Flying on sunshine – a solar jet fuel

Using the simplest of ingredients – sunlight, CO₂ and water – researchers in Europe have brewed jet fuel. Their demonstration – a kind of industrialised photosynthesis – hastens the day we could use solar energy to power cars and planes.

The sun’s energy may be free, clean and green, but efficiently capturing and converting it into liquid fuels is not. Growing plants for biofuels is the simplest way, but competes with food crops for land. And plants just aren’t that efficient at harnessing sunlight. Overall, biofuel production is less than 1% efficient at capturing solar energy.

Now a team of European researchers has pioneered an industrial alternative to photosynthesis that will be many times more efficient, they claim. “It’s the first demonstration of the whole fuel-production chain from sunlight, water and CO₂ to yield kerosene,” says Valentin Batteiger, a researcher at the German aerospace research organisation Bauhaus Luftfahrt who collaborated on the European SOLAR-JET project.

The key to the process, Batteiger says, is the “solar chemical reactor” developed by project members at Swiss university ETH Zürich. The reactor uses sunlight to split water and carbon dioxide, spitting out carbon monoxide and hydrogen. This brew is called syngas, and from there it’s a well-known industrial process to make liquid fuels such as diesel or jet fuel.

Making syngas from CO₂ and water isn’t easy, let alone trying to drive it by sunlight. The reaction needs a lot of energy, and usually produces oxygen as a by-product, which makes a dangerously explosive concoction when mixed with highly flammable syngas. So the oxygen must be scrubbed from the syngas before it can be used to make kerosene.

Rather than trying to remove oxygen at the end of the process, the ETH team developed a two-stage process that stops it forming in the first place. Their trick involves a metal oxide compound called ceria. Normally rich in oxygen, by heating it to around 1,500°C inside the reactor using concentrated sunlight, they could drive off some of its oxygen, which was then be flushed out of the reactor as a gas. After cooling to 700°C, water (steam) and CO₂ were then piped in. The oxygen-starved ceria rips the oxygen from the water to leave hydrogen, and strips CO₂ of half its oxygen to give carbon monoxide – leaving oxygen-free syngas. The gas is piped off to make jet fuel, while the ceria is heated back up to 1,500°C to start the cycle again.

‘The long-term goal is to reach a 15% overall energy efficiency
of solar to hydrocarbon fuel.’

The team’s successful demo, the first to produce a liquid fuel from sunlight this way, is “very significant, in that it proves that this production route is possible, if rather expensive and immature as yet,” says Jim Hinkley who investigates solar fuels at CSIRO in Newcastle. “Good progress has been made in many areas, but it will be some time before we can expect maturity in these systems.”

The SOLAR-JET team has its sights set on the aeroplane industry – responsible for around 2% of global CO₂ emissions – as planes will probably always have to run on liquid fuels. Batteries are simply too heavy, relative to the energy they can carry, for electric airliners to be feasible. “Batteries will never fly, so to speak,” says Hinkley. “Planes will probably always need hydrocarbon fuels because of their high energy density unless some new propulsion system is invented à la Star Trek.”

For now the team has produced just a cup-full of kerosene, with an overall efficiency of about 1%. “The long-term goal is to reach a 15% overall energy efficiency of solar to hydrocarbon fuel,” says Batteiger. “Part of the efficiency improvement will be due to scaling up to a larger size – thermal processes are more efficient at a large scale,” he explains. Further efficiencies can come from capturing the heat lost when the reactor cools to 700°C and recycling it to push the reactor back up to 1,500°C to begin the next cycle.

The team envisages using solar towers, heated by a surrounding field of mirrors to concentrate sunlight, to power the process at scale. The towers could be built in the desert, where they would not compete for the same land area as agricultural food. But building solar towers is expensive, says Batteiger. “Our economic analysis reveals that double digit overall energy efficiencies are required in order to produce fuel at competitive production cost,” he says. “Anything below 10% overall process efficiency will be of little commercial interest.”

“The challenges are not insignificant,” says Hinkley. “But certainly there is no technical barrier, rather a financial one that renewable technologies often have to face.”