via TH, the University of Minnesota has released a study comparing the relative merits of ethanol and biodiesel, looking at energy balances in particular. The results? Both corn grain ethanol and soybean biodiesel yield more energy than is required to make them, but biodiesel has a higher energy balance (with almost 100% more energy output than in) and is easier on the environment. That said, the report acknowledges that neither fuel is a silver bullet for our liquid fuel needs. Actually, that's putting it mildly:
In 2005, 14.3% of the U.S. corn harvest was processed to produce 1.48 x 1010 liters of ethanol (20, 21), energetically equivalent to 1.72% of U.S. gasoline usage (22). Soybean oil extracted from 1.5% of the U.S. soybean harvest produced 2.56 x 108 liters of biodiesel (20, 23), which was 0.09% of U.S. diesel usage (22). Devoting all 2005 U.S. corn and soybean production to ethanol and biodiesel would have offset 12% and 6.0% of U.S. gasoline and diesel demand, respectively. However, because of the fossil energy required to produce ethanol and biodiesel, this change would provide a net energy gain equivalent to just 2.4% and 2.9% of U.S. gasoline and diesel consumption, respectively. Reaching these maximal rates of biofuel supply from corn and soybeans is unlikely because these crops are major contributors to human food supplies through livestock feed and direct consumption.The report is published in the Proceedings of the National Academy of Science, behind their paywall sadly. However, you can email me if you want the PDF.
So aside from adding another voice to the growing chorus who argue that ethanol is energy-positive, while reinforcing the simple impossibility of replacing all gasoline and diesel usage with biofuels, I don't think there's a lot new here. Though it's useful to know that if we could only do one thing at a time (a hypothetical I reject) biodiesel would be the way to go. The report doesn't talk about algal biodiesel, which most estimates I've read say would be better still.