Source: http://arstechnica.com/science/2014/02/m...

The major advantage of Concentrated Solar Power is the potential for storage. Since the sun’s energy is already being converted to heat, it’s relatively straightforward to store some of this heat to create electricity later. A variety of storage media are used, including oil and beds of packed rock, but the most common is molten salt. These salts are highly effective at retaining thermal energy and can be heated to very high temperatures (over 1000 degrees Fahrenheit), making storage extremely efficient.

With thermal storage, these plants can either produce at maximum efficiency during the day or store some of the energy as heat to convert later when the sun isn’t shining. This adds not only flexibility to the power system—since the production curve can be tailored to match the demand curve—but also stability in case of cloudy or stormy conditions.