Comparative analysis of system performance of thermally integrated pumped thermal energy storage systems based on organic flash cycle and organic Rankine cycle

Pumped thermal energy storage (PTES) technology has become an increasingly attractive research field because of its low cost, long lifetime, and lack of geographical restrictions. However, thermally integrated PTES (TIPTES) systems that integrate low-grade heat sources are more promising because they can significantly improve system performance. In this study, TIPTES systems driven by geothermal water employ the organic flash cycle (OFC-TIPTES) and organic Rankine cycle (ORC-TIPTES) as discharge cycles. Five different working fluids, including R123, Butane, Cyclohexane, R1233zd(E), and Butene, are selected as candidates. Parametric study is performed to analyze the effects of main parameters on system thermodynamic and thermo-economic performance. Then, comparative study is conducted based on the multi-objective optimization resutls to search out the optimal cycle structure and working fluid. Results indicate that when the thermal storage temperature increases, the variation trends of the exergy efficiency and levelized cost of storage (LCOS) are identical, but the variation trends of the power-to-power efficiency are different for two systems. And a higher evaporation temperature increases the thermodynamic efficiency of both systems. For the OFC-TIPTES system, a maximal thermodynamic efficiency exists at an optimal flash temperature. The multi-objective optimization results indicate that the ORC-TIPTES has higher power-to-power efficiency, exergy efficiency, and lower LCOS than the OFC-TIPTES. Butane and Cyclo-hexane yield the final solutions for power-to-power efficiency and LCOS for the OFC-TIPTES (33.55 % and 0.83 $.kWh(-1)) and the ORC-TIPTES (68.42 % and 0.34 $.kWh(-1)), respectively. The exergy destructions of the throttle valve before the flash tank and condenser of the heat-pump cycle are maximal, that is, 232 kW and 692 kW for the OFC-TIPTES and ORC-TIPTES systems, respectively. The costs of the condenser of the OFC/ORC subsystem, turbine, and compressor are higher for all components of both systems. A comparative analysis shows that the ORC-TIPTES system exhibits better performance than the OFC-TIPTES system in terms of thermodynamics and economics.