Energy storage device based on a hybrid system of a CO2 heat pump cycle and a CO2 hydrate heat cycle

A new large-capacity energy storage device (with a storage capacity of several megawatt-hours or more) based on a hybrid cycle of a CO2 heat pump cycle and a CO2 hydrate heat cycle is investigated using an experiment-based numerical analysis. In the charging mode of the CO2 heat pump cycle, the work of the compression process is input with surplus electricity from renewable energy sources. Liquid CO2 is stored in a tank after the condensation process. In the discharge (power generation) mode, the liquid CO2 stored in the tank is heated by outside air, and the expanded gas is supplied to the expander, which then operates the generator to obtain electricity from the expansion work. However, if the outside temperature is low, the expansion of liquid CO2 will be insufficient. The abovementioned CO2 hydrate heat cycle is added to the CO2 heat pump cycle. The resulting energy volume density of the proposed system is 80–140 kWh/m3, with a charge–discharge efficiency of 60%–106%, and the equipment costs should be similar to those of pumped storage. At ambient temperatures of <10 °C, the hybrid system with energy storage by gas hydrates is more effective, with a higher energy volume density and charge–discharge efficiency than previous systems.