Biofuels such as ethanol offer an alternative to petroleum for powering our 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. So what’s the best way to maximise “miles per acres” from biomass?
Researchers from the Carnegie Institution for Science say the best bet is to convert the biomass to electricity, rather than ethanol. They calculate that, compared to ethanol used for internal combustion engines, bioelectricity used for battery-powered vehicles would deliver an average of 80 per cent more miles of transportation per acre of crops, while also providing double the greenhouse gas offsets to mitigate climate change.
“It’s a relatively obvious question once you ask it, but nobody had really asked it before,” said Chris Field, director of the Department of Global Ecology at the Carnegie Institution and co-author of the study. “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 maximising efficiency and minimising adverse impacts on climate.”
Field and his fellow researchers — Elliott Campbell of the University of California, Merced, and David Lobell of Stanford’s Program on Food Security and the Environment — 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. They based their analysis on publicly available data on vehicle efficiencies from the US Environmental Protection Agency and other organisations.
Their conclusion? Bioelectricity was the clear winner in the transportation-miles-per-acre comparison, regardless of whether the energy was produced from corn or from switchgrass, a cellulose-based energy crop. For example, a small SUV powered by bioelectricity could travel nearly 14,000 highway miles on the net energy produced from an acre of switchgrass, while a comparable internal combustion vehicle could only travel about 9,000 miles on the highway.
“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 also found that bioelectricity and ethanol 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,” Campbell said. “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 CO2 per acre, relative to a similar-sized gasoline-powered car. Across vehicle types and different crops, this offset averages more than 100 per cent 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 via 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.”