The aviation industry may miss its 2050 goal of net-zero emissions. Four U.S. major airlines including American, Delta, Southwest, and United—recognized a 15 percent surge in the use of jet fuel in the five years prior to the pandemic. That number is expected to rise despite continuous goals to improve engine efficiencies.
Scientists and engineers at the Swiss Federal Institute of Technology (ETH) in Zurich have witnessed successful testing of kerosene. Kerosene was produced from water vapor and carbon dioxide directly from air using solar energy, representing a successful alternative to fossil-derived fuels. In addition, it can be used with current storage and distribution infrastructures, and engines.
Synthesis gas (or syngas), a specific mixture of carbon monoxide and hydrogen is also a known alternative to fossil-derived fuels. Syngas is created by Fischer-Tropsch (FT) synthesis, a chemical reaction transforms carbon monoxide and water vapor into hydrocarbons. The ETH research team discovered a method to split water and carbon dioxide using a metal oxide redox cycle that can produce renewable syngas. They process was conducted on a rooftop solar refinery at the ETH Machine Laboratory in 2019.
The current pilot-scale solar tower plant was set up at the IMDEA Energy Institute in Spain. It increases the 2019 solar reactor experiment by 10, says ETH engineering professor Aldo Steinfeld, who led the study. The fuel plant aligns three subsystems—the solar tower concentrating facility, solar reactor, and gas-to-liquid unit.
The mirrors rotate to follow the sun that concentrates solar irradiation into a reactor on top of the tower. The reactor is lined with reticulated porous ceramic made of ceria (or cerium(IV) oxide). The concentrated sunlight generates a high temperature setting of about 1,500 °C, which is hot enough to split captured carbon dioxide and water to produce syngas. The syngas is then processed to kerosene in the gas-to-liquid unit. A control room operates the whole system.
“The present pilot fuel plant is still a demonstration facility for research purposes,” says Steinfeld, “but it is a fully integrated plant and uses a solar-tower configuration at a scale that is relevant for industrial implementation.”
“The solar reactor produced syngas with selectivity, purity, and quality suitable for FT synthesis,” stated the paper author and conveyed stability for various sequential cycles. A value of 4.1 percent solar-to-syngas energy efficiency, which Steinfeld says is a record value for thermochemical fuel production, even though better efficiencies are required to make the technology economically competitive.
“The measured value of energy conversion efficiency was obtained without any implementation of heat recovery,” he says. The heat accounted for more than 50 percent of the solar-energy input. “This fraction can be partially recovered via thermocline heat storage. Thermodynamic analyses indicate that sensible heat recovery could potentially boost the energy efficiency to values exceeding 20 percent.”
More research is needed to optimize the ceramic structures lining the reactor, something the ETH team is actively working on, by looking at 3D-printed structures for improved volumetric radiative absorption. “In addition, alternative material compositions, that is, perovskites or aluminates, may yield improved redox capacity, and consequently higher specific fuel output per mass of redox material,” Steinfeld adds.
The continued challenge for researchers is scaling-up of the technology for higher solar-radiative power inputs.
“Airlines and airports would be required to have a minimum share of sustainable aviation fuels in the total volume of jet fuel that they put in their aircraft,” Steinfeld says. Solar kerosene can be mixed with fossil-based kerosene, starting small, as little as 1 or 2 percent. This would raise the fuel costs minimally at first, adding “only a few euros to the cost of a typical flight,” as Steinfeld puts it. “By the time solar jet fuel reaches 10 to 15 percent of the total jet-fuel volume, we ought to see the costs for solar kerosene nearing those of fossil-derived kerosene,” he adds.
We may not have to wait too long for solar fuel flights, as Synhelion is working on commissioning the first industrial-scale solar fuel plant in 2023. The company has also teamed up with the SWISS airline to test a flight solely using its solar kerosene.