Spray-cooling concept for wind-based compressed air energy storage

Wind turbine output energy varies over time with local wind speed and is typically inconsistent with grid power demand. Without energy storage, the resulting difference between rated (peak) power and average power output leads to over-sizing of electrical generator and transmission lines. This conventional arrangement can be avoided if wind turbines can be coupled with energy storage to eliminate the output variations and instead produce their average power on a continuous basis. This would allow a smaller, lower-cost, constant-speed generator and a reduced capacity transmission system sized only for average power output. To accomplish this goal, this study discusses a concept for a storage system for a 5 MW off-shore wind turbine, which integrates a spray-based compressed air energy storage with a 35 MPa accumulator. The compressor employs a liquid piston for air sealing and employs water spray to augment heat transfer for high-efficiency. The overall compression is proposed in three stages with pressure ratios of 10:1, 7:1, and 5:1, all operated at 1 Hz to maintain moderate liquid surface acceleration. Based on a simple and fundamental description of the system, compression efficiency was found to be strongly dependent on droplet surface area, which can be achieved through either high mass loading or small drop sizes. The simulations also show that direct injection spray can increase overall three-stage compression efficiency to as high as 89%, substantially better than the 27% associated with a conventional adiabatic compression at the same pressure ratio. In addition, this study introduces a key performance parameter, termed the Levelization Factor, which can be used to quantify the impact of storage on wind energy systems. However, experiments and simulations based on 3-D geometries with design details are needed to determine the potential of this concept.