What’s the most energy-efficient crop source for ethanol?
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:
Ethanol and Biodiesel Yield per Acre from Selected Crops Fuel Crop Fuel Yield (gallons) Ethanol Sugar beet (France) 714 Sugarcane (Brazil) 662 Cassava (Nigeria) 410 Sweet Sorghum (India) 374 Corn (U.S.) 354 Wheat (France) 277 Biodiesel Oil palm 508 Coconut 230 Rapeseed 102 Peanut 90 Sunflower 82 Soybean 56 (author’s estimate)
Note: Crop yields can vary widely. Ethanol yields given are from optimal growing regions. Biodiesel yield estimates are conservative. The energy content of ethanol is about 67 percent that of gasoline. The energy content of biodiesel is about 90 percent that of petroleum diesel.
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.
Ethanol from sugar beets in France comes in at 1.9 energy units for each unit of invested energy. Among the three principal feedstocks now used for ethanol production, U.S. corn-based ethanol, which relies largely on natural gas for distillation energy, comes in a distant third in net energy efficiency, yielding only 1.5 units of energy for each energy unit used.
Another perhaps more promising option for producing ethanol is to use enzymes to break down cellulosic materials, such as switchgrass, a vigorously growing perennial grass, or fast-growing trees, such as hybrid poplars. Ethanol is now being produced from cellulose in a small demonstration plant in Canada. If switchgrass turns out to be an economic source of ethanol, as some analysts think it may, it will be a major breakthrough, since it can be grown on land that is highly erodible or otherwise not suitable for annual crops. In a competitive world market for crop-based ethanol, the future belongs to sugarcane and switchgrass.
The ethanol yield per acre for switchgrass is calculated at 1,150 gallons, higher even than for sugarcane. The net energy yield, however, is roughly 4, far above the 1.5 for corn but less than the 8 for sugarcane.
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.
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