On the applications of energy analysis and second law analysis

On the applications of energy analysis and second law

analysis

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Willeboer, W. (1986). On the applications of energy analysis and second law analysis. (EUT - BDK report. Dept.

of Industrial Engineering and Management Science; Vol. 20). Technische Universiteit Eindhoven.

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Eindhoven

University of Technology

Netherlands

Department of Industrial Engineering and Management Science

On the Applications of

Energy Analysis and

Second Law Analysis

by

W. Willeboer

Report EUT/BDK/20

ISBN 90-6787-021-4

Eindhoven 1986

CIP-GEGEVENS KONINKLIJKE

BIBLIOTHEEK, DEN HAAG

Wiileboer, W .

On the applications of energy analysis and second law

analysis / by W . Willeboer . - Eindhoven : University

of Technology . - (Report / EUT, Eindhoven University of Technology? Department of Industrial Engineering &

Management Science, ISSN 0 1 67-9708 ; BDK/20)

Mrt lit . opg .

ISC?P! 90-6757-0 2 1- 4 SISO 6 44 .2 UDC 6 2 0 .9

ON THE APPLICATIONS OF ENERGY ANALYSIS AND SECOND LAW ANALYSIS

W .Wílleboer January 1986

CONTENTS SUMMARY 1 . INTRODUCTION PART I 2 . ENERGY ANALYSIS 2 .1 . Historical perspective 2 .2 . Definition and aims 2 .3 . Methods and conventions

2 .4 . Applications and later developments 2 .5 . Evaluation

References PART II

3 . SECOND LAW ANALYSIS 3 .1 . History

3 .2 . The exergy concept 3 .3 . Applications 3 .4 . Discussion References

3

4

SUMMARY

In this report, two methods of analysis in the field of energy utilization are discussed . Although the two methods are of quite different natures, it appears that, with respect to their application, the following conclusions apply to both of them :

They provide a fundamental improvement of the insight into the real backgrounds of the energy use and energy savings . Based upon this insight, certain limits can be indicated : limits to energy efficiencies and limits to (overall) energy savings .

The results are not useful in energy conservation projects for existing installations of individual industrial enterprises, but advantages of the methods under consideration are found especially in long term forecasting, in research for future production processes and plants, etc .

The usefulness of the methods in economic considerations is limited .

So it is evident that energy analysis and second law analysis can be applied in every consideration on energy utilization and energy saving, but their application is useful only in certain problems . In ordinary management and engineering questions they add few relevant information to that obtained from conventional analysis methods .

1 . INTRODUCTION

Until the oil crisis of 1973, for most industries energy was a production factor that nobody specially cared for : fuels and electricity were cheap and abundantly available . However, due to the rapid price changes and the reduced availability of energy during the past ten years, this image has changed drastically . Today, energy is a subject that receives more or less special attention in almost every industrial enterprise . Many techniques and methods for managing the energy use and utilizing it better have been developed and introduced . This report discusses two of them : "energy analysis" and "second law analysis" . These methods are tools of a general nature for investigations in the industrial energy use .

The reason for treating these methods in this report is that they are considered and presented by some workers in the field as a "must" in almost every consideration in which energy is concerned, while many practitioners in industry consider them as too academic and too theoretical or at least too complicated for practical use . The aim of this report is to give a brief description of each of the methods, completed by a view of the different applications, and, based upon that, to discuss the relevances and

the advantages of the method in the alternative fields of application . After that, recommendations are given with respect to the use of the methods .

Because both methods have quite different natures and aims, they are treated separately : Part I (Chapter 2) deals with energy analysis and Part II (Chapter 3) discusses second law analysis .

The subject of this report was treated more briefly and with a less general aim in [2 .39], Chapter 2 . This report gives descriptions and applications in more detail and the discussion of the relevance of the method is not restricted to a certain field of application .

PART I

2 . ENERGY ANALYSIS

' 2 .1 . Historical perspective

For over a hundred years scientists and engineers have been interested in the energy used by machines and processes (e .g . [2 .24], [2 .27], [2 .37]) . In recent decades, technologists have been trained more and more to consider energy use in the design of processes . Especially in energy-intensive industries like electricity generation and certain branches of the chemical industry, there has always been much interest in the efficiency of processes, due to their high energy costs . However, when considering energy requirements for making a certain product, until about 1970, it was very unusual to include the related energy used outside the factory itself . The inputs of raw materials etc . to the processes were thought of in tons and in terms of money . Although some people like Soddy [2 .34] and Chenery [2 .8] made suggestions that the production theory should include energy in physical terms rather than in terms of money , this subject never attracted much great interest because energy represented no more than 4-6% of the cost

of the inputs in an industrial economy [2 .22] .

However, due to the sudden rise (by more than 200 percent) of the price of oil in 1973, several people started to look at the total amount of energy utilised in making goods and services, taking into account the processes all the way from ores in the ground to the final product . Early signals, with respect to this thought, had already been published in 1964 by Gehrecke [2 .13] ; he made such analyses in order to determine the share of energy in the total costs of industrial products . Rather alarming thoughts on the general situation with respect to energy use were published by Berry [2 .3] and Makhijani and Lichtenberg [2 .28] . The latter report was even discussed in the U .S . Senate in 1971 [2 .9] . From 1973 on, more attention was paid to this subject and it led to the idea that energy is embodied in goods or services, so that one can talk about the energy requirement of products in terms of MJ per kg .

