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Biofuels: Energy Balance
October, 2003  

(Click here for a PDF, printable version of this document)

Does ethanol have a negative net energy balance? (Requiring more energy to produce than it provides)

The energy balance of ethanol is found by taking the amount of energy contained in a gallon of ethanol (roughly 76,000 Btu) and subtracting the amount of energy that goes into producing a gallon of ethanol. Critics of ethanol have argued that it has a negative net energy value (NEV), meaning that ethanol requires more energy to make than it actually produces.  However, over the years numerous studies have shown that ethanol does indeed have a positive NEV.  Most recently, a 2002 study by the US Department of Agriculture that accounts for gasoline and diesel fuel use, fertilizers and a variety of other energy inputs in the production, concluded that the energy balance of ethanol is 1.34:1.1  This means that ethanol “yields 34% more energy than it takes to produce it, including growing the corn, harvesting it, transporting it and distilling it into ethanol.”  These data are consistent with a study by Dr. Bruce Dale, Michigan State University (2002), and a study by Argonne National Laboratory (1999). 

The positive ratio is due mostly to technological advances in the ethanol production process. Advances in the areas most critical in determining NEV : corn yields, changes in agricultural practices resulting in reduced energy inputs, and advances in the corn to ethanol conversion process.  According to USDA, “energy requirements for producing a gallon of ethanol are falling over time,” and that higher energy costs will provide incentives for industries to “become more energy efficient, which will continue to push the NEV of corn ethanol higher.”  Here are a few specifics as to why:

Corn yield plays a critical role in determining the energy balance of starch-based ethanol. In fact, a 1 percent increase in corn yield raises NEV by 0.37 percent.   Importantly, with the exception of a few bad years, corn yields have been increasing over time since 1975.  This means that farm resources are being used much more efficiently because less energy (fossil fuel) is being put into the growing process, while more ethanol is being produced. 

                     U.S. corn yield - bushels/acre

Ethanol plants are the largest fossil-energy-consuming component in the corn-to-ethanol fuel cycle.  Today’s ethanol plants use far less energy than in the past.  According to USDA, the majority of ethanol plants in production today have been extensively modernized utilizing the latest advances in ethanol production technology.  In fact, USDA reported that by 1991 changes in production patterns (larger plants and energy efficiency innovations) “reduced the processing energy required to produce a gallon of ethanol from 120,000 Btu in 1981 to 43,000 Btu in 1991.”2 This, combined with substantial electricity conservation efforts through cogeneration and alcohol dehydration have resulted in “considerable energy savings.” All of these factors translate into a higher net energy balance for ethanol. 

According to USDA, fertilizer accounts for about 45% of the energy required to grow corn.  However, the use of fertilizer in grain production, which includes chemical inputs such as nitrogen, potash and phosphate, has been in general decline since the early 1980’s.3  In the years from 1985-2000, nitrogen used per planted acre of corn declined from 140 lbs. to 132 lbs; phosphate from 60 lbs. to 47 lbs. per acre; and potash from 84 lbs. to 51 lbs. per acre. The most significant of these decreases is nitrogen, as “it has a much higher average energy requirement than phosphorous and potash fertilizers.”

           Reduced energy use intensity of ethanol plants: 4

A 1999 study by Argonne National Laboratory found the energy balance of cellulosic ethanol to be in excess of 60,000 Btu per gallon.5 Given that feedstocks for cellulosic ethanol are essentially waste products like corn stover, rice bagasse, forest thinnings or even municipal waste, there are relatively few chemical and energy inputs that go into the farming of feedstocks for cellulosic ethanol.  A secondary factor, although to a much lesser extent, is the fact that cellulosic ethanol plants will presumably produce “extra” energy that can be fed into the power grid.  Doing so will effectively displace the use of electricity produced in power plants, which for the most part rely upon fossil fuels.

Then why do I keep hearing that ethanol has a negative energy balance?

Controversial research from Dr. David Pimentel of Cornell University concludes that ethanol has a negative net energy balance.  Several recent studies have challenged the methodology, transparency, and statistical basis of Dr. Pimentel’s findings.  Here are a few reasons why: 6

Þ Dr. Pimentel’s corn yield statistics date from 1992, meaning that the study does not take into account recent advances in the efficiency of corn growing.  Corn yields have increased by over 10% since then with significantly lower inputs such as fertilizer, pesticides, etc., per bushel.7

Þ Dr. Pimentel’s figures for energy used in the ethanol conversion process date from 1979.  Today’s ethanol plants use far less energy per gallon of ethanol produced.

What is biodiesel’s energy balance? 

A 1998 joint study by the U.S. Department of Energy (DOE) and the U.S. Department of Agriculture (USDA) concluded that biodiesel yields 3.2 units of fuel product energy for every unit of fossil energy consumed in its life cycle.8   In other words, the biodiesel life cycle produces more than three times as much energy in its final fuel product as it uses in fossil energy.

The production process of biodiesel and diesel is practically the same in terms of efficiency in the conversion of raw materials into fuel.  The difference is that biodiesel is able to use renewable resources in its production – soybeans and rapeseed oils, or used frying oil and unwanted animal fats – while conventional diesel relies on fossil fuel resources.  In fact, petroleum diesel’s life cycle yields only 0.83 units of fuel product per unit of fossil energy consumed.