exergy and industrial ecology—part 1: an exergy-based definition of consumption and a...

Exergy Int. J. 1(3) (2001) 146–165www.exergyonline.com

Exergy and industrial ecology—Part 1: An exergy-baseddefinition of consumption and a thermodynamic

interpretation of ecosystem evolution

Lloyd Connelly a∗, Catherine P. Koshland b

a Department of Mechanical Engineering, University of California, Etcheverry Hall Rm. 4152, Berkeley, CA 94720, USAb Energy and Resources Group, University of California, Barrows Hall Rm. 310, Berkeley, CA 94720, USA

(Received 3 March 2000, accepted 10 February 2001)

Abstract—A central theme of industrial ecology (IE) is the idea of using the cyclical resource-use patterns observed in mature,biological ecosystems as a model for designing increasingly mature ‘industrial ecosystems’ whose productivity relies less on resourceextraction and waste emission. In this two part series, we will use a thermodynamic interpretation of ecosystem evolution tostrengthen this biological–industrial (B–I) ecosystem analogy. We begin by describing limitations in the current analogy and discussingresulting implications for the development and implementation of IE principles. We propose that these limitations arise largely from apoor definition of resource consumption. We then show that defining resource consumption as a process of removing exergy from aresource provides a basis for interpreting ecosystem evolution as a process of allowing consumption to occur with decreasing levelsof depletion, i.e., a process of ‘de-linking’ consumption and depletion. We use thermodynamic principles to deduce the limitationsand interrelation of several strategies for de-linking consumption and depletion. Lastly, we explore the benefits and limitations ofusing the proposed interpretation of ecosystem evolution as an analogy for the development of industrial systems. 2001 Éditionsscientifiques et médicales Elsevier SAS

Nomenclature

e specific energys specific entropyT temperature

Greek letters

ε specific exergy

Subscripts

o ground state

1. INTRODUCTION

Modern and newly industrialized societies extract, ex-pend, and expel massive quantities of natural resources.

∗ Correspondence and reprints.E-mail addresses: [email protected] (L. Connelly),

[email protected] (C.P. Koshland).

Over the last century, depletion–oriented patterns of re-source use have co-evolved with a complicated andhighly interrelated set of demographic, economic, socio–political, technical, and even philosophical factors. Theseresource use patterns are now deeply ingrained in the in-frastructures and cultures of modern industrial societiesand are spreading to newly industrialized nations. Al-though resource consumption has created unprecedentedlevels of material wealth for some, many of the regionaland global environmental crises we confront today arestrongly related to high rates of anthropogenic resourcedepletion; see, for example, [1].

Global warming, ozone layer depletion, and the widespectrum of ecological and human health problems as-sociated with the release of toxic substances to air andwater are all products of depletion-oriented, ‘extract, ex-pend, and expel’ patterns of resource use. Strong moti-vation for reducing resource depletion may therefore bebased not on Malthusian ‘we are going to run out of re-source x’ arguments, but rather on the benefit of avoidingthe unforeseen symptoms of ongoing resource transfor-mation.

146 2001 Éditions scientifiques et médicales Elsevier SAS. All rights reservedS1164-0235(01)00021-8/FLA

L. Connelly, C.P. Koshland / Exergy Int. J. 1(3) (2001) 146–165

Efforts to reduce depletion and move toward more sus-tainable patterns of resource use have intensified in recentyears with the emergence of a new field of study called in-dustrial ecology (IE); see, for example, [2–5]. One focusof IE is to advance design–oriented, inter-industry strate-gies for resource conservation that emphasize avoidanceor reuse of waste products with the goal of developingmore mature industrial ecosystems that exhibit increas-ingly cyclical resource-use patterns analogous to thoseobserved in mature, biological ecosystems. 1 In the IE lit-erature, an expanded role for waste avoidance and reuseis explored not only on the level of finding new sinks forexisting waste streams, but also by attempting to incor-porate re-usability into product and process design [3].

As discussed in [6], IE shows much promise but re-mains unexplored and ambiguous on many levels. Oneimportant limitation in the biological–industrial (B–I)ecosystem analogy is the lack of a rigorous, physical in-terpretation of resource consumption and associated am-biguity about the interrelated roles and limitations of re-source conservation strategies such as waste cascadingand recycling. In this two part series, we attempt to ad-dress this limitation by introducing a thermodynamic de-finition of consumption and a related interpretation ofecosystem evolution. We begin by identifying specificweaknesses in the existing B–I ecosystem analogy andshowing that they arise from a poor definition of resourceconsumption. We then show that defining resource con-sumption as a process of removing exergy from resourcesaccounts for the first and second law implications of con-sumption and thereby provides a rigorous basis for de-scribing ecosystem evolution as a process of de-linkingconsumption from depletion through increased cascad-ing, recycling, exergy efficiency, and renewed exergy use.This interpretation of ecosystem evolution is not intendedto be comprehensive, but rather useful for coordinatinganalogous resource conservation and waste avoidancestrategies in industrial systems.

We also discuss the limitations and interrelation ofthese four resource conservation strategies, and then ex-

1 As discussed in [2], the distinction between industrial and biologicalecosystems is becoming less clear. Indeed, many ecosystems today re-flect anthropogenic and ‘natural’ influences. Here we focus on systemsthat are far more industrial and less mature on an evolutionary scaleand hence refer to them as industrial ecosystems only to distinguishthem from the more mature, predominantly biological ecosystems fromwhich lessons for achieving more sustainable resource consumption aresought. Note also that the term ‘industrial ecosystem’ does not neces-sarily refer to a fully mature industrial system with complete materialcycling. In this series, industrial ecosystem refers to any specific set ofindustrial processes that one may choose to call a system. On an evo-lutionary scale, that system may be immature (resource depleting) ormature (non-depleting).

plore the broader benefits and limitations of using theproposed interpretation of ecosystem evolution as a pre-scriptive analogy for industrial development. Finally, weshall discuss potential applications of the framework—some of which will be developed and demonstrated inPart 2—and also caution that the de-linking of consump-tion from depletion cannot be interpreted as a generalmeasure of an industrial system’s environmental compat-ibility.

2. AN UNDERDEVELOPED ECOSYSTEMANALOGY

The emerging field of industrial ecology 2 beganlargely with the idea that sustainable resource flows inmature, biological ecosystems provide industrial societyan excellent model of resource use with profound impli-cations for environmental regulation, product design, andeven the way we think about the sustainability of resourceconsumption. The basic model of ecosystem evolutionunderlying the B–I analogy was proposed in [5] and isshown in figure 1.

In this model, a maturing ecosystem—be it a biolog-ical ecosystem or an industrial analog—passes throughthree phases as material flows gradually become less lin-ear (or open) and more cyclical (or closed). A simpletype 1 biological ecosystem could, for example, containchemoautotrophic bacteria that ‘feed’ off of inorganicchemicals. 3 Some modern day archea that live in hot

2 In this series, we use the phrases ‘field of industrial ecology’ and‘industrial ecology literature’ to refer to the body of work dealingexplicitly with the concept of an industrial ecology. The majority ofthis work proceeds from a group of papers published in Vol. 89 of theNational Academy of Sciences. It is not our intent to suggest that IEsubject matter (sustainability, resource recycling, process integration,etc.) is unique to the field of industrial ecology. Rather, our intention isto focus on the contributions of these new studies to an expansive bodyof literature. Although many have insisted that the field of industrialecology predates the term, we prefer to see IE not as a category butas an emerging field of study that has begun to develop and apply apotentially fruitful analogy between biological and industrial systemevolution. Working from this definition, we ask what added value doesthis field bring to the broad arena of environmental science, and inwhat ways may this contribution be strengthened by a thermodynamicinterpretation of resource consumption.

