The primary complaint against solar power—that it, you know, requires the sun—is perfectly valid. But Arizona's new Solana Generating Station, the largest capacity solar thermal plant on the face of the Earth, has just provided a $1.4 billion counterpoint. Thanks to its massive molten salt reserves, this plant keeps producing power even after lights out.

Constructed by Spanish energy giant Abengoa, the 280MW Solana Generating System is unlike other solar thermal plants. Conventional solar-thermal plants employ an array of mirrors to reflect and focus the sun's energy onto a single point, heating a working fluid that boils water to generate steam that runs a turbine. The obvious limitation is that as soon as the sun sets, the water stops boiling, the turbine stops turning, and energy production ceases until the morning.

Solana, on the other hand, produces power for an additional six hours after the sun has gone down thanks to its huge molten salt storage tanks. So, during the day, Solana works like a conventional solar thermal plant with its 2,700 mirrors focusing the Sun's rays onto a synthetic oil, heating it to 735 degrees F. This working fluid then flows into steam boilers to heat water and drive a 140 MW turbine. But that working fluid isn't done yet; any residual heat from the fluid is then siphoned off by a set of six tanks holding a combined 125,000 metric tons of salt. These tanks stay a toasty 530 degrees F during the day and since they're located adjacent to the boilers, can be used to heat more water during the evening hours—without the aid of the sun.

"Solana is a monumental step forward in solar energy production," Don Brandt, the president of APS, the local utility, told CleanTechnica. "This provides a huge boost toward our goal to make Arizona the solar capital of America." They'll have to keep building capacity, in that case. The world's overall largest solar farm at Ivanpah is expected to have all 375 MWs ready in the next few months. Good thing, then, that Solana can work around the clock. [Abengoa via Cleantechnica]