The first papers with actual values for certain products appeared, e .g . from McGowan and Kirc.hhoff [2 .18], Berry and Fels [2 .4] and Chapman [2 .6], which were all about the energy needed to produce an automobile, and from Leach and Slesser [2 .27], about the energy used in agriculture . Moreover, Odum published a book [2 .28] in which he introduced an energy theory of value : he proposed an economic system in which the value of commodities is expressed in terms of energy and not in terms of money . With reference to these - and other - individual activities that had started, in 1974 the International Federation of Institutes for Advanced Studies (IFIAS) organized a workshop [2 .22] where some of the principal practitioners of this new field, which was baptised "Energy Analysis", came together . The aim of this workshop was to propose a set of definitions and guidelines for energy analysis . So in fact this workshop was the point "where it all started" .

2 .2 . Definition and aims

At the IFIAS workshop in 1974, energy analysis was defined as "the determination of the energy sequestered in the process of making a good or service within the framework of an agreed set of conventions or applying the information so obtained" .

A distinction was made between the direct and the indirect energy use of a production system : The direct energy use is the energy used in the production processes ; it enters the production system as fuel, electricity etc . The indirect energy use is the energy that was used in making the raw materials and production equipment itself ; in fact it is the energy embodied in the raw materials, the semi-fabricated articles, the production installatations, etc . A clear distinction between the direct and the indirect energy use can only be made on the basis of a clear definition of the system boundaries of the production system considered . The fact that energy analysis considers also the indirect energy use is the main difference with conventional energy studies .

The purpose of energy analysis can be summarized in the following points : - First of all, energy analysis is a descriptive technique : it is

concerned with describing present industrial energy use . Such a description provides information needed to compare different practices and to examine new designs . This can be done either, at company, or at national level . An essential point is the fact that, in principle, all direct and indirect energy use of the production system must be included .

- It can serve as a means of identifying the constraints to a system . The most important of these is the lower limit of the energy requirements for a process, which can be determined by means of thermodynamic calculations . By comparing these theoretical energy requirements with those of the present technology, one gets an indication of the extent to which the given technology could be developed .

- With respect to decision-making, energy analysis is complementary with economics : it provides information that cannot be derived from purely economic data . For instance, the question of the impact of energy price changes on the cost of the final product can be answered only by applying energy analysis .

Stimulated by the ideas of Odum [2 .28] some people thought of a more important role for energy analysis in the economic system : they favoured a method of valuation based on energy minimization alone (e .g . Hannon [2 .19] and the Technocracy Organization in the United States [2 .35]) . However, many others in the field did not support these ideas . Because of the different opinions with respect to the role of energy analysis in economic allocation, a second IFIAS-workshop was organized, which especially dealt with the interface between energy analysis and economics [2 .23] . The main conclusion of this workshop, attended by economists and scientists, was that "one of the principal roles of energy analysis is to furnish information

that may be utilised in the allocation of the scarce resource energy, but that this important function should not be interpreted as implying an energy theory of value" . A value system based on that single factor "energy" was considered to be unsatisfactory for analyzing modern economies .

The general conclusions of the IFIAS-workshop in 1975, later on confirmed by Slesser [2 .33], were that energy analysis provides information for the physical evaluation of production systems and products, it indicates thermodynamic limits, and it is useful for a second order "feel" in economic questions . Moreover, i t can be used as a powerful tool i n technology assessment studies .

-2 .3 . Methods and conventions

In [2 .22] the conventions that were made in 1974 are summarized with respect to the most important practical aspects of energy analysis . These conventions concerned units of account, system boundaries, the inclusion of man-power, the methods to be used, etc . With respect to the unit of account, it was decided that, in principle, Gibb's free energy (in terms of Joules) is the best measure ; however, because of practical difficulties, for fuels, gross enthalpy can be used, as it is sufficiently accurate in that case . The energy requirements to be considered can be expressed in the following terms :

- gross energy requirement (GER), defined as the amount of energy source which is sequestered by the process of making a good or service . It may be expressed in terms of enthalpy (GER) or free energy (GFER)

- net energy requirement (NER) . This is the GER less the gross energy of combustion of the product

- process energy requirement (PER) : the net energy requirements of all inputs and all stages of the process .

- gross energy requirement for fuel (GER fuel) is the GER of the total amount of the fuel input that is required to furnish one unit of delivered energy

- net energy requirement for fuel (NER fuel) is the GER for fuel le ;s a credit for any unconsumed energy in the waste products after energy transformation

- energy requirement of energy (ERE) is the ratio of energy sequestered to deliver a unit of energy divided by the unit of energy . Thus : ERE > 1 .

No such clear conventions could be made with respect to the boundaries of any system considered in the analysis . In principle, it is always necessary to make the boundaries as wide as possible, but, in practice, it depends on the situation . In the energy input for producing a certain commodity, the following levels can be distinguished :

1 . The direct energy use of the production process 2 . The energy embodied in the material inputs

3 . The energy required to make the process equipment and the other parts of the production system .

4 . The energy required to make the machines that make the machines etc .

It will be evident that, in determining the data of the subsequent energy input levels, one will always reach a level where the outcomes are smaller than the accuracy of preceding levels . Which level this will be depends on the situation .

An important point when performing an energy analysis is the way in which the numerical data can be obtained . In fact, the choice of which kind of system can be analysed depends on the type of data available . An energy analysis can be made for a complete industrial sector, a certain factory, one process, or a certain product . Boustead and Hancock [2 .5] distinguished the following methods of deriving data for an energy analysis (also indicated in [2 .22]) :

- Analyzing general statistics, i .c . the statistics that are regularly published by trade associations, companies and governments . For instance, Chapman applied this method in [2 .36] . It will be clear that the outcomes of this method can be no better

-than average values with a high degree of aggregation in respect of different products, different processes, etc .

