Liquid silicon for grid-scale energy storage

Dec. 10, 2018

By Steve Bush

CAMBRIDGE, MA—White-hot liquid silicon could be the key to storing the vast amounts of energy needed to run a renewables-based national power grid, according to MIT, which claims it “would be vastly more affordable than lithium-ion batteries” and “would cost about half as much as pumped hydroelectric storage, the cheapest form of grid-scale energy storage to date”.

Liquid silicon has been chosen because it is not as corrosive at high temperature than metal salts.

It would reside at 2,000°C in an insulated 10m-wide tank made from graphite.

Tubes with heating elements connect this to second tank at 2,400°C.

When electricity is needed, the high-temperature liquid silicon is pumped through an array of tubes into the less hot tank.

Because of the temperature, those connecting tubes emit light and, in an elaborate assembly, this is picked up by an array of high-grade multi-junction photo-voltaics.

The scheme is dubbed TEGS-MPV (thermal energy grid storage-multi-junction photovoltaics).

“It’s basically an extremely intense light source that’s all contained in a box that traps the heat,” said researcher Asegun Henry, pointing out that insulation is key: “The stuff is glowing white hot on the inside, but what you touch on the outside should be room temperature.”

Could the silicon corrode the tank walls by forming silicon carbide?

Yes, is the answer, but only for a short while, as testing at 2,150°C for an hour revealed the first silicon carbide formed creates a protective layer.

“It sticks to the graphite and forms a protective layer, preventing further reaction,” said Henry. “So you can build this tank out of graphite and it won’t get corroded by the silicon.”

To make the tanks out of multiple graphite parts, a sealing technique has been proposed where pieces of graphite are screwed together with carbon fibre bolts and gaps sealed with ‘grafoil’—a flexible graphite.

Last year, the team developed a pump that could conceivably pump liquid silicon – and is in the Guiness Book of World Records as the pump with the highest heat tolerance, said MIT.

According to the university, a single storage unit could enable a city of 100,000 homes to be powered entirely by renewable energy, without the need for an altitude difference and a dam.

“This is geographically unlimited, and is cheaper than pumped hydro, which is very exciting,” summerised Henry.

The work is published as ‘Thermal energy grid storage using multi-junction photovoltaics‘ in Energy & Environmental Science.

Is it really possible to do this with photovoltaics?

“We report promising initial experimental results that suggest it is feasible and could meet the low cost required to reach full penetration of renewables,” according to the paper’s abstract.

Electronics Weekly has the complete article

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