development of system concepts

8/11/2019 Development of System Concepts

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Energy and Buildings 42 (2010) 16011609

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Energy and Buildings

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e n b u i l d

Development of system concepts for improving the performanceof a waste heat district heating network with exergy analysis

Herena Toro, Dietrich Schmidt

Fraunhofer Institute for Building Physics, Department of Energy Systems, Gottschalkstrasse 28a, 34127 Kassel, Germany

a r t i c l e i n f o

Article history:

Received 26 January 2010

Received in revised form 31 March 2010Accepted 8 April 2010

Keywords:

District heatWaste heatExergy efficiencyEnergy efficiencyBuildingsImproved performance

a b s t r a c t

The building sector is responsible for a great share of the final energy demand and national CO 2 emis-sions in countries like Germany. Nowadays, low quality thermal energy demands in buildingsare mainlysatisfied with high-quality sources (e.g. natural gas fired in condensing boilers). Exergy analysis, pur-suing a matching in the quality level of energy supplied and demanded, pinpoints the great necessityof substituting high-quality fossil fuels by other low quality energy flows, such as waste heat. In thispaper a small district heating system in Kassel (Germany) is taken as a case study. Results from prelim-inary steady-state and dynamic energy and exergy analysis of the system are presented and strategiesfor improving the performance of waste-heat based district heating systems are derived. Results showthat lowering supply temperatures from 95 to 57.7 C increases thefinal exergy efficiencyof the systemsfrom 32% to 39.3%. Similarly, reducing return temperatures to the district heating network from 40.8 to37.7 C increases the exergy performance in 3.7%. In turn, the energy performance of all systems studiedis nearly the same. This paper shows clearly the added value of exergy analysis for characterising andimproving the performance of district heating systems.

2010 Elsevier B.V. All rights reserved.

1. Introduction

The built environment is responsible for around 40% of the finalenergy use in Germany [1]. Particularly space heating and cool-ing and domestic hot water supply represent the biggest shareof energy demands in residential buildings. For these demands,mainly fossil fuels are used (e.g. condensing boilers), causing greatCO2 emissions and thereby making a more efficient use of energyin this sector absolutely necessary. Exergy analysis allows for thedetection and quantificationof the improving potential of complexenergy systems [2,3] andhas been widelyused forthe optimizationof thermodynamic systems (e.g. power plants) since the middleof the last century[4].Similarly, applying the exergy concept tothe building sector delivers more complete information on the useof the energy flows and opens up room for further insight and

improvements within this field[5,6].Exergy is the maximum theoretical work obtainable from the

interaction of a system with its environment, until a state of equi-librium is reached between them [7]. Consequently, exergy is ameasure of the potential of a given energy flow to be transformedinto high-quality energy. Exergy demands for space heating (SH)and domestic hot water (DHW) production in buildings are low,

Abbreviations:DH, districtheating;DHW,domestic hotwater;SH, spaceheating. Corresponding author.

E-mail address:[email protected](H. Toro).

due to the low temperature level demanded for these applications.In most cases, however, this demand is met by high grade energysources, such as fossil fuels or electricity which could instead beused for other higher-grade applications (e.g. power production).Therefore, besides the well known issue of energy saving, a widemargin for exergy saving exists within the built environment. Sub-stituting high-quality fossil fuels used to supply energy demandsin buildings strongly reduces this margin, i.e. increases greatly theexergy performance of the built environment. Thereby the exergyapproach allows reducing CO2 emissions caused by the buildingsector.

Waste heat available e.g. from combined heat and power pro-duction (CHP) plants is a low quality energy flow suitable forsupplyingthe requestedenergy at an appropriate quality level. Theuse of waste heat with low exergy content allows a good matching

between the exergy level of the demand and supply sides and rep-resents, thus, a very efficient manner of supplying thermal energydemands in buildings.

Several authors have performed steady-state exergy analysisof district heating systems [812]. In[11]the effect of differentreference temperatures on the performance of a district heatingsystem is studied. Variable reference temperatures between 0and 25 C are chosen for the reference environment. Values of theenergy and exergy efficiency for these conditions vary between3849% and 4547% respectively. Exergy efficiency varies only 2%being less sensitive than energy efficiency. In[12]the energy andexergyperformance of a geothermal district heating systemin four

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1602 H. Toro, D. Schmidt / Energy and Buildings 42 (2010) 16011609

Nomenclature

Symbols.

