Die Pilotanlage im spanischen Móstoles produziert CO2-freies Kerosin mithilfe von Solarenergie.

Global air traffic is fueling the climate crisis: Before the pandemic, it emitted more than 900 million tons of CO2 emissions – exceeding Germany’s total greenhouse gas emissions in the past year.

The kerosene used in the aircraft tanks comes from crude oil and when it is burned it leads to more climate-damaging carbon dioxide in the atmosphere. Relaxed travel restrictions in the wake of the corona pandemic are now inviting long-distance travel again, and many people willing to travel are causing the number of flights to skyrocket again.

Flying is considered to be one of the fastest growing sources of emissions of all – because with increasing prosperity in the world, more and more people are pursuing their desire to travel by plane. But the aviation industry still faces an unsolved problem: Airplanes are hardly as easy to electrify as cars.

And models with potentially climate-neutral types of propulsion such as hydrogen aircraft would only be ready for the market in many years – if at all. So flying will remain harmful to the climate for a long time to come, and payments to offset emissions are expensive and only help the climate to a limited extent.

In a new study, scientists are now taking an important step on the long road to climate-neutral flying: A solar tower power plant in Móstoles, Spain, has produced CO2-free kerosene with a liquefaction plant. This is reported by a team led by researcher Stefan Zoller from ETH Zurich in the journal “Joule”. The pilot plant described resembles a kind of solar refinery.

“We are the first to demonstrate the entire thermochemical process chain in a fully integrated solar tower system from water and CO2 to kerosene,” said co-author Aldo Steinfeld on Tuesday. In previous studies, researchers often synthesized the green kerosene on a much smaller scale in laboratories.

And this is how the whole process works: The sun shines on 169 reflectors on the bottom of the tower, so-called heliostats. These then reflect the sun’s rays in bundles onto the solar reactor at the top of the tower. The concentrated solar energy fuels so-called redox reactions in the reactor, which is lined with a porous structure made of cerium oxide. The cerium oxide converts water and externally supplied CO2 into synthesis gas.

A liquefaction plant then processes the synthesis gas into hydrocarbons, including kerosene and diesel. At the end of the process, green kerosene is produced from solar energy, water and existing CO2. When it is burned, the fuel does not have any additional impact on the climate because the carbon dioxide used to produce it was already in the atmosphere.

According to the researchers, the said kerosene can be used directly in aviation – in fuel storage, logistics and in aircraft tanks. “With our solar technology, we have shown that we can produce synthetic kerosene from water and CO2 instead of from fossil fuels,” explained Steinfeld.

However, a lot of energy is still lost in the tested pilot plant. The efficiency of the solar reactor is measured by the ratio of the amount of synthesis gas produced to the solar energy required. This is just four percent.

According to co-author Steinfeld, the plant is only intended for research purposes and therefore only produces around one liter of CO2-free kerosene a day. The researcher told the Tagesspiegel: “A commercial solar fuel system could use ten heliostat fields and thus produce 34 million liters of kerosene per year.”

According to Steinfeld, the area required for this would require 3.8 square kilometers. The researcher puts the annual global kerosene requirement at an estimated 414 billion liters. If this were to be covered by the technology and solar systems described, 45,000 square kilometers would have to be paved – that corresponds to an area that is slightly smaller than Lower Saxony.

With its low energy efficiency, the solar refinery suffers from the same problem as synthesis processes for green e-kerosene or other electric fuels. Unlike the solar tower power plant in Móstoles, Spain, the latter works with electricity from wind or solar energy. The solar reactor described in the study does not first have to convert the sun’s energy into electricity.

Academic and business experts largely agree that hydrogen and synthetic fuels such as green kerosene will be a necessary complement to a climate-neutral economy. Such fuels could then be used not only in aviation, but also in other sectors of the economy that are difficult to electrify, such as shipping or trucks.

By 2050, the vast majority of airlines want to become CO2-neutral – at least that’s what the International Air Transport Association (IATA) decided last year. IATA accounts for more than 80 percent of global air traffic. CO2-free fuels such as green kerosene are also to be used to achieve the climate target.