Thermo-economic optimization of an artificial cavern compressed air energy storage with CO2 pressure stabilizing unit

In recent years, the attention of engineers has been increasingly attracted to the compressed air energy storage with artificial cavern as it frees the conventional system from the dependence of salt cavern, greatly reducing the limiting factors of project location. However, the current issues are how to enhance the reliability and safety of the artificial cavern due to the cyclical pressure loading and to cut down the cost of cavern construction. The present study overtures an isobaric system for doing this via designing pressure stabilizing unit utilizing the CO2 phase change process. The specific volume of saturated liquid is an order of magnitude lower than that of the saturated gas and this characteristic contributes much to reduce the cavern size. The system thermodynamics and economics are calculated numerically and the findings are discussed to identify whether the additional complexity of isobaric gas storage are worth to do. It is recommended that the air storage pressure, CO2 storage pressure and CO2 liquefaction pressure should be positioned in sequence at 6.5 MPa, 6 MPa and 9 MPa as the optimal design conditions. In this case, the system efficiency is 69.92 %, the levelized cost of storage is 0.1332 $/kWh, the dynamic payback period is 7.26 years and the investment return ratio is 1.58. Compared to the isochoric system with equivalent efficiency, the air storage pressure in proposed system is highly reduced with just 0.65 times of original value. More importantly, the constant pressure in the cavern operation process significantly upgrades its reliability and safety. The size of cavern volume is narrowed with a decline rate of 31.86 % and the system levelized cost of storage scales back with an amplitude of 0.0138 $/kWh. To be matched with, the investment return ratio is increased by 54.9 %.