Performance of a high-temperature transcritical pumped thermal energy storage system based on CO2 binary mixtures

Pumped thermal energy storage is a novel energy storage technology with features of high efficiency, geographical independence and suitable for bulk capacity energy storage. As a subset of pump thermal energy storage system, the transcritical CO2 arrangements have received widespread attention due to their excellent thermodynamic performance. However, the high cost and low efficiency of heat exchange progress caused by CO2 phase transition hinder the transformation of this system from scientific theory to practical application. CO2 binary mixtures is innovatively employed as working fluid in proposed system. The temperature of mixture is variable rather than remaining constant during the evaporation and condensation progresses. R134a, R290, R600 and R601 are selected respectively to mix with CO2 to form the new working fluids. Advanced thermodynamic and economic models are established to investigate the performance of novel systems. Results indicate that the system driven by CO2/R134a with a refrigerant mass fraction of 0.15 exhibits the optimal overall performance. At the optimal point of the 10 MW/8h system, the round trip efficiency and levelized cost of storage are 66.18 % and 0.146 $/kWh, respectively. Moreover, the investigation of system performance with various energy storage capacities is put forward. It is shown that when the energy system capacity reaches 300 MW, the levelized cost of storage can be reduced to 0.122 $/kWh. To adapt to different peak and valley periods in different regions, systems with different charge and discharge times are investigated and it is found that the charge duration should better be equal to the discharge duration.