Paper N. 107 1 Optimizatin f Transcritical CO 2 Refrigeratin Cycle with Parallel Cmpressin Ecnmizatin Jahar Sarkar Department f Mechanical engineering, Institute f Technlgy, B.H.U, Varanasi, UP-221005; Tel: 91-9919787557 E-mail: jahar_s@htmail.cm Keywrds: transcritical CO 2 cycle, parallel cmpressin ecnmizatin, theretical mdeling, ptimizatin, perfrmance imprvement Abstract Use f parallel cmpressin ecnmizatin is ne f the prmising cycle mdificatins t imprve the COP fr transcritical CO 2 refrigeratin system. The maim aim f the present study is t ptimize f carbn dixide refrigeratin cycle with the additin f parallel cmpressin ecnmizatin fr air-cnditining applicatins. The cycle COP imprvement ver the basic expansin cycle and effect n ptimum cycle high side, which is related the system design issues are presented as well. The cling COP imprvement using parallel cmpressin ecnmizatin can be realized if the rati f cling effect imprvement and ecnmizing cmpressr wrk is mre than the COP f basis expansin cycle. Study shws that there is sme ptimum ecnmizer where the COP imprvement is maximum. Althugh fr simultaneus ptimizatin f gas cler and ecnmizer, tw sets f ptimal values can btained fr maximum cling COP and COP imprvement ver the basic cycle. Fr studied gas cler exit temperatures, the ptimum ecnmizer s gives narrw values lies between 60 t 63 bar. At the ptimum cnditins, bth the cling COP and COP imprvement cmpared t basic cycle decreases with increases in gas cler exit temperature. Fr the given ranges f perating cnditins in this study, the maximum COP imprvement ver the basis expansin cycle is btained as 32.8% thrugh the theretical analyses. The use f parallel cmpressin ecnmizatin in transcritical CO 2 cycle is prfitable fr nt nly COP imprvement als fr lwer ptimum high side. Nmenclature COP cefficient f perfrmance COP b COP f crrespnding basic cycle h specific enthalpy (kj/kg) q specific refrigeratin effect (kj/kg) w c specific cmpressr wrk (kj/kg) x vapur quality COP COP imprvement (%) 1. INTRODUCTION While the synthetic refrigerants exhibit cnsiderably high ODP and GDP, the envirnment friendly natural refrigerant carbn dixide has gained interest lately as alternative refrigerants fr refrigeratin and aircnditining applicatins because f their zer ODP and negligible GWP [1]. As the COP f the basic transcritical CO 2 cycle is cnsiderably lwer than that f ther cnventinal refrigerant because f very high thrttling lss due t high drp cmpared t ther refrigerant based system [2], the cycle mdificatin fr COP imprvement became an essential part f research. There are seral reasns fr mdifying the basic single-stage transcritical cycle, including imprvement f COP, capacity enhancement fr a given system and cmpnent size, adaptatin f the heat rejectin temperature prfile t given requirements and keeping the rati and discharge temperature f the cmpressr within limit. In principle, a large number f mdificatins are pssible, including staging f cmpressin and expansin, splitting f flws, use f internal heat exchange, and wrk-generating expansin instead f thrttling [3]. Recent research prgress in different cycle mdificatins using internal heat exchanger, multistage cmpressin, wrk recvery expansin machine, vrtex tube expansin dice, ejectr expansin dice and thermelectric dice are rised in pen literatures [4-5].
