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32 M.-H. Yang, R.-H. Yeh / Renewable Energy 68 (2014) 25e34 4.5. Effects of seawater temperature in ORC Higher warm seawater temperatures and lower cold seawater temperatures increase the net output power and efficiency of an OTEC system. To investigate the influence of cold seawater tem- peratures at the heat-exchanger inlet, the variation of Teva,o, Tcon,o, gmax, and thermal efficiency for an OTEC system are shown in Fig. 8(a)e(d). To collect colder seawater, the cold seawater extracting inlet has to reach deeper and therefore the pump will consume more power. The lengths of the cold seawater pipe are assumed to be 750 m, 900 m, 1050 m, and 1200 m to correspond to Tcwi 1⁄4 5 C, 6 C, 7 C, and 8 C, respectively. Fig. 8(a) and (b) show that the values of both Teva,o and Tcon,o increase with Tcwi from 5 C to 8 C at Twwi 1⁄4 28 C. Furthermore, the influence of Tcwi on Tcon,o is stronger than on Teva,o in an OTEC system. Inter- estingly, the Tcon,o values from high to low are for R600a, R134a, R152a, R717, and R245fa, respectively. The Teva,o values varied similarly, but the Teva,o distribution of R245fa is slightly higher than that of R717. The maximal objective parameter gmax, and the net thermal efficiency hth, which correspond to Tcon,o and Teva,o, both decreased with Tcwi from 5 C to 8 C as shown in Fig. 8(c) and (d). Although R717 shows the best results in objective parameter analysis, R600a has the highest net thermal efficiency among the five working fluids. Fig. 9(a)e(d) show the influence of warm seawater inlet tem- perature of the evaporator on optimal operating temperatures and their corresponding maximal objective parameter and thermal ef- ficiency. Fig. 9(a) and (b) reveal that the increments of Teva,o are larger than those of Tcon,o from Twwi 1⁄4 25 Ce28 C for each working fluid. The results in these figures show that the gmax values from high to low are for R717, R152a, R600a, R134a, and R245fa. The values of gmax for R152a and R600a are close both in variation of Twci and Twwi. By contrast, the values hth, which correspond to gmax, from high to low are for R600a, R717 R152a, R245fa, and R134a, respectively. From these results, one can deduce that R245fa per- forms worst in object parameter analysis, and R134a has the lowest net thermal efficiency for an OTEC system. These results also indicate that R600a is most suitable for operating at high evapo- rating and condensing temperatures in an OTEC system. To analyze the application of seawater thermal energy in the OTEC system, the variation of optimal seawater temperature dif- ference DTw,o in relation to Twci and Twwi are shown in Fig. 10(a) and (b). It should be noted that in this study, the temperature differ- ences, DTw, between the inlet and outlet for warm seawater of the evaporator, and for cold seawater of the condenser, are assumed to the same. The DTw,o of the OTEC system with R717 are both the highest among corresponding values for all the working fluids at various Twci and Twwi, because of the high thermal conductivity, as explained before. It is interesting to demonstrate that compared with other working fluids, the OTEC system has lowest DTw,o with R600a both at various Twci and Twwi. This phenomenon also reveals that the thermal energy absorbed from warm seawater and dis- charged to cold seawater is the lowest for R600a even though R600a has the highest thermal efficiency. Furthermore, DTw increased slightly with Twwi but decreased markedly with Twci, because lower cold seawater temperature increases pump power consumption obviously. This result also indicates that the cold- seawater inlet temperature, Twci, has a stronger effect than the Fig. 8. The influences of Tcwi on (a) Tcon,o, (b) Teva,o, (c) gmax, and (d) hth at Twwi 1⁄4 28 C.PDF Image | Analysis of optimization in an OTEC plant using ORC
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