Optimization of the thermodynamic model of a solar driven Aqua - ammonia absorption refrigeration system

Transcription

1 nd WSEAS/IASME Internatonal Conference on RENEWABLE ENERGY SOURCES (RES') Corfu, Greece, October -, Optmzaton of the thermodynamc model of a solar drven Aqua - ammona absorpton refrgeraton system J. ABDULATEEF, K.SOPIAN, M.YAHYA, A. ZAHARIM and M. ALGHOUL Solar Energy Research Insttute (SERI) Unversty Kebangsaan Malaysa 4 Bang, Selangor MALAYSIA Abstract: - The am of ths study s to smulate a solar sngle effect ammona-water absorpton refrgeraton system. The nfluences of operatng condtons and effectveness of heat exchanger on the thermal loads of components, coeffcents of performance(c, ) and effcency rato () are nvestgated. It s concluded that the c and values ncrease wth ncreasng generator and evaporator temperatures but decrease wth ncreasng condenser and absorber temperatures. The () value vares wth these temperatures. Also, the effectveness of heat exchanger determnes the maxmum temperature that can be used n order to obtan the maxmum out of the system. Key -Words: Effcency rato; effectveness; performance parameters; solar refrgeraton Introducton Durng recent years, research amed at the development of technologes that can offer reductons n energy consumpton, peak electrcal demand and energy costs wthout lowerng the desred level of comfort condtons has ntensfed. By reason that solar refrgeraton technologes have the advantage of removng the majorty of harmful effects of tradtonal refrgeraton machnes and that the peaks of requrements n cold concde most of the tme wth the avalablty of the solar radaton, the development of solar refrgeraton technologes became the worldwde focal pont for concern agan. Theoretcal and expermental studes have been conducted and reported by varous authors to optmze the performance of absorpton refrgeraton cycles usng NH -H O as refrgerantabsorbent. Serra et al. [] used a solar pond to power an ntermttent absorpton refrgerator wth NH -H O soluton. It s reported that generaton temperatures as hgh as o C and evaporaton temperatures as low as o C could be obtaned. The thermal workng under such condtons was n the range of.4-.. Bulgan [] optmzed an aqua-ammona absorpton refrgeraton system (ARS) n the lght of the frst law of thermodynamcs. A theoretcal model was developed for the ARS. The coeffcent of performance () was maxmzed for varous evaporator, condenser and absorber temperatures. Sun [] performed a thermodynamc analyses of dfferent bnary mxtures consdered n absorpton refrgeraton cycle, and the performances were compared. Srkhrn et al. [4] presented a lterature revew on absorpton refrgeraton technology such as varous types of absorpton refrgeraton systems, researches on workng fluds and mprovement of absorpton processes. Ben Ezzne et al. [5] presented the modellng, thermodynamc smulaton and Second Law analyss of an ammona-water double-effect, double generator absorpton chller. Results ndcated that the absorber, soluton heat exchangers, and condenser have the most potental to mprove chller effcency. In the study of Abdulateef et al. [], the thermodynamc propertes of ammona based bnary mxtures (NH -H O, NH -LNO, NH -NaSCN) were gven, and the performances of the cycles were compared. In the present study, a parametrc thermodynamc analyss of a sngle stage solar absorpton refrgeraton system s performed. Coeffcents of system performance (c, ) and effcency rato () are compared at varous operatng temperatures. The nfluence of heat exchanger effectveness on the thermal loads of components, soluton temperatures and performance parameters are also nvestgated. ISSN: ISBN:

