ARTICLE IN PRESS

�CorrespondE-mail addr

Assessment of the greenhouse gas emissions from cogenerationand trigeneration systems. Part I: Models and indicators

Gianfranco Chicco�, Pierluigi Mancarella

Dipartimento di Ingegneria Elettrica, Politecnico di Torino, Corso Duca degli Abruzzi 24, I-10129, Torino, Italy

Abstract

The diffusion of cogeneration and trigeneration plants as local generation sources could bring significant energy saving and emission

reduction of various types of pollutants with respect to the separate production of electricity, heat and cooling power. The advantages in

terms of primary energy saving are well established. However, the potential of combined heat and power (CHP) and combined cooling

heat and power (CCHP) systems for reducing the emission of hazardous greenhouse gases (GHG) needs to be further investigated. This

paper presents and discusses a novel approach, based upon an original indicator called trigeneration CO2 emission reduction (TCO2ER),

to assess the emission reduction of CO2 and other GHGs from CHP and CCHP systems with respect to the separate production. The

indicator is defined in function of the performance characteristics of the CHP and CCHP systems, represented with black-box models,

and of the GHG emission characteristics from conventional sources. The effectiveness of the proposed approach is shown in the

companion paper (Part II: Analysis techniques and application cases) with application to various cogeneration and trigeneration

solutions.

Keywords: Cogeneration; Emission reduction; Energy saving; Greenhouse gases; Trigeneration

1. Introduction

Cogeneration or combined heat and power (CHP)systems are well known for potentially providing consider-able primary energy saving with respect to the separateproduction (SP) of the same amount of heat (fromconventional combustion heat generators) and electricity(from conventional power plants) [1,2]. In recent years,there has been an increasing diffusion of various small-scale technologies (with electrical rated power below1MWe) for distributed generation (DG) [3], such asmicroturbines (MTs) and internal combustion engines(ICEs). This has allowed for implementing a wider rangeof cogeneration applications with rated power smaller thanthe ones of traditional industrial users and district heatingsystems [4,5].

Trigeneration or combined cooling heat and power(CCHP) systems [6–9] are based upon CHP systems

ing author. Tel.: +39 011 090 7141; fax: +39 011 090 7199.

ess: gianfranco.chicco@polito.it (G. Chicco).

coupled to absorption chillers fired with heat produced incogeneration. Hence, the heat that in several applicationswould be wasted in the summertime because of lack ofthermal request can be effectively exploited to producecooling power, typically for air conditioning purposes. Thisallows for better and longer utilization of the prime moverwith subsequent energy and economic benefits with respectto the CHP-only system [3,6,7]. There are several potentialsmall-scale users whose needs are suitable for trigenerationsystems, such as universities, hospitals, shopping malls,hotels, restaurants, and so forth [6–8,9].Cogeneration and trigeneration systems, as a conse-

quence of their enhanced energy performance, can alsobring important benefits in terms of greenhouse gas (GHG)emission reduction with respect to the SP. This can occurparticularly in those countries where electricity generationis mainly based upon non-renewable sources [10]. How-ever, few papers are available on the topic [10,11], in spiteof the increasing importance of coping with the globalwarming issue. Thus, further investigations and detailedgeneral analysis models are needed.

ARTICLE IN PRESS

Nomenclature

CC combined cycleCCHP combined cooling heat and powerCHP combined heat and powerCO2ECR CO2 emission characteristic ratioCO2EEE CO2 emission equivalent efficiencyCOP coefficient of performanceDG distributed generationEUF energy utilization factorEU15 european union (15 countries)FC fuel cellGHG greenhouse gasGT gas turbineGWP global warming potentialICE internal combustion engineLHV lower heating valueMT microturbinePES primary energy savingSP separate productionTCO2ER trigeneration CO2 emission reductionTEWI total equivalent warming impactTPES trigeneration primary energy saving

Symbols

m mass (g)p pollutantF fuel thermal content (kWht)Q heat (kWht)R cooling (refrigeration) (kWhc)W electricity (kWhe)X generic energy vector (kWh)e correction factorZ efficiencym emission factor (g/kWh)

Subscripts

c coolinge electricityeq CO2-equivalentt thermaly cogenerationz trigeneration

In this work, a novel and general framework forassessing the GHG emission reduction from cogenerationand trigeneration systems is presented. The proposedapproach resorts to an original set of indicators, formu-lated in analogy to the evaluation of the primary energysaving in trigeneration [12,13]. The trigeneration CO2

emission reduction (TCO2ER) indicator is introduced toassess the CO2 emission reduction from trigeneration (andcogeneration as a sub-case) with respect to conventional SPof electricity, heat and cooling power. The focus is set onCO2 emissions, but extensions based on the globalwarming potential (GWP) [4,10,14] for linking otherGHGs to the equivalent CO2 emissions are also discussed.

