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The USDA's contradictory 2004 report is flawed
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The USDA's 2004 report contradicts its 2002 report dramatically and for only one reason. The 2004 report adopts a method of analysis rejected by the 2002 report. This method erroneously overstates corn ethanol's net energy balance. Articles subsequently published in Science and the Proceedings of the National Academy of Sciences have not followed suit. The mistake is explained in full below.
The two USDA reports in question:
The Energy Balance of Corn Ethanol, an Update, 2002. PDF
The 2001 Net Energy Balance of Corn Ethanol, May 2004. PDF
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The curious case of the USDA-2004 outlier result
In June 2004, the USDA revised their estimate of ethanol's net energy from 25% up to 40%. Why the big change?
More than the entire change is explained by their switch to a methodology they rejected in 2002. The effect of data updates alone actually reduced net energy.
Several things are peculiar about this change of method.
1. The "new" method had already been considered and rejected in 2002.
2. The reasons for that rejection are simple and correct.
3. The "new" method has since been rejected by Argonne and UCB.
4. The USDA-2004 report of only 5 pages does not mention that the entire shift
in results (or even most of it) is due to this change in method.
5. The USDA-2004 report does not explain why the previous USDA rejection of the
method was in error.
The change was in how much of the energy used by the ethanol production plant should be assigned to the ethanol co-products instead of to ethanol itself. Here is an example of the "new" (previously rejected) USDA method.
Suppose it takes 10 units of energy to make a gallon of ethanol and 30 more units of energy to ready its co-product for sale. (Suppose the cost of 30 units of energy is $1 and the co-product sells for $1.50, so the co-product makes sense.)
Suppose that, if the co-product were not made, a substitute product would be purchased which requires only 10 units of energy to produce.
Accurate conclusion: Since the ethanol co-product replaces something made with 10 units of energy, it can save only 10 units of energy.
USDA's "new" method credits the coproduct with saving the 30 units of energy it takes to make it instead of the the 10 units it actually saves. This is backwards.
This backwards over-credit is then given to ethanol. This method is simply wrong as was recognized previously by the USDA.
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What if the USDA had not changed methods in 2004?
In Table 6, of the 2002 report, the "Energy ratio without coproduct energy credits, 1996" is listed as 1.08 on average. In Table 3, of the 2004 report, the same "Energy ratio without coproduct energy credits, 2001" is reported as 1.06.
Before taking account of coproducts with the new method, the 2004 report actually shows a lower Net energy for ethanol.
After accounting for coproduct energy, Table 7 in the 2002 report shows an Energy ratio of 1.34, while Table 4 in the 2004 report shows an Energy ratio of 1.67. Consequently the entire improvement (and a bit more) is due to the coproduct energy and the way it is treated.
Note: An energy ratio of 1.34 means 1 unit in produces 1.34 units out, so net energy is 0.34 units out of 1.34, which is 25%. Simiarly an energy ratio of 1.67 corresponds to 40% net energy.
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Now the backup.
The June 2004 report explains: "In the previous studies, we used a replacement method to allocate total energy to ethanol and byproducts. For this report, we used ASPEN Plus, a process simulation program." They give no justification, only this one example of what the ASPEN Plus program does:
Starch accounts for 66 percent of the corn-kernal weight. Only starch is converted to ethanol. Therefore, 66 percent of energy used to produce and transport corn is allocated to ethanol and 34 percent to byproducts.
But what if a pound of byproduct made this way requires 10,000 Btu of energy, but when made in the normal way (not as part of ethanol production) it requires only 4,000 Btu to make? Then how much energy has been saved by making this pound of byproduct? Only 4,000, because that's how much energy would have been used had we not been making ethanol.
The energy savings credit should have nothing to do with the amount of energy used (10,000) but only with the amount of energy saved (4,000). The new method is just wrong, the old "replacement method" was right. It is correctly explained in the 2002 report:
"A fourth method, based on the replacement value of coproducts, is the method chosen for our final results. Energy credits are assumed to be equal to the energy required to produce a substitute for the ethanol coproduct."
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http://zfacts.com/p/769.html | 01/18/12 07:22 GMT
Modified: Thu, 19 Apr 2007 20:31:36 GMT
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