2 The perfrmance f transcritical CO 2 refrigeratin cycle can be als imprved by seral percentages thrugh the successful use f parallel cmpressin ecnmizatin. In parallel cmpressin refrigerating system, refrigerant vapur is cmpressed t supercritical discharge in tw separate nn-mixing streams; ne cming frm an ecnmiser and the ther cming frm the main apratr, s sme cylinders f cmpressr are used t cmpress vapr frm intermediately (in ecnmizer). The parallel cmpressin system will have wide applicatin fr autmtive air cnditining, windw air cnditiners and small water chillers, where it is nt apprpriate t use screw r scrll cmpressrs [6]. Thrugh the theritical analysis and experimentatin fr air cnditining applicatin with additin f superheat fr the primary cmpressin path, Bell [7] shws that the ecnmized carbn dixide system utperfrms a similar hydrcarbn system in terms f efficiency and capacity under sme cnditins and mst efficient fr lwer gas cler exit temperature fr fixed discharge. Althugh detailed ptimizatin studies are scare. In the present study, ptimizatin studies n transcritical CO 2 refrigeratin cycle with the additin f parallel cmpressin ecnmizatin have been dne fr aircnditining applicatins. The cling COP imprvement ver the basic expansin cycle and effect n ptimum discharge are presented as well. Figure 1: Pressure-enthalpy diagram f parallel cmpressin system 2. THEORETICAL MODELING The -enthalpy diagram f a transcritical CO 2 cycle with parallel cmpressin ecnmizatin is shwn in Fig. 1. After the expansin f transcritical fluid frm states 6 t 7 thrugh primary expansin valve, the liquid (state 8) and vapur (3) are separated in ecnmizer. The liquid is again expanded thrugh anther expansin valve and it extracts heat t give useful cling effect in the apratr (frm states 9 t 1). In the cmpressr, the saturated vapurs frm apratr as well as ecnmizer are cmpressed simultaneusly t states 2 and 4, respectively. Then bth the supercritical fluids mix (state 5) and enter t the gas cler fr heat rejectin t the external fluid (sates 5 t 6). The internal heat exchanger is nt used in this study due t its negligible effect n perfrmance [7]. By parallel cmpressin, quality f refrigerant decreases in apratr and bth refrigeratin effect and cmpressr wrk increase, but increase in cmpressr wrk is less than increase in refrigeratin effect, hence COP increases. The entire system has been mdeled based n the energy balance f individual cmpnents f the system. Steady flw energy equatins based n first law f thermdynamics have been emplyed in each case and specific energy quantities are used. The fllwing assumptins have been made in the thermdynamic analysis: 1. Heat transfer with the ambient is negligible. 2. Cmpressin prcess is adiabatic but nn-isentrpic. 3. Evapratin and gas cling prcesses are isbaric. 4. Separatin and mixing prcesses are isbaric. 5. Refrigerant at apratr utlet is saturated vapr. Fr bth the expansin prcesses in expansin valves, h6 = h7 & h8 = h9 (1) Fr unit ttal mass flw rate, the mass flw rates thrugh the ecnmizing and main cmpressrs are x and 1-x, respectively, where, vapur quality x is given by, h7 h8 x = (2) h3 h8 The specific enthalpy f CO 2 at the gas cler inlet can be fund by, h = xh + 1 x h (3) ( ) 5 4 2 Specific refrigerating effect f the apratr: ( 1 )( ) q = x h h (4) 1 9 Specific wrk input t the cmpressr: w = x h h + 1 x h h (5) c ( ) ( )( ) 4 3 2 1 The cling COP fr parallel cmpressin ecnmizatin is given by: q w COP = (6) c
3 and the cling COP f crrespnding basic cycle is given by, b ( ) ( ) COP = h h h h (7) 1 6 2 1 Hence, the COP imprvement cmpared t basic cycle: COP = 100 ( COP COPb) COPb (8) Nw, fr n COP imprvement, ne can write, 1 x (1 x) ( h6 h8) COP = COPb = COPb = (9) 1 x x( h4 h3) Hence, cling COP imprvement can be realized if the rati f cling effect imprvement and ecnmizer cmpressr wrk is mre than the COP b, i.e. (1 x) ( h6 h8) COPb (10) xh ( 4 h3) Based n the thermdynamic analysis presented abve, a simulatin cde was delped t investigate the effect f different perating parameters, which can aluate the cling COP and COP imprvement ver basic cycle fr given apratr temperature, gas cler exit temperature, cmpressr discharge and ecnmizer. The cmpressr utlet cnditins were aluated using fixed isentrpic efficiency. This cde was integrated with the thermdynamic prperty subrutine CO2PROP, delped based n Span and Wagner equatins [8], t cmpute relant thermdynamic prperties f carbn dixide. COP imprvement (%) 30 25 20 10 5 0 90 bar 95 bar 85 bar t = 5 C, t = 40 C c 50 55 60 65 70 Ecnmizer (bar) Figure 2: COP imprvement with ecnmizer at different discharge 3. RESULTS AND DISCUSSION The perfrmance f the transcritical carbn dixide cycle with parallel cmpressin ecnmizatin, being studied fr air-cnditining applicatins, have been aluated fr varius