Answer: It can generate electricity at night.
The facility is, according to builder Terresol Energy, "The world’s first commercial-scale Concentrated Solar Power plant featuring central tower receiver with thermal storage capabilities"
A Concentrated Solar Power plant uses mirrors, called heliostats, to focus sunlight on a single point. Gemasolar, as the facility is known, directs the sun's rays to a point in a central tower, generating tempartures up to 1650 degrees Fahrenheit. This in turns warms molten salt tanks, which create steam to turn turbines for up to 15 hours without sunlight.
Click here to learn more about the Gemasolar facility.