Q heat transfer rate (W)Ex exergy rate (W)m mass flow rate (kg/s)c specific heat capacity (kJ/kgK)Ex exergy (J)

P electric energy (J)Q heat transfer (J)T absolute temperature (K) temperature (C) exergy efficiency energy efficiencyHT specific heat transfer coefficient of building

(W/m2 K)U heat transfer coefficient (W/m2 K)

Indexes

0 referenceave averagedem demand

DH district heatingDHW domestic hot waterel electricFH floor heatingfin finalheater heaterHP heating periodin inletp constant pressureprim primarypumps pumpsret returnsec secondarySH space heating

sp setpointsteady steady-state

winter days is analyzed. Average outdoor temperatures for eachof the four winter days are considered as reference temperatures.Energy and exergy efficiencies are on the ranges of 3742% and4246% respectively. In [8] the authors investigate the influence ofdifferent supply and return temperatures from the district heatingnetwork in the overall performance of a district heating system.Outdoor air temperature for a typical winter day in Germanyis regarded as reference temperature (0 C). For a given returntemperature of 30 C, exergyincreases from25 to 50% if the districtheating supply temperatures are lowered from 130 C to 40 C.

However,districtheatingsystemsoperateat temperatures closeto the reference environment and, thus, their performance may bestrongly influenced by the dynamic behaviour of the system andoutdoor air conditions (taken as reference for exergy analysishere).In this paper,the energyand exergyperformanceof differentpossi-ble configurations of a small district heating system taken as a casestudyare compared.To allowan accurate comparison andshowthedifferences in the performance of the analyzed systems dynamicenergy and exergy analysis are mandatory. Dynamic energy andexergy assessment of the systems is performed here with TRNSYSSimulaton Environment[13].Results show clearly the benefits ofthe exergy approach for evaluating district heat supply systems inbuildings.

The paper is organized as follows: in Section 2 the district heat-

ing system case study used here is briefly introduced. A simplified

Table 1

Main assumptions describing the single family houses in the case study.

Magnitude Value Unit Magnitude Value Unit

U external walls 0.28 W/m2K Infiltration rate,n 0.6 h1

U ground floor 0.30 W/m2K Internal gains 5 W/m2

U roof 0.17 W/m2K in,FH 32 C

HT 0.35 W/m2K ret,FH 27 C

Theexergy input into thedistrict heat exchangerwouldneedto be evaluatedin thiscase as a function of the quality factor of the fossil fuel used in the heat plant.

steady-state assessment of the behaviour of district heating sys-tems is presented in Section 3. Main conclusions derived fromit are compared to findings in the literature. Based on the rec-ommendations from steady-state analysis four different hydraulicconfigurations are derived and modelled dynamically in TRNSYS.These configurations are presented in detail in Section4.Resultsfrom dynamic energy and exergy assessment are presented in Sec-tion 5. Main conclusions from the analyses are summarized inSection6.

2. Description of the case study

The small neighborhood of Oberzwehren, planned to be erectedin Kassel (Germany), is taken here as case study. In the initial plansit consisted of 24 single family houses. District heating supply andreturn pipes from the local utility company circulate close to theresidential area. It is planned to use the return pipe, i.e. with loweravailable temperatures, to supply domestic hot water and spaceheating demands. District heating in Kassel is mainly waste heatfrom co-generation power plants.

The singlefamily housesare definedas free-standingtwo storeybuildings with a net useful area of 184.4m2. In Table 1 mainassumptions describing the buildings are shown. They representwell-insulated new buildings complying with requirements fromthe German standard[14].

Small DHW storage tanks of 200 l are considered in each house.

This allows reducing peak loads for DHW supply significantly from42kW for each single family house, to 7 kW peak power per house.According to[15]for DHW supply in single family houses a tem-perature of 50 C at the outlet of the DHW supply element mustbe ensured at all times. An electric heater located at the outlet ofthe tank is foreseen as back-up system for this purpose. For DHWsupply a simultaneity factor of 0.39 is considered[16].

A centralized heat exchanger unit is planned to supply heat tothe small neighbourhood as shown inFig. 1.In this way, the dis-trict heating network from the local utility company is decoupledfrom the building appliances and systems installed, i.e. mass flowrate and temperature drop in the district heating network are notdirectly determined by the mass flow rates and temperature dropsin the building systems (e.g. floor heating systems). All houses are

connected in parallel to the local distribution network (secondaryside of the heat supply), as shown inFig. 1.

Fig. 1. Simplified scheme of district heat supply to the studied neighbourhood of

Oberzwehren (Germany).


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H. Toro, D. Schmidt / Energy and Buildings 42 (2010) 16011609 1603

With this configuration the main energy and exergy inputs intothe system are the heat flow from the primary side of the heatexchanger, i.e. from the district heating network, and the pumpingenergy both in the primary and secondary sides. Pumping energyfor low temperature district heating systems is estimated to bearound 0.51% of the thermal energy input in the network[8,17].As it is shown in Section5,pumping energy for the secondary side(i.e. for the local heat distribution within the neighbourhood) rep-resents a marginal part of the energy and exergy supply. Havingmuch shorter pipes than the secondary side, less pressure lossescan be presumed in the primary side. Thereby, pumping energy inthe primary side is expected to be even smaller and is disregardedin this study.

