An Engineering Approach to Identifying Key Environmental ... - Informit

1986 National Environmental Engineering Conference Melbciurne 17-19 March 1986

An Engineering Approach to Identifying Key Environmental Factors: The Environment as a Set of Heat Engines T.L. LUSTIG Partner. Environmental Management. Kensington N.S.W

1. INTRODUCTION

A heat engine can be defined as a mechanism which utilises energy and materials with low entropy (resources) and transforms them into energy and materials with high entropy (waste) in the process of doing work. Environmental processes can in general be seen to be heat engines, each having a source of low entropy (often the sun), each producing work (eg. blowing, flowing, growing) and each producing waste.

hot reservoir

reservoir

By considering an environmental problem in terms of its thermodynamics, we can often elicit those processes or effects which are important.

2. ENTROPY AND HEAT ENGINES 2.1. What is Entropy?

Figure 1. Schematic diagram of heat engine which uses energy Q to produce work W and waste Q-W.

Entrcrpy can be defined in several ways. One definition which should suffice here is as a measure of the unavailability of energy for doing work (Georgescu-Roegen, 1971). If entropy is high, the energy is not very useful as a resource. For example, there is an enormous amount of energy in the ocean, but little of it can be used, since the temperatures are mostly too low to run generators. We say that this energy has high entropy. (Strictly speaking, we should say that it has a relatively high entropy, since the temperature differences from its surrounds is low. Were there some way of exploiting the temperature difference between tropical and polar sea waters, we might say that the energy in the tropical seas has relatively low entropy.)

e Entropy S

2.2 Heat Engines

A heat engine is often depicted schematically as in Figure 1. The simplest sort of engine is comprised of a hot reservoir (a source of energy with low entropy), a cold reservoir (a sink for energy with high entropy), and a mobile medium (a container of gas, chemicals, a magnetic system, an electrical system etc.) which shuttles between the two reservoirs thereby producing work. (More complex heat engines, which take account of changes to materials as well as to energy can be formulated if necessary (Lustig, 1983).) The operation of the heat engine is taken to be a cycle, often depicted as in Figure 2, the axes can be S-T (entropy vs. temperature), P-V (pressure vs. volume), p-N (chemical potential of component vs. amount of component) etc. The steps in a cycle are as follows:-

Figure 2. S-T diagram showing cycle of heat engine.

1. A-B: The medium contacts the hot reservoir and takes in heat at a temperature somewhat less than that of the reservoir. 2. B-C:

The medium leaves the reservoir and (nearly) adiabatically (ie. at nearly constant entropy) changes state (eg. expands - if the medium . is a gas), transferring energy out of the system as useful work. 3. C-D: The medium contacts the cold reservoir and emits heat at a temperature somewhat more than the temperature of the cold reservoir. 4.

D-A: The medium leaves the

reservoir

and

adiabatically changes state (eg. compresses), taking in energy from the store of useful work, and increasing temperature to nearly that of the hot reservoir. The net work put out by the engine is proportional to the area ABCD in Figure 2. 3. ENVIRONMENTAL HEAT ENGINES In viewing environmental processes from a thermodynamic perspective, it is often useful to consider them as falling into one of three categories: - abiotic (non-living) processes;

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biotic (living but not human social) processes;

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and (human) social processes.

3.2. Biotic Processes An ecosystem can be viewed as a heat engine, where the hot reservoir is the sun and the cold reservoir is the albedo for enrgy flowing through the system, and is a convenient low point such as the sea for the materials. The removal of (high entropy) waste heat and materials to the "cold reservoir" is normally by several pathways, such as by radiation, evapotranspiration, erosion, leaching and transport of nutrients. 3.2.1. Effects of environmental impacts on entropy production Some adverse impacts such as clearing vegetation can be seen to entail an increase in entropy production, not only for abiotic processes, but biotic ones as well.

3.1. Abiotic Processes Abiotic processes can be seen to be examinable thermodynamically, in that processes such as the hydrological cycle, erosion, sedimentation, open-channel flows, thermal currents and natural chemical processes can all be seen to involve a change in entropy, and for abiotic processes at least, adverse environmental impacts appear to be accompanied by an increased entropy production. (Entropy production is the rate of increase of entropy .)

Let us consider what happens to the energy from the sunlight which shines on a tree. Approximately 95% of the energy is reflected or reradiated back into space. The remaining 5% is used for photosynthesis (Odum, 1971). The energy stored as biomass in the tree will remain for many decades, until the tree dies. (Once the tree dies, most of the energy will find its way out to space within a relatively short time.) The overall rate of increase of the entropy from when the energy leaves the sun to when it is finally dissipated into space is quite low, since it has taken many years.

