Lithium-ion batteries are the ones consumers are most familiar with, so it seems like the obvious choice to scale them up for grid-scale energy storage – as Tesla did with the world's biggest battery in Australia. But since lithium is relatively hard to come by, it may not be the best choice. Researchers at MIT have outlined a new design they call a "sun in a box," which stores energy as heat in molten silicon and harvests it by tapping into the bright light it emits.
The new system, which the team calls Thermal Energy Grid Storage-Multi-Junction Photovoltaics (TEGS-MPV), is based on the molten salt batteries that sit at the heart of grid-scale energy storage systems like concentrated solar. But there are a few problems with salt as a storage medium – for one, it becomes quite corrosive when the heat is cranked up.
"The reason that technology is interesting is, once you do this process of focusing the light to get heat, you can store heat much more cheaply than you can store electricity," says Asegun Henry, lead researcher on the study. "This technology has been around for a while, but the thinking has been that its cost will never get low enough to compete with natural gas. So there was a push to operate at much higher temperatures, so you could use a more efficient heat engine and get the cost down."
Salt tops out at about 1,000° F (538° C), after which its damaging effects become too problematic. So the MIT team looked for a new material that could store more heat, which in turn raises the energy density of the system. They eventually settled on silicon, which can be heated to over 4,000° F (2,200° C) and is abundant to boot.
The TEGS-MPV system would be built with two heavily-insulated tanks, each made of graphite and measuring 33 ft (10 m) wide. One tank stores the liquid silicon at a relatively "cool" temperature of 3,450° F (1,900° C). To heat it up, the silicon is pumped out of that tank through tubes exposed to heating elements that are powered by external energy sources. The warmer silicon then passes into the second tank, which stores it at a much hotter temperature of about 4,350° F (2,400° C).
When it comes time to harvest that energy, the TEGS-MPV does so in an interesting way. With molten salt systems, a heat exchanger uses the heat to boil water, creating steam that drives a turbine to produce electricity. But in this case, the system doesn't tap into the heat but the light – at those temperatures molten silicon shines extremely brightly. The white-hot liquid is pumped through tubes that emit the light, which is then captured by specialized solar cells known as multijunction photovoltaics and converted to electricity. The silicon, now cooling down again, is pumped back into the first tank to start the cycle over.
The team says one of these TEGS-MPV systems could be enough to power 100,000 homes. Ideally, that energy would come from renewable sources like wind or solar, but it could effectively be sourced anywhere. The design can also be implemented almost anywhere, and would be much cheaper – apparently about half the price of pumped hydroelectric, the current champion in terms of energy storage cost.
"This is geographically unlimited, and is cheaper than pumped hydro, which is very exciting," says Henry. "In theory, this is the linchpin to enabling renewable energy to power the entire grid."
One of the issues that the team foresaw is that the molten silicon might react with and corrode the graphite tank, so to test it out, the researchers built a mini tank. When it was filled with silicon heated to 3,600° F (1,980° C) for an hour, they found it did react with the graphite to form silicon carbide. But rather than damaging the tank, this actually created a thin protective layer, the researchers say.
The research was published in the journal Energy & Environmental Science.
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