[tt] EROI below five critical

[Eugen Leitl]

http://resourceinsights.blogspot.com/2008/09/net-energy-cliff.html

Sunday, September 14, 2008

The net energy cliff

Charles Hall, the father of the energy return on investment (EROI) concept,
once told me that our current society would probably not be able to function
if the EROI for the entire society slipped below five.

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.