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Fig. 7. The influences of DTw on (a) Teva,o and Tcon,o and (b) corresponding gmax at Twci 1⁄45CandTwwi 1⁄428C. of g can be seen to increase with evaporation temperature, reach a maximum, and finally decrease. This result indicates that an optimal objective parameter exists with which maximal net power output per unit area of heat exchangers can be obtained from the system. For convenience, the optimal condensing and evaporating temperatures, which have the corresponding maximal objective parameter gmax in the OTEC system, are denoted as Tcon,o and Teva,o in this work. Note that gmax 1⁄4 0.203 is obtained at the corre- sponding optimal evaporating temperature Teva,o 1⁄4 22 C for R717 at Tcon 1⁄4 10.7 C. Table 2 The gmax and its corresponding to Teva,o, Tcon,o and DTw,o for an OTEC system with Tcwi 1⁄45CandTwwi 1⁄428C. Conversely, high condensation temperature decreases the net power output because the enthalpy of the turbine outlet increases, and the enthalpy difference between the turbine inlet and outlet decreases, as shown in Fig. 5(a). The figure also shows that R600a has largest net power output at Tcon 1⁄4 8e14 C and Teva 1⁄4 22.3 C. Furthermore, R134a and R245fa have small Wnet, values that differed only slightly. Similarly, in Fig. 5(b), the At curves show a tendency to decline with condensation temperature. This is because as the condensation temperature rises, the temperature difference between cold seawater and the working fluid in the condenser increases, resulting in a reduction in the total heat transfer area. The At of R245fa is the largest among the working fluids, as mentioned earlier. This is because of the weak thermal conductivity of R245fa. Fig. 5(c) demonstrates the influence of Tcon on g at Teva 1⁄4 22.3 C for each working fluid. As expected, with increasing condensation temperatures, the values of g increase first, then peak, and finally decrease. The maximal value, gmax 1⁄4 0.203, occurs at Tcon,o 1⁄4 11.1 C for R717. In objective parameter evaluation, R717 performs clearly better than the other working fluids tested. To indicate the optimal operating temperatures of the ORC system, the distributions of g for various evaporation and condensation temperatures at DTw 1⁄4 2.1 C are presented in Fig. 6(a)e(e) for each working fluid. As expected, a series of optimal evaporation temperatures, Teva,o, and optimal condensation tem- peratures, Tcon,o, can be obtained for the maximal ratio of Wnet to At. These contours of g show the variations of optimal operating temperatures for the working fluids. These figures show that the gmax 1⁄4 0.204 of R717 is the largest among the five working fluids for DTw 1⁄4 2.1 C. 4.4. Effects of seawater temperature difference in ORC To illustrate the influence of seawater temperatures different on operating temperatures and g, the variations of optimal conden- sation and evaporator temperatures from DTw 1⁄4 1e4 C are revealed in Fig. 7(a) at Twwi 1⁄4 28 C and Tcwi 1⁄4 5 C. In this study, seawater temperature difference between the inlet and outlet, DTw, is assumed to be the same in the evaporator and condenser for warm and cold seawater. As the seawater temperature difference increases in the heat exchangers, the optimal evaporation tem- peratures rise and the optimal condensation temperatures decline. Notably, optimal evaporation and condensation temperatures of R600a in all cases are larger than those of others working fluids, and the optimal operational temperatures of R134a and R245fa are smaller than those of other fluids. Thus, R600a is suitable for higher operational temperatures of the ORC system. Reducing condensa- tion temperature and enhancing evaporation temperature augment the power output in the ORC system, but the changes also increase the heat transfer rate in the condenser and evaporator. These lead to an increase in the sizes of the heat exchangers of an OTEC system and in the cost of the apparatus. Fig. 7(b) shows the gmax variation of an OTEC system in relation to DTw under optimal operational temperatures; with increasing DTw, the gmax values of all working fluids rise to their peak values and then decline. The variation of gmax of R152a and R600a are close to various Teva,o and Tcon,o. The maximal objective parameter, gmax, and its corresponding optimal operating temperatures, Teva,o and Tcon,o, and DTw,o of the OTEC system at Tcwi 1⁄4 5 C and Twwi 1⁄4 28 C, are obtained numerically and displayed in Table 2. With various Teva,o and Tcon,o, the DTw,o values from high to low are for R717, R152a, R245fa, R134a, and R600a. Although the optimal evaporation temperatures, Teva,o, are similar for all the working fluids, both Teva,o and Tcon,o of R600a are higher than corresponding values of other fluids. M.-H. Yang, R.-H. Yeh / Renewable Energy 68 (2014) 25e34 31 Teva,o (C) Tcon,o (C) DTw,o (C) gmax (kW/m2) R717 22.32 10.78 1.61 0.2068 R134a 22.6 11.02 1.56 0.1157 R245fa R152a 22.41 22.62 10.69 10.98 1.58 1.591.5 0.1011 0.1282 R600a 22.75 11.13 0.1283PDF Image | Analysis of optimization in an OTEC plant using ORC
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