A novel coupled system of compressed air energy storage system and air separation unit: Thermodynamic and economic evaluation

Air separation units (ASUs) are energy-intensive systems, with the energy consumption of air compressors accounting for more than 70 % of the total energy consumption. In addition, instability in the demand for liquid oxygen and nitrogen markets constrains the economic performance of ASUs. To address these issues, this paper proposes a novel system that couples a thermal-storage compressed air energy storage (TS-CAES) system with an ASU, where TS-CAES provides high-pressure air for the ASU and reduces the compression energy consumption of the ASU, which is totally different from the traditional coupling mode between the liquid air energy storage system and the ASU. Meanwhile, a portion of the produced liquid nitrogen is recycled back into the TS-CAES system for power generation, balancing liquid nitrogen production with market demand. A detailed thermodynamic and economic model of the coupled system is developed, and the system performance is analyzed comprehensively. The results show that the coupled system exhibits good thermodynamic and economic performance, with energy efficiency of 83.19 %. The TS-CAES achieves a round-trip efficiency of 72.13 % when operating alone, while this efficiency decreases to 55.99 % under an air extraction ratio of 0.2287. When all of the liquid nitrogen is sold, the dynamic payback period of the ASU is 2.41 years, whereas selling 70 % results in 3.78 years. Sensitivity analysis indicates that increasing the inlet air pressure to the ASU improves the ASU's exergy efficiency and reduces the product energy consumption. Higher liquid product selling proportion and lower valley electricity prices can improve economic performance.