Shedding Light on Solar Fuel Efficiencies

In their review "Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for improvement" (13 May, p. 805), R. E. Blankenship et al. compare the conversion efficiency of photosynthesis with that for production of hydrogen by photovoltaics and subsequent water electrolysis (PV-EL). They conclude that the latter technology is more efficient, but also point out ongoing and future research directions that could lead to substantial conversion efficiency increases in plants and microorganisms. We agree that careful analysis of current limitations for a system is valuable as a guide to researchers working on solutions to bottleneck problems. However, using a single figure of merit may discount other important considerations, and could prematurely identify technology winners and discourage research in other promising areas.