Published online 7 May 2009 | Nature | doi:10.1038/news.2009.450

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Crops give more kilometres per hectare if used to power electric vehicles.

Electric cars powered by biomass could be greener than cars that run on biofuel. Punchstock / Cultura

Vehicles propelled by biomass-fired electricity would travel farther on a given crop and produce fewer greenhouse-gas emissions than vehicles powered by ethanol, researchers report today.

Burning biomass to produce electricity is generally more efficient than converting it into ethanol. And electric vehicles — although often more expensive to make and maintain than many vehicles with internal combustion engines — are also more efficient at converting that energy into motion.

In the current study, the researchers, led by Elliott Campbell of the University of California, Merced, modelled the entire system all the way from crop cultivation to vehicle propulsion, comparing cumulative greenhouse-gas emissions for both biofuels and bioelectricity. They found that the bioelectric route came out ahead of both corn ethanol and advanced cellulosic ethanol made from switchgrass.

“In all cases, the electricity pathway uses a lot less land to achieve the same amount of transportation.” Elliott Campbell

University of California, Merced

"We expected that electricity would look better than corn ethanol, but it was surprising to see that this was also the case for the more advanced second-generation ethanols," Campbell says. "In all cases, the electricity pathway uses a lot less land to achieve the same amount of transportation."

The study, published in Science1, suggests that, on average, an electric vehicle powered by biomass will travel 81% farther than an internal-combustion vehicle powered by cellulosic ethanol if both are produced from the same area of cropland. Moreover, the reductions in greenhouse-gas emissions using bioelectricity are more than double those calculated for the cellulosic ethanol.

Biomass bonus

Sonia Yeh, an engineer at the University of California, Davis, says that the study results, although not surprising, have been framed in a new way — in kilometres and emissions per hectare — that allows for a direct comparison between bioelectricity and liquid biofuels. "I think it reinforces what we know," she says, "but they kind of introduce a new and interesting unit to look at the land impact".

Yeh says that the study bolsters California's approach to reducing emissions in the transportation sector. The state's 'low carbon fuel standard', adopted last month, sets a greenhouse-gas standard for fuels and then allows companies to pick their technologies, which include electric transport.

By contrast, US fuel policy is focused on biofuels. The federal mandate ramps up from 9 billion gallons of biofuels in 2008 (compared with almost 138 billion gallons of gasoline) to 36 billion gallons of biofuels by 2022. On 5 May, the US Environmental Protection Agency proposed greenhouse-gas standards for various biofuels, but made no provision for electric transport.

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However, there are proposals to deploy something like California's low carbon fuel standard at the national level. Democrats in the House of Representatives have included such provisions in the climate legislation that is currently under consideration, and President Barack Obama supports the Californian approach as well.

Jeremy Martin, a senior researcher at the Washington DC office of the Union of Concerned Scientists, says it's too early to tell how successful electric vehicles will be or what kind of role advanced biofuels might have in the future. He says the key is to have a flexible policy that sets a goal and then allows all technologies to compete.

"The paper helpfully demonstrates that there is more than one technology pathway that can put the agricultural sector to work in the transportation sector," Martin says. "We would be foolish to put all of our eggs in either basket."