Design and thermodynamic performance analysis of a new liquid carbon dioxide energy storage system with low pressure stores

Highlights: • A novel liquid CO₂ energy storage system with low pressure stores is proposed. • The sensible and latent cold energy of CO₂ after expansion is separately stored. • The efficiency and energy density are 51.45% and 22.21 kW h/m³ at design condition. • A peak value of efficiency and energy density exists as discharge pressure varies. • The optimum allocations of compression and expansion ratios equal to 0.7. Liquid CO₂ energy storage system is currently held as an efficiently green solution to the dilemma of stabilizing the fluctuations of renewable power. One of the most challenges is how to efficiently liquefy the gas for storage. The current liquid CO₂ energy storage system will be no longer in force for high environmental temperature. Moreover, the CO₂ storage pressure is usually high with resulting in the high requirements on component materials. A novel liquid CO₂ energy storage system with low pressure stores is thus proposed in this paper. The sensible cold energy is stored by liquid methanol and the latent cold energy is stored in the latent cold storage for the sake of liquefying the discharging CO₂ after expansion. The mathematical model of the system is established for thermodynamic study. The analysis results indicate that the round trip efficiency and energy density of the system can be respectively 51.45% and 22.21 kW h/m³ at the typical default conditions. The round trip efficiency increases with a rise in charging pressure first and then appears a level-off with a striking inflection point. Moreover, the inflection point moves toward right for a larger discharging pressure. There is a peak value of the system round trip efficiency and energy density when the discharging pressure is changed. The allocations of compression ratio and expansion ratio should equal to 0.7 to reach the maximum value of round trip efficiency.