Input-output analysis . This method is based on an input-output table for an economy, which is a square matrix of numbers showing the transactions of commodities taking place between different sectors of industry . Such an analysis yields data that is more detailed than that obtained from analyzing general statistics . The form of an energy input-output table differs from that of a regular input-output table, see [2 .21] . The most obvious difference is the fact that the dimension of the elements is units of energy in stead of monetary units . An example of this method is Herendeen's "Energy input-output model for the United States" [2 .20] .

Process analysis . Analyzing the processes either, by measuring, or deriving performance data, or by deducing data from published papers or books, offers the possibility of giving the data as much detail as desired . As a consequence, the -detailed- results have more applications than a descriptive one alone . An example of such a process analysis, (although limited to just one process stage) is given in [2 .39] .

Of course the applied method has to take into account the aim of the analysis .

2 .4 . Applications and later developments

In the second half of the Seventies, a general interest in energy analysis grew quickly all over the world . For instance, in the Netherlands, research

in this field was started by Van Gool [2 .16] .

A lot was published during these years, on very divergent subjects . Some of the publications dealt with theoretical backgrounds, general methods of application, etc . like for instance [2 .5], [2 .11] and [2 .33] . A great deal of the publications concerned applications for the proposed methods, like the reports of analysis in [2 .36] and [2 .38] . These energy analyses considered very divergent processes and products . Some of them were about fuels, many were related to materials (steel, aluminium, copper, polymers, etc .), others concerned power stations (especially nuclear ones) ; further, there were studies,about foods, transportation systems, etc . From these studies, the following conclusions were drawn with respect to the benefits of energy analysis :

- The analyses gave an insight into the importance of the different contributions to the total energy use of commodities, for instance, the ratio between direct and indirect energy use .

- For processes, energy analysis shows directly that the maximum theoretical energy efficiency is lower than the maximum thermodynamic efficiency which ignores the indirect energy use . Unlike the thermodynamic efficiency which ignores the indirect energy use . Unlike the thermodynamic optimum, the point with maximum energy efficiency (considering gross energy requirement) appears at finite values of the process parameters . This is illustrated in fig . 1 .

f

energy use

per unit of

product

max energy

efficiency

Fig: 2.1

gross energy

requirements

indirect energy use

direct energy use

max theoretical

variable

energy efficiency .

--~

process

max

thermo-dynamic

efficiency

Derivation of the minimum gross energy requirements .

In some cases, the analysis offered little new evidence, because the indirect energy use was of minor consequence . However, in fact, such a conclusion is still important with respect to decisions on possible changes .

For many energy-using processes there is a possible trade-off between the capital investment and energy use ; parameters were derived by Phung and Van Gool [2 .30] . Energy analysis makes it obvious that only a part of the energy used is available for substitution : the theoretical minimum gross energy requirement (fig .1) can never be replaced by other production factors [2 .17] . With respect to the exploration of fuels, thermodynamic limits could be determined : for instance, it was found that, using current technology, North Sea oil fields smaller than 100 .000 -200 .000 tons of recoverable oil cannot yield net energy production . It was found out that rapid building programs for nuclear power stations may result in negative net energy production during their first decade, because of the large indirect energy use combined with the high rate of building [2 .7] .

Especially with respect to the energy embodied in materials, the effect of different recycling ratios could be calculated . For instance, in [2 .10] it was found that, in the Dutch situation, recycling of concrete hardly changed the gross energy requirement of new concrete .

In some cases, the inaccuracies in the data used for the analysis were so great that no conclusions could be drawn from the comparison of different production methods .

Generally speaking, the advantages of energy analysis over conventional energy studies (which do not include indirect energy use) were limited as far a5 it concerned company level . However, at more aggregated levels like industrial sectors, national and international politics interesting facts and trends were discovered which could be of great importance for medium and long term planning and decisions .

-In the meantime, a theoretical discussion by energy analysts and economists concerned the identity and the aims of energy analysis . This discussion is best restored by the contributions of Webb and Pearce, and Common, in [2 .36] . The general conclusion that can be drawn from this discussion is that energy analysis, as opposed to economics, cannot be used for evaluative purposes or optimizing the resource allocations . However, as a descriptive tool, it provides information suitable for decision-making, that cannot be obtained from economic data .

Later developments in the theory of energy analysis are few . Papers that have been published recently, e .g . [2 .12], [2 .32], deal with methods for deriving data for energy analysis, the quality of this data and the assumptions to be made .

With regard to this last point, especially Schaefer [2 .32] remarked that a lot of published data concerning energy requirements of materials etc . (for instance [2 .21) was useless because insufficient details were given of the system boundaries and the pre assumptions that had been made . Further, the continual appearance of papers on applications of energy analysis techniques e .g . in [2 .1], [2 .26] and [2 .31] makes it evident that it has been accepted to a certain extent as an important tool in searching for and evaluating the impact of energy in production systems, industrial sectors, etc . An illustration of the fact that the results of energy analysis are used in -national - politics is given in [2 .25] .

Le Goff [2 .14, 2 .15] has been developing methods and concepts for investment choices to economise the energy used in processes, deriving so-called conservation paths, etc . in which the role of the indirect energy use is considered, especially the applications have, however, up to now been limited to rather simple examples .