ecnmizer, cmpressr discharge and gas cler exit temperature. The fllwing input parameters have been taken fr the study: apratin temperature = 5 C (typically used fr aircnditining applicatins), isentrpic efficiency = 75% fr bth main and ecnmizing cmpressrs. Fr the calculatin f ptimal values f cmpressr discharge and ecnmizer s, 0.2 bar step was taken in numerical simulatin. COP imprvement (%) 35 31 27 23 19 t = 5 C, t = 40 C Imprvement COP 85 90 95 100 Cmpressr discharge (bar) c 3 2.7 2.4 2.1 1.8 1.5 Cling COP Figure 3: Effect f discharge n perfrmance at ptimum ecnmizer The variatin f COP imprvement ver basic cycle with ecnmizer fr different discharge at gas cler exit temperature f 40 C is shwn in Fig. 2. As the vapur quality decreases with increase in ecnmizer, but the refrigeratin effect decreases due t increase in liquid enthalpy, and the ecnmizing cmpressr wrk decreases due t decrease in quality. Initially, the COP imprvement increases due t less predminant effect f refrigeratin effect, whereas, after sme ptimum, the liquid enthalpy increases very fast (near the critical, slpe change f saturatin line is very fast) and the COP imprvement declined. When the ecnmizer inlet tuches the saturated liquid line, the cycle becmes basic cycle and cycle becmes unrealistic if tuches vapr line. When Eq. (10) is nt satisfied, COP imprvement is negative. Fr discharge f 85 bar
4 (Fig. 2), imprvement becmes negative after 66 bar and unrealistic after 69.6 bar. Maximum imprvement (%) 35 31 27 23 19 COP imprvement Cling COP 2.6 2.4 2.2 2 1.8 1.6 Optimum COP Figure 4: Variatin f cling COP and COP imprvement with gas cler exit temperature fr maximum imprvement Discharge (bar) 1 110 105 100 95 90 85 80 Discharge Ecnmizer 64 63 62 61 60 59 58 Ecnmizer (bar) Figure 5: Variatin f discharge and ecnmizer s with gas cler exit temperature fr maximum imprvement The variatins f cling COP and the COP imprvement ver the basic valve expansin cycle with cmpressr discharge fr ptimum ecnmizer are shwn in Fig. 3 at gas cler exit temperature f 40 C. Interestingly, the discharge gives tw ptimal pints: ne fr maximum COP and anther fr maximum imprvement. This can be attributed by the fact that the discharge ptimizatin based n maximum COP is due t the S-shape nature f istherm line near the critical pint and ptimizatin based n maximum imprvement is due t the slpe changing nature f saturatin curve and the bth ptimal pints may nt be the same. The ptimum discharge based n maximum COP gives the higher value than that based n maximum imprvement. Optimum imprvement (%) 20 19 18 17 16 COP imprvement Cling COP 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2 1.9 1.8 Maximum COP Figure 6: Variatin f cling COP and COP imprvement with gas cler exit temperature fr maximum COP The variatins f cling COP and COP imprvement ver basic cycle and crrespnding ptimum cmpressr discharge and ecnmizer based n maximum COP imprvement with gas cler exit temperature are shwn in Figs. 4 and 5, respectively. The results shw that the variatin in the gas cler exit temperature has a significant effect n system perfrmance and perfrmance imprvement. The effect n ptimal discharge is als very significant; hwer effect is mderate fr ptimal ecnmizer. It can be nted that the variatin trends f ptimum s are rerse. With 10 C increase in gas cler exit temperature, the ptimum discharge increases by 22 bar, whereas, ecnmizer decreases by nly 3 bar. With increase in gas cler exit temperature, as the ptimum discharge increases, the vapur quality increases significantly and hence the imprvement f refrigeratin effect reduces but the ecnmizer cmpressr wrk increases, which lead
5 t reductin f COP imprvement. The results indicate that the design f system fr lwest pssible gas cler exit temperature is mre effective fr nt nly higher cling COP and COP imprvement but als fr lwer ptimum high side. Discharge (bar) 125 120 1 110 105 100 95 90 Discharge Ecnmizer 64 63 62 61 60 59 58 Ecnmizer (bar) Figure 7: Variatin f discharge and ecnmizer s with gas cler exit temperature fr maximum COP Optimum discharge (bar) 135 125 1 105 95 85 75 Maximum COP Maximum imprvement Basic cycle Figure 8: Effect n ptimum discharge by using parallel cmpressin The variatins f cling COP and COP imprvement ver basic cycle and crrespnding ptimum cmpressr discharge and ecnmizer based n maximum cling COP with gas cler exit temperature are shwn in Figs. 6 and 7, respectively. It may be nted that the cling COP decreases significantly frm 2.82 t 1.83 and COP imprvement frm 19% t % with 10 C increase in gas cler exit temperature. As the variatin in the cler utlet temperature has a significant impact n the ptimal design cnditins [1], the ptimum discharge increases sharply frm 96 t 120 bar with the gas cler exit temperature. Hwer, the effect f gas cler exit temperature