Performance characteristics of every component aredescribed through relevant input–output efficiency indica-tors (black-box models [12]). GHG emission characteriza-tion is also carried out in terms of black-boxes by resortingto the emission factor approach [5,15–17]. In particular,different emission factors are considered for all the relevantentries in the most general definition of the TCO2ER

indicator, thus generalizing the approach presented in [10].In addition, the analyses can include different kinds of fuelinput (such as gas, diesel, bio-mass, and so forth) todifferent systems (prime movers, boilers, boilers for SP,and so forth).

The presentation of the work is organized in two parts.The present paper represents Part I and contains themodelling aspects relevant to the formulation of theTCO2ER indicator. In Part II [18] the authors lay downspecific analysis techniques, apply the proposed models tovarious application cases, and discuss the numerical results.

This paper is structured as follows. Section 2 introducesthe general plant structures and black-box efficiencyindicators for trigeneration systems and cogenerationsystems as a sub-case. Section 3 summarizes the emissionfactor approach, recalls the trigeneration primary energy

saving (TPES) indicator [12,13] for the energy evaluationof trigeneration system, and introduces and discusses theTCO2ER indicator. Section 4 contains the final remarks.

2. Trigeneration plant schemes, components, and

performance characteristics

2.1. General structure and components of a trigeneration

plant

A trigeneration plant in its most general structure can beseen as the combination of a cogeneration side and a cooling

side [12,19] (Fig. 1). Focusing on small-scale applications,the plant can be composed of the following main blocks:

�

A CHP group, typically based upon DG equipment suchas ICEs, MTs or fuel cells (FCs) [3–5,15]; � A combustion heat generator group, typically composed

of industrial boilers [4,14], for back-up and thermalpeak-shaving operation. In case, also a heat storagesystem [4,14], operating as a thermal buffer, can beavailable for a more profitable operation of the energysystem. The CHP group, the heat generators, and theheat storage system form the cogeneration side.

� The cooling side for small-scale and air-conditioning

applications is typically composed of single-effect or

ARTICLE IN PRESS

cogenerationside

cooling side

user F

W

Q

W

Q

W

W

R

district cooling F

district heating R

Q

electrical grid

gas distribution

system

Fig. 1. General trigeneration plant layout and energy flows.

double-effect H2O-LiBr absorption chillers [4,8,14,20].Also the triple-effect technology [4,20] is expected to besoon available for wider applications. Mostly, absorp-tion single-effect chillers are fed by hot water, whiledouble-effect ones are fed by superheated water orsteam. Triple-effect chillers would require a higher-temperature heat source, and could be in case direct-fired by the exhaust gases from a CHP unit. It is alsocommonplace to combine absorption and electricchillers together, for instance the former for base-loadand the latter for peak-shaving applications [11,21].From a generalized standpoint [12,19,22,23], furthercooling generation alternatives could be envisaged, suchas engine-driven chillers directly fed by gas [4,8,22].Finally, a cooling storage system could be implementedto increase the flexibility of the system operation froman energy and economic point of view [4,24].

When the plant is not built for stand-alone purposes, it isusually connected to the electric grid, in order to satisfy theenergy needs in any condition (including the stops for outagesand maintenance); this also gives wider opportunities toprofitably run the plant [19,23]. The fuel input, typically gas,can be drawn from a gas distribution system. In the mostgeneral cases, interconnection with a district heating networkor a district-cooling network can occur as well [4,12] (Fig. 1).

About the energy flows in Fig. 1, W is electricity, Q isheat, R is cooling output (typically in the form of chilledwater, for air conditioning purposes), and F is fuel thermalcontent, for instance based on the fuel lower heating value

(LHV). The heat Q could also be supplied at differentthermal levels (for instance, hot water for space heating,and steam to fire an absorption chiller). Likewise, thethermal input F could come from sources other thannatural gas indicated in Fig. 1 (for instance, diesel fuel orhydrogen not produced locally to supply a CHP FC).

