Thermodynamic, economic, and environmental analyses and multi-objective optimization of an organic flash regenerative cycle with an ejector for geothermal heat utilization

Abstract

Based on the existing organic flash regenerative cycle (OFRC), this paper presents a novel OFRC system incorporating an ejector (E-OFRC). The ejector replaces the high-pressure throttle valve for initial flash evaporation and utilizes a low-pressure throttle valve for the second flash evaporation of saturated liquid separated from the first flash. The gaseous working fluid from the second flash is employed as the ejecting fluid. The E-OFRC system employs an ejector to mitigate irreversible losses associated with high-pressure throttle and to drive gaseous working fluids from secondary flash to circulate to enhance overall system performance. Thermodynamic, economic, and environmental models are established, and the impacts of flash pressure (P4), endothermic pressure (P3), and geothermal water temperature (THS,in) on system performance are analyzed and compared with those of the OFRC system. Multi-objective optimization using the Dragonfly algorithm is conducted, while the TOPSIS method is utilized to determine optimal operating parameters for the E-OFRC system. The findings indicate that, in comparison to the OFRC system, the thermal efficiency of the E-OFRC system is only lower under P3, while its thermodynamic, economic, and environmental performance is enhanced under other considered conditions. This underscores the advantages of the E-OFRC system. The optimal operating parameters for the E-OFRC system are THS,in = 423.53 K, P3 = 1.89 MPa, P4 = 0.96 MPa; yielding a net output power of 84.75 kW, a specific investment cost of 2833.54$/kW, and an annual CO2 equivalent emission reduction of 0.807 × 106kg − providing valuable technical support for engineering applications.