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When I proposed an article to Scientific American on energy return-on-investment, also known as EROI, I didn't realize how much legwork would be involved in gathering the numbers needed for an infographic to accompany the story.
On the surface, the measurement of EROI seems simple. It is just the energy output divided by the energy input. (For gasoline, for example, the output would be the energy in a gallon of gasoline, and the input would be all the energy required to make the gasoline—including oil exploration, drilling and refining.)
Despite the simple equation for EROI, however, there is a lot of complexity under the hood. One issue is that there is a range of EROIs in the literature for each energy source. In part this is because various researchers use different methods for calculating the number. The differences often reflect disagreements about how energy intensive various steps are in the process. To get numbers that I thought were reasonable, I consulted dozens of studies to get a sense of the range of EROI figures for each energy source, then figured out which ones seemed middle-of-the-road as well as computed in a way consistent with all the other EROI figures that I used.
Also, there's no single accepted way of calculating EROI, because it depends in part on what you count as an input. Two of the main types of EROI that researchers calculate are also known as the “net energy ratio” and the “external energy ratio.” For sources such as tar sands, there can be a large difference between these two measures because some methods of getting tar sands out of the ground derive a lot of the energy required for the processes from the tar sands themselves (see the paper cited by Adam Brandt below). The “net energy ratio” counts all inputs—whether diesel fuel for a truck or the tar sands themselves. The external energy ratio, on the other hand, only counts the energy that society puts in, and doesn't count what comes from the resource itself. This means the external energy ratio is always higher than the net energy ratio. Whenever possible, I used the external energy ratio because it is most relevant to the question of how much energy we get out, for the energy we put in. (For calculating greenhouse gas emissions, on the other hand, you would want to consider all energy inputs, and the total emissions from them.)
For biofuels, the EROI reported is usually the external energy ratio, and it doesn't include energy derived from, say, burning the stalks of sugarcane to help power the process of refining sugarcane juice into ethanol. So studies of biofuels will sometimes cite the “fossil energy ratio,” which is similar to the “external energy ratio.”
The external energy ratio number was not available for every energy source. For example, with conventional oil only the net energy ratio was available. The difference between these two types of EROIs, however, would likely be relatively small for conventional oil (personal communication, Charles Hall).
There are uncertainties in any EROI estimate, in part because energy companies usually don't report detailed information on their energy consumption. To calculate the energy input, researchers have to make an estimate based on the dollars spent on various processes and goods—such as the cost of steel to line an oil well. To keep the infographic as simple as possible, we did not attempt to show error bars or ranges on the estimates, and generally rounded them off to a single digit. This was meant to reflect the uncertainty in any single estimate as well as the fact that there is not a single, precise EROI for any energy source.