Engineering sustainability: thermodynamics, energy systems, and the environment

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2004; 28:613–639 (DOI: 10.1002/er.988)

Engineering sustainability: thermodynamics, energy systems,and the environment

Geoffrey P. Hammondn,y

Department of Mechanical Engineering and International Centre for the Environment, University of Bath,

Bath BA2 7AY, U.K.

SUMMARY

Thermodynamic concepts have been utilized by practitioners in a variety of disciplines with interests inenvironmental sustainability, including ecology, economics and engineering. Widespread concern aboutresource depletion and environmental degradation are common to them all. It has been argued that theseconsequences of human development are reflected in thermodynamic parameters and methods of analysis;they are said to mirror energy transformations within society. ‘Exergy’, a quantity which follows from theFirst and Second Laws of Thermodynamics, has been viewed as providing the basis of a tool for resourceand/or emissions accounting. It is also seen as indicating natural limits on the attainment of sustainability.The more traditional use of the exergy method is illustrated by a number of cases drawn from the UnitedKingdom energy sector: electricity generation, combined heat and power schemes, and energy productivityin industry. This indicates the scope for increasing energy efficiency, and the extent of exergetic‘improvement potential’, in each of these areas. Poor thermodynamic performance is principally the resultof exergy losses in combustion and heat transfer processes. However, the application of suchthermodynamic ideas outside the sphere of engineering has its critics. The link between the efficiency ofresource utilization, pollutant emissions, and ‘exergy consumption’ is only indirect, and generally providesan insufficient basis for environmental appraisal. Methods of energy and exergy analysis are, therefore,evaluated as appropriate measures of sustainability in and beyond the energy sector. Copyright # 2004John Wiley & Sons, Ltd.

KEY WORDS: thermodynamic analysis; exergy; environmental appraisal; sustainability; U.K. energysector; ecology; economics; resource and emissions accounting

1. INTRODUCTION

1.1. Background

Energy sources of various kinds heat and power human development, but they also put at riskthe quality and longer-term viability of the biosphere as a result of unwanted or ‘second-order’effects (Hammond, 2000b). Many of these side effects of energy production and consumptiongive rise to resource uncertainties and potential environmental hazards on a local, regional and

Received 23 July 2003Accepted 24 September 2003Copyright # 2004 John Wiley & Sons, Ltd.

yE-mail: [email protected]

Contract/grant sponsor: The UK Engineering and Physical Sciences Research Council

nCorrespondence to: Professor G. P. Hammond, Department of Mechanical Engineering, University of Bath, Bath BA27AY, U.K.

global scale. Examples include the depletion of North Sea oil and natural gas resources, thegeneration of smog from urban road transport, the formation of acid rain via pollutantemissions (primarily from fossil fuel power stations), the difficulty of long-term safe storage ofradioactive wastes from nuclear power plants, and the possibility of the enhanced greenhouseeffect from combustion-generated pollutants. Consequently, the energy sector plays a pivotalrole in attempts to achieve sustainable development; balancing economic and socialdevelopment with environmental protection (‘people, planet, prosperity’ in terms of the‘strapline’ adopted by the U.K. Sustainable Development Commission). National governments,as well as regional bodies in the West (such as the European Union and the International EnergyAgency), therefore need to assess the long-term advantages and disadvantages of their availableenergy sources in order to reconcile the pressures induced by the move towards competitivemarkets with the requirements of a sustainable energy strategy. Although it would be desirableon natural resource and environmental grounds to phase out the use of fossil fuels, this may notbe feasible until perhaps the middle of the 21st century.

1.2. The issues considered

The present contribution employs thermodynamic methods of analysis to critically evaluate thesustainability of energy systems and components. Parkin (2000) has argued, influenced by theearlier work of Mueller (1971), that such ideas also underlie the understanding of sustainabledevelopment more broadly. She interprets the Second Law of Thermodynamics in terms of thetendency of everything to return to an elemental state. Natural cycles are seen as combating thisenergy degradation as well as environmental pollution. The link between the efficiency ofresource utilization, pollutant emissions, and ‘exergy consumption’ is real, but not direct.Different resource implications, for example, follow from fossil fuel use as against the adoptionof, typically solar-derived, renewable energy sources (as highlighted recently by Hammond andStapleton, 2001). The former is a ‘capital’ resource that depletes over time, whereas renewablesmay be viewed as energy ‘income’ to the planet. This is not explicitly reflected in thermodynamicanalysis, where the resource bases are taken as essentially equivalent. Engineers and physicalscientists, therefore, have a critical role to play because of their understanding of the scientificprocesses that underpin the natural world. The interconnections between engineeringconstraints and the economic and social domain are illustrated by the sustainability Venndiagram shown in Figure 1 [adapted from a version attributed by Parkin (2000) to ProfessorRoland Clift of the University of Surrey]. Here, thermodynamic limits are represented asunderpinning the environmental sphere. The present assessment, therefore, aims to provide apractical framework for the use of such thermodynamic ideas and analysis in this wider contextof environmental sustainability.

The last book of Stephen J Kline (1999), the distinguished Stanford engineering professor andco-founder of the Stanford Programme on Values, Technology and Society (subsequentlyrenamed Science, Technology and Society), attempted to provide the new generation of teachersand students of thermodynamics with a better understanding of Second Law ideas. In addition,he strove to identify misconceptions in the various definitions of thermodynamic properties,such as entropy, used in engineering, physics, informatics, and biology. Hammond (2003)recently argued that Kline’s work had been rather overlooked, and that it deserved much widerrecognition amongst the thermodynamics community. It suggests that thermodynamic ideas(with their empirical foundations derived from the work of Carnot, Kelvin and Clausius related

Copyright # 2004 John Wiley & Sons, Ltd. Int. J. Energy Res. 2004; 28:613–639

G. P. HAMMOND614

to ‘heat engines’) may not be appropriately applied to other domains outside the area ofenergy systems for which they were first devised. Attempts to use them to determine criteriafor long-term sustainability (see, for example, Mueller, 1971; Parkin, 2000 and Porritt, 2000)can therefore be misleading. The present contribution attempts to illustrate whenthermodynamic concepts and methods of analysis can directly aid an understanding ofsustainable development in contrast to those occasions when their use is merely analogous ormetaphorical.

2. SUSTAINABLE DEVELOPMENT AND THE ENERGY SECTOR

2.1. Sustainable development or sustainability

Over a period of some 15–20 years, the international community has been grappling with thetask of defining the concept of ‘sustainable development’. It came to prominence as a result ofthe so-called ‘Brundtland Report’ produced by the World Commission on Environment andDevelopment (WCED, 1987) under the leadership of the former Prime Minister of Norway, GroHarlem Brundtland. This Commission argued that the time had come to couple economy andecology, so that the wider community would take responsibility for both the causes andconsequences of environmental damage. The WCED (1987) defined sustainable development asmeeting ‘the needs of the present without compromising the ability of future generations to meettheir own needs’. It therefore involves a strong element of intergenerational ethics. Engineershave generally been slow to meet the challenge of making a reality of the notion of sustainability(Hammond, 2000b), although the engineering profession now sees the importance of using their

Ecology andthermodynamics

Society

Economicsand

technology

Area ofsustainability

Figure 1. Sustainability ‘Venn’ diagram for engineers. Source: adapted from Parkin (2000).


ENGINEERING SUSTAINABILITY 615

skills to improve the quality of life. Many writers and researchers have acknowledged that theconcept of ‘sustainable development’ is not one that can be readily grasped by the wider public.However, no suitable alternative has currently been found (Hammond, 2000b).

The sustainable development paradigm has had its critics over recent years (see Doughty andHammond, 2003). Meredith Thring (private communication, 1999) regards the term as anoxymoron; arguing that development per se cannot be sustainable. He would prefer humanity tostrive for a creative and stable world with the aid of ‘equilibrium engineering’ (Thring, 1990).Similar views can be found in developing countries, where their debt burden and inequalities inglobal income distribution are seen as serious obstacles to sustainable development (Amin,1997). On a more fundamental level, Porritt (2000) has recently stressed that such developmentis only a process or journey towards a destination, which is ‘sustainability’. This process cannoteasily be defined from a scientific perspective, although he argues that the attainment ofsustainability can be measured against a set of four ‘system conditions’. Porritt draws these from‘The Natural Step’; an initiative by the Swedish cancer specialist, Karl-Henrick Rob"eert (see, forexample, Broman et al. 2000):

* Condition 1: Finite materials (including fossil fuels) should not be extracted at a faster ratethan they can be redeposited in the Earth’s crust.

* Condition 2: Artificial materials (including plastics) should not be produced at a faster ratethan they can be broken down by natural processes.

* Condition 3: The biodiversity of ecosystems should be maintained, whilst renewableresources should only be consumed at a slower rate than they can be naturally replenished.

* Condition 4: Basic human needs must be met in an equitable and efficient manner.

