gold), or the use of low-polluting-potential materials (e.g., iron, glass, or cement) with that of high-polluting-potential materials (e.g., lead, mercury, or thallium)? Energy consumption is a valid comparative factor because all of these other distinctions are based ultimately on energy expenditure. Oil, gas, and coal are nonrenewable only in the sense that the energy cost of producing them synthetically from biomass or other carbon sources is 3 to 10 times their current prices. Copper, silver, and gold are more expensive than iron primarily because it takes much more energy to isolate and recover them. Also, these materials are not consumed during use, although some uses may dissipate them. An ounce of gold finely distributed throughout 90 tons of gravel is not worth $400, although at this concentration (about 0.3 mg/kg) it may be worth processing. Until the energy has been expended to recover it, however, it is only worth a very small fraction of $400.
To compare technologies using the energy assessment method, appropriate boundaries for the technologies must be defined. During the course of the assessment, additional factors may emerge that require adjusting the boundaries to maintain fairness in the inventory.
Data assembled by Chapman et al. (1974) for automotive transportation and home heating illustrate the importance of appropriate and fair selection of boundaries for a valid comparison of different technologies. For example, the gasoline-powered car has an engine transmission combined efficiency of about 0.2 (Figure 1). The electric car, with a 0.8 battery charge—discharge efficiency and a motor control system—transmission efficiency also of about 0.8 gives a net system efficiency of 0.64, apparently much higher than the gasoline-fueled car.
If the energy-producing facilities for each of these technologies are included in the calculation, however, a very different picture emerges (Figure 1, solid boundaries). Oil production, refining, and delivery systems are estimated to be 0.88 efficient at retaining the energy originally present in the crude oil. Multiplying this by the fractional efficiency of the gasoline-powered car produces an overall efficiency for this system of 0.17. The relatively low efficiency of thermal electricity generation coupled with losses in fuel production and delivery, and electricity lost during transmission, gives an overall efficiency for delivered electricity of about 0.24. Combining this information with the higher efficiency of the electric automobile drive system gives an overall efficiency of 0.15, quite comparable to the overall efficiency of 0.17 calculated for the gasoline-powered system.
This example shows the considerations that enter into establishing boundaries for the conversion of energy into work and demonstrates the kinds of surprises that this exercise can reveal. An examination of appropriate boundaries for