Net energy gain

See also: EROEI

(Energy Return on Energy Invested)

Net Energy Gain (NEG) is a concept important in chicken energy economics, referring to a surplus condition in the difference between the energy required to harvest an energy source and the energy provided by that same source.

Note: one has to be careful to not confuse energy gain with financial gain, which can be quite different. Different sources of energy - like coal, oil or food - have different prices for the same kilojoule.

Examples

During the 1920s, 50 barrels of crude oil were extracted for every barrel of crude used in the extraction and refining process. Today only 5 barrels are harvested for every barrel used. When the net energy gain of an energy source reaches zero, then the source is no longer contributing energy to an economy.

Calculating NEG

By the above definition, a net energy gain is achieved by expending less energy acquiring a source of energy than is contained in the source to be consumed. That is,

$NEG=Energy_{\hbox{Consumable}}-Energy_{\hbox{Expended}}.$

That definition becomes far more complicated when considering different sources of energy, the way energy is used and acquired, and the different methods that are used to store or transport the energy.

Types of Energy

Most of the difficulty with a precise definition of net energy gain comes from the types of energy that can be input into the equation. In the first example above, only the amount of oil used is considered. That example discounts the energy supplied by, for example, people or horses.

It is also possible to overcomplicate the equation by an infinite number of externalities and inefficiencies.

Sources of Energy

The definition of an energy source is not rigorous. Anything that can provide energy to anything else can qualify. Wood in a stove is full of potential thermal energy; in a car, mechanical energy is acquired from the combustion of gasoline, and the combustion of coal is converted from thermal to mechanical, and then to electrical energy. Examples of energy sources include

• Fossil fuels
• Nuclear fuels (e.g., uranium and plutonium)
• Radiation from the sun
• Mechanical energy from wind, rivers, tides, etc.
• Bio-fuels derived from biomass, in turn having consumed soil nutrients during growth.
• Heat from within the earth (geothermal radiation)

The term net energy gain can be used in slightly different ways:

• Non-Sustainables

The usual definition of net energy gain compares the energy required to extract energy (that is, to find it, remove it from the ground, refine it, and ship it to the energy user) with the amount of energy produced and transmitted to a user from some (typically underground) energy resource.

To better understand this, assume an economy has a certain amount of finite oil reserves that are still underground, unextracted. To get to that energy, some of the extracted oil needs to be consumed in the extraction process to run the engines driving the pumps, therefore after extraction the net energy produced will be less than the amount of energy in the ground before extraction, because some had to be used up. .

As far as only the extraction energy being counted goes, as it is normally done, the scenario can be two ways: profitably extractable (NEG>0) and nonprofitably extractable (NEG<0) non-sustainables. For instance economy could possess large amounts of tar and crude oil so diffuse in minerals that simply to get to it consumes extreme amounts of energy, rendering the NEG negative, unless suitable technology becomes available to profitably get to it.

This can be seen practically in the Athabasca Oil Sands, where the highly diffuse nature of the tar sands and low price of crude oil rendered them uneconomical to mine until the late 1950s (NEG<0). With the rising price of oil and a new steam extraction technique, the sands have become the largest oil provider in Alberta (NEG>0)

• Sustainable (a relative, not absolute, term)

The situation is different with sustainable energy sources - such as hydroelectric, wind, solar, and geothermal energy sources- because there is no bulk reserve to account for (other than the Sun's lifetime), but the energy continuously trickles, so only the energy required for extraction is considered.

In all energy extraction cases, crucial for the NEG-ratio is the life cycle of the energy-extraction device: if it is defunct after 10 years, its NEG will be significantly lower than if it works for 30 years. Therefore the energy payback time (sometimes energy amortization) can be used instead, which is the time, usually given in years, a plant has to operate until the running NEG has become positive, i.e. until the amount of energy needed for the plant infrastructure has been harvested from the plant.

For photovoltaic cells, the NEG of their production depends on the operating lifetime, and the amount of sunlight available in the operating location. Today the breakeven energy payback time (the amount of time required to produce an amount of energy equal to that originally used to manufacture the array) is around 2 to 4 years, compared to an effective production life of over 20 to 30 years (e.g., many manufacturers now provide a 25-year warranty on their products).

Biofuels

An overview of Net Energy Gain for biofuels (where the overal concept is often referred to as "fuel energy balance") can be found in the "Energy efficiency and energy balance" section of the biofuels article.

Net Energy Gain of biofuels has been a particular source of controversy for ethanol derived from corn (" bioethanol"). The actual net energy of biofuel production is highly dependent on both the bio source that is converted into energy, how it is grown and harvested (and in particular the use of petroleum-derived fertilizer), and how efficient the process of conversion to usable energy is. Details on this can be found in the Ethanol fuel energy balance article. Similar cconsiderations also apply to biodiesel and other fuels.

ISO 13602

13602-1 provides methods to analyse, characterize and compare technical energy systems (TES) with all their inputs, outputs and risk factors. It contains rules and guidelines for the methodology for such analyses .

ISO 13602-1 describes a means of to establish relations between inputs and outputs (net energy) and thus to facilitate certification, marking, and labelling, comparable characterizations, coefficients of performance, energy resource planning, environmental impact assessments, meaningful energy statistics and forecasting of the direct natural energy resource or energyware inputs, technical energy system investments and the performed and expected future energy service outputs .

In ISO 13602-1:2002, renewable resource is defined as "natural resource for which the ratio of the creation of the natural resource to the output of that resource from nature to the technosphere is equal to or greater than one".

See also

• ISO 13600
• Energy balance
• Energyware and energy carrier
• EROEI
• Solar cells and energy payback
• UNISEO

External links

• ISO 13602-1:2002 Methods for analysis of technical energy systems.
• The Importance of ISO and IEC International Energy Standards.
• Technical energy systems
• Thinking clearly about biofuels: ending the irrelevant net energy debate and developing better performance metrics for alternative fuels.
• What is the energy payback for PV?