Technical feasibility of sustainable fuel production demonstrated (2023)

For the past two years, researchers led by Aldo Steinfeld, professor of renewable energies at ETH Zurich, have been operating a solar mini-refinery on the roof of the machine lab in central Zurich. This unique system can produce liquid transportation fuels, such as methanol or kerosene, from sunlight and air in a multi-stage thermochemical process.

In an interview, project architect Steinfeld and study co-author Anthony Patt, a professor in ETH's Department of Environmental Systems, explain what the experiments revealed, where optimization is needed, and how solar kerosene could succeed in entering the market.

The solar mini-refinery on the roof of an ETH building has now been in operation for two years. How would you summarize this work?
Aldo Steinfeld: We have successfully demonstrated the technical viability of the entire thermochemical process chain for converting sunlight and ambient air into drop-in transportation fuels. The overall integrated system ensures stable operation under real conditions of intermittent solar radiation and serves as a unique platform for further research and development.

In the title of your paperNatureyou refer to "drop-in fuels". What do you mean?
Aldo Steinfeld: Drop-in fuels are synthetic alternatives to petroleum-derived liquid hydrocarbon fuels such as kerosene and gasoline, which are fully compatible with existing transportation fuel storage, distribution and utilization infrastructure. In particular, these synthetic fuels can help to make long-haul aviation more sustainable.

Are these CO2 neutral fuels?
Aldo Steinfeld: Yes, they are carbon neutral because solar energy is used for their production and because they only emit so much CO2₂during their combustion as before was taken from the air for their production. Life cycle analysis of the solar fuel production chain indicates that greenhouse gas emissions are avoided by 80 percent compared to fossil jet fuel and that emissions are close to 100 percent or zero when building materials (e.g. steel, glass) are manufactured using renewable energy .

A refinery that produces fuels from sunlight and air… it sounds like science fiction. How does it work?
Aldo Steinfeld: This is not science fiction; it is based on pure thermodynamics. The solar refinery consists of three thermochemical conversion units integrated in series: first, the direct air capture unit, which co-extracts CO₂an H₂O directly from the ambient air. Second, the solar-powered redox unit, which converts CO₂an H₂O in a specific mixture of CO and H₂so-called syngas. And third, the gas-to-liquid synthesis unit, which ultimately converts the syngas into liquid hydrocarbons.

How was the syngas/methanol yield?
Aldo Steinfeld: Our solar mini-refinery is indeed a "mini" system for research purposes. And while we produced relatively small amounts of fuel, we did so under real field conditions with Zurich's not-so-optimal solar radiation. For example, during a typical day's run, about 100 standard liters of syngas are produced, which can be processed to about half a deciliter of pure methanol. Various parts of the production chain have not yet been optimized. Optimization is the next stage.

What went well, and what didn't go so well?
Aldo Steinfeld: What went exceptionally well is we got total selectivity for splitting H₂O all H₂in ½ O₂, and from CO₂to CO and ½ O₂, i.e. no unwanted by-products of the thermochemical reactions. Furthermore, and crucially for process integration, we were able to adjust the syngas composition for both methanol and kerosene synthesis. However, the energy efficiency is still too low. The highest efficiency value we have measured so far for the solar reactor is 5.6 percent. Although this value is a world record for thermochemical splitting of solar energy, it is not good enough. Substantial process optimization is still required.

How can the system be further improved to increase efficiency?
Aldo Steinfeld: Heat recovery between the redox steps of the thermochemical cycle is essential because it can increase the efficiency of the solar reactor to more than 20 percent. Furthermore, there is room for optimization of the redox material structure, for example by means of 3D printed hierarchically ordered structures for improved heat and mass transfer. We are investing major efforts in both directions and I am optimistic that we will soon be able to report a new energy efficiency record.

For the chemical process, CO₂an H₂O must first be taken from the air and fed into the system. How much energy should be invested for this?
Aldo Steinfeld: The specific energy requirement per mole of CO₂captured are about 15 kJ of mechanical work for vacuum pumps and 500-600 kJ of heat at 95°C, depending on the relative humidity of the air. In principle, we can use residual heat to drive the direct air capture unit. But splitting the H. requires a huge amount of high-temperature process heat₂The en CO₂, and this is provided by concentrated solar energy.

