Design of an artificial cavern compressed air energy storage in isobaric operation

Artificial caverns have been extensively engineered in the compressed air energy storage commercial demonstration projects. However, one urgent issue is how to significantly reduce the air cavern size due to the inevitably large capacity of cushion air. Another problem is the limited efficiency at about 65% in typical cases owing to the low heat storage temperature and throttling losses. Researchers are thus motivated to further develop this type of energy storage considering its inconspicuous thermodynamic and economic characteristics. This article presents three artificial cavern air energy storage systems characterized by high temperature and isobaric operation. A techno-economic model is described in detail to perform an accurate assessment of the performance and capital cost of the systems. It is revealed that the compressed air volume in given systems is only about one-fifth of that in the isochoric system with 4 MPa throttling pressure, typically near 40000 m3 for a 300MW/1500 MWh system at the depth of 200 m. In this case, the efficiency of the three-stage system with Hitec salt is 70.15% and its levelized cost of storage is 0.7521 ¥/kWh when the price of off-peak electricity is 0.3 ¥/kWh. For comparison, the levelized cost of storage of the isochoric system with the same parametric settings is as high as 0.8111 ¥/kWh. Compressors and turbines are the first key components that should be optimized as they contribute to the vast majority of exergy destructions and capital expenditures. Looking forward to the future, the system efficiency will be high up to 78%-80% with rapid development of turbomachines and heat exchangers in the near future. This efficiency is equivalent to or greater than that of the newly constructed pumped hydro storage stations, which proves the great potential to develop the proposed systems.