Boffins build blazing battery bonfire

'Sun in a box' system promises power storage from molten silicon

Energy boffins have proposed an alternative to lithium-ion batteries: Instead of costly electrochemical cells, which have been known to burst into flames, they have devised a "sun in a box" to store energy for power utilities.

Imagine a storage tank 10 meters in diameter filled with molten silicon, alongside a few other components to turn the heat into electricity. That could be tomorrow's grid-scale battery. While converting heat to electricity is inefficient, that loss can be overcome by the potential cost savings – storing heat is 50x to 100x cheaper than storing electricity, the researchers say.

The energy grid isn't set up to store energy, explained Henry, associate professor of Mechanical Engineering at MIT, in an interview with The Register. "The whole grid is operated by predicting demand and dialing in supply," he said.

Having a cost-effective way for utilities to store energy would make the transition to renewable energy sources easier – solar power, for example, can't meet energy demand at night unless it's been stored during the day.

The researchers from Georgia Tech, MIT, and the National Renewable Energy Laboratory – Caleb Amy, Hamid Reza Seyf, Myles Steiner, Daniel Friedman and Asegun Henry – describe their silicon-based battery in a paper, "Thermal energy grid storage using multi-junction photovoltaics," published the journal Energy and Environmental Science.

As the paper puts it, the "storage problem...has emerged as one of the most important technological barriers to decarbonization of the grid and mitigating climate change."

According to Henry, the "sun in the box" – a term for the battery system coined by colleague Shannon Yee at Georgia Tech instead of TEGS-MPV, from the paper's title – represents an attempt to reduce the cost of energy storage at scale. A molten silicon battery can store energy for almost an order of magnitude less than current lithium-ion systems.

Lithium-ion costs run about $300 to $400 per kWh-e, he said, while a molten silicon system looks like it can operate at $30 to $40 per kWh-e. Even with costs for lithium-ion storage projected to drop to around $150 per kWh-e, the paper contends costs will need to be about $50 per kWh-e or less to make energy storage work with renewable sources.

The silicon used isn't semiconductor grade and the cost might be brought down further by adding scrap steel to the mix, said Henry.

Alas, he seemed skeptical that utility-maintained tubs of scorching silicon could serve as a place to toss old electronics in lieu of the current messy recycling process.

Asked about the safety of managing giant vats of fiery silicon, Henry said the infrastructure required would be similar to that used for molten salt. "They dig a big hole and fill it with concrete so if the salt leaks it would be contained," he explains. "These are significant construction projects. We don't expect it to leak because it's essentially a self-healing protective layer of silicon carbide."

With government funding in hand, Henry said he and his colleagues are in the process of negotiating milestones and he anticipates having something to show in about three years.

He allowed that the technology could be disruptive to utilities and their business models, but he suggested it's more likely it will open up new opportunities. ®

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