These sustainability conditions put severe constraints on economic development, and theymay therefore be viewed as being impractical or ‘utopian’ (Doughty and Hammond, 2003).They certainly imply that the ultimate goal of sustainability is rather a long way off whencompared with the present conditions on the planet. However, an interesting feature ofthe advocacy of TNS system conditions in the present context is the claim that they reflectthermodynamic limits. Broman et al. (2000) and others suggest that these conditions addressthe tendency of energy and matter to spread spontaneously. They in turn view this as mirroringthe Second Law of Thermodynamics, or what they call the ‘Law of Entropy’. In reality thelatter property is what the late Stephen J Kline (1999) has called the ‘vulgar’ entropy; thegeneric, but vague or ill-defined, application of entropy to various kinds of disorder (seeHammond, 2003). The Natural Step in effect uses the Laws of Thermodynamics only by way ofa rather loose analogy, or as a metaphor. Indeed Upham (2000) argues that TNS movesbeyond (scientific and other) knowledge in signposting action for the business sector. Hecontends that it represents a political and ethical statement rather than any justifiable scientificconsensus.

2.2. Fossil fuel depletion

The combustion of finite reserves of fossil fuels results in their obvious depletion over time(Hammond, 2000b). It is often claimed, particularly by economists, that the resource carryingcapacity of the planet as a whole is so large that new discoveries offset current production.Although this may be true on a global scale, it is unlikely to apply at the level of the individualnation-state, or even at a regional scale (such as within the European Union). There is


G. P. HAMMOND616

considerable uncertainty over fossil fuel resources in the medium-term. The lifetime and globaldistribution of these vary enormously:

* Oil: OPEC (Middle East)-dominated, 20–40 year life.* Natural gas: CIS (Russian)-dominated, 40–70 year life.* Coal: Widely distributed, 80–240 year life.

These figures are rough estimates assuming current rates of consumption (Hammond, 1998),but they indicate that the sources of fossil fuel supplies for OECD countries, with the exceptionof coal, are rather insecure. If depletion of oil and gas at anything like the rate indicated hereactually occurred, then the price of these fuels would rise. The abundance of coal is likely toplace an upper limit on all fossil fuel prices at the synthetic fuel cost for the foreseeable future(Hammond, 2000b). Nevertheless, this would make the financial case for renewable energysources and nuclear power look much brighter. It has been frequently argued since the ‘oilcrises’ of the 1970s that a nuclear power and/or renewable energy strategy should be adopted asan ‘insurance policy’ against the insecurity of the oil market. In reality these resources are notsubstitutable, particularly in the transport sector (Hammond, 1998).

2.3. Pollutant emissions and global warming

The combustion of fossil fuels results in pollutant emissions that can damage human health andthe environment on a number of levels. Most environmental concern has recently focused onglobal warming (RCEP, 2000): the speculative prospects of global climate change induced bythe emission of the so-called ‘greenhouse’ gases (GHG), principally CO2 emissions, from fossilfuel combustion is an issue of considerable interest. Obviously the main concern is found inthose coastal and island nations at risk of flooding were sea levels to rise as a result of climatechange (Hammond, 2000a). The critical issue is whether the observed global warming is due tohuman activity or simply a natural phenomenon induced, for example, by variations in solaroutput over decades. The U.K. Natural Environment Research Council has summarized thecurrent state-of-the-art in climate change research (NERC, 1997). The British Government’sformer Chief Scientific Adviser, Sir Robert May (1997) [now Lord May of Oxford, President ofthe Royal Society], has advocated action to reduce carbon dioxide emissions on the basis of the‘precautionary principle’ (see, for example, Hammond, 2000b or Porritt, 2000). Not out ofconviction that anthropogenic climate change is currently proven, but because its possibleeffects over the next century may be damaging and large-scale.

Carbon dioxide released into the atmosphere from the burning of fossil fuels is thought tohave a ‘residence time’ of around one hundred years (Hammond, 2000a). Recent trends in CO2

emissions from the U.K. energy sector are depicted in Figure 2 (Hammond, 2000b), along withthe anticipated reduction due to measures already included in the British Government’s 2000Climate Change Programme. These are likely to ensure that the U.K. comes close to thedomestic goal of a 20% cut in carbon dioxide emissions, below the 1990 levels, by 2010.However, the Royal Commission on Environmental Pollution has argued (RCEP, 2000) thatthe U.K. should take the lead in adopting a more ambitious target of reducing these emissionsby some 60% from 1997 levels by about 2050. The 2003 Energy White Paper (DTI, 2003)accepted that Britain should put itself on a path to achieve this goal by adopting various low-carbon options, principally energy efficiency measures and renewable energy sources. Newnuclear power plants are regarded by the Government as uneconomic in the present energy



market conditions, and the problem of longer-term radioactive waste storage as beingunresolved. However, this option will be kept open in case renewable energy technologies do notfulfil their promise in (say) 5–10 years.

2.4. Sustainable energy systems

The intergenerational ethical injunction in the Brundtland report (WCED, 1987), effectively toavoid actions that might degrade the biosphere for future generations, presents specialdifficulties for the energy sector. It implies that governments should conserve depleting fuelresources, and make greater use of renewable energy sources, in line with the Natural Stepsystem conditions. However, the World Energy Council (WEC, 1993) suggested in 1993 thatrenewables might contribute between 3 and 12% of total commercial energy demand by 2020. Amore recent study (Nakicenovic et al., 1998) by the WEC, jointly with the International Instituteof Applied Systems Analysis, has been rather more optimistic about the contribution ofrenewables after 2020. The greatest potential for renewable energy is likely to be in the secondhalf of the 21st Century, with the most optimistic supply projection being 80% of globaldemand by 2100.

In the United Kingdom the government is now committed (DTI, 2003) to developing asustainable energy economy over the longer term, and to taking a lead in reducing CO2

emissions amongst the industrialized (OECD) countries. The main components of this lowcarbon strategy are energy conservation and renewable energy systems, with carbonsequestration and nuclear power in a rather more secondary role. Targets for new renewableelectricity supply have been set at 10% by 2010 and 20% by 2020 from a base of only 1.5%presently. Bridging the renewables gap will be a daunting task. It remains to be seen whethereven the Government’s 2010 target can be met within a competitive market framework. Thereappears to be little prospect, for example, of large-scale projects being funded via this route

MtC

Figure 2. Carbon dioxide emissions from the U.K. energy sector. Source: Hammond (2000b).


G. P. HAMMOND618

(Hammond, 2000b). Even the proposed 8.6GW Weston-Super-Mare to Lavernock Point tidalbarrage scheme, which could meet some 6% of U.K. electricity demand (albeit with potentialecological damage to the Severn Estuary) has not attracted serious investors under presentmarket conditions, owing to high capital costs and long construction periods. Nevertheless, theRoyal Commission on Environmental Pollution suggested that renewable energy mightcontribute between 15 and 25% of total energy demand by 2050 (RCEP, 2000). They envisagethat some 10–15% of total supply would need to be met by intermittent sources; those fromonshore and offshore wind turbines, solar photovoltaic (PV) cells, and wave and tidal streamdevices, as well as the Severn tidal barrage.

For the RCEP (2000) target contribution from renewables to actually be achieved, asignificant reduction in primary energy consumption to between 45 and 75% of the presentfigure would be required. This implies the widespread adoption of energy-saving measuresacross the economy, and this has now been acknowledged in the recent U.K. Energy WhitePaper (DTI, 2003). It is in this area that thermodynamic analysis can make a major contributionto identifying where the improvement potential lies. Indeed Hammond and Stapleton (2001)found, using Second Law or ‘exergy’ analysis, that nearly 80% of this potential is associatedwith electricity generation, together with final energy demand in the domestic sector and intransport. The poor thermodynamic performance in these sectors is due principally to exergeticlosses in combustion and heat transfer processes associated with power generation, spaceheating, and the main transport modes. Various policy instruments will inevitably be required toencourage the introduction of energy efficiency measures in the face of market barriers;including building and product efficiency standards, CO2 emissions trading, tax incentives, andadvice and information (DTI, 2003).

The potential importance of the global warming problem in many minds has resulted inthe case being made for a ‘low carbon’ economy, rather than just a ‘low energy’ one.Technologies for carbon sequestration have therefore been identified as an important elementin any energy R, D & D programme. Such approaches are certainly consistent withthe ‘precautionary principle’ (see, for example, Hammond, 2000b or Porritt, 2000).Japanese industry is well advanced in terms of demonstration plant for CO2 sequestration(Hammond, 2000b), as well as for other pollutants. Integrated coal gasification combinedcycle (IGCC) plants lead to both relatively high thermal efficiencies (greater than 50%)and a reduction of CO2 of better than 20% compared with conventional plant. In the caseof clean coal technology, pressurized fluidized-bed boilers yield high combustionefficiencies together with NOx emission control. In the U.K, the Natural Resources andEnvironment Foresight Panel has called for research on a number of key carbon sequestrationtechnologies.