2 .5 . Evaluation

Relevance of energy analysis

From the preceding sub-sections the following conclusions can be drawn with respect to the relevance of energy analysis in the sense of the definition of IFIAS [2 .22] :

- Energy analysis provides an insight into the total energy required (Gross Energy Requirements) for producing goods or services . Therefore, in fact, it is the only basis for determining the real overall energy savings when changing the production system . Thus, it can be applied in evaluating and decision-making with respect to different alternatives in energy conservation projects .

- For single processes or complete production systems, certain physical limits or boundary conditions can be determined like, for instance, maximum (theoretical) energy efficiency', points of futility where no net energy is produced like minimum ore grade of uranium, minimum oil content in tar sands, etc .

- However, in many cases the insight into these limits does not affect the decisions concerning possible changes to the existing situation, because in general these limits are far beyond the realistic conditions . In selecting processes and installations for completely new plants, some more effects can be expected . But, in general, consideration of the indirect energy use of production will affect todays decisions only in exceptional cases . The insight into physically determined limits and boundaries is important, especially, in technology assessment studies and long

term forecasting and planning . In such cases, the physical limits provide a more reliable basis than economic data .

In this sense, energy analysis is important for developing strategies at the level of national and international politics . With respect to decision-making at company level, the process

analysis method is most important, because it shows which parts of the production system are most important with regard to their energy use . In searching for possible energy conservation measures these parts should get attention .

Moreover, a process analysis like that may be extended in order to provide a tool for estimating the effects of possible process changes .

Recommendations for applying energy analysis

For political decisions concerning energy and resources, it is important to perform an energy analysis of each industrial sector etc . The methods to be used for this purpose are statistical analysis and input-output analysis .

Moreover, at the level of companies and industrial sectors, a knowledge of the total energy sequestered in the products is required for long term planning and for estimating the impact of energy price changes on product costs .

This knowledge can be acquired from input-output analysis and/or process analysis .

Most questions on energy conservation in companies are related to the direct energy use of production processes, especially, when just a part of the total production system is under the control of the company concerned . Process analysis is the most convenient method, then . Starting from such an analysis, thorough investigations can be made afterwards in order to understand the behaviour of the process . Subsequently, the effects of conservation measures can be determined and evaluated .

REFERENCES

[2 .1] Baltic, T .J . and Betters, D .R .

Net energy analysis of a fuelwood energy system Resources and Energy 5, 1983, 45-64

[2 .2] Battelle Columbus Laboratories

Energy use patterns in metallurgical and nonmetallic mineral processing (Phase 7)

US Department of Commerce, Springfield, 1976 [2 .3] Berry, R .S .

The option for survival

Bulletin of the Atomic Scientists 23, May 1971, 22-27 [2 .4] Berry, S . and Fels, M .

Energy costs of an automobile

Bulletin of Atomic Scientists, 1973, 4 [2 .5] Boustead, I . and Hancock, G .F .

Handbook of industrial energy analysis Ellis Horwood, Chichester, 1979

[2 .6] Chapman, P .F .

No overdrafts in the energy economy New Scientist, May 1973, 408-410 [2 .7] Chapman, P .F .

Methods of energy analysis

J .M .Blair (ed .) : Aspects of energy conversion Pergamon, London, 1976

[2 .8] Chenery, H .B .

Process and production functions from engineering data in :W .W . Leontief, e .d ., Studies in the structure of the American Economy .

Oxford University Press, Oxford, 1953, 297-321 [2 .9] The need for electric power

Congressional Record of the U .S .A . 117, 1971, nr .187 [2 .10] Cornelis, R .J .

Het energiegebruik bij de recycling van beton . M .Sc . Thesis, Eindhoven, University of Technology, Dept . of Industrial Engineering, 1982

[2 .11] Duffy, M .C .

Technomorphology, innovation and energy analysis : II Analytical Methods .

Journal of Mechanical Working Technology, 8, 1983, 349-371

-[2 .12] Flaschel, P .

Input-output technology assumptions and the energy requirements of commodities

Resources and Energy 4, 1982, 359-389 [2 .13] Gehrecke, S .

Anteile der Energietr 9 gerkosten am Preis industrieller Produkte

PEK 12 (1964), Heft 6 [2 .14] Goff, P . Ie

Energetique industrielle, vol . 1 Lavoisier, Paris, 1979

[2 .15] Goff, P . Ie

Energetique industrielle, vol . 2 Lavoisier, Paris, 1980

[2 .16] Gool, W . van .

Energie-analyse, betekenis en organisatie Chemisch Weekblad, 1975

[2 .17]

[2 .18]

Pergamon, New York,

and management in industry", European Conference

Gool, W . van

Energy analysis in perspective

in : Reis, A . c .s . : "Energy economics

1984

Proceedings

McGowan, J .G . and Kirchhoff, R .H . How much energy is needed to produce

of the

an automobile? Automotive Engineering 80, 1972, no 7, 39-40

[2 .19] Hannon, B .

An energy standard of value

The annals of the American academy of political and social science, 410, 1973

[2 .20] Herendeen, R .A .

