What Is The Best Way to Turn Plants into Energy?

The environmental case for ethanol from corn continues to weaken. Turning the food crop into ethanol would not be the best use of the energy embedded in the kernels' carbohydrates, according to a new study in Science. That's because fermenting corn into ethanol delivers less liquid fuel energy for internal combustion engines than does burning the kernels to generate power for electric motors.

"We had been studying the area of land that would be available to grow crops for energy and we were curious to discover the most efficient use of these crops," explains environmental engineer Elliott Campbell of the University of California, Merced, who led the study. "We found that with a given amount of biomass you could produce more transportation and greenhouse gas offsets with electricity than with ethanol."

The new study shows that burning biomass to produce electricity rather than converting it to ethanol (made from corn kernels or the other parts of the plant, so-called cellulosic ethanol) delivers 81 percent more miles per acre of transportation in electric vehicles than ethanol burned in internal combustion, even taking into account the lifetime costs of the expensive batteries available today. "The input energy to produce an electric vehicle was 1.5 times the energy to produce an [internal combustion vehicle]," Campbell says. "The batteries currently require large energy inputs in the vehicle production component of our life cycle assessment."

On average, looking at a wide variety of source crops (corn kernels to switchgrass), ways to convert plants to energy, and vehicle sizes (ranging from compact cars to SUVs), bioelectricity delivered 56 percent more energy for transportation per acre, even including the fact that making ethanol produces other useful products, such as cattle feed. To take just one example: a small truck powered by bioelectricity could travel almost 15,000 city and highway miles (24,000 kilometers) compared with just 8,000 comparable miles (13,000 kilometers) for an internal combustion equivalent.

From the atmosphere's point of view, growing biomass to burn in a power plant and using the electricity to move a car avoids 10 tons of carbon dioxide emissions per acre, or 108 percent more emission offsets than ethanol. "One other aspect of the electricity pathway is that most emissions are concentrated in one location, which provides perhaps an opportunity for more control of the emissions," Campbell notes. "It also perhaps locates [other air pollution] emissions in a place where impacts might not be as harmful as where cars are driven today."

via What Is The Best Way to Turn Plants into Energy?: Scientific American.