A Brayton Pumped Thermal Energy Storage System Based on Supercritical Carbon Dioxide Recompression Reheating Discharge Cycle

Abstract

In recent years, renewable energy such as solar energy and large-scale energy storage, which is a very important technology to compensate for solar energy's fluctuation due to weather issues, have been extensively investigated. In this paper, a Pumped Thermal Energy Storage (PTES) cycle based on a supercritical carbon dioxide (sCO2) Recompression Reheating cycle and energy pump with a recuperator has been proposed and analyzed. Molten salt with varying temperatures of 565 °C to 730 °C has been used for energy storage. The pressure ratios have been fixed in the discharge cycle as 2.5, and in the charging cycle, it varies in order to find the optimum operation condition. Parametric studies have been made to determine the best performance of the new system. Molten salt temperature, split ratio, pressure ratio, and intermediate pressure have been varied in the calculation. Exergy analysis has been developed in order to determine exergy destruction in all components. Roundtrip efficiencies have been calculated over a wide range of operating conditions. Different working fluids such as argon, carbon dioxide, and nitrogen were used in both cycles. Performance was determined for different combinations of working fluids. It is concluded that for best performance working fluid for energy pump (charging) should be argon and carbon dioxide should be the working fluid for discharging cycle. For this combination operating at optimum molten salt temperature, intermediate pressure and split ratio in the discharging cycle, the roundtrip efficiency is 66%, which is the maximum.