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Drop-in fuels from sunlight and air

Abstract

Aviation and shipping currently contribute approximately 8% of total anthropogenic CO2 emissions, with growth in tourism and global trade projected to increase this contribution further1,2,3. Carbon-neutral transportation is feasible with electric motors powered by rechargeable batteries, but is challenging, if not impossible, for long-haul commercial travel, particularly air travel4. A promising solution are drop-in fuels (synthetic alternatives for petroleum-derived liquid hydrocarbon fuels such as kerosene, gasoline or diesel) made from H2O and CO2 by solar-driven processes5,6,7. Among the many possible approaches, the thermochemical path using concentrated solar radiation as the source of high-temperature process heat offers potentially high production rates and efficiencies8, and can deliver truly carbon-neutral fuels if the required CO2 is obtained directly from atmospheric air9. If H2O is also extracted from air10, feedstock sourcing and fuel production can be colocated in desert regions with high solar irradiation and limited access to water resources. While individual steps of such a scheme have been implemented, here we demonstrate the operation 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 (for example, methanol and kerosene), with a modular 5 kWthermal pilot-scale solar system operated under field conditions. We further identify the research and development efforts and discuss the economic viability and policies required to bring these solar fuels to market.

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Fig. 1: Simplified process chain of the solar fuel system.
Fig. 2: Representative day run of the solar redox unit for co-splitting H2O and CO2.

Data availability

The main data supporting the findings of this study are available within the paper and its extended data figures. Source data are available with this paper.

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Acknowledgements

This work was funded in part by the Swiss Federal Office of Energy (grant no. SI/501213-01), the Swiss National Science Foundation (grant no. 200021-162435) and the European Research Council under the European Union’s ERC Advanced Grant (SUNFUELS, grant no. 320541) and ERC Starting Grant (TRIPOD, grant no. 715132). We thank P. Basler, T. Cooper, Y. Gracia, P. Good, G. Ambrosetti, A. Pedretti, D. Rast, M. Schmitz, N. Tzouganatos and M. Wild for their contributions to the technology development.

Author information

Authors and Affiliations

Authors

Contributions

R.S., D.R., F.D., P.H., A.M., P.F. and A.S. designed the system’s components. R.S., A.M. and D.R. executed the experiments. J.L. and A.P. performed the economic/policy analyses. P.F. and A.S. managed and co-supervised the project. A.S. conceived the project idea and wrote the manuscript with input from all authors.

Corresponding authors

Correspondence to Philipp Furler or Aldo Steinfeld.

Ethics declarations

Competing interests

ETH Zurich has license agreements with its spinoff companies Climeworks and Synhelion, and owns the following patents: EP 09007467.5, WO2010/091831: Gebald, C., Wurzbacher, J. & Steinfeld, A., Amine containing fibrous structure for CO2 capture; PCT/EP2014/001082: Steinfeld, A., Scheffe, J., Furler, P., Vogt, U. & Gorbar, M., Open-cell materials for use in thermochemical fuel production processes; EP16194074, WO2018/073049: Steinfeld, A., Furler, P., Haselbacher, A. & Geissbühler, L., A thermochemical reactor system for a temperature swing cyclic process with integrated heat recovery; EP18195213.68: Ackermann, S., Dieringer, P., Furler, P., Steinfeld, A. & Bulfin, B., Process for the production of syngas. P.F. is the CTO of Synhelion; P.F. and A.S. are shareholders of Synhelion.

Additional information

Peer review information Nature thanks Albert Harvey, Christian Sattler and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Photographs of the solar fuel system at ETH Zurich.

a) The solar redox unit, comprising the primary sun-tracking solar paraboloidal concentrator coupled to a secondary planar rotating reflector, and the two solar reactors at the foci. b) The two solar reactors, water-cooled calorimeter, and Lambertian target for solar radiative power measurements (seen via the secondary reflector).

Extended Data Fig. 2 Representative solar redox cycle producing syngas with composition suitable for methanol synthesis.

a) Temporal variation of the nominal cavity temperature, total pressure, and outlet gas flow rates during a single redox cycle. b) Temporal variation of the cumulative species concentration and yield of solar syngas collected during the oxidation step. Operation conditions − During the reduction step: Qsolar = 5.1 kW, inlet flow 0.5 L/min Ar, Treduction-end = 1450 °C, total pressure ≤ 25 mbar. During the oxidation step: Qsolar = 0 kW, inlet flows 0.4 L/min CO2 + 9.8 g/min H2O, Toxidation-start = 900 °C, total pressure = 1 bar.

Source data

Extended Data Fig. 3 Representative solar redox cycle producing syngas with composition suitable for FT synthesis.

a) Temporal variation of the nominal cavity temperature, total pressure, and outlet gas flow rates during a single redox cycle. b) Temporal variation of the cumulative species concentration and yield of solar syngas collected during the oxidation step. Operation conditions − During the reduction step: Qsolar = 4.1 kW, inlet flow 0.5 L/min Ar, Treduction-end = 1450 °C, total pressure ≤ 50 mbar. During the oxidation step: Qsolar = 0 kW, inlet flows 0.2 L/min CO2 + 9.8 g/min H2O, Toxidation-start = 800 °C, total pressure = 1 bar.

Source data

Extended Data Fig. 4 Syngas yield (H2 in orange, CO in green, CO2 in black) for each of the 152 consecutive solar redox cycles.

L denotes standard liters.

Source data

Extended Data Fig. 5

Cyclic variation (blue data points) and cumulative (black curve) molar ratio H2:COx for the 152 consecutive redox cycles of Extended Data Fig. 4.

Source data

Extended Data Fig. 6 Simplified layout of a commercial-scale solar fuel plant with ten solar towers, each for 100 MWthermal.

DAC: Direct Air Capture; GTL: Gas-to-Liquid.

Extended Data Table 1 Summary of syngas quality for the experimental runs of Extended Data Fig. 2 for methanol synthesis and Extended Data Fig. 3 for FT synthesis
Extended Data Table 2 Support policy instruments

Source data

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Schäppi, R., Rutz, D., Dähler, F. et al. Drop-in fuels from sunlight and air. Nature 601, 63–68 (2022). https://doi.org/10.1038/s41586-021-04174-y

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