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White-Hot Blocks as Renewable Energy Storage?

Thermal batteries could be the cheap and simple option

4 min read

Concrete Bricks
Blocks made from graphite or ceramics (akin to the concrete blocks pictured here) may be a promising medium for thermal storage of renewable energy generated by intermittent solar and wind energy sources.

In five years, operating a coal or natural gas power plant is going to be more expensive than building wind and solar farms. In fact, according to a new study by Bloomberg New Energy Finance, building a new solar farm is already cheaper than operating coal and natural gas plants in many regions of the world. 

Yet a full shift to intermittent energy sources desperately calls for low-cost, reliable energy storage that can be built anywhere. Some nascent startups believe the answer lies in the process that lights up toaster coils by electrically heating them to scorching temperatures.

Antora Energy in Sunnyvale, Calif., wants to use carbon blocks for such thermal storage, while Electrified Thermal Solutions in Boston is seeking funds to build a similar system using conductive ceramic blocks. Their vision is similar: use excess renewable electricity to heat up the blocks to over 1,500°C, and then turn it back to electricity for the grid when needed.

To beat the cost of the natural gas plants that today back up wind and solar, storing energy would have to cost around $10 per kilowatt-hour. Both startups say their Joule heating systems will meet that price. Lithium-ion batteries, meanwhile, are now at approximately $140/kWH, according to a recent study by MIT economists, and could drop to as low as $20/kWH, although only in 2030 or thereafter. 

Justin Briggs, Antora’s co-founder and Chief Science Officer, says he and his co-founders Andrew Ponec and David Bierman, who launched the company in 2018, considered several energy-storage technologies to meet that goal. This included today’s dominant method, pumped hydro, in which water pumped to a higher elevation spins turbines as it falls, and the similar new gravity storage method, which involves lifting 35-ton bricks and letting them drop.

In the end, heating carbon blocks won for its impressive energy density, simplicity, low cost, and scalability. The energy density is on par with lithium-ion batteries at a few hundred kWh/m3, hundreds of times higher than pumped hydro or gravity, which also “need two reservoirs separated by a mountain, or a skyscraper-sized stack of bricks,” Briggs says.

Antora uses the same graphite blocks that serve as electrodes in steel furnaces and aluminum smelters. “[These] are already produced in 100 million ton quantities so we can tap into that supply chain,” he says. Briggs imagines blocks roughly the size of dorm fridges packed in modular units and wrapped in common insulating materials like rockwool.

“After you heat this thing up with electricity, the real trick is how you retrieve the heat,” he says. One option is to use the heat to drive a turbine. But Antora chose thermophotovoltaics, solar cell-like devices that convert infrared radiation and light from the glowing-hot carbon blocks into electricity. The price of these semiconductor devices drops dramatically when made at large scale, so they work out cheaper per Watt than turbines. Plus, unlike turbines that work best when built big, thermophotovoltaic perform well regardless of power output.

ANTORA Antora Energy’s graphite blocks store renewably-generated energy at temperatures exceeding 1000º C, eventually converting that back to electricity via their proprietary thermophotovoltaic heat engine.ANTORA ENERGY

Thermophotovoltaics have been around for decades, but Antora has developed a new system. Richard Swanson, one of the company’s advisors, was an early pioneer of the technology in the late 1970s. The efficiency with which the devices convert heat into electricity was stuck in the 20s until the Antora team demonstrated a world-record 30% efficiency in 2019. They did that by switching from silicon to higher-performance III–V semiconductors, and by using tricks like harnessing lower-energy infrared light that otherwise passes through the semiconductor and is lost. Antora’s system recuperates that heat by placing a reflector behind the semiconductor to bounce the infrared rays back to the graphite block.

The technology has caught on. Antora has received early-stage funding from ARPA-E and is an alum of the Activate entrepreneurial fellowship program and Shell/NREL GameChanger accelerator program. More recently, they have gotten funding from venture capitalists and the California Energy Commission [PDF] to scale up their technology, and will build a pilot system at an undisclosed customer site in 2022.

Electrified Thermal Solutions, which is part of Activate’s 2021 cohort and was founded in 2020, is much younger. The company’s cofounders Joey Kabel and Daniel Stack chose ceramic blocks as their thermal storage medium. Specifically, honeycomb-shaped ceramic blocks used today to capture waste heat in steel plants. Since ceramics don’t conduct electricity, they dope the bricks to make them conductive so that they can be electrically heated to 2,000°C.

Stack says they plan to target a wide market for that stored heat. They could use it to drive a gas turbine for electricity, or to run any other high-temperature process such as producing cement and steel.

The duo is still working out some technical challenges such as keeping the ceramic from oxidizing and vaporizing over time. Eventually the system should have a lifetime of 20-plus years, another big advantage over batteries. They are now building a benchtop prototype, Kabel says, but the final full-scale system should look like a large grain silo that should store about 600 KWh/m3, matching Antora’s energy density.

It will be a few years before either company is ready to build a full-scale installation.

If they can prove themselves, though, these companies could pave a way for a cost-effective storage technology for the 21st century electrical grid. “We want to decarbonize the industrial and electric sector by replacing the combustion process with a renewable heating system,” Stack says. 

Update (23 June 2021): This story was updated to include results from a new report on the cost of solar electricity from Bloomberg New Energy Finance. 

The Conversation (2)
Thomas Moore
Thomas Moore04 Feb, 2022

Interesting. I've discounted thermal storage because it seems inevitably lossy over anything but short term. But also, getting the energy out into useful form is clearly a challenge. At 30% efficiency, it might be just as well to electrolyze water, bottle hydrogen, and then use it in fuel cells or turbines...

1 Reply