3 Indeed, it is hypothesized that the earliest prokaryotic life waschemoautotrophic bacteria that evolved 3.5 to 4 billion years ago andobtained the thermodynamic work potential (i.e., the exergy) requiredto drive their endergonic metabolic reactions through the oxidation ofavailable inorganic chemical substances such as hydrogen sulfide orcompounds of iron [7].

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Figure 1. Evolutionary history of resource flows in biological ecosystems. Most industrial ecosystems are still type I. Traditionalinterpretation of resource flows is shown above arrows; exergetic interpretation is shown below. Terms used to categorize exergyflow are defined in table I. Figure adapted from [3, 5].

springs rely on the following reaction:

FeS + H2S → FeS2 + H2 + exergy

If this ecosystem does not contain any processes toregenerate iron sulfide and hydrogen sulfide, then once

these compounds are depleted, the bacteria that requiredthem would die and the ecosystem would be consideredunsustainable with respect to these bacteria. An industrialanalog might contain a paper mill that relies on anunsustainably harvested wood source.

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At the other end of the spectrum, a fully mature typeIII biological ecosystem will generally have evolved acomplicated food web that maintains a balance betweenthe inputs and outputs of the metabolic reactions sustain-ing heterotrophic non-photosynthetic life, and photosyn-thetic life forms. Modern ocean, forest, or jungle ecosys-tems are often broadly classified as type III—though spe-cific elements in these ecosystems may be out of bal-ance. A type III industrial analog could be an aluminumproduct industry based on complete recycling of post-consumer aluminum where all aluminum recovery, re-manufacture, and distribution processes were powered byrenewed energy sources. Type II ecosystems will fall be-tween these two extremes; they are not fully sustainablewith respect to resource depletion but limited forms ofresource cycling are present. As these examples suggest,any rigorous measure of ecosystem maturity would re-quire careful definition of boundary conditions and in-cluded/excluded elements [7].

Many new ideas and renewed interest in old ideas havefollowed this provocative analogy: moving away fromcrisis-driven, end-of-pipe approaches to waste reduction,and toward preventative, design-oriented approaches thatfocus more on the environmental burdens of products(and the process that produce them) over their entire lifecycle; reducing resource depletion and waste generationthrough system-level, inter-industry approaches to prod-uct design and process integration (industrial symbio-sis, eco-industrial parks); re-thinking waste as a productwithout a market and attempting to build such marketsby designing waste streams for feedstock requirementsand vice versa; and rethinking environmental regulationsthat restrict waste reuse and inter-industry cooperation inwaste reduction efforts; see, for example, [3–5, 8–10].

Although a promising array of new thinking, newresearch, and new design tools have already begun toemerge from the existing B–I ecosystem analogy, thisanalogy has not been fully developed and therefore re-mains confined and potentially misleading. In this sec-tion, we will advance three critiques of IE’s ecosystemanalogy: the description of cyclical resource flows in bio-logical ecosystems and hence the analysis and discussionof analogous waste reuse strategies in industrial ecosys-tems are superficial, the model of evolving resource flowsis incomplete, and finally the focus on increased cyclingas a central characteristic of ecosystem evolution is mis-placed. We then show how all of these limitations may betraced to a core deficiency in the B–I ecosystem analogy:an inadequate definition of resource consumption.

2.1. Weaknesses in thebiological–industrial ecosystemanalogy

The first and most apparent weakness in the existingB–I ecosystem analogy is the casual approach to the defi-nition of resource reuse. Although the promotion of cycli-cal resource use patterns and the role that waste avoid-ance and reuse plays in advancing them are major themesof industrial ecology, the processes underlying resourcecycling and reuse in evolving biological ecosystems havenot been clearly defined, i.e., the distinct quality reduc-tion and quality gain phases of resource cycles and thephysical laws governing these phases have not been ex-plicitly incorporated in the analogy. As a result, resourceflows amongst plants and organisms with interrelated,mutually dependent feeding habits are referred to looselyas food chains and food webs while the directionality andform of resource transformation in these networks, andthe relation between these transformations and the ad-vancement of resource cascades and resource cycles inindustrial systems are not addressed.

We are not suggesting that these processes are notunderstood, but rather that the casual description of re-source cycles in biological ecosystems creates ambigu-ity in the description of waste reuse strategies in indus-trial systems. The IE literature commonly (and correctly)refers to processes such as recycling, reuse, loop closing,waste exchanges, and cascading as means to conserve re-sources and reduce waste emissions; see, for example, [3,8–12]. Although it is well understood that these practicespromote resource conservation, neither the relation be-tween these terms and the actual physical processes theyrefer to, nor the benefits and limitations of the physi-cal processes themselves are actually discussed. Indeed,terms like recycle, reuse, and cascade are often used in-terchangeably and with little or no stated definition. As aresult, the suggested and plausible outcomes of proposedreuse options at times appear to conflict [6, 8–10].

For conveying the general idea that diverting wastestreams to secondary use usually reduces resource de-pletion and waste emissions, this casual approach is cer-tainly adequate. We propose, however, that in order todevelop a deeper understanding of sustainable resourceuse and the analytical tools necessary to advance it, amore rigorous framework for defining consumption, cy-cling, cascading, and their interrelated roles in ecosystemevolution will be beneficial. Indeed, the development ofsuch a framework is one specific way that the B–I anal-ogy could be used to contribute new thinking to the exist-ing realm of inter-disciplinary environmental research.

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The second shortcoming in the existing B–I ecosystemanalogy is one of misplaced focus. Although increasedresource cycling is a component of ecosystem evolution,other closely related evolutionary processes are left outof IE’s ecosystem analogy almost entirely. The result isnot simply that the IE literature focuses on one resourceconservation strategy (increased cycling) within statedlimits and goes no further. Rather, the limitations ofresource cycling in reducing depletion are simply notaddressed, thereby giving the impression that cycling isthe central and even the only physical process involvedwith the evolution of sustainable resource consumptionin biological—and hence industrial—ecosystems. Theconsequences of this focus are readily apparent in the IEliterature where waste avoidance strategies and design forre-usability are common themes, while other interrelatedstrategies for reducing resource depletion and wasteemissions such as increasing renewed energy use andincreasing energy efficiency receive far less attention.

Lastly, the false centrality of resource cycling in theB–I ecosystem analogy overshadows a physical phenom-enon that is central to the evolution of resource flows inbiological ecosystems: the de-linking of resource con-sumption from resource depletion. As we will showin subsequent sections, mature biological ecosystemsachieve non-depleting resource consumption not simplybecause they achieve full resource cycling, but becausethey evolve a series of interrelated physical processesthat together allow resource consumption to occur with-out depletion. In this multi-faceted evolution, cyclical re-source use plays a complementary but limited role along-side resource cascading, more efficient use of resources,and increased reliance on solar radiation.

2.2. The core deficiency of IE’secosystem analogy

Although several limitations in the existing B–I ecosys-tem analogy have been described, this analogy is not asflawed as the above critique may seem to suggest. In-deed, we propose that all of the limitations discussedhere evolve from one core deficiency: a weak definitionof resource consumption. Although the advancement ofIE concepts is motivated largely by environmental prob-lems associated with extract-expend-expel patterns of re-source consumption, the IE literature provides no defini-tion of resource consumption, and provides only qualita-tive terms such as residuals, wastes, or pollution to dis-tinguish a consumed waste from the associated uncon-sumed feedstock. Consumption is described in terms of

its consequences (such as depletion of resources or accu-mulation of wastes) or in terms of perceived changes inthe consumed material (such as loss of value or reducedusefulness) [10, 13].

We are not suggesting that consumption is not under-stood. Rather, we propose that current descriptions of re-source consumption and its relation to system evolutionare insufficient for developing the B–I ecosystem analogyto its full potential.

