Energy 28 (2003) 1315–1334www.elsevier.com/locate/energy

Extended exergy accounting applied to energy recovery fromwaste: The concept of total recycling

Enrico Sciubba∗

Universita di Roma 1, ‘La Sapienza’, Dipartimento di Meccanica e Aeronautica, via Eudossiana 18, Roma 00184,Italy

Abstract

A novel systematic approach to the evaluation of energy conversion processes and systems, based onan extended representation of their exergy flow diagram is presented and discussed in this article. Themethod constitutes a substantial generalisation of Szargut’s cumulative exergy consumption procedure, andprovides a coherent and consistent framework for including non-energetic quantities like capital, labourand environmental impact into an engineering optimisation procedure (the apposition ‘extended’ refers tothese enhanced capabilities). It is argued that some of the issues that are difficult to address with a purelymonetary or even with a thermo-economic approach can be resolved in a straightforward manner byextended exergy accounting (‘EEA’ in this article). As an indication of the potential of the method, ageneral, qualitative example is offered of the application of EEA to the evaluation of a technical alternativebetween a non-integrated waste recycling and an integrated waste recycling and incineration facility. 2003 Elsevier Ltd. All rights reserved.

1. Introduction

In the second half of the 20th century (beginning in the late 1950s), several researchers indepen-dently proposed different methods for the energy-based assessment of the ‘balance’ and of the‘performance’ of anthropic systems. The motivation of these studies was rooted mostly in indus-trial economics, but there were both implicit and explicit environmental concerns that permeatemost of the published works on these topics.

In the very first of these approaches ([7,17,40], ca. 1965–70), energy conversion systems werethe target of a detailed analysis based on Second Law concepts, which showed that the relevantdesign procedures of the time were intrinsically flawed by their failure to recognise the realsources of irreversibility in processes and components, and that the idea of ‘conversion efficiency’

∗ Tel.: 1-39-064-4458-5244; fax:+1-39-644-585-249.E-mail address: enrico.sciubba@uniroma1.it (E. Sciubba).

0360-5442/$ - see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0360-5442(03)00111-7

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Nomenclature

O Exergy flow output, Wcex Specific exergetic cost, J/Jc Mass concentratione Specific exergy of a stream, J/kgE Total exergy flow, Wee Specific extended exergy, J/kgEE Total extended exergy flow, Wh Specific total enthalpy, J/kgI Exergy input flow, WKCap Exergetic equivalent of capital, MJ/KLab Exergetic equivalent of labour, MJ/(work hour)s Specific entropy, J/(kg °K)T Temperature, °Kw Specific work, J� Extended exergy conversion efficiencyh Exergetic conversion efficiencym Chemical potential, J/kg

based solely on First Law was erroneous and misleading. This method later evolved into the so-called ‘availability analysis’ [1,27], known today more properly as ‘exergy analysis’ [18,24], andits impact on the energy conversion system community has been profound. It is difficult to finda modern design standard that does not make direct or indirect use of exergetic concepts in itssearch for an ‘optimal’ configuration. The very same method has been applied to ‘complex sys-tems’ , a term which was taken to indicate a process constituted by several concurrent transform-ations [13,25,40], industrial settlements [24], a complete industrial sector [4,40], and even anentire nation [11,26,32,47]: in stark contrast with its success in the analysis of single plants andcomponents, the degree of acceptance of the exergetic analysis of complex systems among indus-trial planners and energy policymakers has been remarkably low, and it is only in the most recentyears that this method has caught the attention of energy agencies [21,23]. One of the reasonsfor this low appeal of exergy analysis with legislators might be the fact that the method producesresults of a purely technical nature, which do not immediately convey to non-technical analyststhe idea of being capable of directly impacting the energy conversion- or the productive structureof a system.

This inability of standard exergy analysis to determine real design optima when monetary costsare brought into the picture has been addressed, in the domain of plant and process analysis, bya theory of joint economic- and thermodynamic character, properly named ‘ thermoeconomics’ ,developed to industry standards in the last decade or so [6,44] on the basis of earlier theoreticalformulations [10,12,39]. In this approach, efficiencies are calculated via an exergy analysis, and‘non-energy costs’ (capital, interest, overhead, labour, maintenance, insurance, etc.) are related tothe technical and thermodynamic parameters of the process under consideration: the optimisation

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consists in determining the design point and the operative schedule that minimise the overall(monetary) cost, under a proper set of financial, normative, environmental and technical con-straints. One of the goals of EEA is to proceed beyond thermoeconomics, and to develop aformally complete costing theory based indifferently on an exergetic or on a monetary metric(that is, a general pricing method in which kJ/kg or kJ/kW are perfectly and consistently equivalentto /kg and /kW, respectively): this concept is discussed in Section 4.3 and 4.4 below.

