International Journal of Mechanical and Materials Engineering (IJMME), Vol. 3 (2008), No.1,

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1 Internatonal Journal of Mechancal and Materals Engneerng (IJMME, Vol. (008, No.1, OPTIMUM DESIGN FOR SOLAR ABSORPTION REFRIGERATION SYSTEMS AND COMPARISON OF THE PERFORMANCES USING AMMONIA-WATER, AMMONIA- LITHIUM NITRATE AND AMMONIA-SODIUM THIOCYANATE SOLUTIONS ABSTRACT J. M. Abdulateef, K. Sopan and M. A. Alghoul Department of Mechancal and Materals Engneerng Unversty Kebangsaan, 400 Bang, Selangor, MALAYSIA Solar absorpton refrgeraton systems ncreasngly attract research nterests. The most common cycles are H O-LBr and NH -H O, absorpton machnes that have served as standards for comparson n studyng and developng new cycles and new refrgerant-absorbent pars. In recent years, research has been devoted to mprovement of the performance of ammona-water absorpton refrgeraton systems. In the present study, thermodynamc propertes for ammona-water, ammona-lthum ntrate and ammonasodum thocyanate solutons are compled and are used n cycle smulaton. Detaled thermodynamc desgn data and optmum desgn results are presented. Thermodynamc propertes of these bnary fluds are expressed n polynomal equatons. The performances of these three cycles aganst varous generator, evaporator, absorber and condenser temperatures are compared. The results show that the ammona-lthum ntrate and ammona-sodum thocyanate cycles gve better performance than the ammona-water cycle, not only because of hgher COP values, but also because of no requrement for analyzers and rectfers. The ammona-sodum thocyanate cycle cannot operate at evaporator temperatures below -10 C for the possblty of crystallzaton. These results form a source of reference for developng new cycles and searchng for new refrgerant-absorbent pars. They can also be used n selectng operatng condtons for exstng systems and achevng automatc control for mantanng optmum operaton of the systems. Keywords: Solar energy; NH -H O; Performance; Lthum ntrate; Absorpton; Refrgeraton. NOTATION COP Coeffcent of performance CR Crculaton rato E Effectveness h Enthalpy (kj/kg m Mass flow rate (kg/s P Pressure (kpa Q Thermal energy (kw X Ammona mass fracton n soluton T Temperature (K W Work nput to pump (kw Greek v Specfc volume (m /kg Densty (kg/m Subscrpts abs Absorber cond Condenser evp Evaporator ex Soluton heat exchanger gen Generator l Lqud me Mechancal v Vapor 1. INTRODUCTION Durng the last few decades, an ncreasng nterest, based on research and development, has been concentrated on utlzaton of non-conventonal energy sources, namely solar energy, wnd energy, tdal waves, bogas, geothermal energy, hydropower, hydrogen energy, etc. Among these sources, solar energy, whch s an energy source for coolng applcatons, s a hghly popular source due to the followng facts: drect and easy usablty, renewable and contnuty, mantanng the same qualty, beng safe, beng free, beng envronment frendly and not beng under the monopoly of anyone. The absorpton refrgeraton system, whch has some advantages, such as slent operaton, hgh relablty, long servce lfe, smpler capacty control mechansm, easer mplementaton, and low mantenance, s wdely acknowledged as a prospectve canddate for effcent and economc use of solar energy for coolng applcatons. The bnary systems of NH -H O and H O-LBr were well known as workng flud pars to be appled both n refrgeraton and ar-condtonng. The advantage for refrgerant NH s that t can evaporate at lower temperatures (.e. from -10 to 0 C compared to H O (.e. from 4 to 10 C. Therefore, for refrgeraton, the NH -H O cycle s used. Theoretcal and expermental studes have been conducted to optmze the performance of absorpton refrgeraton cycles usng NH -H O as refrgerant- 17

