Study on Solar-Assisted Cascade Refrigeration System

Transcription

1 Avalable onlne at Energy Proceda 16 (2012) Internatonal Conference on Future Energy, Envronment, and Materals Study on Solar-Asssted Cascade Refrgeraton System Ln Wang, Ahua Ma, Yngyng Tan, Xaolong Cu, Hongl Cu Insttute of Ar-Condtonng and Refrgeraton Henan Unversty of Scence and Technology Luoyang, Chna Abstract Energy-conservaton and envronmental protecton are keys to sustanable development of domestc economy. The solar-asssted cascade refrgeraton system s developed. The system conssts of electrcty-drven vapor compresson refrgeraton system and solar-drven vapor absorpton refrgeraton system. The vapor compresson refrgeraton system s connected n seres wth vapor absorpton refrgeraton system. Refrgerant and soluton reservors are desgned to store potental to keep the system operatng contnuously wthout sunlght. The results ndcate that the system obtans pretty hgher COP as compared wth the conventonal vapor compresson refrgeraton system. COP of the new-type vapor compresson refrgeraton system ncreases as sunlght becomes ntense Publshed Publshed by by Elsever Elsever B.V. Ltd. Selecton Selecton and/or and/or peer-revew peer-revew under under responsblty responsblty of Internatonal of [name organzer] Materals Scence Socety. Open access under CC BY-NC-ND lcense. Keywords- solar energy; refrgeraton; power consumpton; cascade; energy storage 1. Introducton Solar-drven vapor compresson refrgeraton has advantages n energy conservaton and envronmental protecton, however, conventonal solar-drven vapor compresson refrgeraton has many dsadvantages, such as low effcency, ntermttent operaton, too large unt sze and too hgh captal cost [1-3]. As compared wth the solar-drven vapor compresson refrgeraton, the electrcty-powered vapor compresson refrgeraton features hgher energy consumpton and lower ntal nvestment, t s unfavorable to energy conservaton snce t consumes hgh-grade electrcal energy. Besdes, the popularty and applcaton of electrcty-powered vapor compresson refrgeraton have been one of the man reasons of summer and wnter electrcty demand peak n Chna n recent years. In partcular, 60% of the total electrcty capacty s generated by coal. Not only does t exacerbate the depleton of fossl fuel, but also t can produce dangerous gases such as carbon doxde, ntrogen oxdes and sulfur oxdes, whch cause the greenhouse effect and deterorate the global envronment [4]. In order to solve the above problems n the Publshed by Elsever B.V. Selecton and/or peer-revew under responsblty of Internatonal Materals Scence Socety. Open access under CC BY-NC-ND lcense. do: /j.egypro

2 1504 Ln Wang et al. / Energy Proceda 16 (2012) conventonal electrcty-powered vapor compresson refrgeraton system and solar-drven one, a solarasssted cascade refrgeraton system s analysed. 2. Features of the proposed system Fgure 1 shows schematcally the proposed cascade refrgeraton system whch ncludes the solardrven vapor absorpton refrgeraton unt and electrcty-powered vapor compresson refrgeraton unt. Lthum bromde aqueous soluton s used as absorpton workng pars n the vapor absorpton refrgeraton unt, whle HFC134a s used as refrgerant n the vapor compresson refrgeraton unt. In the coolng mode, the vapor compresson refrgeraton unt s cascaded wth the vapor absorpton refrgeraton unt. Low grade solar energy s used as heat source to drve the vapor absorpton refrgeraton unt and low temperature refrgerant water s obtaned to cool the condenser n the vapor compresson refrgeraton unt. As refrgerant water whch s used as coolng medum s at low temperature, condensng temperature decreases for the vapor compresson refrgeraton unt. Therefore, lower condensng temperature ncreases COP of the cascade refrgeraton system. Refrgerant and soluton reservors are desgned to store potental to keep the system operatng contnuously wthout sunlght. 8c a 8b a generator 2-condenser 3- refrgerant reservor 3a-soluton reservor 4-U-type throttle valve 5-condensaton evaporator 6-absorber 7-soluton pump 9-soluton heat exchanger 8-compressor 10-throttle valve 11-flter drver 12-four ways reversng valve 13-compressor 14-solar thermal collector Fg.1 schematc dagram of the solar-asssted cascade refrgeraton system 3. Thermodynamc mathematcal models In order to evaluate the cycle performance of the cascade refrgeraton system, thermodynamc model of components whch consttute the cascade refrgeraton system s establshed. The cascade refrgeraton system operates n the coong mode or heatng mode, but only thermodynamc performance of the system n coolng mode s evaluated n the paper. Accordngly, the thermodynamc model of the system n the coolng mode s presented. Based on the prncples of conservaton of mass and energy, the mathematcal models of vapor absorpton refrgeraton system and vapor compresson refrgeraton system are presented. 3.1 vapor absorpton refrgeraton system (VARS) Accordng to conservaton of mass, total mass balance equaton and soluton mass balance equaton for the vapor absorpton refrgeraton system are shown as follows respectvely:

