Energy Returned Over Energy Invested
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Description
Energy Returned Over Energy Invested, or EROEI, is the ratio of energy spent accessing an energy source to the energy gained from the energy source. The EROEI of different energy sources varies widely, and tends not to be linked with their economic worth.[1]
Detailed description
For finite resources, such as fossil fuels, the EROEI tends to decline as the reserves that are easier to get to are tapped first. Supposedly the EROEI of oil extraction was at one time around 100:1. For substances with lower energy density, this can vary widely between regions since transporting the fuel over long distances can be prohibitively expensive.
The EROEI also tends to decline if the resource in question is used or processed in a different manner. Using a resource such as oil directly would result in a much higher EROEI than refining it and using it's energetic products because each alteration of it can only reduce the net energy available from it and consequently it's EROEI. For example, in the state of California roughly two million barrels of oil per day, or around 700 million barrels of oil per year, were refined in the late nineties[2], and in doing so roughly 37,000 million kWh of energy[3] was used. As a result the EROEI of oil given it's current uses is at most about 30:1.
Because of the differences in use, comparing the EROEI of a resource during different time periods can result in at best uninformative, and at worst, disingenuous results. Energy quality is also a consideration since energy that has a high efficiency of conversion can result in more useful work done. An example is the electricity from wind power compared to the diesel fuel from oil refining. While under optimal conditions they may both have an EROEI of around 30:1 before use, the electricity can be converted to mechanical work with roughly twice the efficiency of the diesel fuel, resulting in more work done per unit energy invested. In terms of physics we would state that electricity has greater exergy, or usable amount of energy given our application, than diesel fuel. Conversely, the diesel fuel could be said to have greater entropy than the electricity with respect to the mechanical work being done.
Here is a list of EROEI according to energy source.
