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John O | December 2018

Researchers design thermal energy storage device that uses molten silicon in insulated tanks


By Josh Perry, Editor
[email protected]

 

Researchers at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass. have demonstrated a novel renewable energy storage system that stores excess thermal energy produced by solar or wind power in the form of white-hot liquid silicon contained in heavily-insulated tanks.

 


MIT researchers propose a concept for a renewable storage system, pictured here, that would store solar and wind energy in the form of white-hot liquid silicon, stored in heavily insulated tanks. (Duncan MacGruer/MIT)

 

According to a report from the school, this concept, called TEGS-MPV (Thermal Energy Grid Storage-Multi-Junction Photovoltaics), would be more affordable than lithium-ion batteries and “half as much as pumped hydroelectric storage,” which is currently the most cost-effective grid-scale energy storage concept.

 

The project began as a way of improving the efficiency of concentrated solar power, which store heat in tanks filled with molten salt that is heated to around 1,000°F. The article explained, “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.”

 

Salt can be corrosive to the tanks, so the researchers turned to molten silicon, which can withstand temperatures higher than 4,000°F. They created a pump last year that can move the silicon and set a record for the highest temperature threshold.

 

TEGS-MPV does not use mirrors, which are standard in concentrated solar systems, but rather uses joule heating to convert electricity from wind or solar sources into thermal energy. The system includes a 10-meter-wide graphite tank filled with liquid silicon stored at 3,500°F and is connected to a second tank by a series of tubes exposed to a heating element that raises the temperature of the silicon to 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,” the article said. “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 research was recently published in Energy and Environmental Science. The abstract stated:

 

“As the cost of renewable energy falls below fossil fuels, the key barrier to widespread sustainable electricity has become availability on demand. Energy storage can enable renewables to provide this availability, but there is no clear technology that can meet the low cost needed. Thus, we introduce a concept termed thermal energy grid storage, which in this embodiment uses multi-junction photovoltaics as a heat engine.

 

“We report promising initial experimental results that suggest it is feasible and could meet the low cost required to reach full penetration of renewables. The approach exploits an important tradeoff between the realization of an extremely low cost per unit energy stored, by storing heat instead of electricity directly, and paying the penalty of a lower round-trip efficiency.

 

“To understand why this tradeoff is advantageous, we first introduce a general framework for evaluating storage technologies that treats round trip efficiency, as well as cost per unit energy and power, as variables.”

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