Some examples are: erosion

the soil particles lose potential energy, thus gaining entropy;

channel straightening the rate of loss of potential energy of the water is increased: water pollution mixing of impurities with water can be shown to result in an increase in entropy; air pollution as for water pollution; adverse impacts on hydrological cycles eg. increasing runoff, reducing evapotranspiration (by clearing trees etc.), reducing groundwater storage, draining swamps result in an accelerated entropy production. (Hydrological cycles can be looked on as massive heat engines powered by the sun (the hot reservoir) and which serve to transport waste heat from the earth's surface to outer space (the cold reservoir) (cf. Tsuchida and Murota, 1979).) By the same token, works which serve to reduce the entropy production are often regarded as beneficial to the environment. Some examples of these are retarding basins, groundwater recharge, rehabilitation of swamps, reduction of erosion, and reduction of air and water pollution. Recycling can also be considered in thermodynamic terms. In such cases, the high entropy wastes have some properties which can be regarded as having low entropy with respect to other processes. One of our most ubiquitous examples is our use of a waste product from plants, oxygen.

By contrast, if the tree is cleared and replaced with a grass such as sugar cane, the energy will be stored as biomass for perhaps one or two years at most. The rate of increase of entropy is then perhaps one or two orders of magnitudes higher than before. Hence, the clearing of vegetation can result in a greater entropy production. However, not all adverse environmental effects are accompanied by an increased entropy production. For example, the invasion of weeds will often result in a decreased entropy production, particularly if the new plants are eaten less than the original ones. In such cases, there is an additional effect to be taken into account, namely the disturbance to the ecosystem as a whole. 3.2.2. Ecological fitness This term is somewhat different from the concept of Darwinian fitness, which refers to the potential for a particular species to survive. By ecological fitness is meant the potential for an ecosystem to maintain itself (Lustig, 1983). Ecological fitness has two components, which might be described as "present fitness" - the ability to withstand short-term but extreme perturbations - and "future fitness" - the ability to adapt to long-term semi-permanent environmental shifts. "Present fitness" can be represented on an entropy-temperature (S-T) diagram as a kind of thermodynamic "fat", being the area between Curve A and Curve B in Figure 3. The inner curve (Curve A) represents work cycles by the ecosystem (to produce biomass), while the outer curve (Curve B) indicates the environmental conditions (resources, temperatures) that the ecosystem must survive within. As long as Curve B does not cross. Curve A, the ecosystem should survive. If the environmental conditions change too much (Curve C), the ecosystem

is liable to degrade. The "future fitness" of an ecosystem can be considered in much the same terms as the alterations to a set of heat engines. If we represent the thermodynamic processes of an ecosystem as Curve A in Figure 4, we see that it could also be depicted as a set of component heat engines which operate both in series and in parallel. Evolution can be looked on as an adding or subtracting of one or more of these components (Curve B in Figure 4 ) . so that if environmental conditions alter sufficiently slowly, the ecosytem has a chance to adapt by adding or subtracting appropriate components. The future fitness of an ecosystem can thus be defined as the rate of change of the (thermodynamic) environmental conditions that it could cope with.

Let us consider a heat engine as in Figure 1. If it operates without friction or wear (as is normally assumed in thermodynamics), then with a constant load and constant temperatures of the hot and cold reservoirs, it would complete each cycle in the same time, since the period would depend only on the transmissitivities of energy across its boundaries and the inertias of its components. This heat engine would thus be a perfect clock. However, we would not be able to use this clock to indicate the direction of time: we could never tell which of two snapshots of the engine was taken earlier, since it would always look the same. Let us now think of a different heat engine, which has friction and wear, and thus requires repairs and maintenance. This clock will not keep perfect time, since a steady-state cycle will be impossible to maintain indefinitely. On the other hand, this engine could be used to give us the direction of time, since we could detect increasing amounts of wear and/or repairs as time passed. We see then, that depending on its characteristics, a heat engine can be used to depict two aspects of time, the time period represented by its mechanical action, and the time sequence represented by the changes or transitions it undergoes.

B

1

Personal time can be seen to be homologous to the behaviour of heat engines (Lustig, 1983) through the well-known connection between entropy and time (Eddington, 1935).

