"Various authors have reported conflicting values for the energy return on investment (rE) of ethanol manufacture. Energy policy analysts predisposed to or against ethanol frequently cite selections from these studies to support their positions. This literature review takes an objective look at the disagreement by normalizing and comparing the data sets from ten such studies. Six of the reviewed studies treat starch ethanol from corn, and four treat cellulosic ethanol. Each normalized data set is also submitted to a uniform calculation of rE defined as the total product energy divided by nonrenewable energy input to its manufacture. Defined this way rE > 1 indicates that the ethanol product has nominally captured at least some renewable energy, and rE > 0.76 indicates that it consumes less nonrenewable energy in its manufacture than gasoline. The reviewed corn ethanol studies imply 0.84 < rE < 1.65; three of the cellulosic ethanol studies imply 4.40 < rE < 6.61. The fourth cellulosic ethanol study reports rE = 0.69 and may reasonably be considered an outlier."

Paraphrasing Bob Hirsch in his most recent talk, making ethanol from corn is a process by which a certain amount of energy in the forms of natural gas and diesel fuel are used to create an equivalent amount of energy in the form of ethanol, with the primary output being money from government subsidies.

"To study the potential effects of increased biofuel use, we evaluated six representative analyses of fuel ethanol. Studies that reported negative net energy incorrectly ignored coproducts and used some obsolete data. All studies indicated that current corn ethanol technologies are much less petroleum-intensive than gasoline but have greenhouse gas emissions similar to those of gasoline. However, many important environmental effects of biofuel production are poorly understood. New metrics that measure specific resource inputs are developed, but further research into environmental metrics is needed. Nonetheless, it is already clear that large-scale use of ethanol for fuel will almost certainly require cellulosic technology."

Comments: Reading the details of this study, one discovers that the authors have assumed that the fossil fuel processing to yield one hundred units of ethanol energy includes 40 units of coal and 30 units of natural gas, plus some oil and other sources. What's the net yield from photosynthesis? "Only 5 to 26% of the energy content is renewable."

For more details, please review

• Looking at Biofuels and Bioenergy, LETTERS edited by Etta Kavanagh in Science, Vol 312 [2006 June 23]

  Take special note of "THE METHODOLOGICAL FLAWS IN A. E. FARRELL ..." and "IN THE NET-ENERGY ANALYSIS IN THEIR REPORT “Ethanol can contribute to energy and environmental goals” (27 Jan., p. 506), A. E. Farrell et al. do not (i) define the system boundaries, (ii) conserve mass, and, consequently, (iii) conserve energy. Most of the current First Law net-energy models of the industrial corn ethanol cycle are based on nonphysical assumptions and should be discarded...."

• The Real Biofuel Cycles, a thorough report on the flaws in this Junk Science article, by Tad Patzek, an Engineering Professor at UC Berkeley.
• More on biomass potential. More on ethanol.
• World’s Forests Continue to Shrink [2006 April 4]
• A poem, The Builders, from The Seaside and the Fireside, by Henry Wadsworth Longfellow (1807-1882), comes to mind under the circumstances. [Ed.]

Junk Science: Editorial, in the same issue of Science: Getting Serious About Biofuels by Steven E. Koonin, Science [2006 January 27]

"Credible studies show that with plausible technology developments, biofuels could supply some 30% of global demand in an environmentally responsible manner without affecting food production."