‘Sun-in-a-box’ creates power after dark

MIT NEWS writes that “engineers have come up with a conceptual design for a system to store renewable energy, such as solar and wind power, and deliver that energy back into an electric grid on demand. The system may be designed to power a small city not just when the sun is up or the wind is high, but around the clock.”

It’s being called sun-in-a-box because it heats silicon – sand – to 4,500º F (2,500º C) at which point it is liquid and literally white hot. The silicon is then pumped through tubes surrounded by special solar panels that can take that white light and create electricity in the same way regular panels do.

The system pumps white light

The concept came out of trying to figure out a more efficient ‘thermal solar’ system. Thermal solar plants, the ones with the mirrors aimed at a central tower, use the focused sunlight from those mirrors to heat salt in the tower to a molten state, which is then pumped around to boil water and create steam under pressure to drive a turbine that generates the electricity..

This reelectrify article about the largest of these plants in the world, being built in Australia, gives a more detailed description. BTW, using something to boil water to drive a turbine is essentially how other power plants work – coal plants burn coal to create the heat to boil it, natural gas plants burn gas, nuclear plants use nuclear energy.

Mirrors called heliostats focus sunlight on a tower, melting salt within the tower.

The big difference with the MIT plan is that it essentially drives light through the system, not heat.

The team had to develop a Guinness Book of Records pump

If you think it might be tricky trying to contain silicon that hot, you’d be correct. It’s about as hot as the edge of the earth’s core, and you can check out these images of what 4,000º did to a NASA heat shield. But in their tests the MIA researchers put the molten silicon in a graphite container in which a thin protective coating was created in reaction.

If you think it would be also be tricky trying to heat silicon to that temperature you would also be correct. So the way the system works is that the silicon, still molten, is kept at a colder temperature (a mere 3,500º F!) and then heated up to the white hot state only when required. For instance, at night.

The world record holder for “Highest operating temperature liquid pump”

And if you think it would be tricky trying to pump the molten silicon you would be correct for the third time. So the team had to develop a pump with the highest heat tolerance on record — a feat that is noted in “The Guinness Book of World Records.” 

A single system could enable 100,000 homes to be powered entirely by renewable energy.

Asegun Henry, the Robert N. Noyce Career Development Associate Professor in the Department of Mechanical Engineering. says “The system could be paired with existing renewable energy systems, such as solar cells or wind, to capture excess electricity during the day and store it for later use. Consider, for instance, a small town in Arizona that gets a portion of its electricity from a solar plant.

Say everybody’s going home from work, turning on their air conditioners, and the sun is going down, but it’s still hot,” Henry says. “At that point, the photovoltaics are not going to have much output, so you’d have to have stored some of the energy from earlier in the day, like when the sun was at noon. That excess electricity could be routed to the storage system we’ve invented here.

The researchers estimate that a single storage system could enable a small city of about 100,000 homes to be powered entirely by renewable energy.

There are obviously other aspects of the design that the MIT team had to address. You can read a more detailed article about how the sun-in-a-box works and its development (it is officially named TEGS-MPV, for Thermal Energy Grid Storage-Multi-Junction Photovoltaics) »» on the MIT NEWS site

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