Thermo-economic performance assessment of a liquid CO2 energy storage system with different sensible heat storage materials

Abstract

Liquid CO₂ energy storage systems offer a promising solution for large-scale energy storage, where the selection of heat storage materials plays a critical role in system performance. This paper investigates the effects of various heat storage materials on the thermo-economic performance of a liquid CO₂ energy storage system, including L-QB300, HITEC molten salt, HITEC XL molten salt, solar salt, Therminol 66, Therminol VP-1, and rapeseed oil. A thermo-economic model is developed, and multi-objective optimization is employed to analyze how these materials affect system performance. The findings indicate optimal configurations for high-pressure tank pressure, low-pressure tank pressure, booster pump pressure rise, and heat storage material split ratio, all of which enhance round-trip efficiency. Specifically, increasing high-pressure tank pressure and throttle valve pressure drop improves energy storage density, while elevated low-pressure tank pressure has a reducing effect. An optimal heat storage material split ratio also maximizes energy storage density. Additionally, configurations that minimize the levelized cost of electricity are identified for high-pressure tank pressure, low-pressure tank pressure, and split ratio, although a higher throttle valve pressure drop results in increased costs. When compared to a pressurized water system, the round-trip efficiency, energy storage density, and levelized cost of electricity improve by 1.00–1.59 %, 4.42–7.47 %, and 0.45–3.05 %, respectively. Among the heat storage materials evaluated, HITEC molten salt demonstrates the best overall performance, achieving a round-trip efficiency of 60.38 %, an energy storage density of 17.7 kW·h/m³, and a levelized cost of 0.1554 $/kW·h.