Sun in a box

MIT engineers have come up with a conceptual design for a system that could store renewable energy and deliver it back into an electric grid on demand. Such a system could power a small city not just when the sun is up or the wind is high, but around the clock.

The new design stores heat generated by excess electricity from solar or wind power in large tanks of molten silicon, and then converts the light from the glowing metal back into electricity when it’s needed. The researchers expect it would be vastly more affordable than lithium-ion storage systems.

The system consists of two large, heavily insulated, 10-meter-wide tanks made from graphite. One is filled with liquid silicon, kept at a “cold” temperature of almost 3,500 °F (1,927 °C). A bank of tubes, exposed to heating elements, then connects this cold tank to the second, “hot” tank. When electricity from the town’s solar cells comes into the system, this energy is converted to heat in the heating elements. Meanwhile, liquid silicon is pumped out of the cold tank, collects heat from the heating elements as it passes through the tubes, and enters the hot tank, where it is now stored at a much higher temperature of about 4,300 °F (2,371 °C).

When electricity is needed (say, after the sun has set), the hot liquid silicon—so hot that it’s glowing white—is pumped through an array of tubes that emit that light. Specialized solar cells, known as multi-junction photovoltaics, then turn that light into electricity, which can be supplied to the town’s grid. The now-cooled silicon can be pumped back into the cold tank until the next round of storage—so the system effectively acts as a large rechargeable battery.

“One of the affectionate names people have started calling our concept is ‘sun in a box,” says mechanical engineering professor Asegun Henry. “It’s basically an extremely intense light source that’s all contained in a box that traps the heat.”

Henry says the system could be sited anywhere, regardless of a location’s landscape. This is in contrast to pumped hydroelectric systems, currently the cheapest form of energy storage, which require locations that can accommodate large waterfalls and dams to store energy from falling water.

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