Isobaric compressed air energy storage system: Water compensating cycle or CO2 compensating cycle?

Isobaric operation of air storage can remove the throttling losses existing in isochoric reservoir, making full use of the storage volume and lowering system construction cost. The water cycle and CO2 cycle are two of the most commonly configurations to stabilize the pressure in the air storage unit. The choice between the two cycles depends on whether the technical complexity and costs are justified by performance gains. This paper mainly focuses on developing the MAP design, a kindly selection diagram plotting the better value of performance indicators between the two systems, to determine the more favorable compensating cycle in the constant storage operation of the compressed air. The analysis results indicate that higher air storage pressure increases the system efficiency. The levelized cost of storage is provided with a valley value when the air storage pressure is at 6.6 MPa for the CO2 cycle and 14 MPa for the water cycle. The optimized systems can share a comparative efficiency of 68.04 % and 68.07 %, and the levelized cost of storage is 0.8237 ¥/kWh for the CO2 cycle and 0.7869 ¥/kWh for the water cycle. The water is suggested to be the compensating fluid for the isobaric system when the water machine efficiency is higher than 0.85 or else opting for CO2 proves to be a more economically viable choice. •Water and CO2 stabilizing cycles are examined for compressed air energy storage.•The system selection MAP diagram for the two configurations is given.•The optimized system can be provided with an efficiency of about 68 %.•The levelized cost of storage of the water compensating system is 0.7869 ¥/kWh.•Generally water compensating system is suggested for higher machine efficiency.