Ethanol's energy contribution: real or imagined?

Oct. 6, 2003
The average of 19 attempts to quantify the net energy value (NEV) of making ethanol from corn over its full life cycle indicates a net energy loss of 8%.

The average of 19 attempts to quantify the net energy value (NEV) of making ethanol from corn over its full life cycle indicates a net energy loss of 8%. But the US Congress acts as if ethanol will provide energy security to America. Let's examine two recent attempts to see if ethanol's energy contribution is real or imagined.

The variations in NEVs stem from the assumptions used to calculate them. The assumptions are a function of the analyst's biases. In an unbiased analysis, errors will tend to offset each other.

Energy balance calculations

Even though it is comparing apples and oranges, consider the 2002 US Department of Agriculture's dry mill ethanol energy balance calculated as an NEV of 0.37 (contributes energy) vs. the 2003 dry mill ethanol energy balance derived by David Pimentel of Cornell University as an NEV of -0.17 (wastes energy).

Energy units complicate the comparison. USDA used high heating value (HHV) btu. Pimentel used low heating value (LHV) btu. In practice, LHV is the more correct. The data to make precise conversions are not readily available. Thus we will rely on each to be consistent and then compare the NEVs. When one balance omits an input, ethanol's HHV-LHV ratio is used to adjust it for use in the other's adjusted balance.

USDA states, "Dry millsUare built primarily to manufacture ethanol." This means all the energy inputs should be allocated to ethanol. In making this correction, we find that the NEVs decrease to 0.13 and –0.22 for the USDA and Pimentel, respectively.

USDA alleges that Pimentel's energy input for corn is too high. Then the agency uses too low a corn energy input. Additional corn production is less efficient than average production. Using USDA's corn energy input from the least-efficient state, Nebraska (25,871 HHV btu/gal,) rather than the average corn energy input (20,758 HHV btu/gal) lowers USDA's NEV to 0.09. Putting the Nebraska corn energy input into Pimentel's calculation improves his NEV to –0.16.

Pimentel omitted ethanol-distribution energy. In 2002, DOE estimated 5,000 LHV btu/gal of ethanol was used for distribution. Including this reduces his NEV to –0.20. The USDA used 1,588 HHV btu/gal ethanol. With more ethanol flowing to the coasts, their distance assumptions seem low. Using the US Department of Energy's calculation for distribution energy in USDA's calculation drops the agency's NEV to 0.02.

Pimentel includes the energy value embodied in farm machinery and ethanol production facilities; USDA does not. But expanding facilities consumes energy. If we insert half of Pimentel's infrastructure-addition energy inputs into USDA's calculation, USDA's NEV becomes 20.05, which implies ethanol from corn via the dry mill process is a net energy loser.

Conclusions

The conclusions to be taken from this exercise are:

  • Ethanol's net energy contribution is a function of the assumptions used to calculate the balance.
  • Offsetting adjustments add credibility to Pimentel's analysis.
  • America does not need more dry mill ethanol plants.
  • Mandating more ethanol from corn should not be a part of the comprehensive energy bill before the US Congress.