Abstract
Underwater compressed air energy (UW-CAES) systems own plentiful merits of high system efficiency, high energy density and stable operation. In terms of research gap of its coupling properties of thermodynamics and economics, along with research lack focusing on detailed design parameters, the comprehensive thermodynamic and economic coupling model of UW-CAES systems are developed in this paper, specifically including more details of heat exchanger structure, thermal storage medium and an improved energy density. In addition, the discharging power capacity is maintained constant in the analysis for practical design. Then, the effect of key parameters on the whole system's thermodynamic and economic performance is deeply investigated in which the coupling characteristics among parameters are focused on. Results show that the studied UW-CAES system has a good thermodynamic performance with a high system efficiency of 0.7074 and an enhanced high energy density of 26.07 MJ/ m3 considering thermal storage tanks. The system investment can be achieved as low as $3.983 million when water is thermal storage medium. Sensitive analysis shows that it is beneficial to improve the whole system performance with larger underwater pipe diameter, and the discharging pressure should not be lower than 30 bar to avoid efficiency and economic deteriorating. The growing discharging power will increase system investment, while decreasing levelized cost of electricity (LCOE) significantly. Generally, the economy of UW-CAES systems with thermal oil is much worse than counterparts with water. As the thermal storage medium is water, there exist optimum economic ranges of discharging pressure, NTU, length-width ratio and part number.
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