Exergy As A Measure Of Material Quantity And Quality

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Almost everybody uses mass as the measure of quantity applicable to material substances. On the surface of the earth, the mass of an object is proportional to its weight, which can be measured quite easily. To be precise, weight is equal to mass times the force of gravity.9 However, mass is not particularly interesting in resource accounting, except for comparisons of changing requirements for specific materials or groups over time (as illustrated in Section 3.2), or similar comparisons between countries. Aggregate mass is also probably proportional to the energy (exergy) requirements for mining and transportation. Yet many authors have attempted to establish the importance of 'dematerialization' as a strategy for achieving long-run sustainability (for example, Herman, Ardekani, and Ausubel 1989, 1990) (Wernick 1994; von Weizsaecker, Lovins, and Lovins 1998). Other authors have attempted to justify the total mass of materials consumed by an economy as a measure of their potential harmfulness (Factor Ten Club 1994 and 1997; Hinterberger and Schmidt-Bleek 1999; Schmidt-Bleek 1993).

However, in either context, total mass as such is almost irrelevant. Most of the mass of extractive resources consists of fossil fuels, biomass or abundant and relatively inert materials such as sand and gravel, limestone and iron ore. On the other hand, apart from fossil fuels, and iron, aluminum and silicon, it is scarcer metallic elements such as copper, molybdenum, cobalt, chromium, nickel, silver and platinum, plus reactive halogens (chlorine, bromine, fluorine) that are most essential to industrial activity. And, along with combustion products and pesticides, it is comparatively tiny amounts of highly toxic by-product metals such as arsenic, cadmium, lead and mercury that dominate the environmental health literature (for example, Nriagu and Davidson 1986; Nriagu and Pacyna 1988).

Yet, for reasons of familiarity, mass is the usual - virtually universal - measure of physical quantity for all material substances used in the economic system. Clearly it is inconvenient to keep separate accounts for all the different categories of materials. This has prompted efforts to aggregate material flows, using total mass as a measure in a macroeconomic context (Adriaanse et al. 1997; World Resources Institute 2000). But the value of such aggregates is questionable, to say the least, due to the very important differences between materials as disparate as hydrocarbons, crops, inert construction minerals, toxic metals and reactive chemicals.

Table 3.1 Typical chemical exergy content of some fuels

Fuel

Exergy coefficient

Net heat. value

Chemical exergy

(KJ /kg)

(KJ /kg)

Coal

1.088

21 680

23 588

Coke

1.06

28 300

29 998

Fuel oil

1.073

39 500

42 383.5

Natural gas

1.04

44 000

45 760

Diesel fuel

1.07

39 500

42 265

Fuelwood

1.15

15 320

17 641

Source: Expanded from Szargut et al. (1988).

Source: Expanded from Szargut et al. (1988).

However it is not necessary to aggregate mass flows. As pointed out by several authors, another measure, called exergy, is available and more suitable for the purpose (Wall 1977; Ayres and Ayres 1998). Unfortunately, exergy is still an unfamiliar term, except to engineers, chemists or physicists. Exergy is a measure of potential work: specifically it is the maximum amount of work that can theoretically be recovered from a system as it approaches equilibrium with its surroundings reversibly (that is, infinitely slowly). In effect, exergy is also a measure of distance from equilibrium, which makes it a measure of distinguishability of a subsystem from the surroundings. But it is really what non-technical people usually mean when they speak of energy?

When people speak of energy consumption or energy production, it is usually exergy that they mean. The exergy embodied in a fuel can be equated approximately to the heat of combustion (or enthalpy) of that fuel. But an important difference is that exergy cannot be recycled; it is used up, or 'destroyed', to use the language of some thermodynamicists. On the other hand, energy is always conserved; it cannot be destroyed. There are several kinds of exergy, including physical exergy (kinetic energy) and thermal exergy (heat). However for our macroeconomic purposes - as in this book - only chemical exergy need be considered. The exergy content of various fuels is given in Table 3.1.

Combustion is a process whereby a substance reacts with oxygen rapidly and generates combustion products - such as carbon dioxide and water vapor - that subsequently diffuse and thus equilibrate with the atmosphere. Combustion generates heat, which can do useful work by means of a Carnot-cycle heat engine. Of course, oxidation need not be rapid. Rusting of iron is an example of slow oxidation. Heat is generated, but so slowly that it is not noticeable. But iron (like most other metals) in finely divided form, with a lot of surface area, will burn and liberate heat rapidly at a high enough temperature. Similarly, the respiration process in animals is another form of oxidation. This is why the energy - actually exergy -content of food is expressed in units of heat energy, namely calories.

There are some economically important processes that are essentially the reverse of combustion, in the sense that chemical exergy is concentrated (but not created) and embodied in a target substance. Photosynthesis is an example where exergy from solar radiation is captured and embodied in carbohydrates, which are combustible chemical substances. Carbo-thermic reduction of metal ores and ammonia synthesis are other examples. In the metals case, a metal oxide in contact with red-hot carbon is converted to a pure metal plus carbon dioxide. The exergy of the smelted metal is less than the exergy of the fuel used (for example, coke) because the combination of oxygen from the metal oxide with carbon from the coke is disguised combustion. In the ammonia case, natural gas plus air is converted to ammonia plus carbon dioxide by a series of catalytic processes at high temperatures and pressures, which also amount to disguised combustion.

There are other non-combustion processes that can do work, in principle. So when salt is dissolved in water, some heat is generated and work could be done if the heat were not rapidly diffused away. Desalination is the reverse of this diffusion process, and quite a lot of heat is required for the purpose of separating salt from water. It follows that any useful material that is present in concentrations above the average in the air (if it is a gas) or the ocean (if it is soluble) or the earth's crust (if it is neither a gas or soluble) also embodies some exergy. Thus, pure rainwater contains some exergy as compared to seawater, which has zero exergy by definition. Pure salt also contains some exergy for the same reason. Similarly pure oxygen or pure nitrogen contains some exergy, whereas the mixture that is air has zero exergy content, by definition. Finally, mine overburden has little or no exergy if it is chemically indistinguishable from the surrounding earth or rock.

Fuels, hydro-power, nuclear heat and products of photosynthesis (biomass) - crops and wood - are the major sources of exergy input to the economy. Most other materials have very little exergy in their original form, but gain exergy from fuels, as in metal reduction or ammonia synthesis. Nevertheless, the exergy content of materials is an interesting comparative measure, especially in contrast to the traditional measure (mass).

We emphasize that the exergy content of fuels and other raw materials can be equated to the theoretical maximum amount of physical work that can be extracted from those materials as they approach equilibrium reversibly. We will point out later that the actual amount of useful work done by the economic system is considerably less than the theoretical maximum. Moreover, the ratio of actual to theoretical maximum can be regarded as the technical efficiency (as opposed to economic efficiency, a very different concept) with which the economy converts raw materials into finished materials. This, in turn, as we will demonstrate later, can be regarded as rather a good measure of the state of technology. Over time, technical efficiency is a useful measure of technological progress or total factor productivity (TFP).

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