Industrial ecology and energy systems: a first step

Resources, Conservation and Recycling 24 (1998) 51–63

Industrial ecology and energy systems: a first step Micah D. Lowenthal, William E. Kastenberg * Department of Nuclear Engineering, 4153 Etche6erry Hall, Uni6ersity of California, Berkeley, CA 94720 -1730, USA Received 23 March 1998; accepted 5 May 1998

Abstract This work is intended to contribute to the foundations for formalizing industrial ecology analyses of energy systems (systems for energy generation, transfer, or transformation) and to examine how the tools for performing these analyses can also enhance the field of industrial ecology in other applications. We discuss requirements for studying materials and energy cycling in industrial processes, with particular emphasis on energy generating systems, through explicit inclusion of entropy concepts in industrial ecology considerations. This perspective is intended to contribute to the theoretical basis for industrial ecology, to the development of tools for comparing the ecological (human and environmental health, and institutional) impacts of energy generating and other industrial processes, and to possible changes in engineering curricula with emphasis on design. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Industrial ecology; Energy; Electricity

1. Introduction The term ‘industrial ecology’ refers to a set of tools, principles, and perspectives borrowed and adapted from ecology for the analysis of industrial systems including the impacts on society and the environment of the systems’ material, energy, and information flows. Industrial ecology perspectives can help to optimize industrial * Corresponding author. [email protected]

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systems,1 but standard ecology models have some conceptual limitations in analysis of energy systems (systems for energy generation, transfer, or transformation) and most industrial ecology analyses in the past have focused on manufacturing processes which result in a ‘material’ product (automobiles to spacecraft) rather than on energy (e.g. electricity) as a product [1]. Others have conducted selection-oriented industrial ecology analyses or comparative life cycle assessments of sets of energy-production systems, such as Ref. [2]. But we have a different aim. In this research we assess the implications and demands of industrial ecology analyses of energy systems appreciating the particular characteristics of energy as a product. This work is a conceptual extension of work initiated in June 1996 by a group at the University of California at Berkeley on inertial confinement fusion [3] regarding energy and material flows in the choice of materials for key components. It is our hope that this work will contribute to establishing a solid conceptual foundation for future studies that examine and compare specific energy systems.

2. Flows in ecosystems and industrial systems Fig. 1 shows typical representations of type II and type III ecosystems as defined by Graedel and Allenby [5]. In this framework, an ideal ecosystem, a type III system, has interconnected feed and waste streams but, unlike a type II ecosystem, is completely closed: it takes in energy (usually sunlight) but no net material resources are consumed and no net wastes are generated. While the lines with arrows are not labeled, they generally represent material flows and, in better analyses, include energy flows. But if one is following energy as well as materials then type III systems as depicted in Fig. 1 do not exist even in nature: the laws of thermodynamics dictate that there must be a sink for the energy (Fig. 2). Nature is not an ideal system, by thermodynamic definitions. Further, in thermodynamic analyses we find that it is not enough to account for material and energy flows (this is like using only the first law without the second law), but that entropy, too, must be included. Without entropy, all processes appear to be reversible. Most life-cycle assessments and industrial-ecology analyses do implicitly consider entropy, but it is only recently that explicit treatment of entropy has been stressed as a necessary component of industrial ecology analysis, and this is mostly by use of the concept of exergy in the context of material recycling [6].2 There is a need to examine energy systems in 1 By optimize we mean to lend to efficient use of energy and material resources, thereby reducing negative societal and environmental impacts. 2 Exergy is essentially the free energy, relative to a ground state. Free energy is the correct concept to use in this context: it is just the systems energy that has the potential to do work in the given environment. For fixed quantities of materials, free energy is a function of the configuration and motion of the components of the system. Gibbs free energy is the quantity relevant to chemical reactions, changes in the configuration of electrons in atoms and molecules. Nuclear transformations are somewhat more complicated but involve binding energy in an analogous role for changes in the configuration of nucleons (or in the kind of nucleons). Kinetic and gravitational potential energy are the quantities relevant to movement of mass (i.e. hydropower).

