Net Energy Gain is an important concept in energy economics, referring to the difference between the energy required to harvest the energy source against the energy provided by using that source.
Examples
As an example, 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 NEG reaches unity, then that source is no longer contributing energy to an economy.
In the early days of PV cells the NEG of their production was actually below 1. Today it is above 1 but no-one knows by how much as the effective productive life of a PV cell is not known. The most modern estimate is that after 2 years service a PV cell has equalled the energy required to manufacture and install it.
Calculating NEG
The term net energy gain can be used in slightly different ways: if the energy content of non-renewables is taken into account, they will always have a NEG below one; If only the extraction energy is counted, it can be higher.
Crucial for the NEG is the life cycle of the product: if it is defunct after 10 years, its NEG will be significantly lower than if it works for 30 years. Therefore, 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. To calculate the NEG for society as a whole, in theory all externalities would have to be taken into account.
Comparing technologies
From an older study (1991) of the German "Forschungsstelle für Energiewirtschaft":
| Type of Power Plant
| Payback (Months)
|
| Nuclear
| 0.7
|
| Coal
| 0.7
|
| Wind:
|
| Wind @ 7 m / s
| 2.5 - 7.5
|
| Wind @ 5.5 m / s
| 3.8 - 11.4
|
| Wind @ 4 m / s
| 6.3 - 22.7
|
| Photovoltaic:
|
| Monocrystalline @ 1000/2200 W / m2
| 44/87
|
| Multicrystalline @ 1000/2200 W / m2
| 43/85
|
| Amorphous @ 1000/2200 W / m2
| 28/56
|
See also