Thermodynamic performance of subcritical double-pressure organic Rankine cycles driven by geothermal energy

Abstract

Abstract In the utilization of hot dry rock geothermal energy, the temperature of production well varies with the injection temperature of the recharge well and operation time. Taking into account of the variation of production well temperature, three kinds of double-pressure Organic Rankine Cycle (DPORC) with different layouts are constructed in present work, and the effects of different layouts on the thermodynamic performance of DPORC are probed. The results indicate that present DPORCs can effectively improve the thermodynamic performance of the system, and the net output power of the systems can be improved by at least 5.68%, 6.42% and 15.75%, compared with those of single-pressure Organic Rankine Cycle (SPORC). In addition, one of proposed DPORCs can reduces the irreversible loss in system and has better thermodynamic performance. When the injection temperature of the recharge well remains constant, the net output power, thermal efficiency and exergy efficiency reach maximum at the same k (the ratio of absorbed heat in high-pressure stage cycle to that in DPORC). After 40\xa0years of operation, the net output power of the three arrangements and SPORC decrease by 434.07\xa0kW, 438.04\xa0kW, 463.94\xa0kW and 465.72\xa0kW, respectively. Therefore, it’s significant to reduce the temperature drop of hot dry rock production well to maintain the stable output of the system. It’s found that the exergy loss in system is mainly concentrated in evaporator, condenser and turbine. Reasonable layout can reduce the exergy loss of the system. The variation trend of cycle perfection with k is the same as other parameters do, and the maximum cycle perfection is achieved when the net output power of system is near the maximum. With the increase of running time, the cycle perfection of DPORC increases slightly. Specifically,\xa0the cycle perfection of one proposed DPORC rises in a wavy manner.