3. First preliminary analysis: steady-state behaviour

Preliminary steady-state analyseshave beencarried out in orderto understand the exergetic behaviour of the district heating sup-ply. As a first simplification, it is assumed that the secondaryside operates under given conditions, i.e. pumping energy whichdepends on the mass flow rates and temperature levels chosenremains unchanged. The main variable is then, the thermal energyinput from the primary side district heating pipe. Steady-state

behaviour of thedistrictheatingsystem is, thus,characterised by itsexergy efficiency calculated as shown in Eq. (1).Eq.(1)representsa rational exergy efficiency, since only the final demands of SH andDHW (i.e. desired output) are regarded as the output from the sys-tem and only the usable output from the district heating system isregarded as input (i.e. the exergy containedin the returnmass flowrate of the district heating pipes is not regarded as input in the sys-tem) [4]. Eq. (1) characterises the overall exergy performance of thedistrict heating supply, i.e. from supply to final demand. In otherwords, all exergy losses happening in the different energy conver-sion processes in the supply chain (i.e. heat exchangers, thermallossesin thepipes, etc.) areincludedin theexpression of theexergyefficiency in Eq.(1).

DH,steady =Exdem,SH,ave+ Exdem,DHW,ave

Exin,DH=

QSH,ave

1 T0,HP,ave

Tr+

Qdem,DHW,ave(Tdem,DHWTnet)

(Tdem,DHW Tnet) T0,HP,aveln

Tdem,DHWTnet

mprim,DH cp

(Tin,prim,DH Tret,prim,DH) T0,HP,aveln

Tin,prim,DHTret,prim,DH

, (1)

whereExdem,SH,ave is the average exergy load forspaceheating(SH),

calculated as a function of the average energy load for SH.QSH,ave

whichherehasavalueof76.43kW,andtheabsolutevalueofdesignindoor air temperatureTr(assumed to be 293 K). The average loadfor domestic hot water (DHW) supply Exdem,DHW,ave is calculated

as a function of the average energy load for DHW supply.QDHW,ave

which here has a value of 9.54kW, the absolute value of the tem-perature demanded for DHW supply Tdem,DHW(i.e. 323K) and thetemperature from the local cold waternetwork Tnet(assumed to be283K). Exin,DH is the thermal exergyinput from the primary side of

the district heating network. It is important to remark that Exin,DHcan be calculated in this way as long as it is waste heat available,otherwise, e.g. if it would be heat from a fossil fuel powered heatplant the expression for estimating the exergy input shown here isnot valid.1 Primary side mass flow rate mprim,DH, inlet and returntemperatures Tin,prim,DHand Tret,prim,DHare varied in order to checkthe efficiency of district heat supply for different operating con-ditions.T0,HP,aveis the absolute value of the average outdoor airtemperature during the heating period (HP), i.e. October to April,which here has a value of 4.8 C. For estimating the primary side

1 Theexergy input input into thedistrictheat exchangerwouldneed to be evalu-ated in this case as a function of the quality factor of the fossil fuel used in the heat

plant.

Fig. 2. Exergy efficiency for different primary side supply and return temperatures(in,DH and ret,DH respectively) and primary side mass flow rates. Only thermalenergy flows are regarded here.

mass flowrate required to supplythe given SH andDHW demands,Eq.(2)is used.

mprim,DH =(QSH,ave+ Qdem,DHW,ave) (1 +fls,SH+DHW)

cp (Tin,prim,DH Tret,prim,DH) , (2)

where the factor fls,SH+DHWdepicts the share of thermal losses inthe supply network and DHW storage tanks, and is considered as0.17 in this case.

Fig. 2shows steady-state exergy efficiencies for supplying theaverage SH and DHWloads at differentsets of operatingconditions(inlet, outlet temperatures and mass flow rates). Mass flow ratesand temperature levels inFig. 2correspond to the primary side ofthe heat exchanger inFig. 1.Continuous lines indicate the exergyefficiency for different primary side return temperatures. Dashedlinesrepresent exergy efficiencies for different primary side supplytemperatures.

