Ethanol's energy return on investment: A survey of the literature 1990 - Present

Various authors have reported conflicting values for the energy return on investment (r(E)) 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 r(E) > 1 indicates that the ethanol product has nominally captured at least some renewable energy, and r(E) > 0.76 indicates that it consumes less nonrenewable energy in its manufacture than gasoline. The reviewed corn ethanol studies imply 0.84 <= r(E) <= 1.65; three of the cellulosic ethanol studies imply 4.40 <= r(E) <= 6.61. The fourth cellulosic ethanol study reports r(E) = 0.69 and may reasonably be considered an outlier.