Surface processes optimisation in a novel CO2-based electrothermal energy and geological storage trigeneration system

Electrothermal energy storage is a promising technology for high penetration of renewable energy. In recent years, the integration of this energy storage system with geological CO2 storage has been introduced. The system consists of a reversible heat pump formed by transcritical CO2 cycles with thermal storage at two temperature levels, enabling the simultaneous operation of geological CO2 storage and the storage/production of renewable electrical energy. This work focuses on studying high and low-temperature thermal energy storage. Step heating on the high-temperature side allows for better integration of the supercritical and subcritical temperature profiles of the CO2 and the thermal storage fluid. Thermal storage at different temperature levels provides a higher turbine inlet temperature, improving the efficiency of the power production cycle and increasing heating applications such as district heating or domestic hot water. Considering four high-temperature tanks, round-trip efficiency increases from 52.8 to 55.4 %. It presents a thermal demand coverage range of about 20–150 °C, with temperature increases of approximately 30 °C. The phase change temperature shift on the low-temperature side directly impacts electric power production and enables new cooling applications. The system's efficiency increases as the low-temperature phase change temperature decreases, reaching 58.7 % at −30 °C. Using alternative configurations in the transcritical CO2 cycle, such as the recuperative cycle and multi-stage compression and expansion, high-efficiency values can be maintained with lower system requirements.