n ptimum ecnmizer is negligible. This can be attributed that althugh the shape f istherm changes with change in temperature, the shape f saturatin curve is unaltered. Results shw that fr higher gas cler exit temperature, the system is nt prfitable in term f cling COP and COP imprvement as well as cst due t high ptimum discharge. Results clearly shw that the use f parallel cmpressin ecnmizatin is mre prfitable at lwer gas cler exit temperature due t higher COP imprvement. The effect f using parallel cmpressin ecnmizatin n the variatin f ptimum discharge with gas cler exit temperature is shwn in Fig. 8. The variatin is very similar t the basic valve expansin cycle, hwer, the ptimum values f high side reduces significantly with the use f parallel cmpressin ecnmizatin as ident frm Fig. 8 and the use f parallel cmpressin ecnmizatin in transcritical CO 2 air-cnditining system is ecnmical in term f lwer high side als. The ptimum discharge based n the maximum COP imprvement is significantly lwer than that n maximum cling COP due t the reasn discussed abve. It is very interesting t nt that fr fixed cmpressr discharge, COP imprvement at ptimum ecnmizer increases initially then decreases with increases in gas cler exit temperature and give sme ptimal value f gas cler exit temperature. Althugh fr simultaneus ptimizatin f gas cler and ecnmizer, bth the cling COP and COP imprvement cmpared t basic cycle decreases with increases in gas cler exit temperature. Present results shw that the maximum COP imprvement ver the basis expansin cycle is btained as 32.8%. 4. CONCLUSIONS A detailed ptimizatin studies n transcritical CO 2 refrigeratin cycle with the additin f parallel cmpressin ecnmizatin have been dne fr air-cnditining applicatins. The cling COP imprvement ver the basic expansin cycle and effect n ptimum discharge are presented as well. Study shws that the cling COP
6 imprvement using parallel cmpressin ecnmizatin can be realized if the rati f cling effect imprvement and ecnmizing cmpressr wrk is mre than the COP f basis expansin cycle. Fr simultaneus ptimizatin f discharge and ecnmizer, tw sets f ptimal values can btained fr maximum cling COP and COP imprvement ver the basic cycle. Althugh the ptimum discharge varies significantly with gas cler exit temperature, the ptimum ecnmizer gives narrw values lies between 60 t 63 bar. With the increase in gas cler exit temperature, bth the cling COP and COP imprvement at ptimum cnditins decrease, whereas the ptimum discharge increases. The ptimum discharge based n the maximum COP imprvement is significant lwer than that n maximum cling COP. The design f system fr lwest pssible gas cler exit temperature is mre effective fr nt nly higher cling COP and COP imprvement als fr lwer ptimum high side. Fr the studied ranges f perating cnditins, the maximum COP imprvement ver the basis expansin cycle is btained as 32.8%. The use f parallel cmpressin ecnmizatin in transcritical CO 2 aircnditining system is prfitable fr nt nly COP imprvement als fr lwer ptimum high side. References [1] Kim, M.H., Pettersen, J., Bullard, C.W., 2004, Fundamental prcess and system design issues in CO 2 vapr cmpressin systems, Prgress Energy Cmbustin Science, 30, n.2: 119-174. [2] Sarkar, J., Bhattacharyya, S., Ramgpal, M., 2005, Transcritical CO 2 heat pump systems: Exergy analysis including heat transfer and fluid flw effects, Energy Cnversin & Management, 46, n.13-14: 2053-2067. [3] Lrentzen, G., 1994, Rival f Carbn dixide as a refrigerant, Internatinal Jurnal f Refrigeratin, 17, n.5: 292-300. [4] Grll, E.A., 2006. Recent advances in the transcritical CO 2 cycle technlgy. 8 th Natinal & 7 th ISHMT- ASME Heat & Mass Transfer Cnference (January 4-6), IIT Guahati, India. [5] Grll, E.A., Kim, J.H., 2007, Riew f recent advances tward transcritical CO2 cycle technlgy, HVAC&R Research, 13, n.3: 499-520. [6] Pearsn, S.F., 2005, Imprved transcritical refrigeratin cycle, Eurpean Patent EP1498667. [7] Bell, I., 2004, Perfrmance increase f carbn dixide refrigeratin cycle with the additin f parallel cmpressin ecnmizatin, 6 th IIR Gustav Lrentzen Natural Wrking Fluid Cnference, Glasgw UK. [8] Span, R., Wagner, W., 1996, A new equatin f state fr Carbn dixide cvering the fluid regin frm triple pint temperature t 1100 K at up t 800 MPa, Jurnal Physical. Chemical Reference Data, 25: 09-96. Bigraphy Authr received his M. Tech. and Ph.D. degrees in Mechanical Engineering frm Indian Institute f Technlgy Kharagpur in 2001 and 2006, respectively. After, he jined the faculty f Institute f Technlgy, Banaras Hindu University and wrking as a Lecturer in Mechanical Engineering Department till date. Authr has published 12 research papers in internatinal jurnal and 12 in natinal and internatinal cnferences in seral areas f Heat Transfer and Refrigeratin.