Preliminary thermodynamic analysis of a carbon dioxide binary mixture cycled energy storage system with low pressure stores

The liquid CO2 energy storage has considerable potential for power supply-demand management, but its low energy density, harsh condensation condition and high operation pressure are substantial obstacles. It is the first time to design energy storage system with high energy density and low-pressure stores by cycling the CO2 binary mixtures. By using CO2 mixtures, the pressure in storage tanks can be as low as ambient pressure (0.1 MPa) and two-tank cold energy storage with liquid storage materials can be used to complete the evaporation and condensation processes of working fluid in subcritical pressure. A large number of refrigerants are discussed and nine of them are selected as preliminary candidate additives into CO2. The performance of newly proposed system cycled with these additives is further analyzed by utilizing an in-house code developed with established thermodynamic models. The numerical results demonstrate that the mass fraction of additives is suggested to be higher than 0.12, otherwise there is high required capacity of cold storage material although a larger energy density. By and large, the mixtures CO2/R161 (0.7/0.3) and CO2/R1270 (0.82/0.18) are the first two most suitable selections among the CO2 mixtures, the round trip efficiency and energy density of which at basic design condition are 52.95% and 29.74 kWh/m3 and 52.12% and 29.83 kWh/m3, respectively. Both the optimal charge and discharge pressures can be identified for round trip efficiency while the energy density increases monotonically with charge pressure and the increasing trend is first steep and gradually becomes slow.