3 Ln Wang et al. / Energy Proceda 16 (2012) o X Go X o G G = 0 (1) G = 0 (2) Accordng to conservaton of energy, energy balance equaton for each component of the vapor absorpton refrgeraton system s wrtten as follows Q G h G h = 0 (3) + o o Where: G, G o s mass flowrate of soluton nto and from the component, kg/s; X, X o s concentraton of soluton nto and from the component, %;Q s heat transferred n the generator, condenser and the condensaton evaporator, kw; h, h o s soluton(or refrgerant water) nlet and outlet enthalpy to the component, kj/kg. Heat removed from the condenser: = D( h ) 6 h7 (4) Coolng capacty of the evaporator: Energy balance for the absorber: Q a Q c Q c = D( h ) 8 h9 (5) + Gh ( h (6) 1 = Dh9 G D) Energy balance for the soluton heat exchanger: G h h ) = ( G D)( h ) (7) Energy balance for the generator: Q g ( h5 5 = Dh6 + G D) Gh ( h (8) Energy balance equaton for the vapor absorpton refrgeraton system: Q + Q = Q + Q (9) g e The effcency of solar radaton collector [5]: η = T T I (10) sc ( a ) / Where: η s energy collecton effcency of the flat plate collector; I s the solar radaton ntensty, sc W/m; T and T a are the collector nlet and outdoor ar temperature. 3.2 vapor compresson refrgeraton system (VCAS) Coolng capacty n the evaporator: Power consumpton of the compressor: Compresson rato: Coeffcent of performance (COP): where : h e o he, h com o hcom, Q 0 r ( e, o e, P e c a = G h h ) (11) = G h h ) (12) r ( com, o com, ε = / p 0 (13) p k COP = η η Q / P (14),, s enthalpy of refrgerant nto the evaporator and from the evaporator, kj/kg;,, s compressor dscharge and sucton enthalpy, kj/kg; G r s mass flow rate of refrgerant R134a, kg/s; P o, P k s evaporaton pressure and condensaton pressure, Pa; η m, η s mechancal effcency and ndcated effcency. m 0 e