Lower supply temperatures increase the exergy efficiency of theheat supply, i.e. lower thequality at whichthe heat flowis supplied

to the single familyhouses allowinga bettermatchingof theenergysupply and demand. Lower return temperatures also increase sig-nificantly the exergy efficiency of the heat supply. Similar resultswere found by Dtsch and Bargel[8]and in[10].It is important toremark that for a given supply temperature, lower return temper-atures lead to an increase in the exergy efficiency. In other words,maximizing the degradation of the thermal potential of the pri-mary mass flow increases the exergy efficiency. These trends andstrategies for improved operation of district heating networks arecoherent with those found in[8].However, Dtsch and Bargel[8]analyze a district heating network without hydraulic separation bymeansofaheatexchanger,i.e.thewholedistributionnetworkistheprimary side of energysupply. In consequence, different mass flowrates and temperature drops have a strong influence on the elec-

tricity demand for the pumps in the network and pumping energy


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Fig. 3. Exergy efficiency for different reference temperatures. The same conditions of primary side supply and return temperatures (in,DH and ret,DH respectively) andprimary side mass flow rates as inFig. 2are considered: (a) a reference temperature of 0 C is considered; (b) a reference temperature of 10 C is considered.

demands were included in the balances. For each set of operat-ing conditions , Dtsch and Bargel[8]size a distribution network,

whereby pressure losses and electricity demands for the pumpscan be estimated. This leads to optimum values of the tempera-ture drop at the primary side of about 10 K, with lower values ofthe temperature difference leading to a greater importance of thepumping energy and,thereby, decreasing theexergy efficiency. Yet,for a given supply temperature, lower return temperatures increasethe exergy efficiency, being similar to conclusions fromFig. 2.

As stated above, inFig. 2the average outdoor air temperatureduring the heating period (4.8 C) has been regarded as referencetemperature. In Fig.3 the influence of the referencetemperature onthe results from exergy analysis is graphically shown. In Fig. 3(a)and (b) a reference temperature of 0 C and 10 C are consideredrespectively. The variation of the thermal energy losses in the pipeshasbeenestimatedinaccordancewith [8]. Itcanbeclearlyseenthatlower reference temperatures (i.e.Fig. 3(a) corresponding to e.g.colder climates) increase the exergy efficiency of the district heat-ing system as compared to Fig. 2. Exergy efficiencies in Fig. 3(a) arein very good agreement with those shown in[8],where the samereference temperature is chosen. In turn, increasing the referencetemperature decreases the exergy efficiency of the district heat-ing system (seeFig. 3(b)). This trend is mainly due to the greaterreduction in the exergy demands (numerator in Eq. (1))as com-pared to the exergy supply (denominator in Eq. (1)) for higherreference temperatures. Yet, regarding the inlet and return tem-peratures from thedistrict heating pipe, thesame trendsas in Fig.2can be observed.

From Fig.2, two mainstrategies to increase theexergy efficiencyof district heating supply can be derived:

1. Minimizereturn temperature to the districtheating network:this can be achieved by proper sizing of the heat exchangerbetween the district heating pipe and the local heat distributionnetwork. However, minimum achievable return temperaturesto the district heating pipe strongly depend on the buildingsystems, e.g. low temperature space heating systems allowlower return temperatures. Therefore, the use of appropriate(i.e. low temperature) building systems is of great importancefor promotingthis strategy. The use of low temperature systemssucceeds only if existing heating loads are low enough. For thispurpose an improved building shell is necessary, i.e. increasedinsulation level and careful design are therefore a must.

2. Minimize supply temperature from the district heating net-

work: supply temperatures are determined by the temperature

profile available from the district heating pipe and cannot bedirectly influenced. However, according to German regulations

forDHW preparation a supplytemperature of 50

C must alwaysbe ensured[15].In turn, maximum required inlet temperaturefor space heating is 32C. Thus, a way to reduce the supply tem-perature for thelowtemperatureuse (i.e.space heatingdemand)is by cascading the energy demands according to their requiredtemperature level.

4. Models for dynamic energy and exergy analysis

According to the strategies developed from preliminary steady-state analysis shown inFig. 2, four different system configurationshave been derived and are analyzed. A schema of their configura-tion and main features are shown inFig. 4:

- System I corresponds to a conventionalhigh temperature districtheat supply, with a supply temperature at the primary side of95 C. DHW and SH supply are combinedly delivered by a singlecentralized heat exchanger.

- System II corresponds to a low temperature district heat supplywith primary side supply temperatures between 50 and 65 C.DHW and SH supply is also done with a single centralized heatexchanger.

- System III is a low temperature district heat supply with primaryside supply temperatures between 50 and 65 C. DHW and SHsupply is performed via separated heat exchangers. In this way,each of the heat exchangers can be sized separately for mini-mizingprimary side returntemperatures at its specific operationconditions. Following, lower outlet temperatures at the primary

side of the heat exchanger are expected.- In System IV a three way valve connects the primary side return

from DHW supply with the primary side for SH supply. If returntemperatures and mass flow rates from DHW supply are highenough to supply SH demands mass flow from DHW supply willcirculate also through the SH heat exchanger. Otherwise, massflow rate is withdrawn directly from the main district heatingpipe. In this way, cascading of DHW and space heating demandis achieved. The setpoint for the secondary side of DHW supplyhas been kept as a function of the primary side inlet temper-ature similarly as in the previous cases (see Eq. (3)).However,primary side mass flow rate from the DHW heat exchanger canonly be used for supplying SH demands (i.e. cascaded) if its tem-perature is higher than the required SH setpoint. Thus, secondary

side setpoint for SH supply has been minimized: it is defined as


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Fig. 4. Four system configurations for district heat supply studied here.

a function of the required inlet temperature for the floor heating

(FH) systemssp,FH(see Eq.(4)).