3. A THERMODYNAMICINTERPRETATION OF CONSUMPTION

The term consumption is ubiquitous in modern soci-ety. Indeed, we commonly speak of quantities rangingfrom paper to time as being consumed and even refer topeople as consumers. Like many terms with broad, con-textual definitions, the word consumption is subject to awide range of interpretations. Ask an economist to defineconsumption, and you will probably be told about usinga product or service to increase your utility; ask an en-vironmentalist and you will likely be told that consump-tion is a process of destroying natural resources; ask aphysicist and you may hear that consumption is a processof increasing a substance’s entropy. No single interpreta-tion of consumption is inherently better or more accuratethan the rest—each interpretation corresponds to a dis-tinct phenomenon and therefore has relevance in an ap-propriate context.

To provide a more accurate description of the phys-ical processes underlying the sustainable cycling of re-sources, we will advance a physical interpretation ofconsumption that is grounded in thermodynamic princi-ples. 4 In [14] it was shown that exergy removal providesa uniform, non-resource specific measure of consump-tion that uses first and second law principles to accountfor transfers and irreversible losses of resource quality. 5

In this section, we briefly review the development of an

4 In this context, sustainable cycling means cycling that occurswithout resource depletion.

5 Unless explicitly indicated, the word consumption will henceforthbe used strictly to describe the thermodynamic interpretation of con-sumption (as exergy removal). The term resource encompasses anyform of matter or energy. As will be described in this section, removalof exergy from a resource does not imply that all exergy removed fromthat resource is lost. When a resource’s exergy content is reduced, ex-ergy is removed from that resource by loss and possibly by transfer ofexergy to other resources. The ‘first law’ is the conservation of energy.The ‘second law’ is the required loss of total work potential in all realenergy transformation processes.

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exergy-based measure of resource consumption, intro-duce a closely related exergy-based measure of resourcedepletion, and offer a thermodynamic interpretation ofecosystem evolution as a process of de-linking consump-tion from depletion.

3.1. Two aspects of consumption

To accurately define consumption, two independentphenomena underlying all consumptive processes shouldbe quantified. The first, throughput, simply defines thequantity (or flow rate) of material passing through aconsumptive process. To define consumption fully, theextent of resource degradation—i.e., the extent to whicha consumptive process removes resource quality—mustalso be considered. This aspect of consumption is criticalbecause it defines the extent to which a consumedresource would have to be upgraded before it could bereused at a given quality level.

As discussed in [14, 15] consumption rate may beexpressed as the product of the resource throughput(quantity/time) and the extent of degradation per unitof resource quantity. The set of throughput-degradationcombinations resulting in the same rate of consumptionare shown as lines of constant consumption in figure 2.

Although material throughput may simply be mea-sured on a mass flow basis, finding a consistent andmeaningful measure of specific degradation (i.e., degra-dation per unit mass) is less straightforward. Since re-sources serve a variety of purposes, no single mea-

Figure 2. Lines of constant consumption [14]. Shifting frompoint A to point B reduces resource throughput but increasesconsumption rate.

sure of degradation can consistently relate the physi-cal changes a resource undergoes during consumptionto loss of perceived usefulness or value. 6 Narrow mea-sures of resource quality must therefore be chosen forspecific purposes, and the limitations of those measuresmust be recognized. Since the purpose of this paper isto strengthen the B–I ecosystem analogy by account-ing for the first and second law implications of resourceconsumption and thereby relate consumption to otherprocesses such as cycling, cascading, and depletion, weseek a physical indicator of degradation to measure thatwhich is transferred or irreversibly lost from a resourceduring any physical transformation.

3.2. An exergy-based definition ofresource consumption

As discussed in Appendix A, the property exergy de-fines the maximum amount of work that may theoreti-cally be performed by bringing a resource into equilib-rium with its surroundings through a reversible process.Unlike mass and energy, exergy is not a conserved prop-erty. Exergy may therefore be transferred between sys-tems and removed from existence completely, i.e., lost.As shown in figure 3, the term ‘loss’ does not refer toa reduction of system exergy caused by the transfer ofexergy outside of an arbitrary system boundary. In thispaper, exergy loss refers to the destruction of exergy byirreversible processes occurring within a system; exergyloss is absolute, not relative.

Regardless of whether the exergy in a resource isused to perform work or not, it is always exergy thatis lost during any physical transformation of matter orenergy. Identifying as exergy the resource quality that islost during all forms of resource transformation allowsa preliminary thermodynamic interpretation of resourceconsumption: Resource consumption causes exergy loss.

This definition may be improved by looking moreclosely at the requirements of the second law. Althoughthe second law ensures that all real processes must resultin an irreversible loss of work potential, the irreversiblenature of this loss does not prohibit a particular resource’s

6 Even an economic measure of resource value depends on temporalvariations in supply and demand and may be distorted due to lackof information, externalities and other market failures. A potentiallyuseful byproduct may be perceived as having zero or even negativeeconomic value simply because those who could profitably use thewaste are unaware of its existence or are uncomfortable with the idea ofsubstituting a ‘waste’ for virgin feedstock.

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Figure 3. The consumption of resource A is accompanied by the upgrade of resource B. The consumption of resource A is expressedin terms of removal, transfer, and loss of exergy. In general, consumption need not be associated with upgrade. Consumption, i.e.,removal of exergy—may occur entirely by loss.

work potential from increasing. The requirement that ac-cessible work potential decrease means only that the sumof all exergy changes for resources and their surroundingsmust be negative: i.e., total exergy must always decrease.The exergy of a specific resource may therefore be in-

creased by the transfer of exergy from other resources, aslong as that transfer results in additional exergy loss; seefigure 3. Any resource that receives exergy by transferfrom another resource is upgraded, i.e., the resource isphysically transformed. Exergy transfer to bauxite from

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TABLE IDistinguishing among four categories of resources.

Term Criteria ExamplesStock resource If consumed, the amount available decreases. If not consumed, the

amount available does not decrease over a relevant time scaleBauxite, coal, oil, geothermal exergy, fis-sionable uranium

Flow resource The amount available decreases steadily irrespective of whether ornot it is consumed for a useful purpose

Solar radiation and related exergy sourcessuch as wind, hydroelectric; tidal ex-ergy; methane emissions from decayingbiomass

Renewedresource

Once consumed, resource is upgraded (i.e., returned to its pre-consumed state) by direct or indirect transfer of exergy from solarradiation. No limit to supply exists over a relevant time scale

Sustainably harvested biomass; any fullycycled resource

Non-renewedresource

Once consumed, no upgrade occurs over a specified time frame.A limit to supply exists over a relevant time scale

Deforestation without re-seeding; fossilfuels

fossil fuels, for example, occurs during the refining of vir-gin aluminum via the Bayer and Hall–Héroult processes.

The ability to increase a specific system’s exergyhas important implications for the science of resourcemanagement and industrial ecology. Once a resource hasbeen consumed, a recycling operation may increase thequality of the consumed resource without violating thesecond law of thermodynamics as long as it produces agreater loss in resource quality elsewhere. To account forthis, our preliminary interpretation of consumption mustbe refined. Since a resource’s accessible work potentialmay decrease as a result of both loss and transfer, itis more accurate to say that: Consumption removes aresource’s exergy by transfer and loss.

