Z
u Z
0 b. a£ kd Q.
I.OI
i, 0
^
A_
^ a
IZ
a
~ SINGLE EFFECT CYCLE J
u u. L W 0 (J
O.~K
0
Water- LiBr
/~
Methmwl- LiBr. ZnBrz
i
0.20 320
530
I 540
t
I 350
I , ~$60
I 570
,
I
i
Ta
K
~
6ENERATOR TEMPERATURE
J 590
i
I I 400
I 410
J
I 4ZO
Fig. 2. Variations of the cooling coefficients of performance as a function of the generator temperature for single effect and double effect generation absorption cycles. - - A - - methanoI-LiBr. ZnBr2; - - Q - water-LiBr.
the system works for a wide range of generator temperatures. As we go on increasing TA or Tc, the COP value is reduced as well as the range of operation is restricted. There is an optimum generator temperature corresponding to maximum COP value and then COP decreases slowly with the increase of the generator temperature. The variation of intermediate parameters viz. solution concentrations, flow rates, pressures and component heat transfer rates as a function of operating temperatures are shown in Table 3. FIXED
CONDITIONS:-
GENERATOR T E M P .
TQ-
E V A P O R A T O R TEMP. T i m
370
K
285 K
HEAT E X C H A N G E R S EFFECTIVENESS
]~HE m
1.8
0.90
leZ
o
C.)
,.' IJ
TA'L300 K
T;" 305K
1.7
Z ,1[
=E E
o
Ik
|.6
E
hi G. 1, O
--o---
1.5
I=E to m
~.
Water-L! Br M e t h a n o l - L i B r . Zn Br2
u
,-: Ik
1.4
to 0 U
1.5
290
; 295 CONDENSER ( T e ) / A B S O R B E R
i 500 (Ta )
TEMPERATURE
I 305 K
Fig. 3. Variations of the generator heat input and refrigerating capacity per unit mass of the strong solution as a function of condenser/absorber temperatures for a double effect generation cycle. - - Q - water-LiBr; - - & - - methanol-LiBr-ZnBr2.
24
KAUSrllK e t
S.C.
FIXED
al.
CONDITIONS:-
GENERATOR TEMR TG==370 K EVAPORATOR TEMP. TE = 283 K HEAT EXCHA N6ERS
700
EFFECTIVENESS
-\
T~ " 0 . 9 0 HI[ 1,2
\ - - - - O - - - Water- tiBr
\
600
\\
~
Methonol-kiBr. ZnBrz
\
\N
400
" ~'O
qG
\®
500
200
,oo
~'-~300
"" " ~ ' ~ . --,
K
"~ '~' "~.-
qE
"'--.
°--
~
"~.~
-
\
"
I zglJ
""
1 300
'
I 505
I 510
CONOENSER (T¢)/ABSORBER (TA) TEMPERATURE K Fig. 4. V a r i a t i o n s o f the cooling C O P as a function o f condenser/absorber temperatures for a d o u b l e effect generation cycle. - - Q - - w a t e r - L i B r ; - - A - - methanol-LiBr.ZnBr,.
It can be seen that the operating low pressures are little higher than that for water-LiBr case and hence the methanol-LiBr.ZnBr2 mixture can be used to produce lower evaporating temperatures. Thus the methanol-LiBr.ZnBr2 mixture has the advantage of working at low temperatures which may have the scope for refrigeration and airconditioning applications. In addition, Table I. Parametric thermodynamic analysis for methanol LiBr.ZnBr, double effect cycle Gen. 268
= TA= 293 K TE(K) 273 278 283
1.46 1.47 1,47 1.47 1.46 1.46
1.51 1.53 1.53 1.52 1.51 1.50 1.49 1.48
Tc
temp. TG(K) 345 350 360 370 380 390 400 410 415
1.59 1.59 1.58 1.57 1.55 1.54 1.52 1.50 1.49
1.63 1.62 1.60 1.59 1.57 1.55 1.53
Tc
268
1.39 1.41 1.41 1.41
= T, = 298 K TE(K) 273 278 283
1.44 1.46 1.47 1.47
1.46 1.52 1.52 1.52 1,51 1.50
273
303 K 278 283
1.40 1.42
1.50 1.51 1.51 1.51 1.49 1.48
Tc = T A =
268
1.58 1.58 1.57 1.56 1.55 1.53 1,52
1.45 1.46 1.46 1.45
345 350 360 370 380 390 400 410 415
268
1.46 1.47 1.47 1.47 1.46 1.46
T~ = 293 K 273 278 1.51 1,53 1.53 1.52 1.51 1.50 1.49 1.48
1.59 1.59 1,58 1.57 1.55 1,54 1.52 1.51
283 1.63 1.62 1.61 1.59 1.57 1.55 1.53
268
1.42 1.43 1.43
= TA= 308 K 273 278 283
1.38 1.40 1.40
1.43 1.45 1.45 1.45 1.44
T4 = 308 K 273 278
283
1,42 1.44 1,45 1.45 1.39
1,44 1.49 1.50 1.50 1.50 1.49 1.45
1.33
Table 2. Parametric thermodynamic analysis for methanoI-LiBr.ZnBr 2 doable effect cycle Gen. Temp. TG (K)
Tc
268
T~ = 298 K 273 278
283
1.52 1.53 1.53 1.53 1.52 1.51
1.59 1.59 1.59 1.57 1.56 1.55 1.53
1.41 1.47 1.49 1.49 1.48
TA=
268
1.38
273
1.40 1.43 1.44 1.44
303 K 278
283
1.43 1.48 1.49 1.49 1.49
1.53 1.54 1.54 1.54 1.53 1.52
(T c
= 293 fixed)
268
1.37 1.39
Double elT~t generation absorption system
~' ~ ~
r-: r-:
r-: r-: r =
¢--:, e-:
e,4
~
N
e~ ¢.,4 o~
i
~ o
u
il
°~
o
,erw~
~¢.,i
iI
25
26
S.C. KAUSHIKet al.
