Solar fuels and photosynthesis: Efficiency
Why even the best leaf wastes over 95% of sunlight
What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.Plants can store at most 4.6-6% of sunlight as chemical energy, and a soybean crop manages about 1.6%. Artificial routes already beat this in the laboratory (19.3% for photoelectrochemical hydrogen), but larger outdoor systems reach only about 4-6%.
Chart: Efficiency ranges
Theoretical limit In service RecordEfficiency by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| Theoretical maximum, C3 photosynthesis (wheat, rice, trees) | 4.6%1 | Theoretical limit | Full-spectrum sunlight to energy stored in biomass, at 30 C and 380 ppm CO2 |
| Theoretical maximum, C4 photosynthesis (maize, sugar cane) | 6%1 | Theoretical limit | Same measure; the C4 advantage disappears as CO2 nears 700 ppm |
| Soybean crop in the field | 1.6%2 | In service | Full-spectrum sunlight to biomass over a season, from 3.2% of photosynthetically active light (calculated); the best observed values are about one third of the theoretical limit |
| Record photoelectrochemical water splitting | 18.5 to 19.3%3 | Record | Simulated sunlight to hydrogen energy: 19.3% in acid, 18.5% at neutral pH (2018) |
| Photocatalytic water splitting (artificial photosynthesis) | 6.2 to 9.2%4 | Record | Sunlight to hydrogen: 9.2% in pure water under concentrated light at about 70 C, about 7% in tap water and seawater, 6.2% in a larger 257 W outdoor system |
| Solar thermochemical fuel plant | 4.1%5 | Record | Concentrated sunlight to energy in syngas (hydrogen and carbon monoxide) at a solar tower, without heat recovery |
Where the energy is lost
| Step | Loss |
|---|---|
| Sunlight outside the photosynthetically active band (400-700 nm) | 51.3% of incident solar energy2 |
| Reflection and transmission by leaves | about 4.9%2 |
| Photochemical inefficiency (energetic blue photons relax to red-photon energy) | about 6.6%2 |
| Carbohydrate synthesis, photorespiration and respiration | most of the remainder, bringing the ceiling to 4.6% (C3) or 6% (C4)21 |
Photosynthesis wastes most sunlight because chlorophyll uses only visible light, and the plant spends much of what it captures building sugars, maintaining itself and (in C3 plants) undoing a wasteful side-reaction with oxygen called photorespiration. Artificial systems can absorb more of the spectrum and skip the plant's maintenance costs, which is why they already exceed photosynthesis in the laboratory, but the best devices use costly materials and have not been proven at scale.
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