Sun in a Box: Scientists Discover New Ways to Store Renewable Energy

Engineers from the Massachusetts Institute of Technology have developed a system that restores renewable energy, such as solar and wind power, and is able to deliver that energy back into an electric grid as needed.  

The new design, which is conceptual at this point, gathers excess electricity from solar or wind power and stores heat in large tanks that are filled with a white-hot molten silicon. The light from the silicon is then converted into electricity upon demand. Because of this, the new design, described in a study published yesterday in the Energy and Environmental Science journal, would dramatically cut costs on lithium ion batteries, which are the current method for storing renewable energy. Similarly, the new design would cut the cost in half on pumped grid-scale hydroelectric storage, which is presently the cheapest form available.

“Even if we wanted to run the grid on renewables right now we couldn’t, because you’d need fossil-fueled turbines to make up for the fact that the renewable supply cannot be dispatched on demand,” said Asegun Henry, the Robert N. Noyce Career Development Associate Professor in the Department of Mechanical Engineering. “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.”

The researches initially set out to find a way to increase efficiency for concentrated solar power – a type of renewable energy. Henry and his team added an extra step to the conversion of light directly into electricity. Their design converts light into heat first, and only then 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,” said Henry.

The published article describes the process as the following: the heat is stored in large tanks containing molten salt, which can be heated to 537 degrees Celsius. When the demand for electricity arises, the hot molten salt is put through a specialised machine called the heat exchanger, where the heat becomes steam. Then, a turbine converts steam into electricity.

Henry says that while the technology itself is not new, “the thinking has been that its cost will never get low enough to compete with natural gas. 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.”

The problem was that heating the salt to extreme temperatures would result in a corrosion of the tanks that stored them. The researchers then focused on materials that could handle higher temperatures. They chose silicon, as its easily available and can easily handle temperatures of more than 2,204 degrees Celsius.

The next step was developing the pump that could withstand the heat from pumping liquid silicon through a new renewable storage system. They succeeded, and the pump they developed has even been noted in “The Guinness Book of World Records.” The next step in the design was to develop a full energy storage system that would use this pump.

The new system the researchers have described in their conceptual paper is called the TEGS-MPV, the Thermal Energy Grid Storage-Multi-Junction Photovoltaics. The proposal is to convert electricity generated by sunlight or wind into heat via joule heating, a process by which an electric current passes through a heating element.

The applicability of the design is in that it can combined with the renewable energy systems that are in existence, to capture excess electricity during the day and store it for later usage.

Henry cites a small Arizona town that partly uses 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 said. “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.”

“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 says.  “It’s basically an extremely intense light source that’s all contained in a box that traps the heat.”

For the project to come to life, researchers say that they first needed to develop tanks that are thick and strong enough to insulate the molten liquid within.

“The stuff is glowing white hot on the inside, but what you touch on the outside should be room temperature,” Henry says. Henry proposed graphite for the tanks, although there was risk of corrosion when graphite reacted with silicone. That possibility was tested, and the researchers found that corrosion does not happen.

“It sticks to the graphite and forms a protective layer, preventing further reaction,” Henry says. “So you can build this tank out of graphite and it won’t get corroded by the silicon.” The researchers also found a solution for preventing leaks by using grafoil, a type of flexible graphite that can act as a high-temperature sealant.

“Innovation in energy storage is having a moment right now,” says Addison Stark, associate director for energy innovation at the Bipartisan Policy Center, and staff director for the American Energy Innovation Council. “Energy technologists recognize the imperative to have low-cost, high-efficiency storage options available to balance out nondispatchable generation technologies on the grid. As such, there are many great ideas coming to the fore right now. In this case, the development of a solid-state power block coupled with incredibly high storage temperatures pushes the boundaries of what’s possible.”

The research is promising: Henry and his team estimate that a single storage system can power 100,000 homes by renewable energy only. What’s more, the system can be used anywhere in terms of geography – unlike the current hydroelectric pumps, that need to be near waterfalls and dams.

“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.”

Photo credit: MIT

Jessica Zeitz