Terrafore Looks to Cut Molten Salt Energy Storage Costs in Half

A new chemistry that lets salt expand and contract inside a capsule could bring salt storage into more realistic price ranges for solar thermal plants

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Terrafore Looks to Cut Molten Salt Energy Storage Costs in Half

As we have seen in recent months, energy storage is becoming a pretty big deal. California has the country's first energy storage mandate in place, and plants like Solana in Arizona have started trying to incorporate storage in from the beginning. Solana uses molten salt energy storage, a common idea wherein salts are heated, retain that energy for relatively long periods of time, and then discharge it by heating steam to turn a turbine. Solana, a concentrating solar thermal plant, can keep running for six hours after the sun drops below the horizon.

Storage like that, though, is still expensive. A company called Terrafore Technologies wants to cut the price almost in half. Terrafore was an exhibitor at the Advanced Research Projects Agency–Energy Summit this week in Washington, D.C., and the company's CEO Anoop Mathur told me he was hoping to raise $5 to $10 million (maybe from the gaggle of venture capital folks that wandered the Summit's halls) in order to scale up his process.

That process, essentially, involves creating a capsule inside which a salt such as potassium nitrate could expand and contract. The capsule itself—think of a peanut M&M, around 10 or 12 mm in size—is made of clay along with some proprietary additives surrounding a polymer layer. The polymer layer (specifically a methyl cellulose polymer), coated on to the spherical salt "prills", is designed to gasify at temperatures below the melting point of the salt. The clay layer is added on above the solid polymer, and then the whole clay-surrounding-polymer-surrounding salt package is heated to a point at which the polymer gasifies and escapes through the pores of the clay, yielding some hollow space inside (see this video for a full explanation). Further heating lets the salt melt and expand into the hollow space; the salt then cools and solidifies and can melt again to act as the thermal storage mechanism. With this system, three different salt capsule types of differing temperatures can be stored in a single tank instead of the two tanks that Solana and traditional salt storage must use—essentially one tank for the hot salts storing energy, and one for colder salts that have just discharged that energy.

"I think the tech is proven," Mathur said. He has calculated that this system could yield a 43 percent reduction in costs over existing molten salt storage, largely due to the need for only one tank. In fact, it would put the technology well on its way to the $15/kWh target for thermal storage set by the Department of Energy's SunShot Initiative. The new idea has been tested through 5000 cycles to this point, though Mathur noted the manufacturing process for the capsules—so, the central piece of this price-cutting puzzle—has yet to be scaled up. Hence, the quest for $10 million in funding.

Storage is going to be a critical part of a true renewables revolution, so big price reductions like the one this promises are crucial.

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