Solar Absorption Refrigeration System Using New Working Fluid Pairs

Transcription

1 INERNAIONAL JOURNAL OF ENERGY, Issue, Vol., Solar Absorption Refrigeration Syste Using New Working Fluid Pairs Jasi M. Abdulateef, Kaaruzzaan Sopian, M. A. Algoul, Mod Yusof Sulaian, Azai Zaari and Ibrai Aad Abstract- Absorption refrigeration systes powered by solar energy increasingly attract researc interests in te last years. In tis study, terodynaic analyses for different working fluid pairs are perfored. A coputer siulation odel as been developed to predict te perforance of solar absorption refrigeration syste using different working fluid. e odel is based on detailed ass and energy balance and eat and ass transfer for te cycle coponent. Detailed terodynaic properties for aonia- water, aonia-litiu nitrate and aonia-sodiu tiocyanate are expressed in polynoial equations and used in cycle siulation. e perforances of tese tree cycles against various generator, evaporator, and condenser teperatures are copared. e results sow tat te aonia-litiu nitrate and aonia-sodiu tiocyanate cycles give better perforance tan te aonia-water cycle. e aonia-sodiu tiocyanate cycle cannot operate at evaporator teperatures below - C for te possibility of crystallization. Increasing condenser teperatures cause a decrease in syste perforance for eac cycle. Wit te increase in evaporator teperature, te values for eac cycle increase. ese results can serve as a source of reference for developing new cycles and searcing for new working fluids pairs. ey can also be used in selecting operating conditions for existing systes and acieving autoatic control for aintain optiu operation of te syste. Keywords- perforance; absorption; solar energy; NH - LiNO ; crystallization; refrigeration; generator I. INRODUCION URING te last few decades, an increasing interest, D based on researc and developent, as been concentrated on utilization of non-conventional energy sources, naely solar energy, wind energy, tidal waves, biogas, geoteral energy, ydropower, ydrogen energy, etc. Aong tese sources, solar energy, wic is an energy source for cooling applications, is a igly popular source due to te following facts: direct and easy usability, renewable and continuity, aintaining te sae quality, being safe, being free, being environent friendly and not being under te onopoly of anyone. Manuscript received April, ; Revised version received October,. Autors are wit Solar Energy Researc Institute, University Kebangsaan Malaysia, 4 Bangi, Selangor, MALAYSIA e absorption refrigeration syste, wic as soe advantages, suc as silent operation, ig reliability, long service life, sipler capacity control ecanis, easier ipleentation, and low aintenance, is widely acknowledged as a prospective candidate for efficient and econoic use of solar energy for cooling applications. Also, te absorption refrigeration cycle is usually a preferable alternative, since it uses te teral energy collected fro te sun witout te need to convert tis energy into ecanical energy as required by te vapor copression cycle. In addition, te absorption cycle uses teral energy at a lower teperature tan tat dictated by te vapor copression cycle. e binary systes of NH - H O and LiBr-H O were well known as working fluid pairs to be applied bot in absorption eat pups and in absorption refrigerators currently. eoretical and experiental studies ave been conducted to optiize te perforance of absorption refrigeration cycles using NH - H O and LiBr-H O as refrigerant- absorbent cobination. e advantage for refrigerant NH is tat it can evaporate at lower teperatures (i.e. fro - to C copared to H O (i.e. fro 4 to C. erefore, for refrigeration, te NH -H O cycle is used. Researc as been perfored for NH -H O systes teoretically [-4] and experientally [5, ]. ese studies sow tat te NH -H O syste exibits a relatively low. Efforts are being ade to searc for better working fluid pairs tat can iprove syste perforance. It is proposed tat NH - LiNO and NH -NaSCN cycles can be alternatives to NH - H O systes [, ].erefore, in te present study, te coparisons of te perforances of NH -H O, NH -LiNO and NH -NaSCN absorption cycles driven by solar energy are perfored. It is oped tat tese results could serve as a source of reference for designing and selecting new absorption refrigeration systes, developing new working fluid pairs and optiizing suitable operating conditions. II. CYCLE PERFORMANCE ANALYSIS e absorption cycle powered by solar energy is illustrated in Fig.. Low-pressure refrigerant vapor fro te evaporator is absorbed by te liquid strong solution in te absorber. e pup receives low-pressure liquid weak solution fro te absorber, elevates te pressure of te weak solution and delivers it to te generator. By weak solution (strong solution is eant tat te ability of te solution to absorb te refrigerant vapor is weak (strong according to ASHRAE definition [9]. In te generator, eat fro a ig-teperature source by solar energy drives off te refrigerant vapor in te weak solution. e liquid strong solution returns to te absorber troug a trottling valve wose purpose is to provide a pressure drop to

