INTERNATIONAL JOURNAL OF ENERGY, Issue 3, Vol.1, 2007 Solar Absorption Refrigeration System Using New Working Fluid Pairs Jasim M. Abdulateef, Kamaruzzaman Sopian, M. A. Alghoul, Mohd Yusof Sulaiman, Azami Zaharim and Ibrahim Ahmad Abstract- Absorption refrigeration systems powered by solar energy increasingly attract research interests in the last years. In this study, thermodynamic analyses for different working fluid pairs are performed. A computer simulation model has been developed to predict the performance of solar absorption refrigeration system using different working fluid. The model is based on detailed mass and energy balance and heat and mass transfer for the cycle component. Detailed thermodynamic properties for ammonia- water, ammonia-lithium nitrate and ammonia-sodium thiocyanate are expressed in polynomial equations and used in cycle simulation. The performances of these three cycles against various generator, evaporator, and condenser temperatures are compared. The results show that the ammonia-lithium nitrate and ammonia-sodium thiocyanate cycles give better performance than the ammonia-water cycle. The ammonia-sodium thiocyanate cycle cannot operate at evaporator temperatures below -10°C for the possibility of crystallization. Increasing condenser temperatures cause a decrease in system performance for each cycle. With the increase in evaporator temperature, the COP values for each cycle increase. These results can serve as a source of reference for developing new cycles and searching for new working fluids pairs. They can also be used in selecting operating conditions for existing systems and achieving automatic control for maintain optimum operation of the system. Keywords- performance; absorption; solar energy; NH3LiNO3; crystallization; refrigeration; generator I. INTRODUCTION URING the last few decades, an increasing interest, based on research and development, has been concentrated on utilization of non-conventional energy sources, namely solar energy, wind energy, tidal waves, biogas, geothermal energy, hydropower, hydrogen energy, etc. Among these sources, solar energy, which is an energy source for cooling applications, is a highly popular source due to the following facts: direct and easy usability, renewable and continuity, maintaining the same quality, being safe, being free, being environment friendly and not being under the monopoly of anyone. D The absorption refrigeration system, which has some advantages, such as silent operation, high reliability, long service life, simpler capacity control mechanism, easier implementation, and low maintenance, is widely acknowledged as a prospective candidate for efficient and economic use of solar energy for cooling applications. Also, the absorption refrigeration cycle is usually a preferable alternative, since it uses the thermal energy collected from the sun without the need to convert this energy into mechanical energy as required by the vapor compression cycle. In addition, the absorption cycle uses thermal energy at a lower temperature than that dictated by the vapor compression cycle. The binary systems of NH3H2O and LiBr-H2O were well known as working fluid pairs to be applied both in absorption heat pumps and in absorption refrigerators currently. Theoretical and experimental studies have been conducted to optimize the performance of absorption refrigeration cycles using NH3H2O and LiBr-H2O as refrigerant- absorbent combination. The advantage for refrigerant NH3 is that it can evaporate at lower temperatures (i.e. from -10 to 0°C) compared to H2O (i.e. from 4 to 10°C). Therefore, for refrigeration, the NH3-H2O cycle is used. Research has been performed for NH3-H2O systems theoretically [1-4] and experimentally [5, 6]. These studies show that the NH3-H2O system exhibits a relatively low COP. Efforts are being made to search for better working fluid pairs that can improve system performance. It is proposed that NH3LiNO3 and NH3-NaSCN cycles can be alternatives to NH3H2O systems [7, 8].Therefore, in the present study, the comparisons of the performances of NH3-H2O, NH3-LiNO3 and NH3-NaSCN absorption cycles driven by solar energy are performed. It is hoped that these results could serve as a source of reference for designing and selecting new absorption refrigeration systems, developing new working fluid pairs and optimizing suitable operating conditions. II. CYCLE PERFORMANCE ANALYSIS The absorption cycle powered by solar energy is illustrated in Fig. 1. Low-pressure refrigerant vapor from the evaporator is absorbed by the liquid strong solution in the absorber. The pump receives low-pressure liquid weak solution from the absorber, elevates the pressure of the weak solution and delivers it to the generator. By weak solution (strong solution) is meant that the ability of the solution to absorb the refrigerant vapor is weak (strong) according to ASHRAE definition [9]. In the generator, heat from a high-temperature source by solar energy drives off the refrigerant vapor in the weak solution. The liquid strong solution returns to the absorber through a throttling valve whose purpose is to provide a pressure drop to 82 Manuscript received April 20, 2007; Revised version received October 11, 2007. Authors are with Solar Energy Research Institute, University Kebangsaan Malaysia, 43600 Bangi, Selangor, MALAYSIA 1. H2O. Issue 3. 1. The thermodynamic properties at outlet of generator to inlet of absorber in Fig. these properties are interdependent and are necessary for computer simulation of absorption refrigeration systems. The cycle Auxiliary Heater Controller performance is measured by the coefficient of performance (COP). 2007 maintain the pressure difference between the generator and the absorber. maintaining the pressure difference between the condenser and the evaporator. temperature. energy balances for the absorber. concentration. as shown in Fig. that CR = m7 m1 (7) The energy balance for the solution heat exchanger is as follows: III. a solution heat exchanger is normally added to the cycle. Vol. 1 are determined by NH3. which is defined as the refrigeration rate over the rate of heat addition at the generator plus the work input to the pump. NH3-LiNO3 and NH3-NaSCN absorption refrigeration cycles. is defined as the mass flow rate of solution from the absorber to the generator to the mass flow rate of working fluid (refrigerant). For NH3-H2O. The schematic illustration of the solar absorption refrigeration system In order to use equation (1). condenser and evaporator yield (5) (6) 1− X 7 m8 = m1 X7 − X8 Qabs = m4 h4 + m10 h10 − m5 h5 Qcond = m1 (h1 − h2 ) Qevp = m1 (h4 − h3 ) (12) (13) (14) The mass flow ratio of the system. and other properties can be calculated 83 . SOLUTION PROPERTIES The thermodynamic properties for NH3-H2O. circulation ratio. mass and energy conservation should be determined at each component. enthalpy and density. 1. that is COP = Qevp Q gen + Wme (1) 1 Generator Condenser 7 2 Heat Exchanger 9 6 3 10 Absorber 5 4 Evaporator Storage Tank Solar Collector 8 Fig. NH3LiNO3 and NH3-NaSCN solutions are pressure. LiNO3 and NaSCN are absorbents.INTERNATIONAL JOURNAL OF ENERGY. For the generator. The high-pressure refrigerant vapor condenses into liquid in the condenser and enters the evaporator through a throttling valve. the flow rates of the strong and weak solutions can be determined: m7 = 1 − X8 m1 X7 − X8 h6 = h5 + ( P6 − P5 )v6 Wme = ( P6 − P5 )v 6 (10) (11) Finally. NH3 is the refrigerant. the mass and energy balances yield: T9 = E ex T6 + (1 − E ex )T8 h7 = h 6 + m8 (h8 − h9 ) m6 (8) (9) m7 = m1 + m8 m7 X 7 = m1 X 1 + m8 X 8 Q gen = m1 h1 + m8 h8 − m7 h7 (2) (3) (4) The energy increase by pumping is From equations (2) and (3). In order to improve cycle performance. 15)2 + D( T − 273. concentration and enthalpy is as follows [7]: h( T . 1. Issue 3. X ) = 2046.44 − 1.15)3 where (37) 18. the two phase equilibrium pressure and temperature of the refrigerant NH3 are linked by the relation: A = 16.65 − 2621.3826 X C = 10 −3 (1.16 (21) The relation among temperature. A = 15.15 )2 (33) B LogP = A − T where (18) 3.255 − 4 X 2 + 3. NH3-LiNO3 or NH3-NaSCN solutions. X ) = A + B(T − 273 .15)2 + D( T − 273.2 NH3-H2O solution The relation between saturation pressure and temperature of an ammonia-water mixture is given as [10]: ρ( T .03(1 − X ) (22) The relation among specific volume.4 NH3-NaSCN solution The relation between saturation and temperature of an ammonia-sodium thiocyanate mixture is given as [7]: A = 7.7 X + 1540.54 A = −215 + 689( X − 0.7 X 3 (19) (20) LnP = A + where B T (34) The relation among temperature.9291X 2 − 3. A = 79.72 − 1072X + 1287.15) + C( T − 273.33 X + 10 X − 3.15125 + 3.362 X 3 B = 2013.3 NH3-LiNO3 solution The relation between saturation pressure and temperature of an ammonia-lithium nitrate mixture is given as [7]: ρ(T.15 ) − 0.15) i i =0 6 (15) The specific enthalpies of saturated liquid and vapor NH3 are expressed in terms of temperature as follows: hl (T ) = ∑ bi (T − 273.4348X + 2256 .9 X 2 − 295.54 − X ) 2 if X ≤ 0. 2007 based on the binary mixture of NH3-H2O.4081− 2.33 X ) 2 3 −5 v( T .29 + 3.9 X 2 − 194.65 X 0. X ) = 100∑ ai ( i =1 16 ni T − 1) mi X 273. temperature and concentration is given as.767 X + 0.7266 − 0.8 − 2155.67 X 3 B = 2.5 − 1.859( 1 − X )3 B = −2802 − 4192(1 − X ) 3 (25) (26) The relation among temperature.15) i hv ( T ) = ∑ ci ( T − 273.099 + 2.15 )i i =0 i =0 6 6 (16) (17) A = −215 + 1570(0. 3.3965 X ) D = 10 −5 (3. concentration and enthalpy is as follows [7]: where X is the ammonia mole fraction and is given as follows: h( T .015 X + 17.15 )i X j (23) The solution density is related to concentration and temperature as [7]: 3.3463 ( T − 273. Vol.INTERNATIONAL JOURNAL OF ENERGY.15) + C(T − 273 .6341X + 5.0039( T − 273.2814X + 7.0637X 3 B = −3.92( 1 − X )3 (35) (36) h(T .15)3 where (27) P (T ) = 10 3 ∑ ai (T − 273.15) + C( T − 273.519− 2400 .298629 X B = −2548. X ) = A + B( T − 273.4552 X 2 − 3. X ) = ∑ j =0 3 ∑a i =0 3 ij ( T − 273.15) 2 where (42) A = 1707 . concentration and enthalpy is as follows.5137X 3 C = 10 −2 ( 1.06 X 3 ) (38) (39) (40) (41) D = 10 (−3.54) 2 if X ≥ 0. X ) = A + B( T − 273.1674 X 3 84 LnP = A + where B T (43) (44) (24) .015 X X = 18.54 B = 1.1 Refrigerant NH3 In the usual ranges of pressure and temperature concerning refrigeration applications.93333 X ) (28) (29) (30) (31) (32) The solution density is related to concentration and temperature as [7]: 3.5083X 2 − 930.982 X 2 + 0.22 − 1409. 5 100 40.8 100 30 -5 25 64. 5.8 52.4 100 P. Tcond = 30°C. The corresponding comparison of circulation ratios vs evaporator temperatures is given in Fig.INTERNATIONAL JOURNAL OF ENERGY. Again. 4 gives the comparison of COP values vs evaporator temperatures for NH3-H2O.5 0. 2 shows the comparison of COP values vs generator temperatures for NH3-H2O.8 4. Vol. with the effectiveness of the solution heat exchanger of 80%. Table 1.2 0. (45) Absorber sol inlet 40.4 X 3 ) All coefficients of equations are listed by Sun [11]. Tabs = 25°C and Tevp = -5°C.3 shows the corresponding comparison of circulation ratios vs generator temperatures. which is the temperature range for refrigeration. the NH3-NaSCN cycle gives the best performance.5 100 100 100 52.2 33. 3. and the NH3-H2O cycle has the lowest COP values.51 1 1 1 3.07 kg/min for NH3-H2O.8 100 30 -5 25 68.1 85 Tgen.2 52. it is shown that the circulation ratio for the NH3-NaSCN cycle is higher than the other two cycles.07 5. It is shown that.4 0.5 2.5 33.Thermodynamic properties at various states in absorption cycles driven by solar energy Fluid state NH3-H2O cycle Generator ref exit Condenser ref exit Evaporator ref exit Absorber sol.5.5 37. This means that either the solution pump needs to run faster or a bigger pump is required.7 48. C 0 50 60 70 80 90 100 110 120 130 Fig.07 4.5 100 100 100 48.95 3.8 m. Fig. 1. . NH3-LiNO3 and NH3-NaSCN respectively.9 0. 3. a bigger pump is needed for the NH3-NaSCN cycle. the performance of the NH3-H2O cycle is better than that of the NH3-LiNO3 cycle.6 COP 80 0.7 0. Fig. exit Generator sol inlet Generator sol exit Absorber sol inlet NH3-LiNO3 cycle Generator ref exit Condenser ref exit Evaporator ref exit Absorber sol. 120 100 0.1X − 3.8 37. the COP values for each cycle increase. The COP values for these three cycles increase with generator temperatures. for generator temperatures higher than 80°C.01 1 1 1 5. 2007 C = 10 −3 (5. Table 1 shows the comparison of the various thermodynamic states in the cycle operating at Tgen = 100°C. Fig. 2. and the NH3-H2O cycle has the lowest COP. exit Generator sol inlet Generator sol exit T. the computer program was used to search for different operation conditions with which an absorption cycle reaches its maximum performance. Kg/min 1 1 1 3.6 X 2 − 5. Kpa 1167 1167 354 354 1167 1167 354 1167 1167 354 354 1167 1167 354 1167 1167 354 354 1167 1167 X% 100 100 100 52. For the NH3-LiNO3 cycle a lower generator temperature can be used than for the others. For high evaporator temperature.95 and 5. RESULTS AND DISCUSSION In order to provide details optimum operating conditions for solar absorption refrigeration systems.0 0. It is illustrated that the circulation ratio for the NH3-NaSCN cycle is higher than for the other two cycles. the differences among them are not very remarkable. C Fig. exit Generator sol inlet Generator sol exit Absorber sol inlet NH3-NaSCN cycle Generator ref exit Condenser ref exit Evaporator ref exit Absorber sol. For evaporator temperatures lower than zero.3 0. This means that more refrigerant can be boiled off in the generator for the NH3-H2O cycle than for the other two.0 Tgen.5 3. Issue 3.1 CR 50 60 70 80 90 100 110 120 130 60 40 20 0.01 3. NH3-LiNO3 and NH3-NaSCN absorption cycles. oC 100 30 -5 25 66 100 40.8 As a result. Variation of COP with generator temperature The total solution amounts circulated are 3. Variation of CR with generator temperature However. the NH3-NaSCN cycle gives the best performance. With the increase in evaporator temperature. NH3-LiNO3 and NH3-NaSCN absorption cycles. 1.1 IV.7 36.51 2.95 3.8 354 36. 8 0. C Fig. The results are calculated using computer program based on thermodynamic properties data for the working fluids. 1.1 0. ammonia-lithium nitrate and ammonia-sodium thiocyanate solar absorption cycles are presented. The effect of absorber temperature is similar to that of condenser temperature. Fig. C COP values. it cannot operate below -10°C evaporator temperature because of the possibility of crystallization [7]. C Tevp. Variation of CR with evaporator temperature NaSCN and NH3-LiNO3 cycles show better performance than the NH3-H2O cycle. The results show that the ammonia-lithium nitrate and ammonia-sodium thiocyanate cycles give better performance than the ammonia-water 86 10 15 20 30 35 40 45 50 cycle.7 0. Variation of COP with condenser temperature V. 7 illustrates the corresponding comparison of circulation ratios vs condenser temperatures. Variation of CR with condenser temperature Therefore. NH3-LiNO3 and NH3-NaSCN absorption cycles.9 0.3 0. 0 not 5only because of 25 higher Tcond. but also because of no requirement for analyzers and rectifiers. 5.0 NH3-H2O 20 NH3-LiNO3 NH3-NaSCN 0. Generally speaking.2 CR 0 10 20 30 40 50 15 10 5 0.5 0. Issue 3.2 0. Fig. the ammonia-sodium thiocyanate cycle cannot operate at evaporator temperatures below 10°C for the possibility of crystallization.9 0.6 COP 0.7 Fig.4 0. 6 illustrates the comparison of COP values vs condenser temperatures for HN3-H2O. 2007 1.1 0.0 0 Tcond.0 0. However. the performance for the ammonia-lithium nitrate and ammonia-sodium thiocyanate cycles are similar. 7. 30 NH3-H2O NH3-LiNO3 25 NH3-NaSCN 20 0. 6. C Fig. with the latter being slightly better than the former. Increasing condenser temperatures cause a decrease in system performance for each cycle. for the NH3NaSCN cycle. 4. It is hoped that these results can serve as a source of reference for comparison in developing new cycles and new working .5 0.4 0. Vol. Alternative refrigerant-absorption pairs are being developed for improving system performance. both the NH31. they are suitable alternatives to the ammoniawater cycle.INTERNATIONAL JOURNAL OF ENERGY. Variation of COP with evaporator temperature Fig. The ammonia-water absorption cycle is mainly used for refrigeration temperatures below 0°C.6 16 12 COP 0.3 0.8 0. CONCLUSIONS Detailed thermodynamic design data and optimum results to compare the performance of ammonia-water. The still circulation ratio for the NH3-NaSCN cycle is higher than for the other two cycles. however. For condenser temperatures ranging from 20 C to 40 C.0 -20 -15 -10 -5 0 5 10 15 20 CR 8 4 0 -20 -15 -10 -5 0 5 10 15 20 Tevp. The advantages for using the NH3-NaSCN and NH3LiNO3 cycles are very similar. No. E.. 1981. No. 231236. D. pp. Heat Recovery Systems & CHP.pp. Houston.32. pp. 15. [11] Sun.211-221. Vol. No. Vol. Da-Wen. 357-368. 1984. Vol. P. 1998. pp. Energy. 6. 135-143. [8] Rogdakis. TX. Vol. p. T. [7] Infante Ferreira. Chapter 40. Solar Energy.INTERNATIONAL JOURNAL OF ENERGY. 477-484.9. E. 1995. and Bugarel. E. A. Vol. A. A. Thermodynamic design data and optimum design maps for absorption refrigeration systems. pp.17. [4] Sun. 1. Applied Thermal Engineering. 204-212. GA 30329.2. N. R. Thermodynamic design data for absorption heat pump systems operating on ammonia-lithium nitrate. Ammonia Absorption Refrigeration in Industrial Processes. 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