In the solar thermochemical process, water is split into hydrogen and oxygen using direct solar heat, rather than electricity, inside a high temperature solar reactor which receives highly concentrated solar flux from a solar field of heliostats that focus the  highly concentrated sunlight into the reactor. 
The two most promising routes are the two step cerium oxide cycle and the copper chlorine hybrid cycle. For the cerium oxide cycle the first step is to strip the CeO into CeO at more than 1400 °C. After the thermal reduction step to reduce the metal oxide, hydrogen is then produced through hydrolysis at around 800 °C. The copper chloride cycle requires a lower temperature (~500°C), which makes this process more efficient, but the cycle contains more steps and is also more complex than the cerium oxide cycle.
Because hydrogen manufacture requires continuous performance, the solar thermochemical process includes thermal energy storage. Another thermochemical method uses solar reforming of methane, a process that replicates traditional fossil fuel reforming process but substitutes solar heat.
In a November 2021 publication in Nature, Aldo Steinfeld of Swiss technological university ETH Zurich reported an artificial photosynthesis where carbon dioxide and water vapour absorbed from the air are passed over a cerium oxide catalyst heated by concentrated solar power to produce hydrogen and carbon monoxide, transformed through the Fischer-Tropsch process into complex hydrocarbons forming methanol, a liquid fuel.
Scaling could produce the   of aviation fuel used in 2019 with a surface of : 0.5% of the Sahara Desert.
One author, Philipp Furler, leads specialist Synhelion, which in 2022 was building a solar fuel production facility at Jülich, west of Cologne, before another one in Spain.
Swiss airlines, part of the Lufthansa Group, should become its first customer in 2023.