Taking carbon dioxide, water and sunlight as its only inputs, a solar thermal tower in Spain produces carbon-neutral, sustainable versions of diesel and jet fuel. Built and tested by researchers at ETH Zurich, it’s a promising clean fuel project. Fossil fuels can be replaced with batteries or hydrogen in cars and trucks – but aircraft are trickier.
With more than 25,000 commercial airliners in service today, and service lifetimes around 25 years, airlines are looking to carbon-neutral fuels to bring down their emissions. Carbon-neutral fuels are drop-in replacements for today’s kerosene Jet-A fuel; they mix in with regular fuel and get burned in jet engines as per normal, producing the normal amount of carbon emissions.
The difference is that rather than pulling that carbon straight out of the ground, carbon-neutral fuels grab CO2 from elsewhere; it’ll still end up in the atmosphere, but at least it does some useful work before it gets there, and every gallon burned is a gallon of conventional fuel that wasn’t burned.
There are a lot of ways to make carbon-neutral fuels – and not all of those are acceptable for other reasons.
Landfill waste-to-jet-fuel plants are popping up here and there, taking municipal garbage or old cooking oil and using that as a feedstock to create syngas, which can be refined into synthetic fuels.
The pyrolysis process usually involved requires a lot of energy – either dirty energy or clean energy that could be used elsewhere – and the feedstock is so wildly random that the resulting fuels sometimes need an extra, energy-intensive cleaning step before they’re ready to go save the planet in a Dreamliner.
Another way is to capture carbon directly from other emissions sources, and convert that into fuel.
This can be done by using green electricity to power an electrolyser, then mixing the resulting hydrogen with carbon monoxide to create syngas, which can then be refined into fuels – but there are energy losses at each of these steps.
This syngas is fed to a Gas-to-Liquid unit at the bottom of the tower, which produced a liquid phase containing 16% kerosene and 40% diesel, as well as a wax phase with 7% kerosene and 40% diesel – proving that the ceria-based ceramic solar reactor definitely produced syngas pure enough for conversion into synthetic fuels.
To give you a sense of the size of the problem here, a Boeing 787 Dreamliner has a fuel capacity up to 126,372 L, on which it can fly up to 14,140 km – roughly the distance from New York to Ho Chi Minh City.
These things don’t necessarily have to replace all the fuel in question – synthetic fuel can be blended with regular fuel in whatever quantities it’s available, and every bit helps reduce overall emissions.
“This solar tower fuel plant was operated with a setup relevant to industrial implementation, setting a technological milestone towards the production of sustainable aviation fuels.”
“The solar tower fuel plant described here represents a viable pathway to global-scale implementation of solar fuel production,” reads the study.