The Chingshui geothermal field is the largest known productive geothermal area in Taiwan. The purpose of this paper is to delineate this geothermal structure by integrating geophysical data and borehole information. The existence of a magma chamber in the shallow crust and shallow intrusive igneous rock results in a high heat flow and geothermal gradient; furthermore, the NE deep fault system within the meta-sandstones provides meteoric recharge from a higher elevation to artesianally drive the geothermal system. There is evidence that geothermal fluid deeply circulated within the fracture zone and was heated by a deeply located body of hot rock. The geothermal reservoir of the Chingshui geothermal field might be related to the fracture zone of the Chingshuihsi fault. It is bounded by the C-fault in the north and Xiaonanao fault in the south. Based on information obtained from geophysical interpretations and well logs, a 3-D geothermal conceptual model is constructed in this study. Further, the geothermal reservoir is confined to an area that is 260 m in width, N21°W, 1.5 km in length, and has an 80°dip toward the NE. A high-temperature zone is found in the SE region of the reservoir, which is about 500 m in length; this zone is located near the intersection of the Chingshuihsi and Xiaonanao faults. An area on the NE side of the high-temperature zone has been recommended for the drilling of production wells for future geothermal development.
SUMMARYTheoretical analyses of natural and conventional working fluids-based transcritical Rankine power cycles driven by low-temperature geothermal sources have been carried out with the methodology of pinch point analysis using computer models. The regenerator has been introduced and analyzed with a modified methodology considering the considerable variation of specific heat with temperature near the critical state. The evaluations of transcritical Rankine cycles have been performed based on equal thermodynamic mean heat rejection temperature and optimized gas heater pressures at various geothermal source temperature levels ranging from 80 to 1201C. The performances of CO 2 , a natural working fluid most commonly used in a transcritical power cycle, have been indicated as baselines. The results obtained show: optimum thermodynamic mean heat injection temperatures of transcritical Rankine cycles are distributed in the range of 60 to 70% of given geothermal source temperature level; optimum gas heater pressures of working fluids considered are lower than baselines; thermal efficiencies and expansion ratios (Exp r ) are higher than baselines while net power output, volume flow rate at turbine inlet (V 1 ) and heat transfer capacity curves are distributed at both sides of baselines. From thermodynamic and techno-economic point of view, R125 presents the best performances. It shows 10% higher net power output, 3% lower V 1 , 1.0 time higher Exp r , and 22% reduction of total heat transfer areas compared with baselines given geothermal source temperature of 901C. With the geothermal source temperature above 1001C, R32 and R143a also show better performances. R170 shows nearly the same performances with baselines except for the higher V 1 value. It also shows that better temperature gliding match between fluids in the gas heater can lead to more net power output.
A detailed thermodynamic and techno-economic comparison is presented for a CO 2 -based transcritical Rankine cycle and a subcritical organic Rankine cycle (ORC) using HFC245fa (1,1,1,3,3-pentafluoro-propane) as the working fluid driven by the low-temperature geothermal source, in order to determine the configuration that presents the maximum net power output with a minimum investment. The evaluations of both Rankine cycles have been performed based on equal thermodynamic mean heat rejection temperature by varying certain system operating parameters to achieve each Rankine cycle's optimum design at various geothermal source temperature levels ranging from 80 o C to 120 o C. The results obtained show that the optimum thermodynamic mean heat injection temperatures of both Rankine cycles are distributed in the scope of 55% to 65% of a given geothermal source temperature level, and that the CO 2 -based transcritical Rankine cycle presents 3% to 7% higher net power output, 84% reduction of turbine inlet volume flow rate, 47% reduction of expansion ratio and 1.68 times higher total heat transfer capacity compared with the HFC245fa-based subcritical ORC. It is also indicated that using the CO 2 -based transcritical system can reduce the dimension of turbine design. However, it requires larger heat transfer areas with higher strength heat exchanger materials because of the higher system pressure.transcritical Rankine cycle, organic Rankine cycle (ORC), low-temperature geothermal, CO 2 , HFC245fa Citation:Guo T, Wang H X, Zhang S J. Comparative analysis of CO 2 -based transcritical Rankine cycle and HFC245fa-based subcritical organic Rankine cycle using low-temperature geothermal source.
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