EEA is capable of properly addressing environmental issues, taken in their extended meaningof ‘ impact of anthropic activities on the pre-existing environment’ . Almost four decades ago, thisissue was the banner of an entire generation, but only few researchers were able to develop theirconcern about the degradation of our biosphere into a scientifically acceptable theory. A forerunnerof this movement was Odum [30] who must be credited as the first author to have consideredglobal energy accounting in complex systems, addressing the question of ‘valuing’ (and conse-quently pricing) inputs which were considered ‘non-energetic’ , as solar radiation, land surface,crops, fisheries and labour. The approach chosen by Odum and its school is based on the definitionof a sort of ‘cumulative energetic index’ (later called ‘embodied energy’ or ‘eMergy’ [31]) thatrequires the introduction of a set of energy quality indices (the ‘ transformities’ ) to quantify the‘quality’— in the current meaning of the word—of different energy carriers. Since the ‘ referencebasis’ of all energy fluxes is—according to Odum—the amount of solar radiation impinging ona certain region, some discrepancies are brought into the ‘emergy balances’ by the non-uniquelydefined transformities. In spite of the large number of published applications, it seems that emergyanalysis may be a good tools for the global assessment of biotic systems, but is so distant fromthe engineering concept of energy flow and of efficiency that its use as a design procedure ispresently being questioned. In their most recent works, emergy supporters try to amplify thesimilarities between emergy and exergy [31,42]: but since some of the differences are striking(emergy in general does not enjoy the additive property, is not conserved like energy, and itstransformities must be either assumed or calculated from field studies), it is dubious that the twoanalyses may ever converge any further.

Another method was developed in the 1970s to assess the environmental impact of a givenprocess or technology: life cycle analysis (‘LCA’ : for a recent review, see Ref. [16]). This methodis a development of a previous concept called ‘net energy analysis’ [22], which was aimed at thecalculation of the net energy output of a given energy conversion device, ‘net’ here meaning thatthe balance is computed over the entire useful life of the system, and that the total amount ofenergy and materials used in the fabrication process, in the maintenance, and in the decom-missioning or disposing of the device is subtracted from the gross overall energy generation. Inthe course of the years, LCA has been expanded to include items as labour (accounted for on apurely monetary basis), environmental damage (mostly, taken to be equal to the actual or foreseenclean-up costs), and recycles (if byproducts of a process P1 are useful inputs for another processP2, their life-cycle equivalent energy content is considered to be positive in the balance of P1).LCA has undeniable merits but, being essentially a first-law type of analysis, its validity is taintedby its inability to account for different types of energy carriers.

A natural extension of LCA is therefore the exergetic life-cycle analysis, or ‘ELCA’ [9], whichperforms a lifelong analysis of a plant or process using exergy as a quantifier, thus expandingthe capabilities of the ‘energetic LCA’ . Both in ELCA and in LCA, all economic issues areentirely left out of the picture (the quantifiers are ‘exergy’ and ‘energy’ , respectively).

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More recently [3,33] an exergetic approach to the calculation of environmental costs has beenproposed: the general idea is that of establishing exergy as the only proper measure of environ-mental impact. Notice that environmental costs (in the monetary sense) can be included in thermo-economic analyses by just expanding the control volume beyond the plant under consideration,to include a portion of the biosphere, named ‘ immediate surroundings’ [28]: process calculationsmust be then accordingly extended by assuming that the effluents from this enlarged controlvolume are at average conditions equal to those of the general environment (zero-impact) or tothe legal pollution limits.

In this sense, the environomic approach [14,15,46] represents already a significant extensionof thermoeconomics, though its ‘environmental penalty’ functions suffer from their direct depen-dence on monetary cost.1

Extended exergy accounting incorporates some elements of all of the above-describedapproaches, so that it can be properly considered a further development of the pre-existing theoriesand methods of ‘engineering cost analysis’ . Some justifications of the name chosen for this newmethodology are in order: the attribute ‘extended’ refers to the additional inclusion in the exergeticbalance of previously neglected terms (corresponding to the so-called non-energetic costs, to lab-our and to environmental remediation expenditures); the word ‘accounting’ has been indepen-dently suggested [3,37] as a reminder that exergy does not satisfy a balance proper, in that theunavoidable irreversibilities of real processes irrevocably destroy a portion of the incomingexergy.

2. Description of the method

Consider a process P (Fig. 1) in which a certain well-defined material output is produced: theinputs to the process are represented in the most general case by a stream of raw materials (I1),an energy supply (I2), a monetary inflow (I3) (capital and operating costs) and human labour (I4).

Fig. 1. Definition of extended exergy accounting: exergy diagrams of a generic process.