3. THE MATHEMATICAL AND PHYSICAL FRAMEWORK FORTHERMODYNAMIC ANALYSIS

3.1. Energy analysis

In order to determine the primary energy inputs needed to produce a given amount of productor service, it is necessary to trace the flow of energy through the relevant industrial system. Thisis based on the First Law of Thermodynamics; the principle of conservation of energy, or the



notion of an energy balance applied to the system. It involves the entire life-cycle of the productor activity from ‘cradle-to-grave’. The system boundary should strictly encompass the energyresource in the ground (for example, oil in the well or coal at the mine), although this is oftentaken as the national boundary in practice. Thus, the sum of all the outputs from this systemmultiplied by their individual energy requirements must be equal to the sum of inputs multipliedby their individual requirement. This process consequently implies the identification of feedbackloops, such as the indirect, or ‘embodied’, energy requirements for materials and capitaloutputs. It is therefore sometimes termed ‘net energy analysis’.

Energy conservation suggests that for a steady-state process the First Law may be representedby (Hammond and Stapleton, 2001):X

ðhþ keþ peÞinmin �X

ðhþ keþ peÞoutmout þX

Q� W ¼ 0 ð1Þ

where min and mout denote the mass flow across the system inlet and outlet, respectively, Qrepresents the heat transfer across the system boundary, W is the work (including shaft work,electricity, and so on) transferred out of the system, and h, ke, and pe denote the specific valuesof enthalpy, kinetic energy, and potential energy, respectively. Equation (1) is commonly knownas the ‘steady flow energy equation’, and is represented schematically in Figure 3. This energybalance can be simplified, assuming negligibly small changes in kinetic and potential energy andno heat or work transfers, to (Hammond and Stapleton 2001):X

Hi;in ¼X

Hj;out ð2Þ

where Hi.in represents the various energy (or enthalpy) streams flowing into the system, andHj,out the different energy outputs. If all these inputs and outputs are taken into account(whether or not all the outputs are actually ‘useful’) then the First Law energy efficiencybecomes:

Z ¼X

Ej;out

,XHi;in ¼ Hout=Hin ¼ 1 ð3Þ

This is not a very helpful expression, as many of the energy output streams will be in the form of‘waste heat’. A more practical definition for the energy efficiency is one along the lines(Hammond, 1998):

Overall energy system efficiency ¼energy supplied to final consumers

primary energy consumed100% ð4Þ

Control Volume

Qe

Qother (or Wother)Hin

Wuseful

Qb

T0

Figure 3. An energy balance for a simple control volume or unit operation.


G. P. HAMMOND620

Here ‘primary energy’ is the energy resource input into the whole system. There aremany feedback loops in the energy system whereby primary energy sources (includingfossil fuels, uranium ore, and hydro-electric sites) and secondary derivatives (suchas combustion and nuclear-generated electricity) themselves provide upstream energyinputs into the ‘energy transformation system’. This is that part of the economy wherea raw energy resource is converted to useful energy, which can then meet downstream ‘final’or ‘end-use’ demand (Slesser, 1978). ‘Renewable’ energy sources are taken to mean thosethat are ultimately solar-derived: mainly solar energy itself, biomass resources, and windpower. Thus, energy inputs and outputs to the whole system can be equated via (Hammond,1998):

Primary energy ¼ downstream end-use or delivered energy

þ upstream waste ð5Þ

The energy efficiency defined in Equation (4) may be rewritten in terms of the mathematicalnotation employed here to yield:

Z ¼ ðHoutÞuseful=Hin51 ð6Þ

The value of Z for the U.K. energy system as a whole has remained pretty constant at about69% over the 30 year period 1965–1995 (Hammond, 1998) despite quite dramatic changes in theinternational and domestic energy scene.

3.2. Exergy: a measure of the thermodynamic quality of an energy carrier

First Law or ‘energy’ analysis, sometimes termed ‘fossil fuel accounting’ (see, for example,Chapman, 1976; Slesser, 1978), takes no account of the energy source in terms ofits thermodynamic quality. It enables energy or heat losses to be estimated, but yields onlylimited information about the optimal conversion of energy. In contrast, the Second Lawof Thermodynamics indicates that, whereas work input into a system can be fully convertedto heat and internal energy (via dissipative processes), not all the heat input can be convertedinto useful work. [This Second Law ‘asymmetry’ also dictates that, although heat can flowdown a temperature gradient unaided, shaft work or an electrical energy input is required inorder for heat transfer to take place from a cold to a hot reservoir (as in the case of aheat pump).] The Second Law of Thermodynamics, therefore, suggests the need for thedefinition of parameters that facilitate the assessment of the maximum amount of workachievable in a given system with different energy sources. ‘Exergy’ is the available energyfor conversion from a donating source (or reservoir) with a reference to a specified datum,usually the ambient environmental conditions (typically 1 bar and 5–258C). In a sense itrepresents the thermodynamic ‘quality’ of an energy carrier, and that of the waste heat or energylost in the reject stream. Electricity, for instance, may be regarded as an energy carrier having ahigh quality, or exergy, because it can undertake work. In contrast, low temperature hotwater, although also an energy source, can only be used for heating purposes. This distinctionbetween energy and exergy is very important when considering a switch, for example,from traditional internal combustion (IC) engines to electric, hybrid, or fuel cell vehicles.Thus, Hammond (2000a) has argued that it is important to employ exergy analysis(see, for example, Kotas, 1985; Szargut et al., 1988) alongside a traditional First Law energy



analysis in order to illuminate these issues. It provides a basis for defining an exergy efficiency,and can identify exergetic ‘improvement potential’ within systems.

Exergy is lost or degraded in every irreversible process or system. Consequently, an exergybudget on a control volume can be formulated in an analogous manner to the First Law energybalance, Equation (1), as (Rosen and Dincer, 1997):X

einmin �X

eoutmout þX

ðEQ � EWÞ � I ¼ 0 ð7Þ

where EQ and EW denote the exergy transfer associated with Q and W, respectively, I is thesystem exergy consumption or irreversibility, and e represents the specific exergy. It isrepresented schematically in Figure 4. Equation (7) can also be simplified like its First Lawequivalent to yield (Hammond and Stapleton, 2001):X

Ei;in >X

Ej;out ð8Þ

Thus, the exergy loss or irreversibility rate (van Gool, 1992) of the system is given by

I � DElost ¼ Ein � Eout > 0 ð9Þ

Kline (1999) argues that this ‘irreversibility’, perhaps denoted better by the termexergy ‘degradation’ or ‘destruction’ (Hammond, 2003), can be interpreted as the dissipated‘available energy’ (or exergy) that ends up as random thermal fluctuations of the atomsand molecules in the exit flow of mechanical devices. He illustrates this process by wayof examples drawn largely from the sort of rotating fluid machines with which he wasmost familiar (essentially kinetic energy converters). A slightly different interpretationmay be needed to understand exergy dissipation in the course of fossil fuel combustion(Hammond, 2003). Nevertheless, Kline rightly argues that this phenomenon canonly be understood as an interaction between processes at both the macroscopic andmicroscopic scales.

The exergy function itself is an ‘extensive’ property (Rosen and Dincer, 1997; Hammond andStapleton, 2001) which is defined by reference to a ‘dead’ or equilibrium state (in terms oftemperature T0, pressure P0, and species component mio):

E ¼ ðH � HoÞ � ToðS � SoÞ þXi

Niðmi � mioÞ ð10Þ

T0

Control Volume

Ee =

−

eT

T01 Qe

IEin = Hin

Wuseful

Eb =

−

bT

T01 Qb

Tb

Te

Figure 4. An exergy budget for a simple control volume or unit operation.


G. P. HAMMOND622

where S denotes entropy and Ni is the number of moles of species i. Variations in speciesconcentration are not usually significant in problems related to the macro-scale analysis ofenergy systems. Consequently, a truncated mathematical expression can be used to calculate‘physical’ or ‘thermomechanical’ exergy states:

E ¼ ðH � HoÞ � ToðS � SoÞ ð11Þ

The choice of the reference state has been the subject of some divergence of opinionin the literature. Rosen and his co-workers (Rosen, 1992; Rosen and Dincer, 1997)employed standard temperature and pressure (To=298K (258C) and Po=1 atm) for hisexergy analysis of the Canadian and Turkish economies, whereas Wall (1987, 1990) adopted158C as the datum for his country studies of Sweden and Japan. Nevertheless, a more commonbasis for heat load calculations in mainland Britain is to assume a winter outsidedesign temperature of about �18C. This was the reference condition recently adopted byHammond and Stapleton (2001) for their exergy analysis of the U.K. energy system. It is thesame as the ‘dead state’ temperature adopted by Reistad (1975) for exergy analysis of spaceheating in the U.S.A

3.3. The exergy method in practice: some useful parameters and tools

An exergy efficiency, c, can be defined as (Hammond and Stapleton, 2001):

c ¼ Eout=Ein ¼ 1� I=Ein51 ð12Þ

It should be noted that this expression is strictly analogous to Equation (3), rather than thepractical First Law (energy) efficiency defined by Equation (6). Comparison with the formerequation indicates that, in any real engineering system (which is irreversible) exergy is degradedand the exergy efficiency is consequently less than unity. van Gool (1992) has noted that themaximum improvement in the exergy efficiency for a process or system is obviously achievedwhen DElost is minimized; see Equation (9). Consequently, he suggested that it is useful toemploy the concept of an exergetic ‘improvement potential’, IP, when analysing differentprocesses or sectors of the economy. It is given by

IP ¼ ð1� cÞðEin � EoutÞ ð13Þ

This expression was recently used by Hammond and Stapleton (2001) to evaluate theimprovement potential of critical elements of the U.K. economy.

