What does that mean? First, a quick review. It takes energy to get energy. EROI is a measurement of how efficient a process, an enterprise or a society is in obtaining energy. EROI is usually expressed in a ratio, say, 20 to 1. That would mean that the process being studied produced 20 units of energy for every one unit expended. As it turns out, that's about what conventional crude oil returns.
Hall estimates that the United States is currently running on an EROI of just under 40 to 1. This looks like a fairly substantial margin of safety over the 5 to 1 that might lead to societal breakdown. But worrisome developments in the oil, natural gas and coal fields may send us rushing toward that figure.
A post earlier this year on The Oil Drum suggests that the EROI for natural gas in North America is dropping like a stone. This is, in part, reflected in the price of natural gas which is up fourfold in this decade. It is also reflected in the number of wells and the number of total feet drilled just to maintain production. We are having to drill faster and deeper just to stay even. The recent uptick in U. S. supplies may represent a small flattening of the EROI decline, but those supplies are the product of furious drilling and huge exploration expenditures.
The tar sands, presumed to be the great energy savior for North America, have long been a low EROI source of oil. Estimates range from 1 to 1 to about 7 to 1. Work by Charles Hall and his students posted on The Oil Drum gives a tentative estimate of 5.2 to 1 based on admittedly incomplete data.
Coal has a very high return when used to generate electricity, around 80 to 1. But evidence now suggests that in the United States at least, not only has the energy content per ton of coal declined by more than 30 percent since 1955, but the total energy content of coal mined in the country is now falling despite rising coal tonnage.
But what about nuclear? Hall and his students once again attempted to calculate the EROI. Others have made claims of 1.86 to 1 to 93 to 1. The very high estimates appear to leave out many steps in the nuclear fuel and construction cycle. Some contend that the EROI of nuclear is favorable enough--perhaps 11 to 1--to argue for expansion of nuclear power. But, if one takes into account all the energy that will be expended over time storing nuclear waste and guarding the waste and the mothballed nuclear plants in the future, the EROI could drop below 1. Essentially, we get the benefit now, and future generations get both the security and energy expenditures.
On its current trajectory, nuclear power may not even maintain its share of world energy production. It would certainly be useful to know what the true EROI of nuclear power is in order to assess its importance to our energy future.
Solar power has promise as shown in this chart compiled by Hall and his students. But, the estimated EROI ranges are so wide that it would be difficult to promise that solar photovoltaic could consistently provide returns above 5 to 1.
This chart provides an estimate of above 70 to 1 for wind power in one location. EROI in this case, of course, depends heavily on whether the wind generators are located in ultra-windy Denmark or not-so-windy Japan. The main problem with wind and solar, however, is that they are intermittent; the energy produced is difficult to store for use during nighttime or low-wind conditions.
Finally, hydroelectric has a very high EROI. While there is still room for some expansion of hydro power in the developing world, most of the good sites have already been taken in North America and Europe.
And, this brings us to the idea of the net energy cliff. If our energy transition away from fossil fuels does not result in their replacement by high EROI sources of energy with the necessary versatility and storage characteristics, or if such replacements are possible, but delayed too long, then we may be facing a net energy cliff.
It may seem that the difference between an EROI of 40 to 1 and one of, say, 30 to 1 would be comparable to a move from 20 to 1 to 10 to 1. But the mathematics say otherwise. In a society that has an EROI of 40 (which is approximately what the United States is thought to have) about 2.5 percent of the economy is devoted to gathering energy for the other 97.5 percent. If an economy has an EROI of 30 to 1, then the portion of the economy involved in gathering energy rises to about 3.3 percent. This is a significant jump, but probably manageable. However, an EROI that drops from 20 to 1 to 10 to 1 results in the doubling of the part of the economy devoted to securing energy from 5 percent to 10 percent. A further drop to an EROI of 5 to 1, puts 20 percent of the economy within the general classification of energy gathering. This is the net energy cliff.
A drop to an EROI of 5 in today's American economy would mean that the energy sector of society would have to grow eightfold. If the drop came quickly, it would be very difficult to adapt. If the EROI were to drop to, say, 3, this would imply that potentially every third person would be involved in gathering energy in some fashion. Such a society would have little resemblence to the one we now inhabit.
The net energy cliff shows us how important EROI is when considering energy alternatives. Even very large resources such as the tar sands and oil shale become problematic when one considers their EROI.
There appear to be two ways forward then. One is to hope for breakthroughs which increase the energy returns of alternative energy sources. A second is to rework our infrastructure and our way of living so that our society can better withstand a significant overall decline in EROI should it develop.