<P> Compression can be done with electrically powered turbo - compressors and expansion with turbo' expanders' or air engines driving electrical generators to produce electricity . </P> <P> The storage system of a CAES (Compressed Air Energy Storage) is one of the most interesting characteristics of this technology, and it is strictly related to its economic feasibility, energy density and flexibility . There are a few categories of air storage vessels, based on the thermodynamic conditions of the storage, and on the technology chosen: </P> <Ol> <Li> Constant Volume Storage (Solution mined caverns, aboveground vessels, aquifers, automotive applications, etc .) </Li> <Li> Constant Pressure Storage (Underwater pressure vessels, Hybrid Pumped Hydro - Compressed Air Storage) </Li> </Ol> <Li> Constant Volume Storage (Solution mined caverns, aboveground vessels, aquifers, automotive applications, etc .) </Li>

Which of the following is an ideal form of stored energy