“Sun in a box” Might Store Green Energy for the Grid

published: 2018-12-27 9:30 | editor: | category: News

Even though lithium batteries have been the leading mainstream energy storage system, when we need grid-scale energy storage, lithium batteries may not be the best choice because of limited high-density lithium reserves. Limited lithium reserves hint higher costs. To resolve this issue, a research team from Massachusetts Institute of Technology (MIT) proposed a new solution called "Sun in a box" which provides a low-cost 24-hour non-stop heat-generated molten silicon energy storage system. The “Sun in a box” system will be significantly more affordable than lithium-ion batteries.

Solar energy and wind power are both popular green energy technologies. However, they are both intermittent energy sources. When wind doesn't blow, or when the sky is heavily cloudy, wind power generation and solar power generation both cannot work. In addition, when the solar power is excessive, if it can be effectively stored and be used at a later time when electricity is less sufficient, a renewable energy storage system would be a perfect solution. In particular, li-ion batteries have been the leading technology of energy storage, but lithium batteries' prices are higher. For a grid-scale system, massive installation of li-ion batteries aren't cost effective.

To resolve the problem, MIT researchers have developed a new system, named "Thermal Energy Grid Storage-Multi-Junction Photovoltaics (TEGS-MPV)". The idea is to store extra renewable energy in the form of white-hot liquid silicon, which is contained by heavily insulated tanks. Later on, the system will convert the light emitted by molten silicon into electricity.

TEGS-MPV is similar to a molten salt battery or a concentrated solar plant. However, in the TEGS-MPV system, the conventional molten salt is replaced with molten silicon. Because when molten salt is heated to 538℃, the salt will seriously corrode the stainless steel tanks. Besides, silicon can be heated to 2,200°C and increase equipment's energy density.

The system needs tanks that are thick and durable enough to insulate the molten silicon within. When the heated silicon is inside, what you touch on the outside should be room temperature. Besides, tanks made by graphite can prevent itself from being corroded by silicon. A TEGS-MPV system consists of two 10-meter-wide tanks (made from graphite). One of it is filled with "cooler" (1,900°C) liquid silicon. The other tank stores "hotter" (2,400°C) liquid silicon. A bank of tubes will be exposed to heating elements and connect the cooler tank to the hotter tank. When electricity from solar cells enters the system, the energy is converted to heat in the heating elements. At the same time, liquid silicon will flow from the cooler tank and be heated up when it passes through the bank of tubes exposed to the heating elements, and then it will enter the hotter tank.

When the TEGS-MPV system generates electricity, it doesn't use the traditional method. A traditional way is to use the heat to boil water, creating steam that drives a turbine to generate electricity. Rather, the system converts molten silicon's light into electricity. The molten silicon emits bright white lights with a high temperature. MIT scientists utilize specialized solar cells known as multi-junction photovoltaics to capture lights and convert them into electricity. Then electricity will be transmitted to a central power grid. After the silicon is cooled down, it is pumped back into the first tank to begin the cycle again.

One issue with the system is that the molten silicon might react with and corrode graphite tank. To test this possibility, the MIT team built a mini tank. When the tank was filled with molten silicon that was heated to 1,980°C for an entire hour, the graphite tank reacted with the molten silicon and formed silicon carbide. The result made the scientists quite satisfied. According to the researchers, the silicon carbide did not corrode the tank. Rather, it formed a thin, protective layer.

The research team predicts that one TEGS-MPV system can provide electricity to 100,000 households. This system's design is geographically unlimited. In other words, it can be built anywhere. Theoretically speaking, this system can store renewable energy (such as PV power and wind power). Besides, this system's cost is cheaper than other leading options, about half the price of a pumped hydroelectric station. It might become the next-generation low-cost and long life-cycle grid-scale energy storage technology. This research paper was published in the journal Energy & Environmental Science.


 (Photo credit: MIT. Article by DaisyChuang)

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