Parametric analysis of a reheat carbon dioxide transcritical power cycle using a low temperature heat source

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1 2011 2nd International Conferene on Environmental Engineering and Appliation IPCBEE vol.17 (2011 (2011 IACSIT Pre, Singapore Parametri analyi of a reheat arbon dioxide tranritial power yle uing a low temperature heat oure Hanfei Tuo 1 1 Mehanial Siene and Engineering, Univerity of Illinoi at Urbana Champaign Abtrat. CO 2 tranritial rankine power yle ha been widely invetigated reently, beaue of it better temperature glide mathing between enible wate heat oure and working fluid in vapor generator, and it deirable qualitie, uh a moderate ritial point, little environment impat and low ot. A reheat CO 2 tranritial power yle with two tage expanion i preented to improve baeline yle performane in thi paper. Firt law analyi i arried out to invetigate parametri effet on reheat yle performane. The main reult how that reheat yle performane i enitive to the variation of medium preure and the optimum medium preure exit for maximizing work output and thermal effiieny, repetively. Reheat yle i ompared to baeline yle under the ame ondition. More ignifiant performane improvement by reheat are obtained at lower turbine inlet temperature and higher maximum yle preure. Work output improvement i muh higher than thermal effiieny improvement, beaue extra wate heat i required to reheat CO 2, whih redue the thermal effiieny. It i found that reheat yle ha great potential to improve thermal effiieny and epeially work output of a CO 2 tranritial power yle uing a low-grade heat oure. Keyword: Tranritial yle, Carbon dioxide, Reheat, Low-grade heat oure, Sytem optimization 1. Introdution The utilization of low-grade indutrial wate heat beome more attrative due to the world inreaing eletriity demand. Superritial power yle how high potential to reover uh low grade heat, beaue working fluid temperature glide above the ritial point provide a better temperature profile math in the vapor generator with le irreveribility. Mot previou tudie fou on thermodynami analyi and optimization of onventional arbon dioxide tranritial power yle. Chen et al. [1] found that the arbon dioxide tranritial power yle had a lightly higher work output than did an organi rankine yle (ORC with equal mean thermodynami heat rejetion temperature. Zhang et al. [2] tudied a imilar yle powered by olar energy for both power and heat generation. They alo onduted experimental tudy to validate the feaibility of thi yle [3]. Cayer et al. [4] optimized peifi work output of a tranritial yle by invetigating the turbine inlet temperature on yle performane. Two tranritial yle were optimized by Baik et al. [5] for power output through examining turbine inlet temperature and preure. Wang et al. [6] ued a generi algorithm and an artifiial neural network to optimize a few important parameter with exergy effiieny a an objetive funtion. Previou tudie indiated that higher turbine inlet temperature produed more work output a well a better thermal effiieny. Thi i beaue CO 2 exhibit a teeper lope for the iobari urve of the highpreure region than in the low preure region, and thu peifi work output i inreaed when arbon dioxide i expanded at a fixed expanion ratio but a higher inlet temperature [7]. Thu, thi indiate the performane of tranritial yle an be improved by reheat yle where CO 2 i only firtly expanded to medium preure and then reheated to maximum temperature before 2nd tage expanion. Dotal [8] invetigated a reheat CO2 brayton yle and analyed it improvement over baeline yle. However, none of the publihed tudie on tranritial CO2 power yle have invetigated effet of reheat on yle 33

