# Solar thermochemical fuel production

Solar thermochemical fuel production is the conversion of sunlight, water, and carbon dioxide into chemical fuels by using concentrated solar radiation as high-temperature process heat to drive redox reactions, most commonly a two-step cycle in which a metal oxide alternately releases and reabsorbs oxygen to split H₂O and CO₂ into hydrogen and carbon monoxide. The products, hydrogen alone or syngas (a mixture of H₂ and CO), can be converted by established gas-to-liquid processes into kerosene, diesel, methanol, and other drop-in fuels whose hydrocarbon chains match those of today's fossil fuels.<sup>[1](https://www.sciencedirect.com/science/article/pii/S0048969723046302)</sup>

| Key fact | Detail |
|---|---|
| Core reaction | Two-step redox cycle: solar-driven reduction of a metal oxide releases O₂; re-oxidation with H₂O or CO₂ yields H₂ or CO<sup>[2](https://pubs.rsc.org/en/content/articlehtml/2017/ta/c7ta04000h)</sup> |
| Benchmark material | Non-stoichiometric ceria (CeO₂), valued for fast kinetics, crystallographic stability, and cyclic durability<sup>[2](https://pubs.rsc.org/en/content/articlehtml/2017/ta/c7ta04000h)</sup><sup> • </sup><sup>[3](https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2025.1671615/full)</sup> |
| Operating conditions | For ceria, reduction typically at 1500 °C and 0.01 bar; oxidation at 1000 °C and 1 bar<sup>[4](https://www.solarpaces.org/how-hot-solar-aviation-fuel-from-h2o-and-co2-has-taken-off/)</sup> |
| Demonstrated efficiency | Best two-step cycles have achieved solar-to-fuel efficiencies of only 4–6%, excluding solar collection and spillage; a dual-scale porosity ceria cavity reactor reached 5.25% at 4 kW and 5.6% at 50 kW<sup>[3](https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2025.1671615/full)</sup> |
| Theoretical limit | Estimated between 20% and 35%, because high-temperature heat is stored directly in chemical bonds without intermediate conversion steps<sup>[3](https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2025.1671615/full)</sup> |
| Projected costs | Techno-economic modeling puts long-term jet fuel and diesel at 16–38 €/GJ (0.6–1.3 €/L) with plant efficiencies of 13–19%<sup>[1](https://www.sciencedirect.com/science/article/pii/S0048969723046302)</sup> |
| Milestone | First synthesized "solar" jet fuel produced in April 2014 in the EU-funded SOLAR-JET project<sup>[5](https://www.dlr.de/en/media/publications/miscellaneous/2014/press-release-solar-jet-european-collaboration-for-the-first-time-demonstrates-the-entire-production-path-of-solar-kerosene_1437/%40%40download/file)</sup> |

## The two-step redox cycle

The cycle uses a non-volatile, non-stoichiometric metal oxide as an oxygen carrier. In the endothermic reduction step, concentrated sunlight heats the oxide, which releases oxygen:

\[ \mathrm{MO}_{x} \rightarrow \mathrm{MO}_{x-\delta} + \tfrac{\delta}{2}\,\mathrm{O}_{2} \]

In the exothermic oxidation step, the reduced oxide is re-oxidized with steam or carbon dioxide at a lower temperature, regenerating the oxide and producing fuel:

\[ \mathrm{MO}_{x-\delta} + \delta\,\mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{MO}_{x} + \delta\,\mathrm{H}_{2} \]

\[ \mathrm{MO}_{x-\delta} + \delta\,\mathrm{CO}_{2} \rightarrow \mathrm{MO}_{x} + \delta\,\mathrm{CO} \]

Here δ is the oxygen non-stoichiometry, which determines how much fuel is generated per cycle.<sup>[2](https://pubs.rsc.org/en/content/articlehtml/2017/ta/c7ta04000h)</sup><sup> • </sup><sup>[6](https://pubs.rsc.org/en/content/articlehtml/2023/cp/d3cp00609c)</sup> Separating the two half-reactions allows each to be run at its own optimal thermodynamic conditions, the main structural advantage of the two-step approach over one-step routes.<sup>[3](https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2025.1671615/full)</sup>