1 The idea of linking the environmental ‘cost’ of a process to the exergetic content of its byproducts was introduced in Ref. [2]:but the concept developed therein (the assumed proportionality between toxicity and exergetic content) has been since discounted asbeing an undefendable assumption.

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The outputs consist of the desired products (O1 and O2), of some energy rejection to the environ-ment (O3), of byproducts (O4), and of some waste (O5): since physical exergy is not conservedin real processes, there will be a flow representing the exergy loss, indicated by El. There mayof course be several types of raw materials, several types of different energy inputs, etc.: for thetime being, we shall be content with this rather simple (but complete for our purposes) descriptionof P. Assume that it is indeed possible to attribute an exergetic value to each one of the inputand output fluxes (using the procedures discussed in Sections 3 and 4 and more extensively inRef. [36]), and that the process internal structure is known. This means that the transfer functionof P, i.e., a formal expression that links the outputs with the inputs [34], is known explicitly, sothat given the proper process design specifications, the ‘O’ fl uxes are known if the ‘ I’ fl uxes are,and vice versa [36,40,44]. The primary conversion efficiency of P can be computed as the ratioof the exergetic value of the useful output (O1 + O2) to the sum of the inputs that concurred toproduce it:

h �

�useful

Ok

�j

Ij

�O1 � O2

I1 � I2 � I3 � I4

. (2)

The total exergetic cost of the outputs O1 + O2 is defined as the amount of input required(‘spent’ ) to generate the output, and is the reciprocal of the conversion efficiency:

cO1�

1h

�

�j

Ij

�useful

Ok

�I1 � I2 � I3 � I4

O1 � O2

. (3)

If a portion of both the energy discarded, say aO3 and of the waste materials, say bO4, areused again (‘ recycled’ ) in some other process belonging to the productive structure of which Pis part, the primary conversion efficiency (expressed by Eq. (2)) remains the same, but in realterms the value of the overall process efficiency must reflect this increased ‘usefulness’ of thewaste streams:

e �

�useful � recycled

Ok

�j

Ij

�O1 � O2 � aO3 � bO4

I1 � I2 � I3 � I4

. (4)

The exergetic cost of the global unit output can again be computed as the reciprocal of theoverall process efficiency:

cO1+aO2+bO3�

1e

�I1 � I2 � I3 � I4

O1 � O2 � aO3 � bO4

. (5)

We see thus that in EEA, unlike in other current costing methods, it is not necessary to apportionthe cost between the main and the ‘secondary’ outputs, because all of the expenditures incurred

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in during the process of fabrication of both O1 and O2 have been calculated in homogeneousterms, so that, for example, the specific exergetic cost of the stream (1�aO3) has been alreadydiscounted for in the expression of the cost of the ‘useful’ and ‘ recycled’ products.2

The very same considerations apply to a chain of technological processes that represents thefabrication of some generic product (O6 in Fig. 2): formulae equivalent to Eqs. (1–5) can be usedto calculate the conversion efficiency of a given technological chain if its structure and the transferfunction of each fabrication step are known. Notice that the analysis may be broadened by per-forming either one of the following steps:

� While maintaining unchanged the chain structure and the type of the individual processes Pi,modify some of the design parameters for one process Ps, to assess its influence on the overallconversion efficiency: this is a sensitivity study;

� Modify the efficiency of one or more of the processes Pi of the fabrication chain, and recomputethe overall conversion efficiency. This constitutes a comparison of different technological scen-arios;

� Compare two different technological chains that produce the same output, to assess their relativemerits for what resource conversion is concerned. This amounts to performing a comparisonbetween different production technologies.

It is also apparent that the procedure can be nested, i.e. applied at different levels of aggregationin a productive structure [4,35]: for example, each one of the three processes ‘extraction’ , ‘pre-treatment’ and ‘fi nal’ shown in Fig. 2 could be analysed in detail (‘disaggregated’ ). Or, severaltechnological chains could be joined (‘aggregated’ ), and each one of them treated as a singleprocess whose transfer function is known: since the results are all expressed in homogeneousunits (kW per unit or per unit mass), they can be easily transferred from one aggregation levelto another, resulting in a general and very powerful analysis tool.

The above-described procedure is still in essence the ‘cumulative exergy consumption’ method

Fig. 2. Extended exergy flows of a technological chain based on an extracted resource.

2 The usual ‘cost allocation’ , and specifically the one adopted in thermo-economics, can of course be introduced in EEA as well:in this case one recovers the ‘structural thermo-economic theory’ of Valero [44]. Such an assumption is though against the ‘spirit’of EEA, which maintains that all products must be assigned a proportional share of the exergetic production factors (capital, labour,materials, energy and environmental costs).

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[40]: to show where and to what measure extended exergy analysis differs from it, it is necessaryto analyse in detail how the exergetic contents of individual streams are calculated.