2.2. Input–output black-box approach to trigeneration

equipment modelling

In order to describe the trigeneration equipmentcharacteristics, an input–output black-box approach [12] is

particularly suitable. Each CCHP component can bemodelled according to its efficiency performance indicator[1,4,5], in general energetically defined as relevant output-to-input energy ratio. Hence, in a combined system withvarious cascaded equipment it is possible to track back,starting from the user’s needs (final outputs), the inputenergy requested by each machine, onto the plant input.For instance, an absorption chiller produces cooling powerfrom a thermal input; this input can be generated by theCHP unit, in turn fed by gas, which is the original plantinput.One of the major upsides of this approach is that a

detailed description of the machine and of its internalcharacteristics, that is, of the actual content of the ‘‘black-box’’, can be avoided. Of course, this can be done providedthat the performance models are comprehensive of all theinformation needed to characterize the system correctly,including the off-design behaviour.

2.3. Performance indicators for CCHP equipment

The characteristics of the CHP prime movers can beeffectively and synthetically described by means of theelectrical efficiency ZW, the thermal efficiency ZQ, andthe energy utilization factor (EUF) indicator representingthe overall cogeneration efficiency [1,5,15]:

ZW ¼W y

F y

; ZQ ¼Qy

Fy

; EUF ¼ ZW þ ZQ, (1)

where the subscript y is used to point out cogeneration. Theabove efficiencies depend upon the technology, the loadinglevel, the outdoor conditions, the heat recovery system, andthe enthalpy level at which the heat must be provided to theuser. In particular, the thermal efficiency may decreaseconsistently if heat (for instance hot water at 70 1C) isrequired at higher temperature than the nominal one or inthe form of steam [5,25], as apparent from equipmentmanufacturers’ catalogues.Regarding the cooling equipment, the performance is

usually described by means of the specific coefficient of

performance (COP). The COP can be generally defined asratio of the desired cooling energy output R to the relevant

ARTICLE IN PRESS

input (electrical energy Wc for electric chillers, thermalenergy Qc for steam-fed, hot water-fed or exhaust-fedabsorption chillers [4,14,22]):

COP ¼R

W c

ðelectric chillersÞ; (2)

COP ¼R

Qc

ðsteam-fed; water-fed or exhaust-fed

absorption chillersÞ; ð3Þ

where the subscript c points out that the final use of therelevant input is cooling production.

In general, the performance characteristics of the chillersmay depend significantly upon the outdoor conditions andthe temperature of the ambient to keep cooled, as well astheir loading level [4,14]. In addition, for absorptionchillers the COP could drop consistently if the firingsource does not comply with the specific thermal con-straints [4,26–28]. Firing temperatures are typically about80–90 1C for single-effect, 120–160 1C for double-effect,and 160–200 1C for triple-effect chillers [4,22]. At the sametime, the thermal efficiency of small-scale units such asMTs and ICEs could be quite low for generating high-temperature hot water or steam. Consequently, the overall

performance of a trigeneration system should be thor-oughly evaluated, above all for small-scale units.

3. Trigeneration primary energy saving and GHG emission

saving evaluation

3.1. The emission factor approach for CO2 and general

GHG emission evaluation

The assessment of any pollutant emission from acombustion device can be assessed through an energy

output-based emission factor approach [5,16,17]. Accordingto this approach, the mass mp

X of a given pollutant p

emitted while producing the energy output X can beworked out as:

mXp ¼ mX

p X , (4)

where mpX is the energy output-based emission factor, that

is, the specific emissions of p per unit of X, in (g/kWh). Forinstance, the output X can be electrical energy W (kWhe),useful thermal energy Q (kWht), or cooling energy R

(kWhc). The emission factor mpX depends upon several

operating and structural variables, such as the specificequipment, partial load operation, age, state of mainte-nance, outdoor conditions, pollutant abatement systems,and so forth [5,16].

If complete combustion is assumed, CO2 emissions canbe worked out according to the characteristics of thechemical reaction, being a function of the carbon contentin the fuel and of its LHV (i.e., a function of the fuel itself)[5]. Thus, for a given fuel, the emission factor mF

CO2referred

to the primary energy F released when burning the fuel canbe considered constant at first approximation. The CO2

emission factor referred to the energy output can be thenevaluated in every operating point through the relevantenergy efficiency model (Section 2.3). Besides the energyefficiencies, also mF

CO2can actually change when the CHP

unit operates in generic off-design conditions. In this case,typically the combustion characteristics worsen, bringingabout incomplete combustion and thus a decrease of CO2

specific emissions [5,16]. However, this behaviour can beneglected at first approximation, as done in several studies[5,10,29]. In particular, considering a constant mF

CO2is

conservative for what concerns the assessment of CO2

emission reduction brought by a combined system withrespect to the SP.For more detailed analyses, specific experimental measure-

ments in situ for the given equipment should be carried out toevaluate the actual CO2 emission factor for various operatingconditions. In fact, it would be in general tough to drawgeneral analytical models, since the results are relevant to thespecific equipment combustion characteristics.