This thermodynamic interpretation of consumptionprovides a basis for measuring extent of resource degra-dation in a wide range of physical resource transforma-tions including irreversible mixing to form a solution,heat transfer across a finite temperature difference, unre-strained expansion, and irreversible chemical reactions.For each of these processes, expressions for the rate ofexergy removal as a function of the physical propertiesof the pre- and post-consumed resource have been de-veloped; see, for example, [16–19]. In some consump-tive processes—such as irreversible mixing—exergy isremoved only by loss. In others—such as exothermicchemical reaction—exergy may be removed by transferto other resources and by loss. 7

7 In real processes, exergy loss always accompanies exergy transfer.

3.3. The relation between consumptionand depletion

To develop a thermodynamic interpretation of re-source depletion that may be directly related to theprocess of exergetic consumption introduced here, it isnecessary to look beyond the conservation laws of massand energy. Indeed, these laws ensure that none of themass or energy content of resources on our planet hasbeen or ever will be lost in a thermodynamic sense. Theterm depletion will therefore be used here to describe asecond law phenomenon arising from irreversible loss ofexergy.

The extent to which resource consumption contributesto resource depletion depends both on what happens toa resource after it is consumed and what would havehappened to the resource if it were not consumed. Ifan upgrade process coincides or follows a consumptiveprocess, it is also important to consider the time frameover which both processes occur. As shown in table I, afirst step toward defining the relation between consump-tion and depletion is to separate resources into four over-lapping categories: renewed, non-renewed, stock, andflow. In defining these categories, exergy from solar ra-diation is considered to be effectively without limit inboth quantity and rate. Solar radiation is therefore con-sidered an exergy source that allows other resources tobe indefinitely renewed—i.e., over a relevant time scale;the sun’s fusion-based exergy is not depleted. 8 This ap-proach to resource categorization and its implications for

8 Solar irradiation intercepted by earth greatly exceeds current andprojected world exergy demand.

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TABLE IIUsing resource categories to determine the relation between consumption and depletion.

Does consumption of aresource contribute to

depletion?

Flow resource Stock resource

NO NORenewed resource Wind, tidal, hydro-potential exergy are supplied irre-

spective of whether or not their exergy is transferred toother resources. Also, the source of the exergy flow isrenewed

Sustainably harvested biomass or fully cycled solventsmay be consumed without depletion

NO (but supply limited) YESNon-renewed resource Methane emissions from decaying, non-renewed bio-

mass are lost irrespective of whether are not they areconsumed for a useful purpose

Fossil fuels may be depleted

natural resource management have been explored by sev-eral researchers [20–24].

As shown in table II, these four categories are used todetermine when consumption results in depletion. Con-sumption never contributes to the depletion of a renewedstock resource since by definition the consumed resourceis upgraded to its pre-consumed state by transfer of ex-ergy from solar radiation. Neither does consumption con-tribute to the depletion of a non-renewed flow resourcesince this form of resource exergy is steadily supplied ir-respective of use. However, while the consumption of anon-renewed flow resource does not contribute to deple-tion of that resource, neither can a non-renewed flow re-source be considered a sustainable source of exergy sinceit will eventually deplete itself.

The third category, renewed flow resources, refersto steadily supplied resources whose exergy source iscontinually renewed by solar radiation. 9 It is clearthat consumption of renewed flow resources does notcontribute to depletion since this category of resource isboth renewed and supplied irrespective of whether or notwe chose to consume it.

Only the consumption of non-renewed stock exergycontributes to resource depletion. Since consumptionalways causes exergy loss, the consumption of non-renewed stock exergy always causes the loss of non-renewed stock exergy. Since this exergy is not renewed,it is depleted. 10 The relation between consumptionand depletion described in table I may be expressed

9 Solar radiation itself is strictly a non-renewed flow resource, thoughas discussed earlier its source is considered effectively limitless on arelevant time scale.10 Since resource consumption may occur through a mix of exergy loss

and exergy transfer to other resources, the rate of non-renewed stockexergy depletion may be less than the rate of non-renewed stock exergy

as follows: Consumption of non-renewed stock exergycauses resource depletion.

This relation—although concise—can only be estab-lished with respect to explicit spatial and temporal bound-ary conditions. Clear boundary conditions must be es-tablished in distinguishing between ‘renewed’ and ‘non-renewed’ resources, and hence between which formsof consumption contribute to depletion and which donot. Since these distinctions may change dramatically atlonger time scales, the period over which resource con-sumption and upgrading occurs must be clearly estab-lished. Even a fossil fuel, for example, may be considereda renewed resource on a time scale of millions of years.

In addition, estimates of exergy depletion rates ina chain of processes will depend on which forms ofindirect exergy consumption are included and excluded.Does the cumulative exergy cost of electricity includethe exergy used to construct the power plant, and if soover what length of time is this one-time exergy costannualized so that it may be included in the overall exergycost of providing electricity? These types of questionsare not necessarily problematic—one-time exergy costsfor a coal-fired power plant may be annualized over anestimated thirty year plant life time—but they cannot beanswered without clear boundary conditions.

3.4. Ecosystem evolution as a processof de-linking consumption fromdepletion

In any ecosystem, resources ranging from living bio-mass to fossil fuel and mineral deposits are consumed,

consumption. This distinction will be developed quantitatively in Part 2of this series.

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i.e., the physical processes underlying all ecosystem ac-tivity cause resource exergy to be continually transferredand lost. It follows directly that if ecosystem activity re-lies on the consumption of non-renewed, stock resources,then the activity must eventually cease or find renewedsources of exergy to rely on. In other words, exergy con-suming ecosystem activity—be it the metabolic processesof life or the resource transforming activities of mod-ern industry—must eventually occur without resource de-pletion if it is to be sustainable. The evolution of re-source use in maturing biological and industrial ecosys-tems may therefore be partially characterized as a processof allowing resource consumption to occur with decreas-ing levels of resource depletion, i.e., as a process of de-linking consumption from depletion. 11 This thermody-namic interpretation of ecosystem evolution may be usedto strengthen the form and significance of the B–I ecosys-tem analogy as it provides a quantifiable measure of sys-tem progress towards a mature state in which resourceconsumption is fully de-linked from resource depletion.

This interpretation of ecosystem evolution representsjust one of many different lenses through which the com-plex processes involved with the evolution and mainte-nance of ecosystem function may be viewed. The pro-posed interpretation does not capture many aspects ofecosystem evolution; but neither is it intended to: the rel-evant criterion for judging this framework is not its com-prehensiveness, but rather its usefulness in the develop-ment of new perspectives and new analytical tools thatwill support the intelligent implementation of resourceconservation strategies.

4. STRATEGIES FOR DE-LINKINGCONSUMPTION FROM DEPLETION

In the field of industrial ecology, the processes in-volved with de-linking consumption from depletion inevolving, biological ecosystems are used as resource con-servation strategies for de-linking consumption from de-pletion in immature industrial systems. In this section,we will use a deductive approach to discuss the interre-lation among four strategies for de-linking consumptionfrom depletion: cascading, cycling, efficiency gains, andrenewed exergy use. We are not attempting to ‘rediscov-er’ these common conservation strategies, but rather to

11 Depletion rate may also be lowered by reducing consumption rate.Here, we focus on the processes that allow consumption to occurwithout depletion. Both approaches are accounted for in the quantitativeanalysis introduced in Part 2 of this series.

demonstrate that the relation between these strategies andthe proposed exergy-based definition of ecosystem evolu-tion follows directly from first and second law principles.

The four conservation strategies will be discussed inthe context of a simple, hypothetical industrial ecosystemconsisting initially of two solvent consumption processesand the chain of industrial processes required to deliversolvent to inlet of these two processes. 12 For the sakeof explanation, we assume that one solvent consumingprocess requires lower purity feedstock than the other. Allsolvent feedstocks are derived from non-renewed, fossilsources, and all solvent leaving the two consumptiveprocesses is emitted to the atmosphere. This is thereforea type I industrial ecosystem.

