Wednesday, July 22, 2009

EROEI - Energy Returned on Energy Invested

EROEI (Energy Returned on Energy Invested), ERoEI, EROI (Energy Return On Investment) is the ratio of the amount of usable energy acquired from a particular energy resource to the amount of energy expended to obtain that energy resource. It is an important metric to decide on the strategy for the transition to renewable energy.

The Oil Drum has great posts on EROEI.

As production from fossil fuel deposits starts to decline, we are being forced to procure energy from other sources such as wind, tidal, solar, bio-fuel and nuclear. It is important to understand the energy efficiency of new energy procurement systems, if industrial society as we know it is to survive the next great energy transition away from fossil fuels.

The Energy Return on Energy Invested (ERoEI) provides one measure of the efficiency of energy procurement and is quite simply defined as:

Energy procured / Energy used to procure energy

The chart shows how the proportion of net energy available for society to use varies with ERoEI. There is in fact much uncertainty in the data displayed and many large gaps in knowledge. The shape of the curve shows that for ERoEI above 10, the bulk of energy procured is available to society – to power industry, transportation, schools and hospitals. With falling ERoEI below 10 there is an exponential increase in the amount of energy required to procure energy with a corresponding decline in net energy available for society.

High per-capita energy use is considered desirable as it is associated with a high standard of living based on energy-intensive machines. A society will generally exploit the highest available EROEI energy sources first, as these provide the most energy for the least effort. With non-renewable sources, progressively lower EROEI sources are then used as the higher-quality ones are exhausted.

For example, when oil was originally discovered, it took on average one barrel of oil to find, extract, and process about 100 barrels of oil. That ratio has declined steadily over the last century to about three barrels gained for one barrel used up in the U.S. (and about ten for one in Saudi Arabia). The EROEI of wind energy is about 20:1 which is driving its adoption.

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