Scaling up to industrial scale: is it feasible?
Aldo Steinfeld: Certainly. For upscaling, a heliostat field aimed at a solar tower can be used. The current solar mini-refinery uses a 5 kW solar reactor, and while a 10x scale solar reactor has already been tested in a solar tower, another 20x scale is needed for a 1 MW solar reactor module. The commercial-sized solar tower accommodates a range of solar reactor modules and, in particular, can leverage the solar concentration infrastructure already in place for commercial solar thermal power plants.

Do you and your company take care of this?
Aldo Steinfeld: No, this is up to our industrial partners. We at ETH focus on the more fundamental aspects of the technologies. But we also take care of technology transfer to industry, for example by licensing patents. Two spin-offs have already emerged from my group, founded by former PhD students: Climeworks commercializes the technology for CO₂capture from the air, while Synhelion commercializes the technology for the production of solar fuel from CO₂.

Anthony Patt, as co-author of the study, explored how solar fuels can enter the market and become competitive. What kind of policies would be needed to make this happen?
Anthony Patt: Our analysis of policy instruments shows that there is a need for technology support similar to what has been for solar and wind energy. Both cost about ten times as much to build and run as fossil generators when governments first started supporting them. The current price ratio for solar kerosene compared to fossil is of the same order. A comparison with other renewable energy technologies shows that with a similar support mechanism it should be possible to reduce the cost of solar kerosene to the current cost of fossil jet fuel.

What are the main barriers?
Anthony Patt: The hardest part is overcoming the high initial price barrier. Carbon taxes are unlikely to be effective. If we taxed fossil jet fuel in such a way that the cost to airlines is the same as solar fuels, which would be necessary, it would mean making it ten times more expensive. No one would want to pay this extra cost of flying and politicians would not be willing to impose this burden on people. For solar and wind energy, however, other policy instruments fit the context much better. They imposed a small additional cost on total electricity consumption and used this revenue to fund the costs that wind and solar added to the system. Similarly, thanks to the current market dominance of fossil fuels, we should only need to impose a small additional cost of flying to fund investments in renewable fuel production. This would certainly help the solar reactor and solar fuels gain a foothold in the market.

What do you think would be the ideal policy instrument to help solar fuels enter the market?
Anthony Patt: The most appropriate tool for the fuel market would be a quota system. This would work as follows: airlines and airports would have to have a minimum share of renewable fuels in the total volume of fuel they put into their aircraft. This would start small, e.g. like 1 or 2 percent. It would increase overall fuel costs, but only minimally; the initially small quota would only add a few Swiss francs to the cost of a typical European flight. The quota would increase each year, eventually reaching 100 percent, meaning only solar fuels would be burned. The increasing quota would lead to investments, which in turn would lead to falling costs, just as we saw with wind and solar. By the time solar fuels reach 10-15 percent of fuel volume, we should see costs for solar fuels close to that of fossil kerosene. It is a strategy that is politically feasible and easy to implement.

Which locations are suitable for large production facilities?
Anthony Patt: A solar reactor needs direct sunlight, with no clouds in the way. It makes sense to build them in arid environments, such as those in southern Spain and North Africa, the Arabian Peninsula, Australia, in the southwestern United States, in the Gobi Desert in China or in the Atacama Desert in Chili. The process chain does condense water from the air as one input, but even desert air is moist enough to provide the required amounts. Finally, desert land is relatively cheap, with no competing uses. Solar fuels would be a global commodity, similar to today's fossil fuels, and would indeed rely on the same basic shipping and delivery infrastructure.

Aldo Steinfeld: Suitable locations are regions for which the annual direct normal solar radiation exceeds 2000 kWh/m²per year. Unlike biofuels, which are constrained by resource availability, global jet fuel demand can be met by using less than one percent of global dry land, which does not compete with food production. To put this in context, in 2019, global aviation kerosene consumption was 414 billion litres; the total land footprint of all solar installations required to fully meet global demand would be approximately 45,000 km², equivalent to 0.5 percent of the area of ​​the Sahara desert.