An energy input-output model for the United States, User's Guide

C .A .C . report nr . 67

University of Illinois, Urbana, 1963 [2 .21] Heuvel, P . van den

Energy input-output tables 1960-1979

for the Netherlands based on energy balances Economisch Instituut Tilburg, February 1983 [2 .22] Energy analysis

Workshop on methodology and conventions IFIAS report nr .6, Stockholm, 1974 [2 .23] Energy analysis and economics

IFIAS Workshop report nr . 9, Stockholm, 1975

-[2 .24] Jevons, W .S . The coal question

MacMillan and Co . f London, 1906 [2 .25] Energie en economie, onderzoekrapport

Krekel van der Woerd Wouterse, Rotterdam, 1983 . [2 .26] Lange, K .

Energíeeinsparung und Fertigungstechnik

Werkstattstechnik, Zeitschrift fur industrielle Fertigung 68, 1978, 535-537

[2 .27] Leach, G . and Slesser, M .

Energy equivalents of network inputs to agriculture Strathclyde University, 1973

[2 .28] Makhijani, A .B . and Lichtenberg, A .J . Energy and well-being

Environment 14, 1972, nr . 5, 10-18 [2 .29] Odum, H .

Power, Environment and Society Wiley Inter-Science, New York, 1971 [2 .30] Phung, D .L . and Gool, W . van

On industrial energy conservation policy : the trade-off between energy savings and additional i nvestments

Materials and Society, 4, 1980, nr . 4 .

471-479

[2 .31] Plant, R .A . and Herendeen, R .A .

Empirical energy requirements for several ethanol-from-grain operations

In R .A .Fazzolare (Ed .) : Beyond the energy crisis, opportunity and challenge .

Pergamon Press, Oxford, 1981, 1159-1164 [2 .32] Schaefer, H .

Kumulierter Energieverbrauch von Produkten . Methoden der Ermittlung - Probleme der Bewertung Brennstoff-Wárme-Kraft 34, 1982, 7

[2 .33] Slesser, M .

Energy analysis : its utility and limits IIASA report RM-78-46, Laxenburg, 1978 [2 .34] Soddy, F .

Wealth, Virtual Wealth and Debt Allen & Unwin, London, 1926

[2 .35] Technocracy, Technological social design Technocracy Inc . Savannah, 1975

[2 .36] Thomas, J .A .G . Energy analysis

IPC, Guildford, 1977

-[2 . 1 7] Thompson, W . (Lord Kelvin)

British Association for the Advancement of Science Transactions (A), 527, 1881

[2 .38] Energy accounting of materials, products, processes and services

9th International TNO conference TNO, Rotterdam, 1976

[2 .39J Willeboer, W .

Priorities in energy conservation Dissertation Eindhoven, 1985

PART II

3 . SECOND LAW ANALYSIS 3 .1 . History

Thermodynamics is based upon several empirical laws which have been known and applied for a very long time . Two of them are well known, to wit the first and the second laws of thermodynamics . The first law of thermodynamics is the law of conservation of energy : it states that energy can neither be created nor destroyed, but merely converted from one form to another .

The second law of thermodynamics says that work can be completely converted into heat, but heat cannot be completely converted into work*) . In fact this implies that all real processes are irreversible ; reversible processes are idealized boundary cases of real processes . The degree to which a certain energy conversion process is irreversible is expressed in the quantity of entropy : the stronger the irreversibility, the larger the entropy created . The opposite of the entropy concept is the availability or potential to do work**) . Although this concept has been applied for at least a century [3 .16], it is only in recent decades that it has received the real attention of scientists and engineers . Analytical techniques ana methods were developed in order to facilitate the application of the principles of the second law, and to express the efficiency of energy conversion processes not only in terms of quantity, but also in terms of quality . This field is called "second law analysis" or "exergy analysis" . The term "exergy" was introduced by Rant in 1953 [3 .2] . In fact "exergy" is

synonymous with "availability" : exergy is the maximum (useful) work that can be extracted from a system in any process which brings it into equilibrium with its environment (see also subsection 3 .2) . From about 1960 on, more and more research was done in the field of the general principles of second law analysis, for instance by Baehr [3 .1,3 .2] and Fratzscher [3 .14, 3 .15], and in the field of application which were, at first, limited to refrigeration [3 .19], combustion [3 .9, 3 .281 and steam cycles for electricity generation [3 .5, 3 .22] . After the so-called energy crisis of 1973, the general interest in second law analysis grew quickly and so did the number of applications in divergent fields of engineering and fundamental research . Another application, which is very important, is the use of the exergy concept in teaching engineering thermodynamics (e .g . [3 .23]), because this concept gains a better understanding of the consequences of the second law of thermodynamics in all kinds of technical processes than the entropy concept .

3 .2 The exer concept

In the conventional thermodynamic analysis of processes, energy flows and energy conversion efficiencies, etc . are expressed only i n terms of energy

*)A more general formulation was given by Baehr [3 .2] who used the terms "unlimited convertible energy forms" and "limited convertible energy forms" instead of "work" and "heat" .

**)In German literature : "Technische ArbeitsfUhigkeit" [3 .16]

-or enthalpy, which are in fact quantitative measures . No distinction is made between the energy flows of different types . One of the problems that occur is that no reference or standard to judge the thermodynamic performance of a process is made . For instance : a thermal efficiency of 70% is bad for a central heating boiler, but a thermal efficiency of "only" 45% is very good for a power station . Another problem is that thermal efficiencies and energy balances provide no information about where and to what extent energy is dissipated : all "kinds of" Joules, whether dissipated or not, are simply added together, or compared with each other .