Thermodynamics set hard requirements. [Water splitting](https://www.edgepedia.org/water-splitting) requires a reduction enthalpy greater than 2.51 eV (242 kJ mol⁻¹). Materials with high reduction enthalpies reduce only at high temperature but give high H₂O conversion, while those with low reduction enthalpies reduce more easily but need a larger temperature swing or yield less fuel, so intermediate reduction enthalpies are desirable.<sup>[2](https://pubs.rsc.org/en/content/articlehtml/2017/ta/c7ta04000h)</sup> The temperature and pressure swing between the two steps determines the oxide's oxygen exchange capacity, and thereby the fuel yield per cycle.<sup>[4](https://www.solarpaces.org/how-hot-solar-aviation-fuel-from-h2o-and-co2-has-taken-off/)</sup>

## Redox materials

**Ceria dominates.** Candidate oxides include ZnO, mixed metal ferrites (MFe₂O₄), hercynite spinels (FeAl₂O₄), perovskites (ABO₃), and ceria (CeO₂), but ceria has become the benchmark because of its rapid kinetics and long-term stability. On reduction or oxidation, ceria generates or annihilates oxygen vacancies in its lattice rather than undergoing a phase change; this contributes to its stability, and the vacancies contribute to its fast kinetics.<sup>[2](https://pubs.rsc.org/en/content/articlehtml/2017/ta/c7ta04000h)</sup> Its limitation is that a reasonable reduction extent requires temperatures above 1400 °C and oxygen partial pressures below 10⁻⁴ bar. Heating ceria at 1500 °C under inert gas flow yields δ of 0.04–0.06, only about 10% of the theoretical maximum reduction yield, which limits fuel production per unit mass.<sup>[2](https://pubs.rsc.org/en/content/articlehtml/2017/ta/c7ta04000h)</sup><sup> • </sup><sup>[7](https://www.mdpi.com/1996-1944/16/9/3582)</sup> The maximum non-stoichiometry without changing ceria's fluorite structure has been reported as 0.286 at 1000 °C.<sup>[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC10590985/)</sup>

**Doping and alternatives.** Substituting transition or rare-earth metals into the ceria lattice introduces defects and additional oxygen vacancies, improving reduction conditions; Zr-doped ceria is a prominent example, though thermodynamic analyses show dopants do not always improve performance. Predictions give ceria a solar-to-fuel yield of 12.9% at a 1500 °C reduction temperature, with Zr-doped CeO₂ lower at 10.1% under realistic operating conditions.<sup>[9](https://pubs.rsc.org/en/content/articlepdf/2023/cp/d3cp00609c)</sup><sup> • </sup><sup>[10](https://pubs.rsc.org/en/content/articlehtml/2025/cc/d5cc00262a)</sup> Perovskites reduce at more moderate temperatures and to larger extents because of their lower oxygen vacancy formation enthalpy, but they oxidize more poorly, demanding higher H₂O/CO₂ flows and incurring energy penalties; experimental studies also show that favorable thermodynamics do not guarantee practicality when reduction or re-oxidation kinetics are unfavorable.<sup>[3](https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2025.1671615/full)</sup><sup> • </sup><sup>[6](https://pubs.rsc.org/en/content/articlehtml/2023/cp/d3cp00609c)</sup> Pure ceria shows slow reduction but fast re-oxidation, while perovskites and doped ceria reduce faster and re-oxidize more slowly, a trade-off often managed by raising oxidant concentrations.<sup>[6](https://pubs.rsc.org/en/content/articlehtml/2023/cp/d3cp00609c)</sup>

