Analysis of heat transfer characteristics of a novel liquid CO2 energy storage system based on two-stage cold and heat storage

As the installed capacity of renewable energy such as wind and solar power continues to increase, energy storage technology is becoming increasingly crucial. It could effectively balance power demand and supply, enhance allocation flexibility, and improve power quality. Among various energy storage technologies, liquid CO₂ energy storage (LCES) stands out as one of the most promising options due to its advantages such as high round-trip efficiency (RTE), high energy storage density (ESD), safety, stability, and longevity. Within the system, the cold and heat storage units play a critical role in determining the overall performance of the system and are particularly important among its various components. In this paper, a novel LCES system is proposed and the heat transfer characteristics are analyzed in detail. Then, the impact of key parameters on the liquefaction ratio and RTE is discussed. The results indicate that the RTE, ESD, and exergy efficiency of the system are 56.12%, 29.46 kWh/m³, and 93.73% under specified design conditions, respectively. During the gas–liquid phase change process of carbon dioxide or when it is in a supercritical state, the related heat transfer processes become more complex, leading to increased energy loss. The analysis of key parameters of the Linde-Hampson liquefaction unit reveals that as the liquefaction temperature decreases, both the liquefaction ratio and RTE increase. While the liquefaction pressure has a minimal impact on the liquefaction ratio, it significantly affects RTE, with an optimal liquefaction pressure identified.