4 1506 Ln Wang et al. / Energy Proceda 16 (2012) Results and dscusson Based on meteorologcal parameter on July 2nd of some year n Zhengzhou area, Chna, codes usng the Vsual C++ are programmed to calculate the cycle performance of the cascade refrgeraton system accordng to the thermodynamc mathematcal models. In the standard workng condtons, the parameters for VCRS are gven as: (1) Evaporatng temperature =35.9 ; superheated temperature=5 ; (2) Condensng temperature =25 ; subcoolng temperature=5 ; (3)The actual vapor compresson process n the compressor s consdered as a non-sentropc process. Indcated effcency of the compressor s equal to 0.80, and the motor effcency of the compressor s equal to Numercal computatons are terated by bsecton method. Tab 1 shows the results of thermodynamc performance of the cascade refrgeraton system obtaned for the normal workng condtons. Tab.1 Results of thermodynamc performance desgned for the standard workng condtons Parameter Value Parameter Value Generatng temp ( ) Mass flowrate of refrgerant R134a (kg/s) 0.28 Mass flowrate of Evaporatng temp refrgerant water for VARS ( ) (kg/s) Condensaton temp for VARS ( ) Unt mass power nput to the compressor (kj/kg) Heat load at the condensaton evaporator (kw) COP of VCRS for the new cascade refrgeraton Fg 2 shows the relatonshps of solar radaton ntensty and generaton temperature wth the tme of day. It s seen that solar radaton ntensty began to ncrease and then decreased from 9:00 to 16:00, when t reached a maxmum value of 900 W/m2 at about 13:00. Generaton temperature depends on hot water outlet temperature to the solar radaton collector, so t also began to ncrease and then decreased from 9:00 to 16:00 wth the varaton of the solar radaton ntensty, and t reached a maxmum value of 70.8 at about 13: solar radaton(w/m 2 ) Inte nsty radaton of sloar Ge nerat on temp. 8:30 9:30 10:30 11:30 1 2:30 13:30 14:30 15:30 16:30 T me of t he day outdoor ar temp( ) Fg. 2 Solar radaton ntensty and generaton temperature vs. tme

5 Ln Wang et al. / Energy Proceda 16 (2012) Fg 3 shows the relatonshps of outdoor ar temperature and coolng water temperature wth the tme of day. Solar radaton ntensty has a sgnfcant nfluence on outdoor ar temperature. It s noted that outdoor ar temperature began to ncrease and then decreased from 9:00 to 16:00 wth the varaton of solar radaton ntensty, and t reached a maxmum value of 32.8 at about 15:00. Heat of condensaton and absorpton for VARS s rejected to the atmosphere through the coolng tower, so coolng water temperature s dependent on both heat removed through coolng tower and outdoor ar temperature. When coolng water flowrate s kept constant, coolng water outlet temperature to the coolng tower s 32 at 9:00, whle t reaches a maxmum value of 32.9 at 12:00. It ndcated that, at that tme, hgher coolng capacty needs to be provded by the cascade refrgeraton system, and more heat of condensaton and absorpton for VARS s removed owng to lower solar radaton ntensty and lower refrgeraton effcency, so coolng water temperature rses to the maxmum at 12:00. Temp.( ) :30 9:30 10:30 11:30 12:30 13:30 14:30 15:30 16:30 The t me of day coolng water temp. outdoor ar temp Fg.3 Outdoor ar temp. and coolng water temp. vs. the tme of day COP of VCRS and CHP VCRS CHP VA RS COP of VARS :30 9:30 10:30 11:30 12:30 13:30 14:30 15:30 16:30 The tme of day Fg.4 COP vs. the tme of day Fg 4 shows the relatonshps of COP of VARS, VCRS and conventonal vapor compresson refrgeraton system (CVCRS) wth the tme of day. The cascade refrgeraton system s made up of

