Which is more efficient at harvesting the sun’s energy, plants or solar cells? This salient question and an answer are the subject of an article published in the May 13 issue of the journal Science.

Although both photosynthesis and photovoltaics harvest energy from the sun, they operate in distinctly different ways producing different fuels. It is not a simple task to find common ground between the two in order to compare energy conversion efficiency.

“In order to make meaningful comparisons between photosynthesis (which provides stored chemical potential) and photovoltaic technology (which provides instantaneous electrical power), we considered photovoltaic driven water electrolysis to yield hydrogen using existing technology as an example of artificial photosynthesis,” explained co-author Thomas Moore, director of the Center for Bioenergy and Photosynthesis at ASU.

“The hydrogen produced by the artificial system is thermodynamically equivalent to the sugar produced by photosynthesis. The take-home from this point is that the artificial system out performs the natural one, but on the basis of potential for efficient solar energy conversion as measured by the land area required for a given energy output, both technological and biological processes could in principle offer similar outcomes.”

Photovoltaic technology uses fundamental principles combined with advances in materials to achieve record efficiencies of solar-to-electrical power conversion and thereby hydrogen production from water electrolysis.

Photosynthesis, on the other hand, originated in an environment where it was rapidly selected for, as it provided early life forms with a means of self-contained energy production. However, as with many evolutionary adaptations, it is far from a perfect, ideal system for the production of energy, and certainly is not optimal for providing solar-derived fuel to support human activities and economies.

All natural photosynthetic organisms contain light-gathering antenna systems in which specialized pigments (typically several hundred molecules) collect energy and transfer it to a reaction center where photochemistry takes place.

With so many pigments absorbing light energy, the capacity of the photosynthetic apparatus to process the energy is quickly exceeded. In leaves in full sun, up to 80 percent of the absorbed energy must be dumped to avoid its diversion into toxic chemical reactions that could damage or even kill the plant.