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Photovoltaic hydrogen production efficiency

 T. Goreau

 Submitted to SCIENCE June 25 2011, declined without reason or review, July 7

 Robert E. Blankenship and 17 others (SCIENCE, 13 May, 2011, Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for improvement, 332:805-809), make a very simple and fundamental error in calculating the efficiency of photovoltaic cells for water electrolysis to produce hydrogen that greatly overestimates net efficiency.

 In computing energy efficiency of the process they state: "This assumes there is no mismatch between the photovoltage generated by the PV array and the voltage required for electrolysis". But almost all solar panels on the market produce much higher voltages than is needed for electrolysis, and so are intrinsically inefficient for that purpose.

 When energy efficiency is computed as weight of hydrogen produced per kilowatt-hour of energy input, the efficiency is inversely proportional to the voltage above the threshold voltage for electrolysis of water.

 Only about 1.23 volts are needed for electrolysis (ignoring relatively minor junction potentials). Most solar panels on the market have traditionally produced 17-18 volts, and therefore waste more than 93% of the energy they produce when used for hydrogen production. Production of these panels is now being discontinued by manufacturers, who are switching to production of higher voltage solar panels that will be even more inefficient for water hydrolysis.

 Considerable increases in efficiency can be easily made by reducing the input voltage to more closely match the end use requirements.