"It's a relatively obvious question once you ask it, but nobody had really asked it before," said Chris Field of the Carnegie Institution. "The kinds of motivations that have driven people to think about developing ethanol as a vehicle fuel have been somewhat different from those that have been motivating people to think about battery electric vehicles, but the overlap is in the area of maximizing efficiency and minimizing adverse impacts on climate."
There have been concerns that growing crops as an energy source for transportation can have direct or indirect effects on land use, potentially raising food prices and releasing carbon. So maximizing the transport mileage obtained from any given area of land growing biomass would appear to be a good plan.
The main reason for bioelectricity's relatively good performance is the low (roughly 40%) efficiency of internal combustion engines.
"The internal combustion engine just isn't very efficient, especially when compared to electric vehicles," said Elliott Campbell of the University of California, Merced. "Even the best ethanol-producing technologies with hybrid vehicles aren't enough to overcome this."
The researchers found that bioelectricity and ethanol also differed in their potential impact on climate change. "Some approaches to bioenergy can make climate change worse, but other limited approaches can help fight climate change," said Campbell. "For these beneficial approaches, we could do more to fight climate change by making electricity than making ethanol."
Using an acre of switchgrass to power an electric vehicle, for example, would prevent or offset the release of up to 10 tonnes of carbon dioxide per acre, compared to a similar sized gasoline-powered car. It would also be possible to capture and sequester carbon dioxide released by bioelectricity plants; this would be much harder to do on an individual vehicle basis for biofuel-powered cars.
The researchers used the Energy and Resources Group Biofuel Analysis Meta-Model (EBAMM) to assess the whole life cycle for a range of vehicle types and energy sources, assessing the energy used to produce the fuels and vehicles as well as the energy given out by the different technologies.
"We found that converting biomass to electricity rather than ethanol makes the most sense for two policy-relevant issues: transportation and climate," said David Lobell of Stanford University. "But we also need to compare these options for other issues like water consumption, air pollution, and economic costs."
The researchers reported their work in Sciencexpress.