Heat demand by industry is the world’s largest energy end-use, the IEA calculates. So how can we supply that with renewable energy? For some heat needs, it is possible today, using concentrated solar thermal (CST). Many industrial processes need heat at between 150ºC and 400ºC, which CST can meet commercially today. Some examples are in the pharmaceutical and textile sector, brick, paper, food processing and hospital uses.

Even industries like mining and steel and cement that require higher temperatures also operate some medium heat processes under 400ºC. The World Bank finds that technology to replace half the global use of fossil fuels with solar heat is already possible for many industrial and agricultural processes and space heating needs.

High temperature solar heat is not yet commercial

But processes that run at very high temperatures like 1,000ºC to 2,000ºC, and that must operate day and night, pose a bigger challenge. For these needs, a decade of research is now bearing fruit and beginning to result in the first joint pilot projects in partnership with firms willing to try out novel solar alternatives to burning gas or coal. Such industries need continuous high temperature heat for mining, to produce cement or steel or to process ores for steel and aluminum or refine oil to aviation fuel.

These partnerships have built on a decade of international government-funded research into how very high temperature concentrated solar thermal could supply continuous heat processes for day or night solar at very high temperatures year round – for hydrogen production, for example. With so much research, yet so little widely known, now is a good time to take stock of where results are currently, what looks promising, and where the challenges are for future very high temperature solar thermal research.

To meet the need for an assessment of where we stand, Sylvain Rodat and Stéphane Abanades who lead research at the Processes, Materials and Solar Energy Laboratory at PROMES-CNRS in France put together a review of all the high temperature CST research to date, just published at Renewable and Sustainable Energy Reviews. Their paper; On the path toward day and night continuous solar high temperature thermochemical processes A review offers a state-of-the-art assessment of the various proposed solutions to help guide productive research in the future.

”There’s a need to do a review because we have done a lot of research about high temperature reactors, but to date there has been no scale up and so the question is why and I think that it is because we cannot yet provide continuous solar processes,” explained Rodat.

“The industrial processes like reforming or cracking for hydrogen, or carbon black production, are all continuous processes. We need them round the clock and so you cannot stop this kind of equipment every day because it takes some days to startup again.”

The solar flame

Among solutions to this problem of continuous heat – such as thermal storage or hybrid operation – (using a hydrogen flame for example) the review includes not just the research papers published, but also all the patents filed to date. Rodat himself has patented the intriguing “solar flame” and just published with Abanades a reactor design using an inert hot gas acting as a flame for continuous operation at night.

“When you produce a flame you are just burning a fuel with oxygen,” he noted. “A combustion flame is just a gas at high temperatures – say 1,000°C and 2,000°C, depending on the oxygen ratio. However, with CST we can also heat a gas to high temperature. And since there is no combustion, we will have no CO2. It will be a clean flame. We can heat for example nitrogen, so we could have a flame of high temperature nitrogen with the same effect of a combustion flame, but without any combustion products like nitrogen oxides or CO2. If we just heat the gas we will have no CO2.”

But getting there is a challenge, he added: “The theory is quite simple: We just replace the combustion flame by a hot gas. But in fact it is not so easy to heat a gas to a very high temperature. It is easier to heat a liquid or a solid.”

To solve the difficulty of heating gases to over 1,000°C, a new technological work-around is to instead heat particles suspended in the gas in particle receivers. Many patents have been filed since 2010 for different ways to keep various kinds of particles hot enough to provide high temperature heat.

The paper notes: “This technology could even be more performant for continuous thermochemical processes. Indeed, in this case, the absorbed heat by the particles can be directly used in the solid-gas processes so there is no need for a particle/gas heat exchanger as it is the case to run a gas cycle.”

Among the ideas enumerated in the review, some stand out for their promise, and some have led to research spin-off startups like Synhelion, which has partnered with ENI to produce aviation fuel using solar thermochemistry, with receiver solar-to-heat efficiencies of over 80% calculated for temperatures up to 1,800 Kelvin.

Thermochemistry in molten metals or salts

An example of a promising line of research noted in their review is making hydrogen in molten tin “provided that carbon can be continuously stripped off at the surface and valorized, which is still challenging” the paper states. An international team is generating hydrogen in a solution of molten tin, which has a very wide temperature operating range from 232°C up to 2,600°C, a far wider operating range than today’s CSP plants producing electricity; of between 290°C and 565°C. Methane bubbled in a molten tin column yielded 78% hydrogen at 1,175°C.

“I would like to go further on this direction because I think it is promising,” said Rodat. “But there’s not many researches carried out in this field to date. I identify this process as a promising way because we can store heat in the molten metal and also we have more inertia in the system so we can have a better control of the temperature. Also it is a good way for methane cracking because we can have a good heat transfer coefficient between the gas and the liquid metal.”

Molten salts are commercially proven in CSP, where a heat exchanger transfers its heat to a steam generator for power generation. But this review reveals research into performing thermochemistry directly within the hot molten salt solution.

“Yes, we can indeed use molten salt to provide directly heat in high temperature processes,” he agreed. “If the composition is well-controlled we should be able to make some reaction in molten salts without any reaction with the molten salts, keep them in their original form. The idea is this molten salt can catalyze but be as stable as possible. We have some works that demonstrate that we can make reaction inside the molten salt. It is not simple, of course it is a challenge. Depending on the reactions we can observe some molten salt degradation, and that is a point to be addressed in future research.”

The advantage of conducting the chemical reaction directly inside the molten salts is better thermal efficiency, as you can use the heat directly without a heat exchanger. Like much of the research cited in the review, most of this advanced solar thermochemical research is at a technical readiness level (TRL) of between 3 to 6 on a 1 to 9 scale where 9 equals ready to deploy commercially.

This kind of research holds the promise of a fully decarbonized economy, by replacing the use of fossil fuels for high temperature heat day and night. For low and medium heat, the readiness level is already high. But getting there for high temperatures will take investment in innovation and talented research and development up to pilot scale trials.

“The main point is to be able to scale up continuous processes,” said Rodat. “Because some works are existing now but most of them are less than 10 years old. While I wanted to show where there are some possibilities, I hope it will give more chance for the teams to study this.”

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