SustainX’s idea is to compress air and store it in tanks which could be placed anywhere; for example adjacent to a wind farm as shown in their illustration above. This differs from the more common idea of storing the compressed air underground, which has plenty of uncertainty associated with it and not all that many locations that will work. They compress and decompress the air isothermally (so it doesn’t heat up) using renewable sources (nominally excess wind and solar generation) when it is not needed on the grid, or when it could be served to the grid more profitably at peak times. To make it isothermal they use a piston and crankshaft device connected to an electrical machine, spraying water into each cylinder to absorb or capture excess heat. Construction of a 1.5 MW demonstration plant at their headquarters in New Hampshire was announced last September (http://bit.ly/1k4ZE8l0) but there are no new releases on their website since.
Below is some discriptive information from SustainX’s most recent patent application.
Storing energy in the form of compressed gas has a long history and components tend to be well tested and reliable, and have long lifetimes. The general principle of compressed-gas or compressed-air energy storage (CAES) is that generated energy (e.g., electric energy) is used to compress gas (e.g., air), thus converting the original energy to pressure potential energy; this potential energy is later recovered in a useful form (e.g., converted back to electricity) via gas expansion coupled to an appropriate mechanism. Advantages of compressed-gas energy storage include low specific-energy costs, long lifetime, low maintenance, reasonable energy density, and good reliability.
If a body of gas is at the same temperature as its environment, and expansion occurs slowly relative to the rate of heat exchange between the gas and its environment, then the gas will remain at approximately constant temperature as it expands. This process is termed “isothermal” expansion. Isothermal expansion of a quantity of high-pressure gas stored at a given temperature recovers approximately three times more work than would “adiabatic expansion,” that is, expansion where no heat is exchanged between the gas and its environment—e.g., because the expansion happens rapidly or in an insulated chamber. Gas may also be compressed isothermally or adiabatically.
An ideally isothermal energy-storage cycle of compression, storage, and expansion would have 100% thermodynamic efficiency. An ideally adiabatic energy-storage cycle would also have 100% thermodynamic efficiency, but there are many practical disadvantages to the adiabatic approach. These include the production of higher temperature and pressure extremes within the system, heat loss during the storage period, and inability to exploit environmental (e.g., cogenerative) heat sources and sinks during expansion and compression, respectively. In an isothermal system, the cost of adding a heat-exchange system is traded against resolving the difficulties of the adiabatic approach. In either case, mechanical energy from expanding gas must usually be converted to electrical energy before use.
The whole patent is here: bit.ly/1rFYkIh