Solar fuels and photosynthesis: Techniques
Three laboratory routes racing to outdo the humble leaf
What this tab covers The methods used to capture this energy: how each works, and how they compare on efficiency, cost, life-cycle emissions, maturity and scale.Photosynthesis is the only solar fuel route at global scale. Three artificial routes make hydrogen or hydrocarbons directly from sunlight, water and CO2; all are at research or pilot stage, competing with PV electricity feeding an electrolyser.
Comparison
| Method | Efficiency | Cost | Life-cycle emissions | Maturity | Scale |
|---|---|---|---|---|---|
| Photosynthesis (agriculture and forestry)126 | Maximum 4.6% (C3) or 6% (C4); soybean about 1.6% (calculated) | Not applicable (see the Biomass tile) | Removes CO2 while growing; net effect depends on land use (see the Biomass tile) | mature | Agriculture uses 48 million km2, 44% of habitable land |
| Photoelectrochemical water splitting3 | Record 19.3% | No commercial cost | No life-cycle figure | research | Laboratory devices |
| Artificial photosynthesis (photocatalytic particles and CO2 reduction)4 | 9.2% indoors; 6.2% in a larger outdoor system | No commercial cost | No life-cycle figure | research to pilot | Outdoor prototypes only |
| Solar thermochemical reactors5 | 4.1% sunlight to syngas | No commercial cost | No life-cycle figure; carbon comes from captured CO2 | pilot | A 50 kW solar tower with 169 reflectors at IMDEA Energy, Spain, which ran the full chain to kerosene |
Photosynthesis is cheap and self-replicating but inefficient and land-hungry. The artificial routes could make fuel on land unsuitable for crops, but none is close to market: outdoor efficiencies of 4-6% are below a PV panel driving an electrolyser, and durability is unproven. The market is large but slow: almost 100 Mt of hydrogen was produced in 2024, less than 1% of it low-emission. Solar kerosene targets aviation, which has few other low-carbon options.
How each method works
Photosynthesis (agriculture and forestry)Chlorophyll absorbs red and blue light and uses it to split water, releasing oxygen; the Calvin cycle then fixes CO2 into sugars, later built into starch, oil and wood.126
Photoelectrochemical water splittingA semiconductor electrode in water absorbs light and passes charges to catalysts (here rhodium) that split water into hydrogen and oxygen: a solar cell and electrolyser in one device, protected by a titanium dioxide coating.3
Artificial photosynthesis (photocatalytic particles and CO2 reduction)Light-absorbing semiconductor particles or nanowires (indium gallium nitride in the record device) carry catalysts and split water with no wires; related systems aim to reduce CO2 to fuels.4
Solar thermochemical reactorsMirrors concentrate sunlight about 2,500 times onto ceria, which at about 1,500 C gives up oxygen; the cooled oxide then strips oxygen from water and CO2, leaving syngas that is made into kerosene by established processes.5
Sources
- What is the maximum efficiency with which photosynthesis can convert solar energy into biomass? (Zhu, Long and Ort, Current Opinion in Biotechnology 19) University of Illinois Experts, 2008
- Improving Photosynthetic Efficiency for Greater Yield (Zhu, Long and Ort, Annual Review of Plant Biology 61, 235-261) University of Illinois (RIPE), 2010
- Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency (Cheng et al., ACS Energy Letters 3, 1795-1800) American Chemical Society, 2018
- Solar-to-hydrogen efficiency of more than 9% in photocatalytic water splitting (Zhou et al., Nature 613, 66-70) Nature, 2023
- A solar tower fuel plant for the thermochemical production of kerosene from H2O and CO2 (Zoller et al., Joule 6(7), 1606-1616) Cell Press, 2022
- Half of the world's habitable land is used for agriculture Our World in Data, 2024