3.2. The TPES indicator for primary energy saving

evaluation in trigeneration

Several performance indicators have been presented inthe literature to evaluate the CHP plant characteristics [1].In particular, the primary energy saving (PES) indicator isdefined as

PES ¼F SP � Fy

FSP¼ 1�

Fy

W y=ZSPe þQy=ZSP

t

, (5)

where Fy represents the fuel thermal input to thecogeneration plant, while FSP is the fuel thermal input tothe conventional SP of the cogenerated electricity Wy (in anequivalent power plant with reference electrical efficiencyZe

SP) and the cogenerated heat Qy (in an equivalent boilerwith reference thermal efficiency Zt

SP). The PES indicatoris extensively adopted, also from a regulatory standpoint[30,31].Following the approach to classical trigeneration out-

lined in [32,33], the authors in [12,13] have proposed theadoption of the TPES indicator as a generalization of thecogeneration PES to compare the energy saving perfor-mance from different CCHP systems. In particular, if anelectric chiller is assumed as reference technology for theSP of cooling power, the TPES is defined as [13]

TPES ¼FSP � F z

FSP

¼ 1�F z

W z=ZSPe þQz=ZSP

t þ Rz= ZSPe COPSP

� � , ð6Þ

where the subscript z points out the trigeneration entries.Concerning the reference efficiency values, figures of

ZeSP¼ 0.4 (about the power system average efficiency in

Italy, including line losses) and ZtSP¼ 0.8–0.9 (average

boiler efficiencies) could be adopted for evaluating theenergy system performance [30]. In alternative, the CCHP

ARTICLE IN PRESS

system could be compared to the best available technolo-gies, setting Ze

SP¼ 0.55–0.6 (combined cycles, CCs) and

ZtSP¼ 0.95 (modern high-efficiency boilers). Likewise,

average values or cutting-edge values could be chosen forthe reference electric chiller performance. However, theperformance of a chiller strongly depends on the outdoortemperature and on the condenser typology (water-cooledor air-cooled, in particular) [4,14]. Therefore, it would besuitable to consider reference values relevant to the specificapplication. In any case, COPSP figures from 3 to 5represent a typical range of values.

According to the TPES definition (6), cogeneration canbe analysed as a sub-case with Rz ¼ 0, thus obtaining theclassical PES (5).

3.3. The TCO2ER indicator for CO2 emission reduction

evaluation in trigeneration

Following the lines that brought to the TPES definition[13], here the authors introduce the novel TCO2ER

indicator for CO2 emission reduction evaluation in CCHP(and CHP) systems. The TCO2ER is generally defined as

TCO2ER ¼ðmCO2

ÞSP � ðmCO2Þz

ðmCO2ÞSP

, (7)

where ðmCO2Þz is the CO2 mass emitted while burning the

fuel input to the trigeneration plant, and ðmCO2ÞSP is the

CO2 mass emitted while producing the same trigeneratedenergy vectors in reference SP technologies.

The TCO2ER can be more specifically expressed in termsof relevant emission factors as

TCO2ER ¼ 1�ðmF

CO2ÞzFz

ðmWCO2ÞSPW z þ ðm

QCO2ÞSPQz þ ðm

RCO2ÞSPRz

,

(8)

where ðmWCO2ÞSP, ðm

QCO2ÞSP and ðmR

CO2ÞSP are the energy-

output related emission factors for the SP of electricity,heat and cooling, respectively. These emission factors areconventionally evaluated, also depending upon the purposeof the study, as illustrated in the numerical applications inthe companion paper [18]. The term ðmF

CO2Þz refers to the

specific fuel input to the trigeneration system. Again,cogeneration can be analysed as a sub-case, correspondingto Rz ¼ 0 in (8).

The expression (8) allows for running general parametricanalyses of different types, considering different emissionfactors for the SP and different inputs to the CCHP energysystem. A relevant simplification in the TCO2ER expressioncan be carried out considering that the SP of cooling powertypically occurs in electric chillers, as in (6), so yielding

TCO2ER

¼ 1�mF

CO2

� �zFz

mWCO2

� �SP

W z þ mQCO2

� �SP

Qz þ mWCO2

� �SP

1COPSP Rz

.

