MIT's conceptual "sun-in-a-box" energy storage system plugs into molten silicon

MIT's conceptual "sun-in-a-box" energy storage system plugs into molten silicon
A render of MIT's new grid-scale energy storage system
A render of MIT's new grid-scale energy storage system
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A render of MIT's new grid-scale energy storage system
A render of MIT's new grid-scale energy storage system

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.

Source: MIT

Chris Coles
For a start, the test for the container material was not at the advised temperature for the system of 4350F or 2400C. Then add the requirements for overrun and one might need to verify the system with temperatures as high as 20% above those cited herein. Now they describe passing the liquid Silicon through "tubes" ... and do not describe the material from which the tubes are formed; let alone the material forming the heating elements.
I for one would want to see a very detailed analysis of the design parameters before I would believe that the proposed energy storage system is a viable proposition; particularly for very long term use. It is one thing to test below temperature for an hour; quite another to satisfy a design that must need to operate for decades without intervention.
Captain Danger
So solar energy is converted to electrical energy at %18 eff The Electrical energy is used to melt silicon at %95 eff Melted silicon is pumped through transparent tubes that can withstand 4000+deg heat Light from these tubes is converted to electricity at %45 eff. There are enough of these tubes and multijunction photovoltaics to generate about 150 megawatts of power (roughly enough for 100,000 homes). sounds great esp after the rigorous testing of building a mini tank and heatin it ot about %75 of the target temperature for an hour.
It sounds like some idea a first year student "team" with a limited budget and mechanical skills came up with.
I admit I have not read all the attached literature yet so maybe I am wrong, but it sounds pretty pie in the sky to me.
IMHO, best way to store energy is probably flywheels! What really needed is, R&D on flywheel energy storage devices of different sizes, from small battery sizes to very large grid sizes! Imagine, if they used magnetic levitation/bearing, in vacuum, to reach truly extreme rotation speeds (to store energy, far more of any other kind of battery)!
I love how we keep coming up with giant leap innovations. I did not know Silicon glowed at temperature.
Douglas Bennett Rogers
I think they will need a gravity system with graphite lifts and boats and under vacuum. The PV's can be placed over the top of the boats. They will need massive cooling. The silicon could be the liquid first wall of a nuclear fusion reactor.
You have to wonder why the team at MIT fail to cite nor even acknowledge in passing the prior work done by the researchers at the Solar Energy Institute of the Polytechnic University of Madrid years ago. This really isn't an MIT thing.
What is the theoretical end efficiency and projected life span of a unit that would power 100k homes?
I had a friend, many years ago, who came up with an idea that sounds much safer than a thousand degree tank of silicone. He used sand in aluminum pouches. It could have scaled up or down, for home use or grid use. Our federal agencies would not consider it back in the early 90's. I could never understand why not, other than it didn't involve the usual cast of big businesses and corrupt politicians. He had all the technical details laid out and formulas worked out. He even had a small stove unit in his house. Cheap and safe but no cigar.
Silicon melts at just over 1400C so providing they could come up with some sort of heat exchanger, that's more than enough to drive a turbine directly at about 40% efficiency. That's better than you'd get from a PV system.
If this silicon energy storage proves viable, I see no reason not to also use it in solar thermal power plants, instead of molten salt. This would greatly improve their efficiency. The MIT concept can be used in more places, but in very sunny areas, why not CSP, aka solar thermal, base load solar plants. And there is not the energy loss of conversion from electricity to heat or light, and back to electricity, that Captain Danger talked about.
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