This story was corrected on 28 May 2009.
13 May 2009—Like money, gasoline doesn’t grow on trees. But researchers are working to make it practical to grow fuel on the farm. So scientists from the University of California, Merced; Stanford University; and the Carnegie Institution for Science, in Stanford, Calif., are examining how to make the most of each bit of land devoted to fuel crops.
In research published in the 7 May online edition of Science , the team concluded that turning biomass directly into electricity by, say, burning it along with coal and then using the electricity to power electric vehicles, yields, on average, 80 percent more distance per hectare than turning the same amount of biomass into liquid fuel. So a small SUV powered by bioelectricity from a hectare’s worth of switchgrass could travel 60 000 kilometers. The same vehicle powered by ethanol produced with the same amount of feedstock would go about 30 000 km.
Why such a big difference? J. Elliott Campbell, a professor of environmental science and engineering at the University of California, Merced, and the study’s lead author, has a simple explanation. ”The internal combustion engine just isn’t very efficient, especially when compared to electric vehicles,” he says. How inefficient is it? So much so that biomass-generated electricity will carry you farther even though the ethanol from a hectare of switchgrass contains roughly one-third more energy than what a power plant could extract from the biomass.
Campbell says there is no level of efficiency for converting biomass to ethanol achievable in the near future where bioelectricity-powered EVs don’t win. ”Even when we’re considering an ideal scenario—where roughly 41 percent of the energy content of the biomass ends up in the fuel tank—the results seem to favor the biomass-to-electricity pathway,” he says.
Just as important, say the researchers, is the difference in bioelectricity’s and ethanol’s impact on climate change. On average, they concluded, the bioelectricity pathway results in more than twice the reduction in emissions per unit area produced by ethanol.
In highway driving, using a hectare’s worth of ethanol to power the same small SUV from the earlier example would prevent 8000 kilograms of carbon dioxide equivalents from being emitted; using bioelectricity would keep 13 600 kg of CO 2 out of the atmosphere, Campbell and his colleagues found. The difference is even more pronounced in city driving, with ethanol and bioelectricity offsetting 8700 kg and 20 900 kg, respectively.
Does this count ethanol out? ”It’s still too early to definitively say,” cautions Michael Wang, a researcher at Argonne National Laboratory’s Center for Transportation Research, near Chicago. Who knows, he asks, when researchers will hit upon a conversion technology that results in much more ethanol for every bit of biomass harvested, develop a battery pack that allows for hundreds of kilometers of travel between very rapid rechargings, or when polygeneration will allow for the simultaneous generation of electricity and the production of liquid fuel? He notes that because there is a multiplicity of key assumptions behind each such analysis, and because researchers have yet to find common ground for what suppositions are critical, it would be premature to declare one a winner.