Off-Design Performance of Variable-Speed Compressed Air Energy Storage System Under Different Operation Schemes

Abstract

In practical operation of compressed air energy storage systems, the off-design performance caused by pressure fluctuations within the air storage reservoir persists throughout the entire energy storage and release process, limiting their flexibility and reducing performance. To improve off-design performance and expand the operational range, a novel variable-speed compressed air energy storage (VS-CAES) system with five-stage compressors and five-stage turbines is proposed. The off-design performance of various variable-speed operation schemes is analyzed and compared for the first time. Moreover, the optimal variable-speed operation scheme is obtained with advantages of the optimized variable-speed scheme over the constant-speed scheme summarized. Results show that the last stage variable-speed scheme offers the best off-design performance. Significantly, the optimal operation scheme is identified as front 4 stages constant-speed & last stage variable-speed for both energy storage process and energy release process. The impact of variable-speed operation on the compressor exceeds that on the turbine in terms of off-design performance for VS-CAES. Specially, the optimal scheme increases exergy efficiency by 1.50% in the energy storage process, while 0.15% for the energy release process. Moreover, variable-speed operation expands the system’s operating range and optimizes off-design performance. Compared to constant-speed operation, the system with the optimal variable-speed scheme achieves a 1.56% increase in round-trip efficiency in the 70% load regulation range. In terms of component performance, the optimal scheme expands the pressure ratio of the fifth-stage compressor from 1.48-2.60 to 1.08-2.55, raising the minimum efficiency at the start of energy storage from 79.20% to 83.36% and at the end of the energy release process from 80.78% to 91.59%.

Keywords: Compressed air energy storage, Variable-speed, Off-design performance, Operation schemes, Performance analysis