3. Methodological remarks

3.1. Choice of exergy as the universal quantifier

Exergy has several qualities that make it a very convenient basis for an energy-accounting para-digm:

1. Physical exergy is defined for a stream of matter at a certain thermodynamic state ‘1’ as,3

e1 � h1�h0�T0(s1�s0) � �i

(mici�m0c0) (6)

Once the reference state ‘0’ has been selected, its value is uniquely determined by the respectivestate parameters.4

2. Physical exergy enjoys the additive property. It is therefore possible to calculate the value ofthe ‘cumulative physical exergy content’ of a product by summing up all the contributions tothe different streams that were used in its fabrication, starting from the original values of themineral ores that constituted the initial inputs in the process.

3. If a proper ‘Earth average chemical reference state’ is defined, the chemical portion of thephysical exergy of a mineral ore is either zero (if the ore is at that composition) or attains avalue exactly computable on the basis of the ore’s chemical composition, its physical state andthe Gibbs energy of formation of its constituents [1,40,45].

4. If an effluent stream of a generic process is required to have a zero impact on the environment,the stream must be brought to a state of thermodynamic equilibrium with the reference statebefore being discharged into the environment. Assuming no recovery is performed, the mini-mum amount of exergetic ‘expense’ required to perform this task by means of ideal transform-ations is by definition equal to the physical exergy of the stream that is destroyed in the ‘ return-to-equilibrium’ process [35].

5. As a consequence of the above points, an invested exergy value can be attached to any product,and specifically to mechanical, thermal and chemical equipment: this invested exergy is equalto the sum of the cumulative physical exergy of product, and the ‘environmental disposalexergy’ necessary to the ideal, zero-impact disposal of the equipment.

6. Notice that extended exergy enjoys additivity and is not conserved in real processes.

3 Eq. (6) contains only the thermodynamic and chemical portions of the exergy content of the stream: if other forces and gradientsare present and relevant, additional terms (magnetic, nuclear, molecular) may be added. Thence the use made here of the apposition‘physical’ as a reminder of this restricted definition.

4 See though reference [19] for a novel extension of the derivation of the concept of exergy that makes it somewhat independentfrom a rigid definition of a fixed ‘environment’ .

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Fig. 3. The correct control volume for EEA analysis.

3.2. Boundaries of an EEA analysis in the space and time domains

In EEA, the proper control volume CV for the analysis of a process P must be chosen farenough from the physical boundaries of the real process to include a sizeable portion of theenvironment, the ‘ immediate surroundings’ [27]. The extension of this portion depends on thetype and quantity of effluents: in principle, the immediate surroundings must be large enough toallow for the ‘ treated effluents’ (in the sense of point 4 in Section 3.1) to exit the CV in a stateof zero physical exergy. Furthermore, the CV must also include the portion of the environmentwhence the original materials were extracted, so that their initial exergetic value may be computed(Fig. 3).5

As far as its time-extension is concerned, EEA has many similarities with life cycle analysis:the time window over which the calculation of the exergy flow diagrams is performed is ofrelevance in the analysis (Fig. 4). Time variations of some of the input parameters cannot be

Fig. 4. The time window for extended exergy accounting.

5 This requirement may impose a very impractical burden on actual process calculations. A practical ‘shortcut’ is that of prescribingthe EE values of the process raw inputs. Such estimates may be inferred from prior knowledge of similar processes or acquired froma previous simulation of the same process.

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neglected, and in essence, the analysis ought to span the life of the product or plant. Of paramountimportance is the choice of the time frame over which mineral ores or fossil fuels are exploited,because the extended exergy of their extraction may vary with time (mining conditions reflectthe relative scarcity of the material, and so do the exergetic costs of fossil fuel extraction). Alsoof importance is the time scale assumed for the degradation of the effluents:

� If a zero-impact is specified, then the effluents ‘clean-up’ must take place over time scalesequal—in order of magnitude—to the production time scales;

� In real processes it is more likely that a conservative approach be taken, and that a certaintime interval be prescribed within which the effects of the effluents must be completely bufferedby the immediate surroundings (‘biodegradability’ ).

Extending this time interval decreases the clean-up costs, but in effect constitutes a substantialderangement from the ‘zero-impact’ assumption: An excessively short time interval fails toappreciate (and exploit) the buffering capacity of the biosphere. Care must be exercised whenevaluating these two issues: no general rule is apparent, and decisions have to be made on a case-specific basis. The choice of the time window is particularly important when a society as a wholeis examined: aside from obvious considerations on the time-dependency of the structure of theindividual sectors in which a society articulates itself, a proper window choice is mandatory forpolicy-planning consistency [2,8,35]: many published energy studies and projection are taintedby a confusion between steady and unsteady sustainability, that could have been avoided by aproper choice of the time window.

