‘Sun in a box’? Designing for 100% clean power, no emissions

Source: Christa Marshall, E&E News reporter • Posted: Tuesday, December 18, 2018

Scientists are developing a device dubbed “sun in a box” that they say could revolutionize energy storage and slash greenhouse gas emissions by bringing large amounts of renewable power online.

Described this month in the journal Energy & Environmental Science, the design could power a small city of about 100,000 houses with renewable energy, according to researchers from the Massachusetts Institute of Technology, National Renewable Energy Laboratory and Georgia Institute of Technology.

And, they add, it could cost half as much as the cheapest storage option available now and sequester energy for days, unlike existing options like lithium-ion batteries.

“This is one of the most interesting prospects we have to mitigate climate change and solve the storage problem,” said Asegun Henry, an MIT mechanical engineering professor and study co-author.

The “problem” is finding breakthroughs that can sequester energy for much longer than current batteries and also be affordable. “Sun in a box,” as the researchers call it, is one of several efforts to deploy new thermal energy options that store energy as heat for later use.

Henry has been working on the project for five years and recently received a $1.5 million grant from the Department of Energy’s Advanced Research Projects Agency-Energy to develop the technology. The goal was to find a cheaper alternative to molten salt storage used with concentrating solar plants, which may never be cheap enough to compete with natural gas, he said.

Key to the design: white-hot molten silicon that can get about three times hotter than molten salt and still be used as storage. The hotter the liquid, Henry said, the more energy can be converted to electricity.

The team envisions two 1-meter-wide tanks — one holding a “cold” version of liquid silicon, which would still be almost 3,500 degrees Fahrenheit. When electricity demand spikes, a network of tubes would heat the flowing metal to about 4,300 degrees, with power coming from any type of renewable energy.

The liquid would then be piped out of the second, “hot” tank, where specialized solar cells would harvest light from the glowing liquid to generate electricity.

Because the liquid is so hot, “there’s so much light coming out of this, it doesn’t have to be big to get a lot of power,” Henry said.

The system would cost about half as much as pumped hydroelectric storage and even less than lithium-ion batteries, partially because silicon is readily available and inexpensive, Henry said. Conversion to electricity from specialized solar cells is cheaper than a steam-driven turbine, he said.

It was previously known that silicon could store thermal energy, but there was skepticism that any system could work because pumps and storage tanks wouldn’t tolerate such a hot liquid. Last year, the MIT team developed a pump that has the highest heat tolerance ever recorded and could move the hot liquid theoretically.

For this paper, the team discovered via tests that silicon reacts with graphite — which would be used for a storage tank — but instead of corroding it as expected, it created a silicon carbide “skin” that acts as an insulator.

The engineers also addressed a potential leakage problem by demonstrating that carbon fiber bolts could seal graphite enough to prevent seeps of liquid silicon, which was a key challenge holding the technology back.

Like any new technology, “sun in a box” might not make it to commercialization. Until a full pilot is built, it’s unknown whether new technical snafus could develop. Currently, the project is at the conceptual design stage, with lab experiments verifying parts of the design.

The next step is to test more components in a lab, including running the pump with liquid silicon itself, Henry said. There need to be experiments with the solar cells too, he said.

“We’re talking about these things sitting for 30 years or more. It’s one thing to run things for a couple hours. … It’s another to say, ‘Will it behave like this for years?'” Henry said. A lot “could go wrong,” he said.

He said he was looking at a 10- to 15-year horizon to build a full-sized pilot. A smaller prototype could come in the next few years, with help from the ARPA-E money, he said.

In the meantime, others are looking at the concept. This month, Australia-based 1414 Degrees broke ground on the first commercial pilot of a molten silicon system, with partial funding coming from the South Australian government.

The company, named after silicon’s melting point, is attempting to use the technology to store energy generated from biogases created during wastewater treatment. Earlier this month, 1414 Degrees Executive Chairman Kevin Moriarty said he hoped the project would be a foundation for “similar sites across the globe.”

Yayoi Sekine, an analyst at Bloomberg New Energy Finance, said potential breakthrough technologies are important but there haven’t been any yet that can compete with lithium ion batteries in the next five years.

New storage technologies “have to compete at an ever-declining lithium-ion battery price and increasing manufacturing scale scenario, and that is proving challenging,” Sekine said.