A great deal of energy-intensive capital investment goes into the development and expansion of energy sources, and under high-growth rates it is quite conceivable for energy-supply programs to be net consumers of energy. A very important factor here is the rate of expansion; the lower the rate, the better the net energy performance.
It appears that the rate of growth of consumer energy demand E may be a critical factor in evaluating the net energy obtainable from a system because of the strong positive-feedback energy flows F required for rapid system growth. Although net energy is not the only resource that an energy technology must supply, one can conclude that it is the most important.
It is a basic tenet of neoclassical economics that substitution is always possible, which effectively makes scarcity only relative. From biophysical systems reasoning, however, substitution of a resource by another of lower accessibility is not a situation in which the two resources can be considered separately: They exist in a systemic relationship, in which one affects the other. The increased energy requirements of access to new primary energy resources suggest that the cost of delivered energy could increase at an accelerating rate in the future and that substitution is by no means automatic. The extent to which this argument applies to the transition from fossil fuels to renewable energy sources is still an open question.