4. Calculation of the exergetic content of a stream

4.1. Material and energy flows

For material streams, a procedure similar to that originally proposed by Szargut is employedin EEA: a material is assigned an extended exergetic content (‘EEC’ ) given by its ‘ raw stateexergy’ augmented by the sum of all the net exergetic inputs received, directly or indirectly, invarious processes pertaining to its extraction, preparation, transportation, pre-treatment, etc., andincluding the exergetic equivalents of labour, capital and environmental costs (Sections 4.2, 4.3and 4.4). The ‘ raw state exergy’ is computed with reference to an assumed average compositionof the Earth’s crust: there is disagreement among researchers on what exactly this ‘average compo-sition’ ought to be [1,40,45], but for our purposes here it is only important that the choice, what-ever it is, be used consistently. It is interesting to remark that the second part of the exergeticcontent (the ‘process exergy’ ) may assume different numerical values for different productionchains: so, for instance, a forged steel artefact has a different exergetic content as the same artefact,cold rolled. If an input or an output consists of an energy flow (mechanical power, electricalenergy, heat transferred under whatever mode, chemical or nuclear energy, etc.), EEA simplyadvocates the use of the corresponding exergy value for that flux.

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4.2. Labour and human services

The issue of properly valuing the equivalent labour input (taken here to include all service-related, blue and white collar human activities) has been debated for a long time. While at present(in industrial cost accounting [2], but also in thermoeconomics [6,12,41,43]) labour is accountedfor on a purely monetary basis, very credible different approaches have been proposed. Due tospace limitations here, we must refer interested readers to [21,29,34,48]. EEA attaches to labour,and in general to human services, in a certain society (or part thereof) an equivalent exergeticvalue computed as the ratio of the cumulative exergy consumption of that society to the totalnumber of man-hours it supports:

Klabour �Ein,Society

nworkers∗(work hours /year)[J /work hour] (7)

whereas the extended exergy value attached to a certain number Nw of work hours is:

EEN � Klabour∗N.

This formulation leads to an important consequence (Fig. 5): if a certain portion of humansociety could be analysed in isolation from the rest of the world, the workers’ contributions wouldnot cancel out in the overall balance of that portion, because the exergetic input that ‘sustains’the workers is constituted not only by the products generated by their own activity, but also bysome net exergetic inflow of material (e.g. water, wood) and energy (e.g. solar radiation, fossilfuels) provided by the biosphere.6

Fig. 5. Simplified model of a society–environment interaction: the exergetic equivalent of the labour contribution (E4)does not cancel out with the exergetic input into the domestic sector (E2 + E3).

6 It can be successfully argued that a flow of material resources is fully equivalent to a well-defined amount of direct or indirectsolar radiation. For our purposes, the distinction is irrelevant here (but, see Ref. [31,48]).

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4.3. Capital

Parallel to the debate over labour, a similar debate has been staged over the years about howto include capital and, in general, financial costs in the picture. The dominant paradigm is todaythe so-called ‘neo-classical theory of economic price’ (‘NCE’ in the following), which employsmoney and its time-value as a quantifier for goods, services, labour, resources, etc. In spite of theclear inability of neo-classical economic theories to properly deal with the so-called externalities(environmental impact of anthropic activities, effects of dwindling resource basis and of a chang-ing environment, time-related mutations of the biosphere), the rationale often invoked by thosewho support an extension or adaptation of NCE is that, on an average world basis, the contributionof energy-related costs to the global gross national product varies between 2 and 9%, and thattherefore money (i.e. capital) and not energy is the main quantifier of human activities. Thisparadigm of money as a basic metric for energy-related activities is so deep-rooted into our culturethat even technical experts of a specific industrial field tend to attach to a product its monetaryprice, rather than its energetic (and much the less its exergetic!) content.

There is growing concern though that NCE’s inability to deal with some of the new issues ofglobal environmental quality and of resource scarcity may lead to the wrong choices in addressingmedium- and long-term energy policies. Furthermore, a new point of view is coming of age:economic systems are eco-systems that function only because of the energy and material fluxesthat sustain human activities. In fact, all our agricultural, industrial and economic activities canonly exist as long as they exploit (‘use’ ) biophysical resources taken from a reservoir of non-infinite material capacity.7 Thus, it is conceivable to turn around NCE’s point of view, and valuehuman activities on the basis of a different metric, founded on an energy quantifier. This reversalof the scale of values is exactly the issue raised by EEA, that in addition maintains that exergybe the right quantifier. In this perspective, it appears clearly that it is not capital that ought tomeasure the value of a piece of equipment or of a product by attaching a price tag to it, butexergetic content; and that the monetary price ought to reflect this new scale of values.