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order to complete the picture for traditional industrial systems [1]. Thus to see what we can learn about energy systems from ecology models we begin by looking carefully at ecological systems, following flows of materials, energy, and entropy.

3. Energy systems and entropy considerations Taking the edges of the Earth’s biosphere to define the boundaries of the ecosystem for our energy analysis, energy flows in from four major natural sources: by far the largest is solar energy imparted by the sun in the form of radiant energy; gravitational forces transfer translational kinetic energy from the Earth (as a

Fig. 1. Schematic representation of a type II ecosystem and of a type III ecosystem (adapted from Ref. [4]).

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Fig. 2. A more accurate representation of an ecosystem.

whole), the moon and other massive bodies to fluids on earth that are free to move, such as the oceans, causing tides; the Earth’s rotational energy is dissipated both by interaction with the moon and as friction in the earth’s molten core and mantle;3 heat from the Earth’s core flows into the biosphere by conduction (through the crust) and convection (volcanoes, seafloor spreading, hot springs, etc.).4 The two largest of these energy flows into the biosphere are nuclear in origin: solar energy is generated in a nuclear fusion reaction, a reaction that fuses light nuclei to form more tightly bound, heavier nuclei, and energy in the Earth’s core from radioactive decay results from a similar transformation to a more tightly bound nucleus, but from an unstable nucleus. Every one of the flows entails an increase in entropy, as all irreversible energy flows must. Entropy can be thought of as a measure of disorder or, if one is comparing two states, entropy is higher for the state that is closer to equilibrium. A state with energy dispersed is higher in entropy than a state with energy concentrated. The flow of energy from high concentrations, such as the sun and the earth’s core, to low concentrations, such as the biosphere, represents an increase in entropy. The processes that generate energy also generate entropy. In the production of energy we can draw upon the natural energy flows, either directly (solar and geothermal) or indirectly (wind, biomass, and tidal), or we can

3

Earth’s days get longer by about 1.48 ms each century. The Earth’s core may be at temperatures that are higher than the surface of the sun. The heat comes from the original accretion energy from formation of the Earth; from decay of naturally occurring radioactive material such as uranium, thorium, and potassium; from the ongoing solidification of molten rock (latent heat of formation) and from the above-mentioned frictional forces. 4

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extract stored energy (chemical and nuclear fuels). This distinction between tapping into natural flows of energy and extracting stored energy gives a basis for distinguishing ‘renewable’ energy sources from ‘non-renewable’ ones (the term ‘renewable’ is not helpful in this respect). But the categories are not clearly divided: oil and natural gas are forms of fossilized biomass. The difference is in the relative rates of consumption and production. If we were to consume fossil fuels as slowly as they are produced, then these too would be ‘renewables’. New nuclear fuels are no longer naturally created (except in exploding stars), but can be ‘bred’ by deliberate actions, analogous to large-scale cultivation and consumption of biomass, that could make the fuel resources last for millennia.5 Fig. 3 illustrates the energy and materials flows in a fuel-based energy-generation system. A renewable system looks identical, except that there is no fuel cycle (labeled A, in the center of Fig. 3). Material and energy flows in Fig. 3 are indicated by arrows and entropy is loosely represented by vertical position: up and higher on the diagram indicate increase of entropy and higher entropy. Living organisms perform two kinds of physical actions: energy conversion and material transformation. Both of these actions incur irreversible energy losses. Living organisms take in energy directly (as in photosynthesis) or as fuel (as in food) and convert it, with some energy losses, to useful forms that drive movement and sustain life. Conversion of energy from fuels is a kind of material transformation that increases the entropy associated with the material. Organisms can also transform materials in ways that reduce the entropy associated with the materials’ configurations, as in purification of a material or storage of potential energy in chemical bonds (i.e. formation of chemical fuels). But this is only a local reduction in entropy. The organism uses more energy in refining or forming the material than

Fig. 3. Flow of materials in conventional and nuclear electricity production. Dashed lines represent recycling options that might be possible with some technologies. E, energy; M, materials; S, entropy. 5 The supply of fuels for fusion are less abundant than the sun but still so plentiful as to challenge common definitions of sustainability.