"fat"

Figure 3. Heat cycle of ecosystem (Curve A) operating. . within thermodynamic boundaries of environment (Curve B) . Thermodynamic "fat" provides a degree of resilience, but ecosystem could not withstand an environmental shift to as much as Curve C.

It can also be useful to consider personal time in terms of a "mechanical'' aspect and a "transitional" aspect. "Mechanical" uses of time would be such as with machine-like activities (eg. monotonous . work, commuting), or being in machine-like environments (eg. perceiving oneself to be a small cog in a big machine). In such situations, time hardly seems to pass, and like with the frictionless heat engine, we have difficulty in distinguishing one day from the next. Stimulating or creative activities could be taken.as examples of "transitional" uses of time. When we are occupied in something creative or stimulating, we are often unaware of how much time has passed. However, as with the heat engine that does wear, we usually know which of two events preceded the. other.

Figure 4. Heat cycle of ecosystem (Curve A) is in reality made up of many smaller heat cycles which interact with each other. Ecosystem adapts to environmental change by adding an extra component, resulting in alteration of heat cycle to Curve B. 3.3. Social Processes If we make the (somewhat misleading) assumption that most people in our society have adequate food, clothing and shelter, we might consider social impacts in terms of the effects on people's personal time .

Adverse social impacts tend to involve a shift from "transitional" uses of time, which tend to be interesting and enjoyable, to mechanical uses of time, which tend to be boring or even alienating. However, it is not proposed that we should assess such shifts each time we wish to evaluate the social effects of an engineering works. Not only would it be an extremely complex procedure (Lustig, 1983), the effort would not be justified in most practical problems. 'Instead a somewhat less accurate index, the loss (or gain) in enjoyable times which would result from a project, appears to be a generally satisfactory approximation for evaluating the adverse (or beneficial) effects of most engineering schemes. It is also simpler to assess. For example, if smoke from a nearby factory enters the backyard of someone who is thereby prevented from having a barbeque, the social impact might be roughly assessed in terms of the pleasurable times forgone by those who would have participated.

1

1

4. EXAMPLES OF IDENTIFYING KEY ENVIRONMENTAL FACTORS A few examples of how a thermodynamic perspective has helped the writer identify important environmental factors might serve to illustrate its usefulness. 4.1. Identifying a key abiotic factor A resort development is being proposed for a town whose main industry is tourism, and which lies within two hours' drive of the CBD of a major city. The development would have both overnight and day visitors and is expected to have up to 8000 visitors/day

.

This would generate sewerage effluent equivalent to that of a permanent population of 2-2500 people. However, the sewerage system of the town is overloaded and would be very expensive to upgrade to take this extra load. The site is in a natural amphitheatre, and as part of this development, a lake is to be constructed at the bottom of the site to improve the aesthetics and for passive recreation. The watercourse draining the site leads to a waterfall which is an important tourist attraction. As a result of past poor planning, this waterfall is polluted. When considering the overall planning of this project, the writer gave particular thought to the sewage effluent, inasmuch as it was a potential resource whose entropy would increase quite rapidly if fed into the nearby sewer. (The treatment works are at a much lower elevation than the site.) It was suggested to the client that the sewerage be treated on the site to the same quality as that of local unpolluted waterways, then pumped back up to the topmost sections of the site. From there it could be allowed to run down through a series of swamps, cascades and ponds to the lake. This would have the following advantages:1. It would be much cheaper than using the existing sewer.

2. It would be utilising scarce resources more fully. (The restrictions.)

town

has

often

had

water

3. The aesthetics of the site would be enhanced.

4. It would contribute to reducing the degree of pollution in the waterfall downstream.

5. It would help maintain the level of the lake, which is important for aesthetics and for the flora and fauna which is planned for it.

6. The swamps, cascades and ponds would act as a fourth stage of treatment of the sewage. 4.2. Identifying a key biotic factor. I

The writer has been involved in a project to examine the feasibility of and identify reference catchments in Eastern N.S.W. (Fatchen and Lustig, 1985; Lustig and Fatchen, 1985). Such reference areas are being considered for preserving ecological benchmarks, and for comparisons when assessing the degree of disturbance, or alternatively success in rehabilitation of other sites. Part of the investigation required criteria for determining which catchments were representative of which other catchments. In pondering what would be