4.1. Waste cascading

A waste cascade may be described in thermodynamicterms as a process of using resource outputs from one ormore consumptive processes to supply other consumptiveprocess(es) at equal or lower exergy. 13 Waste cascadingreduces resource consumption in two ways: by reducingthe rate of resource exergy loss caused by the dissipationof wastes released to the environment, and by reducingthe need to refine virgin resources. In our hypotheticalindustrial ecosystem, cascading used (i.e., partially con-sumed) solvent from the first to the second solvent con-sumption process eliminates solvent emissions from thefirst process and eliminates the need to refine and supplypure solvent to the second process. The solvent consump-tion rate in the two processes is unchanged, but the rateof resource depletion associated with these processes isreduced.

Although waste cascading reduces demand for otherresources—and hence is an important resource conser-vation strategy as discussed in [25]—cascading does notreturn to a waste the exergy that was removed from it dur-ing consumption. Cascading cannot therefore establish a

12 These solvent consumption processes could for example consistof the mixing of toluene vapors with air and or other impuritiesby irreversible mixing in a rotogravure printing plant. The chain ofindustrial processes supplying the solvent would consist of fossil fuelextraction and delivery, solvent manufacture from fossil feedstocks,and any other components that contribute significantly to resourceconsumption.13 This is not to say that the waste of a process can supply any

feedstock requirement of lower exergy, but rather that any waste thatis reused without upgrading (i.e., without having exergy transferred toit from another resource) must arrive at the inlet of a compatible processat the same or lower exergy level that it had when it left the supplyingprocess.

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resource cycle. Losses associated with the upgrade andsupply of solvent to the top of the cascade, the consump-tion of resources in the two processes constituting thecascade, and dissipation of waste solvent released fromthe bottom of the cascade cannot be avoided. Cascadingcan reduce the linkage between consumption and deple-tion; it cannot fully de-link the two.

4.2. Resource cycling

To reduce emissions from the bottom of a wastecascade (or at the outlet of a single consumptive process)and return this bottom waste to the top of a resourcecascade, the exergy removed from a resource duringconsumption must be returned to it. This process ofexergy loss through consumption followed by exergyreturn through transfer is the basis of resource cycling.

Adding a solvent recycling process and its associatedchain of industrial processes to our hypothetical systemreduces depletion both by eliminating exergy loss fromthe dissipation of released solvents, and by substitutinga post-consumption upgrade path for a virgin resourceupgrade path. An activated carbon solvent separationsystem, for example, will generally be far less exergyintensive that fossil-based manufacture of virgin solvent.

Cycling cannot, however, eliminate depletion. In ac-cordance with the second law, all exergy transfers in real(irreversible) processes must be accompanied by exergyloss (i.e., total exergy must always decrease). Hence, inany real cycling process, the overall resource depletionrate will exceed the rate of exergy loss in the consumptiveprocess whose wastes are being cycled. In our example,the two solvent consumption processes and the exergy re-moved from non-renewed resources for the purpose ofupgrading the solvent would still contribute to resourcedepletion in the case of complete solvent cycling.

4.3. Increasing exergy efficiency

One way to reduce the resource depletion associatedwith cycling is to reduce the losses that accompany thetransfer of exergy to consumed resources by increasingthe efficiency of exergy transfer between resources, i.e.,increasing the fraction of exergy removed from one re-source that is transferred to another. Exergy efficiencymay be thought of as a more accurate measure of en-ergy efficiency that accounts for quantity and quality as-pects of energy flows. Unlike measures of energy effi-ciency, exergy efficiency provides an absolute measure

of efficiency that accounts for first and second law lim-itations (i.e., the second law often prevents energy effi-ciencies from approaching one). In the current example,increasing exergy efficiency in the case of complete cy-cling would involve increasing the efficiency of the sol-vent upgrade process.

Although technological and economic limitations toefficiency gains prevent exergy efficiency from approach-ing one, many industrial processes today operate at verylow efficiencies, and it is widely recognized that signifi-cant room for efficiency improvement remains. However,even if exergy efficiency could be brought to one, theresource depletion associated with solvent consumptionand upgrade in our example would still not be eliminated.The conservation of energy ensures that all exergy trans-ferred to consumed solvent must draw on the exergy ofother resources. Recycling with a 100% exergy efficientupgrade process would therefore result in a depletion rateequal to the consumption rate of the two solvent con-sumption processes. To fully de-link consumption fromdepletion, it is necessary to use resources that supply ex-ergy without being depleted.

4.4. Renewed exergy use

To fully de-link consumption from depletion, theexergy used to upgrade consumption products mustbe derived from renewed exergy sources, i.e., sourcessuch as electricity generated directly or indirectly fromsolar radiation or sources such as sustainably harvestedbiomass feedstocks. 14 In the solvent cycling example,using a sustainably harvested biomass fuel as the exergysource for the solvent upgrade process could in theorycreate a type III solvent cycling system in which a closedsolvent cycle is driven entirely by renewed exergy inputs.In this situation, depletion rate becomes independent ofthe exergy efficiency of the solvent upgrade process.

4.5. Reducing resource depletion: asystem-level approach

In many situations, resource depletion may be reducedby minimizing or avoiding consumption. Depletion mayalso be reduced by de-linking consumption from deple-tion through the implementation of the four interrelated

14 From the thermodynamic perspective discussed here, a fully sus-tainable biomass harvest implies the maintenance of several non-depleting resource cycles including a carbon cycle and necessary nu-trient cycles (nitrogen, potassium etc.).

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strategies discussed. Although each strategy overcomesthe thermodynamic limitations of the previous, they neednot be implemented sequentially. Indeed, partial imple-mentation of several strategies simultaneously will nor-mally be the most cost effective depletion reduction strat-egy.

An analytical framework for relating the implemen-tation of these strategies to an overall measure of sys-tem progress on an evolutionary scale would provide astrong basis for detailed analyses of industrial systemsand means to increase their sustainability. In part 2 of thisseries, we will introduce a non-dimensional measure ofsystem progress—the depletion per unit consumption—and show how it may be expressed as a function of nondi-mensional indicators of the extent to which the above de-scribed strategies are implemented. The resulting analyti-cal framework provides a range of analytical tools to sup-port depletion reduction efforts at an inter-industry level.Before developing such a framework, however, it is es-sential to first ensure that it is based on sound rationalefor reducing resource depletion.

5. REDUCING EXERGETIC RESOURCEDEPLETION: WHY?

We have proposed that the process of de-linkingconsumption from depletion provides an interpretation ofecosystem evolution that can help advance the design ofmore mature industrial systems. We have not, however,discussed why achieving more mature industrial systemsmight be a desirable goal. In this section, we considerseveral potential bases for justifying efforts to reduceresource depletion.

5.1. To conserve stock exergy

Malthus and others have proposed specific resourcelimits to economic growth [26, 27]. Although predictionsof drastic resource shortages have failed to materialize inthe face on on-going technical innovation, does there ex-ist a broader non-resource-specific limit to consumption?Since all resources embody exergy, does the total quan-tity of stock exergy on planet earth represent an outernon-resource specific limit to resource consumption? Ifso exergy depletion should be avoided on the grounds ofconserving our limited resource exergy.

The idea of an entropy-based limit to resource con-sumption was proposed by Georgescue-Roegen [28, 29]and has since been extensively debated; see, for example,

Daly [30, 31], Young [32, 33], Khalil [34–36], Biancardiet al. [37, 38], Mansson [39], Kummel [40], Townsend[41], Lozada [42, 43], Williamson [44], Norgaard [45].In our opinion, the ability to transfer exergy from so-lar radiation to stock resources eliminates any relevant,thermodynamic limit to quantity or rate of resource con-sumption. Limits to exergy supply do not therefore offer areasonable basis for promoting avoidance of stock exergydepletion.

5.2. To conserve economic value

If it were possible to accurately correlate the exergycontent of any resource with that resource’s economicvalue, then arguments for avoiding stock exergy depletioncould be based on purely economic arguments. Anyindustrial system that met consumer needs with a reducedlevel of resource depletion could be considered a moreeconomically efficient system.