In the case of heat transfer at a constant temperature (say Tp) the ‘thermal exergy’ is given by(Rosen and Dincer, 1997):

EQ ¼ ð1� To=TpÞQ ð14Þ

Domestic gas-fired and electric heating equipment is similarly used to generate heat at aconstant temperature. The energy and exergy efficiencies of, for example, electric heaters cantherefore be determined (Rosen and Dincer, 1997) from the earlier definitions implied byEquations (6) and (12) above

Z ¼ Q=We ð15Þ

and

c ¼ EQ=EWe ð16Þ



where We is the electrical energy supplied to the heater. Equation (14) can then be used tosimplify the expression for the exergy efficiency (Rosen and Dincer, 1997):

c ¼ ðð1� To=TpÞQÞ=We

¼ ð1� To=TpÞZ ð17Þ

via Equation (15). Rosen and Dincer (1997) have shown that this expression is alsoapproximately correct for gas-fired heating. It shows that in this situation the exergy efficiencyis directly proportional to the energy efficiency; dependent only on the ratio of the process toenvironmental datum temperatures.

Now, it was suggested above that exergy analysis provides an indication of thethermodynamic quality of an energy carrier. This was formally defined by van Gool (1987)as the ratio of exergy to enthalpy in the flow:

Y �EH

ð18Þ

The same parameter was more recently termed the ‘exergetic potential’ by O’Callaghan (1993).

Thus; for electricity : Y ¼ 1

and for process heat : Y ¼ ð1� T0=TpÞ

Electricity is essentially a ‘capital’ resource that is normally generated in advanced,industrialized countries using either depleting fossil or nuclear fuels (see Hammond, 1996).These latter sources may be contrasted with the renewable (or ‘income’) energy sources, such assolar energy and tidal, wave and wind power. In contrast to electricity (a high quality energycarrier withY=1 as indicated above), low temperature hot water (Y� 0.2) can only be used forheating purposes.

The variation in van Gool’s thermodynamic quality (Y) with the process temperature ratio(Tp/T0) is shown in Figure 5. This was produced using the environmental datum temperatureadopted by Hammond and Stapleton (2001) for their energy analysis of the U.K.: �18C (orT0=272K). They indicated that the exergy efficiency of various domestic heating appliances wasquite sensitive to the choice of this reference temperature, when the process temperature is closeto the selected environmental datum. However, both the exergy efficiency (C) and thethermodynamic quality (Y) are insensitive when plotted against the process temperature ratio;as depicted in Figure 5. Here a very wide variation in Tp/T0 is displayed, and various heatsources are shown for comparison purposes. Their associated process temperatures span therange from liquefied natural gas (LNG) at about �508C to the optical temperature of our Sun ataround +55008C.

van Gool (1987) utilized his definition of thermodynamic quality (Y) in order to developenthalpy/quality diagrams for different power generation and process plant types. Theseindicate where heat losses arise and their quality in exergetic terms. He also employed thequality concept to help analyse the industrial energy demand of several industrialized countries(including the Netherlands and the former West Germany). This facilitates the identification ofthe scope for energy cascading (O’Callaghan, 1993) as a means of improving the thermodynamicperformance of the sector.


G. P. HAMMOND624

4. THERMODYNAMIC ANALYSIS OF ENERGY SYSTEMS

4.1. Central-station electricity generation

A wider range of factors impinge on the choice of fuel or technology for power stations,including both First Law and Second Law generation efficiencies (Table I), security anddiversity of energy sources, and environmental impacts of one sort or another. Reconciliation ofthese conflicting factors is a complex matter that is difficult to resolve by formal methods.Rather than attempting to find an optimal solution, a pragmatic approach is required(Hammond, 2000b), what is often termed ‘satisficing’ in the management literature. It can beargued (see, for example, Hammond, 1998) that the U.K. Government will need to keep thebalance of energy resources under periodic review (perhaps with the aid of external advice) andto intervene in the competitive energy market to offset its deficiencies. Similar views have beenexpressed by Fells (2000).

The relation between the lumped (or sector-weighted) energy and exergy efficiencies forvarious central-station power plants can be determined using detailed or ‘microscale’ processanalysis like that undertaken by Szargut et al. (1988), and those adopted by Hammond andStapleton (2001) for their exergy analysis of U.K. power generation are reproduced in Table I.

Liqu

efie

d N

atur

al G

as

Hot

Wat

er S

tora

ge

Hot

Tre

atm

ent o

f Ste

el

Mel

ting

Poi

nt o

f Tun

gste

n

The

Sun

Unity

- 8

+1

0

-1

-2

-310-1 100 101 102

Reference State: T0 = 272K

The

rmod

ynam

ic Q

ualit

y, Θ

Process Temperature Ratio,TP/T0

Figure 5. Temperature dependence of thermodynamic quality.

Table I. The relationship between the energy and exergy efficiencies for electricity generation.

Power plant type Energy-exergy efficiency relations

Conventional steam c=0.96ZCCGT c=0.96ZNuclear c=ZHydro-electric c=78%, Z=90%

Data source: Szargut et al. (1988).



In all cases the energy and exergy efficiencies are quite similar, but this hides the underlyingcauses for power plant exergy losses or irreversibilities. Reistad (1975) has presented a detailedbreakdown of the energy and exergy losses across each component of a U.S. coal-fired powerstation. His results are shown in Table II, together with the corresponding plant generationefficiencies. It is clear that the major First Law losses arise in the condenser, whereas SecondLaw losses occur in the steam generator, owing mainly to combustion, and in the heatexchangers. The slightly higher generation efficiencies reported by Reistad (1975) in comparisonwith those of Hammond (2000b) result from the rather higher operating temperatures typicallyadopted in US power plant practice (Szargut et al., 1988) in comparison with the U.K.

The energy-exergy efficiency ratios in Table I permitted Hammond and Stapleton (2001) tomake an estimate of the inputs and outputs for U.K. electricity generation in a similar mannerto other national exergy analysis studies (see, for example, Reistad, 1975; Wall, 1987, 1990;Rosen, 1992; van Gool, 1992; Rosen and Dincer, 1997). Fossil fuel and other energy inputs forpower plant may all be regarded as high-grade carriers, and consequently the value of the energyand exergy inputs are essentially the same. Thus,

Ei;in � Hi;in ð19Þ

Major changes have taken place in the U.K. electricity generation sector over the period from1965 onwards (Hammond, 2000b). The privatization of U.K. energy utility companies in the1980s and moves towards the creation of a fully competitive energy market have induceddramatic changes in terms of energy resources employed for electricity generation (Hammond,1998). Relative fuel prices, construction costs and times, and arguably environmental benefitsled to the ‘dash for gas’ and a fall in the amount of indigenous solid fuel consumed at powerstations. The amount of electricity supplied to final consumers nearly doubled during thisperiod, while the efficiency of generation improved from about 30% in 1965 to over 35%currently (Hammond, 1998). It reflected the introduction of more modern plant, particularlycombined cycle gas turbine (CCGT) power stations, and the ‘dash for gas’ for electricitygeneration. The corresponding exergetic improvement potential is shown in Figure 6; brokendown to highlight the improvement potential of the different types of power plant. Hydroelectricand pumped storage systems provide only a very small output compared with other power

Table II. Thermodynamic performance of coal-fired power stations.

Plant components Energy losses (% of plant input) Exergy losses (% of plant input)

Steam generator 9.0 49.0* combustion (29.7)* heat exchanger (14.9)* thermal stack loss (0.6)* diffusional stack loss (3.8)

Turbines �0 4.0Condenser 47.0 1.5Heaters �0 1.0Miscellaneous 3.0 5.5Plant totals 59.0 61.0Generation efficienciesn Z=100–59=41 c=100–61=39

Source: Reistad (1975); U.S. conventional design.nEfficiencies based on gross calorific or higher heating value (HHV) of fuels.


G. P. HAMMOND626

stations and have therefore been omitted from Figure 6. Nevertheless, the relatively poorSecond Law performance of electricity generation, or its large improvement potential, is clear. Itis also wasteful in thermodynamic terms to convert fuels to electricity only to employ it forheating. If heat is required, then it would be far more efficient to burn fossil fuels (for example)to produce heat directly. Chapman (1976) discusses the relative end-use merits of electricity,arguing that (in spite of the lack of detailed statistics) it was possible to estimate that some 25%of electricity in the U.K. was used for heating in the mid-1970s.

4.2. Cogeneration plant or combined heat and power schemes

Large energy losses occur during electricity generation unless it is used in conjunctionwith combined heat and power (CHP) or ‘cogeneration’ systems (Horlock, 1987). Thus, theonly ways to improve the efficiency of the ‘energy transformation system’ significantly(Hammond, 2000b), in the absence of new large-scale hydropower sites, is either to restrictthe use of electricity to power applications (and not for relatively low-temperature heating)or to adopt a greater proportion of CHP plants (Hammond and Stapleton, 2001). TheU.K. Government currently favours consent for the construction of CHP plants, whichproduce both electricity and usable heat, provided that they are suitably sized to meet on-siteor nearby heat requirements. Such schemes have an overall First Law efficiency (Z) of some 80%in contrast with the best recuperative CCGT plant of 59–61% (Hammond and Stapleton, 2001).In fact, all the fossil fuel ‘power’ station designs have a high CHP potential.

