I recently met twice this interesting concept, EROEI, I do not intend to give a better definition than wikipedia so I quote it:
energy returned on energy invested (EROEI or ERoEI); or energy return on investment (EROI), is the ratio of the amount of usable energy acquired from a particular energy resource to the amount of energy expended to obtain that energy resource
The easy and rough way: how much energy you need to get a unit of energy.
- How much energy is neccessary to drill holes for getting oil, and then tranporting, refining,.. before final consumption in form of gas.
- How much energy is needed to build a wind farm and the electricity net associated.
I like this concept because it is very physical and physicists love those things. Apart from that it is something that does not depend on economic circumstances or the influence of big companies in prices, or subsidies… The economy is more prone to account for short term interests and factors, so a more pure measurement system to compare different energy sources is welcomed. Of course it does not substitute the actual prices because nobody will accept it straightforward but it is another tool to takelong term decisions and understanding what’s going on. These 3 links go deeper in that concept, here, hear, and here. And finally this article in neofronteras that explains it very clearly in Spanish, as usual in that blog.
The calculated EROEI valujes range from 100 for oil almost in the surface to almost 1 or even less than one values for cases where the needed energy is more than the obtained. It is considered that values below 10 are doubfully profitable and cases close to 1 should not be profitable at all but could be due to econonomical circunstances. Another factor is the time, that is not considered in this calcaulation and has led to other indexes as EIRR (last figure).
However, actual EROEI figures are not as pure as the concept and differ seiously from one source to other, you can check it in the figures below. Wind mills are ussually in the 20 values, quite good, there is also a coincidence about the lowering value of oil, but nuclear and coal prsent several different values. Photovoltaic is ussualy near the 10 limit. I do not know the cause of the divergence, I suppose it is not easy to calculate or maybe the same factors affecting energy economy are present here too, I will have to check it better.
At least, I agree with the idea that in the long term it is difficult to maintain a cheap low EROEI energy system, it would not be sustainable in any sense.