Net Energy Expenditure: A Method for Assessing the Environmental Impact of Technologies

Martin B. Hocking

Although it is easy to compare the relative environmental merits of bicycling and driving a car, it is more difficult to use life-cycle inventory methods to compare the merits of paper towels, cloth towels, and a hot-air hand dryer, or of paper plates and china used in a cafeteria. In other words, it is often difficult to select a valid approach to compare the environmental advantages of very different technologies, even when they are used to accomplish similar functions.

Various factors such as usefulness, convenience, aesthetics, or the production of waste could be used to weigh the value of parallel technologies; however, a comparison of the net energy expenditure may be the most useful measure for judging environmental performance. This is the approach that is explored in this paper.

Even significant differences in the emission characteristics of competing technologies can be minimized by the expenditure of some additional energy on an emission-control function. For example, 2 percent or more of the energy produced by a thermal power station is consumed by its emission-control activities. Similarly, in sewage treatment plants, the processes used are primarily for emission control and are also, generally, net consumers of energy. Thus, emissions reduction is one way the environmental merits of competing technologies can be improved. Energy conservation also can influence the choice of competing "green" technologies over time.

What is significant about the choice of energy consumption as the primary factor for comparing technologies? Why not compare the consumption of renewable resources (e.g., biomass) with that of nonrenewable resources (e.g., those of fossil origin), or the consumption of a large-reserve resource base (e.g., sand, iron, or salt) with that of a small-reserve resource base (e.g., copper, silver, or

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