In case I, with a conventional high temperature supply, aconstant supply temperature from the district heating network(primary side inFigs. 1 and 2)of 95 C is assumed. For options II,III and IV with low temperature supply, a temperature profile as afunction of outdoor air temperature is assumed. Supply tempera-turein,prim,DHis assumed to vary between 50 and 65

C, accordingto the profile shown inFig. 5(a)[18].Fig. 5(b) shows the setpointinlet temperatures forthe FH systems used forspaceheatingsupplysp,FHas a function of outdoor air temperature.

In order to minimize thepumping power for the secondaryside,minimummassflowratesneedtobeachieved.Forthispurpose,thesetpoint of the secondary side of the network providing DHW andSH to the single family housessp,sec is maximized in cases II andIII. As shown inFig. 4and Eq.(3), the setpoint is chosen as a func-tion of the inlet temperature in the primary side of the supply heatexchangersin,DH, i.e. the supply temperature available from thedistrict heating network. Comparing cases I and II the influence oflower supply temperatures from the network can be investigated.To show clearly the influence of this parameter on the exergy per-formance, all other variables need to be the same in both cases.Thus, thermal energy losses and energy performance of the sec-ondary side needs tobe the same in bothcases. For this aim, incaseI a constant setpoint of 55 C has been chosen for the secondaryside. Higher setpoint temperatures lead to higher temperatures inthe upper layers of the DHW tanks, and thus, higher thermal lossesin the storage tanks occur.In turn, with 55C as setpoint theenergysupplied to both systems and their energy behaviour are similar.

Forsupplying the required temperature to the floor heating sys-temssp,FHa mixing valve for recirculating part of the cold returnwater from FH systems is foreseen in all cases.

sp,sec(tk) = in,prim,DH(tk) 2 (3)

sp,sec(tk) = sp,FH(tk) + 2 (4)

Thefour systems under investigation(Fig.4) havebeen dynami-callysimulated inTRNSYS with a timestep of 3 min. Detailed energyflows for every energy conversion step in the energy supply sys-tems (i.e. heat exchangers, pipes in the networks, storage, etc.)can be obtained. An input output approach has been applied forexergy analysis. This means that exergy flows are not derived andanalyzed in detail for every energy conversion step on the district

heating system. Instead, only the exergy associated to the energy

supply (input) and demand (output) are regarded. The heat input

in the primary side of the heat exchangers and pumping energy inthe secondary side are the main inputs in the system. Results from

Fig.5. (a)Primary sidesupply temperature forthe district heating system as a func-tion of the outdoor air temperature[18]. (b) Setpoint for the supply temperature ofthefloor heating (FH) systems forspaceheating supply as a function of theoutdoor

air temperature.


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dynamic energy assessment required to calculate the exergy flows(energy, mass flow rates or temperatures) are used as input in theequations used to perform exergy analysis for every timestep (tk).Eqs.(5)(9)show the mathematical expressions used to calculatethe exergy of heat input, exergy of SH and DHW demands and thefinalexergyandenergyefficienciesovertheyear(withNtimesteps)respectively. Exergy flows are thereby obtained for every timestep(i.e. 3 min):

Exin,DH(tk) = mprim,DH(tk) cp

(Tin,prim,DH(tk) Tret,prim,DH(tk)) T0(tk) lnTin,prim,DH(tk)Tret,prim,DH(tk)

(5)

Exdem,DHW(tk) = mdem,DHW(tk) cp

Tdem,DHW(tk) Tnet(tk)

T0(tk) ln

Tdem,DHW(tk)

Tnet(tk)

(6)

Exdem,SH(tk) = Qdem,SH(tk)

1 T0(tk)Tr(tk)

(7)

DH,fin =

k=Nk=1

Exdem,SH(tk) +

k=Nk=1

Exdem,DHW(tk)

k=N

k=1

Exin,DH(tk) +

k=N

k=1

Ppumps(tk) +

k=N

k=1

Pel,heater(tk)

(8)

DH,fin =

k=Nk=1

Qdem,SH(tk) +

k=Nk=1

Qdem,DHW(tk)

k=Nk=1

Qin,DH(tk) +

k=Nk=1

Ppumps(tk) +

k=Nk=1

Pel,heater(tk)

(9)

The aim of this study is to compare the energy and exergyperformance of different configurations for district heat supply.Therefore, the systems are compared in terms of final energy andexergy supply and efficiencies.