suitable criteria, the writer thought about each ecosystem as a heat engine with its component parts (plants, animals etc.). For engines in general, if one wished to assess if two engines were equivalent, one would examine their structures and processes. One certainly would not be looking at who manufactured all the components, except perhaps as indications of their likely qualities. It was realised from this that in the same way, those heat engines which we call ecosystems, and which are operating as reference catchments, would be representative of areas with similar structures and processes, but not necessarily with the same species. (This may not seem a dramatic insight, and we would not claim it to be so. However, we were surprised to find how many ecologists had difficulty with this idea. We were freauentlv admonished. advised or criticised about some aspect of our study in terms which indicated that the particular ecologist was looking at the problem from the viewpoint of species rather than ecosystems (eg. the need to preserve a certain species, a function not intended for these reference catchments).) It followed then that a sufficient criterion for an undisturbed area to be set aside as a reference catchment, was that it should be ecologically fit for its environment. Hence the parameters that were ultimately used to classify areas into groups were environmental parameters or indicators of environmental parameters (Fatchen and Lustig, 1985). 4.3. Identifying a key social factor The writer was recently engaged in assessing the social and economic effects of a levee proposed for South Grafton. In the normal cost-benefit analysis for this sort of study, one would tot up the benefits and costs in monetary terms, apply some discount rate and declare whether or not the benefits exceeded the costs. Intangible costs such as loss of life, sickness, disruption to social activities, and cleaning up afterwards would be assigned equivalent monetary values for such an evaluation. Such monetary equivalents are the source of much dispute: those in favour of such a project will tend to argue for a low value of time, while opponents will often want it higher. Moreover the theoretical arguments among economists concerning this problem are ubiquitous in the literature (eg. Layard, 1972). To get around this, the writer reasoned that the key factor in these intangible costs was essentially the loss of enjoyable time. A rough criterion of what constituted an intangible cost was adopted: activities entailing a monetary exchange were considered in the benefit-cost analysis, while activities or losses entailing essentially no monetary exchange were totalled simply in terms of lost enjoyable time. The two results were presented separately (Environmental Management, 1985). It might be noted that a similar approach, of separating time from money, has been put forward by economists (Becker, 1965). The advantage in such a presentation is that any trade-offs between monetary costs and time costs by the decision-maker are made plain. The alternative of assigning monetary values first has the

disadvantage of giving a false air of objectivity to an exercise which, in the final analysis can only be subjective, since the economic analyst must subjectively determine what the monetary value of time is.

Such a perspective can provide a consistent means of eliciting important factors in environmental problems.

5. DISCUSSION

Becker, G.S. (1965) A theory of the allocation of time. The Econ. J., 75, 493-517.

It can be seen that the environment is made up of what can be described as a set of heat engines, mostly powered by the sun. Effects on living and non-living systems can thus be assessed in terms of the changes to thermodynamic processes. Likewise, social impacts are largely those which result in losses (or gains) of enjoyable time, which might be looked on as shifts from (or to) stimulating or creative activities to machine-like uses of time. The examples have illustrated how a thermodynamic perspective can help elicit pragmatic approaches to environmental problems. It is not claimed that this perspective will necessarily bring out solutions to problems which would not have been thought of without it. Indeed, it would be a simple matter of showing where other workers have come up with similar ideas for solving their own problems. What is claimed however, is that if an environmental engineer were to look on the environment much as a mechanical engineer would look on a heat engine, key factors should be able to be discerned in a consistent and systematic manner. Till now, environmental engineers have had to rely on experience gained by the profession with similar projects, to determine what the important factors are. From the writer's own experience, this experience can be augmented with a thermodynamic viewpoint.

6. CONCLUSIONS It can be shown that it is valid to have a perspective on the environment which views the environment as a set of heat engines.

REFERENCES

Eddington, A.S. (1935) The nature of the physical world. Dent, London.

Fatchen. T.J. and Lustig, T.L. (1985) Choosing reference catchments in Eastern N.S.W. National Parks and Wildlife Service of N.S.W., Sydney. Georgescu-Roegen. N. (1971) The entropy law and the economic process. Harvard U.P., Camb., Mass. Layard, R. (Ed.) (1972) Penguin, Harmondsworth.

Cost-benefit analysis.

Lustig. and Fatchen. T.J. (1985) Choosing -. T.L. representative reference catchments. ~ydrologyand Water Resources Symposium, 1985, Sydney. The Institution of Engineers, Australia, National Conference Publication No. 8512. Odum, H.T. (1971) Environment, power and Wiley-Interscience, N.Y.

society.

Tsuchida, A. and Murota, T. (1979) Thermodynamic approach to the economics of water and soil: an oriental viewpoint. (Publication unknown. English translation available from the writer.)