MIT 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. The new design stores excess electricity from solar or wind power as in large tanks of white-hot molten silicon and then converts the light from the glowing metal back into electricity when it’s needed. The researchers estimate that such a system would be vastly more affordable than lithium-ion batteries, which have been proposed as a viable yet, 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. 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 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.

Now, the researchers have outlined their concept for a new renewable energy storage system, which they call TEGS-MPV, for Thermal Energy Grid Storage, using Multi-Junction Photovoltaics. Instead of using fields of mirrors and a central tower to concentrate heat, they propose converting electricity generated by any 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. The system would consist of a large, heavily insulated, a 10-meter-wide tank made from graphite and filled with liquid silicon, kept at a 'cold' temperature of almost 3,500 degrees Fahrenheit. 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, 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 multijunction 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 - acting effectively as a large rechargeable battery.

The system’s design is geographically unlimited, meaning that it can be sited anywhere, regardless of a location’s landscape. This is, in contrast, to pumped hydroelectric - currently the cheapest form of energy storage, which requires locations that can accommodate large waterfalls and dams, in order to store energy from falling water.