Renewable Accounts: Energy returns on biofuels
Last week, an industry colleague forwarded to me an article that criticized corn-based ethanol on a number of fronts.
Among the article’s major criticisms was that corn-based ethanol uses more energy to produce than it delivers, a very powerful claim that resonates well, true or not.
To investigate the issue, scientists often calculate a fuel’s energy return on energy invested, or EROI. The arithmetic is simple because it’s just energy output divided by energy input. However, the results can vary greatly depending on the data and assumptions used.
Estimating EROI requires building an accurate energy budget from field to consumer, which can be a very demanding exercise.
For example, a corn-to-ethanol energy budget requires estimates of the fuel used to power the machinery to plant, spray and harvest the corn and also the energy required to produce the machinery, manufacture the fertilizer and pesticides, and sustain the farm laborer, which is more than a can of pop and a summer sausage sandwich.
Beyond the field, the energy associated with storing and converting the corn into ethanol, including that required to build the biorefinery and transport the biofuel to consumers, typically are included.
All along the way, decisions on how to model the process and what data to use must be made. These decisions typically aren’t cut and dried.
A study by David Pimental and Tad Patzek published in 2005 that estimated a corn-based ethanol EROI of less than 1 was widely reported by the media and continues to be a primary piece of evidence against corn-based ethanol.
However, some scientists criticized the study for undervaluing dried distillers grains and overestimating some energy inputs. Again, this goes back to the impacts of data and assumptions.
The University of Minnesota has estimated an EROI of 1.25, while the U.S. Department of Agriculture estimated dry-mill operations to have an EROI of 1.37.
Intuitively, it doesn’t make sense to support energy-related activities with an EROI of less than 1, but that’s not the case. There are a number of desired, economically profitable activities that have EROI’s of less than 1. A good example is the generation of electricity, which has an EROI of about 0.4. This is substantially less than 1 but, as a consumer, I want kilowatts of electricity delivered to my home, not a ton of coal dropped in my driveway.
One of the biggest challenges is assigning energy to coproducts. This is especially important for corn-based ethanol because it contains about the same amount of energy as dried distillers grains and corn oil on a per-bushel basis. This is energy that could be used as fuel.
So, is corn-based ethanol an energy sink? Most studies say no, but it depends on the numbers used, the assumptions made and, in many cases (unfortunately), the argument one is trying to make.