2 nd WSEAS/IASME Internatonal Conference on RENEWABLE ENERGY SOURCES (RES') Corfu, Greece, October -, Cycle descrpton Fg. shows the schematc dagram of the solar drven aqua-ammona absorpton refrgeraton cycle. Hgh-pressure lqud refrgerant () from the condenser passes nto the evaporator (4) through an expanson valve () that reduces the pressure of the refrgerant to the low pressure exstng n the evaporator. The lqud refrgerant () vaporzes n the evaporator by absorbng heat from the materal beng cooled and the resultng low-pressure vapor (4) passes to the absorber, where t s absorbed by the strong soluton comng from the generator () through an expanson valve (), and forms the weak soluton (5). The weak soluton (5) s pumped to the generator pressure (). Heat from a hgh-temperature source by solar energy used n the generator to separate the bnary soluton of water and ammona by causng ammona to vaporze. The remanng soluton () flows back to the absorber and, thus, completes the cycle. By weak soluton (strong soluton) s meant that the ablty of the soluton to absorb the refrgerant vapor s weak (strong), accordng to the ASHRAE defnton []. For the current study, t s assumed that the refrgerant vapor leavng the generator s % ammona. In order to mprove cycle performance, a soluton heat exchanger s normally added to the cycle, t s an energy savng component. Solar Collector Controller Auxlary Heater Storage Tank For the generator, the mass and energy balances yeld: m & + () m & X X + X () The flow rates of the weak and strong solutons can be determned from equatons () and (), respectvely: X X m & () X X X X m & (4) X X Generator Soluton Heat Exchanger 9 Fg.. Solar absorpton refrgeraton system 5 Absorber 4 Condenser Evaporator The crculaton rato (CR) can be defned as: CR = (5) The components thermal loads of the ARS are expressed as follows: Q gen h + h h () Q abs 4h4 + h 5h5 () Q cond ( h h) () Q evp ( h4 h) (9) The energy balance for the soluton heat exchanger s as follows: T9 = ε T + ( ε ) T () h = h + ( h h9) () The pump work for the weak soluton leavng the absorber may be expressed as: h = h5 + ( P P5 ) v () W me = ( P P5 ) v () The system performance s measured by the coeffcent of performance (): = (4) + Wme The effcency rato () s defned as the rato of the coeffcent of performance to the Carnot coeffcent of performance (c). The Carnot coeffcent of performance s the maxmum possble coeffcent of performance of an ARS under gven operatng condtons. T T T gen abs evp = c ( ).( ) (5) Tgen T cond Tevp = () C The performance ncrease rato (PIR) of the system usng heat exchanger s expressed as follows: PIR = () Where s the coeffcent of performance wthout heat exchanger (ε = ) and s the coeffcent of performance wth heat exchanger (ε > ). Soluton propertes.. Refrgerant NH In the usual ranges of pressure and temperature concernng refrgeraton applcatons, the two ISSN: ISBN:

3 nd WSEAS/IASME Internatonal Conference on RENEWABLE ENERGY SOURCES (RES') Corfu, Greece, October -, phase equlbrum pressure and temperature of the refrgerant NH are lnked by the relaton: P ( T ) = a ( T.5) () = The specfc enthalpes of saturated lqud and vapor NH are expressed n terms of temperature as follows: h ( T ) = b ( T.5) (9) l = = h ( T ) = c ( T.5) () v All coeffcents of equatons above are lsted by Sun []... NH -H O soluton The relaton between saturaton pressure and temperature of an ammona-water mxture s gven as: B LogP = A () T Where: A =.44.X +.9X +.X () B =. 55.X X 94.X () The relaton among temperature, concentraton and enthalpy s as follows: T n m h( T, X ) = a ( ) X (4). = where X s the ammona mole fracton and s gven as follows:.5x X =.5X +.( X ) (5) The relaton among specfc volume, temperature and concentraton s gven as: j v( T, X ) = a ( T.5) X () j = = j All coeffcents of equatons above are lsted by Sun []. 4 Results and dscusson 4.. The effects of operatng temperatures The effects of the generator, evaporator and condenser temperatures on the thermal loads of the components are shown n Fgs. -4. In these calculatons, the operatng temperatures ranges are selected as follows: T gen =- o C, T evp =-- o C, T cond =-45 o C, T abs =5 o C, ε = 5 % and refrgerant mass flow rate =. Kg/sec. As t can be seen from Fg., when the generator temperature ncreases, the generator and absorber thermal loads (Q gen and Q abs ) decrease. If the generator temperature gets hgher, the concentraton of the soluton leavng the generator decrease, and hence, the CR decreases, as can be seen from equatons ()-(5). Moreover, the weak soluton temperature and, hence, the enthalpy (h ) s ncreased by the strong soluton n the. The generator thermal load s decreased by both decreasng the CR and ncreasng h. The enthalpy of the ammona vapor (h ) leavng the generator decreases wth ncreasng generator temperature, and hence, condenser thermal load (Q cond ) decreasng from 9.9 kw to.9 kw. mal Load, KW Ther 5 4 Tcond=Tabs=5 C, Tevp=-5 C, ε =.5 Qcond 9 Tgen, C Fg.. Varaton of thermal loads wth the generator temperature The evaporator thermal load does not change wth generator temperature and remans as a constant value of. KW. The evaporator temperature affects the low pressure of the system. If the evaporator temperature rses, the concentraton of the weak soluton ncrease whle the CR decrease. They cause a decrease n the absorber thermal load; on the other hand, the decreasng of CR decreases the generator thermal load (Fg. ). A small ncrease n evaporator outlet enthalpy (h 4 ) also causes a small amount of ncrease n the evaporator thermal load (from.5 kw to 9. kw). The condenser thermal load remans unchanged as 9.59 kw. Thermal Load, KW 5 4 Tgen=9 C, Tcond=Tabs=5 C, ε = Tevp, C Fg.. Varaton of thermal loads wth the evaporator temperature Qcond ISSN: ISBN:

