One of the problems with solar and wind energy is their intermittancy. The sun doesn't shine at night and the wind doesn't blow all the time. But the idea of compressing air and storing it underground for later use to run generators is become more viable.A review at Wired magazine focuses on compressed air storage for wind energy, but it should be just as viable for solar energy. Here in Arizona, we have extensive salt basins where salt caverns could be hollowed out to serve as storage vaults for compressed air to be released during peak times or after the sun sets. [right, Iowa compressed air storage experiment]