Given the energy saving potential of cogeneration schemes indicated above, it is usefulto examine the thermodynamic behaviour of an individual CHP plant more closely.Bilgen (2000) recently carried out a parametric study of a cogeneration cycle incorporating acombined cycle gas turbine and heat recovery steam generator (HRSG). This was based ona nominal 22MW industrial turbine set manufactured in the U.S.A, which typically operateson a power-to-heat ratio of 0.92. It is illustrated schematically in Figure 7. Turnkey CCGTplant typically have an electricity generation, or exergy, efficiency (LHV) that varies withsize; 40–60% over the range 7–790 MW (Richard Hotchkiss, Innogy plc, private communica-tion, 2002). In the 22 MW size range, an efficiency of about 45% would be expected basedon U.K. experience. Nevertheless, the thermodynamic results of Bilgen’s U.S. parametricstudy (he also undertook an engineering cost evaluation) are reproduced in Figure 8. Here, it

1000

1200

1400

1600

800

400

600

200

01965 1970 1975 1980 1985 1990 1995 2000 2005

PJ

Year

NuclearCCGT

Conventional Steam

Sector Total

UK ELECTRICITY GENERATION: IMPROVEMENT POTENTIAL

Figure 6. Exergetic improvement potential within the U.K. power generation sector. Source:Hammond and Stapleton (2001).



can be seen that the First Law (energy) efficiency, defined via Equations (4) and (6) in ananalogous way to the parameter that Horlock (1987) terms the ‘energy utilization factor(EUF)’, falls sharply with power-to-heat ratio and the proportion of process steam extracted.In contrast, the exergy efficiency is insensitive to these parameters. This is because it reflectsthe ability to perform work and the efficiency of power generation only. Nevertheless,CHP is clearly desirable on fossil fuel resource productivity grounds. It is, therefore, evidentthat this is a case where exergy analysis on its own is insufficient, and reinforces the argu-ments of Hammond and Stapleton (2001) against the primacy of Second Law considera-tions. They should simply form part of a much more comprehensive toolkit of quantitativeand qualitative methods for evaluating energy and environmental issues (Hammond,2000a).

EXTRACTEDSTEAM

CONDENSATERETURN

POWER

GENERATOR

ST

EA

MT

UR

BIN

E

TO CONDENSERPOWER

GENERATOR

TU

RB

INE

HEAT RECOVERYGENERATOR

FUEL

AIR

COMBUSTIONCHAMBER

CO

MP

RE

SS

OR

STACKLOSSHEAT

∼

∼

Figure 7. Combined cycle gas turbine plant (with and without extracted steam for process heating; dashedline). Source: adapted from Bilgen (2000).

ENERGY (η)

EXERGY (ψ)

22 MW INDUSTRIAL CHP PLANT

80

70

60

50

401 2 4 6 10 20

100

80

60

40

20

0

The

rmod

ynam

ic E

ffici

enci

es(%

)

Ste

am E

xtra

cted

(%

)

Power-to-Heat Ratio

Figure 8. Thermodynamic performance of combined heat and power plant. Source:adapted from Bilgen (2000).


G. P. HAMMOND628

4.3. Improving energy productivity in industry

In order to analyse energy usage and its effectiveness within the industrial sector, Hammond andStapleton (2001) subdivided the multitude of processes into four broad categories: lowtemperature (Tp5394K), medium temperature (Tp=394–692K), high temperature(Tp>692K), and mechanical drives. Typical First and Second Law efficiencies in thesecategories are illustrated in Figure 9. These are end use values, and obviously the net energy andexergy efficiencies would need to account for losses in power generation. Consequently, the useof electricity to power mechanical drives is not as attractive as it appears, but is largely anecessary engineering requirement. A knowledge of fuel and electricity shares in each of theprocess categories (Hammond and Stapleton, 2001) enables the thermodynamic inputs andoutputs associated with the industrial sector to be estimated. The overall industrial sector exergyefficiency (�46% in the mid-1990s) is significantly lower than the corresponding energyefficiency (�69%), although Hammond and Stapleton (2001) observed that the disparity is notas large as in the domestic sector, where space heating requirements predominate. They showedthat exergy losses in industry, as a proportion of the energy input, are rather smaller than ineither electricity generation or the domestic sector. Nevertheless, there is still considerable scopefor thermodynamic improvements in industry.

There is obviously a need to stimulate improvements in resource use efficiency generally, andto encourage energy conservation from the ‘bottom-up’. Such an approach would need to becoupled with measures to reduce the rate of consumption of fossil fuels, and stimulate anexpansion in the use of renewable energy sources (RCEP, 2000). It would involve a consumer-oriented market approach, coupled with intervention by way of a portfolio of measures tocounter market deficiencies; economic instruments, environmental regulation, and land useplanning procedures. Scenarios such as the ‘dematerialization’ or ‘Factor Four’ project,advocated by von Weizsacker et al. (1997), suggest that economic welfare in the industrial worldmight be doubled while resource use is halved; hence the Factor 4. This would involve a

%

10090

8070

60

50

4030

20

100

Low

Tem

pera

ture

Hig

hTe

mpe

ratu

re

Hig

hTe

mpe

ratu

reFue

l

Fue

l

Fue

l

Fue

l

Fue

l

Fue

l

Ele

ctric

ity

Ele

ctric

ity

Ele

ctric

ity

Ele

ctric

ity

Ele

ctric

ity

Ele

ctric

ity

Mec

hani

cal D

river

s

Mec

hani

cal D

river

s

Low

Tem

pera

ture

Energy Efficiencies Exergy Efficiencies

Figure 9. Thermodynamic efficiencies of industrial processes. Source: Hammond and Stapleton (2001).



structural shift from energy intensive manufacturing to energy frugal services (Hammond,2000b). Britain has moved some way in this direction, with a 40% improvement in primaryenergy intensity since 1965. Increases in resource use efficiency to the Factor 4 level (and theU.K. Foresight Programme is contemplating Factor 10 over the long term) would have theenormous knock-on benefit of reducing pollutant emissions that have an impact, actual orpotential, on environmental quality. In reality such a strategy requires a major change(‘paradigm shift’) to an energy system that is focused on maximizing the full fuel/energy cycleefficiency, and minimizing the embodied energy in materials and products by way of reuse andrecycling (Hammond, 2000b). In order to make such an approach a practicable engineeringoption, it would be necessary to use systems analysis methods to optimize the energy cascade(Hammond, 2000b; O’Callaghan, 1993). Thermodynamic analysis will be an importanttechnique for identifying process improvement potential. The tools developed by van Gool(1987 and 1992) and his co-workers based on the notion of ‘thermodynamic quality’, Equation(18), can play a key role here. They employed, as previously noted, enthalpy/quality diagrams toidentify the scope for energy (or heat) cascading within the industrial sector of several developedcountries.

5. THERMODYNAMIC IDEAS AND ENVIRONMENTAL SUSTAINABILITY:MULTIDISCIPLINARY DIMENSIONS

5.1. Thermodynamics, analogy and metaphor

Practitioners in a variety of disciplines with interests in environmental sustainability, includingecology, economics and engineering, have drawn on thermodynamic concepts. Widespreadconcern about resource depletion and environmental degradation are common to these differingareas of study. It has been argued that the deleterious consequences of human developmentare reflected in thermodynamic ideas and methods of analysis (see, for example, the earlywork of Mueller (1971) at the U.S. Goddard Space Flight Center); they are said to mirrorenergy transformations within society. Mueller (1971) draws a parallel between the resourceflows in economics and energy (as well as implicitly exergy) flows in thermodynamics. Thisleads him to an, arguably rather dubious, analogy between the ‘technology of man’ andheat engines. Such ideas have inspired the environmental campaigner, Sara Parkin (2000)[a co-founder of the sustainable development charity ‘Forum for the Future’ with JonathanPorritt], and others to believe that thermodynamic principles or laws may act as a guidefor engineers in the quest for environmental sustainability. In the context of ‘The Natural Step’,energy and matter are seen as having a tendency to disperse. Entropy (another SecondLaw extensive property of matter, that is related to exergy via Equations (10) or (11)) is regardedas a measure of this disorder in a closed or isolated system. The Earth is such a closed systemin terms of matter, but an open one from the perspective of the incoming solar energy thatdrives living plants via photosynthesis. This underpins the notion of ‘capital’ and ‘income’energy resources for the planet (such as fossil fuels and solar energy, respectively), and isbehind the first of the TNS system conditions. Outside the realm of energy systems,thermodynamic concepts are typically employed in terms of an analogy with, or resemblanceto, physical processes. Alternatively, their use may be regarded, as colleagues at the Universityof Bath have suggested (Stephen Gough and William Scott, Centre for Research in Education


G. P. HAMMOND630

and the Environment (CREE), private communication, 2003), as metaphorical: beingimaginatively, but not literally, applicable.

