Finite-time thermodynamics modeling and analysis on compressed air energy storage systems with thermal storage

The charging and discharging processes of compressed air energy storage (CAES) systems are operated separately, and their characteristics depend on time strongly. In addition, CAES systems typically consist of certain scales of thermal storage and air storage units. The size of these units has a significant effect on system performance. As time and size factors are usually involved and analyzed through finite-time thermodynamics, it can be applied for analyzing and optimizing CAES systems properly. In this paper, the finite-time thermodynamics models for CAES systems with thermal storage (TS-CAES) including single-stage and multi-stage configurations are first established. Equations of TS-CAES system efficiency are innovatively derived, with time and size items being clearly expressed and decoupled. Moreover, the well-built unbalanced factors among compressor/expander stages are used for investigating the coupling relationship. The system efficiencies with charging/discharging time, heat exchange area, unbalanced factors and other key parameters are calculated. The finite-time thermodynamic boundary of TS-CAES system is obtained. It is found that the effect of finite time and finite size on the system efficiency is in a certain range, and the optimal heat transfer conductance under a certain finite storage/release time is attained. System efficiency decreases obviously with the increase of pressure/expansion ratio unbalance. The equilibrium of the pressure loss coefficient rather than the absolute value of the pressure loss can achieve higher efficiency. Positive matching of pressure ratio/expansion ratio and compressor efficiency/expander efficiency can result in obvious positive effects.