4 nd WSEAS/IASME Internatonal Conference on RENEWABLE ENERGY SOURCES (RES') Corfu, Greece, October -, The hgh pressure of the system ncreases, and the concentraton of the strong soluton ncreases when the condenser temperature ncreases. Wth ncreasng strong soluton concentraton, the CR ncreases, and n ths case, the thermal loads of both the generator and absorber ncrease (Fg. 4). The enthalpy of the saturated lqud (h ) leavng the condenser ncreases wth ncreasng condenser temperature. Thus, t causes a small amount of decrease n the condenser and evaporator thermal loads. Thermal Load, KW The varatons of the coeffcents of performance (c and ) and effcency rato () wth operatng temperatures are gven n Fgs. 5-. The hgh c and values are obtaned at hgh generator and evaporator temperatures (Fgs. and ). As s seen from the equaton (5), the performance of the Carnot cycle gets better wth ncreasng generator and evaporator temperatures. Snce the ncrease n c s faster than that n, the value gradually decreases., c and Tgen=9 C, Tevp=-5 C, Tabs=5 C, ε =.5 Fg. 4. Varaton of thermal loads wth the condenser temperature c Qcond 4 5 Tcond, C Tevp=-5 C, Tcond=Tabs=5 C, ε =.5. 9 Tgen, C Fg. 5. Varaton of performance parameters wth the generator temperature, c and c Tgen=9 C, Tcond=Tabs=5 C, ε = Tevp, C Fg.. Varaton of performance parameters wth the evaporator temperature It s seen from Fg., that the c and values decrease wth ncreasng condenser temperature. When the temperatures of the condenser ncrease, the thermal load of the generator rses, and the performance of the system gets worse. Whle the value ncreases wth ncreasng the condenser temperature up to 4 o C, t decreases above 4 o C due to the relatvely rapd decrease of., c and Tgen=9 C, Tevp=-5 C, Tabs=5 C, ε = Tcond, C Fg.. Varaton of performance parameters wth the condenser temperature The effect of absorber temperature s smlar to that of condenser temperature. The generally speakng, the condenser and absorber temperatures should be at a smlar level. 4.. The effects of soluton heat exchanger Fg. shows the varaton of outlet temperature wth heat exchanger effectveness. As known, f the effectveness ncreases, the heat exchange between the weak and strong solutons ncreases, and as a result of ths, the temperature of the strong soluton (T 9 ) decreases and that of the weak soluton (T ) ncreases. Wth an ncrease n the weak soluton temperature enterng the c ISSN: ISBN:

