Thermodynamic and economic assessment of a reversible organic Rankine heat pump system for energy storage

This study presents the energy, exergy, and thermo-economic analysis of a hybrid waste-heat-assisted pumped thermal energy storage (PTES) system. Although PTES is a promising solution for large-scale energy storage, its deployment is constrained by moderate round-trip efficiency and economic challenges, motivating waste-heat integration and working fluid optimization. The system operates as a heat pump during charging and as an organic Rankine cycle during discharging. A MATLAB-based simulation incorporating particle swarm optimization was developed to evaluate the system under first- and second-law formulations. The cost formation of exergy streams was also examined through thermo-economic analysis. Both pure and zeotropic working fluids were investigated. For the pure working fluid case, the power-to-power efficiency, heat pump COP, and ORC energy efficiency were 0.7196, 6.9916, and 0.1029, respectively, while for the zeotropic case these values increased to 0.7799, 7.0545, and 0.1106. A parametric study assessed the effects of storage temperatures, component isentropic efficiencies, and evaporator inlet temperature. Results show that reducing reservoir temperature difference, improving isentropic efficiencies, and increasing evaporation temperature enhance performance and reduce the levelized cost of energy storage. Cyclohexane/pentane exhibited the highest efficiencies and lowest cost among the mixtures analyzed, highlighting the thermodynamic and economic benefits of waste-heat integration and zeotropic mixtures in PTES systems.