When Solana was completed in 2013 it was the largest thermal solar plant in the nation. It produces 280 megawatts, using the circulating heated oil system known as parabolic troughs, with concave mirrors in rows that track the sun, heating up a heat transfer fluid, generally Therminol, that makes steam, and then power. Located west of Gila Bend, Arizona, it was built by the Spanish company Abengoa, which is building another 280-megawatt plant of the same type at Harper Lake, in California. The cost of these plants is close to $2 billion, each.