It must be stressed that EEA does not demand for a utopic cancellation of monetary prices. Itis clear, on the contrary, that the very structure in which we live and function today demands forthe conservation of the ‘price-tag’ concept in common social and industrial activities. What EEAadvocates is that this tag be calculated on the basis of the extended exergetic content EEC of acommodity or service, corrected for environmental impact (see Section 4.4). It is also immediatelyclear what the numerical relation between the EEC and the price of a product ought to be. Sincethe EEC is expressed in kJ/unit, and the price in /unit, the conversion factor KCapital is the recipro-cal of the cost of the unit of exergy, which for each country is easily computed as the ratio ofsome measure of monetary circulation to the global exergy input.8 As proposed in Ref. [36]:

7 It is very convincingly argued in Ref. [2] that, for a given global resource base it is the (solar) total exergy flux that sustains asociety. The technological state-of-the-art and the recycle rate determine in effect only the energy-pro-capite level at which the societycan sustain itself. These considerations are not invalidated—over anthropic timescales—by the bounds set by the Georgescu-Roegen[20] ‘material entropy production’ theory (see [2,36].

8 It is important to note that: (a) Two global monetary circulation indicators are published by the central bank of most countries:M1 and M2. They represent the total adjusted cash circulation in an economy in a given period (here, we use the annual averagevalue). M2 excludes short-term credits, and is therefore a few percentage points lower than M1. (b) It is therefore irrelevant inpractice which measure of monetary circulation is taken to compute KCapital. What is important is that we abandon here the conceptof gross national product (GNP) as a measure of the economic wealth of a society. (c) A problem does exist in choosing the correct

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KCapital �Ein,Society

M2[J / ]

whereas the extended exergy corresponding to a monetary flux C is given by:

EEC � KCapital∗C.

When processes are analysed, it is the exergetic equivalent of the capital expenditure that mustbe inserted in the balance as an input, and the monetary revenues must be converted to an exergeticoutput. It is certain that for most processes this substitution will show a discrepancy in the exergybalance. These are adjustments caused by the present over- or underestimates of the real exergeticcontent of materials, feedstock and energy flows. It would be auspicious that the economic andthe exergetic value become consistent in the long run, meaning that the two scales have reacheda sort of fixed parity. Of course, the very same definition of the exergy-equivalent implies thatdifferent countries (or regions) may have different KCapital, due to their different social and pro-ductive structures.

4.4. Environmental impact

It was the attempt to include in a deterministic economic treatment the complex issues relatedto the environmental impact that stretched the NEC theory, forcing its users to incorporate vagueand volatile concepts like ‘global risk assessment’ , ‘willingness to pay’ , and similar, which con-tributed to expose the weak point of the theory, and detract from the credibility of any process-or plant economic analysis. A generic chemical pollutant must be disposed of in a relatively shorttime, and the technological effort to reduce it to a set of components of zero environmental impacthas in effect an extremely high monetary cost. Therefore, once a substance is acknowledged asharmful, it becomes ‘ regulated’ , i.e. legal upper limits are set to its free release into the environ-ment. This is equivalent to setting—with the current technology—an upper tolerable price to theclean up for that particular effluent. However, if the cumulative amount of the emissions of thatpollutant, say over a year, is such that its influence on the overall balance of the biosphere isnon-negligible, then the only way to make a non-zero limit ‘acceptable’ is to assess the risks tomankind in terms of (monetary) health- and life-expectancy parameters, and decide to set an upperbound to the expenditures in such a way to remain below a certain statistical probability ofincurring in that risk. Since in reality there is no ‘clean’ technology, this economical burden istransferred both ways in the economy: downwards, affecting the price of resources that are per-ceived to be ‘more or less clean’ , and upwards, increasing the price of products seen as ‘ lessenvironmentally friendly’ . But ‘willingness to pay’ and the attitude towards a ‘sustainable resourceexploitation’ are different in different countries, and may well vary in time. Moreover, the disturb-ances caused by restricted or relaxed environmental regulations affect regions possibly far awayfrom the point of origin. Thus, the system becomes intrinsically unfair, because not only thepollution, but also the health risks are transferred from a ‘willing’ region to its ‘unwilling’ neigh-bours. Only recently a solution to this situation has been sought, by trying to link the monetary

‘exergy input’ indicator. We choose Wall’s approach [47], and consider the total exergetic input due to all incoming resources. Whileother choices are indeed possible ([3,30]), they seem to introduce an unnecessary bias into the procedure.

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structure of the environmental levies to some energetic consideration. This is the rationale of the‘pollution commodity trading’ and of the ‘exergy tax’ [5,21,23,33,48].

