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A solar oven to make cement more sustainable

A new project based on CSP technology could contribute to the reduction of emissions generated by the production of cement and other materials.

by Chris Dalby
12 March 2020
10 min read
byChris Dalby
12 March 2020
10 min read

One of the hindrances of solar energy has long been that current solar panel technology cannot produce the very high temperatures needed to power industrial processes such as making cement, glass or steel.
California-based Heliogen might soon change all of that. The startup has been working secretly on a solar "oven" that can generate temperatures over 1,000 degrees Celsius. Based on CSP (concentrating solar power), this oven sees hundreds of solar mirrors redirect sunlight toward a steam turbine.
The energy from the sun superheats water into steam that generates power. By finding a powerful backer—Bill Gates—Heliogen is committed to improve CSP technology.
Once Heliogen can produce temperatures of 1,500 degrees Celsius, it could produce an immediate zero-carbon alternative for heavy industries. This is not negligible when considering cement production alone accounts for a sizable amount of all carbon emissions.
In 2015, for example, the cement industry emitted 2.8 billion tons of carbon, accounting for 8% of all global emissions. This dwarfs that of the airline industry (2.5%) and almost rivals the emissions from agriculture (11%).
Without innovations to make its production cleaner, the impact of cement to greenhouse gases is unlikely to drop, especially since global use of cement is actually accelerating. The international concrete and cement market grew at an average pace of 7% from 2014 to 2018, and is estimated to speed up to 11% a year through 2022, boosted by the housing demand and low-interest rates for mortgages in developed countries.
Various ways have been suggested to lower the emissions from cement such as using cleaner, alternative materials in its production, making more efficient cement kilns or powering them with biofuels. But Heliogen's advances are still one of the more notable advances.
The potential of Heliogen made headlines in November 2019 when it was announced that Bill Gates was investing in the startup. In recent years, Gates has been a patron for potential game-changing energy projects, including a hefty investment in Commonwealth Fusion Systems, which is making strides in nuclear fusion and also boasts backing from Eni.


The cement industry is responsible for a significant proportion of CO2 emissions

How does the solar oven work?

The "solar oven" built by Heliogen in the Mojave Desert in California is comprised of 400 solar mirrors (heliostats), each equipped with an algorithmic conductor that directs sunbeams to a target on a nearby tower. The tower itself, described as "an industrial eye of Sauron", collects sunlight at temperatures of 1,800 degrees Celsius—"roughly a quarter of the surface temperature of the sun".
This scorching heat can then be applied to heavy industrial processes, providing the energy necessary to make cement, steel or glass. The appeal is evident—replacing the fossil fuels and heavy emissions of the current cement industry with clean energy would make this technology a potential show-stopper. "We are rolling out technology that can beat the price of fossil fuels and also not make the CO2 emissions," explained Bill Gross, Heliogen's founder and CEO to CNN Business. "And that's really the holy grail".
Heliogen has already installed a small cement oven atop the Heliomax tower to prove that limestone can be broken down into lime and carbon dioxide and later reassembled into cement, along with other materials. It worked, although a first attempt was successfully carried out by Solpart, a French-based effort, in 2018.

Global obstacles

As with any potential groundbreaking technology, Heliogen does face some real challenges. Going from proof of concept to widespread application is one. The fact that these solar ovens require an abundance of space is another. The final obstacle is that the quantities Heliogen has currently produced are but a fraction of what would be needed to produce cement at an industrial scale.
As both Solpart and Heliogen have admitted, scaling the technology is going to be challenging. The cement industry requires thousands upon thousands of tons of cement to be produced around the clock. Can solar ovens take this constant load at affordable prices? More research is needed to prove that this computer-powered solar mirror situation can be replicated globally.
What works in Heliogen's testing site may not be universally applicable. Even if the technology can be produced at scale and delivered at a cost-effective price, global variations in weather patterns, topological conditions and the individual size of cement companies are all going to matter.
Speaking to Wired, Jan Baeyens, manager of European Powder and Process Technology, explained that the variability in available sunlight would mean that many cement companies would be better off seeking a hybrid solution to manufacturing their product, likely blending solar with biomass. And even Heliogen’s leaders admit that only about half the world's cement plants have the space for a Heliomax installation even if they were interested.

