Fleeing Vesuvius. Gillian Fallon
more generally, the start of the industrial revolution.
Coal represented something new. For the first time, energy from a different time period was accessible and, more importantly, available on a larger time scale. Before coal, the available energy was limited to the proportion of the transient energy flows the technology of the day could capture. Coal (and the other fossil fuels) enables us to access a stock of energy sequestered over millions of years in the distant past and, to release that energy over a few short centuries.
Just as tools enabled early man to exceed the physical limits of his body by focusing the energy of his muscles, fossil fuel enables us to use more energy than we could obtain from current natural flows by tapping into vast stocks of ancient energy. The rate at which we are drawing down this ancient stock can only lead to its depletion. The characteristics of this depletion are already becoming apparent, years before its total exhaustion. As the stock diminishes, it becomes harder to extract energy from it. In other words, more energy is required by the extraction process, which reduces the net energy available to society.
Net Energy
A tree must gather more energy from the sun through its leaves that it expends constructing the foliage. Similarly, a fox must gain more energy consuming the hare than it took to chase it down. Our exploitation of fossil fuels is no different. In order to extract fossil fuels and utilize their embodied chemical energy, the amount of energy expended must be less than the amount we get to use. In the early days of its exploitation, a resource is abundant, easily discovered and takes little energy to extract. The principle of “best first” is adopted automatically, so the large coal seams near the surface and the large onshore oil fields are both the first to be discovered and easiest to exploit. This ease of exploitation results in large amounts of net energy as relatively little energy needs to be expended to extract the fuels.
As the resources become depleted, however, the task becomes harder. In the case of oil, new extraction is increasingly coming from deep-water deposits. The recently announced Keathley Canyon discovery in the Gulf of Mexico is under 1,259 m of water and the well depth is 10,685 m below the sea bed;1 that’s a greater distance below the surface of the earth than Everest rises above it. Unconventional resources such as shale oil and Canada’s tar sands require the use of a lot of energy to produce a useful product while coal-to-liquids, biofuels and gas-to-liquids require a great deal of post-extraction processing before the fuels can be used.2 The net energy — the energy return on invested (EROI) — delivered by all these processes is much less than the return from, say, the first oil fields in Texas.
Illustration 2 summarizes the concepts of surplus energy and the EROI ratio. Eout represents the magnitude of energy available after the energy extraction costs, Ein, have been accounted for. This is the energy available to society.
Energy Has an Energy Cost
FIGURE 2. An energy source can rarely be used directly. An energy extraction process is required to discover, extract and process the resource before its energy is available to society. This process consumes energy itself, a deduction from the energy otherwise available. The energy return on invested is the ratio of surplus energy to energy required to drive the process.
EROI is a dimensionless ratio. If the extraction of 50 barrels of oil takes the energy equivalent of 1 barrel of oil, the ratio is 50:1 and 98% of the embodied energy in the source is net energy available to society. This ratio has dramatically declined over time. Professor Charles Hall at the State University of New York has calculated that for oil extracted in the US:
The EROI for oil...during the heydays of oil development in Texas, Oklahoma and Louisiana in the 1930s was about 100 returned for one invested. During the 1970s it was about 30:1, and from about 2000 it was from 11 to 18 returned per one invested. For the world the estimate was about 35:1 in the late 1990s declining to about 20:1 in the first half decade of the 2000s.3
This decline has occurred almost invisibly as total extraction has increased. This has been possible as the decline from 100:1 to 30:1 to ~11–18:1 only represents a move from 99% energy availability to 97% to 93%, a trivial change in the face of the magnitude of total production which increased almost four-fold. There has been a large increase in net surplus energy compared with a small decrease in the EROI. However, projecting forward, this is not a linear system. Illustration 3 illustrates how the net energy available declines rapidly as the EROI continues to fall.
Impact of Declining EROI on Energy Availability
FIGURE 3. When an energy resource’s EROI is high (>10) then most of the gross energy is available as net energy to society. Over time the EROI declines; however, the net energy does not fall significantly until the “knee” of the curve is reached at approximately 5:1. Once the knee is reached, a rapidly increasing amount of gross energy is consumed by the extraction process itself until it is no longer energetically profitable to continue to extract the resource.
Very low-EROI sources (Canadian tar sands, for example, at <5:14) are already being used; their exploitation is sustained through energy cross subsidy from high EROI sources like natural gas. Large volumes of water (2–4.5 barrels of water for every barrel of synthetic crude) are also required in this case so it is likely that extraction rates will not depend on the tar sand resource at all but rather on other inputs.5 This works in the short term, for a small volume, and whilst the gas and water is available, but does not guarantee the continued exploitation that some assume going forward.
Calculating EROI
Calculating EROI is not simple, largely because our current system is denominated in monetary terms, not energy terms. Two significant challenges are energy quality and system boundaries.
To a physicist, energy is a simple concept. Measured in joules (after 19th-century physicist James Prescott Joule), it quantifies the amount of work performed on the environment; work against gravity to raise an object, work performed to increase the temperature or velocity of an object, for example. Quality does not come into it. However, for practical applications energy can be considered to vary in quality, complicating direct comparison. The ten megajoules of chemical energy released as heat when 3 kg of coal is burnt cannot power a television for a day because the heat cannot be used directly. An indication of relative energy quality can be obtained from market price. The price for a megajoule of electricity is typically around three times higher than that of a megajoule of natural gas, and represents a willingness to waste as much as two-thirds of the primary energy in the gas when converting it to a higher quality energy, electricity.
System boundaries are particularly troublesome. A simple analysis may look at an oil well and consider the electrical energy used to pump the oil from beneath the ground compared with the energy content of the resulting oil. This is reasonable, and returns the EROI on the day the measurements were taken. However, energy will also have been expended in discovering the oil field, drilling the well (including the three preceding dry holes) and in the manufacturing and transporting of the pumping equipment itself. This will produce a fairer result because to extract oil, one must first discover it. This line of thought can be extended to include the energy costs of the petroleum engineer’s education, food and health care.
Finally, simply producing surplus net energy with an EROI ratio greater than one still is not enough. A barrel of oil at the wellhead cannot be used as it stands. First, it must be refined into products such as petrol or diesel and transported to where it is required. Secondly, the infrastructure with which to use this fuel must be manufactured; the cars, trucks and the very road surface upon which they travel.
The energy used to extract the energy is only one part of the picture. Further energy must be expended in order to use the energy. Too often EROI discussion is centered upon whether a proposal is greater than unity, whether it breaks even and provides a net energy surplus. This breakeven point is not nearly enough though. If our energy system were merely to break even, human civilization would do