The article, and the comments here, are missing an important fact that really helps clarify how these compressed-air energy storage systems work: These systems

The article, and the comments here, are missing an important fact that really helps clarify how these compressed-air energy storage systems work: These systems use the compressed air as input into a natural gas turbine. You can think of the underground storage as a giant intercooler that stores the output of the first (compressor) half of a natural gas turbine, and then feeds that compressed air into the second (combustion) half. See a good explanation here: http://energystorage.org/compressed-air-energy-storage-caes http://energystorage.org/compressed-air-energy-storage-caes