Zoller et al, 2022
The Swiss team’s solar tower fuel plant during operation at IMDEA Energy in Spain
RESEARCHERS at ETH Zurich, the Swiss Federal Institute of Technology, have successfully created a synthetic version of kerosene, the main ingredient in jet fuel, using solar energy, water and carbon dioxide in a fully integrated solar tower setup.
The global fuel consumption by commercial airlines has been increasing every year since 2009 and had reached an all-time high of 432bn L in 2019 before the pandemic thwarted global travel plans.
That fuel is made primarily of kerosene, a hydrocarbon derived from petroleum, which when burned releases CO2. According to the IEA, around 1027m t of CO2 emissions were produced from fossil jet kerosene combustion in 2019, and although the figure took a tumble during lockdowns, air travel is now once again booming. This renewed interest in flying is expected to bump-up global energy-related CO2 emissions from aviation to 3.5% by 2030, compared with 2.5% reported in 2019.
With countries pushing to achieve carbon neutrality by 2050, industries are now looking at renewable or alternative fuels such as biofuels to help end our reliance on fossil fuels. Blended biofuels – lower-carbon sustainable aviation fuel (SAF) with fossil jet fuel – has been in use since 2008, but only five airports have regular biofuel distribution today.
Another promising alternative which relies even less on fossil fuels is syngas. Syngas is produced by the conversion of hydrogen and CO into liquid hydrocarbons using a catalyst. This method, known as the Fischer-Tropsch (FT) process can produce hydrocarbons of different molecular weights from organic acids and alcohol, to methane and higher molecular olefins and paraffins.
Taking this idea one step further by making it even more environmentally friendly, a team of Swiss researchers has been working on an experiment to break down water and CO2 collected from the air using concentrated solar energy.
What first started off as a rooftop experiment in Switzerland has now grown to an array of 169 sun-tracking spherical reflectors which concentrates around 50 kW of solar radiative power – equivalent to 2,500 Suns – onto a solar redox reactor mounted on a tower near Madrid, Spain.
The concentrated energy is then directed into a 16cm wide aperture on the reactor at the top of the tower and is used to thermally reduce the redox material and cerium oxide at temperatures of around 1,500°C. When CO2 and water enter the chamber, the reduced ceria is then reoxidised and CO and hydrogen are generated. These in turn are processed to liquid hydrocarbon fuels using the Fischer–Tropsch unit at the base of the tower.
So far, the solar reactor generates around 1 L/d of kerosene, but the researchers estimate that covering an area slightly bigger than Switzerland in larger versions of their solar plant could one day provide enough aviation fuel to satisfy global demand.
“This is still a demonstration for research purposes, but this time it is at a technical size and uses a solar tower configuration that is relevant for industrial application,” said lead researcher Aldo Steinfeld, at ETH Zurich.
The researchers said that in the new experiment at IMDEA Energy in Spain, a 4.1% solar-to-syngas energy conversion efficiency was achieved. While this is still a low value for making the technology economically attractive, it is a record value for thermochemical conversion of solar energy into syngas, Steinfeld said.
In the longer term however, the research team say that the systems efficiency has the potential “to reach competitive values of over 20%”. This could be achieved by applying heat recovery, and by improving the volumetric absorption of the porous structures within the cavity-receiver.
In this experiment the cavity-receiver contains a reticulated porous ceramic (RPC) structures made of ceria a cylindrical structure of interlocking RPC bricks made of ceria. Cavity-receivers in solar reactors play a dominant role in the conversion between light and heat, and its performance can directly affect the efficiency of the whole power generation system.