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.