Performance of a CO2-mixture cycled energy storage system: Thermodynamic and economic analysis

Liquid state storage is one of the most features for CO2 energy storage. How to effectively liquefy the gaseous CO2 after generating the power severely restricts its further development. In this paper, the refrigerants are employed as the additives to the CO2 working fluid and utilizing the temperature glide of zeotropic mixtures the two-tank cold energy storage can be designed to complete the condensation and evaporation processes of the gaseous working fluid. The numerical simulations for the system are completed by using an in-house code with the built mathematical models. The round trip efficiency and the economic indicators including levelized cost of storage, net present value, dynamic payback period and savings to investment ratio are analyzed in detail. The results demonstrate that increasing the mass fraction of refrigerants is negative to the system efficiency. The efficiency of the system with working fluid CO2/R32 is obviously insensitive to the additive mass fraction, with just a 3.5% change ratio when mass fraction of R32 is varied from 0.05 to 0.5. There is always an optimized mass fraction to reach the best economic performance for each additive. The mixtures CO2/R32 and CO2/R161 are firstly rec-ommended as the working fluid pairs of the designed system with the suggested mass compositions of about 0.65/0.35 and 0.85/0.15, respectively. The charge pressure is advocated at 15 MPa and the discharge pressure is suggested at 13-16 MPa. Furthermore, the proposed energy storage system is favorable to adapt the surrounding temperatures.