Transient Thermodynamic Assessment of the World's First Grid Connected UWCAES Facility by Exergy Analysis

Experience with Toronto's Underwater Compressed Air Energy Storage (UWCAES) facility has shown the technology to be more versatile than originally anticipated. Beyond typical steady-state operations, potentially valuable ancillary grid service roles can be assumed in the transient phase of its operation. This study examines the challenge to operate efficiently during UWCAES ramping events. A conventional and advanced exergy analysis is here conducted for the world's first grid connected UWCAES facility located in Toronto, Canada. A conventional exergy analysis showed that under real working conditions, the exergy efficiency of the plant is low during start-up. The exergy destruction rate, under unavoidable conditions, 2 minutes after the start of the transient phase is 42%, and this decreases asymptotically to 25%. An advanced analysis of exergy efficiency through a complete charge/discharge cycle suggested that the first improvement priority be given to the Heat Exchangers group. This should then be followed by the Ancillary, Compressor, Turbine, Motors and Generator and Pipelines groups. In terms of the rate of exergy destruction the analysis indicated that improvement priority for the Compressor group was higher than that for Heat Exchangers followed by Ancillary, Motors and Generator, Turbine and Pipelines groups. The advanced exergy analysis also revealed that 67% of the exergy destruction was endogenous and avoidable, highlighting the significant potential for performance improvement. Moreover, it was shown that with improvement in the system's component efficiencies, the plant exergy efficiency could be exogenously improved. This effect could further reduce the total exergy destruction to 76%, where only 9% is due to component-component interaction.