La conversión de materias vegetales o biomasa en energía eléctrica podría ser una tecnología más eficiente que la obtención del etanol, según un estudio que publica la revista Science.
El estudio encabezado por Elliott Campbell, de la Universidad de California en Merced, y Christopher Field, del Instituto Carnegie, determinó que una cosecha de biomasa puede llevar a un vehículo eléctrico más lejos que a uno que consuma etanol.
Las preocupaciones por el precio del petróleo y las gasolinas y los efectos a largo plazo de las emisiones de gases que contribuyen al calentamiento atmosférico han estimulado la investigación científica y la experimentación tecnológica en busca de alternativas con fuentes de energía renovables y no contaminantes para el transporte.
Sobre el uso de la biomasa se han desarrollado dos tecnologías: la conversión en etanol para motores de combustión interna y la conversión en electricidad para vehículos con baterías.
La pregunta que se plantearon estos investigadores es sencilla: ¿cuál de las dos tecnologías rinde más kilómetros por hectárea?
Y su respuesta es que los vehículos motorizados con baterías rinden un promedio de un 80 por ciento más kilómetros de transporte por hectárea de cultivos, al tiempo que reducen a la mitad las emisiones de gases que contaminan la atmósfera.
"Es una pregunta relativamente obvia, una vez que uno la ha hecho, pero la realidad es que nadie la había planteado antes", dijo Field, director del Departamento de Ecología Global en el Instituto Carnegie.
"Las motivaciones que llevaron a las personas a pensar en el desarrollo del etanol como combustible para vehículos han sido un tanto diferentes de las que llevaron a otras personas a considerar los vehículos con baterías eléctricas, pero donde ambos esfuerzos se tocan es en el área de maximizar la eficiencia y minimizar los impactos adversos para el clima", agregó.
Los investigadores ejecutaron un análisis del ciclo de vida tanto de la tecnología de bioelectricidad como del etanol, tomando en cuenta no sólo la energía producida por cada tecnología, sino también la energía que se consume en la producción de los respectivos vehículos y sus combustibles.
"La bioelectricidad fue, sin duda, la ganadora en la comparación de kilómetros de transporte por hectárea de cultivo, sin importar que la energía se produjera del maíz o de pastos autóctonos (Panicum virgatum o "switchgrass")", señaló el artículo.
Por ejemplo, un vehículo todo terreno pequeño que emplee bioelectricidad puede recorrer 22.530 kilómetros en autopista con la energía neta producida por media hectárea de pastizales, en tanto que un vehículo comparable con motor de combustión interna puede recorrer sólo 14.500 kilómetros con esa misma fuente de energía.
"El motor de combustión interna, simplemente, no es muy eficiente, especialmente si se le compara con los vehículos eléctricos", dijo Campbell.
"Aun las mejores tecnologías de producción de etanol y el uso en vehículos híbridos (que combinan motor de combustión y motor eléctrico) no son suficientes para superar esta diferencia", agregó.
En cuanto al impacto ambiental, los investigadores también encontraron diferencias entre el uso de biomasa para la producción de electricidad y el uso de esos materiales para la conversión en etanol.
Bioelectricity Promises More ‘Miles Per Acre’ Than Ethanol
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. How can we maximize our "miles per acre" from biomass?
Researchers writing in the online edition of the journal Science on May 7 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% 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," says study co-author Chris Field, director of the Department of Global Ecology at 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."
Field, who is also a professor of biology at Stanford University and a senior fellow at Stanford’s Woods Institute for the Environment, is part of a research team that includes lead author Elliott Campbell of the University of California, Merced, and David Lobell of Stanford’s Program on Food Security and the Environment. The researchers 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 US Environmental Protection Agency and other organizations.
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. (Average mileage for both city and highway driving would be 15,000 miles for a biolelectric SUV and 8,000 miles for an internal combustion vehicle.)
"The internal combustion engine just isn’t very efficient, especially when compared to electric vehicles," says Campbell. "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 CO2 per acre, relative to a similar-sized gasoline-powered car. Across vehicle types and different crops, this offset averages more than 100% 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," says Lobell. "But we also need to compare these options for other issues like water consumption, air pollution, and economic costs."
"There is a big strategic decision our country and others are making: whether to encourage development of vehicles that run on ethanol or electricity," says Campbell. "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 Food Security and Environment Project, The University of California at Merced, the Carnegie Institution for Science, and a NASA New Investigator Grant. .
* to be published in the May 22, 2009 print edition.
What Is The Best Way to Turn Plants into Energy?
A new study compares biofuels with bioelectricity
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."
Of course, such a bioelectricity future for transportation would also rely on widespread availability of cars and trucks with batteries and electric motors. "A great deal of innovation must happen in vehicle and power transmission technologies to make that a reality," argues Renewable Fuels Association spokesman Matt Hartwig, an ethanol trade association that owns an ethanol-electric hybrid car. "In the meantime, Americans still need liquid transportation fuels. If the goal is to have more of those gallons come from renewable sources rather than imported oil, fuels like ethanol are the only technologies that are having an impact today."
He adds: "In theory, you could have a plug-in hybrid with a renewable fuel powered [internal combustion engine] and eliminate the need for petroleum all together."
The Obama administration seems to agree, granting $786 million in 2009 for biofuels research and setting up the Biofuels Interagency Working Group to study how best to meet the renewable fuel standard mandated by Congress that will require increasing the amount of renewable fuels, such as ethanol, to 36 billion gallons by 2022.
But the U.S. Environmental Protection Agency (and the California Air Resources Board) have noted that turning corn into ethanol can actually be a significant source of greenhouse gas emissions and other unintended environmental effects, largely by driving the expansion of agriculture and its attendant pollution—as evidenced by previous studies published in Science.
All use of biomass—whether for ethanol or electricity—runs the risk of displacing food crops, however, as well as the need for large amounts of water. "Both pathways could be totally disastrous if these types of impacts can’t be avoided," Campbell admits. "This is going to be a constrained area of land and amount of biomass, so how much transportation and greenhouse gas offsets can we milk out of this constrained land? It looks like the electricity pathway might get us more bang for the buck."
And burning biomass for electricity while capturing the CO2 emissions from such a power plant can actually result in carbon-negative power generation—taking CO2 out of the atmosphere. "By sequestering the flue gas CO2 at the power plant, the bioelectricity pathway could result in a net removal of CO2 from the air," the researchers wrote, and that could help with the problem of ever-rising levels of the greenhouse gases causing climate change.