ð9Þ

The expression (9) contains only two CO2 emissionfactors for the SP, namely, for electricity generation andfor heat generation. The emissions from cooling powergeneration are now assessed through the electricity-relatedemissions and the reference electric chiller efficiency.

3.4. Conceptual differences between TPES and TCO2ER

A main conceptual difference between the TPES and theTCO2ER has to be pointed out. When evaluating theenergy saving though the TPES, the analysis mainly refersto the rationale of trigeneration as a combined process formultiple energy vector production. Thus, the selection ofthe reference efficiencies for the SP is related in primis towork out the potential of the combined process withrespect to conventional benchmarks. The analysis runthrough the expression (6) is performed in terms of primaryenergy saving, no matter the actual fuels and/or technol-ogies involved. For instance, the combined plant can befuelled with natural gas, the heat SP can be referred to anactual average mix of boiler technologies and fuels, and theelectricity separate generation can be referred to an actualaverage mix of power plant technologies and fuels. In anycase, the TPES analysis finally yields a numerical value ofprimary energy saving. What actually matters are only theefficiencies and not the fuel inputs to the various systems.In this respect, the specific technologies are only addressedfor assessing the relevant efficiencies.Differently, when evaluating the CO2 emission perfor-

mance through the TCO2ER not only the technologies, butalso the fuel inputs to the different systems are relevant. Infact, different fuels exhibit different carbon chemicalcontents and thus their burning would generate differentCO2 emissions [4,5]. In addition, if the electricity SPmakes reference to the power system in a given context(e.g., a given country), the actual generation mix can bebased upon various alternatives. In particular, electricityproduction from renewable sources such as wind or sun, aswell as from nuclear energy, is virtually characterized byzero CO2 emissions, if the analysis excludes the contribu-tion from the embedded energy needed for building theplant [4,34,35]. Thus, the equivalent overall emissions frompower systems can be quite low if a consistent quota ofelectricity is generated through renewable or nuclearsolutions.

3.5. Discussion on the rationale and use of the TCO2ER

indicator

On the premises drawn in Section 3.4, the selection of theentries in (9) must be carried out with even more care thanfor the entries in (6), considering the purpose of theanalysis. For instance, in [10] it is assumed that thefuel input to the combined (co- or tri-generation) systemis the same as for the heat SP. Thus, the GHG emis-sion reduction is a consequence of the primary energysaving brought by the combined multi-generation process.

ARTICLE IN PRESS

However, from this standpoint also the separate generationof electricity should be evaluated on the basis of the samefuel input. In addition, the electrical efficiency in generalincreases with the capacity and changes with the technol-ogy. Thus, a fair comparison should also consider the sametechnology in the same size range for electricity-onlygeneration [36]. For instance, a gas-fired cogenerative CCshould be compared to a non-cogenerative gas-fired CCplus a gas-fired boiler. In this case, the profitability of thecogeneration process would be evaluated also taking intoaccount that setting up a cogenerative CC (in whichthermal power is recovered) might bring about a decreasein electrical efficiency [4,5]. In alternative, the comparisoncould be run regardless the technology and the size. Forinstance, the separate generation of electricity could beassessed in terms of best available technology (e.g., a gasMT could be compared with a gas CC). However, thisseems a less suitable approach for actual evaluation of theperformance of small-scale combined plants.

It is possible to point out a theoretical meaningful resultin the hypothesis that the same fuel is adopted for all thesystems involved in the analysis (combined and separategeneration). In this case, in fact, taking into account themodelling considerations of Section 3.1, the TCO2ER

would assume the same numerical value of the TPES.Hence, the CO2 emission reduction would depend onlyupon the relevant efficiencies considered in the analysis.Such an approach seems the most suitable for evaluatingthe contribution to the struggle against global warming ofcogeneration and trigeneration as combined processes ableto generate a manifold output.

