Performance analysis and additive screening of a liquid carbon dioxide mixture energy storage system coupled with a coal-fired power plant

Liquid carbon dioxide (CO2) energy storage is a promising technology for balancing grid supply and demand, but liquefaction in high temperature environments is a substantial dilemma. In this study, a novel liquid CO2 mixture energy storage system coupled with a coal-fired power plant is proposed to broaden the liquefiable ambient temperature range, where condensate and feedwater without phase change are used for compression heat recovery and CO2 mixture heating. Firstly, eight refrigerant additives are selected to blend with CO2; then, comparative performance analysis between CO2 mixtures and pure CO2 is conducted, followed by parametric studies; finally, a double-layer decision-making framework based on multi-objective optimization is adopted for additive screening across various ambient temperatures. The results show that a moderate amount of additive favors system safety but degrades thermodynamics and economy, well at high temperatures the performance of CO2 mixture improves and absolutely outperforms pure CO2. CO2/difluoromethane (CO2/R32) reduces the system operating pressure by 20.95 % over pure CO2 at ambient temperature of 298.15 K, accompanied by only 5.78 %, 0.76 % and 9.92 % deterioration in round-trip efficiency, energy storage density and levelized cost of electricity, and exhibits optimal adaptation to varying additive mass fraction. The working fluid unit cost and heat exchangers are the main factors causing these variances. In response to environmental changes, CO2/pro- pylene (CO2/R1270) and CO2/R32 emerge as the two most adaptable mixtures, with the former preferred at low ambient temperatures (288.15-293.15 K) and the latter at high ambient temperatures (298.15-308.15 K). Among all additives, R600 and R161 consistently rank lowest in priority regardless of ambient temperature.