The system relies on the transfer of molten silicon between two cylindrical graphite tanks, each about 10m wide. Silicon in the ‘cool’ tank would be stored at around 1,900°C, then pumped through a network of heated pipes into the second tank, reaching temperatures close to 2,400°C. The elements heating the pipes would be powered by excess energy from the grid, theoretically from renewable sources such as wind and solar.
To feed the stored energy back to the grid, molten silicon from the second tank – now glowing white hot - is passed through a system of pipes that emit light. This light is then captured by highly efficient multi-junction solar cells. Though commonly referred to as the ‘sun in a box’, the system’s official name is Thermal Energy Grid Storage-Multi-Junction Photovoltaics, or TEGS-MPV. The research is published in the journal Energy & Environmental Science.
“One of the affectionate names people have started calling our concept, is ‘sun in a box,’ which was coined by my colleague Shannon Yee at Georgia Tech,” said corresponding author Asegun Henry, an associate professor at MIT’s Department of Engineering. “It’s basically an extremely intense light source that’s all contained in a box that traps the heat.”
The system was inspired by concentrated solar plants that use molten salt as a storage medium. However, molten salt can only be heated to around 550°C before it becomes corrosive, meaning its efficiency is limited. Silicon, one of Earth’s most abundant elements, can operate safely at a much higher range. Key to the overall concept is a pump that can withstand such temperatures, and last year the team developed a pump that holds the Guinness World Record for the highest heat tolerance. Another area of concern was how the silicon would react with the graphite tanks. Testing on a miniature tank with silicon heated to 2,000°C for 60 minutes, the team found that a layer of silicon carbide formed, but actually acted as a non-reactive protective layer.
According to the researchers, a scaled-up version of the 'sun in a box' system using 10-metre tanks would allow a small city of 100,000 homes to be powered by renewable energy. Furthermore, it would not rely on natural geography in the way that pumped hydro does, as well as costing significantly less, claim the team. Pumped hydro is currently the cheapest form of grid-scale storage in widespread use.
“This is geographically unlimited, and is cheaper than pumped hydro, which is very exciting,” said Henry. “In theory, this is the linchpin to enabling renewable energy to power the entire grid.”
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