5. Results and discussion

Fig. 6shows the specific final energy and exergy supply for thefour cases studied. Even in the cases with low temperature dis-trict heating supply, with a temperature profile between 50 and65 C, the temperature level is high enough to ensure that most ofthe time DHW tanks are heated up to the required level of 50 Cwithout additional energy input. Additional electricity required topower the instantaneous heater and guarantee the 50 C requiredamounts 435 kWh/a in the cases with low temperature supply, i.e.0.098kWh/m2a, being a marginal energy input as it can be seen inFig. 6.

In terms of final energy and for the low temperature districtheating supply, pumping energy represents around 0.1% the ther-mal energy input into the local network. In the literature these

values are estimated to be around 0.5% and 1% [8,17].The reasonfor the significantly smaller values in the models developed is thathere a criteria of maximum fluid velocity of 11.5 m/s was cho-sen for sizing the network pipes. Since the safety of energy supplywas a key issue in the systems investigated, this was done to makesure that thermal losses in the network are not underestimated.On the other hand, as stated in Section2,pumping energy in thesecondary side is one of the main inputs in the system therebyinfluencing strongly its performance. Criteria chosen here to sizethe network aim also at minimizing the pumping energy, therebyallowing an optimized performance of the systems. In turn, veloci-ties of up to 2.5 m/s in the main pipes would be acceptable [8]. Thisleads to bigger pipes and lower pressure losses, i.e. lower pumpingpower required. Pipes with bigger diameters required for the net-

works sized with the criteria used here (i.e. with target velocities

of 11.5 m/s) are estimated to cost around 17% more than simi-lar networks sized with conventional criteria. However, the pumpswould be smaller in the networks used here (since maximum pres-sure lossesare also smaller) and thereby cheaper. Estimations donefor case II show that the lower prices for the pumps might evencompensate greater costs for the pipes in the hydraulic network.

Final energy input required to supply the different casesamounts 92.0, 91.7, 92.3 and 93.0 kWh/m2a for cases I,II,III, and IV

respectively. The greatest difference can be found between cases IIandIV and amounts only 1.4% of the total energysupply.In terms offinalenergysupply,asitcanbeseenin Fig.6(a) all systems analyzedare equivalent. Thereby the energy performance (efficiency) of allcases is very similar, as it can be seen in Table 2.

In turn, the exergy supplied is significantly different for the dif-ferent systems studied: 15.8kWh/m2a for case I, 12.8kWh/m2aforcase II, 11.8 kWh/m2a for case III and 11.9kWh/m2a for case IV.

The greatest difference can be found between cases I and III andamounts 25.6% of the exergy supply in case I, being thus a relevantreduction. This leads to an increased exergy performance for caseIII as compared to the rest of cases (seeTable 2).

Fig. 6. (a)Finalenergy supplyfor thefour cases studied and(b) Final exergysupply

for the four cases studied.


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Table 2

Final energy and exergy efficiencies for the four cases studied. Annual average values of the primary side supply and return temperatures are also shown for each case.

(Final) energyefficiency,DH,fin[%]

(Final) exergyefficiencyDH,fin[%]

Primary side supplytemperaturein,primDH[

C]Primary side returntemperatureret,primDH[

C]

I 1HX, 95 C 81.7 32.0 95.0 32.4II 1HX, 6550 C 81.9 39.3 57.7 40.8III 2HX, 6550 C 81.4 43.0 57.7 37.7IV 2HX cascading, 6550 C 80.7 42.4 57.5 37.8

Table 2shows the final energy and exergy efficiencies for thefour cases analyzed. In addition, annual average values for the pri-mary side inletin,primDH,aveand return temperaturesret,primDH,avefor the cases studied are also shown for being representative forthe operating conditions in each case.

Case I has the lowest exergy efficiency, being between 10.4and 7.3% lower than the rest of systems studied. Case II repre-sents a system with the same hydraulic configuration but withlower supply temperatures (5065C) and shows an exergy inputof12.8kWh/m2a,representing a reductionof 19%in the finalexergyinput as compared to case I. This shows the importance of reduc-ing supply temperatures in district heating networks for increasingtheir exergy performance. Cases II, III and IV have a more sim-

ilar exergy supply (Fig. 6b) and, in consequence, a more similarperformance (Table 2).