EEA advocates a substantially different approach: consider a process P (Fig. 6a), and assumethat its only effluent is a stream that contains hot chemicals, some of which not present in theenvironment. To achieve a zero environmental impact, these chemicals must be brought first tothermal, and then to chemical equilibrium with the surroundings, and this can be technicallyachieved in several ways. In such processes, usually no exergy recovery is implemented, andtherefore the exergetic ‘cost’ of the zero impact, i.e. the exergy that must be wasted in the clean-up, is equal to the physical exergy of the effluent. This exergetic cost corresponds to the exergyideally required to cool the effluent and break it up into its constituents such that each one ofthem is in equilibrium conditions with the surroundings. Real effluent treatment processes requirea much higher exergetic input than ideal ones: as shown schematically in Fig. 6b, the additionalprocess Pt requires an energetic input, possibly some auxiliary materials, labour and investedexergy, so that its output will have a zero physical exergy. These additional exergetic expendituresrequired by Pt must be charged to the effluent O2, whose extended exergy will now be higherthan its original one, because for any real process Pt the net exergetic input will be higher thanthe annihilated physical exergy. Therefore, the overall conversion efficiency of the joint process(P + Pt) is decreased. There may be effluents for which at least a part of the chemical decompo-sition reactions take place ‘spontaneously’ , in a reasonably short time and in the immediate sur-roundings of the emitting source: but, as correctly remarked in [23], in such cases (Fig. 6c) thereactions must draw on some exergy source within the environment (which can be a certainparticular chemical, oxygen, water, solar radiation, or even a biological system), and this exergyconsumption must be charged to the useful products of the main process.

EEA leads to the formulation of some alternative strategies to deal with the treatment of efflu-ents, and a discussion is given in [33]. The important issue here is that it is possible to consistentlyincorporate the effects of effluent treatment into the extended exergetic balance of a process, andthat EEA provides guidelines as to the minimum resource consumption necessary to achieve zeroimpact. Notice that, if an acceptable level of pollutant is specified, then the minimum exergeticexpenditure will be proportional to the difference between the values of the physical exergies ofthe effluent stream between the point of its release and the ‘ regulated’ state point. This last con-sideration shows that EEA can be usefully applied to real industrial cases without reference toits physical exactness.

5. An example of application of the extended exergy accounting method as a design andplanning tool

To show the potential of the EEA method, let us consider an admittedly rather abstract techno-logical comparison, between two possible solutions to the problem of waste disposal [38]. Thefirst one is what we could call a non-integrated, sustainable option (Fig. 7a): the waste producedin the domestic sector of a society is collected, parsed and selected. The organic portion is thensent to a composting unit, while other components (glass, metals, paper, wood, plastics) are separ-ately recycled into the relevant portions of the industrial sector. The residual fraction of the refuse(with today’s standards, less than 10% in weight) is then sent to a waste treatment plant, which

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Fig. 6. (a) The effluent O2 is not at reference conditions. (b) Real treatment of O2. Each one of the final effluents isat its reference conditions. (c) Only a portion of the clean-up is performed by man-made treatment processes: theremaining takes place spontaneously in the immediate surroundings.

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Fig. 7. (a) ‘Total’ waste recycling: a non-integrated, sustainable option. (b) ‘Total’ waste recycling: An integrated,sustainable option.

incinerates the combustible portion producing some process heat, and chemically treats ashes andresidues to produce an inert final product.

The second solution, which we shall call an integrated, sustainable option, is similar to theprevious one in the first part of the process. The selected refuse is though split in two parts: one(indicated as 1�a in Fig. 7b) is recycled to the industrial sub-sectors, and the remaining portion(a) is incinerated together with industrial residues of medium-high calorific value. The ashes andthe residues are then converted into an inert final product.

Let us assume that both processes produce a ‘zero impact’ , which is a rather strong, but not

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technically unrealistic assumption. Both processes would therefore abide by the current definitionof ‘sustainable technology’ . How can we correctly assess the relative merits of each process? Toanswer this question, at least qualitatively, let us examine the way three different methods ofengineering process optimisation approach the problem. Table 1 is a synthetic representation ofthis examination.

5.1. Engineering economic analysis

According to this procedure, we should separately analyse the capital and operating costs ofeach process, and select the one with the lower overall ‘production costs’ . Adopting this point ofview, we would then compare the performance of both processes over a, say, 20-year period, andcompute the cumulative net value of the produced energy. Then, the price of the product ‘heat’or/and ‘work’ generated over this lifetime can be calculated via the usual engineering economiccalculations. Since the ‘upstream’ portion is common to both processes, the differences would bedetected in the recycling and in the incineration/inertisation facility. In detail:

� The total mass flow of the recycle is lower in option B, and thus all transportation expensesshall proportionally be lower (even if the reduced transportation costs are partially offset bythe necessity of channelling the industrial high-LHV residuals to the incinerator).

� The size of the incineration/inertisation unit per unit of mass of original domestic refuse is alsosmaller for this option, so that there is another advantage here.

