Spray-cooling for wind-based compressed air energy storage

Energy systems can benefit from compact and efficient energy storage technologies. In particular, energy storage is well suited for off-shore wind turbines whose output energy variability is typically inconsistent with grid power demand. Furthermore, accommodating peak power generation can lead to over-sizing of electrical generator and transmission lines. It would be more efficient and economical if off-shore wind turbines could be sized for average power and could produce this power on a continuous basis. This would allow the traditional wind turbine generator and transmission lines can be replaced by a smaller, lower-cost, constant-speed generator and a transmission system sized for average power output. This study analyzes a compressor to build and maintain compressed air energy storage for a 35-MPa accumulator sized for a 5 MW off-shore wind turbine. The compressor employs a liquid piston for compression and water spray for heat transfer to achieve near isothermal behavior and efficiency. The overall compression is achieved in three stages with pressure ratios of 10:1, 7:1, and 5:1 under 1-Hz working frequency. The results indicate that droplet surface area plays a critical role in system performance and that high mass loading and small drops can increase overall system efficiency by as much as 50%, as compared to conventional air compressor systems.