Net energy gain

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Net Energy Gain is a concept important in 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 - command 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_} - Energy_}.]

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 independent of time.

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
• Radiation from the sun
• Mechanical energy from wind, rivers, tides, etc.
• Bio-fuels derived from biomass
• Heat from the earth

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

• Non-Renewables

From a theoretical perspective, if the energy content of non-renewables is taken into account, they will always have a NEG-ratio below one; but if only the extraction energy is counted, as it is normally done, it can be less than or higher than one.

To better understand this, assume an economy has a certain amount of finite oil reserves that are still underground, unextracted, thus one could theoretically account for it all, and say this economy owns x amount of energy contained in this oil. But 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 economy will own less compared to before extraction, because some had to be used up. There is no 100% efficient extraction process, therefore the NEG-ratio is always less then one, from a theoretical perspective, if the energy content of the non-renewables is accounted for.

As far as only the extraction energy being counted goes, as it is normally done, the scenario can be two ways: profitably extractable (NEG-ratio>1, NEG>0) and nonprofitably extractable (NEG-ratio<1, NEG<0) non-renewables. 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-ratio much below 1, unless suitable technology becomes available to profitably get to it.

• Renewables

The situation is different with renewable energy sources - such as hydro, wind, solar, biomass - because there is no bulk reserve to account for (other than the Sun's lifetime), but the energy continuosly 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, the number of months/years a plant has to operate until it has a positive energy balance.

In the early days of photovoltaic cells the NEG of their production was actually negative - one would have had to assume unreasonably long lifetimes before the invested energy was recovered. Today the breakeven energy recovery time (the amount of time required to recover an equivalent amount of energy as was used in manufacturing the cell) is around 2 to 5 years, compared to an effective production life of 20 to 30 years - some manufacturers provide a 25-year warranty on their products.

See also

• Energy balance
• Biodiesel
• EROEI

 

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