Entropy is not an easy concept to grasp, particularly when it has been so widely usedand abused. It was originally developed by Rudolf Clausius (ca. 1864) from a consideration ofthe Carnot cycle for an ideal heat engine. This original ‘energetic’ (Clausius) entropy reflectsthe fact that, although heat can flow down a temperature gradient unaided, shaft workor an electrical energy input is required in order to induce heat transfer to take place from acold to a hot reservoir: Clausius’ inequality. However, the idea of entropy has fascinatedwriters in disciplines far removed from engineering and the physical sciences. Many analogousproperties have been proposed. Stephen Kline (1999) identifies five microscopic ‘entropies’(including Gibbs’ statistical entropy), two information functions (Shannon’s and Brillouin’sthe so-called ‘entropies’), and what he amusingly denoted as the ‘vulgar’ entropy.Kline interprets Gibbs’ statistical entropy as a useful measure of the ‘spread-outness’ ofrandom molecular fluctuations amongst various microstates within the constraints of thephysical boundaries of a system. However, he criticizes the attribution of the term ‘entropy’to information functions as an error of typology; saying it is like equating ‘apples with oranges’.In addition, Kline demonstrates that Brillouin’s ‘entropy’, a probabilistic quantitywidely employed in the field of informatics, was built on the foundation of a sign error in afamous 1929 paper by Leo Szilard, another Nobel Laureate in Physics. These are not uniquecriticisms, and Kline points to earlier reservations by the likes of Denbigh, Fast, Pierce, andPopper.

5.2. Environmental economics and the entropy law

Perhaps the first discipline outside engineering to seriously adopt thermodynamic ideaswas economics; actually the sub-set that has become known as environmental economics.The system studied in economics is the individual firm or the consumer (Hammond, 2000a).Transactions between the firm (or consumer) and the rest of the world are described in termsof the quantities and prices of the commodities exchanged. Prices in this neoclassicaleconomic model are supposed to reflect the ‘value’ that society places on an economicgood. Thus, economic practitioners claim that their discipline is ‘normative’: it suggeststhe optimal course of action to be taken in the allocation of resources, whereas thermodynamicanalysis is ‘prescriptive’. However, environmental economists have employed thermo-dynamic ideas to devise for alternative accounts of sustainability by analogy to physical ornatural processes, such as energy usage. There is a well-developed literature, dating back tothe early 1970s, that amounts to the postulation of an ‘Energy Theory of Value’ (S .oollner,1997), although this has been largely rejected because choices about (First Law) energyuse do not reflect the full complexity of human behaviour and value judgements. However,it was soon recognized that it is Second Law properties, such as entropy and exergy, whichmore realistically reflect dissipative processes. Georgescu-Roegen (1971) was at the forefrontof this movement with his advocacy of ‘the Entropy Law’, effectively the SecondLaw of Thermodynamics, as a measure of economic scarcity. He viewed economic systemsas ones in which energy is conserved, but in which entropy increases (or exergy degrades).A brave attempt to investigate interdisciplinary approaches to long-term energy problemsand the employment of thermodynamic concepts was made in a workshop organized under theauspices of the Dutch Energy Study Centre (van Gool and Bruggink, 1985). Here similarities



and differences between the physical sciences and economics were explicitly investigated, andmany enduring insights obtained. S .oollner (1997) produced a comprehensive review and critiqueof the use of thermodynamic ideas in environmental economics. He again drew attention to theinsight that energy and related properties can bring to economics and sustainability via the useof analogies and the setting of absolute limits respectively. An important medium-term exampleof the latter (Slesser, 1978; S .oollner, 1997) is the dominant use and finite nature of fossil fuelresources. But there is no direct link between thermodynamic properties and the characteristicsof economic systems. The former cannot explain the latter, let alone forecast the future paths ofcomplex economies (S .oollner, 1997).

5.3. Resource flows and exergy

Several thermodynamicists have viewed the exergy function as a potential tool for resourceand/or emissions accounting (see, for example, Szargut et al., 1988; Dincer and Rosen, 1999).Material resources are viewed as having chemical exergy, because of their disequilibriumwith the surrounding environmental conditions. When they undergo processing, these materialscause waste and therefore environmental damage. An increase in process efficiency willreduce the amount of waste and the exergy degradation. Thus, exergy is seen as a measureof the ‘value’ of the material, whereas process wastes have a potential to cause adversechanges in the environment. Dincer and Rosen (1999) term the exergy encapsulated inmaterials (positive) ‘constrained exergy’, in contrast to the (negative) ‘unconstrained exergy’associated with emissions to the environment. They suggest that this provides a means ofresource and emissions accounting, albeit one that needs further exploration. Szargut et al.(1988) describe the cumulative consumption of ‘natural exergy resources’ as the ‘coefficient ofecological cost’. Depleting or non-renewable (fossil and nuclear) fuels represent the largestcomponent of this ecological cost. However, replacing these fuels by ‘income’ energy resources,like nuclear fusion or solar energy, would enable these sustainable energy technologies to beutilized to extract fuels from lean raw materials. It would obviously require greater exergyconsumption, but this would be of little consequence in a world driven by income sources (seealso Lovins, 1977).

Szargut et al. (1988) argue that expressing waste products in terms of exergy, in a mannermore recently employed by Dincer and Rosen (1999), may be oversimplified. Certainly theenvironmental toxicology, or ‘ecotoxicology’, of waste products or pollutant emissions isunlikely to be reflected solely by their exergy content. Other environmental appraisal techniques,such as environmental life-cycle assessment (LCA), would be required to properly evaluate theseemissions (Hammond, 2000a). There is clearly much that needs to be done in developing theseexergy-based ideas and analysis techniques before they can be practically applied. They alsosuffer from the same criticism that can be levelled at the notion of an ‘Exergy Theory of Value’(by, for example, Hammond and Stapleton, 2001). Exergy is a measure of the maximumtheoretical useful work that is obtainable from a thermal system (as it is brought intoequilibrium with its surrounding environment), and this may be not be the only or relevantcriterion in a particular situation.

An innovative attempt to analyse different societies in terms of energy and exergy flowdiagrams has been made by Sciubba (1995). He examined the sustainability of a variety of socialstructures ranging from ‘primitive’ tribal groups, via industrial (and ‘post-industrial’) societies,to a future envisaged as being dominated by a highly ‘robotized’ or cybernetically controlled


G. P. HAMMOND632

social organization. In essence, energy and exergy were employed here as ‘technology levelindicators’. Sciubba recognized that his model was an oversimplification of thecomplex interactions that arise between human societies and the natural world. Nevertheless,he believes this energy/exergy approach can adequately represent these various interactions.Consequently, the model could be used to examine alternative societal arrangements thatmight be ‘leaner’ in terms of resource extraction, whilst being as ‘comfortable’ aspresent industrialized societies. Not all forms of societal organization were found to beself-sustaining, with certain size and technology-related restrictions applying to most societies.Sciubba (1995) argues that neither resource scarcity nor biosphere capacity appears to constrainhuman development, although many energy analysts and environmentalists (for example,Goldemberg, 1996; Lovins, 1977; Parkin, 2000; Porritt, 2000) would suggest that the contrary isthe case.

5.4. Ecology and ‘free energy’

In the field of ecology, strictly the branch of the natural sciences that deals with therelation between biological organisms and their physical surrounding, the concept of Gibbsfree energy or function (G) is used in preference to exergy (Haynie, 2001). It is definedmathematically as

G ¼ H � TS

The connection between this thermodynamic property and the physical (or thermomechanical)part of exergy can to seen by comparison of this expression with the truncated Equation (11).Gibbs free energy is again the maximum work that is available from a natural or other system,but it is not determined by reference to the surrounding environmental conditions. The deadstate temperature is effectively taken to be absolute zero (�2738C). However, in many cases, it isthe change in free energy (DG) that is significant to the problem being considered, and this isnearly the same as the corresponding change in physical exergy (DE). [They are identical whenT=To.]

Schneider and Kay (1994) have argued that the evolution of life from primitive tocomplex organisms involves processes similar to those governed by the SecondLaw of Thermodynamics. They propose a new, ‘thermodynamically oriented’ paradigm forthe life sciences, including explanations for the origins of life, biological growth, and patternsof biological evolution apparent in the fossil record. However, this approach has generatedsome controversy. Corning and Kline (1998) have expressed their exasperation with this lineof reasoning. They argue that life is too complex and is sensitive to processes at a detailedlevel. Exergy (or, in their terms, ‘available energy’) analysis has therefore been misusedwhen applied to the biological domain. It focuses on energy waste, whereas perhaps amore important consideration is the way in which energy is captured and utilized duringthe struggle to survive. These arguments mirror those that Kline (1999) employed moregenerally in relation to the adoption of thermodynamic concepts in other disciplines,particularly in informatics. A cybernetic perspective leads Corning and Kline (1998) to suggestthat certain law-like ‘bioeconomic’ principles are more likely to constrain thermodynamicprocesses in living systems. Natural selection may favour organisms that improve energycapture.



