The researchers also say that over the lifecycle of the fuel -- from growing the switchgrass to burning the ethanol in a car engine -- it emits 94 percent fewer greenhouse gas emissions than gasoline.
"This is a significant study. It's the best available data we have on growing switchgrass for an energy crop," said Andy Aden, a research engineer at the National Renewable Energy Laboratory who was not involved in the research.
Ethanol has been touted as a cleaner-burning fuel that will reduce U.S. dependence on oil as well as greenhouse gas emissions. But most ethanol in the United States is produced from corn, and critics say that it takes nearly as much fossil fuel energy to produce the corn as it provides, and that growing corn as an energy crop will compete with food production.
Ethanol produced from feedstock such as switchgrass or wood chips -- called cellulosic ethanol -- could solve some of these problems. Switchgrass grows on marginal farmland that's not suitable for food crops; requires less fertilizer, fuel and other energy inputs to grow; and byproducts of the crop can be burned for fuel at the refinery, replacing fossil fuels.
However, the techniques for refining cellulosic ethanol are not as developed as those for corn ethanol, and no commercial plants currently produce cellulosic ethanol.
Still, the government is betting on these technologies. The energy bill passed by Congress and signed into law by President Bush last month mandates a six-fold increase in ethanol production -- to 36 billion gallons -- by 2022, and mandates that 21 billion gallons of that will come from feedstock other than corn.
Switchgrass is a perennial grass that grows naturally on roadsides and the edges of fields. In the new study, published Wednesday in the journal Proceedings of the National Academy of Sciences, researchers from the University of Nebraska and the U.S. Department of Agriculture teamed with paid farmers on 10 farms in Nebraska, South Dakota, North Dakota and Minnesota. The farmers planted switchgrass in fields ranging in size from about seven to 23 acres and grew it as a crop. They recorded all of the resources that they used in the planting and harvesting, from seeds to tractor fuel to pesticides to fertilizer, as well as the amount of switchgrass they harvested.
The researchers then used that information, together with a model that calculates the amount of ethanol that can be produced per kilogram of switchgrass, and found that the switchgrass could provide 540 percent more energy than went into producing it.
Previous efficiency estimates ranged widely, but were all based on switchgrass grown on small test plots that did not require the use of tractors and other farming equipment. This study was the first to estimate switchgrass' energy potential using data gleaned from farming, researcher Ken Vogel said.
"We kept track of everything, and replaced estimates in the model with actual data," Vogel said. "So this is the first field-scale test of what the energy can potentially be."
The researchers did, however, still have to rely on estimates of the amount of ethanol produced per kilogram of switchgrass, because no commercial switchgrass processing plants exist yet. The estimates come from laboratory tests and pilot-scale processing plants.
Some critics argue that is the study's biggest weakness, and they question the model's claim that one kilogram of switchgrass can produce .38 liters of ethanol.
"Experimentally you can do it, but [in commercial processing] you wont get .38 liters per kilogram," says David Pimentel, an ecology professor at Cornell University who produced a model in 2005 that suggested that it would take more fossil fuel energy to produce cellulosic ethanol than that ethanol would provide.
But Vogel disagrees, and says that the model his paper uses relies on reasonable estimates from laboratory and pilot-scale tests.
"This value is very realistic. It is an estimate, but not an outlandish estimate," he says.
Andy Aden agrees: "We're actually pretty far along," he says. "Here [at the National Renewable Energy Laboratory] on site we have 10 years experience running this on a pilot scale."
And cellulosic ethanol processing will soon move to the commercial scale. The U.S. Department of Energy announced last February that it will spend up to $385 million to partially fund six cellulosic ethanol refineries. In November construction began on the first plant, the Range Fuels biorefinery in Soperton, Ga., which will produce ethanol from wood waste.