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would be gained by degrading the material to its initial state and the waste energy carries the increase in entropy that arises from inefficiencies in the process in addition to the balance of the entropy reduction in the material. Every action by living organisms generates waste energy that is in a degraded, less useful form (e.g. heat energy at a lower temperature). Some of this energy may be used to drive other processes and organisms, but with each successive action the net entropy increases and the energy is degraded further, until it is no longer useful. Thus, even if all of the energy flowing into the biosphere were taken up by organisms for reduction of local entropy, the system would still need to reject waste energy (heat). The Earth radiates thermal energy into its heat sink, outer space. The ‘greenhouse effect’, which raises concerns about global warming, is simply reflection by chemical wastes in the atmosphere of too much of that thermal radiation: chemical wastes from our consumption of energy are clogging our sink for our waste energy and reflected sunlight. As the above discussion suggests, the difference between a degraded or dissipated state of a material and a valuable state of the same material is often simply that the former is higher in entropy [7].6 Energy allows us to export entropy or refine materials. Two different approaches can turn a degraded material into a valuable material: one is to find a process that can use the material in its degraded state and the other is to refine the material. In an ideal system, these two approaches are combined. The first, while preferable to disposal, does not tend to lead to a closed system unless a refining step is incidental to the process. Take as an example coal ash from coal-fired power plants: its use in concrete is preferable to directly disposing of the material both because of the sequestration of the residual toxins and because of the additional discovered and exploited utility. But the material is unlikely to see another incarnation in the industrial system without undergoing some refining step. Ideally, industrial refining systems are the industrial equivalents of photosynthesis: energy is used to transform stable, degraded material into more reactive, high-grade material. From a material transformation standpoint, the chief currency is energy. In our analysis, energy is the product and the quality of the energy is as important as the quantity of energy considered. Electricity is a form of energy that is of high quality. Virtually 100% of the electricity flowing through an electric heater can be used for the intended purpose: heating. A heater that burns coal can recover approximately 80% of the energy content of the coal for heat. In both cases we are converting the energy from a higher quality to a lower quality, namely heat. Degraded energy, like a degraded material, diminishes in utility. Like a degraded material, degraded energy can be refined (see below for a discussion of heat pumps) but, also like degraded material, the cost is still more energy. As with degraded material, one can make waste energy valuable by finding a process that uses it in its 6 It is important to note that higher entropy states are not necessarily less valuable and that keeping materials more pure does not necessarily save the energy of refinement. A good example is glass. Pure silica can be formed into glass by melting, but stronger glass can be formed with less energy if certain impurities are introduced into the mixture, lowering the melting temperature.

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degraded state. This is the basis for cogeneration plants and feedwater heaters in power plants.7 In fuel-based systems we specifically transform refined materials into degraded materials for energy, rather than degrade energy for material transformations. One’s objectives in generating electricity differ, then, from one’s objectives in a typical manufacturing process. In order to get a better handle on these systems and how they manipulate energy we need to have more detailed definitions of entropy. Taking a historical perspective, we first understood entropy in a relative sense. That is, we could calculate the difference in entropy, dS, between two states using the expression, dS= dQ/T, where S is entropy, dQ is heat transferred into the system, and T is the temperature at which the heat is exchanged. In a reversible process, the total entropy of a system and its surroundings does not change: % dSi =0

(1)

i

Further, for a process that changes the state of a system, the change in entropy for the system is only given by dQ/T if the system is ideal and if the change is carried out reversibly.8 Because thermodynamics is a study of equilibria it cannot give us relations for calculation of entropy change in irreversible systems, except to establish minimum values. In other words, we know that the change in entropy of a system undergoing an irreversible change must be greater than the change in entropy for an ideal system undergoing a reversible change. Thus using classical thermodynamics we can utilize ideal systems to represent general principles about entropy and efficiency and their relative increase or decrease. Statistical thermodynamics offers a more rigorous and demanding definition of entropy. We define V(E) to be the number of configurations of the system such that the system has total energy E. This is called the thermodynamic probability. If two systems are in communication with each other and the total energy between them is fixed, ET, the probability of finding a particular distribution of energy between the two systems is proportional to the product of the thermodynamic probabilities of the two states, expressed as P(E) 8V1(E)V2(ET −E)

