Different writers use different criteria to measure the net energy performances of processes such as these in delivering a flow of energy E to an economy (refer to figure 6.1) from a gross resource flow G, using a feedback flow F.8 The net energy criterion, in whatever form, is the means of indicating the physical accessibility of a resource. It is the amount of effort, measured in energy terms, required to extract the resource and deliver it in the appropriate form to the consumer.9
Over the past century or so in most industrializing countries, economies have gone from an almost complete dependence on biomass-based fuels (mainly wood) to coal, then to oil and natural gas. Each transition was characterized by change to a more accessible energy source, but that change was not possible until technology had developed to the stage at which the greater level of accessibility was achievable, 10 Thus, coal was not accessible in large quantities until the technology of underground mining was developed, and open-cast coal could not be won in quantity until large earth-moving machines were developed. Oil and gas were not accessible until drilling, transportation, and refining technologies had been developed. All of these depended heavily on the development of systems for long-distance transportation of both raw materials and products.
In each case, the accessibility of the new resource (measured by both net energy and economic methods) was better than for the previous one. This meant that costs were lower, so substitution, once started, could proceed rapidly. This outcome is believed by some workers from the political-economic viewpoint to be the result of factors internal to economic systems. To the extent that technological developments were required, that is true. The underlying physical accessibility mechanism is a separate component, however, and should not be excluded from the argument.
There is increasing evidence that energy is becoming more difficult to obtain and process into readily usable forms. Worldwide, oil discoveries per meter of well drilled are declining, and exploration is being done in ever more inhospitable places,11 Many countries have substantial reserves of fossil fuel (mainly coal), but the majority is usually of low quality. In other words, resources are not homogeneous, and humankind has (quite reasonably) exploited the easily accessible, high-quality resources first. Lower-quality, less-accessible resources are left for future generations, in the expectation that the price mechanism will offset any shortages.
The second law of thermodynamics tells us that as an economy changes its sources over time from high-quality (high-availability) energy sources to low-quality (low-availability) sources, the efficiency with which work may be done by those resources will decline. In addition, the move from resources that are easy to mine to those that are deeper and/or more difficult to mine means that a greater amount of economic (e. g., engineering) effort will have to be expended in order to tap those resources and supply useful net energy to the economy. In our terminology, this means that at the same time, the resources are also becoming less accessible. In both cases (refer to figure 6.1), a greater flow of either G or F or both is needed to maintain a constant flow E. In time, the energy transformation industry will have to grow and absorb a steadily increasing quantity of economic activity just to maintain the system.
If there is a steady increase in the resources that need to be expended from the economy (flow F) in order to obtain a given quantity of energy E, then in the long term, one can envisage a situation in which no net energy is supplied. In other words, the energy content (embodied in buildings, machinery, maintenance, and operation) of the economic effort needed to extract energy and deliver it could rise to such a level that as much embodied energy is flowing back into the energy supply industry as is being supplied to the economy. If F equals E, a point of futility has been reached at which all that is happening is that a lot of socioeconomic effort is going into a process equivalent (in energy terms) to going around in circles.
When resources are relatively inaccessible, the primary resource flow will be greater than it was for the earlier, high-accessibility case. Thus, the rate of depletion of primary resources will also increase for a given (constant) flow of net energy to the economy. Pollution and other externalities will also be greater due to the increased rejection of unavailable energy.
As an example of what could happen, consider a simple coal mine. Let us assume that the coal seam is vertical, starting at the surface. (Although this scenario is exaggerated to make a point, it represents very much the sort of thing that will happen in the long term, because coal that is closest to the surface will always be extracted first, leaving deeper seams until later.)
At the start of the mining operation, only simple materials and resources are needed to mine the coal and deliver it to trucks or railway wagons. As the coal face recedes, the total work that has to be done to get the coal to the surface increases because of the increased vertical height up which it has to be lifted. The greater the height, the heavier and more complex the lifting mechanism has to be, and the faster it has to run just to maintain a constant rate of delivery of coal at the surface. At this stage, a substantial proportion of the energy in the coal delivered to the surface will be needed just to build, run, and maintain the machinery needed to get the coal out. This is quite separate from the effort expended in other activities, such as pumping water to keep the mine dry and taking the precautions necessary to ensure safety at the coal face, where the working conditions are becoming ever more inhospitable.