2 absorbent combnaton have already been reported by varous authors. Uppal et al. (198 bult a small capacty solar-powered NH -H O absorpton refrgerator to store vaccnes n remote locatons. In the same perod, Stacovc (198 developed an ntermttent sngle-stage solar absorpton system for fsh preservaton. Actual thermal COP of could be acheved at generaton and condensaton temperatures of 80 and 4. o C, respectvely. Rogdaks and Antonopoulos (199 studed a two-stage NH -H O absorpton-refrgeraton system, whch may be used to produce refrgeraton temperatures as low as 70 C. The system could be operated at three pressure levels so that the heat released by the hgh-pressure condenser and by the medum-pressure absorber rejected to the ambent, whle heat released by the low-pressure absorber was receved at the medum-pressure evaporator. A method and a correspondng computer code developed to smulate the operaton of the proposed system. For 10 C ambent temperature, the theoretcal coeffcent of performance ranges from 0 to 5%, whle the lowest temperatures range from 70 to 0 C; the correspondng theoretcal refrgeraton load ranges from 00 to 1100 kj/kg of refrgerant produced n the generator. Serra et al. (199 used a solar pond to power an ntermttent absorpton refrgerator wth NH -H O soluton. It s reported that generaton temperatures as hgh as 7 o C and evaporaton temperatures as low as o C could be obtaned. The thermal COP workng under such condtons was n the range of The detaled thermodynamc cycle of the NH -NaSCN absorpton refrgeraton unt was presented by Rogdaks and Antonopoulos (1995. Correlatons developed to express the coeffcent of performance and the coolng capacty n terms of the requred evaporaton temperature and the avalable ambent temperature. A nomograph was also presented, whch showed n a compact form the behavor of the NH -NaSCN system and allowed drect estmaton of the man characterstcs. Bulgan (1995 optmzed of the aqua-ammona absorpton refrgeraton system (ARS n the lght of the frst law of thermodynamcs. The system conssted of an evaporator, a generator, a condenser, a pump, expanson valves and two heat exchangers. A theoretcal model was developed for the ARS. The coeffcent of performance (COP was maxmzed for varous evaporator, condenser and absorber temperatures. Smulatons of H O-LBr and NH -H O absorpton refrgeraton systems carred out by Sun (199. Thermodynamc propertes for H O-LBr and NH -H O soluton was compled and used n cycle smulaton. Detaled thermodynamc desgn data and optmum desgn maps also presented. Horuz (1998 conducted an nvestgaton to analyze the Vapor Absorpton Refrgeraton (VAR systems usng ammona-water and water-lthum bromde solutons. The comparson of the two systems was presented n respect of the coeffcent of performance (COP, the coolng capacty and the maxmum and mnmum system pressures. It was concluded that the VAR system usng water-lthum bromde soluton provded better performance than the system usng ammona-water soluton. Some ponts should be consdered such as; the danger of crystallzaton and mpossblty of operatng n very low temperatures because of the use of water as the refrgerant. Sun (1998 performed a thermodynamc analyses for ammona-water, ammona-lthum ntrate and ammonasodum thocyanate cycles. The performances of these three cycles are compared. It was found that ammona-lthum ntrate and ammona-sodum thocyanate cycles are sutable alternatves to ammona-water absorpton systems. The performance of the ammona sodum thocyanate cycle was slghtly better than that of the ammona-lthum ntrate cycle. Hammad and Habal (000 desgned a solar-powered absorpton refrgeraton cycle usng NH -H O soluton to cool a vaccne cabnet n the Mddle East. A year round smulaton ndcated that thermal COP ranged between 0.5 and 0.5 wth generaton temperature at o C and the cabnet nsde temperature at 0-8 o C. De Francsco et al. (00 developed and tested a prototype of kw NH -H O absorpton system n Madrd for solar-powered refrgeraton n small rural operatons. The test results showed that unsatsfactory operaton of the equpment wth COP lower than In Mexco, a theoretcal study of an ntermttent absorpton refrgeraton system carred out by CO. Rvera and W. Rvera (00.The desgned system drven by a compound parabolc concentrator operated wth LNO mxture n order to avod a rectfer. The results show that n a typcal Mexco weather, t was possble to produce up to 11.8 kg of ce and the thermal COPs were between 0.15 and 0.4 dependng on the generaton and condenser temperatures. The modellng, thermodynamc smulaton and Second Law analyss of an ammona-water double-effect, doublegenerator absorpton chller was presented by Ben