(2)

According to statistical thermodynamics, the entropy of a system is given by the relation, S =k ln V, where k is the Boltzmann constant and ln is the natural

7 In power plants that use steam conversion, the steam delivered to the turbines must be ‘dry’ (superheated) so as not to degrade the turbines and their efficiency. Between a high pressure turbine and a low pressure turbine the steam passes through a moisture separator and a reheater. The hot water extracted from the steam by the moisture separator is generally used to heat the feedwater for the primary heat transfer from the boiler. 8 What we mean by an ideal system is one that obeys the first and second laws of thermodynamics and that undergoes only reversible changes. In addition to excluding obvious irreversible losses like friction, for changes to be reversible they must be driven by vanishingly small differences and occur over an infinite time.

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Fig. 4. A simple heat exchanger.

logarithm. From these expressions we see that states that are more probable are also higher in entropy. Equilibrium is reached between two systems when the entropy (and the probability) is maximized. This probabilistic description lends itself to discussion of systems in terms of their capacity to change spontaneously. Systems in equilibrium do not change spontaneously whereas systems far from equilibrium have a great capacity for such change. High quality energy, as a result, is perishable: it spontaneously seeks a degraded state. As a system makes transitions toward equilibrium it can do useful work or it can disperse energy as waste. An interesting feature of the statistical formulation is that at T= 0°K a system consisting of a pure substance (in crystalline form with no defects) must have a particular configuration: every particle must be in its lowest quantum state. Thus at T= 0°K, V =1 and S =0. This is called the third law of thermodynamics and it gives us an absolute measure of the entropy of a system, whether it is an atom, an ingot, or a solar panel. As the systems get more complex, however, the calculations become exponentially more complicated, and while we can identify some basic principles — such as that a mixture of substances is higher in entropy than the same substances in a separate, pure state—precise analysis of industrial-scale systems is cumbersome using statistical thermodynamics. Looking now at energy systems involving thermal conversion (as opposed to photovoltaic or mechanical conversion of energy), the most basic component, the heat exchanger, illustrates the introduction of irreversibility and the limits on efficiency. Consider a hot fluid and a cold fluid at temperatures TH and TC, respectively, flowing at equal rates in opposing directions on opposite sides of a perfectly conducting barrier (Fig. 4). In an ideal system heat is transferred isothermally, thus TH1 =TC2 and TH2 = TC1 and the heat transferred is proportional to the difference between the temperature of the hot source, TH1, and the temperature of the cold source, TC1. But in order for net heat to flow in a finite time period, one must have a temperature difference between the two fluids at the point of transfer. Thus TH1 \TC2 and TH2 \TC1. The effectiveness of an actual heat exchanger is considered to be the actual heat exchanged divided by the heat exchanged in an ideal system. For identical mass flows and heat capacities in the two fluids this effectiveness, hx is given by hx =

TC2 −TC1 TH1 −TC1

(3)

TC2 is a function of the area of the heat exchanger and the flow velocity. If we now look at the whole thermal conversion cycle (Fig. 5) we can examine first an ideal cycle and then a real cycle and account for entropy generation.

