Thermodynamic performance investigation of environmentally friendly working fluids in a geothermal integrated pumped thermal energy storage system

Abstract Among the available energy storage technologies, pumped thermal energy storage (PTES) is emerging as a potential solution for large-scale electrical energy storage with high round-trip efficiencies and no geographical limitations. However, PTES requires a low-cost, high-temperature heat source to achieve reasonable round-trip 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, isobutene, R1233zd(E), R1234ze(Z), R1224yd(Z), HFO1336mzz(Z), n-hexane, and n-pentane was investigated. For geothermal fluid outlet temperatures between 60 °C and 120 °C and geothermal fluid inlet and outlet temperature differences across the evaporator between 20 °C and 60 °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.