Thus, as the depth of the mine increases, the amount of net energy delivered to the surface declines because more and more of the mine's output is needed just to drive the mining machinery itself. To ensure a constant supply of net energy at the surface (as is normal for economies), the rate of mining will therefore have to increase. It is not difficult to appreciate that the mining rate will accelerate under such circumstances. The faster the miners try to get the coal out, and the greater are the demands placed on the mine by the economy, the faster the process approaches the stage at which the whole operation is energetically pointless. Clearly, the end result is stalemate, in that the mining operation does not actually do anything for the economy other than supply employment. Beyond the point at which the net energy yield is zero, the operation is in fact taking more than it supplies. This will happen irrespective of the amount of coal left in the mine. After a certain stage, therefore, further mining is pointless and the resource is economically and energetically useless. 12
It is possible to predict from dynamic energy analysis the depth at which the work required to lift the coal exactly equals that available from the energy in the coal lifted. The lower the quality of the energy resource, the sooner this will happen. Peat and lignite will be accessible at shallower depths than will bituminous coal and anthracite. The direct and indirect energy requirements of the other parts of the mining activity will ensure that in practice, the depth that is minable will be very much shallower than that calculated from the energy requirements of lifting alone.
Many people who hold the political-economic viewpoint rely on improvements in technology at this stage to ensure that the economy will always get what it wants. Such a view is not supported by physics. It is quite possible that lifting machinery and water pumps will be improved, but there is an absolute minimum energy requirement for lifting a ton of coal to the surface and pumping a liter of water that is absolutely unalterable by technology because it is a consequence of the second law of thermodynamics. The most that can be hoped for from technology is that it will enable mining to go a little deeper; the constraint remains.
The further question may then be asked of whether we can envisage making more radical changes in technology. For example, would underground gasification enable the coal resource to be made available with a lower expenditure of feedback energy? The answer is not clear because of so many unknowns. However, we know that such methods are unsuitable for many coal seams (e.g., those with geological faulting) and also incur large increases in depletion rate, wastage, and pollution due to the low efficiency with which the coal is converted into gas. The capital structures needed to carry out such operations would also be substantial.
There is a considerable literature relating to the "breeder" type of nuclear reactor, which has been claimed to produce more fuel than it consumes. For some people, this possibility has engendered confidence that humans can continue to treat the world's resources as limitless. In fact, the claims are illusory, resulting from a misunderstanding of some fairly simple facts of physics. The fuel for conventional (fission) nuclear power plants is the uranium isotope U-235. But the most common form of uranium in the world (around 99 percent of known reserves) is the nonfissionable form U-238. To obtain fuel for nuclear plants, the small quantities of U-235 have to be separated out, producing an "enriched" uranium.
It was soon realized that if a neutron from a fission reaction is captured by an atom of U-238, it is converted into plutonium 239, a highly fissionable material. The so-called breeder reactor (of which a few prototypes exist) converts nonfuel U-238 into fuel Pu-239. It does not make fuel out of nothing; it is simply a resource-upgrading plant that embodies energy in one material in order to turn it into a more "useful" one. All that the process does is multiply the known reserves of fissionable material by a factor that is substantial but by no means unlimited. That Pu-239 is an extremely dangerous material, and one that is very suitable for making nuclear weapons, is a further reason for us to be on our guard against sloppily worded claims as to the importance of the breeder option. The further fact that breeder reactors are both costly and troublesome to operate is yet another reason for caution.
The "ultimate," unlimited form of energy is believed to be nuclear fusion. In this, deuterium, a naturally occurring isotope of hydrogen, would serve as fuel for plants that would duplicate the reactions that occur in the interior of the sun--at temperatures of millions of degrees. Although claims of breakthroughs in this area have been with us since I left school some thirty-five years ago, fusion reactor programs have so far produced nothing more than the ability to absorb vast amounts of money. They have also absorbed vast amounts of energy in construction and operation of ever more elaborate experimental machines. Whether they will ever show an economic or energetic benefit nobody knows, but even if they do, the results will be available only to a few rich nations; fusion power will almost certainly be too expensive (and too dangerous) for anybody else. This option involves too many unhatched eggs to be part of realistic plans.