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Even in an ideal system, entropy is generated by the creation of thermal energy in the high-temperature reservoir; every other step in an ideal cycle is reversible. That heat is transferred to the working fluid at a constant temperature, increasing the entropy of the fluid by QI/ TH. The fluid then undergoes an isentropic expansion that yields work, WO. The Kelvin–Planck statement of the second law of thermodynamics is that no cycle can produce net work with only a single thermal reservoir. This is the same as saying that no system can convert heat to work with 100% efficiency. Thus the working fluid must reject heat (at constant temperature) before being isentropically compressed (by the pump exerting work, Wp, on the fluid) and beginning the cycle again. The rejection of heat from the working fluid is the opposite of heat transfer to the fluid and entails a decrease in entropy of the fluid by QO/TL. In this cycle entropy does not accumulate in the fluid and the expansion and compression steps are isentropic, therefore QO/TL = QI/TH = dS. This ideal cycle is called the Carnot cycle and the Carnot efficiency, hc, is given by the equation below. Q −QO QI =THdS, so hc = I (4) QI T − TC T QO =TCdS, so hC = H = 1− C (5) TH TH In real systems the heat exchangers do not function isothermally, the fluid experiences friction, and the expansion and compression are not truly isentropic or even adiabatic. Nonetheless, the conclusion that lowering the temperature of the heat sink increases the efficiency holds true for real systems as well. The heat deposited in the low-temperature reservoir, QO, is of lower quality than the heat deposited in the working fluid, QI, by the high-temperature reservoir. The quality is a measure of the capacity of the system to use the heat to perform useful work. If we compare equal quantities of energy in the two reservoirs we find the difference is quality or concentration, as measured by temperature, and hence the capacity of the energy to be used for work.

Fig. 5. Energy flow in a thermal conversion cycle. C, converter (e.g. a turbine or piston); HX, heat exchanger.

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Fig. 6. Simple diagrams of energy processes including (A) a generator and (B) a heat pump.

The above discussion concerns thermal conversion of energy to useful work (e.g. mechanical or electrical). A complementary example is that of using work to refine thermal energy. Fig. 6 is a representation of a thermal conversion engine running in two different modes. The first is a standard illustration of a power plant. The second is a representation of a heat pump. In a heat pump we use work to move energy from a low-temperature reservoir to a high temperature reservoir, as in the operation of an air conditioner or a refrigerator. Q1 must equal the sum of Q2 and W. In an ideal system, Q1/TH − Q2/ TL =0. Putting these together gives Q2 TL = W TH −TL

or

Q1 TH = W TH − TL

(6)

The effectiveness in this case depends on the intent of the process. The quantity Q2 is important for refrigeration purposes and the quantity Q1 is important for heating purposes. Fig. 7 depicts the lines of constant heating effectiveness for an ideal heat pump moving heat from a low-temperature reservoir to a high temperature reservoir (note that the heat pump is most effective where the energy difference is the smallest). An ideal refrigeration system that cools a compartment surrounded by a 294-K environment to 273 K does so at an effectiveness of 13. That is, every joule of work can remove 13 J of heat from the compartment. An ideal heating system utilizing a heat engine at the same temperatures would heat at an effectiveness of 14: the joule of work is added to the 13 J of heat removed from the environment. Thus an ideal heat pump under these conditions delivers 14 times the heat that an electric heater would, given the same amount of energy. Real heat pumps are not ideal. In order for an electric heat pump serving as a heater to be superior to a standard electric heater at the above temperatures it would need to have an efficiency greater than 7.14% or 1/14. Commercially available models have a net effectiveness or ‘coefficient of performance’ (the

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product of the efficiency and the thermodynamic effectiveness) between 3 and 4, corresponding to an efficiency of around 20–25% in temperate areas. Looking back now at the earlier example comparing electric heaters to coal heaters, we note that electricity is not a source of energy, but a form of energy. To compare the efficiency of heat sources we should look at the whole system. We now compare the heat provided by 1 t of coal to the heat provided by an electric heater with electricity produced by 1 t of coal. The approximate energy equivalent of 1 t of coal is 2.93× 1010 J. A heater that burns coal and yields its heat directly to a compartment might operate with an efficiency of 80%, yielding 2.34× 1010 J to the compartment. A coal-fired power plant extracts nearly all of the energy content of the coal, say 95%, and converts that heat at an efficiency of 40% to electricity. Transmission and distribution losses might cut out 5% (in 1993 the average electrical transmission efficiency in California was 95.5% [8]) and the heater itself operates at an efficiency of 100%. (2.93 × 1010 J) ×(95%) × (40%)× (95%)×(100%)= 1.06× 1010 J The inefficiencies of conversion make direct heating more efficient. We may still choose the electric heater if the coal-fired power plant is cleaner, or reduces exposures to air pollution. But it is only with the heat pump that electricity competes with respect to efficiency (3 × 1.06× 1010 = 3.18× 1010 J), and then only for a limited range of reservoir temperatures. A heat pump displays many of the same characteristics as a material refinement system: as the difference between the quality of the two reservoirs increases, the work required to increase the difference increases. A distinction between refinement of energy and refinement of materials is that the work itself is energy and ultimately adds to the heat rejected into the high temperature reservoir. In material systems the work also manifests as heat and results in a higher temperature product.

