Plants turn sunlight into an electric current with about 90% efficiency. How?
By D.J.P. | LONDON
As renewable energy sources go, solar is the big one: the sun delivers about as much energy to the Earth every hour as mankind consumes in a year. But our ability to harness sunlight remains limited, despite decades of research. The very best photovoltaic cells, the kind seen on many roofs, convert sunlight to electrical energy with an efficiency of around 35%; for more affordable cells the figure is closer to 20%. Plants accomplish the same process with about 90% efficiency during the first stage of photosynthesis. How do they do it so effectively?
Photosynthesis starts within leaves, in sprawling structures aptly called antenna complexes. These are partly made up of molecules of chlorophyll, which give plants their green colour. As sunlight hits the leaf, packets of light called photons bash into the chlorophyll molecules. In doing so they knock electrons out of their orbits around magnesium atoms. Electrons have a negative charge, so their loss creates positively charged magnesium ions. Once separated, each electron-ion pair is known as an exciton; the photon’s energy has, in effect, been poured into it.
Efficiently converting light into electricity requires preserving the exciton's energy as it travels deep within the so-called “reaction centre” of the leaf. The exciton’s energy can then be redeployed into reshuffling the constituent atoms within water and carbon-dioxide molecules into simple sugars and oxygen. The key to preserving the exciton’s energy lies in way it travels to the “reaction centre”. Researchers discovered to their great surprise in 2007 that plants use a bit of trickery from the realm of quantum physics to help excitons find their way. Rather than bumping randomly through a forest of chlorophyll molecules until they happen to reach their destination, excitons are in what are known as “coherences”. In effect, each exciton spreads out over all possible paths simultaneously, and then funnels down through the most efficient route.
That one of the weirdest effects in quantum physics should be relevant to one of life’s most basic processes has spurred researchers to look for more examples, creating a burgeoning field called quantum biology. But knowing more about how photosynthesis works may also help in the business of renewable energy, because when it comes to turning light into electrical energy, photosynthesis beats all human technologies in efficiency. Photosynthesis also performs another useful trick: it splits water into oxygen and hydrogen, which can be used as a renewable fuel with good environmental credentials: burning it just produces water. So figuring out how to copy the photosynthetic machinery of plants—with all their quantum tomfoolery—could be the brightest idea in renewable energy.