� On an overall societal point of view, the industrial sectors will benefit from the recycling onlyif the ‘cost’ of the recycled material is lower than the cost of extraction, transportation andpreparation of the equivalent raw materials. Under ideal conditions, Option A is a favourite inthis regard.

� The fuel costs would be lower for option B, because the mass flow rate of expensive high-quality fuel required to ignite and burn the low-LHV refuse is strongly reduced by the adoptionof high-LHV industrial residuals (here, too, allowance must be made for the additional expensesrequired by the more sophisticated combustion control and gas clean-up devices).

From the above considerations, concisely represented in Table 1, we conclude therefore thatoption B is likely to be more convenient than option A. It is well known though that, if a detailedcalculation of the cost of the generated steam and electricity is carried out, there is no economicconvenience in implementing this type of projects as an industrial enterprise, unless a ‘waste tax’

Table 1Schematic comparison of the two waste-disposal options

Cost Item Option A Option B ‘Better’ option

Capital Higher Lower BLabour Lower Higher AMaterial Indifferent Indifferent /Energy (fuel + transportation) Higher Lower BInduced benefits Higher Lower B

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is levied, so that the operator of the plant receives monetary compensation to make up for theeconomically deficitary operation of his plant. Normally, an ‘ incentive to recycle’ type of govern-ment action would be necessary to ‘ force’ the actual implementation of this technology. As aresult, it may well be that a process with an overall negative economic balance be forced intoproduction on the basis of monetary/regulatory issues.

5.2. Thermo-economic analysis

Though more correct than energy analysis in assessing the real efficiency of either option, athermo-economic analysis would lead to the same results. Option B would in all likeliness beselected, on the basis of the same considerations as those developed in Section 5.1 above. Only,thermo-economics makes it possible to assign a correct ‘production cost’ to both the process heatand the electricity generated by the process. Also the savings in fossil fuels generated by therecycling would be assessed in a more exact way, because they could be correctly split between‘process heat’ and ‘motive power’ savings, which have different exergetic factors. In conclusion,a thermo-economic analysis would probably lead to the same result as the previous analysis, andsuggest the pursuing of option B: but both the ‘waste tax’ and the ‘ incentive to recycle’ wouldassume two different monetary values from those calculated with purely monetary methods.

5.3. Extended exergy accounting

An EEA assessment can be developed under the following guidelines.

5.3.1. Exergetic equipment costHere, Option B would be the winner. The quantitative ‘gap’ between the two alternatives though

would not depend on monetary factors alone, but on several other factors, one on them being forinstance the labour intensity of each device, another being the extended exergy needed to decom-mission the plant, etc.

5.3.2. Operating costsIn this case, it is likely that option A, intrinsically less labour-intensive, would be the better

one. The fuel costs are comparable, because EEA attaches a non-negligible cumulative cost tothe ‘ industrial residuals’ which option B employs as fuels.

5.3.3. Comparison of the life-time extended exergetic ‘effectiveness’ of the processFrom an EEA point of view, all processes have an overall negative exergy balance over their

lifetime (Fig. 4). The fact that option B is likely to attain a better exergetic efficiency does notnecessarily make it automatically the better option. Since EEA heavily weights the ‘extractionexergy’ , it is possible that option A, which maximises the recycling, obtains a better EEA rating.The decision depends on the results of a lifetime comparison: if EEex,saved represents the lifetimesavings in the extraction, transportation and pre-treatment extended exergy costs and EEconsumed

represents the overall value of the extended exergy deficit over the plant lifetime, the ‘optimal’process will be the one for which the difference between EEconsumed and EEex,saved takes thesmaller value:

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�lifetime

{�EEconsumed��EEex,saved}dt � minimum.

6. Conclusions

A novel approach to the computation of the ‘costs’ of technical processes is proposed and someof its consequences explored.

The analysis shows to be both reasonable and productive, in that it offers several advantageswith respect to the current evaluation techniques:

1. EEA expresses all costs in congruent units (kJ/(kg of product), or kJ/(kJ of work)), so thatthese costs can be directly added;

2. EEA does not need conversion factors or complex statistical theories to compute labour andenvironmental costs;

3. EEA leads to results that are directly comparable with those produced by the current (thermo-)economic theory;

4. EEA ‘costs’ can be directly converted into monetary costs by pricing the kJ of exergy inputin the societal structure under consideration. The claim here is that such a price can be computedas the ratio of a proper measure of the global monetary circulation in the system under consider-ation to its corresponding total resource consumption. Such a definition leads to two importantconsequences: first, it characterises both in time and in space a cost/exergy balance of a process(because the same process would have different costs in different countries and, in the samecountry, at different times). Second, it projects an immediate perception of the imbalancesbetween different economies, and of the specific reasons thereof.

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