6. CONCLUDING REMARKS

6.1. Engineering sustainability

There are various ways in which the energy system interacts with the requirements of sustainabledevelopment. A strict interpretation of the Brundtland Commission injunction, or ‘TheNatural Step’ system conditions (Broman et al., 2000; Parkin, 2000; Porritt, 2000;Upham, 2000), would mean a rapid changeover to renewable energy, and the conservationof non-renewable sources (fossil fuels and uranium). This in turn could lead to a significantreduction in pollutant emissions, an unwanted side-effect of the energy sector, that give riseto damaging impacts at local, regional and global scales. Only in this way could the biosphere beprotected for future generations. However, dramatic changes to the nature of the energysystem appear unlikely in the short to medium-term. It has been argued that it is impractical toachieve the very strict system conditions laid down under TNS (see also Doughty andHammond, 2003; Hammond, 2001). Even Parkin (2000) acknowledges that the timescale forachieving sustainability could be in the range 2050–2100, or longer.

Current U.K. measures to combat climate change will reduce CO2 emissions (see Figure 2)to the domestic target of a 20% fall below 1990 levels by about 2010. But this is amodest achievement compared with the perceived global need to reduce GHG emissionsby some 60% to stabilize the climate. There is obviously a need to stimulate improvements inresource use efficiency generally, and to encourage energy conservation from ‘bottom-up’,if the ambitious 2003 U.K. Energy White Paper targets are to be met. The White Paper(DTI, 2003) charts a new path for energy policy with a focus on low-carbon options;principally energy conservation measures and renewable energy technologies for the long-term (with the nuclear power option kept open and in reserve). It will require a portfolioof measures to counter market deficiencies; economic instruments, environmentalregulation, and land use planning procedures (DTI, 2003; Hammond, 2000b). Arobust transitional energy strategy is clearly needed with a focus on energy efficiency (a movein the direction of Factor 4 or more technologies), and minimizing significantlypollutant emissions. The elements of such a strategy will change over time, and the ‘optimal’mix at any given instant will be uncertain when viewed from the present (Hammond, 1998,2000b). Engineers have much to contribute in terms of identifying opportunities for processimprovement using thermodynamic and other related means of analysis, such as those discussedhere.

Actions taken to reduce pollutant emissions from power stations, industrial processes, and theother sectors of the economy would have benefits on both a local and global scale. Measures tolimit acid precursors from electricity generation, for example, will also reduce GHG emissions.Policies of this type are therefore of a ‘win–win’ nature (Hammond, 2000a). Central governmentneeds to stimulate implementation, and develop an enhanced ‘systems modelling’ capability toensure that the sum of the parts meet national targets (Hammond, 2000a). Thus, theenvironmental and sustainable development challenges of the 21st Century might be met by wayof a mixture of vision and realism. Thermodynamic analysis may again have an important roleto play in this wider scale of effort. It also provides ‘evidence-based’ means of analysing movestowards, and criteria for, sustainability. This suggests that ‘The Natural Step’ systemsconditions may not be quite as scientifically based as is sometimes argued. Viewing the First andSecond Laws of Thermodynamics as representing the conservation and dispersion of energy and


G. P. HAMMOND634

matter is perhaps limited to the domain of the metaphor, rather than an approach grounded inscience.

6.2. Thermodynamics and sustainability: energy systems and beyond

Thermodynamic methods will undoubtedly form an indispensable part of the ‘toolkit’ needed tosecure a sustainable future (Hammond, 2003); see also Figure 1 (Parkin, 2000). It is clear fromthe discussion here that exergy analysis can provide an important tool for the understanding ofcomplex energy systems. It was used by Reistad (1975) to identify the true nature of losses inpower plant. He noted that enthalpy losses arise in the condenser, and therefore offer littleprospect of improvement other than by way of a ‘bottoming cycle’. However, exergy analysisindicates that Second Law losses are associated with combustion processes and with heatexchangers. Making improvements at that end of the cycle will have the ‘knock-on’ benefit ofalso giving rise to higher First Law efficiencies. Hammond and Stapleton (2001) argue that thefeasibility of such changes is not as important as a proper comprehension of the thermodynamicprocesses involved. Now it is possible, of course, to identify Second Law-type improvementpotential without explicitly adopting exergy analysis. Indeed Chapman (1976) correctlydiscerned the waste inherent in using nuclear-generated electricity for space heating ratherthan for electrical appliances or mechanical drives. He employed First Law energy analysis, butsupplemented this via an implicit understanding of the Second Law issues. Hammond (2000a)consequently advocated the use of exergy analysis as one tool amongst several quantitativeapproaches that should be employed to study energy systems, in addition to the more traditionalFirst Law energy analysis. The components of this ‘sustainability toolkit’, which would alsoinclude environmental LCA and cost–benefit analysis (CBA), all have their particularadvantages and disadvantages (Hammond, 2000a). However, there is a tendency for somethermodynamicists to elevate Second Law analysis to a pivotal position. Gaggioli (1980), forinstance, views exergy as representing thermodynamic ‘value’, and regards the Second Lawefficiency as the ‘true’ efficiency. This is not warranted, and Hammond and Stapleton (2001)have argued that it should be discouraged.

On First and Second Law thermodynamic grounds the rank order for the construction of newfossil-fuelled power plant would be CHP or cogeneration schemes, CCGTs, and integrated coalgasification combined cycle (IGCC) plants, reflecting the highest to lowest conversionefficiencies, respectively. Modern IGCC plants have conversion efficiencies of 48–51% andlead to a reduction in CO2 emissions of better than 20% compared with conventional coal-firedplant (Hammond, 2000b). Hydropower and wind turbines also have high thermodynamicefficiencies; the former being around 65–75%. Unfortunately, the U.K. has already exhaustedall its favourable sites for large-scale hydropower and pumped storage schemes. Although thereare many suitable locations for onshore wind energy generators in the British Isles, they aremeeting significant community resistance at local planning enquiries owing to their perceivedeffects of landscape disruption and noise emission (Hammond, 2000b). The utilization ofoffshore wind turbine arrays may avoid these difficulties, but only at higher life-cycle financialcosts. Both offshore wind ‘farms’ and the growing of energy crops (biomass) as a primary usefor agricultural land have been given a strong endorsement by the RCEP (2000) as possiblelong-term energy options. Solar energy systems, such as modern grid-connected PV devices, arediffuse and have low conversion efficiencies compared with their 5500K potential. Nevertheless,they are renewable rather than ‘depletable’ energy systems. Consequently, Lovins (1977) has



argued that renewable energy sources in general should be viewed as having a premium overfossil fuels and uranium, with their finite life. This is another reason for not giving primacy tothe results of exergy analysis. An important finding, reinforced by the study of Hammond andStapleton (2001), relates to the thermodynamic inefficiency of using electricity (from whateversource) to provide low-grade space heating.

Several ways in which thermodynamic concepts have been utilized by practitioners outside theenergy sector have been examined. Many other disciplines have attempted to invoke these ideasin the name of environmental sustainability, not least in fields such as ecology andenvironmental economics. Concern about resource depletion and environmental degradationis common in these disciplines. Human development and its ecological consequences are seen bysome analysts to mirror energy transformations within society. The concept of ‘exergy’, whichfollows from the First and Second Laws of Thermodynamics, has been viewed as providing thebasis of a tool for resource and/or emissions accounting, as well as indicating natural limits onthe attainment of sustainability. However, the application of these thermodynamic ideas outsidethe sphere of energy systems, from which they were first derived, is not without its critics. Theseconcepts are often employed by way of analogy, or simply in a metaphorical sense. ‘Exergyconsumption’ is only indirectly linked to the efficiency of resource utilization. The relative‘ecotoxicology’ of pollutants is also unlikely to be a function of their exergy, and would need tobe assessed via other environmental appraisal techniques (such as LCA). Clearly it is necessaryto apply the methods of energy and exergy analysis with some care when trying to drawconclusions about the criteria for, and pathways to, sustainability.

6.3. Energy, sustainability, and the international dimension

The present work has focused primarily on energy-related considerations in the developedor industrialized countries, particularly the U.K. It is certainly important that these countriesplay their full part in maintaining environmental sustainability as they currently emit the bulkof pollutants into the atmosphere world-wide. But sustainable development must also beviewed in a global context. The resource base of the planet is not well defined; limits are oftenunclear until they are almost reached. One of the great challenges for the 21st century is,therefore, to improve dramatically the efficiency of resource use, particularly of non-renewableenergy, across the planet so that humankind will ‘tread lightly on the Earth’ (Hammond, 2000a).Clearly the industrial nations, whose societies are by far the most resource intensive, will need totake the lead. It will require difficult decisions for the West in terms of market intervention tostimulate the development of sustainable technologies, and possibly to induce changes inlifestyles.