2 INERNAIONAL JOURNAL OF ENERGY, Issue, Vol., aintain te pressure difference between te generator and te absorber. e ig-pressure refrigerant vapor condenses into liquid in te condenser and enters te evaporator troug a trottling valve, aintaining te pressure difference between te condenser and te evaporator. In order to iprove cycle perforance, a solution eat excanger is norally added to te cycle, as sown in Fig.. e cycle Auxiliary Heater perforance is easured by te coefficient of perforance (, wic is defined as te refrigeration rate over te rate of eat addition at te generator plus te work input to te pup, tat is Q Qevp + W = ( gen e Solar Collector Controller Storage ank Generator Heat Excanger Condenser 9 Absorber 5 4 Evaporator Fig.. e sceatic illustration of te solar absorption refrigeration syste In order to use equation (, ass and energy conservation sould be deterined at eac coponent. For te generator, te ass and energy balances yield: + + Q gen + = ( = ( = (4 Fro equations ( and (, te flow rates of te strong and weak solutions can be deterined: = (5 = ( e ass flow ratio of te syste, circulation ratio, is defined as te ass flow rate of solution fro te absorber to te generator to te ass flow rate of working fluid (refrigerant, tat CR = ( e energy balance for te solution eat excanger is as follows: = E + ( E ( 9 ex ex = + ( 9 (9 e energy increase by puping is = + ( P P v ( = ( 5 5 W e ( P P5 v Finally, energy balances for te absorber, condenser and evaporator yield Q abs Q cond Q evp = + 55 ( = ( ( = ( ( III. SOLUION PROPERIES e terodynaic properties for NH -H O, NH - LiNO and NH -NaSCN solutions are pressure, teperature, concentration, entalpy and density, tese properties are interdependent and are necessary for coputer siulation of absorption refrigeration systes. For NH -H O, NH -LiNO and NH -NaSCN absorption refrigeration cycles, NH is te refrigerant, H O, LiNO and NaSCN are absorbents. e terodynaic properties at outlet of generator to inlet of absorber in Fig. are deterined by NH, and oter properties can be calculated

3 based on te binary ixture of NH -H O, NH -LiNO or NH -NaSCN solutions.. Refrigerant NH In te usual ranges of pressure and teperature concerning refrigeration applications, te two pase equilibriu pressure and teperature of te refrigerant NH are linked by te relation: i P ( = a i (.5 (5 e specific entalpies of saturated liquid and vapor NH are expressed in ters of teperature as follows: i l ( = bi (.5 i v ( = ci(. 5 ( (. NH -H O solution e relation between saturation pressure and teperature of an aonia-water ixture is given as []: LogP B A = ( A = (9 B = ( e relation aong teperature, concentration and entalpy is as follows; i ni (, = ai ( (. is te aonia ole fraction and is given as follows:.5 =.5 +.( ( e relation aong specific volue, teperature and concentration is given as; v(, INERNAIONAL JOURNAL OF ENERGY, Issue, Vol., = j= a ( ij i j. 5 (. NH -LiNO solution e relation between saturation pressure and teperature of an aonia-litiu nitrate ixture is given as []: B LnP = A + (4 A 9 59 B = 49( ( =. +. ( (5 e relation aong teperature, concentration and entalpy is as follows []: (, = A+ B( 5. + C( 5. + D( 5. ( A = 5+ 5(.54 if. 54 ( A = 5 + 9(.54 if. 54 (9 B = ( C = ( ( D = 5 (.9 ( e solution density is related to concentration and teperature as []:. 5 ρ(, = (. 5. 9(. 5 (.4 NH -NaSCN solution e relation between saturation and teperature of an aonia-sodiu tiocyanate ixture is given as []: B LnP = A + (4 A = (5 B = ( ( e relation aong teperature, concentration and entalpy is as follows []: (, = A+ B( 5. + C( 5. + D( 5. ( A = ( B = (9 C = ( (4 5 D = (. +. (4 e solution density is related to concentration and teperature as []: ρ (, = A+ B(.5 + C(.5 (4 A= (4 B = (44 4

