A widely cited 1998 life cycle study by the U.S. Department of Energy and the Department of Agriculture concluded that "Biodiesel yields 3.2 units of fuel product energy for every unit of fossil energy consumed in its life cycle" while "petroleum diesel yielded only 0.83 units of energy per unit of fossil energy consumed". Time for an update: A soon to be published 2007 study by the USDA and the University of Idaho finds that the energy balance of biodiesel is "a positive ratio of 3.5-to-1".
What has changed? Was the first study wrong?
The many changes that have occurred in the U.S. biodiesel and agricultural industries since the 1990s prompted researchers at the University of Idaho to update the study in cooperation with the USDA. Both the 1998 and 2007 study are based on biodiesel production from soybeans, which according to U.S. Census data is responsible for more than 80 percent of 2007 estimated biodiesel production. [...]
"The bottom line is that the energy balance of biodiesel has definitely improved in the last decade," says University of Idaho Department Head of Biological and Agricultural Engineering Jon Van Gerpen, [...] "The increase in soybean yields and a decrease in herbicide use greatly contributed to the increased energy balance. Meanwhile, energy used for crushing soybeans is significantly lower than what was reported in the NREL study."
So that's good news! We're getting more efficient and getting more energy out of the same amount of soybeans. Of course, the best biodiesel is made from waste grease and we should do everything we can to use all of it (from restaurants and industrial food producers) and not throw it away. There's also algal biodiesel that looks promising. But it's good to learn that even biodiesel made from 'virgin' soybeans is this energy-positive (unlike corn ethanol - and it also has the bonus of fixing nitrogen in the soil).
Thanks to Collin Dunn for the tip.
::Biodiesel Yields Even Higher Energy Balance , via ::Biodiesel is even better than we thought in 1998, now returns 3.5 units of energy
See also: ::Green Basics: Biodiesel, ::i-Spec Q-100: A Biodiesel Analyzer in Your Pocket, ::Find Biodiesel With Your Cell Phone
Update: Syntec Biofuel: Closer to Fuel from Waste (with nod to commenters in this post).