The exergy concept was introduced in order to overcome these difficulties : exergy is a measure that takes into account the quantity as well as the quality of energy flows or systems . Similar concepts are found in literature under the names of : availability [3 .25], "technische Arbeitsfghigkeit" [3 .1b], or essergy [3 .12] . There are some differences between these functions ; they relate to the boundary conditions and assumptions with respect to the environment of the system [3 .29] . A definition of exergy has already been given in the preceding subsection : "exergy is the maximum useful work that can be extracted from a system in any process which brings the system into equilibrium with its environment"

[3 .7] . The consequences of this are different for different forms of energy :

Mechanical or electrical exergy is equal to the mechanical or electrical energy of the system

Heat exergy must be expressed as a function of the environmental temperature Ts . The maximum work that can be extracted from a system at a certain temperature Ts is the work generated by a Carnot-cycle, for which the efficiency is given by :

T S T

When an amount of heat Q is transferred at temperature T, the related heat exergy is defined by :

T

Ex =Q* ( 1 - Ts )

The chemical exergy of a fuel is mostly assumed to be equal to the heating value of the fuel . In fact, it may differ, depending on the state of the environment [3 .25]

So it is evident that different groups of energy forms can be distinguished on the basis of their convertibility into work [3 .2] . The above mentioned energy forms belong to two groups, viz . : energy forms that are completely convertible into work and energy forms that are only partially convertible . Moreover, there is another group : energy forms that are not at all convertible into work . This group concerns the energy of systems that are in equilibrium with the environment . The exergy of such systems equals zero . In the light of these considerations the word "Anergie" (anergy) was introduced by Rant [3 .2, 3 .27] . Exergy and anergy are complementary quantities : the part of energy that is not exergy, is anergy . Thus :

exergy + anergy = energy

Up to now, the term "anergy" has rarely appeared i n literature ; exceptions 17

-are [3 .3], [3 .26] .

With the help of the exergy concept, a measure of performance for a system is defined as the ratio of the exergy gained to the exergy expended by the system during a process . This ratio is denoted as effectiveness [3 .7], exergetic efficiency [3 .10, 3 .231, or second law efficiency t3 .25] . An important use of these second law efficiencies is to assess the performance of a machine, plant, or industry relative to the average level for like machines, plants, or industries . Such assessments on the basis of exergy are more reliable than those based on energy ("first law efficiency") because with exergy the different forms of energy used are added on a comparable basis .

Another advantage of second law efficiencies is that they show the theoretically potential improvements to the processes although it is evident that a second law efficiency of 100% is an impossible goal . By determining the second law efficiency of each process or process part, one gets information on the irreversibilities in each part of the system . This information can be used in searching for ways of improving the efficiency of the whole system .

In order to give an indication of the second law efficiencies of domestic energy use, table I presents some calculation results of Brzustowski and Golem [3 .7] for a dwelling with a heat demand of 7 kW and an electricity demand of 3 kW . The following assumptions were made :

- temperature inside the house 20°C - ambient temperature 0°C

- (second law) efficiency of electricity generation and supply : 0 .25 - thermal (first law) efficiency of gas heating furnace : 0 .60

- second law efficiency of an electric heat pump : 0 .50 - second law efficiency of a fuel cell using gas : 0 .50

Table I,Second law efficiency of domestic energy ' systems using electricity and natural gas (from [3 .7]) .

System configuration

second law efficiency

electric heating, electrical power

from grid 0 .087

gas furnace heating, electrical power

from grid 0 .143 electric heat pump heating

electric power also from grid 0 .220 gas supplied to fuel cell to produce i

electricity and 3 kW of waste heat, 4 kW of heat supplied by gas furnace

1

0 .268 gas supplied to fuel cell to produce 5 kW

electricity and 5 kW waste heat, 2 kW of

electricity used for electric heating 0 .348

-3 .-3 Applications

Publications on the application of second law analysis and the exergy concept have appeared from about 1960 onwards e .g . [3 .9, 3 .11, 3 .13] .

However, until about 1980 such publications were rare and, moreover, they were limited to the classical examples of heat exchangers, refrigerators and

steam cycles . Especially in the years 1980 to 1982, quite a lot of papers appeared on the application of the exergy concept, in divergent industries and processes . Most of the applications concerned only the physical aspects, but some of them dealt with the relationship between exergy and economic analysis too .

Exergetic considerations of heat transfer processes and equipment were published by e .g . Elsner [3 .9], Baehr [3 .3] and Bejan [3 .4] . Elsner applied the exergy concept in the economic optimization of heat exchangers and feedwater preheaters in steam cycles . Bejan gave a thorough theoretical analysis of the phenomena and the irreversibilities that can occur in heat

transfer processes . He presented a (physical) optimization for a heat exchanger and took into account the exergy losses due to heat transfer irreversibilities as well as those due to fluid flow of the participating media (pumping power) . Baehr treated (among other things) an exergetic comparison between conventional heating systems and heat pumps . Each of these authors concluded that second law analysis and exergetic efficiency improved the insight into the performance of the process : th6 irreversibilities and the real origins of energy degradation and losses are shown clearly . Examples of the second law analysis of combustion processes were given by Rant [3 .28] and Brzustowski [3 .8] . As Rant concluded, there is just one way to improve combustion efficiency : by raising the combustion temperature . It is clear that one doesn't need the exergy concept to draw conclusions like that . Also the results of Brzustowski's research are not surprising : high flame temperatures, good mixing of fuel and air and high heat transfer coefficients improved the exergetic efficiency of combustion ;

Second law analysis has often been applied to integrated process systems like, for instance, refrigeration cycles ([3 .19, 3 .21]), steam cycles for electricity generation [3 .5, 3 .22] and chemical plants ([3 .20, 3 .30]) . In each of these examples, exergy analysis gives a very clear picture of the

performance of each section of the plant or the process, and the irreversibilities are allocated . In special cases, optimization of the process components can be achieved without considering the effects of the changes on the complete process : in [3 .5] and [3 .9] the optimization of feedwater preheaters was performed by taking into consideration only the exergy losses in the preheaters, assuming that changes in these exergy

losses would equal the change of exergy losses in the complete cycle . However, this is an exception, as Van Lier stated [3 .23] . In general, changing a certain process part will change the exergy efficiency of the

rest of the process . Thus, the application of second law analysis, instead of conventional process analysis, will not decrease the computational effort .