4 INERNAIONAL JOURNAL OF ENERGY, Issue, Vol., C = ( All coefficients of equations are listed by Sun []. (45 Absorber sol inlet IV. RESULS AND DISCUSSION In order to provide details optiu operating conditions for solar absorption refrigeration systes, te coputer progra was used to searc for different operation conditions wit wic an absorption cycle reaces its axiu perforance. able sows te coparison of te various terodynaic states in te cycle operating at gen = C, cond = C, abs = 5 C and evp = -5 C, wit te effectiveness of te solution eat excanger of %. As a result, a bigger pup is needed for te NH -NaSCN cycle. Fig. sows te coparison of values vs generator teperatures for NH -H O, NH -LiNO and NH -NaSCN absorption cycles. e values for tese tree cycles increase wit generator teperatures. For te NH -LiNO cycle a lower generator teperature can be used tan for te oters. It is sown tat, for generator teperatures iger tan C, te NH -NaSCN cycle gives te best perforance, and te NH -H O cycle as te lowest CR gen, C Fig.. Variation of wit generator teperature e total solution aounts circulated are.5,.95 and 5. kg/in for NH -H O, NH -LiNO and NH -NaSCN respectively. is eans tat ore refrigerant can be boiled off in te generator for te NH -H O cycle tan for te oter two. able.erodynaic properties at various states in absorption cycles driven by solar energy Fluid state, o C P, Kpa %, Kg/in NH -H O cycle Generator ref exit Condenser ref exit Evaporator ref exit Absorber sol. exit Generator sol inlet 5..5 Generator sol exit.5.5 Absorber sol inlet NH -LiNO cycle Generator ref exit Condenser ref exit Evaporator ref exit Absorber sol. exit Generator sol inlet Generator sol exit.5. Absorber sol inlet NH -NaSCN cycle Generator ref exit Condenser ref exit Evaporator ref exit Absorber sol. exit Generator sol inlet Generator sol exit gen, C Fig.. Variation of CR wit generator teperature However, te differences aong te are not very rearkable. Fig. sows te corresponding coparison of circulation ratios vs generator teperatures. It is illustrated tat te circulation ratio for te NH -NaSCN cycle is iger tan for te oter two cycles. is eans tat eiter te solution pup needs to run faster or a bigger pup is required. Fig. 4 gives te coparison of values vs evaporator teperatures for NH -H O, NH -LiNO and NH -NaSCN absorption cycles. Wit te increase in evaporator teperature, te values for eac cycle increase. For evaporator teperatures lower tan zero, wic is te teperature range for refrigeration, te NH -NaSCN cycle gives te best perforance, and te NH -H O cycle as te lowest values. For ig evaporator teperature, te perforance of te NH -H O cycle is better tan tat of te NH -LiNO cycle. e corresponding coparison of circulation ratios vs evaporator teperatures is given in Fig. 5. Again, it is sown tat te circulation ratio for te NH -NaSCN cycle is iger tan te oter two cycles. 5

