Thermodynamic analysis of a novel hybrid thermochemical-compressed air energy storage system powered by wind, solar and/or off-peak electricity

Abstract

In this paper, a hybrid energy storage system based on integrated thermochemical and compressed air energy storage is proposed. This hybrid system can store energy from wind, solar and/or off-peak electricity simultaneously. In the energy charging process, the concentrated solar heat is used to provide heat for the endothermal reduction of tricobalt tetroxide to cobalt monoxide. Meanwhile, wind energy is employed to drive a series of compressors for compressing the input ambient air. The products (cobalt monoxide and compressed air) are then stored temporarily. In the energy discharging process, the compressed air is released to oxidize cobalt monoxide which generates high-quality heat. The superheated and pressurized air is then used for electricity generation via air turbines. In conventional compressed air energy storage, natural gas is employed for superheating the compressed air. By contrast, the proposed hybrid system completely eliminates the use of fossil fuel and replaces it with the exothermal oxidation heat of cobalt monoxide, which originates from solar energy. Moreover, cobalt oxides can be stored at ambient temperature for an extended period without any insulation requirements, which renders the system to be a long-term energy storage solution. According to the thermodynamic analysis, the hybrid system can achieve a round trip efficiency of 56.4% with an energy storage density of 3.9 kWh/m3. Meanwhile, the proposed system has an overall exergy efficiency of 75.6% with the largest exergy destruction process taking place in the intercoolers. The sensitivity analysis demonstrates that the round trip efficiency is mostly sensitive to the reactivity of cobalt monoxide, the isentropic efficiency of compressors and turbines, and cavern operating pressures.