We are developing 24 hour high temperature steam storage via capped 6 inch pipes pile driven 100 feet into sand, spaced 3 feet apart. There is no insulation, and the sand is part of the thermal storage. The other part is the pressurized water in the pipes which flashes to steam when released. The source steam is from a field of solar dishes.
http://www.harbornet.com/sunflower/pvdish.html