10: Energy Balance

The following is an Excerpt from Sustainable Ethanol: Biofuels, Biorefineries, Cellulosic Biomass, Flex-Fuel Vehicles, and Sustainable Farming for Energy Independence

Energy Balance:
Is Ethanol Renewable?

In order to convert matter or energy from one form to another or move it from one place to another, energy must be expended. This principle is behind the concept of energy balance or net energy ratio—energy obtained from a system divided by the energy put into a system. It’s supposed to help us compare different energy systems. Unfortunately, net energy ratio is not an ideal comparison tool because of the different qualities of various energy inputs and outputs.

All BTU’s are Not the Same

When computing net energy ratio, we need to assign some unit of energy to each energy carrier involved. Generally, energy in and out is measured in British Thermal Units (BTU’s). One BTU is the heat energy needed to raise the temperature of one pound of water from 60°F to 61°F at one atmosphere pressure.[i] The number of BTU’s for an energy carrier such as ethanol or gasoline, then, is a measure of how well it can heat water when burned. This is probably as good a common denominator as we could use, but it does not take some important factors into account. “All BTU’s are not created equal,” as Dr. Bruce Dale of Michigan State University puts it.[ii]

Different energy carriers are not interchangeable. A pile of coal with the same BTU content as a gallon of gasoline will not get you down the road if you put it in your car’s gas tank. A BTU worth of gasoline will cost you more than a BTU worth of coal because of our thirst for liquid transportation fuels. If we are going to make comparisons based on net energy ratio, we should do so in a way that reflects our goals—our reasons for wanting to replace gasoline with an alternative like ethanol. Dr. Dale proposes two such “metrics”—Fossil Energy Replacement Ratio and Petroleum Replacement Ratio.[iii]

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Fossil Energy Replacement Ratio

It is the fossil energy inputs that are non-renewable and can cause pollution of water and air. That’s why most energy balance studies actually consider the Fossil Energy Replacement Ratio (FER) rather than the total energy ratio, whether they call it that or not. In order to calculate FER, energy delivered to the consumer is divided by fossil energy inputs.

The majority of recent studies looking at ethanol from corn kernels show a positive FER (over 1.0), meaning more BTU’s are available in the ethanol than in the fossil fuels that went into producing that same ethanol. Of the 13 major studies on the subject completed between 1998 and 2005, 9 showed a positive net energy balance for corn ethanol.[v] In 2001, the USDA calculated an industry average 1.7 net energy ratio for corn ethanol (19 state average).[vi] Dr. Dale uses the conservative 1.4 FER figure in his comparisons. This means 1.4 BTU’s are delivered to the end consumer for every 1 BTU of fossil fuel input into the corn ethanol production system.

Higher FER means less fossil fuels were consumed for each available BTU. In other words, an energy carrier with a higher FER displaces more fossil fuels. In calculating FER, BTU’s from direct solar energy are not counted against ethanol. We can be fairly certain the sun will continue to shine and plants will continue to convert sunlight into biomass from year to year. Input from sunlight is what gives ethanol a positive Fossil Energy Replacement Ratio. Sunlight also sustained the plant life that became fossil fuels, but that took millions of years.

Petroleum Replacement Ratio

Petroleum is often imported from potentially unstable sources and shipped through areas vulnerable to environmental damage. Most fossil fuel inputs for U.S. ethanol production, on the other hand, are sourced from within North America—mainly coal and natural gas. Petroleum Replacement Ratio (PRR) reflects the degree of reliance on petroleum for a given energy carrier. In order to calculate PRR, energy delivered to the customer is divided by petroleum inputs. It counts only the petroleum BTU input against the BTU’s delivered to the final customer. An energy carrier with a higher PRR displaces more petroleum. Dr. Dale calculates a PRR of 20 for ethanol from corn kernels. For every 20 BTU’s delivered to the consumer, only 1 petroleum BTU went into the corn ethanol production system. This number reflects the fact that corn ethanol production relies on petroleum to a very small extent, making it desirable for our economy and security.

See chapter 10 of Sustainable Ethanol for these topics:

Rating Cellulosic Ethanol
Comparing Ethanol and Gasoline
Fuel Economy and Energy Balance
Variables and Trends
Figure 10-1: Calculating Fossil Energy Replacement Ratio for Corn Ethanol
Figure 10-2: Calculating Petroleum Replacement Ratio for Corn Ethanol
Figure 10-3: Energy Balance Ratios for Ethanol and Gasoline
Figure 10-4: U.S. Natural Gas Wellhead Price, 1996–2007

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Notes

[i]. U.S. DOE Energy Efficiency & Renewable Energy, Biomass Energy Data Book Glossary, http://cta.ornl.gov/bedb/glossary.shtml.
[ii]. Bruce E. Dale, “Biofuels: Thinking Clearly about the Issues,” Presented at the 4th Annual Life Sciences & Society Symposium at the University of Missouri, Columbia, March 14, 2007; Also see www.everythingbiomass.org.
[iii]. Ibid.
[v]. Michael Wang, “The Debate on Energy and Greenhouse Gas Emissions Impacts of Fuel Ethanol,” University of Chicago Argonne National Laboratory, 2005, 24.
[vi]. Hosein Shapouri, “Net Energy Balance of Biofuels,” Presented at the 4th Annual Life Sciences & Society Symposium at the University of Missouri, Columbia, March 15, 2007.