Efforts to associate resource energy content witheconomic value have a long (and much criticized) history.Smith [46] discusses three waves of interest in energy-centric thinking beginning with early works in the laterpart of the 19th century, the technocracy movementin the 1930s, and then work with net energy analysisin the in the 1970s. Early examples of energy-centricthinking include Spencer [47] who discussed all activityas ultimately arising from energy, Boltzmann [48] whodescribed all life as the struggle for free energy, andOstwald [49] who advanced a ‘monistic philosophywhich reduces all things to forms and flows of energy.’In 1922, Lokta [50] discussed life evolution as a questfor free energy. Bernt [51] discusses the technocracymovement lead by Howard Scott in the 1930s andits relation to Odum’s later work with energy theoriesof value [52]. Bernt also discusses several underlyingmotivations and problems with such theories.

Any consistent association between a resource’s en-ergy or exergy content and the economic value of that re-source would have strong implications for the proposedexergy-based framework. 15 On a superficial level, it ap-pears that such an association may actually exist. Manyexergy-intensive resources such as petroleum productsand manufactured chemicals, for example, are generally

15 Indeed, although the cited works discuss relations between energyand value, it is generally a resource’s useful work potential that isactually referred to, i.e., exergy is implied although the more commonterm energy is used.

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more valuable on a mass basis than low exergy com-pounds such as ores and salts. However, the exergy con-tent of a resource cannot account for variations in re-source supply and demand. The value of gold changesdaily, but its specific exergy is invariable. In addition, ex-ergy cannot account for value added to resources throughthe process of resource combination. The value of a per-sonal computer, for example, cannot be adequately ex-pressed in terms of the sum total exergy content of allits components. Such a total would remain constant, evenif the computer’s value were drastically reduced with ahammer. 16 Hence, as a basis for promoting the avoid-ance of exergy depletion, the very limited association ofexergy with value is inadequate.

5.3. To prevent the environmental harmthat released exergy will cause

Some researchers have suggested that exergy contentmay be correlated with a resource’s potential for harm-ing the environment. 17 If this were true, than the mostexergy conserving cascading and cycling options wouldbe those that most reduced the release of harmful com-pounds to the environment. Indeed, emissions avoidancepriorities could be ranked on an exergy basis and therebyallow the reduction of exergy emission to be used asan objective function for environmentally benign processdesign.

Ayres et al. [68] suggests that ‘the exergy-content of awaste residual can be interpreted as its potential for doingharm by driving uncontrolled reactions in the environ-ment,’ and according to Brodyanski [18]. ‘The lower (asubstance’s) exergy, i.e., the nearer are its parameters tothe corresponding parameters of environment, the lowerare the exergy losses and the lower will be the harmfulecological consequences.’ In addition, Kummel [53] sug-gests that the entropy production associated with pollu-tion avoidance technologies be used to weight pollutionavoidance priorities: ‘The indicator ‘entropy production’would allow comparison of the pollutants from differentenergy carriers on one unified thermodynamic scale. . . .If taxes on energy carriers are to be established not only

16 This and other forms of resource transformation that exergy cannot‘see’ are discussed in Section 6.17 Rosen and Dincer [58] discuss the apparent ‘paradox’ between

efforts to interpret exergy as a measure of value and as a measure ofenvironmental harm potential and also provide an excellent overviewof the theory and application of exergy in the arena of natural resourcemanagement.

in order to stimulate energy conservation but also to pun-ish polluters, the (entropy production) indicator may pro-vide an appropriate rating base.’ Similarly, Kummel andSchussler [54] proposed that ‘the heat equivalents whichare emitted into the biosphere when the pollutants arekept out of or removed from the biosphere by appropriatetechnological processes’ might be used to weight pollu-tion avoidance priorities. Von Spakovsky et al. [55] incor-porate an entropy-based pollution factor defined as ‘therate of entropy change of each pollutant per unit volumeof the environmentally effected region,’ into a detailedmethodology for optimizing and evaluating the design ofenergy conversion systems. See also [56–59].

An association between exergy and environmentalharming potential is not entirely without ground. Highexergy compounds, by definition, are far from equilib-rium with the surrounding environment and for that rea-son have a greater potential for driving processes that willalter the physical character and function of resources inthe environment. Indeed, nearly all of the United StatesEnvironmental Protection Agency’s ‘33/50’ Program’shigh-priority chemicals for emissions reductions are highexergy compounds [60]. 18 On the other hand, benzene,lead, aluminum, ammonia, and hydrochloric acid all havehigh specific exergy but vastly different potential (andmechanisms) for harming human and ecological health.A process modification that reduces resource depletionby substituting a smaller rate of benzene emission for amuch larger rate of acetone emission would increase the‘maturity’ of an industrial ecosystem, as we have definedit, but would also increase emission carcinogenicity.

The primary problems with associating exergy withharming potential arise from the fact that the mereexistence of disequilibrium is far too crude a measureto predict the complicated interaction of compoundswith various aspects of the environment. Hence, as withassociations with economic value, correlations betweenexergy and harming potential may at times align effortsto reduce resource depletion with a secondary benefit.These correlations, however, are far too inconsistentto provide adequate justification for avoiding exergeticresource depletion.

5.4. To prevent environmental change

The idea of defining resource degradation in thermo-dynamic terms and thereby making a direction connec-

18 Compounds include benzene, cadmium compounds, carbon tetra-chloride, chloroform, toluene, xylenes.

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tion between environmental conservation and the avoid-ance of exergy loss has been approached in a variety ofways. Szargut [61] for example has suggested that ‘theindex of cumulative consumption (i.e., loss) of the ex-ergy of unrestorable natural resources can be redefinedas the index of ecological costs.’ Also, Glasby [62] arguesthat the entropic disorder which must arise whenever (ex-ergy) is consumed ‘. . .can be thought of as environmentaldegradation and has been steadily increasing as a conse-quence of industrialization.’ Ayres has also written exten-sively on this topic, e.g., [65, 68], as has Wall and others[17, 22, 24, 59, 63].

In accordance with the second law of thermodynam-ics, exergy loss accompanies all real resource transfor-mation processes. Since the earth’s biosphere consists ofmaterial resources that are subject to the second law, theloss of energy from non-renewed, stock resources musttherefore change the form and character of our environ-ment. In other words, exergy depletion causes environ-mental change. This change may be immediate and obvi-ous as in the case of deforestation, or the change may beslow and long term as in the cases of ozone layer destruc-tion and global warming. In either case, these phenom-ena are the consequences, or symptoms, or an underlyingprocess of environmental change driven by resource de-pleting patterns of resource use. Avoiding the depletionof non-renewed stock resources through the establish-ment of closed resource cycles driven by flow or renewedstock exergy sources would eliminate a significant driverof environmental change. 19 We therefore propose thatthe strongest rationale for avoiding exergy depletion isto avert the unforeseen symptoms of wide-scale resourcetransformation, i.e., to avoid future environmental crisescaused by current resource depletion in immature indus-trial ecosystems.

This rationale is inherently a preventative, non-crisis-specific rationale for the development of more sustain-able industrial systems. As such, it complements thecrisis-specific, reactionary approach to addressing envi-ronmental degradation is too often the modus operandifor environmental policy decisions not just in the UnitedStates, but throughout much of the world. Consider sev-eral common examples. In response to the realizationthat production and subsequent release of CFC’s in theenvironment led to ozone layer depletion, internationalcooperation led to the Montreal Protocol for the reduc-tion of CFC emissions. In response to growing evidence

19 This is not intended to suggest that stopping resource depletionwould end all environmental change. As discussed in the followingsection, exergy does not capture all forms of change. Neither are exergydepletion processes the only drivers of environmental change.

of ecological and human health damage, DDT and nowheavy metal emissions have been increasingly curtailedin a growing number of countries. 20 In response toacidic precipitation and urban smog, efforts to reduceSO2 emissions from coal-fired power plants and reduceNOx and hydrocarbon emissions from internal combus-tion engines continue to progress. More recently, in re-sponse to mounting evidence that increased atmosphericconcentrations of CO2 and other greenhouse gases willlead to global climate change, international efforts to re-duce greenhouse gas emissions are underway. These ef-forts are both necessary and beneficial, but in each casearose in response to a specific crisis.