An advantage (in principle) of second law analysis for process systems with energy cascading etc . is that the exergy used and lost in the different process parts provides a good measure upon which energy cost distributions can be based . Without the exergy concept this is hardly possible . In fact, in such cases, exergy is interpreted as the value of energy . This leads to another field of applications for second law analysis : exergy in the economic analysis . Although some publications appeared in this field (e .g . [3 .9], [3 .26], [3 .31]), the specific role of exergy in these analyses was limited to the fact that fuel costs were expressed as costs per unit of

-exergy instead of energy or fuel . Moran [3 .25] remarked that "these thermoeconomic analyses can be conducted without recognition of the exergy concept as well, but in many cases their formulation is understood more easily if it is used" .

London [3 .24] proposed an engineering methodology for incorporating energy in the economic analysis on the basis of costs of irreversibilities . A very fundamental discussion of the consequences of the second law of thermodynamics with respect to the economic process was given by Georgescu-Roegen [3 .17] . He argued that all economic systems feed upon low-entropy ordered structures such as fuels and materials and consequently present procedures can only continue as long as such low-entropy sources remain available . Recently, the possible links between the exergy concept and the economic sciences were discussed extensively in [3 .18], which gives in fact the state-of-the art of this subject . However, it must be concluded that most of the publications on "exergy and economy" are limited to more theoretical considerations : practical applications can hardly be found (except for instance [3 .311) .

3 .4 . Discussion

The exergy concept is not very well known, not even in the engineer's world . In fact, what is meant by the word "energy" in common parlance corresponds very often with the exergy concept : frequently, one speaks of energy only if it can be applied usefully .

The significance of second law analysis and the exergy concept in energy engineering is discussed now in four areas :

- exergy and irreversibility

- exergy and the potential for improvement

- exergy and the evaluation of processes and process improvements - exergy and economic analysis

Exergy and irreversibility

The exergy concept is an excellent tool for identifying the location and the magnitude of irreversibilities in a process . These insights are not suitable for practical engineering, but they are of great importance for fundamental process research . Second law analysis applied to the existing forms of processes can reveal the places where irreversibilities occur . The identification of "built-in" irreversibilities will be very helpful for finding and designing more efficient production technologies . On the other hand, exergy and the way it affects irreversibilities are of great didactic value . Unlike entropy, the concepts of exergy and anergy can be imagined easily and clearly . Moreover, they can be applied simply in process analyses and evaluations .

Exergy and the potential for improvement

Several authors (e .g . Moran [3 .25], Steeman [3 .30]) argued that the gap between the level of fuel consumption with current practices and the minimum theoretical requirement determined by second law analysis, is a measure of the potential improvement . This is true, but it must be remembered that this is only a theoretical measure . It is evident that exergetic efficiency provides an indication of the potential for improvement that is

-more fundamental than that shown by energetic efficiency . But an exergetic efficiency of 100% is impossible and second law analysis does not provide information on what is really achievable in a certain production method . Thus, the information obtained by second law analysis on the potential improvement is of limited importance still .

Although exergy is theoreticàlly a better measure than energy, it can be somewhat misleading in certain practical situations . For instance, when making an exergy analysis of power generation by means of a condensation steam cycle, it is found that the heat absorbed by the cooling water in the condenser represents no more than about 2% of the exergy input of the plant . From this figure, it might be concluded that utilization of the heat in the condenser can hardly yield any contribution to fuel conservation . This conclusion is wrong : if condenser conditions are slightly up-graded, the heat can be utilized in heating systems, thereby,

reducing the total fuel consumption significantly .

So we see that in practical situations, exergy does not always provide more useful information than energy, with respect to the potential for improvement .

Exergy and the evaluation of processes and process improvement As Baehr [3 .2] stated, one of the advantages of the exergy concept

is that, in order to evaluate a process with respect to the way energy is utilized, it is no longer necessary -like before- to design a comparable process that is completely reversible . An exergy analysis of the process under consideration yields the same information . However, this concerns a general and more theoretical evaluation of the process . In practice, the evaluation of processes and process improvement is aimed at showing the differences in fuel consumption for different alternatives . Efficiency numbers, as such, are less important then . In determining the fuel consumption of processes, exergy analysis and conventional process analysis are equivalent . So, in studying and evaluating practical alternatives for energy conservation, second law analysis is no better than conventional process analysis . Exergy and economic analysis

One of the possible definitions of "exergy" is : the "potential to do work" . Therefore, exergy could be interpreted as the "value of energy" . With this, only a very small step is needed to determine the economic value of energy flows on an exergy basis . This can be applied for internal accounting of energy flows easily, in chemical

plants etc ., e .g . with energy cascading [3 .30], and in plants for cogeneration of power and heat .

