The “solar tower” system produces jet fuel from CO2, water and sunlight

In theory it's possible to create jet fuel from nothing more than water, CO2 and the sun's energy, but do it outside of the laboratory proved to be difficult. Researchers have now created the first fully integrated system capable of doing this at scale in the field.

Aviation accounts for about 5% of global greenhouse gas emissions, and it's proving stubbornly difficult to decarbonize. While other sectors have relied on electrification to switch from fossil fuels to renewable energy sources, aviation's stringent weight constraints make it impossible to rely on battery power anytime in the future. close.

There is a growing consensus that any realistic path to decarbonizing aviation by the middle of this century will require the use of sustainable fuels"d 'top-up', i.e. fuels that work with existing fuels. jet engines and refueling infrastructure. The logic is that any alternative energy source like batteries, liquid hydrogen or liquid ammonia will require unrealistic levels of investment in new aircraft and fuel storage and distribution systems.

Researchers are investigating a wide variety of approaches to making sustainable aviation fuels. The most common today is to create kerosene by reacting animal or vegetable oils with hydrogen. The approach is well established, but renewable sources of these feedstocks are limited and there is competition from biodiesel from the automotive sector.

An emerging approach is to create fuel by directly combining green hydrogen with carbon monoxide derived from captured CO2. It's much more difficult because all the steps involved (electrolyzing water to create green hydrogen, capturing CO2 from the air or industrial sources, reducing the CO2 to CO and combining them to create kerosene) consume lots of energy.

The upside is that raw materials are plentiful, so finding a way to reduce energy needs could open the door to an abundant new source of sustainable fuels. A new factory that uses an array of mirrors to direct sunlight to a solar reactor atop a tower could be a promising approach.

"We are the first to demonstrate the entire thermochemical process chain from water and CO2 to kerosene in a fully integrated solar tower system", Aldo Steinfeld of ETH Zurich, who led the research, said in a press release. "This solar tower fuel plant was operated with a configuration suitable for industrial implementation, establishing a significant technological milestone towards the production of sustainable aviation fuels."

The installation, described in an article published in Joule, includes 169 solar-tracking reflective panels that redirect and focus sunlight into the solar reactor perched atop a 49-foot-tall tower. Water and CO2 are pumped into the solar reactor, which contains a porous structure of cerium, an oxide of cerium, a rare earth metal.

Cerium oxide helps initiate a redox reaction that extracts oxygen from water and CO2 to create a mixture of carbon monoxide and hydrogen called gas of synthesis. Cerium oxide is not consumed by this process and can be reused, while excess oxygen is simply released into the atmosphere. Syngas is pumped through the tower to a gas-to-liquid converter, where it is converted into liquid fuel containing 16% kerosene and 40% diesel.

Using the heat of the sun to drive the entire process, the setup circumvents the huge electrical demands of more conventional approaches. However, the researchers note that the efficiency of their system is still relatively low. Only 4% of captured solar energy has been converted to chemical energy in the syngas, although they see a way to increase this to over 15%.

Overall production levels are also far from what would be needed to reduce fuel demand from the aviation industry. Although the facility takes up space equivalent to a small car park, it was only able to produce just over 5,000 liters of syngas in 9 days. Considering that only 16% of that was then converted...