the prevailing high pressure in this system reduces the pumping cost for methanol-LiBr'ZnBr2 working fluid and will also reduce the size of the equipment for a given capacity. Further more, less evacuation cost and little chances of leakage into the system are also the possible advantage in case of methanoi-LiBr-ZnBr 2 mixture. Although the solution circulation ratio may be of the same order of magnitude. CONCLUSIONS In conclusion, a double effect absorption cycle is a viable available option in the development of cooling cycles for solar applications. Although there is a loss in collector efficiency at the temperatures required for the operation of a double effect absorption cycle but the improvement in the COP is expected to override the loss by a reasonable margin. The major limitation is the choice of the working fluids in the development of these double effect generation systems. A water-LiBr system is restricted to limited range of operating conditions due to the onset of crystallization problem while methanol-LiBr.ZnBr: has the potential advantage of producing lower evaporator temperatures due to low freezing point of methanol and reduced crystallization problem due to the addition of ZnBr2 in LiBr-methanol mixture. Moreover, lower evaporator temperatures ( < 0°C) cannot be achieved with water-LiBr mixtures since water is the refrigerant. If water-NH3 mixture is used as the working fluid, the pressure in first effect generator is excessively high and hence not suitable from a practical point of view. On the other hand the NH3-H_,O system has the advantage of being operated efficiently at higher sink temperatures, allowing the possible use of air cooling. Acknowledgements--The authors gratefully acknowledge the financial support from Tata Energy Research Institute, India.
REFERENCES 1. Sofrata eta/., Solar absorption air conditioning: a critical review, Research report, King Saud University, No. AM-5199, Riyadh (1979). 2. S. C. Kaushik, Solar absorption refrigeration and air-conditioning. Chapter-IV in Reviews in Renewable Sources of Energy, Wiley Eastern Ltd., New Delhi (1982). 3. B. A. Phillips, Absorption cycles for aircooled solar air-conditioning, presented at ASHRAE semi-annual meeting, Dallas (1976). 4. S. C. Kaushik, Advanced absorption cooling systems: Thermal modelling and designing aspects, paper presented at SOLARAS meeting on Solar Building Workshop at K.S.U., Riyadh (1984). 5. E. P. Whitlow and J. S. Swearingen, An improved absorption refrigeration cycle, Gas Age 122(9), 19-22 (1958). 6. K. H. Ritcher, Multistage absorption refrigeration systems, J. Refrigeration, 5, 105 (1962). 7. P. J. Wilbure and C. E. Mitchell, Solar absorption air-conditioning alternatives, Solar Energy 17, 193 (1975). 8. S. C. Kaushik and S. Chandras, Thermal modelling and parametric study of a double effect generation absorption cycle for solar air conditioning, Energy Conserv. Manag (In Press) (1984). 9. G. C. Vliet et aL, A water-LiBr double effect absorption cooling cycle analysis, ASHRAE Trans., 88(1), 811-823 (1982). 10. J. E. Aker and L. F. Albright, An evaluation of alcohol-salt mixtures as absorption refrigeration solution, ASHRAE Trans. Jan 0968). II. S. L. Grassie, Solar operated absorption air-conditioner, PhD. thesis, Queensland University, Brisbane (1976). 12. S. M. B. Gadhi. R. S. Agarwal and S. C. Kaushik, Thermodynamic properties state equations for the alcohol-salt mixtures (methanol-LiBr. ZnBr2) used in absorption air conditioning, paper presented in SOLERAS meeting on Solar Building Workshop at King Saud University, Riyadh (1984).