Concise analytical solution and optimization of compressed air energy storage systems with thermal storage

The evaluation of compressed air energy storage (CAES) system mostly focused on system efficiency and cost, while less attention has been paid to energy density in the past, and each performance expression was complex, making it difficult to obtain clear variation law of multiple indexes with key parameters, as well as the optimal coupling relationship among them. In view of the above problems, the research on concise analytical solution of CAES systems with thermal storage (TS-CAES) is carried out in this paper, in which a dimensionless pressure coefficient K and the parameter Z to describe thermal storage characteristics are established. Based on the above parameters and simplified model, the concise analytical expression of system efficiency and energy density for constant-volume and constant-pressure CAES systems are established. Based on the analytical expression, the influence law of each key parameter on the system evaluation indexes is revealed, and the optimal matching relationship of key parameters is obtained. It is revealed that without considering the heat exchange temperature difference, the system efficiency is only related to the thermal storage temperature, K and the efficiency of compressor/expander. The greater the temperature and K are, the higher the system efficiency is. The higher the temperature and pressure are, the higher the energy density of the two systems is. The energy density is more sensitive to the value of Z with lower temperature and higher pressure. It is also found that there is an optimal K under a certain temperature and pressure to maximize the energy density for constant-volume CAES, and the optimal value of K corresponding to 200 bar and 100 °C is 0.77.