There is little work to cut down a tree in your backyard to use for fire wood. As fewer and fewer trees remain in your courtyard, you have to walk to collect firewood, and you may gradually have to use twigs and bushes instead of logs as trees grow scarcer and scarcer. So you end up spending more and more energy on getting a lower and lower quality of energy for the family supper.
The first fossil fuels were easily available and there were little efforts needed to bring it up, in a similar way as the first trees in the backyard. The Energy Return on Energy Invested (EROEI) was something like 100. That is it took one liter of oil to extract 100 liters of oil. Many oil sources now yield just 10 or 20 liters for each liter used. The net effect is that we have to increase the total energy use just to keep the net energy delivered to society on the same level.
EROEI is often showed with a diagram like this[i]:
What we see is that when EROEI approached 5 there is a dramatic increase in the amount of energy that has to be used to produce energy. In its early days, oil frequently yielded an EROEI in excess of 100:1, meaning that 1% or less of the energy contained in a barrel of oil had to be used to deliver that barrel of oil. Not a bad bargain. Oil production today more typically has an EROEI around 20:1, while tar sands and oil shale tend to be about 5:1 and 3:1, respectively. Perhaps the effect is better seen by re-writing the graph assuming that we want a constant supply of energy (set at 100 in the graph).
3:1-10:1. Wind energy perhaps in the range of 15:1
This has big economic effects. As Richard Heinberg writes in End of Growth
“As EROEI declines over time, an ever-larger proportion of society’s energy and resources need to be diverted towards the energy production sector.”
I will come back to this very soon in another post.And it also has huge environmental effect. For instance, if the US is going to build its energy supply around shale oil and tar sand the gross energy use will increase tremendously, and also its emissions of green house gases.
EROEI is mostly discussed in the phase of energy production, i.e. at the well or mine head. Another aspect of EROEI is also to consider the whole system. For instance, for solar systems, one can calculate the EROEI directly in the panel, but one should include the storage and distribution systems that are an integral part of the system. Finally, one should look at the EROEI all the way to consumption. So if the EROEI of petrol at the pump is 10:1, the EROEI of the use of petrol in the car is only 3:1, as only a smaller part of the fuel is converted into the movement of the car (Garden Earth).
There are many other aspects of relevance when discussing energy systems, for example, the quality of the source and how it can be used. For solar and wind energy there are also big problems associated with intermittence and storage.
For a rather recent EROEI update, read A Review of the Past and Current State of EROI Data Ajay K. Gupta and Charles A.S. Hall.
For related post on this blog
Energy squeeze, Is it a trap, a cliff or just a simple adjustment?
It takes more energy to eat than to farm
Energy and Agriculture
There will be no nuclear power without oil
our energy debt