Ezzne et al. (004. Computer smulaton was carred out n order to determne the stream propertes and the amount of heat and work exchanged wth the surroundngs. Also, smulaton results used to study the nfluence of the varous operatng parameters on the performance coeffcent of the chller. Results ndcated that the absorber, soluton heat exchangers, and condenser have the most potental to mprove chller energy effcency. Efforts are beng made to search for better refrgerantabsorbent pars that can mprove system performance and can be alternatves to NH -H O systems. Therefore, n the present study, the comparsons of the performances of NH -H O, NH -LNO and NH -NaSCN absorpton cycles drven by solar energy are performed. It s hoped that these results could serve as a source of reference for desgnng and selectng new absorpton refrgeraton systems, 18

3 developng new refrgerant-absorbent pars and choosng and optmzng sutable operatng condtons.. CYCLE OPERATION PRINCIPLES The absorpton cycle powered by solar energy s llustrated n Fgure 1. Low-pressure refrgerant vapor from the evaporator s absorbed by the lqud strong soluton n the absorber. The pump receves low-pressure lqud weak soluton from the absorber, elevates the pressure of the weak soluton and delvers t to the generator (ASHRAE, In the generator, heat from a hgh-temperature source by solar energy drves off the refrgerant vapor n the weak soluton. The lqud strong soluton returns to the absorber through a throttlng valve whose purpose s to provde a pressure drop to mantan the pressure dfference between the generator and the absorber. The hgh-pressure refrgerant vapor condenses nto lqud n the condenser and enters the evaporator through a throttlng valve, mantanng the pressure dfference between the condenser and the evaporator. In order to mprove cycle performance, a soluton heat exchanger s normally added to the cycle, as shown n Fgure. 1, t s an energy savng component but not essental to the successful operaton of the cycle. The soluton heat exchanger s mportant for the heat recovery. Wthout heat recovery, the COP values would be much lower. The heat flow pattern to and from the four heat exchange components n the absorpton cycle s that hghtemperature heat enters the generator, whle lowtemperature heat from the substance beng refrgerated enters the evaporator. The heat rejecton from the cycle occurs at the absorber and condenser at temperatures such that the heat can be rejected to the atmosphere. The above descrpton s typcal for a H O-LBr cycle. However, for the NH -H O absorpton cycle, t not only conssts of all the components prevously descrbed, but also a rectfer and dephlegmator. The need for the latter two s occasoned by the fact that water s volatle. When ammona s evaporated off the generator, t also contans some water vapor. When ths water enters the evaporator, t elevates the evaporatng temperature, t may also freeze along the ppelnes. To remove as much water vapor as possble, the vapor drven off at the generator frst flows countercurrent to the ncomng soluton n the rectfer, next, the soluton passes through the dephlegmator and condenses some water-rch lqud dranng back to the rectfer. By ths process, only a small amount of water vapor escapes the dephlegmator and passes from the evaporator to the absorber. For the current study, t s assumed that refrgeraton vapor contans 100% ammona n smulaton for compatblty of results for NH -H O, NH -LNO and NH -NaSCN cycles. The cycle performance s measured by the coeffcent of performance (COP, whch s defned as the refrgeraton rate over the rate of heat addton at the generator plus the work nput to the pump, that s Qevp COP (1 Q W gen me Auxlary Heater Solar Collector Controller Storage Tank Generator 8 Heat Exchanger 7 1 Condenser 9 10 Absorber 5 4 Evaporator Fgure 1 The schematc llustraton of the solar absorpton refrgeraton system In order to use equaton (1, mass and energy conservaton should be determned at each component. For the generator, the mass and energy balances yeld: m m ( 7 1 m8 7 X 7 m1 X1 m8 X 8 m ( Q gen m h (4 1 1 m8h8 m7h7 From equatons ( and (, the flow rates of the strong and weak solutons can be determned: X1 X 8 m7 m1 (5 X X