Fig. 7. Lines of constant heating effectiveness (E, joules of heat/joule of work) for an ideal heat pump moving heat from a low-temperature reservoir (X axis) to a high-temperature reservoir (Y axis).

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4. Concluding comments and principles for industrial ecology for energy systems It is useful to think in terms of refinement when we examine any industrial system because for any life cycle there are both degradation and refinement stages. If in examining a process we consider what is required to reverse the process (and what benefit can be gained from that stage), then we begin to take a truly ecological perspective. Many of the costs of reversing processes are manifestations of entropy effects. We could assign an entropy value to stages in a product’s life cycle. This could be seen as a cost like a negative monetary cost. Such a cost would be calculable and subject to some of the same considerations as monetary costs, such as externalization. If we account for efficiency in energy production and entropy removal, we then have a framework for incorporating energy considerations in cycling of materials. We would also need a utility score for the life cycle in order for entropy value to be meaningful for comparisons. Combining these values would give a sense of the effectiveness of the cycle. From this discussion we can distil some principles of industrial ecology for energy systems. 1. Energy or efficiency gains and penalties must be analyzed for the entire system, product use included. 2. Higher quality energy is, by definition, more useful. But energy transfers and transformations (and even storage) carry a penalty, so energy in the appropriate form (heat, kinetic, electromagnetic) is often better than energy that is of higher quality but in the wrong form. 3. For every transfer or transformation of energy or materials, the cost of reversing the action should be considered and factored into the global system. Acknowledgements The authors gratefully acknowledge the feedback of Lloyd Connelly who reviewed a draft of this paper. This work was funded by Lawrence Livermore National Laboratory (Memorandum Number B291817) and Lawrence Berkeley National Laboratory (Memorandum Number LBL 7544500). References [1] O’Rourke D, Connelly L, Koshland CP. Industrial ecology: a critical review. Int J Environ Pollut 1996;6(2/3):89–112. [2] Rowe RD, Lang CM, Chestnut LG, Latimer DA, Rae DA, Bernow SM, White DE. The New York Electricity Externality Study, Volume I: Introduction and Methods. Dobbs Ferry, New York: Oceana Publications, 1995. [3] Lowenthal MD, editor. Industrial Ecology Analysis of Inertial Fusion Energy. Department of Nuclear Engineering, University of California, Berkeley, UC-BFE-051, 1997. [4] Graedel T. Industrial ecology: definition and implementation. In: Socolow R, Andrews C, Berkhout F, Thomas V, editors. Industrial Ecology and Global Change. New York: Cambridge University Press, 1994;25.

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[5] Graedel TE, Allenby BR. Industrial Ecology. Englewood Cliffs, NJ: Prentice-Hall, 1995. [6] Connelly L, Koshland CP. Two aspects of consumption: using an exergy-based measure of degradation to advance the theory and implementation of industrial ecology. Resour Conserv Recycl 1997;19:199–217. [7] Ayers RU. Industrial metabolism: theory and policy. In: Allenby BR, Richards DJ, editors. The Greening of Industrial Ecosystems. Washington, DC: National Academy of Engineering, 1994:23 – 27. [8] California Energy Commission. California Historical Energy Statistics, December 1995, Sacramento, CA, Publication Number: P300-95-020.

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