The operation principle behind compressed air energy storage is simple. When there is excess electricity in a system, a fluid is compressed in a large impermeable cavity. The fluid remains in the cavity at high pressure until there is a need for power.
When the air is compressed, the heat is not released into the surroundings: most of it is captured in a heat-storage facility. During discharge, the heat-storage device rereleases its energy into the compressed air, so that no gas co-combustion to heat the compressed air is needed. The object is to make efficiencies of around 70% possible.
Most compressed air energy storage systems addressed in literature are large-scale systems of above 100 MW which most of the time use depleted mines as the cavity to store the high pressure fluid. Three main concepts are researched; diabatic, adiabatic and isothermal.
Expansion machines are designed for various compressed air energy storage systems and operations. An efficient compressed air storage system will only be materialised when the appropriate expanders and compressors are chosen. The performance of compressed air energy storage systems is centred round the efficiency of the compressors and expanders.
Appendix B presents an overview of the theoretical background on compressed air energy storage. Most compressed air energy storage systems addressed in literature are large-scale systems of above 100 MW which most of the time use depleted mines as the cavity to store the high pressure fluid.
Conventional CAES systems store energy by driving large electric motors that pump compressed air into a mine. This process is done during off-peak energy demand when it is much less expensive. In addition, during the compression process the air is cooled down before injection in order to accommodate more air in the same space.