Thermodynamic investigation of variable-speed compression unit in near-isothermal compressed air energy storage

Isothermal compression is the state-of-the-art in compressed air energy storage (CAES) technology. The study of cyclic pressurization unit in isothermal CAES is carried out in this paper. The unit can continuously compress air utilizing double vessels operating alternately. In each vessel, the large heat capacity of water and spray cooling is applied to establish a conducive environment for the air achieving the effect of efficient near-isothermal compression. The operating mode of the cyclic pressurization unit is divided into sliding-pressure operation and constant-pressure operation according to the pressure of its outlet. By adopting the characteristic curves of pump, the dynamic model is built for the unit in this paper. Further, the differences between the cyclic pressurization unit in isentropic process and in variable-speed process are quantified and compared in four different cases. In isentropic process, the energy-saving ratio and exergy efficiency of the unit are expected to reach 20.4% and 95.0%, respectively. In variable-speed process, the energy-saving ratio and efficiency of the unit can achieve 7.3% and 73.4%, respectively. This gap is mainly determined by the off-design performance of the pump. Moreover, the vessel volume should be no less than 3 m(3) and 5 m(3) when there is spraying and when there is no spraying. Furthermore, spraying favors the improvement of the worsened compression performance caused by the change of design parameters. The energy-saving ratio can be improved via spraying by 1.3% to 11.0% and 1.8% to 25.6% in the isentropic and variable-speed processes, respectively. Meanwhile, the exergy efficiency can be increased by 1.5% to 9.4% in the isentropic process and 1.4% to 8.4% in the variable-speed by spraying.