2 performane and ompare to baeline yle without reheat. Thu, in thi paper, a reheat tranritial arbon dioxide yle i introdued and optimized. 2. Sytem deription and modeling Fig. 1(a and (b how hemati of a baeline yle and a reheat yle, repetively. In the reheat yle, uperheated vapor from the vapor generator (3 i expanded through 1 t tage turbine to a medium preure (P m. Then, it i reheated again in the vapor generator and expanded through 2 nd tage turbine to a low preure (P, and finally the vapor i ompletely ondened to aturated liquid. A regenerator i not inluded ine reent tudie [9, 10] have hown that a regenerator only produe mall improvement of thermal and exergeti effiieny but ha little influene on the net power output. For the preent tudy, the heat oure i indutrial wate water at a temperature of 100 ºC. The ooling water i at 10 ºC. The following general aumption are formulated for thi tudy without loing generoity: eah omponent i onidered a a teady-tate teady-flow ytem; kineti and potential energie a well a the heat and frition loe are negleted; pump and the turbine ientropi effiienie are both et to be 0.8; aturated liquid i aumed at the ondener exit. (a Baeline yle (b Reheat yle Fig. 1. Shemati diagram of uperritial CO 2 power yle The firt law of thermodynami i ued for energy analyi. The thermal effiieny η and peifi work output W net are final reult for omparion. For the reheat yle, the model and equation for the different omponent are the following: For the pump: h2, h1 ηp h2 h1 (1 WP m ( h2 h1 (2 For the two turbine: h3 h4 ηt,1 h3 h4, (3 h5 h6 ηt,2 h5 h6, (4 Wt,1 m ( h3 h4 (5 Wt,2 m ( h5 h6 (6 For the vapor generator: Qin m ( h3 h2 + ( h5 h4 (7 For the ondener: Q m h h (8 out ( 6 1 For the peifi work output: W + W W Wnet m For the thermal effiieny of the reheat yle: t,1 t,2 P (9 34

3 η 3. Reult and Diuion W + W W t,1 t,1 p (10 Q in Fig. 2 how the T- diagram for reheat yle at a high preure P h 12 MPa. Fig. 3 how the effet of medium preure P m on the peifi work output and thermal effiieny at a turbine inlet temperature of 90 ºC. Epeially, for the ae of P m P h, thi indiate only 2 nd turbine produe work and no reheat i reqired from vapor generator. Thu, the reheat yle work a the baeline yle with the ame ytem performane. It an be een that the peifi work output and thermal effiieny beome higher a inreae of P m, and exibit maximum value at different optimal P m. In addition, the optimal P m at maximum work output i lower than that at maximum effiieny. Thi an be explained that produed work from 1 t turbine derreae but from 2 nd turbine inreae a inreae of P m, thu maximum work output i about kj/kg at the optimal P m 8.49 MPa. However, higher P m redue the reheat input, beaue the temperature after the 1 t tage expanion i inreaed due to the lower expanion ratio and le amount of wate heat i required to reheat CO 2. The required total heat input dereae till relatively fater than work output doe, o maximum thermal effiieny ourr at a light higher P m. 100 R744 Wate heat Temperature C MPa 12 MPa MPa Cooling water Entropy [kj/kg-k] Fig. 2. T- diagram for the reheat yle 3.1 Effet of P h on the optimal P m and yle performane Fig. 3. Effet of P m on reheat yle performane Fig. 4. Optimal P m for maximum W net Fig. 5. Optimal for P m for maximum η Fig. 4 and 5 how effet of varied high preure P h on the yle performane and optimal medium preure P m for maximum work output and thermal effiieny, repetively. A expeted, optimal medium preure P m are higher a inreae of high preure P h. Maximum work output W net rie dramatially when high preure P h inreae from 9 MPa to about 12 MPa, and then a maximum W net about 18.3 kj/kg i obtained at P h MPa and P m MPa. Thermal effiieny inreae monotonially with P h at the given range in thi tudy. Effiieny reahe the maximum value of For omparion of baeline and reheat yle, the improvement of W net and η an be illutrated in Fig. 6 and Fig. 7, repetively. Larger enhanement by reheat for W net and η i ahieved at higher P h. Thi i beaue high-preure iobari urve i teeper than at low preure. For the ae of P h 15 MPa, W net and η are inreaed by approximately 50% and 15%, repetively. Additionally, it an be notied that thermal effiieny improvement i relatively low ompared to work output, beaue extra reheat input ounterat inreaed work output. 35

