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Thinking Clearly about Biofuels: Ending the Irrelevant ?Net Energy? Controversy
15 Aug, 2007 11:50 am
The age of petroleum is ending. If we are to make sound choices about our energy future, we will have to think clearly and carefully. The irrelevant ?net energy? argument revolving around fuel ethanol offers a textbook example of how not to think about alternative fuels.
The critics’ most recent paper¹ concludes that corn ethanol has a -29% net energy. Net energy is defined as ethanol’s heating value (a fixed, known quantity) minus the fossil energy inputs required to produce the ethanol. For accounting convenience, the authors add up all fossil energy inputs as equivalent: one megajoule (MJ) of coal equals one MJ of petroleum equals one MJ of natural gas. This is the fundamental premise of net energy and it is completely wrong. All MJ are not created equal and cannot be added in this way. If all MJ were equal, then energy markets would reflect that fact. But the energy markets do no such thing. At current energy prices, a MJ of natural gas is worth about 3.5 times a MJ of coal, and a MJ of petroleum is worth more than five times a MJ of coal. Clearly, all MJ are not created equal.
These market realities reflect another underlying, fundamental reality: we do not value energy per se but rather the services or “qualities” that the energy provides. For example, the energy in coal cannot directly light our homes. Coal must be converted to electricity in a power plant in order to provide many desired energy services. About 1 MJ of electricity is produced for every 3 MJ of coal burned. The “net energy” of electricity is therefore electricity energy out minus the coal energy used, approximately -200%, much worse than the corresponding figure for ethanol. Are we going to turn off the lights because electricity has a terribly negative net energy? The logic of the “net energy” argument would say “Yes, turn them off!”.
Thus the underlying premise of the net energy argument is wrong. The “net energy” metric is mistaken at the very core—not at its margins. Different energy carriers cannot be compared on straight energy basis. In the real world, the different “qualities” of different energy carriers must be considered. Apples are good for making apple juice; apples are not good for making orange juice. Petroleum is uniquely suited for making liquid fuels; neither coal nor natural gas are nearly so well-suited to make liquid fuels. Thus it is misleading and irrelevant as a public policy guide to use net energy to compare liquid fuel alternatives.
Comparisons of alternatives are central to science and sound policy decisions, but unfortunately the net energy advocates have never published a single comparison of ethanol with other liquid fuels. It is not difficult to do such calculations. Using precisely the same net energy methodology and assumptions of the ethanol critics, one quickly finds that gasoline has a net energy that is less than -45%. Thus ethanol is actually superior to gasoline in its (I repeat: irrelevant) net energy metric.
So are there better metrics than net energy to compare alternative fuels? There is room for discussion on this issue, but two complementary metrics suggest themselves. First, alternative fuels (eg, ethanol) can be rated on their ability to displace petroleum, our most pressing energy security policy issue. To produce 1.0 MJ of ethanol requires about 0.05 MJ of petroleum, while it takes 1.1 MJ of petroleum to produce 1.0 MJ of gasoline (data from Farrell, et al2). Thus ethanol displaces 1.1/0.05 equals 22 MJ of petroleum for every MJ of ethanol produced. We greatly extend our supplies of petroleum by producing corn ethanol. Cellulosic ethanol has similar petroleum displacement numbers relative to gasoline. These values will improve as cellulosic ethanol technology improves.
Second, ethanol could be rated on the total greenhouse gases produced per km driven, our most pressing climate security policy issue. Corn ethanol currently achieves modest greenhouse gas reductions, but new technologies in the field and biorefinery operations are reducing greenhouse gas emissions. Cellulosic ethanol will reduce the life cycle greenhouse gas emissions per km driven by over eightfold compared to gasoline—a huge improvement.
So what have we learned? If we are to make wise decisions as we embark on this brave new world of alternative fuels, we will need to carefully choose our metrics of comparison. We want attractively priced alternative fuels that will reduce total petroleum use and also provide environmental improvements versus gasoline and diesel. These are appropriate metrics for biofuels and other alternatives. Useless, misleading and irrelevant metrics such as net energy must be eliminated from our discourse on fuel alternatives.
1. David Pimentel and Tad W. Patzek Ethanol Production Using Corn, Switchgrass, and Wood; Biodiesel Production Using Soybean and Sunflower Natural Resources Research Vol. 14, No. 1 (2005) pg. 65-76.
2. Alexander E. Farrell, et al. Ethanol Can Contribute to Energy and Environmental Goals. Science. Vol. 311. Pg. 506-508.
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This is great! I recommend publication as it stands. Good to see the references to Pimental, who has made some very salient points about the "ethanol economy". Yes, the "Oil Deartn Era" (as I have called it) is at hand and nobody should be fooled otherwise!
1) Greenhouse gasses (primarily CO2) from the combustion of fuels used in production, including for nitrogen fertilization
2) CO2 resulting from soil respiration as land is cleared for biofuels crops
3) CO2 resulting from soil respiration in annual plowing
4) Nitrogen oxides resulting from plowing
5) methanogenesis from irrigation
Low-Input, High-Diversity biofuels have great advantages in minimizing greenhouse gas losses.
In any case, it is possible that the method of energy accounting proposed by Pimental and Patzek is incorrect; I am not competent to judge that. However, the philosophy is dead on: we must account for energy in and energy out. More importantly, we must account for carbon in and carbon out (and the subtleties of the different forms of greenhouse gases).
It's probably much more appropriate to talk about our new circumstances as The Era of Really, Really Pricy Petroleum. Or maybe the Post Peak Oil Century.
It still want to point out that this is the case for ethanol from corn. Cellulosic ethanol will be a reality in the near future. In the case of ag waste I tend to think of this as energy reclaimation because we will be able to turn corn stover or wheat straw or other waste into ethanol as well. Energy crops like switchgrass use little of the water and fertilizer used on these food crops and present an even better yield.
The Haber process can use syngas from biomass thermochemical processes. Natural gas isn't necessary. It's just cheap and abundant. But woody biomass like poplar and eucalyptus have lots of lignin, and there aren't very many good uses for lignin at present, so turning it into syngas in a Fischer-Tropsch like process is perfectly reasonable.
The limiting factor in all of this is that there is only so much biomass available. At some point we wont be able to cultivate more land or increase yields economically.
Malca and Freire. Renewability and life-cycle energy efficiency of bioethanol and
bio-ethyl tertiary butyl ether (bioETBE): Assessing the implications of allocation. Energy 31 (2006) 3362?3380