However, the expressions (8) and (9) allow for assessingmore general situations. In particular, it is possible to takeinto account that the fuel inputs to a co- or tri-generatorcan be quite different from the input to the SP boilers. Thisapproach is more corresponding to the rationale ofcomparing the GHG impact from a given CHP or CCHPsystem to actual systems available for SP. For instance, theanalysis can be carried out with reference to average

systems for heat production. In this case, considering a mixof fuels corresponding to the actual distribution of heatgenerator typologies, and a relevant weighted average

TCO2ER ¼ 1�mF

CO2

� �zþ mF

CO2eq

� �z

h iFz þ mR

CO2eq

� �zRz

mWCO2

� �SPþ mW

CO2eq

� �SP

h iW z þ

Rz

COPSP

� �þ mQ

CO2

� �SPþ mQ

CO2eq

� �SP

h iQz þ mR

CO2eq

� �SP

Rz

, (11)

emission factor, seems more appropriate. Similarly, elec-tricity could be evaluated on the basis of the actualtechnology mix (including various fuels and virtually zero-emission renewable or nuclear sources) in a given region.Finally, the same holds true for the cooling powergeneration, considering that cooling can be produced in

alternative ways (in some regions, for instance, the share ofthermally-fed chillers with respect to electrically-fed chillerscan be quite high [4]). Such an approach seems moresuitable for evaluating CHP and CCHP plants in a givenenergy framework.On the basis of the variety of rationales discussed above,

for every given plant and operating condition it is possibleto run alternative studies by considering the entriesðmW

CO2ÞSP, ðm

QCO2ÞSP, and COPSP as parameters in (9).

Relevant numerical applications are illustrated in [18].

3.6. Comprehensive model accounting for other GHG

emissions

The models presented in Section 3.3 can be generalizedby considering the emissions of other types of GHG forboth the CCHP system and the SP references. For instance,when dealing with natural gas-fuelled units for cogenera-tion and trigeneration, it could be possible to consider alsoCH4 emissions, that could be in a range of 90% of the totalun-combusted hydrocarbons emitted [5,16]. Similarly,every device burning a certain fuel, such as diesel, coal oroil, generates some types of GHG, in an amount that canbe more or less significant [4,5]. Thus, a more detailedmodel could account for various GHG emission typolo-gies, in particular from the SP of heat and electricity. Inaddition, another relevant source of GHG emissions isrepresented by the refrigerant used in the chillers (in boththe CCHP and the SP systems), which is more or lesssubject to a certain leakage rate [4,10,14].The GWP for a given GHG is evaluated by assuming

CO2 as the reference basis. The emissions of a genericGHG are then expressed in terms of equivalent CO2

emissions [4,10,14]. In particular, it is possible to define theequivalent CO2 emission factor mX

CO2eq as

mXCO2eq ¼ GWPGHGmX

GHG, (10)

where GWPGHG is the CO2 mass equivalent emissions peremitted mass unit of a given GHG, and mGHG

X is theemission factor of the given GHG in order to produce theenergy vector X, as defined in (4).The TCO2ER model (8) can be therefore generalized as

where in particular:

�

ðmFCO2eqÞz accounts for different types of GHG other than

CO2 emitted when burning a given fuel (e.g., methane),so that in general ðmF

CO2eqÞz could be expressed as anaverage weighted value for different GHG.

ARTICLE IN PRESS

�

ðmRCO2eqÞz is related to the GHG emissions corresponding

to refrigerant mass leakage from the absorption chiller[4,10,14].

� ðmW

CO2eqÞSP and ðmQCO2eqÞSP refer to the average equivalent

CO2 emissions from SP of electricity and heat, weightedaccording to different types of GHG.

� ðmR

CO2eqÞSP refers to the equivalent GHG emissions dueto refrigerant mass leakage from the reference electricchiller; this term can be again assessed as an averagevalue weighted on different types of electric chillers andrefrigerants it is possible to adopt for separate coolingproduction.

Clearly, the expression (11) is much more complicated toevaluate than (8). However, as far as emissions from fuelburning are concerned, in general CO2 emissions prove tobe quantitatively much more important than other GHGemissions [4]. Emissions of other important GHG such asmethane, in any case, are typically comparable for thecombined systems and the SP [4,5,16]. Regarding therefrigerant leakage, in H2O–LiBr absorption chillersthe refrigerant is water, with no virtual GWP. On thecontrary, the GWP of some refrigerants used in electricchillers for SP can be consistent (halocarbons have GWP ina range from some hundreds to some thousands [4,14]).However, the current trend is to render the refrigerantleakage almost negligible in modern systems (below 1% onan annual basis [4,10]). Therefore, again the mostsignificant impact is represented by CO2 emissions fromgenerating the electricity needed as input to the electricchillers [4]. This is also confirmed by recent studies basedon integrated indicators such as the total equivalent

warming impact (TEWI) [14].According to the above considerations, regarding the

cooling production the results obtained in terms ofTCO2ER for trigeneration systems with H2O–LiBr absorp-tion chillers are slightly conservative with respect to theseparate cooling generation based upon electric chillers.Likewise, as far as fuel burning is concerned, at firstapproximation GHG emissions other than CO2 can beneglected in the absence of more detailed relevant data.Thus, the analyses carried out in this paper and in Part II[18] mainly consider CO2 emissions. However, a completeGHG emission reduction assessment could be run on thebasis of the model illustrated in this section, provided thatsuitable and reliable information for the specific case isavailable.

