Huge New Solar Thermal Plant Can Keep Running for Six Hours After Sun Goes Down

The Ivanpah plant in the Mojave may have recently snatched away the title of "world's largest," but Abengoa Solar's Solana plant in the desert near Gila Bend, Arizona, still has its share of superlatives. At 280 megawatts, Solana is one of the largest plant using parabolic mirrors in the world, and it is undoubtedly the largest to use substantial thermal storage to keep the juice flowing for hours after the sun goes down. Intermittency is still among the most common complaints about industrial-scale renewable energy, so proving that this storage tech can work is a huge step for the solar industry.

Abengoa announced on Wednesday that the Solana plant "passed commercial operation tests." The first of these involved running the plant's generator at full power while also ramping up the thermal storage system. Next, after letting the solar part of the plant stop once the sun was down, operators fired up the generator and produced electricity for six full hours using only the thermal storage system. Intermittency, you matter not here.

Solana is composed of about 3 200 parabolic trough mirrors spread out over 7.8 square kilometers (3 square miles), with 280 megawatts of total generating capacity. The mirrors concentrate the sun's rays onto a tube filled with a "heat transfer fluid" usually involving oil, which eventually heats water to create steam and turn a turbine. The plant generates enough electricity at its peak to power around 70 000 homes. It's located in a nice sunny spot about 70 miles southwest of Phoenix, an area as good as any when it comes to sunny days. In fact, Gila Bend, Arizona, is turning into a sort of solar power wonderland in recent days: there are now four solar plants approved in the area, though the other three range between only 17 and 32 megawatts. The town even has the "Gila Bend Transmission Initiative" up and running to try and make solar development in the region even more attractive than it already is.

The storage part of Solana is what makes it really interesting. Ivanpah, the 377-megawatt behemoth currently holding the mark as the largest solar thermal plant in the world (and currently ramping up toward full production in the Mojave Desert in California), doesn't have a way to store its generated energy, and most other plants built so far also lack that ability.

But it has become increasingly clear that storing solar power is a worthwhile endeavor; in one recent paper, researchers calculated that the energy return on investment—that is, the amount you get out based on how much energy you put in—is positive for any storage technology paired with solar photovoltaics. Battery storage tends to be the worst form of storage we can use, while pumped hydro and compressed air storage are the best.

Abengoa uses thermal storage, and variations that use steam, molten salts (used at Solana), or other materials, are also now under investigation in a number of places. Of course, the energy return isn't the same as the cost return, and price is what has kept storage from going mainstream with most renewable projects. A huge focus of the industry right now is to bring those costs down, with help from places like the Department of Energy's Advanced Research Projects Agency—Energy. In Solana's case, Abengoa has been testing out storage tech in Spain (notably at the Solucar platform near Seville,  home to a number of pilot projects), and seems to be a bit ahead of some other major solar developers. The $1.45 billion loan from the DOE for Solana likely didn't hurt, however.

Solana will start full commercial operation soon. A power purchase agreement with Arizona's biggest utility, Arizona Public Service, is already in place covering the entirety of the solar plant's output.