MIT study shows corn ethanol's marginal energy benefit; tropical biofuels make more sense

Planting more corn to make ethanol is not a good idea, a new MIT study has found. Controversy over the benefits of using corn-based ethanol in vehicles has been fueled by studies showing that converting corn into ethanol may use more fossil energy than the energy contained in the ethanol produced. The new MIT analysis shows that the energy balance is actually so close that several factors can easily change whether ethanol ends up a net energy winner or loser.

This new analysis is important, because it shows once again that 'first generation' corn ethanol - a true 'lobby fuel' that is excessively subsidized (earlier post) - is a waste of money and energy. As the chief of the International Energy Agency, Claude Mandil, recently said: consumers in the West better import biofuels produced from crops that yield far more useable biomass and fuels, such as those that grow in the tropics (sugarcane, cassava, sweet potatoes, to name but a few). This is the 'green' thing to do (earlier post).

The Bush administration is pushing the use of ethanol as a domestically available, non-petroleum alternative to gasoline. But most U.S. ethanol is now made from corn, and growing corn and converting the kernels into ethanol consume a lot of energy--comparable to what is contained in the ethanol produced. Making ethanol from corn stalks, other agricultural wastes and wild grasses would consume less energy, but the technology for converting them to ethanol may not be economically viable for another five or so years.

So does using corn-based ethanol in place of gasoline actually make energy consumption and emissions go up, as some researchers claim? Why do others reach the opposite conclusion? And how much better would ethanol from "cellulosic" feedstocks such as switchgrass be?

To answer those questions, Tiffany A. Groode, a graduate student in MIT's Department of Mechanical Engineering, performed her own study, supervised by John B. Heywood, Sun Jae Professor of Mechanical Engineering.

Negative energy balance
Using a technique called life cycle analysis, she looked at energy consumption and greenhouse gas emissions associated with all the steps in making and using ethanol, from growing the crop to converting it into ethanol. She limited energy sources to fossil fuels. Finally, she accounted for the different energy contents of gasoline and ethanol. Pure ethanol carries 30 percent less energy per gallon, so more is needed to travel a given distance:
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While most studies follow those guidelines, Groode added one more feature: She incorporated the uncertainty associated with the values of many of the inputs. Following a methodology developed by recent MIT graduate Jeremy Johnson (Ph.D. 2006), she used not just one value for each key variable (such as the amount of fertilizer required), but rather a range of values along with the probability that each of those values would occur. In a single analysis, her model runs thousands of times with varying input values, generating a range of results, some more probable than others.

Based on her "most likely" outcomes, she concluded that traveling a kilometer using ethanol does indeed consume more energy than traveling the same distance using gasoline. However, further analyses showed that several factors can easily change the outcome, rendering corn-based ethanol a slightly "greener" fuel.

Co-products and 'system boundary'
One such factor is the much-debated co-product credit. When corn is converted into ethanol, the material that remains is a high-protein animal feed. One assumption is that the availability of that feed will enable traditional feed manufacturers to produce less, saving energy; ethanol producers should therefore get to subtract those energy savings from their energy consumption. When Groode put co-product credits into her calculations, ethanol's life-cycle energy use became lower than gasoline's.

Another factor that influences the outcome is which energy-using factors of production are included and excluded--the so-called system boundary. This part of life-cycle research evokes a lot of debate. In principle, the system boundary can be extremely broad, even bordering the absurd. (A case of imposing an absurd system boundary would be to include in the calculus the energy needed to grow the food that the biofuel farmers and processing plant managers consume each year so they can carry out their job...). A study performed by Professor David Pimentel of Cornell University in 2003 sets such a very broad system boundary. It includes energy-consuming inputs that other studies do not, one example being the manufacture of farm machinery. This way, his analysis concludes that using corn-based ethanol yields a significant net energy loss. Other studies with more strict system boundaries conclude the opposite.

To determine the importance of the system boundary, Groode compared her own analysis, the study by Pimentel and three other reputable studies, considering the same energy-consuming inputs and no co-product credits in each case.

"The results show that everybody is basically correct," she said. "The energy balance is so close that the outcome depends on exactly how you define the problem." The results also serve to validate her methodology: Results from the other studies fall within the range of her more probable results.

Cellulosic ethanol more promising
Growing more corn may not be the best route to expanding ethanol production. Other options include using corn stover (the plants and husks that are left on the field), or growing an "energy crop" such as switchgrass. While corn kernels are mostly starch, corn stover and switchgrass are primarily cellulose. Commercial technologies to make ethanol from cellulose are not yet available, but laboratory and pilot-scale tests are generating useful data on processing techniques. So how do cellulosic sources measure up in terms of saving energy and reducing greenhouse gas emissions?

Using her methodology, Groode performed an initial analysis of switchgrass and, drawing again on Johnson's work, corn stover. She found that fossil energy consumption is far lower with these two cellulosic sources than for the corn kernels.

Farming corn stover requires energy only for harvesting and transporting the material. (Fertilizer and other inputs are assumed to be associated with growing the kernels.) Growing switchgrass is even less energy intensive. It requires minimal fertilizer, its life cycle is about 10 years, so it need not be replanted each year, and it can be grown almost anywhere, so transport costs can be minimized.

Groode and Heywood now view the three ethanol sources as a continuum. In the future, cellulosic sources such as corn stover and ultimately switchgrass can provide large quantities of ethanol for widespread use as a transportation fuel. In the meantime, ethanol made from corn can provide some immediate benefits.

"I view corn-based ethanol as a stepping-stone," said Groode. "People can buy flexible-fuel vehicles right now and get used to the idea that ethanol or E85 works in their car. If ethanol is produced from a more environmentally friendly source in the future, we'll be ready for it."

Tropical crops: highly positive balance
Compared to corn ethanol's minor energy benefit, biofuels made from tropical crops such as cassava, sweet potatoes, sugar cane or sweet sorghum have a very strong energy balance. Let us take the case of sugar cane ethanol, for which detailed life-cycle analyses have been carried out. The most authoritative study puts the energy balance at between 8 and 10 to 1. This means that for each unit of energy you put into planting, harvesting and processing the canes, you end up with 8 to 10 times more energy in the form of biofuel (earlier post).

Similar numbers can be found for most other tropical biofuel crops. The reasons for this large difference are relatively easy to understand: the agro-ecological circumstances in the tropics -- the amount of sunshine, rain, the length of the growing period, etc... -- combined with the special nature of the crops, are such that the crops' natural biomass productivity is consistently high.

For some crops, such as sweet potatoes, energy balance analyses have been carried out that look at using the easily convertible parts of the crop only (in this case, the starch-rich roots; sweet potato ethanol's energy balance approaches that of sugar cane ethanol); if, as is the case with the MIT study, the 'co-product' credit of the processed biomass was taken into account, the energy balance would be considerably higher still.