**Hybrid routes.** A reducing agent such as methane can be used to lower ceria's reduction temperature below 1000 °C in chemical-looping reforming.<sup>[7](https://www.mdpi.com/1996-1944/16/9/3582)</sup> In a solar-tower demonstration of methane dry reforming via a ceria redox cycle at 800–1000 °C with a 10 kW radiative input, the process reached a peak CH₄ conversion of 70% and a peak solar-to-fuel energy efficiency of 27%, defined against the syngas heating value and the methane fed.<sup>[11](https://pubs.rsc.org/en/content/articlehtml/2023/se/d2se01726a)</sup>

## Reactors and demonstrations

The most demonstrated design is the directly irradiated cavity reactor, in which a window admits concentrated radiation onto a porous redox structure inside. Reticulated porous ceramic (RPC) structures are generally preferred because the material stays fixed in place, maximizing exposed reactive surface and radiative heat transport while avoiding particle transport between steps. Pore size involves a trade-off: larger pores increase solar absorption, smaller pores increase reaction rate through greater surface area.<sup>[3](https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2025.1671615/full)</sup><sup> • </sup><sup>[12](https://pubs.acs.org/doi/10.1021/jacs.5c21750)</sup>

__Demonstrations have moved from laboratory to pilot scale.__ The EU-funded SOLAR-JET project (2011–2015, €2.2 million) produced the first synthesized solar jet fuel in April 2014 by coupling a ceria-based solar syngas reactor with commercial Fischer-Tropsch synthesis (chemical process converting syngas into liquid fuels); its follow-up, SUN-to-LIQUID, demonstrated the entire production chain to CO₂-neutral jet fuel on a solar tower at IMDEA Energía near Madrid.<sup>[5](https://www.dlr.de/en/media/publications/miscellaneous/2014/press-release-solar-jet-european-collaboration-for-the-first-time-demonstrates-the-entire-production-path-of-solar-kerosene_1437/%40%40download/file)</sup><sup> • </sup><sup>[13](https://www.icao.int/SAF/stakeholder-action-groups-projects-solar-jet-solar-chemical-reactor-demonstration-and-optimization)</sup><sup> • </sup><sup>[14](https://www.solarpaces.org/solarpaces-tasks/task-ii-solar-chemistry-research/)</sup> The ETH Zurich solar mini-refinery, a 5 kW thermal pilot facility with direct air capture of CO₂ and H₂O and a catalytic unit for liquid fuel synthesis, produced 12.81 liters of syngas per kilogram of ceria with an H₂/(CO+CO₂) molar ratio of 2.7, but its overall sun-to-liquid efficiency was only about 0.8% when all unit energy consumption was counted.<sup>[6](https://pubs.rsc.org/en/content/articlehtml/2023/cp/d3cp00609c)</sup> The air-to-fuel cycle produces roughly 4 L of syngas per kilogram of CeO₂ per cycle with a solar-to-syngas efficiency around 3–5%, and has been scaled to 50 kW at IMDEA Energy in Spain for CO₂-to-fuel production without the direct air capture component.<sup>[12](https://pubs.acs.org/doi/10.1021/jacs.5c21750)</sup>

Not every design direction survived testing. A 4.4 kW solar reactor implementing the isothermal ceria cycle, operated at 1750 K with 95% recovery of the process gases' sensible heat, achieved a solar-to-fuel efficiency of 1.64%, falling to 0.72% when the energy cost of the nitrogen sweep gas was included; its authors concluded on thermodynamic grounds that the isothermal approach is not attractive for future development.<sup>[15](https://pubs.acs.org/enfuem/article/30/8/6654/825886/Demonstration-of-a-Solar-Reactor-for-Carbon)</sup>

## Efficiency: achieved and possible

__A wide gap separates demonstrations from theory.__ The best experiments reach 4–6% solar-to-fuel efficiency (excluding solar collection and spillage), against theoretical limits of 20–35%; for comparison, mature photovoltaic-electrolysis has reached about 30% solar-to-hydrogen and photo(electro)catalytic approaches above 10%.<sup>[3](https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2025.1671615/full)</sup> A principal loss is sensible heat: in the common temperature-swing mode, large amounts of heat are dispersed between the hot reduction step and the cooler oxidation step, in both the solid (redox material and reactor components) and the gas phases. Implementing solid and gas heat recovery has been estimated to raise cycle efficiencies to as much as 20%.<sup>[3](https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2025.1671615/full)</sup> Material-level constraints add to this: high-temperature sintering and irreversible degradation of the redox material over repeated cycles modify its thermal and mechanical properties.<sup>[3](https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2025.1671615/full)</sup><sup> • </sup><sup>[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC10590985/)</sup>

