Design and techno-economic assessment of a transcritical CO2 pumped thermal energy storage system

Among long duration energy storage systems, Pumped Thermal Energy Storage with transcritical CO2 represents an effective solution being site-independent, with relatively high round-trip efficiencies, and including the possibility to store and dispatch both cold and hot thermal energy. This work proposes the sizing of the main components of a transcritical CO2 Pumped Thermal Energy Storage adopting the same heat exchangers during the charging and discharging phases. The system hot and cold storages are inspired by commercial solutions and consists of a pressurized water and a synthetic oil loop for the hot storage, while an ice slurry tank is employed for the cold storage. A MATLAB routine was developed to properly design the system and optimize its main variables to maximize the round-trip efficiency and minimize the storage costs. The investigated solution is designed with latent cold storage at 0 °C and sensible hot storage between 72 °C and 294 °C, having a maximum pressure of 250 bar resulting in a round-trip efficiency of 54.6 % and a specific cost of 2412 €/kWel,ch for 10 h of storage. The system levelized cost of storage is estimated as a function of the purchase price of electricity and the number of charge/discharge cycles per year, showing values competitive with more conventional long duration energy storage solutions. As the largest share of the overall CAPEX cost is related to the storage systems, a parametric analysis varying the minimum temperature differences of the heat exchangers is carried out to assess their effect on the techno-economic performance of the system.