Photovoltaics (PV) technology is much more efficient than biomass at fuelling a car, say researchers, making a mockery of the US 2005 energy bill that calls for more use of corn ethanol as a biofuel.
Researchers examined three ways of using sunlight to power cars: the traditional method of converting corn or other plants to ethanol; converting energy crops into electricity for vehicles; rather than producing and using photovoltaics to convert sunlight directly into electricity for cars.
"PV is orders of magnitude more efficient than biofuels pathways in terms of land use - 30, 50, even 200 times more efficient - depending on the specific crop and local conditions," says University of California, Santa Barbara professor Roland Geyer. "You get the same amount of energy using much less land, and PV doesn't require farm land."
Even the most efficient biomass-based technique requires 29 times more land than the PV-based alternative in the same locations, says the team. Photovoltaic systems for battery electric vehicles also have the lowest life-cycle greenhouse gas emissions throughout the US, and the lowest fossil fuel inputs, except in locations that have very high hypothetical switchgrass yields.
"The bottleneck for biofuels is photosynthesis," says Geyer. "It's at best one percent efficient at converting sunlight to crop, while today's thin-film PV is at least 10 percent efficient at converting sunlight to electricity."
And with the cost of solar power dropping, says the team, the federal tax credit means that electric vehicles are already competitive.
"What it says to me is that by continuing to throw money into biofuels, we're barking up the wrong tree. That's because of a fundamental constraint, which is the relative inefficiency of photosynthesis," says Geyer.
"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."