4. Final remarks

This paper has presented a novel approach to assess theGHG emission performance from cogeneration andtrigeneration systems. In this respect, the TCO2ER

indicator has been introduced for assessing the emissionreduction brought by the combined energy systems withrespect to conventional references for the SP of electricity,heat and cooling power. The characteristics of all the

equipment involved in the analysis have been modelledthrough black-box models. In particular, the emissionperformance characterization has been carried out bymeans of energy output-related emission factors. Theapproach introduced has been formulated by focusing onthe CO2 emissions as the most relevant GHG. Further-more, it has been extended to account for other GHGemissions from CHP and CCHP systems, such as methanecontained in the thermal equipment exhaust gases, orleakages of GHG substances used as refrigerants in thechillers.By adequately setting the input entries to the TCO2ER

indicator, various typologies of analyses can be run. Morespecifically, the indicator is able to assess the actualemission reduction under general operating conditions ofthe energy system. In particular, the relevant entries can becalculated accounting for the off-design behaviour of thevarious equipment. In addition, by changing the SPreference values, it is possible to perform different analysesaccording to different evaluation rationales. In this light, ameaningful theoretical result refers to the analogy betweenthe TCO2ER indicator, evaluated with reference to onlyCO2 emissions (neglecting the contribution from otherGHG), and the TPES indicator adopted for trigenerationenergy saving assessment. If the same fuel is assumed to bethe input to the combined energy system and to the SPmeans, the two indicators bring the same numerical results;in this case, according to the model developed, energysaving and CO2 emission reduction are coincident.In the companion paper [18], starting from the theore-

tical framework introduced here, specific analysis techni-ques and further indicators are presented. In addition, theeffectiveness of the proposed approach and assessmentmodels is illustrated on various application cases withrelevant cogeneration and trigeneration systems. Para-metric analyses, referred to different possible rationales forassessing the reference SP means, are included to highlightsome key numerical aspects relevant to different CHP andCCHP technologies.

Acknowledgements

This work has been supported by the Regione Piemonte,Torino, Italy, under the research grant C65/2004. Theauthors thank the anonymous reviewers for their insightfulcomments and for the precious advice given to improve thispaper.

References

[1] Horlock JH. Cogeneration—combined heat and power (CHP).

Malabar, FL: Krieger; 1997.

[2] Martens A. The energetic feasibility of CHP compared to the separate

production of heat and power. Appl Therm Eng 1998;18(11):935–46.

[3] Borbely AM, Kreider JF, editors. Distributed generation—the power

paradigm of the new millennium. Boca Raton, FL: CRC Press; 2001.

ARTICLE IN PRESS

[4] Danny Harvey LD. A handbook on low-energy buildings and district

energy systems: fundamentals, techniques, and examples. UK: James

and James; 2006.

[5] EDUCOGEN, The European Educational Tool on Cogeneration.

December 2001. See also /www.cogen.org/projects/educogen.htmS.

[6] Cardona E, Piacentino A. A methodology for sizing a trigenera-

tion plant in Mediterranean areas. Appl Therm Eng 2003;23(13):

1665–80.

[7] Maidment GG, Tozer RM. Combined cooling heat and power in

supermarkets. Appl Therm Eng 2002;22(6):653–65.

[8] Resource Dynamics Corporation. Cooling, heating, and power for

industry: a market assessment, Final Report, US Department of

Energy and Oak Ridge National Laboratory, Vienna, VA, August

2003. See also: /www.cogeneration.netS.

[9] Ziher D, Poredos A. Economics of a trigeneration system in a

hospital. Appl Therm Eng 2006;26(7):680–7.

[10] Meunier F. Co- and tri-generation contribution to climate change

control. Appl Therm Eng 2002;22(6):703–18.

[11] Minciuc E, Le Corre O, Athanasovici V, Tazerout M. Fuel savings

and CO2 emissions for tri-generation systems. Appl Therm Eng

2003;23(11):1333–46.

[12] Mancarella P. From cogeneration to trigeneration: energy planning

and evaluation in a competitive market framework. Doctoral thesis.

Torino, Italy: Politecnico di Torino; April 2006.