For economic evaluation of processes, for instance, in conservation projects, the exergy concept has little value . In such studies, it is only the fuel consumption that accounts . In the foregoing, it has been explained that, for determining fuel consumption, second

law analysis has no specific advantages .

With respect to macro economics, up to now, the exergy concept appears to be useful only for theoretical considerations at a rather high level of abstraction (e .g . Bruggink and Strdbele in

[3 .181) .

-From the discussion of these four points it is clear that the benefits of the exergy concept and the advantages of second law analysis over conventional (first law) process analysis are found especially in the fields of theoretical considerations, process research and engineering education . In practical studies of process changes, conservation projects, comparisons between different alternatives, etc . the exergy concept has few specific advantages .

REFERENCES

[3 .1] Baehr, H .D .

Ein Exergie-Entropie-Diagramm fur Luft Chemie-Ingenieur-Technik 33, 1961, 335-338 [3 .2] Baehr, H .D .

Energie, Exergie, Anergie in : [3 .10]

[3 .3] Baehr, H .D .

Zur Thermodynamik des Heizens

Brennstoff-WArme-Kraft 32, 1980, 9-15, 47-3 .6 [3 .4] Bejan, A .

Second law analysis i n heat transfer Energy 5, 1980, 721-732

[3 .5] Bergmann, E . and Schmidt, K .R .

Ein Stórungsrechenverfahren mit der Exergie in : [3 .10]

[3 .6] Borel, L .

Energy economics and exergy- Comparison of different heating systems based on the theory of exergy

in : Camatini, E . and Kesler, T . (eds) : Heat pumps and their contribution to energy conservation Noordhof, Leiden, 1976

[3 .7] Brzustowski, T .A . and Golem, P .J .

Second law analysis of energy processes Part I : Exergy - An introduction

Transactions of the CSME 4, 1976, 209-218 [3 .8] Brzustowski, T .A .

Toward a second-law taxonomy of combustion processes

Energy 5, 1980,

743-755

[3 .9] Eisner, N .

Die Exergie und thre Bedeutung fur Wárme-technische und energiewirtschaftliche Untersuchungen

Energieanwendung 15, 1966, 193-201, 270-278 [3 .10] Energie und Exergie

Die Anwendung des Exergiebegriffs in der Energie-technik

VDI-Verlag, Di.isseldorf, 1965

[3 .11] Evans, R .B ., Crellin, G .L . and Tribus, M .

Thermoeconomic considerations of sea water demineralization in : K .S . Spiegler (ed .) : Principles of desalination

Academic Press, 1966 .

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A proof that essergy is the only consistent measure of potential work .

Ph .D . Thesis, Ann Arbor, 1969 [3 .13] Frank, W .

Ueberlegungen zur Anwendung der irreversiblen Thermodynamik auf die Optimierung energie-wirtschaftlicher Prozesse .

Energieanwendung 17, 1968, 241-248

[3 .14] Fratzscher, W .

Die Grundsatzliche Bedeutung der Exergie fur die technische Thermodynamik

Dissertation Dresden, 1960 [3 .15] Fratzscher, W .

Zum Begriff des exergetischen Wirkungsgrads Brennstoff-W~rme-Kraft 13, 1961, 486-493 [3 .16] Gasparovic, N .

Schrifttum uber Exergie

Brennstoff-W~rme-Kraft 13, 1961, 502-509 [3 .17] Ceorgescu-Roegen, N .

The entropy law and the economic process

Harvard University Press, Cambridge (USA), 1971 . [3 .18] Cool, W . van and Bruggink, J .J .C . (eds .)

Energy and time in the economic and physical sciences North-Holland, Amsterdam, 1985

[3 .19] Grassmann, P .

Anwendungen von Exergiebetrachtungen in der K~ltetechnik

in : [3 .10]

[3 .20] Kaiser, V .

Energy optimization

Chemical Engineering, 1981, 62-72

[3 .21] Leidenfrost, W ., Lee, K .H . and Korenic, B . Conservation of energy estimated by second law analysis of a power consuming process

Energy 5, 1980, 47-61 [3 .22] Lier, J .J .C . van

Thermodynamische processen i n de centrale en mogelijkheden tot verbetering van deze processen Argus, Amsterdam, 1963

[3 .23] Lier, J .J .C . van

Energietransformaties III : exergetische beschouwingen

Delft University of Technology, 1973

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Economics and the second law : an engineering view and methodology

Int . Journal of Heat and Mass Transfer 25, 1982, 743-751

[3 .25] Moran, M .J .

Availability Analysis

Prentice-Hall, Englewood Cliffs, 1982 [3 .26] O'Callaghan, P .W . and Probert, S .D .

Exergy and economics

Applied Energy 8, 1981, 227-243 [3 .27] Rant, Z .

Die Heiztechnik und der zweite Hauptsatz der Thermodynamik

Gasw5 rme 12, 1963,

297-304

[3 .28] Rant, Z .

Die ExergieverhBltnisse bei der Verbrennung in : [3 .10]

[3 .29] Sorensen, T .S .

The science of energetics in the exergy crisis or how is thermodynamics made really useful

in : R .A . Fazzolare (Ed) : Beyond the energy crisis, opportunity and challenge

Pergamon Press, Oxford, 1981 [3 .30] Steeman, J .W .M .

Energiebesparing in de chemische industrie Energie Spectrum, 1979

[3 .31] Vries, B . de and Nieuwlaar, E .

A dynamic cost-exergy evaluation of steam and power generation

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