Bioelectricity promises more 'miles per acre' than ethanol
Using biofuel crops such as corn or switchgrass to generate electricity for running battery-powered vehicles is a far more efficient way of producing energy than making ethanol with them, according to Stanford researchers.
Compared to ethanol used for internal combustion engines, bioelectricity used for battery-powered vehicles would deliver an average of 80 percent more miles of transportation per acre of crops while also providing double the greenhouse gas offsets to mitigate climate change, the researchers said.
They performed a "life-cycle" analysis of both bioelectricity and ethanol technologies, taking into account not only the energy produced by each technology but also the energy consumed in producing the vehicles and fuels. For the analysis, they used publicly available data on vehicle efficiencies from the U.S. Environmental Protection Agency and other organizations. They sought to answer the specific question, "How can we maximize our 'miles per acre' from biomass?"
"It's a relatively obvious question once you ask it, but nobody had really asked it before," said Chris Field, professor of biology and of environmental Earth system science and a co-author of a paper describing the research, published in the May 7 online edition of Science magazine. "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."
Bioelectricity was the clear winner in the transportation-miles-per-acre comparison, regardless of whether the energy was produced from corn or from switchgrass. (Both plants are usable for ethanol production, although cellulosic ethanol—which can be made from switchgrass—is more efficient to produce than corn ethanol.) For example, a small SUV powered by bioelectricity could travel nearly 15,000 miles on the net energy produced from an acre of switchgrass while a comparable internal combustion vehicle could travel only about 8,000 miles.
Field, who is also director of the Carnegie Institution's Department of Global Ecology and a senior fellow at Stanford's Woods Institute for the Environment, is part of a research team that includes co-author David Lobell, senior researcher at Stanford's Program on Food Security and the Environment, and lead author Elliott Campbell, assistant professor of engineering at the University of California-Merced.
"The internal combustion engine just isn't very efficient, especially when compared to electric vehicles," Campbell said. "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," says Campbell. "For these beneficial approaches, we could do more to fight climate change by making electricity than making ethanol."
The energy from an acre of switchgrass used to power an electric vehicle would prevent or offset the release of up to 10 tons of carbon dioxide per acre, relative to a similar-sized gasoline-powered car. Across vehicle types and different crops, this offset averages more than 100 percent larger for the bioelectricity than for the ethanol pathway. Bioelectricity also offers more possibilities for reducing greenhouse gas emissions through measures such as carbon capture and sequestration, which could be implemented at biomass power stations but not individual internal combustion vehicles.
While the results of the study clearly favor bioelectricity over ethanol, the researchers caution that the issues facing society in choosing an energy strategy are complex. "We found that converting biomass to electricity rather than ethanol makes the most sense for two policy-relevant issues: transportation and climate," Lobell said. "But we also need to compare these options for other issues like water consumption, air pollution and economic costs."
Biofuels such as ethanol offer an alternative to petroleum for powering cars, but growing energy crops to produce them can compete with food crops for farmland, and clearing forests to expand farmland will aggravate the climate change problem.
The carbon impact of those types of changes will have to be part of the life-cycle analyses assessing the full "carbon intensity" of a fuel that will be required under a regulation adopted by the California Air Resources Board on April 23, 2009. The regulation mandates that the overall carbon content of the mix of fuels each manufacturer sells in the state must be reduced 10 percent by 2020. In assessing the true carbon intensity of a fuel, the indirect effects of manufacturing the fuel must be included. For biofuels, this includes the impact of land-use change.
"There is a big strategic decision our country and others are making: whether to encourage development of vehicles that run on ethanol or electricity," Campbell said. "Studies like ours could be used to ensure that the alternative energy pathways we chose will provide the most transportation energy and the least climate change impacts."
This research was funded through a grant from the Stanford University Global Climate and Energy Project, with additional support from the Stanford University Program on Food Security and the Environment, UC-Merced, the Carnegie Institution for Science and a NASA New Investigator Grant.