The challenges facing concentrated solar power

Heliogen is also facing another technological challenge: proving once and for all that concentrated solar power (CSP) is viable. Thus far, CSP has struggled to get off the ground despite being used in a number of ambitious projects. The most notable to date—the Ivanpah solar plant in California—deploys 173,500 heliostats with two mirrors each, redirecting solar energy at three towers. It was the largest solar thermal plant in the world when activated in 2013, although it has since been outpaced by the Noor power plant in Ouarzazate, Morocco.
This mammoth installation will culminate with three separate CSP power plants, providing a combined 500 megawatts, as well as incorporating hours of energy storage. But Ivanpah's troubles almost spelled doom for CSP. By November 2014, the plant was only meeting around half of its generation obligations.
By 2016, there had been no significant improvement, and the California Public Utilities Commission gave Ivanpah a year to meet its 377MW targeted generation capacity. The plant did eventually get back on track, but with a build cost of $2.2 billion, those teething troubles did leave a blot on the perception of CSP. During Ivanpah's preparation, one early investor, Google, said it would be entirely divesting from concentrated solar to focus on increasingly more affordable photovoltaic technology.


The Ivanpah solar power plant


Therein lies another challenge: Ivanpah has no energy storage capacity, meaning it cools down at night. With the rapid development of such storage solutions in recent years, how can CSP integrate this? Starting in the 1990s, the idea of using molten salt as a storage medium was tested at the Crescent Dunes power plant in Nevada, which reportedly made it highly cost efficient to build and run and ensured that 24-hour solar energy was available.
Also in Eni’s laboratories researchers are working to make concentrated solar power more efficient and versatile introducing major engineering improvements including the development of a mixture of molten salts that solidifies at a temperature threshold much lower than the temperature at which the fluids currently used solidify.
Another technique has been developed by Sandia National Laboratories, where a falling particle receiver deploys a "cascade of particles" similar to sand, such as iron oxide and silica, into a solar receiver with the potential of reaching temperatures of up to 1200 degrees Celsius. The idea won an R&D 100 award for innovation in 2016 and has since advanced through several stages of evolution to form the Gen 3 Particle Pilot Plant, although the program still appears to be at the developmental stage.
Indeed, Heliogen may need to decisively reveal how it plans to deal with this CSP energy storage issue, perhaps by aligning with a third-party technology developer. This is unless it runs into the same issues as Ivanpah and needs to cool down at night, something that is unlikely to appeal to its industrial client base.


Eni’s CSP project

Real innovation for the future

Heliogen is not claiming to have immediately unlocked the key to zero emissions cement. While most of the pollution from cement comes from its heating process, the rest of its production chain also releases emissions. Other plans have been created to resolve this.
In 2018, cement industry’s leaders banded together to see how to meet their obligations under the Paris Agreement and the International Energy Agency, and made recommendations for reducing the emissions from cement by 24% by 2050. These range from switching to alternative fuels, as is the case with Heliogen, and using alternative binding materials in cement production, to relying on carbon capture technology to stop emissions from reaching the atmosphere.
The latter is of particular interest to Gross, whose Heliomax technology could help with capture. The Heliomax solar oven releases carbon dioxide, cleaner than the current byproducts of traditional cement ovens, which he said "makes it easier to capture". This will have to be another step along Heliogen's road to commercial success.
The impact of cement on the modern world is difficult to overstate. It is the most widely used man-made substance ever and the second most widely used substance of any kind after water.
Heliogen's solar oven may be a long ways away from being commercially rolled out or finding the right first crop of cement partners. However, its proof of concept was certainly appealing enough to convince Bill Gates. He may soon be just one among many.


The author: Chris Dalby

Energy and political journalist with experience covering politics, energy, oil and gas, mining, finance, business, Latin America, China, and the Olympics.