“The systems that are commercial today are limited to about 565 °C—that’s the molten salt tower plants,” says Mehos. “The tower and optics themselves can hit higher temperatures, but you’re limited by the salt temperature right now.” The new materials can work at temperatures up to 1,200 °C.
Improving the reliability of solar power will also be key to making solar power competitive with fossil fuels. Without storage, the amount of solar power that can be installed on the grid is limited, since utilities need to provide backup generation, or build extra transmission lines, to deliver power from other areas when solar power output drops. So far this isn’t a problem, since solar power accounts for only a small part of the power on the grid. But it could be a serious issue within the decade in places such as California, where renewable energy requirements are leading utilities to install large amounts of solar power.
In the current heat-storage design, salts are heated up above their melting point, up to their highest working temperature (565 ⁰C), and then stored in a large insulated tank. The salt is pumped through a heat exchanger to generate steam, and then kept in another insulated storage tank just above its melting point to keep it from freezing.
The first material Halotechnics plans to bring to market is designed for use in existing solar-thermal plant designs. It operates at the same temperature as the current molten salts, but will cost about 20 percent less. Salts currently cost about $1,000 a ton, and a typical plant uses 30,000 tons of salt, so this could save millions of dollars. Halotechnics plans to test the material in a pilot-scale plant for six months starting this summer and then license the formula for other companies to produce.
Two other materials—one an improved salt mixture, and the other a form of glass—can operate at greatly increased temperatures, reducing the amount of storage material needed and potentially improving efficiency.
“Without an amazing ability to screen samples, it’s an intractable problem. That’s what we’re trying to do with our high-throughput technique,” says Justin Raade, CEO of Halotechnics.
Whereas conventional molten salts melt at 300 °C and can operate up to 565 °C, Halotechnics has developed a molten salt that has the same melting point, but can operate up to 700 °C. The material is being tested for long-term compatibility with the steel pipes and containers used in storage systems, and the company plans to start pilot tests in 18 months. While current materials limit solar-thermal plants to turbines that are about 42 percent efficient, this material could be paired with steam turbines that are 48 percent efficient. A storage system that will work with this material is being developed as part of an NREL project that’s part of the SunShot Initiative.
The last material is a form of glass that melts at 400 °C (typical window glass melts at about 600 °C) and can operate up to 1,200 °C. It could be used to heat up air to drive a gas turbine, with the leftover heat used to drive a steam turbine, much as is done in a natural-gas combined-cycle plant. Such a system could be about 52 percent efficient using existing turbine designs. (Natural-gas combined-cycle plants can reach 60 percent efficiency, but the natural gas burns at temperatures higher than 1,200 °C.)
Eventually, the materials could perhaps even enable a new form of renewable fuel for vehicles. At 1,200 °C, the glass could drive some of the key chemical reactions needed to make fuels such as hydrogen and gasoline made from water and carbon dioxide.
Operating at such high temperatures, however, will bring engineering challenges, including finding relatively inexpensive materials to contain the molten glass. Commercialization of this technology could be many years away.