The task facing the nearly 80% of the world population that live in developing countries isdaunting. They have, in most cases, rapidly growing populations that will drive up energyconsumption and environmental pollution. This will feed back to the whole planet, and therebyalter the climate in the wealthier nations (Hammond, 2000b). Consequently they need assistancefrom industrial countries to promote Third World economic growth, which will in time induce a‘demographic transition’ (WCED, 1987), as well as improving the efficiency of their energysystems. These are matters of interregional and intergenerational ethics, rather than purelyscientific debate. A more equitable sharing of world income and resources is likely to be aprerequisite for sustainable development in the long-term. Environmental sustainability wouldcertainly be aided by the transfer of best-practice, or ‘leapfrog’, energy technologies from the


G. P. HAMMOND636

richer to the poorer regions (Goldemberg, 1996). This will ultimately be in the interests of all thecitizens of ‘Spaceship Earth’.

NOMENCLATURE

E =exergyG =Gibbs ‘free energy’ or functionH =enthalpyh =specific enthalpyI =irreversibility (always 50)IP =exergetic ‘improvement potential’ke =specific kinetic energym =massN =number of moles of speciesP =absolute pressurepe =specific potential energyQ =heat transferS =entropyT =thermodynamic temperatureW =work transfer

Greek letters

e =specific exergyZ =First Law or ‘energy’ efficiencyZ0 =idealized energy efficiencyY =thermodynamic qualitym =chemical potentialc =Second Law or ‘exergy’ efficiency

Subscripts

e =electricali =chemical species iin =process or system inlet boundarylost =property losso =reference environmental state (or ‘dead state’)out =process or system outlet boundaryp =process or device

ACKNOWLEDGEMENTS

This paper has been adapted, in part, from an invited lecture presented at the 7th U.K. National HeatTransfer Conference, Nottingham University, 11–12 September 2001. A version close to the current onewas then presented to an interdisciplinary audience at a seminar for the International Centre for theEnvironment (ICE) at the University of Bath on 9 October 2001. The author’s research on energy systems



and environmental sustainability has been supported by research grants awarded by the U.K. Engineeringand Physical Sciences Research Council (most recently under grants GR/J92910, GR/L02227, and GR/L26858). He would also like to acknowledge the support of British Gas plc, now demerged as BGplc andCentrica plc, who have partially funded his Professorship. However, the views expressed in this paper arethose of the author alone, and do not necessarily reflect the policies of either of the new companies. Finally,the author wishes to acknowledge the care with which Sarah Fuge prepared the typescript and Gill Greenprepared the figures.

REFERENCES

Amin S. 1997. Economic, political and social distortions in the modern world. In Dimensions of Sustainability, Smith P,Tenner A (eds). Nomos Verlagsgesellschaft: Baden–Baden, 19–25.

Bilgen E. 2000. Exergetic and engineering analyses of gas turbine based cogeneration systems. Energy 25:1215–1229.Broman G, Holmberg J. Rob"eert K-H. 2000. Simplicity without reduction: thinking upstream towards the sustainable

society. Interfaces 30:13–25.Chapman PF. 1976. Methods of energy analysis. In Aspects of Energy Conversion, Blair IM, Jones BD, Van Horn AJ

(eds). Pergamon: Oxford, 739–758.Corning PA, Kline SJ. 1998. Thermodynamics, information and life revisited, Part I: to be or entropy. Systems Research

and Behavioural Science 15:273–295.Department of Trade and Industry. 2003. Our Energy Future}Creating a Low Carbon Economy. TSO: London.Dincer I, Rosen MA. 1999. The intimate connection between exergy and the environment. In Thermodynamic

Optimization of Complex Energy Systems. Bejan A, Mamut E (eds). Kluwer: Dordrecht, 221–230.Doughty MRC, Hammond GP. 2003. Cities and sustainability. In Towards an Environment Research Agenda, Winnett

A, Warhurst A (eds). Palgrave Macmillan: Basingstoke, 81–105.Fells I. 2000. Can the energy market protect the environment? Transactions of the Institution of Chemical Engineers Part

B: Process Safety and Environmental Protection 78:328–331.Gaggioli RA. 1980. Thermodynamics: Second Law Analysis. American Chemical Society: Washington, DC.Georgescu-Roegen N. 1971. The Entropy Law and the Economic Process. Harvard University Press: Cambridge, MA.Goldemberg J. 1996. Energy, Environment and Development, Earthscan: London.Hammond GP. 1996. Nuclear energy into the Twenty-first Century. Applied Energy 54(4):327–334.Hammond GP. 1998. Alternative energy strategies for the United Kingdom revisited; market competition and

sustainability. Technological Forecasting and Social Change 59:131–151.Hammond GP. 2000a. Energy and the environment. In Towards an Collaborative Environment Research Agenda:

Challenges for Business and Society. Warhurst A. (ed). Macmillan: London, 139–178.Hammond GP. 2000b. Energy, environment and sustainable development: a U.K. perspective. Transactions of the

Institution of Chemical Engineers Part B: Process Safety and Environmental Protection 78:304–323.Hammond GP. 2003. Book Review: Kline SJ. The Low-Down on Entropy and Interpretive Thermodynamics.

Proceedings of the Institution of Mechanical Engineers Part A: Journal of Power and Energy 217(3):337–339.Hammond GP, Stapleton AJ. 2001. Exergy analysis of the United Kingdom energy system. Proceedings of the Institution

of Mechanical Engineers Part A: Journal of Power and Energy 215(2):141–162.Haynie DT. 2001. Biological Thermodynamics. CUP: Cambridge.Horlock JH. 1987. Cogeneration}Combined Heat and Power (CHP). Pergamon: Oxford.Kotas TJ. 1985. The Exergy Method of Thermal Plant Analysis. Butterworth: London.Kline SJ. 1999. The Low-Down on Entropy and Interpretive Thermodynamics. DCW Industries: La Ca *nnada, CA.Lovins AB. 1977. Soft Energy Paths. Penguin Books: Harmonsworth.May R. 1997. Climate Change. Office of Science and Technology: London.Mueller RF. 1971. Thermodynamics of environmental degradation. Proceedings of the Annual Meeting of the American

Geophysical Union, Washington, DC.Nakicenovic N, Gr .uubler A, McDonald A. 1998. Global Energy Perspectives. CUP: Cambridge.Natural Environment Research Council. 1997. Climate Change: Scientific Certainties and Uncertainties. NERC:

Swindon.O’Callaghan PW. 1993. Energy Management. McGraw-Hill: London.Parkin S. 2000. Sustainable development: the concept and the practical challenge. Proceedings of ICE: Civil Engineering

138:3–8.Porritt J. 2000. Playing Safe: Science and the Environment. Thames & Hudson: London.Reistad GM. 1975. Available energy conversion and utilisation in the United States. Transactions of ASME: Journal of

Engineering Power 97:429–434.Rosen MA. 1992. Evaluation of energy utilisation efficiency in Canada using energy and exergy analysis. Energy

17(4):339–350.


G. P. HAMMOND638

Rosen MA. Dincer I. 1997. Sectoral energy and exergy modelling of Turkey. Transactions of the ASME: Journal ofEnergy Resource Technology 119:200–204.

Royal Commission on Environmental Pollution. 2000. Twenty-second Report: Energy}The Changing Climate. TSO:London.

Sciubba E. 1995. Modelling the energetic and exergetic self-sustainability of societies with different structures.Transactions of ASME: Journal of Energy Resource Technology 177:75–86.

Schneider ED, Kay JJ. 1994. Life as a manifestation of the Second Law of Thermodynamics. Mathematical andComputer Modelling 19(6–8):25–48.

Slesser M. 1978. Energy in the Economy. Macmillan: London.S .oollner F. 1997. A reexamination of the role of thermodynamics for environmental economics. Ecological Economics

22:175–201.Szargut J, Morris DR, Steward FR. 1988. Exergy Analysis of Thermal. Chemical and Metallurgical Processes.

Hemisphere: New York.Thring MW. 1990. Engineering in a stable world. Science, Technology and Development 8(2):107–121.Upham P. 2000. Scientific consensus on sustainability: the case of The Natural Step. Sustainable Development 8:180–190.Van Gool W. 1987. The value of energy carriers. Energy 12(6):509–518.Van Gool W. 1992. Exergy analysis of industrial processes. Energy 17(8):791–803.Van Gool W. 1997. Energy policy: fairy tales and factualities. In Innovation and Technology: Strategies and Policies,

Soares ODD, Martins da Cruz A, Pereira GC, Soares IMRT, Reis AJPS (eds). Kluwer: Dordrecht, pp. 93–105.Van Gool W, Bruggink JJC. (Eds) (1985). Energy and Time in the Economic and Physical Sciences. North-Holland:

Amsterdam.von Weizsacker E, Lovins AB, Lovins LH. 1997. Factor Four: Doubling Wealth, Halving Resource Use. Earthscan:

London.Wall G. 1987. Exergy conversion in Swedish society. Resource Energy 9:55–73.Wall G. 1990. Exergy conversion in Japanese Society. Energy 15(5):435–444.World Commission on Environment and Development. 1987. Our Common Future. Oxford University Press: Oxford.World Energy Council. 1993. Energy for Tomorrow’s World. St Martin’s Press: New York.



Documents

Engineering sustainability: thermodynamics, energy systems, and the environment