Analysis of 5E (energy, exergy, energy level, exergoeconomics, and exergetic sustainability) for a liquid CO₂ energy storage system coupling solar energy and LNG

As the demand for energy transition becomes increasingly urgent, the intermittent challenges faced by renewable energy grid integration urgently need to be overcome, and efficient energy storage technology has become a key support. This paper proposes a liquid carbon dioxide energy storage system coupled with solar energy and liquefied natural gas cold energy, innovatively constructing a 5E evaluation system covering energy, exergy, exergoeconomics, energy level, and exergy environmental sustainability, achieving systematic analysis from energy conversion to economic and environmental impacts. The research results show that the inlet temperature and pressure of Compressor 1 have significant positive effects on system exergy efficiency and unit exergy cost: the exergy efficiency can be improved by up to 4.02%, and the unit exergy cost is correspondingly reduced by 4.21%. Exergoeconomic analysis indicates that the costs of expanders and high/low pressure gas storage tanks in the system mainly come from equipment investment, while the costs of heat exchangers and cold/hot storage units are mainly contributed by exergy destruction during operation. In multi-objective optimization, with the objectives of maximizing energy storage efficiency and minimizing total product unit cost, the Pareto optimal frontier is obtained using the NSGA-II algorithm, and the TOPSIS decision-making method is combined to select the best compromise solution. This point corresponds to an energy storage efficiency of 68.05% and a total product unit cost of 77.71 $/GJ, significantly improving system energy storage performance while ensuring economic viability, providing theoretical basis and optimization direction for the liquid carbon dioxide energy storage systems.