1. The role of money is disputed. Most economists would agree that money is simply a convenient medium of exchange, and that it must be backed by an underlying stock of physical goods. As long as money was explicitly backed by gold, this point seemed self-evident. But now that money is backed only by the 'faith and credit' of the government in power, it is unclear whether money itself is also a component of wealth.

2. Actually it is mass-energy that is conserved. But apart from nuclear fission (or fusion), matter and energy are not interconvertible on the surface of the earth. Hence, as a practical matter, each is conserved separately.

3. The case of batch processes or continuous processes with time variability requires more careful consideration. In general, however, the accounting rule holds: stock changes equal inputs minus outputs. When stock changes are zero, or can be neglected, inputs equal outputs.

4. These conditions can be very helpful in filling in missing data. For instance, chemical engineering textbooks (for example, Faith et al. 1950) tend to provide 'recipes' for standard chemical processes that specify inputs (per unit output) in some detail, but neglect to specify waste products. While a detailed chemical characterization of the wastes requires very complex model calculations or direct measurements, one can derive some useful information about the elementary composition of the wastes.

Percent of total exergy input

Japanese Trade 1920 1960

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010


Figure 3.8b Exergy input sources as percent of total exergy input (Japan, 1900-2004)

5. On the other hand, animal manure generated in large industrialized feedlots is a waste.

6. Actually 51 MMT of the 89 MMT of steel produced in the US in 1993 was recycled scrap. Domestic pig iron inputs were only 48 MMT. The two input streams add up to 99 MMT; the weight difference consists mostly of slag and CO2.

7. It can be argued that food and beverages are also service-carriers, inasmuch as they pass through the body and become wastes almost immediately, except for the tiny fraction that is retained in body mass. Even that is returned to the environment at the end of life, except for the annual incremental increase in the mass of the human population.

8. Glass is manufactured by a thermal process from a mixture of silica (sand), magnesia, kaolin and soda ash (sodium carbonate) plus traces of other metal oxides. Carbon dioxide is released. Portland cement is made by heating (calcining) a mixture of crushed limestone, clay, gypsum and other silicate minerals. Carbon dioxide is released. Concrete is made from cement, sand and other fillers, with added water. Brick and ceramic tiles are made from clay by heating to drive off water. Plaster is produced from natural gypsum by heating to drive off water, but the material is rehydrated (as in the case of Portland cement) to solidify.

9. However, in a more general physics context mass is a quantity only known from its influence. Originally the notion of mass was inferred from the observed fact of inertia. Some objects were more difficult to accelerate, or decelerate, than others. The 'something' that explained this difference was called mass (Newton's law was 'force equals mass times acceleration'). Isaac Newton applied this law to explain planetary orbits by equating the centrifugal force, proportional to mass, with the attractive gravitational force exerted by the sun, also proportional to mass. Later still Einstein proved that mass and energy are interconvertible through his famous formula: energy (E) is equal to mass (m) times the velocity of light (c) squared, probably the second most famous formula in physics. The reality of this interconvertibility was demonstrated over Hiroshima and Nagasaki in August 1945.

10. In effect, there has been a modest dematerialization of energy carriers since 1900. That is to say, the mass/exergy ratio for primary fuels consumed in the US has declined from 0.042 metric tons per tJ (teraJoule) in 1900 to 0.03 metric tons per tJ in 1995. This is an overall decline of 28 percent, due primarily to the increased use of natural gas and reduced use of coal. But, curiously, the minimum point was reached in the decade 1965-75 (0.028 metric tons per tJ). The trend has been rising since that time as coal has increased its share of the electric power generation market since the 'energy crisis' of 1973-4.

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