What's the most energy-efficient crop source for ethanol?
Posted by David Roberts at 12:14 PM on 07 Feb 2006
Biofuel is the hot topic lately in the green blogosphere. There's legitimate dispute about the political and environmental wisdom of plant-based fuels, but at the very least everyone should be starting from a valid, shared set of numbers (oh, to dream).
In an attempt to offer up such numbers, I'm going to ... rip off somebody smarter than me. Namely, Lester Brown, founder of the Worldwatch Institute, founder of the Earth Policy Institute, and author of the recently released Plan B 2.0, which is the best big-picture summary of our environmental situation I've ever read (and I'm only 2/3 through it!). The entire thing can be downloaded for free from EPI's site.
There are two key indicators when evaluating various crops for biofuel: fuel yield per acre and net energy yield of the biofuel, minus energy used in production and refining. This table (taken from Chapter 2) compares crops based on the first indicator:
Here's what the book has to say about the second indicator, net energy yield:
For net energy yield, ethanol from sugarcane in Brazil is in a class all by itself, yielding over 8 units of energy for each unit invested in cane production and ethanol distillation. Once the sugary syrup is removed from the cane, the fibrous remainder, bagasse, is burned to provide the heat needed for distillation, eliminating the need for an additional external energy source. This helps explain why Brazil can produce cane-based ethanol for 60¢ per gallon.
For the most part, I'll leave readers to do what they will with these numbers. But one thing seems quite clear: Corn-based ethanol is a friggin' boondoggle. It's just about the worst source for ethanol, requiring enormous acreage and producing very little energy relative to energy inputs. The recent enthusiasm for ethanol among the powers-that-be in the U.S. has much more to do with massive corporate subsidies than any genuine interest in a sustainable energy future. I'm guessing Bush's talk about switchgrass, etc. is largely to provide cover for these subsidies.