WHAT IS NET ENERGY?

A Net Energy Parable: Why is ERoEI Important?

Posted by nate hagens on August 4, 2006 - 10:00am in The Oil Drum: Net Energy
Topic: Alternative energy
Tags: eroei, eroi, externalities, net energy, sustainability, tainter [list all tags]

Besides water, energy is the most important substance for life on the planet. For most organisms energy is embodied in the food they eat, be it bugs, nuts or gazelles. The excess of energy consumed to energy expended (net energy) has been integral in the evolution of the structure and form of present day organisms.

(extract from original)

Net energy is measured as how much energy is left over after the calories used to find, harvest, refine and utilize the original energy are accounted for. It is a term linked to physical principles and departs in many cases from our current market mechanism of valuing things by price. The alternative energy debate seems to have two firmly entrenched camps - those that acknowledge the importance of energy gain to our society and those who focus on gross energy, energy quality and dollars. This post explores what net energy is, why its important and how its principles may impact the future organization of our society.

To harness and consume energy requires some type of energy investment. This investment is what comprises the difference between gross energy and net energy. There is various nomenclature that describes this concept. Energy profit ratio, surplus energy, energy gain, EROI, and ERoEI all represent virtually the same relationship of how much energy we receive, relative to an energy input(dollars do not factor in). The most referenced metric in the Peak Oil literature is EROI or ERoEI (Energy Returned on Energy Invested), which, in its simplest sense is the ratio:
Energy Output / Energy Input

There is disagreement (sometimes dramatic) in the energy literature not only as to what should be included as energy inputs and outputs (a boundary issue) but how variables are included (how to evaluate co-products, how to include other limiting inputs to an energy technology, etc) These nuances will be covered in a subsequent post.


Net energy is typically given as per unit of energy invested. Thus:
EROI = Net Energy + 1.
(For those of you who've played craps - some tables pay off the hard-ways FOR one and others TO one. EROI and net energy have a similar relationship. EROI is how much energy output FOR an energy input and net energy is the energy output TO the energy input.)

Net energy also can refer to a sum as well as a ratio. For an ethanol process that has an EROI of 1.2:1 -the net energy is just .2, but we can also calculate how much net energy is created for society in a given year or a life-of-resource total. At EROI of 1.2, the 3.9 billion gallons that the US produced in 2005 required 3.29 billion gallons of BTU energy input, resulting in a `net energy' of 610 million gallons. (This post will use net energy and EROI interchangeably - if a sentence uses EROI, just subtract one to get net energy, if I use net energy, just add one to get EROI)


A GROSS vs NET ENERGY GRAPH

Briefly, the above graph shows a theoretical depletable resource which follows the 'best first' concept of resource extraction. The vertical axis is quantity and the horizontal is time. The gross energy resource "X", is the entire area under the curve. ("X" = "A"+"B"+"C"+"D"). Direct energy costs are "D". Indirect energy costs (like tractors and highways and medical insurance and such) are "C". Environmental externalities (in energy terms) are "B". "A" represents the total net energy of the resource after costs have been subtracted. At any given point in time the EROI can be calculated by taking a ratio of the total area divided by the costs (depending on the boundaries). As can be seen, net energy peaks and goes to zero way before the total gross energy is depleted.