[13] Chicco G, Mancarella P. Trigeneration primary energy saving for

energy planning and policy development. Energy Policy 2007;35:

6132–44.

[14] Kreider JF, editor. Handbook of heating, ventilation and air

conditioning. Boca Raton, FL: CRC Press; 2001.

[15] US Environmental Protection Agency. Catalogue of CHP technol-

ogies. See also /www.epa.govS.

[16] Distributed Energy Resources Emissions Survey and Technology

Characterization, E21, Palo Alto, CA, Ameren, St. Louis, MO,

California Energy Commission, Sacramento, CA, New York

Independent System Operator, Albany, NY, and New York Power

Authority, White Plains, NY. See also: /http://www.epriweb.com/

public/000000000001011256.pdfS.

[17] Canova A, Chicco G, Genon G, Mancarella P. Environmental

impact of small-scale distributed cogeneration systems. In: Chicco G,

editor. VI world energy system conference, Torino, Italy, 10–12 July

2006. p. 689–96.

[18] Mancarella P, Chicco G. Assessment of the greenhouse gas emissions

from cogeneration and trigeneration systems. Part II: Analysis

techniques and application cases. Following paper, doi:10.1016/

j.energy.2007.10.008.

[19] Chicco G, Mancarella P. From cogeneration to trigeneration:

profitable alternatives in a competitive market. IEEE Trans Energy

Convers 2006;21(1):265–72.

[20] US DOE. Review of thermally activated technologies, Distributed

energy program report, July 2004. See also /http://www.eere.ener-

gy.gov/de/publications.htmlS.

[21] Minciuc E, Le Corre O, Athanasovici V, Tazerout M, Bitir I.

Thermodynamic analysis of tri-generation with absorption chilling

machine. Appl Therm Eng 2003;23(11):1391–405.

[22] Wu DW, Wang RZ. Combined cooling, heating and power: a review.

Progr Energy Combust Sci 2006;32(5,6):459–95.

[23] Chicco G, Mancarella P. Planning evaluation and economic

assessment of the electricity production from small-scale trigeneration

plants. WSEAS Trans Power Syst 2006;1(2):393–400.

[24] Zhang HF, Ge XS, Ye H. Modeling of a space heating and cooling

system with seasonal energy storage. Energy 2007;32(1):51–8.

[25] Colombo PML, Armanasco F, Perego O. Experimentation on a

cogenerative system based on a microturbine. Appl Therm Eng 2007;

27(4):705–11.

[26] Burer M, Tanaka K, Favrat D, Yamada K. Multi-criteria optimiza-

tion of a district cogeneration plant integrating a solid oxide fuel cell-

gas turbine combined cycle, heat pumps and chillers. Energy 2003;

28(6):497–518.

[27] Kaynakli O, Kilic M. Theoretical study on the effect of operating

conditions on performance of absorption refrigeration system.

Energy Convers Manage 2007;48(2):599–607.

[28] Colonna P, Gabrielli S. Industrial trigeneration using ammonia–

water absorption refrigeration systems (AAR). Appl Therm Eng

2003;23(4):381–96.

[29] Cardu M, Baica M. Regarding the greenhouse gas emissions of

thermopower plants. Energy Convers Manage 2002;43(16):2135–44.

[30] Italian Electricity and Gas Authority website (Autorita per l’energia

elettrica e il gas). See also: /www.autorita.energia.itS.

[31] Directive 2004/8/EC of the European Parliament and of the Council

of 11 February 2004 on the promotion of cogeneration based on a

useful heat demand in the internal energy market and amending

Directive 92/42/EEC. Off J Eur Union 2004;L52:50–60.

[32] Havelsky V. Energetic efficiency of cogeneration systems for combined

heat, cold and power production. Int J Refrig 1999;22(6):479–85.

[33] Heteu PMT, Bolle L. Economie d’energie en trigeneration. Int J

Therm Sci 2002;41:1151–9.

[34] Voorspools KR, Brouwers EZ, D’haeseleer WD. Energy content and

indirect greenhouse gas emissions embedded in ‘emission-free’ power

plants: results for the low countries. Appl Energy 2000;67(3):307–30.

[35] Fthenakis V, Alsema E. Photovoltaics energy payback times,

greenhouse gas emissions and external costs: 2004–early 2005 status.

Progr Photovoltaics Res Appl 2006;14(3):275–80.

[36] Cogen Europe. Position statement on the definition of harmonised

efficiency reference values, February 2005. See also: /http://

www.cogen.org/Downloadables/Publications/Position_Paper_CHP_

Reference_Values.pdfS.