The Oil Drum offers a very exhaustive report on a National Academy of Sciences study about ethanol and water. Here’s part of the Drum’s conclusion:

As discussed often here in the past, biofuels not only have a much lower energy return vis-a-vis conventional crude, but have between one and two orders of magnitude lower in power density, (or how much energy we get per unit of land). Furthermore, in our ‘Burning Water’ paper, (and alluded to here in this NAS report), biofuels also require significantly more water than even the least efficient fossil fuel systems.

There are also concerns about pesticides, nitrate and other environmental impacts. So when replacing energy with a ’substitute’, all other things do not usually remain equal. I commend the National Academy scientists for highlighting what will be a central issue in upcoming natural resource science - that of systems, and tradeoffs.