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Tuesday, August 14, 2007

Net Energy

As oil becomes scarce, the world needs new transportation fuels. As new fuel options develop we need means of assessing which are most effective at replacing petroleum. So far many scientists have used a measure called 'net energy'.

However, Professor Bruce Dale from Michigan State University claims, "Net energy analysis is simple and has great intuitive appeal, but it is also dead wrong and dangerously misleading -- net energy must be eliminated from our discourse." Dale's perspective is published in the first edition of Biofuels, Bioproducts and Biorefining.

Instead, Dale recommends comparing fuels by assessing how much petroleum fuel each can replace, or by calculating how much CO2 each produces per km driven.

A fuel's 'net energy' is calculated by attempting to assess how much energy a new fuel supplies, and then subtracting the energy supplied by fossil fuels needed to create the new fuel. The calculation is often carried out in a way that leaves grain ethanol with a net energy of -29%, giving the impression that it uses more fossil fuels to produce it that the new fuel supplies. Dale claims that this figure is then used by opponents of biofuels to pour scorn on the new products.

The problem with net energy, says Dale, is that it makes an assumption that all sources of energy (oil, coal, gas etc) have equal value. "This assumption is completely wrong -- all energy sources are not equal -- one unit of energy from petrol is much more useful than the same amount of energy in coal...and that makes petrol much more valuable," says Dale.

For evidence, he points to the markets, where a unit of energy from gas, petrol and electricity are worth 3.5, 5 and 12 times as much as a unit of energy from coal, respectively.

"Clear thinking shows that we value the services that energy can perform, not the energy per se, so it would be better to compare fuels by the services that each provides...not on a straight energy basis...which is likely to be irrelevant and misleading," says Dale.

For example, biofuels could be rated on how much petroleum use they can displace or their greenhouse gas production compared with petroleum. His calculations indicate that every MJ of ethanol can displace 28 MJ of petroleum, in other words ethanol greatly extends our existing supplies of petroleum. Using corn ethanol provides an 18% reduction in greenhouse gasses compared with petrol, while fibre-produced ethanol gives a 88% reduction compared to petrol.

"As we embark on this brave new world of alternative fuels we need to develop metrics that provide proper and useful comparisons, rather than simply using analyses that are simple and intuitively appealing, but give either no meaningful information, or worse still, information that misleads us and misdirects our efforts to develop petroleum replacements,"

More about Net energy.

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 - has different prices for the same kilojoule.


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 = EnergyConsumableEnergyExpended.

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

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 than one, from a theoretical perspective, if the energy content of the non-renewables is accounted for.

Due to the immense energy content in the binding energy of nuclear fuel, nuclear fuel will always have a postive Net energy gain which makes it highly desirable in terms of providing non-carbon emitting power over a long period of time.

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 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, 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.

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