Scientists have searched for a sustainable aviation fuel for decades. Now, with emissions from air traffic increasing faster than carbon-offset technologies can mitigate them, environmentalists worry that even with new fuel-efficient technologies and operations, emissions from the aviation sector could double by 2050.
But what if, by 2050, all fossil-derived jet fuel could be replaced by a carbon-neutral one made from sunlight and air?
In June, researchers at the Swiss Federal Institute of Technology (ETH) in Zurich demonstrated a new technology that creates liquid hydrocarbon fuels from thin air—literally. A solar mini-refinery—in this case, installed on the roof of ETH’s Machine Laboratory—concentrates sunlight to create a high-temperature (1,500 degrees C) environment inside the solar thermochemical reactor.
There, carbon dioxide and water extracted from ambient air are converted via a redox cycle to syngas, a specific mixture of carbon monoxide and hydrogen. The syngas produced is then sent to a conventional gas-to-liquid unit for processing into hydrocarbon fuels, such as kerosene, petrol, methanol, and others. The entire process chain is thermochemical, and concentrated solar energy can provide the required process heat. Only a small amount of electricity is needed to operate the pumps.
The kerosene produced with this technology can replace fossil-derived jet kerosene. “It releases only as much carbon dioxide during combustion as was previously extracted from the air for its production,” says Aldo Steinfeld of the ETH. Steinfeld leads the team that developed the technology. Moreover, since solar energy is needed only at the first stage of the redox cycle for creating syngas, the refinery operates two solar thermochemical reactors in parallel to make optimum use of concentrated sunlight.
A large-scale test of ETH’s solar refinery is currently underway in a solar tower of IMDEA Energia near Madrid, Spain, as part of the EU-supported SUN-to-LIQUID project, a four-year research program devoted to exploring ways to de-carbonize the aviation industry. The real test, though, will be how well the fuel integrates into the existing infrastructure and hardware of the aviation industry, and how soon.
“Solar kerosene is a drop-in synthetic fuel that is a completely interchangeable substitute for fossil kerosene,” says Steinfeld. “We can continue to use the existing global infrastructures for kerosene distribution, storage, and utilization.”
The biggest barrier to commercial adoption will likely be price, at least initially. Solar kerosene is estimated to cost about twice the price of fossil kerosene. However, as deployment of solar energy becomes more widespread, the costs should decline. According to the Bloomberg New Energy Outlook 2019, by 2030 the cost of solar and wind energy will undercut energy produced from coal and gas almost everywhere.
If the cost can be lowered, the implications for de-carbonizing aviation could be huge. A recent analysis published by The Guardian finds that even a short-haul flight from London to Edinburgh adds more carbon dioxide to the atmosphere than the mean annual per-capita emissions of Uganda or Somalia.
Figures from the International Air Transport Association (IATA) state that flying will be more popular in 2019 than ever before—there has been a 300 percent increase in the total number of miles flown in the last 30 years. And yet, sustainable aviation fuels accounts for less than 1 percent of total demand [PDF]. The IATA has noted that unlike ground transport, which can be battery powered, aviation has no near-term alternative to fossil fuels—it forecasts that electric commercial aircraft will arrive in 2040 at the earliest.
Even then, “the theoretical limitations of commercial aircraft powered by batteries is about 1,000 miles,” says Steinfeld. “Thus, long-haul flights will continue to use kerosene.” If all goes as planned, commercial production of solar kerosene may start by 2025. For now, two ETH spin-off companies are working on scaling the technology for commercial availability.
One of these is Climeworks, which provides commercial carbon-removal units to capture carbon dioxide from air. The other is Synhelion, which aims to commercialize the new solar refinery for industrial fuel production.
Solar hydrocarbon fuel generation depends on air and sunlight, both abundant sources, and if applied in arid climates, does not compete with food production.
“Theoretically, a plant the size of Switzerland—or a third of the Californian Mojave Desert—could cover the kerosene needs of the entire aviation industry,” says Philipp Furler, director of Synhelion, in a press release. “Our goal for the future is to efficiently produce sustainable fuels with our technology and thereby mitigate…emissions.”