A Solar ‘Breakthrough’ Won’t Solve Cement’s Carbon Problem

ANYONE WHO SAYS there’s nothing new under the sun hasn’t made a recent trip to Lancaster, California. There, on the outskirts of the Mojave desert, 400 giant mirrors, each the size of a large flatscreen TV, twitch in the sunlight. Their reflective faces are turned toward a nearby tower that looms over the lot like an industrial eye of Sauron. Each of these robotic heliotropes are at the beck and call of an algorithmic conductor, which directs them to focus their sunbeams toward a small target on the tower. Here temperatures soar above 1,800 degrees Fahrenheit, or roughly a quarter of the surface temperature of the Sun.

It sounds like a doomsday machine cooked up by a comic book villain, but this is a device meant to help save the world, not end it. It’s called the Heliomax and its superpower is making cement.

The system was developed by Heliogen, a six-year-old startup that just came out of stealth mode this week and is partially bankrolled by Bill Gates’ investment firm. Its goal is to use concentrated solar energy to power industrial processes that require a lot of heat, such as steel and cement manufacturing. These industries are literally and figuratively the foundation of modern civilization, but their toll on the climate has made them prime targets in the push to limit carbon emissions. The cement industry is a particularly bad offender, releasing 2 billion tons of carbon into the atmosphere annually—more than twice as much as the airline industry. Indeed, nine tons of carbon is produced for every 10 tons of cement.

Heliogen’s technology has many possible uses, including as a way to produce hydrogen, but the company plans to work with cement production facilities first. According to Heliogen CEO Bill Gross, the company has already installed a small cement oven on the top of the tower and used it to replicate the most carbon-intensive step in the production of cement. It was a proof-of-concept, but it worked. Now that the company is out of stealth mode, Gross says he and his team have received dozens of inquiries from cement companies around the world and begun the hunt for an industrial partner to test the technology at a commercial scale. “Cement is the most versatile building material there is; if you look around it’s everywhere,” Gross says. “But it accounts for 8 percent of emissions. So we’re starting with cement first because these heavy industries have been untouched by renewables.”

It’s an innovative solution that’s being widely hailed as a breakthrough, but Heliogen isn’t the first to go this route. A European project known as Solpart aims to create a partially solar-powered cement plant in Spain by 2025. Last year, its researchers used an experimental solar reactor in France to replicate the same step Heliogen has now demonstrated.

That carbon-intensive step is called calcination, and it happens early on in production, says Gkiokchan Moumin, a doctoral student at the German Aerospace Center’s Institute for Solar Research and a member of Solpart. To make cement, crushed limestone is fed into a kiln and heated to roughly 1,500 degrees Fahrenheit, at which point it breaks down into lime and carbon dioxide. Powering this step with renewable energy would go a long way toward reducing the cement industry’s carbon footprint—and Solpart proved it was possible.

The Solpart team used the world’s largest solar furnace, in southern France, and tried out two different cement reactor designs. One was a rotating kiln and the other a bubbling fluidized bed, kind of like a meat grinder for rocks. The reactors were placed in a shipping-container-sized receiver and then blasted with 1 megawatt of concentrated sunlight reflected off a segmented parabolic mirror the size of a 17-story building.

Both cement reactors worked, but the overall design falls far short of a commercial facility, says Jan Baeyens, the managing director of European Powder and Process Technology and a member of the Solpart team. First, the system needs some kind of energy storage so cement production isn’t contingent on the sunshine available at that moment. And it needs to bake a whole lot more rock, scaling up from a few dozen kilograms to several thousand tons of cement per day.

Moumin says the latter challenge is one of the Solpart team’s most active areas of research. It’s a problem Heliogen will also have to solve once it links up with a commercial cement facility.

The differences between the two systems are in how Heliogen and Solpart use their arrays of mirrors to concentrate sunlight. Heliogen is using machine vision to constantly tweak the positions of the mirrors so they are always pointed at the most optimal spot. That allows it to achieve roughly the same temperatures with much less energy, ultimately dumping around 300 kilowatts into an area only a little larger than a basketball hoop. “We have a black, silicon carbide plate that the light is shining on and it is glowing white hot from all that energy,” Gross says. “It’s insane.”

For now, both Solpart’s and Heliogen’s systems remain impressive experimental validations of a very futuristic concept. If they are successful in powering cement production with sunlight, it would be a major step toward sustainability in an industry that has struggled to reduce its carbon footprint. “I think more cement plants would like to use alternative fuels,” says Jeremy Gregory, the executive director of MIT’s Concrete Sustainability Hub.

The industry won’t decarbonize easily, however. Solpart’s Baeyens says he is “doubtful” of the prospect of a fully solar cement plant because of the variability in available sunlight, so hybrid operations are more likely, merging solar with alternative fuels such as biomass. Gross, of Heliogen, also acknowledges the limitations of solar power. He estimates only half the world’s cement plants have enough land to host a Heliomax system on site, assuming they wanted to.

But even if every single cement kiln ran on photons, the industry would still produce a wildly unsustainable amount of carbon. Using solar power to run the kilns only accounts for about 40 percent of cement’s carbon footprint. The majority of its emissions come from the chemical processes that occur while the limestone is being heated. So even with entirely solar-powered kilns, global cement production would continue to belch roughly 1.2 billion tons of CO2 annually. To put that in perspective, that is still 30 percent more than the entire airline industry.

A more complete solution would pair the solar kilns with new cement materials or carbon capture technologies. There are plenty of ideas for alt-cement, but so far none have seen widespread adoption. Democratic presidential candidate Andrew Yang, for example, has suggested that the federal government should invest in research on cement blends that can trap airborne carbon. These types of cements already exist; they’re just prohibitively expensive to make.

Carbon capture technologies have also struggled to gain ground due to a host of technological problems and funding woes. Gross sees brighter opportunities for carbon capture with Heliomax. Its kilns release pure CO2, unlike the dirtier exhaust of fossil fuel-powered ovens, which he says makes it easier to capture. The company will still have to demonstrate there’s an economical way to do so.

Heliogen’s solar power system might solve one big part of the cement’s carbon problem, but the industry still needs some more concrete solutions.

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