5 nd WSEAS/IASME Internatonal Conference on RENEWABLE ENERGY SOURCES (RES') Corfu, Greece, October -, generator, the heat load of the generator decreases. Smlarly, wth a decrease n the strong soluton temperature enterng the absorber, the heat rejected from the absorber also decreases. For ths reason, decreasng ratos of both generator and absorber thermal loads ncrease wth the effectveness of the (Fg. 9). 9 Tgen=9 C, Tevp=-5 C, Tcond=Tabs=5 C T9 T PIR (%) 5 4 Tgen=9 C, Tevp=-5 C, Tcond=Tabs= 5 C Temperature, C Effectveness of Soluton Heat Exchanger Fg.. Varaton of soluton temperature wth the effectveness of Tgen=9 C, Tevp=-5 C, Tcond=Tabs=5 C Effectveness of Soluton Heat Exchanger Fg.. Varaton of PIR wth the effectveness of The varatons of the coeffcents of performance and effcency rato wth the effectveness of the are shown n Fg.. Whle the c value does not change wth the effectveness and remans at., the value vares between.4 and.9. Ths means that the ncreasng rato s about 5%, as gven n Fg.. The c remans unchanged; the ncreases wth effectveness, and as a result of ths, the value ncreases (Fg. )... Tgen=9 C, Tevp=-5 C, Tcond=Tabs=5 C c Decreasng Thermal Load (%), c and Effectveness of Soluton Heat Exchanger Fg. 9. Varaton of decreasng thermal load wth the effectveness of The effects of the on the system performance are gven n Fg.. If the effectveness of the s zero (ε = ), normally the ncrease rato (PIR) s also zero. The performance of the system gets better wth an ncrease n the effectveness. For the best case condton (ε =, strong soluton outlet temperature equals weak soluton nlet temperature), the value ncreases up to a rato of 5% Effectveness of Soluton Heat Exchanger Fg.. Varaton of performance parameters wth the effectveness of 5 Conclusons From the above study, the followng results can be drawn: The thermal loads of the generator and absorber decrease, as the generator and evaporator temperatures ncrease. The decrease of the generator thermal load ncreases the value. Also, the c value ncreases wth the generator and evaporator temperatures. Snce the ncrease n the s greater than the ncrease n the c up to the generator temperature of o C, the value ncreases. When the generator temperature s above ISSN: ISBN:

6 nd WSEAS/IASME Internatonal Conference on RENEWABLE ENERGY SOURCES (RES') Corfu, Greece, October -, o C, however, the ncreases and the value References: decreases. [] Serra, FZ., Best, R. and Holland, FA., 99. The thermal loads of the generator and absorber Experments on an absorpton refrgeraton ncrease as the condenser and absorber system powered by a solar pond. Heat temperatures ncrease. The ncrease of the generator thermal load decreases the value. The c Recovery Systems & CHP, Vol., pp decreases wth the condenser and absorber temperatures. The value ncreases up to the condenser and absorber temperature values of about 4 o C, and then t decreases wth hgher temperatures. The ncrease of the effectveness decreases the generator and absorber thermal loads. The decrease rato n the thermal loads of these components reaches 4%. As expected, the decrease n the generator thermal load leads to an ncrease n the performance and effcency of system. The ncreases from.4 to.9 wth use of the. In ths case, the maxmum ncrease n the s 5%. Notaton ARS Absorpton refrgeraton system Coeffcent of performance CR Crculaton rato h Enthalpy (kj/kg) Mass flow rate (kg/s) P Pressure (kpa) PIR Performance ncrease rato Q Thermal energy (kw) Soluton heat exchanger T Temperature (K) X Ammona mass fracton n soluton W Work nput to pump (kw) Greek ε v Effectveness of heat exchanger Effcency rato of the system Specfc volume (m /kg) Subscrpts abs Absorber c Carnot cond Condenser evp Evaporator gen Generator l Lqud me Mechancal v Vapor... State ponts [] Bulgan, A. T., 995. Optmzaton of the thermodynamc model of aqua-ammona absorpton refrgeraton systems. Energy Converson Management, Vol., No., pp [] Sun, Da-Wen, 99. Comparson of the performances of NH -H O, NH -LNO and NH -NaSCN absorpton refrgeraton systems. Energy Convers. Mgmt, Vol. 9, No. 5/, pp. 5-. [4] Srkhrn P., Aphornratana S. and Chungpabulpatana S.,. A revew of absorpton refrgeraton technologes. Renew Sust Energ Rev, Vol. 5, pp. 4-. [5] Ben Ezzne, N., Barhoum, M., Mejbr, Kh., Chemkh, S. and Bellag, A., 4. Solar coolng wth the absorpton prncple: frst and Second Law analyss of an ammona-water double generator absorpton chller. Desalnaton, Vol., pp [] Jasm M. Abdulateef, Kamaruzzaman Sopan, M. A. Alghoul, Mohd Yusof Sulaman, Azam Zaharm and Ibrahm Ahmad.,. Solar absorpton refrgeraton system usng new workng flud pars. Internatonal Journal of Energy, Vol., Issue, pp. -. [] ASHRAE, ASHRAE Handbook, Refrgeraton Systems and Applcatons, Chapter 4, p. 4.. ASHRAE, 9 Tulle Crcle, N. E., Atlanta, GA 9, 994. ISSN: ISBN: