'Sun in a box' concept could store renewable energy for the grid

06 December 2018

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MIT engineers have produced 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 – around the clock.

The new design stores heat generated by excess electricity from solar or wind power in large tanks of white-hot molten silicon, and then converts the light from the glowing metal back into electricity when it is needed.

The researchers estimate that such a system would be vastly more affordable than lithium-ion batteries, which have been proposed as a viable, though expensive, method to store renewable energy. They also estimate that the system would cost about half as much as pumped hydroelectric storage: the cheapest form of grid-scale energy storage to date.

Said Asegun Henry, the Robert N. Noyce career development associate professor in the Department of Mechanical Engineering: "Even if we wanted to run the grid on renewables right now we couldn't, because you'd need fossil-fuelled turbines to make up for the fact that the renewable supply cannot be dispatched on demand.

“We're developing a new technology that, if successful, would solve this most important and critical problem in energy and climate change, namely, the storage problem."

Henry and his colleagues have published their design today in the journal Energy and Environmental Science.

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Record temperatures

The new storage system stems from a project in which the researchers looked for ways to increase the efficiency of a form of renewable energy, known as ‘concentrated solar power’. Unlike conventional solar plants that use solar panels to convert light directly into electricity, concentrated solar power requires vast fields of huge mirrors that concentrate sunlight onto a central tower, where the light is converted into heat that is eventually turned into electricity.

"The reason that technology is interesting is, once you do this process of focusing the light to get heat, you can store heat much more cheaply than you can store electricity," Henry notes.

Concentrated solar plants store solar heat in large tanks filled with molten salt, which is heated to high temperatures of about 1,000 °F. When electricity is needed, the hot salt is pumped through a heat exchanger, which transfers the salt's heat into steam. A turbine then turns that steam into electricity.

"This technology has been around for a while, but the thinking has been that its cost will never get low enough to compete with natural gas," said Henry. "So there was a push to operate at much higher temperatures, so you could use a more efficient heat engine and get the cost down."

However, if operators were to heat the salt much beyond current temperatures, the salt would corrode the stainless steel tanks in which it's stored. So Henry's team looked for a medium other than salt that might store heat at much higher temperatures. They initially proposed a liquid metal and eventually settled on silicon – the most abundant metal on Earth, which can withstand incredibly high temperatures of over 4,000 degrees Fahrenheit.

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Last year, the team developed a pump that could withstand such blistering heat, and could conceivably pump liquid silicon through a renewable storage system. The pump has the highest heat tolerance on record: a feat that is noted in The Guinness Book of World Records. Since that development, the team has been designing an energy storage system that could incorporate such a high-temperature pump.

‘Sun in a box’

Now, the researchers have outlined their concept for a new renewable energy storage system, which they call TEGS-MPV, which stands for Thermal Energy Grid Storage-Multi-Junction Photovoltaics. Instead of using fields of mirrors and a central tower to concentrate heat, they propose converting electricity generated by any renewable source, such as sunlight or wind, into thermal energy, via joule heating: a process by which an electric current passes through a heating element.

The system could be paired with existing renewable energy systems, such as solar cells, 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 invention would store solar and wind energy in the form of white-hot liquid silicon, stored in heavily insulated tanks | Credit: Duncan MacGruer | Massachusetts Institute of Technology

The system would consist of a large, heavily-insulated, 10-metre-wide tank made from graphite and filled with liquid silicon, kept at a ‘cold’ temperature of almost 3,500°F. A bank of tubes, exposed to heating elements, then connects this cold tank to a 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 and further heats up as it passes through the bank of tubes exposed to the heating elements, and into the hot tank, where the thermal energy is now stored at a much higher temperature of about 4,300°F.

When electricity is needed, for instance, after the sun has set, the white-hot liquid silicon is pumped through an array of tubes that emit the said light. Specialised 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: this effectively acts as a large rechargeable battery.

"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," Henry continued. "It's basically an extremely intense light source that's all contained in a box that traps the heat."

For more information, visit news.mit.edu/2018/liquid-silicon-store-renewable-energy-1206.


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