Thermodynamic modeling supports meaningful headroom. A screening model optimizing cycling conditions for non-stoichiometric oxides found ceria reaching 12.9% solar-to-fuel efficiency at 1500 °C reduction temperature, the highest among the candidate oxides examined, and material thermodynamic properties were found to have an enormous impact on optimal cycling conditions.<sup>[16](https://www.sciencedirect.com/science/article/pii/S0038092X2200336X)</sup> Because solar intermittency disrupts continuous operation, researchers also point to compatible thermal energy storage and hybrid solar-electric heating to keep downstream gas-to-liquid processing continuously fed.<sup>[17](https://www.sciencedirect.com/science/article/pii/S2772940022000042)</sup>

## Economics and environmental performance

A techno-economic and life-cycle assessment of an industrial-scale ceria plant using air-captured water and CO₂, designed for round-the-clock operation with high-temperature thermal energy storage, examined six fuels at three high-irradiation sites with a 1 km² heliostat field.<sup>[1](https://www.sciencedirect.com/science/article/pii/S0048969723046302)</sup><sup> • </sup><sup>[18](https://pubmed.ncbi.nlm.nih.gov/37541501/)</sup> In the near-term (2030) scenario at Sierra Gorda, Chile, the minimum fuel selling price was about 76 €/GJ (2.5 €/L) for jet fuel and diesel, 65 €/GJ for methanol, gasoline via methanol, and dimethyl ether, and 42 €/GJ for hydrogen excluding liquefaction. Over the long run, improvements in solar receivers, redox reactors, high-temperature heat recovery, and direct air capture could lift plant efficiency to 13–19% based on which fuel is targeted, and the minimum selling price would come to 16–38 €/GJ (0.6–1.3 €/L) in the case of jet fuel and diesel, and 14–32 €/GJ in the case of methanol, gasoline, and DME. Moving production from Sierra Gorda to Tabernas, Spain, or [Ouarzazate](https://www.edgepedia.org/ouarzazate), Morocco, would raise costs by 22–33%.<sup>[18](https://pubmed.ncbi.nlm.nih.gov/37541501/)</sup> [Greenhouse gas](https://www.edgepedia.org/greenhouse-gas) savings exceed 70% for the fuels and locations considered and can exceed 80% in the near term.<sup>[18](https://pubmed.ncbi.nlm.nih.gov/37541501/)</sup><sup> • </sup><sup>[19](https://csfm.ethz.ch/en/outreach/whats-on/2023/09/sustainable-transportation-fuels.html)</sup>

Solar thermochemical processing has found uses beyond fuels. NASA's Carbothermal Reduction Demonstration used concentrated solar energy to extract oxygen from simulated lunar regolith, confirming carbon monoxide production in a solar-driven reaction intended to support a long-term lunar presence.<sup>[20](https://www.nasa.gov/centers-and-facilities/johnson/sunlight-extracts-oxygen-from-regolith-using-solar-chemistry/)</sup>

