Combining Wind-Driven Air Compression with Underwater Compressed Air Energy Storage

Energy storage is quickly becoming a priority in the energy sector as inflexible renewables penetrate further into the energy mix. The opportunity for novel energy storage solutions has therefore never been greater. Generation-Integrated Energy Storage (GIES) holds several key advantages over systems that separate electricity generation and energy storage. Primarily, a reduced number of energy transformations gives rise to the possibility of greatly improved all-round efficiencies. This paper discusses some existing and proposed technologies for energy generation and storage, as well as the potential for integration between them. A GIES system is then presented that takes advantage of the complimentary natures of wind-driven air compression and underwater compressed air energy storage (UWCAES). It is proposed that an adiabatic, liquid-piston air compressor be powered by an offshore wind turbine floating over deep water. The exergy generated by this compression is then stored in two forms: heat in a gravel packed bed and compressed air in a flexible underwater bag. Both of these forms of energy storage are expected to be relatively low-cost, and the system therefore has the opportunity to be considerably cheaper than if the electricity generation and energy storage were separate, such as with conventional wind turbines and battery plants. Using direct-drive compression also removes the need for an expensive geared transmission. However, to prevent the necessarily large swept volumes involved with direct-drive compression of air from ambient pressure, an initial stage of isothermal air compression is used. Consideration is given to several compression technologies in order to achieve this, including the possibility of wave-powered hydraulic air compression. A medium-pressure compressed air energy bag is also employed prior to the adiabatic compression stage to store this medium-pressure air. This has the added advantage of supplying air to the turbine during times of peak demand, reducing the requirement for electric compression during these otherwise expensive periods.