Values for the exergy efficiencies inTable 2 are between 32and 43%. Similar values can be found in the literature: in [9,12]exergy efficiencies for two geothermal district heating systems arefound to be 50 and 45.66% respectively. Efficiencies in[9,12]how-ever, do not consider the final demand to be supplied, i.e. spaceheating or domestic hotwater supply, as thefinal outputof the sys-tem. Instead, in[9,12]the thermal exergy output from the districtheating system to the building systems is regarded as the output.This leads to slightly higher values in the exergy efficiencies. Onthe contrary, the exergy efficiencies in the present paper considerthe exergy demands for SH and DHW supply as the final output,regarding therefore a further conversion step. Following, exergy

efficiencies presentedhere (seeTable2 and Fig.2) are lower. Dtschand Bargel[8] also follow this last approach, i.e. they regard theexergy of SH demands to be supplied as the final desired outputof the district heating system. Values of the exergy efficiencies in[8]are in very good agreement with those shown in the presentpaper.

In[12]the authors conclude that higher supply temperaturesincrease the exergy efficiency of the district heating system. Thisresults,contradictorytothoseshowninthepresentpaper,areagaindue to the boundary chosen for exergy analysis: as stated above, in[9]the authors regard the exergy supplied by the district heatingsystem as the desired output. In consequence, higher temperaturesof the energy supplied increase the exergyefficiency of the system.Inturn,inthepresentpapertheSHandDHWdemandsareregarded

as the final desired output of the district heating system. Supply-ing these demands with higher temperatures (e.g. by using hightemperature radiators) leads to increased exergy losses inside thebuilding systems. Following this approach, for a given demand at agiven temperature (e.g. SH at 20 C) lowering the supply tempera-ture from the district heating network increases the performanceof the system. Similar conclusions can be found in[8],where thesame approach as in the present paper is followed. This shows theimportance andgreat influenceof thechosen boundaries for exergyanalysis on the results and conclusions that can be derived from it.

Comparing cases II and III the influence of supplying DHW andSH demands separately, i.e. with separate heat exchangers andsec-ondary networks, can be seen. Separate supply (i.e. case III) allowsminimizing primary side return temperatures to the district heat-ing network (see Table 2). Fig. 7(a) and (b) shows the quality factor

oftheenergysupplied FQ,primarysidereturntemperaturesret,primand mass flow rates for both casesII and III. For completeness, heat

rate supplied to the heat exchanger(s).

Qin,DH and the exergy rate

associated to it.

Exin,DH arealso shown. Supplytemperatures arethesame at all times, and therefore not shown inFig. 7.The dynamicbehaviour of the heat transfer is quite similar in both cases. How-ever, it happens at significantly different mass flow rates, and inconsequence, different temperature drops occur in the primaryside, i.e. different return temperatures are found. The grey shad-owed areas in Fig. 7 showthe correlationbetweenthe quality factorof the energysupplied FQand the primary sidereturn temperaturesret,prim,DH: wheneverreturntemperatures arelower foranyof bothcases, quality factors are also lower. This means that the quality oftheenergy supplied decreaseswithlower return temperatures. Theinstantaneous exergy supplied at thistimesteps, however,might behigher if the energy supplied is also greater. Thereby, final exergy

Fig.7. (a)QualityfactorsFQand mass flowratesmfor casesII andIII;(b) Exergyand heat transfer rates supplied Ex and Q and return temperatures ret for

cases II and III.


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Fig.8. (a)QualityfactorsFQ andmassflowrates mforcases IIIandIV;(b) Exergy

and heat transfer rates supplied Ex and Q as well as inlet and return in andret temperatures for cases III and IV.

inputrequiredis reduced to11.7kWh/m2a,representingadecreaseof 8.4% as compared to case II. This shows the influenceof minimiz-ing return temperatures for increasing the exergy performance ofdistrict heating systems.

Fig. 8(a) and (b) show the quality factor of the energy suppliedFQ, primary side inletin,primand return temperaturesret,primandmass flow rates for bothcases III and IVas well asthe heatratesup-

plied to the heat exchangers.

Qin,DHand the exergy rate associated

toit.

Exin,DH. Inlettemperatures arethe same forbothsystems when

cascading is not possible and lower in case cascading of the massflow rate for DHW supply might be used for SH supply in case IV, asshown inFig. 8(b). The grey shadowed areas inFig. 8highlight thecorrelationbetween thequality factor of theenergy supplied FQandthe primary side supply temperatures in,prim,DH: whenever supplytemperatures are lower for case IV, i.e. when cascading is possible,quality factors are also lower despite return temperatures at thosetimestepsare higherfor case IV.Whenlowerinlettemperatures forcase IV succeed mass flow rates for this case increase and becomegreater than for case III. The instantaneous heat transfer is nearlythe same in both cases at all times. In turn, the quality factor asso-ciated to the heat transfer for those situations is lower for case IV.This means that the quality of the energy supplied decreases withlowersupply temperatures. Thisshows the importance of reducing

supply temperatures when possible.

