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2752 Z. Shengjun et al. / Applied Energy 88 (2011) 2740–2754 Fig. 13. Variation of the LEC with the condensing temperature and evaporating temperature. Table 8 Performance comparison of fluids under the optimal operation parameters based on LEC. According to Fig. 14, R152a provided the lowest value followed by R600 and R600a. R123 gave the highest LEC value of 0.062$/ (kWh). In transcritical power cycle system, LEC was a function of the three independent variables specified in Section 5.2. R143a and R125 exhibited the same LEC value of 0.056$/(kW h). And CO2 had much higher operating pressure, which resulted in addi- tional expenses in the plant design, leading to a high objective function value. Two observations may be made about the results for the eco- nomically optimum working fluids. Firstly, the minimum objective function value was 0.053$/(kWh) yielded by R152a at the evaporation pressure and condensation pressure of 1.5 MPa and 0.65 MPa, respectively. The optimizing maximum pressure was significantly lower than the corresponding value determined in previous section when APR was used as the criteria. The reason for this result was the important dependence of the heat exchanger costs on the pressure. Indeed, their prices increased rapidly when the pressure was augmented and the lowest possible pressure was the best from this point of view. On the other hand, at low pressures the heat exchangers area increased and consequently so did their cost. With these two opposite effects, a minimum total cost was reached at a certain maximum pressure lower than the one minimizing the surfaces. Without considering the effect of pressure on the cost, APR ignored the economic difference between subcritical ORC and transcritical power cycle system caused by the pressure, which might lead to erroneous results. This validated the need for an economic optimization to determine the best operation parameters. The cost being habitually the most important issue, it seemed that the optimizations of the previous sections could not lead to the best values for the independent parameters. Secondly, the economic comparison showed that the LEC with R152a was lower, by as much as 17.1%, than that of R123. However, the differ- ence in APR was 45.1%. The reduction in economic performance partially owed to the high operation pressure of R152a, which re- sulted in a high cost of the system. If the cost of turbine and refrig- erant pump were involved in large-scale system, the LEC would be different. From Fig. 14, it could be observed that the LEC value of R143a, R125and R41 in transcritical power cycle was similar with that of R152a in subcritical ORC. Moreover, the transcritical power cycle 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) 0.30 0.35 0.49 0.64 0.64 0.95 0.85 1.50 1.23 1.76 3.84 2.83 4.06 7.90 6.82 11.21 Pout (MPa) 0.10 0.11 0.17 0.27 0.23 0.38 0.30 0.65 0.50 0.77 1.36 0.94 1.49 4.11 4.37 6.58 tmax (°C) 62 62 62 60 62 64 64 60 62 62 79 77 78 84 83 84 Mp Me (L/year) (kg/year) 18,054 60,653 18,285 61,431 18,399 61,816 18,556 62,340 18,662 62,697 17,963 60,348 18,442 61,959 18,387 61,772 19,525 65,598 17,888 60,097 18,374 61,731 22,788 76,560 22,032 74,019 23,491 78,922 19,265 64,725 16,570 55,668 heat exchanger area was required in transcritical power cycle. The heat exchanger area required for R143a was 57.4% and 9.8% larger than that of R152a and R123. As a result, the objective function va- lue of R143a was 23% larger than that of R152a, but 14.8% smaller than R123. 5.4.2. LEC as the objective function The last section of the study addressed the economic perfor- mance and the levelized energy cost(LEC) was used as the indica- tor. As mentioned before, it was not a detailed cost analysis but only an approximate evaluation of the main components’ price. The environment performance of the subcritical ORC and transcrit- ical power cycle system was also estimated. As shown in Fig. 13, the objective function value of R152a reached a minimum one for a specific combination of evaporation and condensation temperature in subcritical ORC cycle. Optimum cycle performance and parameters at the minimum objective function value were listed in Table 8 and Fig. 14, respectively.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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