Return on Energy
Return on energy, also called an energy yield, is a ratio: energy return on energy invested to get it (EROEI). Obtaining less energy than we use to get it is a yield less than 1, an economic dead end. However, dollar analysis can hide this if we burn cheap energy to get expensive energy. Market logic assumes that if we pay more, we can always get more – that nature is unlimited.
Wild animals spend much of their time and energy finding food. If they can’t find enough, they starve: their EROEI drops under 1.
Although EROEI is well-known, programs are usually touted using monetized market comparisons. However, EROEI is a sanity check on the life-cycle viability of alternative energy sources. For example, can a photovoltaic cell system produce more lifetime energy than was required to put the total system in place? Had they known the low EROEI, few investors would have jumped on early corn ethanol programs, and yields are still below 2.0.
We can’t get more energy from any source than nature put in it. We need to ask three key questions. 1) How much energy is in this “storehouse?” 2) How concentrated is the energy in it? 3) How much work must be done to get it (what will be the EROEI)?
Falling energy yields are a major driver of global Compression. The more one can identify the energy required to obtain energy, the more realistic is an EROEI. A graph suggests this trend.
Graph of the Effect of Global Decline of Return on Energy*
*The full article from which this graph was derived: http://onlinelibrary.wiley.com/doi/10.1111/j.1749-6632.2009.05282.x/pdf. Calculations of EROI do not factor in all the energy used as input. Were that done, most ratios would shift slightly to the left. That is, they would be a little lower. (And note that, contrary to impression, nuclear does not have a high EROI. Takes a lot of energy to build a nuke plant and obtain the fuel.)