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Table 7 Performance comparison of fluids under the optimal operation parameters based on APR. mize the utilization of the geothermal. So the favored working fluids in terms of geothermal utilization were the fluids in trans- critical power cycle with R218, R41 and R125. 5.4. Economic analysis 5.4.1. APR as the objective function A cost effective design optimization was conducted for organic Rankine cycle using low-temperature geothermal resources. And the ratio of total heat exchanger area to net power output was used as the objective function. As was shown in Fig. 11, the objective function value of R152a in subcritical ORC reached a minimum for specific values of evap- oration and condensation temperatures. So it was necessary to conduct the optimization process for the two variables (Pin and Pout). Table 7 and Fig. 12 showed the value of the optimum objec- tive function and the operation parameters for different working fluids. In subcritical ORC cycle, R152a had the least objective func- tion value of 1.64 m2/kW, followed by R134a and R600. R123 yielded the highest objective function value of 2.38 m2/kW, which was 45% higher than that of R152a. In order to have a better insight about the selection of the working fluid, the heat exchanger area and the net power output were described in the Table 7. It was ob- served that R123 provided the net power output of 5.4 kW, 3.9% higher than that of R152a. However, the heat exchanger area of R123 was 40% larger than that of R152a, leading to the highest APR value. In transcritical power cycle, R143a exhibited the least APR value of 2.02 m2/kW and was a cost-effective fluid. And R170 provided the highest APR value, about 59% larger than that of R143a. Among the fluids considered, R41 produced the highest net power output of 9.3 kW, but the heat exchanger area was 82.5% larger than that of R143a. As a result, R41 gave the APR value of 2.66 m2/kW, 31.7% higher than that of R143a. To demonstrate the differences in the subcritical ORC and the transcritical power cycle, the economic performance comparison was conducted and the results were presented in Table 7. It was observed that the choice of working fluid could greatly affect the power plant cost. It was indicated in the Table 7 that the fluids in transcritical power cycle system took the advantage of high net power output. For example, the net output power of R143a was 28.7% and 23.8% larger than that of R152a and R123, respec- tively. However, due to the large heat absorption capacity, more Z. Shengjun et al. / Applied Energy 88 (2011) 2740–2754 2751 Refrigerant R123(Sub) R245ca(Sub) R245fa(Sub) R600(Sub) R236ea(Sub) R600a(Sub) R236fa(Sub) R152a(Sub) R227ea(Sub) R134a(Sub) R143a(Tran) R218(Tran) R125(Tran) R41(Tran) R170(Tran) CO2(Tran) Pin (MPa) 0.42 0.49 0.71 0.85 0.91 1.18 1.14 2.06 1.78 2.31 3.99 3.21 4.57 8.02 7.02 11.80 Pout (MPa) 0.10 0.11 0.18 0.27 0.23 0.40 0.30 0.65 0.56 0.77 1.47 1.02 1.57 4.20 4.46 6.58 tmax (°C) 74 74 76 72 76 76 76 74 78 74 84 84 84 79 77 84 A (m2) 12.2 11.1 9.2 10.5 10.1 9.6 10.4 8.7 9.0 9.6 13.7 21.2 20.8 25.0 23.2 6.7 Wnet (kW) 5.4 5.2 4.3 5.8 4.6 5.1 4.7 5.2 3.9 5.4 6.7 6.9 7.9 9.3 7.2 2.6 transcritical power cycle. For example, the EDFc and EDFr,p value of R41 was 68.7% and 29.5 times higher than that of R123. And the EDFtotal value of R41 was 57.1% higher than that of R123. The sec- ond law efficiency could be expressed as Eq. (41) and was shown in Fig. 9. 1=gII 1⁄4 1 þ EDFtotal EDFtotal 1⁄4 EDFh þ EDFc þ EDFr;p þ EDFturbine ð42Þ Due to the high EDFtotal value, the second law efficiency of R41 was 28.3% less than that of R123. So the fluids in transcritical power cy- cle exhibited less second law efficiency. 5.3. Recovery efficiency as the indicator The recovery efficiency was an indicator for evaluating the ratio of available energy recovered from the geothermal water. With the assumption given in Table 3, the maximum theoretical power which could be recovered from the geothermal was 56.7 kW. The parameter optimization was performed on different fluids and the results were presented in Fig. 10. It was shown in Fig. 10 that the highest recovery efficiency was 19.2% delivered by R218 followed by R41 and R125. These fluids in transcritical power cycle recovered much more available thermal power and could maxi- ð41Þ Fig. 12. The APR value of different working fluids under the optimized operation parameters.PDF Image | Performance comparison and parametric optimization of subcritical Organic Rankine Cycle (ORC) and transcritical power cycle system for low-temperature geothermal power generation
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