How solar fuels work: thermochemistry

Solar fuel researchers test the thermochemical reactor designed for Synhelion's jet fuel production

Solar fuel researchers test the solar thermochemical reactor designed for Synhelion’s jet fuel production

Solar fuels are made using thermochemistry driven by direct heat from the sun

Solar thermochemistry is a process in which solar thermal energy drives chemical reactions to produce new compounds such as aviation fuel. Highly concentrated solar heat is reflected off thousands of mirrors in a solar field surrounding a tower receiver to heat a solar reactor. These reactions are used to extract hydrogen from molecules of water (or other hydrogen-containing compounds) and to make chemical compounds like ammonia or hydrocarbon fuels like aviation fuel.

National research laboratories around the world are advancing solar fuels using solar thermochemistry. Many designs are being tested for the solar reactors, heat transfer fluids, and thermal storage capable of very high temperatures ranging from 700ºC to 1500ºC. Research results are shared at annual SolarPACES Conferences and published in peer-reviewed journals such as Energy, Solar Energy, The Journal of Energy Storage, Solar Energies, etc. SolarPACES Task II Solar Chemistry Research is tasked by the International Energy Agency with furthering solar fuel research, which is carried out by affiliated member nations.

The resulting solar fuels would be a sustainable and renewable alternative to traditional fossil fuel-based thermochemical processes that produce many fuels, chemicals, and materials, and has been recognized by at COP26. Thermochemical solar fuels have the advantage over biofuels that the land use is much lower, and the advantage over artificial photosynthesis that first require the conversion of photovoltaic solar energy to electricity, of being more thermally efficient.

The chemistry reactions are heated using very highly concentrated solar flux. To generate the heat, a solar field of mirrors called heliostats focuses thousands of reflections of the sun (many “suns”) up to a solar receiver that heats up to as high as 1500 C, using the heat in an adjacent solar reactor to heat the chemical reaction.

Solar thermochemical production of hydrogen, ammonia, and jet fuel are among the most promising lines of research, with the first commercial applications in aviation fuels that meet Aviation Standards for jet fuel.

Hydrocarbon fuels like gasoline, diesel, and jet fuel consist of hydrogen (H) and carbon (C) elements in various molecular configurations. The raw materials to produce any hydrocarbon fuel can be found in any materials that contain the chemical elements; hydrogen and carbon, such as water and carbon dioxide. Both hydrogen and carbon are found in numerous compounds, so they can be extracted from any materials that contain these elements.