## References

1. [Moretti, C. et al. (2023). Technical, economic and environmental analysis of solar thermochemical production of drop-in fuels. Science of the Total Environment 901, 166005.](https://www.sciencedirect.com/science/article/pii/S0048969723046302)
2. [Principles of doping ceria for the solar thermochemical redox splitting of H₂O and CO₂. Journal of Materials Chemistry A (2017).](https://pubs.rsc.org/en/content/articlehtml/2017/ta/c7ta04000h)
3. [Riberi, M., Orsini, F., Ferrero, D., Santarelli, M. (2025). Reactor modeling for two-step solar thermochemical fuel production: a concise review. Frontiers in Energy Research.](https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2025.1671615/full)
4. [How hot solar aviation fuel took off: interview with Aldo Steinfeld. SolarPACES (2022).](https://www.solarpaces.org/how-hot-solar-aviation-fuel-from-h2o-and-co2-has-taken-off/)
5. [DLR press release: Sunlight to jet fuel, SOLAR-JET demonstrates the entire production path of solar kerosene (28 April 2014).](https://www.dlr.de/en/media/publications/miscellaneous/2014/press-release-solar-jet-european-collaboration-for-the-first-time-demonstrates-the-entire-production-path-of-solar-kerosene_1437/%40%40download/file)
6. [Catalytic enhancement of production of solar thermochemical fuels: opportunities and limitations. Physical Chemistry Chemical Physics (2023).](https://pubs.rsc.org/en/content/articlehtml/2023/cp/d3cp00609c)
7. [A Review of Oxygen Carrier Materials and Related Thermochemical Redox Processes for Concentrating Solar Thermal Applications. Materials 16, 3582 (2023).](https://www.mdpi.com/1996-1944/16/9/3582)
8. [Solar-driven thermochemical conversion of H2O and CO2 into sustainable fuels (2023). PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC10590985/)
9. [catalytic enhancement of production of solar thermochemical fuels:opportunities and limitations](https://pubs.rsc.org/en/content/articlepdf/2023/cp/d3cp00609c)
10. [Two-step thermochemical cycle for solar fuel production from H₂O and CO₂: technological challenges and potential solutions. Chemical Communications (2025).](https://pubs.rsc.org/en/content/articlehtml/2025/cc/d5cc00262a)
11. [Methane dry reforming via a ceria-based redox cycle in a concentrating solar tower. Sustainable Energy & Fuels 7, 1804–1817 (2023).](https://pubs.rsc.org/en/content/articlehtml/2023/se/d2se01726a)
12. [Solar Air-to-Fuel Technologies for a Circular Carbon Economy. Journal of the American Chemical Society (2025).](https://pubs.acs.org/doi/10.1021/jacs.5c21750)
13. [SOLAR-JET project record, ICAO Stakeholder Action Groups.](https://www.icao.int/SAF/stakeholder-action-groups-projects-solar-jet-solar-chemical-reactor-demonstration-and-optimization)
14. [SolarPACES Task II: Solar Chemistry Research.](https://www.solarpaces.org/solarpaces-tasks/task-ii-solar-chemistry-research/)
15. [Demonstration of a Solar Reactor for Carbon Dioxide Splitting via the Isothermal Ceria Redox Cycle. Energy & Fuels 30, 6654–6661 (2016).](https://pubs.acs.org/enfuem/article/30/8/6654/825886/Demonstration-of-a-Solar-Reactor-for-Carbon)
16. [Lou, J. et al. (2022). Thermodynamic assessment of nonstoichiometric oxides for solar thermochemical fuel production. Solar Energy 241, 504–514.](https://www.sciencedirect.com/science/article/pii/S0038092X2200336X)
17. [Solar thermochemical fuels: Present status and future prospects. Solar Compass 1, 100010 (2022).](https://www.sciencedirect.com/science/article/pii/S2772940022000042)
18. [Technical, economic and environmental analysis of solar thermochemical production of drop-in fuels. PubMed record, Sci Total Environ (2023).](https://pubmed.ncbi.nlm.nih.gov/37541501/)
19. [Towards commercial solar production of sustainable transportation fuels. ETH Zurich Center for Sustainable Future Mobility (2023).](https://csfm.ethz.ch/en/outreach/whats-on/2023/09/sustainable-transportation-fuels.html)
20. [Sunlight Extracts Oxygen From Regolith Using Solar Chemistry. NASA Johnson Space Center (2026).](https://www.nasa.gov/centers-and-facilities/johnson/sunlight-extracts-oxygen-from-regolith-using-solar-chemistry/)

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*Topic: Encyclopedia › Technology and the built world › Energy technology › Solar power › Concentrating solar power technologies*

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