Sustainable Fuels from Sunlight: Start with the Arithmetic

Eric McFarland
University of California, Santa Barbara (UC Santa Barbara)

The radiated heat from sustained thermonuclear reactions in our sun provides approximately 30,000 moles of photons per square meter on the earth’s surface each year. Absorption of these photons generates heat which evaporates water and produces winds. Some of the photons are used in plants for photoelectrochemical processes producing biomass. None of the structures, which absorb the photons producing cost-effective sustainable hydroelectric and wind energy or agriculture, were produced by humans. Chemical fuels typically contain approximately 100 kJ per mole of molecular bonds useable in energy production, and are economically sustainable only at costs of under ~ 1-10 cents per mole of oxidize-able C and H. Even with relatively high efficiency processes that convert solar photons into chemical fuels, the value of those fuels is minimal and thus any artificial system that is to be manufactured, installed, and operated can cost very little. Results and concepts for potential solar fuels produced using scalable artificial photoelectrochemical processes will be discussed together with an overview of the economic constraints. Other than wind and hydroelectric power, today we have no commercially scalable means for sustainably producing energy or fuels. This is an applied problem and only through collaborative science and engineering will the big ideas needed to make it happen be developed.

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