In 2005, a decade and a half of environmental lobbying convinced President George W. Bush and the U.S. Congress that corn ethanol was a promising fossil fuel substitute which would reduce both American dependence on foreign oil and greenhouse gas emissions. The 2005 energy bill mandated that 4 billion gallons of renewable fuel be added to the gasoline supply in 2006. That rose to 4.7 billion gallons in 2007 and 7.5 billion in 2012, to the delight of corn farmers, activists and almost no one else.

Since then, life cycle assessments (LCAs) have shown that corn ethanol doesn't reduce CO2 emissions, it actually increases them even without its lower fuel efficiency than gasoline, while growing the corn poses a threat to natural habitats and food supplies as food stocks are turned to fuel and marginal lands are used to keep up with government mandated and subsidized demand. In 2010, fuel ethanol consumed 40 percent of U.S. corn production while environmental claims that mandates and subsidies would inspire research and optimization turned out to be untrue - with a captive market and a guaranteed profit, ethanol companies had no reason to improve. 2012 prices for corn were at record highs. Since the U.S. also accounts for 40 percent of the world's corn, U.S. ethanol production has affected corn prices around the planet.

More electric vehicles have entered the market and are competing with alternative-fuel vehicles. Using electricity generated by coal-fired plants to power the cars defeats the purpose and the resource cost (batteries) for electric vehicles are quite high but what if the energy came from the ultimate clean and renewable source, the sun?  Both have monstrous land use costs but when life-cycle emissions and dollar cost are factored in, photovoltaics look good - relatively. When it comes to energy density, gasoline is still the way to go.

But if you use sunlight efficiency, producing solar energy beats out photosynthesis. This is true. While Mother Nature is spectacular, photosynthesis in plants is only 5% efficient whereas solar panels are 8-10% and gasoline internal combustion is 25% (showing that efficiency is not always the correct way to look at the problem). But if your only choice is biomass or photovoltaics, and your metric is best use of the sun, then photovoltaics are much more efficient than biomass at turning sunlight into energy to fuel a car.

The academics examined three ways of using sunlight to power cars: a) converting corn or other plants to ethanol; b) converting energy crops into electricity for battery powered vehicles rather than producing ethanol; and C) using photovoltaics to convert sunlight directly into electricity for 
battery powered vehicles
. Because land-use decisions are local, they examined five prominent "sun-to-wheels" energy conversion pathways – ethanol from corn or switchgrass for internal combustion vehicles, electricity from corn or switchgrass for  battery powered vehicles, and photovoltaic electricity for
battery powered vehicles
– for every county in the contiguous United States.

Focusing the life cycle assessment in that closed system and on three key impacts – direct land use, life cycle greenhouse gas (GHG) emissions, and fossil fuel requirements – the researchers identified PV electricity for battery electric vehicles as the superior sun-to-wheels conversion method.

"Even the most efficient biomass-based pathway…requires 29 times more land than the PV-based alternative in the same locations," the authors write. "PV BEV systems also have the lowest life-cycle GHG emissions throughout the U.S. and the lowest fossil fuel inputs, except in locations that have very high hypothetical switchgrass yields of 16 or more tons per hectare."

Photovoltaic conversion also has lower greenhouse gas emissions throughout the life cycle than do cellulosic biofuels, even in the most optimistic scenario for the latter. "The bottleneck for biofuels is photosynthesis," says U.C.Santa Barbara Bren School of Environmental Science&Management Professor Roland Geyer . "It's at best 1-percent efficient at converting sunlight to crop, while today's thin-film PV is at least 10-percent efficient at converting sunlight to electricity. Finally, while cost was not a key component of the study, Geyer says, "The cost of solar power is dropping, and our quick calculations suggests that with the federal tax credit, electric vehicles are already competitive."

What does this mean for the future?

"What it says to me is that by continuing to throw money into biofuels, we're barking up the wrong tree," Geyer explains. "That's because of a fundamental constraint, which is the relative inefficiency of photosynthesis. And we can't say that right now, biofuels aren't so great but they'll be better in five years. That fundamental problem for biofuels will not go away, while solar EVs will just continue to get more efficient and cheaper. If they're already looking better than biofuels, in five years the gap will be even greater. A search for a silver bullet is under way through "synthetic photosynthesis," but using genetic engineering to improve the efficiency of photosynthesis is a pipe dream. If there is a silver bullet in energy, I think it's solar power."

Published in Environmental Science & Technology