THERMODYNAMIC PERFORMANCE INVESTIGATION OF ENVIRONMENTALLY FRIENDLY WORKING FLUIDS IN A GOETHERMAL INTERGATED PUMPED THERMAL ENERGY STORAGE SYSTEM

Among the available energy storage technologies, pumped thermal energy storage (PTES) is emerging as a potential solution for large-scale electrical energy storage with no geographical limitations and high roundtrip efficiencies. However, PTES requires a low-cost high temperature heat source to achieve reasonable roundtrip efficiencies. Moreover, organic Rankine cycle-based PTES systems require high performance and environmentally friendly working fluids. In this study, the thermodynamic performance of a geothermal integrated PTES system using environmentally friendly working fluids is investigated. The mathematical model of the geothermal integrated PTES system is developed using the first and second laws of thermodynamics and implemented in Engineering Equation Solver (EES). With the developed model, the thermodynamic performance of the PTES system for different working fluids, including Butene, cyclopentane, iso-butane, R1233zd(E), R1234ze(Z), R1224yd(Z), HFO1336-mzz(Z), R365mfc, n-hexane, and n-pentane was investigated. For geothermal fluid outlet temperatures between 60 degrees C and 120 degrees C and geothermal fluid inlet and outlet temperature differences across the evaporator between 20 degrees C and 60 degrees C, the net power ratio i.e., the ratio of the electrical energy discharged to the electrical energy used to run the charging cycle is between 0.25 and 1.40. This shows that the system has the potential to give back more than 100% of the electrical energy used during charging under certain conditions. High net power ratios are obtained for a combination of high source temperatures and low geothermal fluid inlet and outlet temperature differences.