The motivation for developing thermodynamicallymature industrial systems stems not from specific envi-ronmental concerns but instead from a more systemic ra-tionale for resource conservation based on a the recog-nition that exergetic resource depletion is a driver of en-vironmental change with unpredictable short and long-term consequence. From this perspective, specific en-vironmental problems are seen less as isolated crisesaround which we should organize isolated responses andmore as symptoms of current resource use patterns.

6. WHAT EXERGY CANNOT SEE

Any effort to use the proposed model of ecosystemevolution as a framework for helping plan the develop-ment of more mature industrial systems requires a clearappreciation for the analogy’s many limitations. In thissection, we will focus on two important limitations tothe thermodynamic interpretation of ecosystem evolu-tion. First, the property exergy is blind to many ‘non-exergetic’ forms of consumption. As a result, the modelof sustainable (i.e., non-depleting) cycling based on re-newed exergy transfers does not apply to all forms ofconsumption—specifically those forms that involve irre-trievable as opposed to just irreversible loss. Second, al-though avoidance of exergy depletion has long-term ben-efits associated with the establishment of more sustain-able resource use patterns, it must not be seen as a com-prehensive objective function for environmental benefit.

20 Recently, some DDT bans have been lifted to allow for improvedmosquito control in malaria prone regions. Since malaria eradicationis an important goal, this example clearly shows that avoidance ofexergy depletion cannot be considered a general objective function forenvironmental benefit. This issue is discussed further in Section 6.

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6.1. Non-exergetic forms ofconsumption

Although exergy removal can be used to measure re-source consumption in a wide range of processes, it doesnot capture all forms of change. Physical changes aris-ing from microscopic mixing of gases, liquids, or solids(to form solution), chemical reaction, heat transfer, un-restrained expansion, and other well-defined thermody-namic processes involve exergy loss. Exergy, however,cannot capture loss of information content, or macro-scopic changes in structure and position—all importantways of changing a resource’s characteristics [64–66]. 21

Consider, for example, a room full of chemicals. If,on one humid afternoon, all the labels on the chemi-cal bottles fell off thereby rendering the bottles indis-tinguishable, the usefulness of the chemicals would de-crease dramatically. However, the physical properties ofthe chemicals themselves would not have changed in anyway. Similarly, although a slight variation in the posi-tion of a single component of a complex product suchas a personal computer may render the product useless,such change may not involve any form of exergy removal.Another example is geographical distribution. Aluminumcans strewn in a landfill are far less useful than the samealuminum cans collected in a recycling bin.

It follows that the model of sustainable resource cy-cling discussed earlier cannot render all forms of resourcetransformation sustainable. The reason lies in the dif-ference between irretrievable as opposed to simply ir-reversible loss. Consider two chemicals that are mixedto form a solution. Since the pre-mixed form of bothchemicals is known, the two chemicals may be upgradedthrough a separation process such as distillation. As dis-cussed in Section 3, the upgrading process requires ex-ergy transfer from other resources and, in accordancewith the second law, must result in additional exergyloss. The exergy removed from the mixed chemicals may,however, be transferred back to them from a renewed ex-ergy source, thereby establishing a basis for maintainingcycling without depletion.

Consider, however, a case of species extinction—aform of ‘information’ loss where information in this con-text is the genetic code stored in the DNA nucleotidesequence. Since the pre-consumed form of that which

21 Some researchers argue that exergy can indeed be used to capturesuch forms of change if the necessary mechanisms for calculatingentropy gain associated with such change are used. The relationbetween changes in information content, macroscopic structure, andchanges in entropy remains controversial.

is lost (i.e., the information content of DNA) is gener-ally not known, it cannot be recovered through an ex-ergy transfer process (or any other process). Such a lossis best classified as irretrievable as opposed to merely ir-reversible. Since it is impossible to de-link an irretriev-able resource loss from the depletion of that resource, thisform of depletion can be avoided only by eliminating theprocesses that cause it.

6.2. The limited implications ofdepletion avoidance

Although efforts to reduce resource depletion will helpavoid environmental change and associated environmen-tal crises, these efforts may have collateral effects thatare detrimental to the environment in the short and longterm. 22 Hence, while the image of a robust recyclingsector supplying recovered post-consumer products tohighly efficient manufacturing processes powered largelyby renewed exergy sources is certainly an appealing con-trast to our existing depletion-oriented system of resourceuse, the virtues (or failings) of even a highly mature in-dustrial ecosystem are dependent on much more than thede-linking of consumption from depletion.

A program for substituting renewed biomass fuels fornatural gas consumption may, for example, result in alarge reduction in exergy depletion rate, but could alsoresult in increased air pollutant toxicity. Furthermore,achieving 99% cycling of toxic compounds such aslead would dramatically reduce bulk throughput andresource depletion rate but could increase exposure tolead emissions in recycling facilities [67].

In addition, system boundary conditions could bedrawn to allow the consumption of one resource to beoffset by the transfer of renewed exergy to an entirely dif-ferent resource. A program that harvested old growth red-wood groves and Ecuadorian rainforest while replacinglost biomass on an equivalent exergy basis with a mono-culture pine forest, could be completely non-depletingin strictly thermodynamic terms but also result in bio-diversity loss. Even more extreme, exergy loss throughdeforestation could be ‘offset’ by transferring renewedexergy to bauxite. These examples betray the central ideaof avoiding resource depletion through the establishmentof closed resource cycles driven by exergy transfer from

22 Advancing depletion avoidance may lead to conflicts in muchbroader arenas by raising economic concerns, ethical dilemmas, etc. Wefocus here on the more likely error of assuming that depletion avoidancemay be equated with environmental benefit.

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renewed sources, but they allow consumption to occurwithout depletion.

Using flow-to-stock exergy transfer as a basis fornon-depleting resource consumption involves establish-ing new dynamic equilibria, i.e., cycles that consume re-sources but produce no net change in stock resource ex-ergy. Achieving or approximating a new dynamic equi-librium, does not, however, mean that the operation ofthat equilibrium will be associated with perceptions ofenvironmental ‘good’. Chemicals such as toluene, forexample, may be manufactured from renewed biomasscompounds. Releasing such compounds to waterways ata rate equal to their rate of dissipation to ground statecompounds while also manufacturing the same chemicalsfrom renewed biomass feedstock at the same rate could intheory create a fully mature, non-depleting toluene cycle.

The atmospheric or aquatic concentrations of toluenemaintained by this new dynamic equilibrium could, inan extreme case, result in a new state of ecosystemoperation—an aquatic ecosystem, for example, that is‘sustainable’ but no longer has fish, or an atmospherethat is not being degraded but has high concentrationsof greenhouse gasses. Although resource cycling drivenby renewed exergy sources is generally a beneficial alter-native to depletion-based resource use patterns, just be-cause a new resource cycle is maintained without deple-tion does not mean that any new dynamic equilibriumestablished would be desirable.

It follows directly that while the avoidance of exergydepletion provides an important guideline for developingmore sustainable and less environmentally disruptiveindustrial systems over the long term, any effort toevaluate the overall ‘environmental friendliness’ of anindustrial ecosystem only on the basis of its resourcedepletion rate could be highly misleading. Avoidance ofexergy depletion is one of many variables that should beconsidered in the design and improvement of industrialsystems and is therefore best accounted for in the contextof a multivariable analysis such Life Cycle Analysis(LCA), Design for Environment, Industrial Metabolism,etcetera; see, for example, [3, 12, 24, 68–71].

