The Chilean government recently gave the go-ahead on a massive solar thermal plant that is expected to produce electricity 24 hours a day, seven days a week—a considerable feat for a plant that depends solely on solar energy. The plant, proposed for a site in Chile’s Tamarugal province, would consist of three 150 megawatt solar thermal towers, which become heated as mirrors placed around each tower reflect sunlight onto it.

That heat is transferred to molten salt, which circulates through the plant during the day and is stored in tanks at night. The salt, a mixture of sodium nitrate and potassium nitrate that’s kept at a balmy 1,050 degrees Fahrenheit (566 degrees Celsius), is used as a “heat transfer fluid.” As energy is needed, the salt can be dispatched to a heat exchanger, where it will lend its heat to water to create a super-heated steam. That steam is used to move a traditional steam turbine to create electricity.

Because the molten salt will stay hot for hours in its thermal storage tank—even throughout the night or during a cloudy morning—the molten salt is said to store that thermal energy. Each tower will have 13 hours of energy storage, for a total of 5.8 gigawatt-hours of energy storage capacity.

In total, the proposed Tamarugal plant would be able to provide 450 MW of power continuously, 24 hours a day. The plant could theoretically generate 2,600 GWh annually.

SolarReserve, the US-based company that proposed this project, has also proposed two others—a 260 MW, 24-hour plant near the city of Copiapó in the Atacama Region of Chile, as well as a 390 MW, 24-hour plant in the Antofagasta Region. Mary Grikas, a SolarReserve spokesperson, told Ars via e-mail that Copiapó is shovel-ready, and now Tamarugal is, too, with the Chilean government’s recent approval, which assessed the site for environmental impact. The plant in Antofagasta is still waiting on permitting approval.

A video from SolarReserve on how its solar thermal plants work.

It still might be a while before the plants become a reality, however. SolarReserve now needs to secure Power Purchase Agreements (PPAs) so that the company can be assured that it can sell its power to grid managers in Chile. The next public auction for such agreements is in October. If the company is awarded PPAs, SolarReserve will seek financing through investors to actually build the plants—Tamarugal’s reported investment value is a hefty $2.6 billion, but that number could change after the Power Purchase Agreements have been locked in, Grikas said. She added that if the company is awarded a Power Purchase Agreement in October, SolarReserve hopes to close financing and start construction on Tamarugal by the end of 2018.

Although a 24-hour plant is relatively novel, the technology proposed for Tamarugal isn’t new. In southern Arizona, the Solana solar thermal plant uses concentrated solar power from rows and rows of parabolic mirrors to heat an oil that’s sent to either directly create steam or be transferred to heat molten salt for storage and later use. Ivanpah is another massive solar thermal plant outside of Las Vegas that was built to produce 448 GWh annually. But that plant has hit some bumps in the road—it has struggled to meet its production goals, and last May one of the towers suffered an electrical fire. Pilots have also complained of glare coming from the boiler towers.

But SolarReserve may have an advantage in the solar thermal game. Rather than use a heat transfer fluid to heat molten salt, SolarReserve heats the molten salt directly. The company used this technique on a solar thermal project it built at Crescent Dunes, outside of Tonopah, Nevada. While Ivanpah is still short of its production goals, Crescent Dunes has recently delivered 105 percent of its contracted output, and the plant is operating 2 percent above its expected efficiency, according to industry publication Power Magazine.

Crescent Dunes is smaller than what SolarReserve has planned for Chile. The solar company intends for its Chilean 24-hour solar thermal plants to compete with other base load plants like coal, natural gas, hydrothermal, or nuclear plants. Grikas told Ars that the Tamarugal plant will operate with a 75 to 85 capacity factor, depending “upon final configuration of the steam turbines for each tower.” Comparing that to information from the Energy Information Administration on other renewable sources of energy, that’s a competitive number—on par with geothermal and not as good as nuclear but far above other intermittent renewables like wind and solar photovoltaic panels.

Tamarugal and its brethren are expected to have 30-plus-year lifespans, during which they will use no fossil fuels and need no replacement of molten salt. As an added benefit, when the proposed plants are finally decommissioned, SolarReserve says the molten salt can be used to create “high-grade fertilizer.»

Listing image by SolarReserve

Megan Geuss Megan is a staff editor at Ars Technica. She writes breaking news and has a background in fact-checking and research.