The team has implemented the system in the field and describe their findings in the journal Joule.
“We are the first to demonstrate the entire thermochemical process chain from water and CO2 to kerosene in a fully-integrated solar tower system,” said Aldo Steinfeld, a professor from ETH Zurich and the corresponding author of the paper. Previous attempts to produce aviation fuels with solar energy have mostly been lab-based.
The aviation sector, which accounts for about five per cent of global anthropogenic emissions, relies heavily on kerosene, or jet fuel, which is typically derived from crude oil. Currently, no clean alternative is available to power long-haul commercial flights at the global scale.
“With our solar technology, we have shown that we can produce synthetic kerosene from water and CO2 instead of deriving it from fossil fuels. The amount of CO2 emitted during kerosene combustion in a jet engine equals that consumed during its production in the solar plant,” Steinfeld said in a statement. “That makes the fuel carbon neutral, especially if we use CO2 captured directly from the air as an ingredient, hopefully in the not-too-distant future.”
As a part of the European Union’s SUN-to-LIQUID project, Steinfeld and his colleagues have developed a system that uses solar energy to produce drop-in fuels, which are synthetic alternatives to fossil-derived fuels. The solar-made kerosene is fully compatible with the existing aviation infrastructure for fuel storage, distribution, and end use in jet engines, Steinfeld said. It can also be blended with fossil-derived kerosene.
In 2017, the team started scaling up the design and built a solar fuel-production plant at IMDEA Energy Institute in Spain. The plant consists of 169 sun-tracking reflective panels that redirect and concentrate solar radiation into a solar reactor mounted on top of a tower.
The concentrated solar energy then drives oxidation-reduction (redox) reaction cycles in the solar reactor, which contains a porous structure made of ceria. The ceria converts water and CO2 injected into the reactor into syngas, a tailored mixture of hydrogen and carbon monoxide. Subsequently, syngas is sent into a gas-to-liquid converter, where it is finally processed into liquid hydrocarbon fuels that include kerosene and diesel.
“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,” said Steinfeld.
During a nine-day run of the plant reported in the paper, the solar reactor’s energy efficiency was around four per cent. Steinfeld said his team is working on improving the design to increase the efficiency to values over 15 per cent. One idea being explored is to optimise the ceria structure for absorbing solar radiation and developing methods to recover the heat released during the redox cycles.