The world's first molten salt concentrating solar power plant
'Archimede' demonstration solar plant in Sicily becomes the first to use molten salts to store energy overnight
Here's Wikipedia on Solar Two from 1995:
Solar Two used molten salt, a combination of 60% sodium nitrate and 40% potassium nitrate, as an energy storage medium instead of oil or water as with Solar One. This helped in energy storage during brief interruptions in sunlight due to clouds. The molten salt also allowed the energy to be stored in large tanks for future use such as night time.Solar Two proved it could run continuously around the clock producing power.
[Update: Carlo, the author, responds. It's the Guardian's fault, not his ;-) I'm not surprised.]