4 m X X ( m1 X 7 X 8 The mass flow rato of the system, crculaton rato, s defned as the mass flow rate of soluton from the absorber to the generator to the mass flow rate of workng flud (refrgerant, that m7 CR (7 m1 The energy balance for the soluton heat exchanger s as follows: T E T ( 1 E T (8 9 ex ex 8 m8 h7 h ( h8 h9 m (9 The energy ncrease by pumpng s h h ( P P v ( W me ( P P5 v (11 Fnally, energy balances for the absorber, condenser and evaporator yeld Q abs Q cond Q evp m h (1 4 4 m10h10 m5h5 m1 ( h1 h 1( h4 h (1 m (14. SOLUTION PROPERTIES The thermodynamc propertes for NH -H O, NH -LNO and NH -NaSCN solutons are pressure, temperature, concentraton, enthalpy and densty; these propertes are nterdependent and are necessary for computer smulaton of absorpton refrgeraton systems. For NH -H O, NH - LNO and NH -NaSCN absorpton refrgeraton cycles, NH s the refrgerant, H O, LNO and NaSCN are absorbents. The thermodynamc propertes at outlet of generator to nlet of absorber n Fgure. 1 are determned by NH, and other propertes can be calculated based on the bnary mxture of NH -H O, NH -LNO or NH -NaSCN solutons..1 REFRIGERANT NH In the usual ranges of pressure and temperature concernng refrgeraton applcatons, the two phase equlbrum pressure and temperature of the refrgerant NH are lnked by the relaton: P ( T 10 a ( T 7.15 (15 0 The specfc enthalpes of saturated lqud and vapor NH are expressed n terms of temperature as follows: h ( T b ( T 7.15 (1 l 0 0 h ( T c ( T 7.15 (17 v The coeffcents are lsted n Table 1(Sun, 1998, see Appendx A... NH -H O SOLUTION The relaton between saturaton pressure and temperature of an ammona-water mxture s gven as (Bourseau and Bugarel, 198: B LogP A (18a T A X 0.98X 0.X (18b B X X 194.7X (18c The relaton among temperature, concentraton and enthalpy s as follows, wth coeffcents gven n Table (Patek and Klomfar, 1995, see Appendx A. 1 T m n h( T, X 100 a ( 1 X (19a X s the ammona mole fracton and s gven as follows: X X X 17.0(1 X (19b The relaton among specfc volume, temperature and concentraton s gven as, wth the ftted coeffcents lsted n Table (Sun, 1998, see Appendx A. v j ( T, X aj ( T 7.15 X (0 j NH -LNO SOLUTION The relaton between saturaton pressure and temperature of an ammona-lthum ntrate mxture s gven as (Infante Ferrera, 1984: B LnP A (1a T A (1 X (1b B (1 X (1c The relaton among temperature, concentraton and enthalpy s as follows (Infante Ferrera, 1984: h( T, X A B( T 7.15 C( T 7.15 (a D( T 7.15 A (0.54 X f X (b A 15 89( X 0.54 f X (c B X (d C 10 ( X (e D 10 5 (.9X (f The soluton densty s related to concentraton and temperature as (Infante Ferrera, 1984: 0

5 0.5 ( T, X X 1.4( T 7.15 ( 0.009( T NH -NaSCN SOLUTION The relaton between saturaton and temperature of an ammona-sodum thocyanate mxture s gven as (Infante Ferrera, 1984: B LnP A (4a T A X (4b B (1 X (4c The relaton among temperature, concentraton and enthalpy s as follows (Infante Ferrera, 1984: h ( T, X A B( T 7.15 C( T 7.15 (5a D( T 7.15 A X 187.9X 95.7X (5b B X 7.991X.517X (5c C 10 5 (1.55 4X.0X (5d D 10 (.X 10X.X (5e The soluton densty s related to concentraton and temperature as (Infante Ferrera, 1984: ( T, X A B( T 7.15 C( T 7.15 (a A X 5.508X (b 90.07X B.41X 5.455X.174X (c C 10 (5.1X.X 5.4X (d Table 1 Coeffcents of equatons (15-(17, (Sun, 1998 a, equaton (15 b, equaton (1 c, equaton ( x x x x x x x x x x x x x x x x x x x x x10-10 Standard error 1. x x x10 1 Mean devaton 1.5 x x x10 - Table Coeffcents of equatons (19, (Patek and Klomfar, 1995 m n a m n a x x x x x x x x x x x x x x x x10 0 Table Coeffcents of equatons (0, (Sun, 1998 j a j j a j j a j j a j x x x x x x x x x x x x x x x x10-9 1