7. SUMMARY AND CONCLUDINGREMARKS

The intriguing biological-industrial ecosystem anal-ogy at the core of industrial ecology may be strength-ened by an exergy-based definition of consumption anda thermodynamic interpretation of ecosystem evolution.Interpreting consumption as a process of exergy removal

and ecosystem evolution as a process of de-linking con-sumption from depletion provides a quantitative basis forincorporating first and second law principles into the de-sign of less depleting industrial systems. Specifically, thisapproach allows for improved understanding and analy-sis of the interrelated roles that cycling, cascading, effi-ciency gains, and renewed exergy use play in de-linkingconsumption from depletion.

The avoidance of exergy depletion has been promotedon several grounds. We propose that the strongest ra-tionale for avoiding exergy depletion is to avert the un-foreseen symptoms of wide-scale resource transforma-tion. Exergy depletion causes environmental change. Thesymptoms of this change are well documented: globalwarming, rain forest depletion, ozone layer destruction,etc. Avoiding the depletion of non-renewed, stock re-sources would eliminate a significant driver of environ-mental change and the ongoing environmental crises thataccompany it. This rationale is preventative and precau-tionary in nature and as such complements the normalcrisis-specific, reactionary approach to addressing envi-ronmental problems.

A thermodynamic interpretation of ecosystem evolu-tion also has several limitations. The proposed exergy-based measure of consumption cannot ‘see’ all formsof change, a resource’s exergy content cannot be con-sistently associated with value or harming potential, andthe model of sustainable cycling advanced here does notapply to consumption that results in irretrievable loss.In addition, the longer-term benefits of activities suchas increased cycling of heavy metals may at times con-flict with more immediate ecological and human healthconcerns. The avoidance of resource exergy depletion—although an important objective in its own right—cannottherefore be seen as a universal objective function for en-vironmental benefit.

Exergy depletion avoidance is a goal best advanced inbalance with other environmental and human health ob-jectives. A quantitative framework for measuring exergydepletion can add foresight—i.e., a basis for designing toprevent unforeseen environmental disruption—to multi-variable analyses of industrial systems and product de-sign. In Part 2 of this series, we attempt to take a steptowards this goal by presenting an analytical frameworkthat uses non-dimensional indicators to relate exergeticresource depletion rates to specific resource use prac-tices. It is our intention that this framework eventuallyform one piece of a more comprehensive approach to thedesign and analysis of increasingly sustainable industrialecosystems.

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Acknowledgements

This research was supported by the University of Cali-fornia Toxic Substances Research and Teaching Programand by the US Environmental Protection Agency. Theideas presented in this two part series represent the culmi-nation of over three years of research during which timenumerous people including John Creyts, Diana Bauer,Edgar Hertwich and Dara O’Rourke have contributedvaluable insights and much needed critique. Also of greathelp were inputs from students in UC Berkeley’s courseon Industrial Ecology taught through the Energy and Re-sources Group, and conversations with numerous peopleat the 1998 Gordon Research Conference on IndustrialEcology.

APPENDIX A: CONSUMPTION ANDIRREVERSIBLE LOSS

Mass and energy are never lost in any physical trans-formation. Indeed, resources may be crushed, chemicallyreacted, mixed, and even incinerated, but their total massand energy content is never lost—only rearranged. To de-termine what is lost in resource transformation processes,we need to look beyond the conservation laws and con-sider a profound though often misinterpreted principleknown as the second law of thermodynamics.

Although many definitions of the second law of ther-modynamics have been offered, see, for example, [72–74], for our purposes the following is sufficient: the sec-ond law ensures that accessible work potential is alwayslost in any real process. Accessible work potential—more commonly known as exergy and alternatively asavailability and available energy—defines the maximumamount of work that may theoretically be performed bybringing a resource into equilibrium with its surround-ings through a reversible process. It follows directly thatexergy is a function of both the physical properties ofa resource and its environment (i.e., its predefined ther-modynamic ground state). Unlike mass and energy, ex-ergy may be transferred from one resource to another ormay be lost entirely. Exergy, therefore, is not a conservedproperty. Regardless of whether the exergy in a resourceis used to perform work or not, it is always exergy that islost during any physical transformation of matter or en-ergy.

The common expression for specific exergy (ε) as afunction of the specific energy (e) and specific entropy(s) of both a resource and its environmental groundstate

(specified as eo, so, To) is

ε = (e − Tos) − (eo − Toso) (A.1)

Szargut provides a standardized approach for defininggroundstate conditions for a wide variety of substances[75]. To better understand the significance of equa-tion (A.1), we find it helpful to rearrange the terms asfollows:

ε = (e − eo) − To(s − so) (A.2)

In this expression, (e − eo) represents the quantity of en-ergy that must be removed from a resource to bring thatresource into equilibrium with its surroundings. Hot wa-ter, for example, contains more thermal energy than waterat atmospheric, i.e., groundstate temperature. Similarly, amixture of O2 and CH2 contains more chemical energythan the corresponding groundstate compounds H2O andCO2. In both cases, (e − eo) represents the energy quan-tity differential separating the resource from its ground-state. To cool the hot water or convert the methane-oxygen mixture into a mixture of carbon dioxide and wa-ter, energy (thermal in the first example, chemical in thesecond) must be removed. However, since this first termonly accounts for energy quantity, we cannot say any-thing about how much of the energy removed from a re-source may be used to perform work.

To determine accessible work potential (exergy), it isnecessary to consider the second term in equation (A.2),To(s − so). This term represents the difference in energyquality, i.e., the difference in work potential, that sepa-rates a resource from its groundstate condition. Two idealgases in separate containers, for example, have lower en-tropy than the same gases mixed together. At constanttemperature and pressure, the gases in the mixed and un-mixed state contain the same quantity of energy. The en-ergy of the gases in the unmixed state, however, is ofhigher quality and therefore has greater work potential.If the gases are mixed in an irreversible process, then thework potential in the unmixed gases is lost. Energy quan-tity is unchanged, but the quality of the energy is reduced.

If the gases are mixed through a reversible process,then the work potential in the unmixed gases is not im-mediately lost but instead used to perform work. The re-sulting mixed gases—in accordance with the conserva-tion of energy—will have a lower energy content andhence a reduced temperature. 23 If the energy extracted

23 If the unmixed gases are at the groundstate temperature before areversible mixing process is performed, then the gases are cooled belowthe groundstate temperature during a reversible mixing process becauseenergy is removed from them.

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during the reversible mixing process is used to reversiblyseparate equal quantities of two mixed ideal gases in an-other chamber, then the exergy removed from the mixinggases is transferred to the separating gases. Since energyis added to the separating gases, their temperature rises.By cooling the separated gases back to the groundstatecondition through a Carnot engine and using the workproduced to heat the mixed gases back to the ground-state, the overall result of this series of ideal, reversibleprocesses is the transfer of energy quality from a vesselof mixing gases to a vessel of separating gases, i.e., ex-ergy transfer with no net energy transfer.

Exergy, i.e., accessible work potential, may thereforebe removed from a resource by loss or by transfer to otherresources. In all real processes exergy transfer is alwaysaccompanied by exergy loss. Although energy transfergenerally accompanies exergy transfer, the transfer ofexergy may in theory occur with or without net energytransfer. A more thorough introduction to the concept ofexergy and its use as a measure of resource consumptionmay be found in [14]. A review of exergy-related litera-ture is provided in [15]. Starting points for studying thetheory and practice of exergy analysis include [16–18, 21,75–81].

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