We are at the beginning of a great transition in our energy regime, in which so-called renewable energy sources are in the process of replacing fossil fuel sources of all types (coal, oil, natural gas). Consequently, there is much discussion about the relative merits and demerits of the various energy sources. A key idea underlies this discussion but is seldom mentioned: the concept of energy returned on energy invested (abbrev. ERoEI). Perhaps it is omitted from discussion because it is not widely understood, or, more likely, that it is discouraging for producers and supporters of oil from tar sands and shale. These, of course, are the oils produced at the present time in Canada and the United States. A basic explanation of the ERoEI concept goes like this. Suppose that in a particular oil field in, say, Saudi Arabia, it is found that the investment of one barrel of oil in the production process results in the production of 100 barrels of oil. In this situation, the energy invested (EI) is the energy content of 1 barrel of oil. The energy return (ER) on this investment is the energy content of 100 barrels of oil. ERoEI is simply the ratio of ER to EI: ERoEI = ER/EI= 100. Of the 100 barrels of oil produced, 1 barrel must be considered to offset the 1 barrel invested, leaving 99 barrels that were essentially obtained “for free”. An ERoEI of around 100 was typical of oil fields at the very start of the oil boom, in the latter half of the 19th century. Today, ERoEI for a typical traditional oil field (a field that does not require the use of fracking) is in the vicinity of 40. This falls far short of the early ratios, but still represents a profit of 39 barrels of oil per barrel invested.
Of course, it is quite easy to define ERoEI. It is also relatively easy to measure ER. Unfortunately, EI is much more difficult to measure. It includes contributions from the exploration process (labor, equipment, transportation); from the mining process (construction materials, construction labor, transportation), from the transport of crude oil to refineries; from the refining process; and from the transport of the refined oil to where it is used. Determining accurate numbers for all of these contributions is labor-intensive, and undoubtedly results in a value for EI that is subject to some degree of error. Nonetheless, this effort has been made for most of our current energy sources. The results follow:
Energy Source ERoEI
Hydroelectric 100
Natural gas 67
Coal 60
Conventional oil 20-40
Wind 18
Nuclear 5-15
Solar (PV) 6-12
Shale oil 6
Tar sands oil 4.5
Corn ethanol 0.8-1.6
Please keep in mind that these are approximate numbers. It would be reasonable to assume an error of 20% in each. That said, the data suggest several conclusions. First, hydroelectric provides by far the largest energy returned per unit of energy invested. Unfortunately, expansion of hydroelectric capacity in the US is not possible, as most suitable rivers have already been fully exploited. Second, both shale oil (US) and tar sands oil (Canada) have quite small ERoEI, because so much energy must be invested to produce this hard-to-access form of oil. These energy sources, though much-hyped recently, are clearly the bottom-of-the-barrel; it is only slightly worth the effort to produce them. Third, solar photovoltaic and wind renewable sources have reasonable values of ERoEI which will increase the longer a wind turbine or solar panel is in operation. This is because the energy invested (EI) is all up-front. Once the solar panel or wind turbine is in place, it will produce energy for decades with very little expense for maintenance. Fourth, the conventional fossil fuels clearly have quite high values of ERoEI. Unfortunately they are also highly polluting. The mining and transport of natural gas releases significant amounts of methane to the atmosphere. Methane is a much more potent greenhouse gas than carbon dioxide, so these methane releases exacerbate the global warming problem. Of course, the combustion of the gas produces CO2. Coal has been recognized for years to generate pollution in the form of heavy metals, noxious gases, soot, and radioactive particulates, and when burned generates more CO2 per unit of energy produced than any other fossil fuel. There is no future for coal as a source of energy. Fifth, corn ethanol production should be abandoned as nonviable. An ERoEI of 1 means that all of the energy produced must go to offset the energy invested, with none remaining for other societal uses. It is not worth producing an energy source with ERoEI of 1 or less. With ERoEI of 0.8, corn ethanol produces less energy when burned than is required to produce it.
Following the link below will provide you with an interesting graphic presentation of the ERoEI concept. I will leave that for you to study. The graph very clearly reveals that at ERoEI of 1 or less, all energy produced must be used to find more energy, with none left for other purposes.
http://images.dailykos.com/...