‘Sun in a box’ promises cheaper renewable energy storage

Technology News | December 6, 2018

By Rich Pell

The “sun in a box” concept, say the researchers, could deliver the stored energy back into an electric grid on demand and should be less costly than alternative approaches. 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.

Such a system, say the researchers, would be much more affordable than lithium-ion batteries, which have been proposed as one viable method to store renewable energy. In addition, they also estimate that the system would cost about half as much as pumped hydroelectric storage – the least expensive form of grid-scale energy storage to date.

“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,” says 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 new storage system design stems from research that looked for ways to increase the efficiency of a form of renewable energy known as concentrated solar power, which – unlike conventional solar plants that use solar panels to convert light directly into electricity – uses vast arrays 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,” says Henry, “once you do this process of focusing the light to get heat, you can store heat much more cheaply than you can store electricity.”

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

“This technology has been around for a while,” says Henry, “but 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.”

However, heating the salt much beyond current temperatures would result in the salt corroding the stainless steel tanks in which it’s stored. So, in looking for a medium other than salt that might store heat at much higher temperatures, the researchers initially proposed a liquid metal and eventually settled on silicon – the most abundant metal on Earth – which can withstand temperatures of over 4,000°F.

The researchers then developed a pump that could withstand such a high heat, and that could conceivably pump liquid silicon through a renewable storage system. The resulting pump, say the researchers, has the highest heat tolerance on record – a feat noted in “The Guiness Book of World Records.” Since that development, they have been designing an energy storage system that could incorporate such a high-temperature pump.

They call their new renewable energy storage system “TEGS-MPV” – 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 heat is produced by passing electric current through a resistive conductor or heating element.

The system could be paired with existing renewable energy systems – such as, for example, a solar plant – to capture excess electricity during the day and store it for later use. The system would consist of a large, heavily insulated, 10-meter-wide tank made from graphite and filled with liquid silicon, kept at a relatively “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 excess electricity from the existing solar plant 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, the hot liquid silicon – which is so hot that it’s glowing white – is pumped through an array of tubes that emit that light. Specialized solar cells, known as multijunction photovoltaics, then turn that light into electricity, which can then be supplied to the local grid. The now-cooled silicon can be pumped back into the cold tank until the next round of storage, acting effectively as a large rechargeable battery.

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

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. In addition, say the researchers, the system’s design is – unlike pumped hydroelectric energy storage – geographically unlimited, and can be sited anywhere.

“In theory,” says Henry, “this is the linchpin to enabling renewable energy to power the entire grid.”

For more, see “Thermal energy grid storage using multi-junction photovoltaics.”