Swiss device creates jet fuel from air

A solar-powered system for generating liquid hydrocarbon fuels from the hydrogen and carbon dioxide contained in air is currently in operation on a rooftop of the Swiss Federal Institute of Technology (ETH) in Zurich. 

Electric motors powered by rechargeable batteries can ultimately replace much of our reliance on fossil fuels in ground transportation, advocates believe. However, aircraft and shipping, which contribute about 8% of total human carbon dioxide emissions, are not so easily converted to battery power, because of the long distances and flight times involved.

This situation increases the urgency of finding a carbon-neutral fuel supply to power aircraft and ships. 

A paper by the designers of the synthetic fuel generator appears in Nature magazine, Oct. 21. Authors of the paper are from the Swiss Federal Institute of Technology (ETH), the University of Potsdam (Germany), and Synhelion SA in Lugano, Switzerland.

Pros and cons

As the authors describe the project, it is the first demonstration “of the entire thermochemical solar fuel production chain, from H₂O and CO₂ captured directly from ambient air to the synthesis of drop-in transportation fuels [e.g. methanol, kerosene], with a modular 5-kW (thermal) pilot-scale solar system system operated under real field conditions.” 

If the system can be scaled up to achieve economies of scale and receive some clean energy subsidies, its inventors believe it could possibly provide a carbon-neutral solution to powering long-range aviation and shipping. 

However, to produce enough fuel to power an Airbus A350 carrying 325 passengers on one London-New York roundtrip flight, a scale-up of the current system would require a solar plant capable of producing 1 gigawatt of thermal power to operate for a full day. 

As an example of a commercial scale-up, the authors propose a system of ten solar towers in heliostat fields, each producing 100 MW thermal power. The estimated land footprint for one such plant would be 3.8 square kilometers (1.5 square miles). 

An ideal location would be a low-latitude desert, such as the Sahara, providing high-intensity solar energy. As the hydrogen for the fuel is derived from atmospheric air, there is no significant requirement for water.

Assuming that optimization measures could raise the energy conversion efficiency of the system (currently 0.8%) to 10%, the authors calculate that the proposed 1-gigawatt solar concentrator plant will produce about 34 million liters of kerosene per year. 

“To put this in context,” they write, “2019 global aviation kerosene consumption was 414 billion liters; the total land footprint of all solar plants required to fully satisfy global demand would be about 45,000 square kilometers, equivalent to 0.5% of the area of the Sahara Desert.” 

Fuel costs

Analyses of the entire process show that the fuel would cost 1.20 to 2.00 euros per liter, if it were produced on an industrial scale. The current price of jet fuel (Jet A1) is about 0.54 euros per liter. 

To overcome this disadvantage, the authors call for subsidies and limited quotas requiring use of the carbon-neutral fuel. 

“Given their high initial investment cost,” the authors note, “solar thermochemical fuels require policy support to see widespread deployment, leading to concomitant cost reductions initially through scaling effects and process optimization, and then through mass production of key components and learning-by-doing.”

Because carbon-pricing under current reduction schemes is too low to support the technology, the authors propose a fuel quota scheme mandating aviation fuel retailers or airlines to purchase a certain small portion of their fuel, perhaps 0.1% at first, from carbon neutral sources.

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Remo Schäppi et al. Drop-in Fuels from Sunlight and Air. Nature (2021). DOI: https://doi.org/10.1038/s41586-021-04174-y