4 Fig. 6. W net maximum improvement by reheat Fig. 7. η maximum improvement by reheat 3.2 Effet of turbine inlet temperature T 3 on the yle performane Fig. 8 and Fig. 9 illutrate effet of turbine inlet temperature on the performane of the reheat yle. Obviouly, for the fixed P h, higher turbine inlet temperature lead to larger W net and higher η. Compared to other tudie for baeline yle, dependene of reheat yle performnae on turbine inlet temperature i imilar. In order to quanlify the improvement by mean of reheat, improvement fator for W net and η are defined a: ( Wnet ( Wnet r b εw (11 ( Wnet b ( η ( η r εη (12 ( η Where ubript r and b denote reheat yle and baeline yle, repetively. Fig. 8. work output veru P h at different T 3 Fig. 9. Thermal effiieny veru P h at different T 3 Fig. 10. W net improvement fator veru P h Fig. 11. η improvement fator veru P h Reult are hown in Fig. 10 and Fig. 11, repetively. It an be een that higher improvement of both W net and η are obtained at a lower turbine inlet temperature and a higher P h. Under preent ondition, the maximum improvement of W net and η are approximately 90% and 30%, repetively. Additionally, the improvement of W net i more ignifiantly than that of η. Thi may indiate ome potential appliation of reheat yle. Firtly, reoure of wate heat i relatively heap and even free like wate tream, thu ahieveing high thermal effiieny i le important than genrating the maximum peifi net work output. Seondly, the maximum temperature of wate heat i relatively low, therefore limiting the turbine inlet temperature, ine both improvement fator are higher at lower turbine inlet temperature. 36

5 4. Conluion In thi paper, a reheat tranritial CO 2 power yle ha been invetigated and ompared to a non-reheat baeline yle with repet to net power output, thermal effiieny and exergy effiieny. It i found that medium preure before 2 nd tage expanion i an important parameter to determine reheat yle performane. For the given yle high preure and turbine inlet temperature, two different optimal medium preure exit orreponding to maximum work output and thermal effiieny, repetively. The optimal preure for maximum work output i higher than that for maximum thermal effiieny under the ame ondition. Compared to baeline yle, larger improvement of work output and thermal effiieny by reheat yle are obtained at a lower turbine inlet temperature and/or larger high yle preure. The maximum improvement for work and thermal effiieny are approximately 90% and 30%, repetively. The overall reult how that work output i inreaed by reheat more ignifiantly than thermal effiieny. Thi indiate it i proper to ue the reheat yle under whih ondition maximizing work output i mot onerned. In ummary, the reheat with two tage expanion ha great potential to improve work output and thermal effiieny of a CO 2 tranritial power yle uing a low-grade heat oure. Nomenlature h enthalpy (kj/kg Subript P preure (MPa b baeline yle Q heat input (kw C ondener T temperature (ºC h high preure W power (kw m medium preure Greek letter net net work output ε improvement fator p pump η thermal effiieny r reheat yle η p Pump ientropi effiieny t turbine η t Turbine ientropi effiieny VG vapor generator 5. Referene [1] Chen Y, Lundqvit P, Johanon A, Platell P. A omparative tudy of arbon dioxide tranritial power yle ompared with an organi rankine yle with R123 a working fluid in wate heat reovery. Appl Therm Eng 2006;26: [2] Zhang XR, Yamaguhi H, Uneno D. Thermodynami analyi of the CO 2 -baed rankine yle powered by olar energy. Int J Energy Re 2007;31: [3] Zhang XR, Yamaguhi H, Fujima K, Enomota M, Sawada N. Study of olar energy powered tranritial yle uing uperritial arbon dioxide. Int J Energy Re 2006; 30: [4] Cayer E, Galani N, Nereddine H. Parametri tudy and optimization of a tranritial power yle uing a low temperature oure. Appl Energy 2010;87: [5] Baik YJ, Kim M, Chang KC, Kim SJ. Powered-baed performane omparion between arbon dioxide and R125 tranritial yle for a low-grade heat oure. Appl Energy 2011;88: [6] Wang J, Sun Z, Dai Y, Ma S. Parametri optimization deign for uperritial CO 2 power yle uing geneti algorithm and artifiial neural network. Appl Energy 2009;87: [7] Chen H, Gowami DY, Stefanako EK. A review of thermodynami yle and working fluid for the onverion of low-grade heat. Renew Sut Energ Rev 2010;14: [8] Dotal V. A uperritial arbon dioxide yle for next generation nulear reator. Dotoral thei, Department of Nulear Engineering, MIT, [9] Cayer E, Galani N, Deilet M, Nereddine H, Roy P. Analyi of a arbon dioxide tranritial power yle uing a low temperature oure. Appl Energy 2009;86: [10] Starling KE, Fih LW, Iqbal KZ, Yieh D. Reoure utilization effiieny improvement of geothermal binary yle, phae 1. Semiannual progre report;