5 INERNAIONAL JOURNAL OF ENERGY, Issue, Vol.,..9.. NH-HO NH-LiNO NH-NaSCN..5 CR evp, C Fig. 4. Variation of wit evaporator teperature Fig. illustrates te coparison of values vs condenser teperatures for HN -H O, NH -LiNO and NH -NaSCN absorption cycles. Increasing condenser teperatures cause a decrease in syste perforance for eac cycle. Fig. illustrates te corresponding coparison of circulation ratios vs condenser teperatures. e circulation ratio for te NH -NaSCN cycle is still iger tan for te oter two cycles. For condenser teperatures ranging fro C to 4 C, bot te NH evp, C Fig. 5. Variation of CR wit evaporator teperature NaSCN and NH -LiNO cycles sow better perforance tan te NH -H O cycle. e effect of absorber teperature is siilar to tat of condenser teperature. e advantages for using te NH -NaSCN and NH - LiNO cycles are very siilar, owever, for te NH - NaSCN cycle, it cannot operate below - C evaporator teperature because of te possibility of crystallization [] NH-HO NH-LiNO NH-NaSCN...5 CR cond, C Fig.. Variation of wit condenser teperature cycle, not only because cond, of iger C values, but also because Fig.. of Variation no requireent of CR wit for analyzers condenser and teperature rectifiers. V. CONCLUSIONS Detailed terodynaic design data and optiu results to copare te perforance of aonia-water, aonia-litiu nitrate and aonia-sodiu tiocyanate solar absorption cycles are presented. e results are calculated using coputer progra based on terodynaic properties data for te working fluids. e aonia-water absorption cycle is ainly used for refrigeration teperatures below C. Alternative refrigerant-absorption pairs are being developed for iproving syste perforance. e results sow tat te aonia-litiu nitrate and aonia-sodiu tiocyanate cycles give better perforance tan te aonia-water erefore, tey are suitable alternatives to te aoniawater cycle. Generally speaking, te perforance for te aonia-litiu nitrate and aonia-sodiu tiocyanate cycles are siilar, wit te latter being sligtly better tan te forer. However, te aonia-sodiu tiocyanate cycle cannot operate at evaporator teperatures below - C for te possibility of crystallization. It is oped tat tese results can serve as a source of reference for coparison in developing new cycles and new working

6 INERNAIONAL JOURNAL OF ENERGY, Issue, Vol., fluid pairs. ese results can be used to select operating conditions for tese cycles and realize autoatic control for aintaining optiu operating of tese systes under different conditions. W Subscripts work input to pup (kw Noenclatures Coefficient of perforance CR Circulation ratio E Effectiveness Entalpy (kj/kg Mass flow rate (kg/s P Pressure (kpa Q eral energy (kw Aonia ass fraction in solution eperature (K abs Absorber cond Condenser evp Evaporator ex Solution eat excanger gen Generator l Liquid e Mecanical v Vapor Greek v Specific volue ( /kg ρ Density (kg/ REFERENCES [] Rogdakis, E. D. and Antonopoulos, K. A., Absorption-diffusion acines: coparison of te perforances of NH -H O and NH - NaSCN, Energy, Vol., No.5, 99, pp [] Bulgan, A.., erodynaic design data for absorption eat pup systes operating on aonia-litiu nitrate, Energy Conversion Manageent, Vol., No., 995,pp [] Sun, Da-Wen, Coputer siulation and optiization of aoniawater absorption refrigeration systes, Energy Sources, Vol. 9, No., 99. [4] Sun, Da-Wen, erodynaic design data and optiu design aps for absorption refrigeration systes, Applied eral Engineering, Vol., No., 99, pp.-. [5] Bogart, M., Aonia Absorption Refrigeration in Industrial Processes, Gulf, Houston,, 9. [] Butz, D. and Stepan, K., Dynaic beavior of an absorption eat pup, International Journal of Refrigeration, Vol., 99, pp. 4-. [] Infante Ferreira, C. A., erodynaic and pysical property data equations for aonia-litiu nitrate and aonia-sodiu tiocyanate solutions, Solar Energy, Vol., No., 94, pp. -. [] Rogdakis, E. D. and Antonopoulos, K. A., erodynaic cycles for refrigeration and eat transforer units H O/LiBr, Heat Recovery Systes & CHP, Vol. 5, No., 995, pp [9] ASHRAE, ASHRAE Handbook, Refrigeration Systes and Applications, Capter 4, p. 4.. ASHRAE, 9 ullie Circle, N. E., Atlanta, GA 9, 994. [] Bourseau, P. and Bugarel, R., Absorption-diffusion acines: coparison of te perforances of NH -H O and NH -NaSCN, International Journal of Refrigeration, Vol.9, 9, pp. -4. [] Sun, Da-Wen, Coparison of te perforances of NH -H O, NH - LiNO and NH -NaSCN absorption refrigeration systes, Energy Convers. Mgt, Vol. 9, No. 5/, 99, pp. 5-.