6 1508 Ln Wang et al. / Energy Proceda 16 (2012) VARS and VCRS. Refrgeraton effcency of VARS depends on generaton temperature, condensng temperature and evaporatng temperature for VARS, whle that of VCRS s largely dependent on condensng temperature and evaporatng temperature for the VCRS. As for VARS, COP ncreases and heat removal decreases wth solar radaton ntensty rse, so that varaton trend of COP s largely n accord wth that of solar radaton ntensty. COP s up to the maxmum at 13:00. It follows that generaton temperature or solar radaton ntensty has a consderable nfluence on COP of VARS, when coolng water flowrate and evaporatng temperature s constant. As for VCRS, condensng temperature hnges on evaporatng temperature for VARS, so the varaton trend of COP for VCRS s n lne wth that of solar radaton ntensty. COP of VCRS reaches the maxmum at 13:00, just lke VARS. As for conventonal vapor compresson refrgeraton system(cvcs), condensng temperature has an mportant nfluence on refrgeraton effcency, when evaporatng temperature s constant. Consequently, COP of CVCS s dependent on coolng water temperature or outdoor ar temperature. It s observed that COP of CVCS begns to decrease and then ncreases, because outdoor ar temperature ncreases at frst and then decreased from 9:00 to 16:00 wth the varaton of solar radaton ntensty. For the cascade refrgeraton system, VCRS s cascaded wth VACS, so condensng temperature for VCRS depends on evaporatng temperature for VACS. However, condensng temperature for CVCS s dependent on coolng water temperature. Consequently, condensng temperature for VCRS s 15 lower than that for CVCS, and COP for VCRS s twce hgher than that for CVCS. For nstance, COP of VCRS s 6.1 at 13:00, whle that of CVCS s only 2.1. Power consumpton of the compressor for CVCS s twce hgher than that for VCRS. Solar radaton ntensty has an mportant effect on refrgeraton effcency of the cascade refrgeraton system. The greatest advantage of the cascade refrgeraton system s that refrgeraton effcency of the cascade refrgeraton system ncreases wth the solar radaton ntensty rse. When solar radaton ntensty s the maxmum, coolng load n ar-condtonng rooms s up to the maxmum as well. The greater solar radaton ntensty, the hgher refrgeraton effcency of the cascade refrgeraton system. It follows that varaton trend of refrgeraton effcency s n lne wth that of coolng load n arcondtonng rooms, whch helps to meet the demand of ar condtonng rooms for coolng capacty. As compared wth the cascade refrgeraton system, the refrgeraton effcency of the conventonal vapor compresson refrgeraton system (CVCS) s too low to provde the coolng capacty for the ar condtonng rooms, when solar radaton ntensty s the greatest. Obvously, the cascade refrgeraton system contrbutes to solve the problems that coolng capacty of CVCS decreases wth solar ntensty or outdoor ar temperature rse. 5. Conclusons The solar-asssted cascade refrgeraton system ncludes the solar-drven vapor absorpton refrgeraton unt and electrcty-powered vapor compresson refrgeraton unt. Refrgerant water and soluton reservors are used as energy storage unts to store solar energy, n order to keep the system operatng contnuously wthout sunlght. COP of the cascade refrgeraton system s up to 6.1 wth the solar ntensty of 700W/m2, outdoor ar temperature of 35 and chlled water supply temperature of 7. Power consumpton of the cascade refrgeraton system s 50% lower than that of the conventonal vapor compresson refrgeraton (CVCS) n the coolng mode. The COP of the new vapor compresson refrgeraton system (VCRS) ncreases as sunlght becomes ntense. COP of the cascade refrgeraton system s up to the maxmum when COP of the conventonal vapor compresson refrgeraton system s mnmal.

7 Ln Wang et al. / Energy Proceda 16 (2012) Acknowledgment Ths work was fnancally supported by the Natonal Natural Scence Foundaton of Chna (Grant No ) and Henan Provnce Natural Scence Foundaton (Grant No.2008A48001). References [1] S. Rosek, F.J. Batlles, Integraton of the solar thermal energy n the constructon: Analyss of the solar-asssted arcondtonng system nstalled n CIESOL buldng, Renewable Energy, vol.34, 2009,pp [2] M. Mazloum, M. Naghashzadegan, K. Javaherdeh, Smulaton of solar lthum bromde water absorpton coolng system wth parabolc trough collector, Energy Converson and Management. Energy Converson and Management, vol.49, 2008, pp [3] R.J. Romero, W. Rvera, R. Best, Comparson of the theoretcal performanc of a solar ar condtonng system operatng wth water/lthum bromde and an queousternary hydroxde, Solar Energy Materals & Solar Cells, vol.63, 2000, pp [4] Ln Wang, Ahua Ma, Xwen Zhou, Yngyng Tan, Xaona Yan and Wang Yu, Envronment and energy challenge of ar condtoner n Chna, The proceedngs of the Internatonal Conference of Envronmental Polluton and Publc Health, Chna, vol.2, No. 5, 2008, pp [5] B.J. Huang, J.M. Chang, V.A. Petrenko, K. B. Zhuk, A solar ejector coolng system usng refrgeraton R141b, Solar Energy, vol.64, No.4-6, 1998, pp