6 4. RESULTS AND DISCUSSION In order to compare the performance of ammona-water, ammona-lthum ntrate and ammona-sodum thocyanate absorpton cycles, a computer smulaton program was developed based on the above analyses, coupled wth the thermodynamc property equatons (15-(. The cycle operatng condtons are; T gen = 90 C, T cond =T abs = 5 C and T evp = -5 C wth mass flow rate of refrgerant, m 1 =0.01 Kg/s. Effectveness of the soluton heat exchanger of 80%. Snce ammona s the refrgerant n these three cycles, hgh operatng pressures are observed. It can be seen that the man energy consumpton occurs at the generator and that the mechancal work requred for the pump s very small and can be omtted for general calculatons or when nformaton on soluton densty s not avalable. Snce the energy suppled to the evaporator s the same for these three cycles, the energy consumpton at the generator determnes the level of COP values. Fgure shows the comparson of COP values vs generator temperatures for NH -H O, NH -LNO and NH -NaSCN absorpton cycles. The COP values for these three cycles ncrease wth generator temperatures. There exsts a low generator temperature lmt for each cycle. Each cycle cannot be operated at generator temperatures lower than ts lmt. For the NH -LNO cycle a lower generator temperature can be used than for the others. Ths s an mportant pont for utlzng solar energy snce flud temperatures for flat plate solar collectors are generally below 90 C. It s shown that, for generator temperatures hgher than 80 C, the NH -NaSCN cycle gves the best performance, and the NH -H O cycle has the lowest COP. However, the dfferences among them are not very remarkable. For low generator temperatures, the NH - LNO cycle gves the best performance. Fgure shows the correspondng comparson of crculaton ratos vs generator temperatures. It s llustrated that the crculaton rato for the NH -NaSCN cycle s hgher than for the other two cycles COP 0.5 CR Tgen, C Fgure Varaton of COP wth generator temperature Tgen, C Fgure Varaton of CR wth generator temperature NH-HO NH-LNO 0.8 NH-NaSCN COP 0.5 CR Tevp, C Tevp, C Fgure 4 Varaton of COP wth evaporator temperature Fgure 5 Varaton of CR wth evaporator temperature 0

7 Ths means that ether the soluton pump needs to run faster or a bgger pump s requred. If the generator temperature approaches ts low temperature lmt, the crculaton rato ncreases dramatcally. Therefore, t s hghly mpractcal to operate a cycle at a generator temperature too low, although t s stll operable as shown n Fgures and. Fgure 4 gves the comparson of COP values vs evaporator temperatures for NH -H O, NHa-LNO and NH -NaSCN absorpton cycles. Wth the ncrease n evaporator temperature, the COP values for each cycle ncrease. For evaporator temperatures lower than zero, whch s the temperature range for refrgeraton, the NH - NaSCN cycle gves the best performance, and the NH - H O cycle has the lowest COP values. However, for hgh evaporator temperature, the performance of the NH -H O cycle s better than that of the NH -LNO cycle. The correspondng comparson of crculaton ratos vs evaporator temperatures s gven n Fgure 5. Agan, t s shown that the crculaton rato for the NH -NaSCN cycle s hgher than the other two cycles. Fgure llustrates the comparson of COP values vs absorber temperatures for NH -H O, NH -LNO and NH - NaSCN absorpton cycles. Increasng absorber temperatures cause a decrease n system performance for each cycle. Fgure 7 llustrates the correspondng comparson of crculaton ratos vs absorber temperatures. The crculaton rato for the NH -NaSCN cycle s stll hgher than for the other two cycles COP 0.5 CR The comparson of COP values vs condenser temperatures for NH -H O, NH -LNO and NH -NaSCN absorpton cycles s shown n Fgure 8. The effect of condenser temperature s smlar to that of absorber temperature. For condenser temperatures rangng from 0 o C to o 40 C, both the NH -NaSCN and NH -LNO cycles show better performance than the NH -H O cycle Tabs, C Fgure Varaton of COP wth absorber temperature 0 Also,, for low condenser temperatures, the COP values for the NH -NaSCN cycle are the hghest, whle for hgh condenser temperatures, the NH -LNO cycle has the hghest COP values. The correspondng comparson of crculaton ratos vs condenser temperatures s gven n Fgure 9. Generally speakng, the condenser and absorber temperatures should be at a smlar level Tabs, C Fgure 7 Varaton of CR wth absorber temperature NH-HO NH-LNO NH-NaSCN COP 0.5 CR Tcond, C Fgure 8 Varaton of COP wth condenser temperature Tcond, C Fgure 9 Varaton of CR wth condenser temperature

8 The results of Fgures -9 show that the system performance for the NH -NaSCN and NH -LNO cycles s better than that for the NH -H O cycle; however the mprovement s not very remarkable. Consderng the fact that, for the NH -NaSCN and NH -LNO cycles, no analyzers and rectfers are needed, these two cycles are sutable alternatves to the NH -H O cycle. For the NH -NaSCN cycle, t cannot operate below - 10 C evaporator temperature because of the possblty of crystallzaton (Infante Ferrera, CONCLUSIONS In ths study, an alternatve refrgerant-absorpton pars are beng developed for mprovng system performance. Ammona-water, ammona-lthum ntrate and ammonasodum thocyanate absorpton refrgeraton cycles are analyzed, wth ther thermodynamc propertes expressed n polynomal equatons. The results are calculated usng a computer smulaton program whch developed and coupled wth the thermodynamc propertes of theses solutons. The performances of these three cycles aganst varous generator, evaporator, absorber and condenser temperatures are compared. The results show that the ammona-lthum ntrate and ammona-sodum thocyanate cycles gve better performance than the ammona-water cycle, not only because of hgher COP values, but also because of no requrement for analyzers and rectfers. Therefore, they are sutable alternatves to the ammona-water cycle. Generally speakng, the performance for the ammona-lthum ntrate and ammona-sodum thocyanate cycles are smlar, wth the latter beng slghtly better than the former. However, the ammonasodum thocyanate cycle cannot operate at evaporator temperatures below -10 C for the possblty of crystallzaton. It s hoped that these results can serve as a source of reference for comparson n developng new cycles and new refrgerantabsorbent pars. REFERENCES ASHRAE, ASHRAE Handbook, Refrgeraton Systems and Applcatons. Chapter 40, p ASHRAE, 1791 Tulle Crcle, N. E., Atlanta, GA 09. Bulgan, A. T., Optmzaton of the thermodynamc model of aqua-ammona absorpton refrgeraton systems. Energy Converson Management, Vol., No., pp Ben Ezzne, N., Barhoum, M., Mejbr, Kh., Chemkh, S. and Bellag, A., 004. Solar coolng wth the absorpton prncple: frst and Second Law analyss of an ammona-water doublegenerator absorpton chller. Desalnaton, Vol. 18, pp Bourseau, P. and Bugarel, R., 198. Absorpton-dffuson machnes: comparson of the performances of NH -H O and NH -NaSCN. Internatonal Journal of Refrgeraton, Vol. 9, pp De Francsco, A., Illanes, R., Torres, JL., Castllo, M., De Blas, M. and Preto, E., 00. Development and testng of a prototype of low-power water-ammona absorpton equpment for solar energy applcatons. Renewable Energy, Vol. 5, pp Horuz, I., A comparson between ammona-water and water-lthum bromde solutons n vapor absorpton refrgeraton systems. Int. Comm. Heat Mass Transfer, Vol. 5, No. 5, pp Hammad, M. and Habal, S., 000. Desgn and performance study of a solar energy powered vaccne cabnet. Appl Therm Eng,Vol. 0, pp Infante Ferrera, C. A., Thermodynamc and physcal property data equatons for ammona-lthum ntrate andammona-sodum thocyanate solutons. Solar Energy, Vol., No., pp. 1-. Pa Patek, J. and Klomfar, J., Smple functons for fast calculatons of selected thermodynamc propertes of the ammona-water system. Int. J. Refrg, Vol. 18, No. 4, pp Rogdaks, E. D. and Antonopoulos, K. A., 199. Performance of a low- temperature NH -H O absorptonrefrgeraton system. Energy, Vol. 17, No. 5, pp Rogdaks, E. D. and Antonopoulos, K. A., Thermodynamc cycle, correlatons and nomograph for NH -NaSCN absorpton refrgeraton systems. Heat Recovery Systems & CHP, Vol. 15, No., pp Rvera, CO. and Rvera, W., 00. Modelng of an ntermttent solar absorpton refrgeraton system operatng wth ammona/lthum ntrate mxture. Sol Energy Mater Sol Cells, Vol. 7, pp Stacovc, MD., 198. An autonomous solar ammona-water refrgeraton system. Solar Energy, Vol., pp Serra, FZ., Best, R. and Holland, FA., 199. Experments on an absorpton refrgeraton system powered by a solar pond. Heat Recovery Systems & CHP, Vol. 1, pp Sun, Da-Wen, 199. Thermodynamc desgn data and optmum desgn maps for absorpton refrgeraton systems. Appled Thermal Engneerng, Vol. 17, No., pp Sun, Da-Wen, Comparson of the performances of NH -H O, NH -LNO and NH -NaSCN absorpton refrgeraton systems. Energy Convers. Mgmt, Vol. 9, No. 5/, pp Uppal, AH., Norton, B. and Probert, SD., 198. A lowcost solar-energy stmulated absorpton refrgerator for vaccne storage. Appl Energy, Vol. 5, pp