Fuel from light and air – solar refinery produces syngas, methanol or kerosene from CO2 and sunlight
Feasibility confirmed: The production of fuels with the help of sunlight and CO2 from the air works and could also turn out to be large-scale wages. This is now confirmed by the results of two years of test operation of such a solar refinery on the roof of the ETH Zurich. It produces syngas, methanol or kerosene solely from the CO2 and water in the air and the energy from the sun.
So far, most fuels have been made from the fossil fuel crude oil. Kerosene, petrol and the like therefore make a major contribution to human greenhouse gas emissions and thus to anthropogenic climate change. But there is another way: If fuels are produced from plant material, plastic waste or from CO2, this lowers their climate footprint and can even make them climate-neutral. The latter, however, requires efficient technologies for capturing CO2 from air or exhaust gases and a climate-friendly energy supplier.
A solar refinery on the laboratory roof
A system that has been in operation since 2019 is on the roof of the machine laboratory at ETH Zurich. The small solar refinery only needs air, sunlight and a catalyst to produce syngas, methanol or kerosene from atmospheric CO2. Team leader Aldo Steinfeld from ETH and his team are now reporting how well this works after this “sun-to-liquid” system has been in operation for two years.
“We were able to successfully demonstrate the technical feasibility of the entire thermochemical process chain for converting sunlight and ambient air into drop-in fuels,” explains Steinfeld. Production has proven to be stable and reliable under real field conditions, even with the less than optimal solar radiation from Zurich. According to the researcher, the technology is now ready for transfer to industry.
In three steps to fuel
Specifically, the solar refinery uses a process consisting of three thermochemical conversion units connected in series. In the first step, CO2 and water are extracted from the ambient air by means of amine scrubbing. To do this, the system filters around 2,000 cubic meters of air per hour and uses it to extract eight kilograms of pure CO2 per day and, depending on the humidity, 20 to 40 kilograms of water. The team reports that around 30 to 60 percent of its original CO2 content is removed from the air that is sucked in.
The CO2 obtained from the direct air capture is compressed to up to twelve bar and fed into the solar redox unit together with the water. In this, focused sunlight is used to split CO2 and H2O with the help of a ceramic catalyst made of cerium oxide and convert them into syngas – a mixture of hydrogen and carbon monoxide. “In a representative day-to-day operation, the amount of syngas produced is around 100 standard liters,” confesses Steinfeld.
The syngas can be used directly or it IS converted into methanol or kerosene in the third process step using common chemical processes. “Our mini solar refinery is a research facility; accordingly, it only produces small amounts of fuel, ”explains Steinfeld. The approximately 100 liters of syngas per day resulted in another half a deciliter of pure methanol.
Hardly any emissions, but efficiency can still be increased
The positive thing about the results so far: “The ecological balance of the production chain of solar fuels shows that greenhouse gas emissions can be avoided by 80 percent compared to fossil kerosene and that they go to zero if the materials for the construction of the production facilities such as glass and steel are used are made with renewable energies, ”explains Steinfeld. In addition, there are no undesirable by-products and the composition of the syngas can be tailored to the synthesis of methanol or kerosene.
The catch, however: “The energy efficiency is still too low. So far, the highest degree of efficiency we have measured for the solar reactor is 5.6 percent, ”reports the ETH researcher. “Although this value is a world record for solar thermochemical cleavage, it is not good enough.” However, ER and his team estimate that and optimize the structure of the redox catalyst.
Ripe for use on a grand scale
Nevertheless: According to Steinfeld and his team, the technology of the sun-to-liquid process is now ready for industrial use. “For example, a commercial-scale solar refinery could consist of ten heliostat fields, each of which collects around 100 megawatts of thermal solar energy,” the researchers explain. “With an efficiency of around ten percent, such a system could already produce 95,000 liters of kerosene per day – enough to bring an Airbus A350 from London to New York and back.”
Such large solar systems would ideally be built in sunny desert areas. “The solar process chain needs water from the air as a starting material, and this is available in sufficient quantities even in the dry desert air. In addition, desert land is cheap and there are usually no other usage requirements than in densely populated areas, ”says Steinfeld.
To fully meet the global demand for kerosene, according to the researchers’ calculations, it would need around 45,000 square kilometers – around 0.5 percent of the area of the Sahara.
© ETH Zurich
Quota solution against the initial hurdles
However, the costs for fuel from solar production would initially be significantly higher than for conventional kerosene from petroleum because of the high investments in the systems. The more solar fuel is generated and used, the cheaper it becomes. Based on analyzes of the process chain, scientists estimate that the fuel would cost 1.20 to 2 euros per liter if produced on an industrial scale. “According to our calculations, solar jet fuel WILL cost the same as fossil kerosene with a share of 10 to 15 percent,” says co-author Anthony Patt from ETH Zurich.
In order to overcome the initial cost hurdle, he and his colleagues propose a kind of quota system: “Airlines and airports are obliged to add a prescribed minimum amount of solar aviation fuel to every liter of kerosene they fill up. At the beginning, for example, this proportion is one to two percent, ”explains Patt. The resulting price premium for a flight ticket for intra-European flights would be a few euros. “The successive increase in the quota then leads to the price of solar kerosene falling drastically – as we have observed with wind and solar energy,” says Patt. (Nature, 2021; doi: 10.1038 / s41586-021-04174-y)
Source: ETH Zurich, Institute for Advanced Sustainability Studies eV (IASS)