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Sun-in-a-box developed by MIT as an energy storage device

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Traditionally MIT engineers like to think outside the box when coming up with conceptual designs. This time, however, is different…

A system of storing renewable energy dubbed the “sun-in-a-box” has been designed using white hot molten silicon to store energy as heat, and release it as light.

Sounds bizarre? We’ll walk you through it. 

The Sun-In-A-Box is a design by MIT that will heat silicon to store energy
(Image via MIT)

How it works

The system uses a large, heavily insulated graphite tank, around 10 metres wide, that is filled with liquid silicon. This is kept at a “cold” temperature of just 1,900°C (3,500°F).

A bank of tubes exposed to heating elements, powered by energy from solar cells, connected this tank to a second “hot” tank. The silicon is pumped through the tubes and heated to around 2,400°C (4,350°F).

At this temperature the silicon is white hot. So white that it is actually glowing brightly enough for the light to be captured by specialised solar cells and converted into electricity. 

So solar energy is concentrated and converted to heat, before being turned back in to light to create electricity when needed.

The silicon can then be pumped back to the cool tank to be re-heated and reused as needed.

The researchers estimate that a single storage system could provide enough electricity for a small city of around 100,000 homes.

Current concentrated solar power projects reflect energy to a tower of molten salt to store as heat

(Concentrated solar farm in Australia using a tower of molten salt to store energy as heat - image via SolarReserve)

Where did the idea come from?

The design stems from a concentrated solar power project that used molten salt to store the heat. Unlike traditional solar plants that convert light directly into electricity, these variations concentrate sunlight from huge mirrors on a central tower, which can convert the energy into heat for storage.

“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,” said Asegun Henry, an Associate Professor at MIT’s Department of Engineering.

With the molten salt systems, the stored energy is used to heat water and create steam to power turbines. The problem with it is that once it reaches 537°C (1000°F) it begins to corrode the stainless steel tanks that store it. 

Silicon, however, can withstand much higher temperatures. Tests on graphite tanks were undertaken to see how the two materials would react at super-high temperatures. They discovered after 60 minutes that though silicon carbide did begin to form, it stuck to the graphite, creating a thin protective layer rather than corroding the tank. 

Cheap, efficient energy storage technology is essential for maximising the potential of renewable energy
(Image via Wiki)

Why is this needed? 

Solar and wind can be generate energy effectively, but can’t operate around the clock. Energy that isn’t directly used as electricity needs to be stored. 

“Even if we wanted to run the grid on renewables right now we couldn’t, because you’d need fossil-fueled turbines to make up for the fact that the renewable supply cannot be dispatched on demand,” says Henry.

The most common storage uses lithium-ion batteries, such as the gigantic Gannawarra Energy Storage System in Australia or GE and Arenko’s 41MW battery system in the UK. However, lithium is a difficult material to come by, making these systems extremely expensive. 

Other systems have been developed using innovative methods, such as gravity batteries using towers of concrete blocks, or disused mines.

But according to MIT these are less effective and less cost efficient than their Sun-In-The-Box. Silicon is the most abundant metal on Earth, making it much cheaper to use in large quantities than lithium. 

The design would also require less infrastructure than gravity-based systems or even the concentrated solar systems that use molten salt - since the modified solar panels reduce the need for steam tanks and turbines. 

They even claim that the proposed system is more cost-effective than pumped hydroelectric - currently the cheapest form of energy storage, and doesn’t depend on locations with large waterfalls and dams. 

“This is geographically unlimited, and is cheaper than pumped hydro, which is very exciting. In theory, this is the linchpin to enabling renewable energy to power the entire grid.”

With the design still at the conceptual stage, there’s a long way to go before we can see the sun-in-a-box in use. But each development like this will put us a step closer to resolving the biggest issue in renewable energy - cost-effective, reliable storage.

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Recent Comments
There are already molten silicon facilities being build ( But those operate at lower temperatures and just use the heat cycle. Why would upgrading to a way higher heat level and also using the photo voltaic in combination with heat be more efficient?
adrian, 12 December 2018
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Sun-in-a-box developed by MIT as an energy storage device - Time to read 4 min
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