Unfortunately, cascading between DHW and space heating heatexchangers succeeds only 485h/a of the 3594 h/awhenspace heat-ing energy demand exist and it represents only 2% of the totalenergyand exergy supplied to space heating. Therefore, the benefitof this improved exergy performance due to cascading representsa very small amount of the total energy and exergy supplied.

Whencascading is notpossible,return temperatures areslightlyhigher for case IV as for case III, leading to also slightly higher qual-ity factors. In addition, due to the control strategy of the secondaryside network for SH supply in case IV higher mass flow rates arerequired,leadingto higher pumpingpower demands:electricity forpumping amounts 0.047 kWh/m2a in case III and 0.122kWh/m2ain case IV. These adverse factors are responsible for the worse per-formance of case IV as compared to case III. Yet, pumping powerfor operatingthe secondary network would notvary if cascadingofDHWand SH demands was possible more often. In turn, if frequentcascadingwould reducethe quality of the heat supplied in a greatershare than the increase in pumping energy system IV would showthe best performance of all cases. This requires, however, develop-ing suitable system concepts promoting cascaded use of the massflow rate.

6. Conclusions

Fourdifferent systems for districtheat supply havebeen dynam-ically analyzed. As a first step, simplified steady-state exergyanalysis was performed to show the exergetic behaviour of a dis-trict heat supply system. System concepts studied were developedbased on conclusions from this analysis. Main conclusions andtrends derived from this preliminary steady-state analysis havebeen confirmed by results from dynamic analysis.

The energy performance of the systems studied is very similar.Maximum differences in the final energy efficiency of the systemsstudied amount only1.2%.Their exergy performance shows, in turn,significant differences with values for the final exergy efficienciesfrom 32% to 43%. These results show that exergy analysis is a morepowerful tool for depicting the performance of the systems ana-

lyzed than mere energy analysis. By depicting the quality level ofthe energy supplied, exergy analysis allows usingavailable thermalenergyflowsin a more efficientway,i.e. degradinglower amount oftheir potentialto deliver work(exergy). This might bea suitabletoolfor organizing available energy flows in district heating networksin a more efficient way, where energy supplied and demanded canbe matched in terms of their quality level.

The main conclusions related to the performance of districtheating systems obtained from this study can be summarized asfollows:

- Results from dynamic analysis show that reducing the inlet tem-peratures from 95 C to an average annual value of 57.7 C allowincreasing the exergy efficiency of the district heating system in

7.3%. Similar behaviour is confirmed by simplified steady-stateanalysis (Fig. 2).

- Reducing the average annual return temperature in 3.1 C leadsto an increase in the exergy efficiency of 3.7%. Thus, it can bestated that lower average annual return temperatures to the dis-trict heating network lead to an increase in the exergy efficiencyof the system. One strategy to achieve lower returntemperaturesconsists on supplying SH and DHW demands with separate heatexchangers. Thisallows sizing and operatingeach heatexchangerso as to minimize return temperatures to the district heatingnetwork at all times.

- Supply temperatures have a greater influence on the exergyperformance of the district heating system studied than returntemperatures. This has been shown by the comparison of cases

III and IV (cascaded supply of DHW and SH demands). When


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cascading between DHW and SH demands is possible lower sup-ply temperatures and higher return temperatures are found forcase IV. At lower supply temperatures (and despite simultane-ous higher return temperatures) similar heat transfer happensat lower quality levels (FQ), i.e. less exergy input is required toprovide the same thermal energy demands. This indicates thatthe first step for a more efficient district heat supply should beto reduce supply temperatures. For this aim, suitable networkconcepts need to be developed which allow energy demandsat different temperature levels to be supplied at a suitabletemperature.

- Due to the low simultaneity between DHW and SH demandscascading between DHW and SH heat exchangers succeeds only485h/aof the3594h/awhenspace heating energydemandexists,representing only 2% of the total energy and exergy supplied tospace heating. Therefore, the benefit of the improved exergy per-formance due to cascadingrepresents a very small amount of thetotal energyand exergysupplied. Yet, if cascadingwouldsucceedmore often the performance of case IV might greatly increase.Thus, hydraulic concepts promoting a cascaded supply of DHWand SH demands (e.g. by means of storage concepts) need to bedeveloped and investigated.

- For cascaded supply between DHW and SH demands higher

mass flows are required on the secondary side for SH supply. Anincrease in thepumpingpower required to operate the secondarynetwork follows. In the case studied, the increase in the pumpingenergy was greater than the benefits from cascaded supply froman exergy perspective. Thus, the performance of cascaded supply(case IV) as compared to separated supply (case III) is reduced in0.6%.

Acknowledgement

The authors warmly thank the German Federal Foundation forEnvironment and the German Federal Ministry of Economy andTechnology for their financial support.

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