Belgium, and especially in Sweden and Russia since at the beginning of 20th century. Forexample, the pipe network of district heating system in Moscow is about 600 km [2]. As districtheating and cooling systems spend great quantities of fuel at national economies level to provide

Applied Thermal Engineering 24 (2004) 10091017* Corresponding author. Tel.: +90-442-2314849; fax: +90-442-2360957.1. Introduction

Faster urbanization caused by industry revaluation made to emerge the idea nding a remedyto human needs from a centre in addition to the social services like water supply, sewer system,public transportation and district heating system. The rst district heating system was built atLockport (New York, USA) in 1877 [1]. Since then it has been spread on the various Country ofEurope. The district heating systems have been used increasingly in Germany, Denmark, Holland,Abstract

In this paper, energy and exergy losses forming in the heat distribution network of district heating

systems were evaluated. For this purpose, equations of energy and exergy losses were identied and by

using them, heat distribution network of the university campus which has a heating network of pipes 11,988

m in length and 65250 mm in diameter was analyzed. It was found that exergy losses forming during heat

distribution were about 16% of total exergy in the system and temperature of hot water supplied and

returning was found to be the most important factor aecting these exergy losses.

2003 Elsevier Ltd. All rights reserved.

Keywords: Energy loss; Exergy loss; District heating networkEvaluation of energy and exergy losses in districtheating network

Kemal Comakl *, Bedri Yuksel, Omer ComaklDepartment of Mechanical Engineering, Ataturk University, 25240 Erzurum, Turkey

Received 5 July 2003; accepted 25 November 2003

www.elsevier.com/locate/apthermengE-mail address: kcomakli@atauni.edu.tr (K. Comakl).

1359-4311/$ - see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/j.applthermaleng.2003.11.014

1010 K. Comakli et al. / Applied Thermal Engineering 24 (2004) 10091017Nomenclature

CC the channel circumference length (m)DT diameter of insulated pipe (m)Ex Exergy (kJ)kc thermal conductivity of channel material (W/mK)ki thermal conductivity of insulation material (W/mK)ks thermal conductivity of soil (W/mK)L length of channel (m)Pr prandtl numberQc heat provided for consumer (kJ)Qi heat transferred to the water by heating plant (kJ)heating cooling and domestic hot water in buildings. These systems attracted great attention in theliterature [17].District heating is important not only because of the use of energy sources more eciently but

also being received with energy need more regularly, suciently and cheaper than other ways. Indistrict heating system, heat generated in a plant is transported to the consumers in an extensivearea. Thus, it is provided to heat a region consisting of many buildings and hot water from acentre. The district heating system, transport from the heat plant by using primary pipe network,via substation, to secondary pipe network where heat is nally distributed to consumer.In this system, energy and exergy losses are form in distribution pipes owing to carrying o the

heat through large distances. Energy and exergy losses because of pipeline highly aect theadvantages of heating systems economically. Hence, heat losses from pipelines should be reducedto minimum level. Heat losses in heat distribution network were computed 810% with a studycarried out by Poredos and Kitonovski [8].

QLoss heat losses in pipe network (kJ)Ra Rayleigh numberRb resistance of channel hole (mK/W)Rc resistance of channel (mK/W)Ri resistance of insulation material (mK/W)ri radius of insulated pipe (m)rp radius of pipe (m)Rs resistance of soil (mK/W)Ta daily average outer temperature (K)tc the channel thickness (m)Tc;r temperature of consumer return water (K)Tc;s temperature of consumer supply water (K)Tr temperature of return water (K)Ts temperature of supply water (K)Wp work of circulation pump (kJ)

withitrans

coldest cities in Turkey. In one season, approximately 10,000 tons fuel-oil is consumed in theheating system. Because the pipe network is about 12 km, which is assumed to be quite long, there

K. Comakli et al. / Applied Thermal Engineering 24 (2004) 10091017 1011have been energy and exergy losses considerably in the heating network system.

2. System description and analysis

The system consists of a branched pipeline network for distributing the heat from the heatingplant to the consumer. The essential element of such a system is the pipeline, which enables thetransport of energy and which is the source of the heat losses. Another important part of thesystem is the heat stations, where the heat is transferred from a high to a low temperaturemedium, resulting in decreased heat quality. In the analysis, the pipeline network was the insu-lated pipes with diameter from 65 to 250 mm. The total length of the pipeline network is 11,988 m.The average pressure in the primary network (Ts 175 C, Tr 110 C), was 15 bar, while in thesecondary network (Tc;s 85 C, Tc;r 65 C), the pressure depended on the atmosphere pressure.Heat transfer from the primary to the secondary network in heat stations was via shell-tube heatexchangers.

2.1. Energy balance

According to Fig. 1, the heat supplied to the consumer is

_Qi _Wp _Qloss _Qc 1where, Qi shows the heat transferred to the water by heating plant, Wp is work which has beenthermodynamics, which is concerned with the conversation of energy. Exergy analysis is based onsecond law of thermodynamics. Many researchers propose that the thermodynamic performanceof a process is best evaluated with exergy analysis [9].From the thermodynamics point of view, exergy is dened as the maximum amount of work,

which can be produced by a system, or a ow of matter or energy as it comes to equilibrium with areference environment. Unlike energy, exergy is not subject to a conservation law (except forideal, or reversible, processes). Rather, exergy is consumed or destroyed, due to irreversibilities inany real process. The exergy consumption during a process is proportional to the entropy createddue to irreversibilities associated with the process. Exergy analysis is a method that uses theconservation of mass and conservation of energy principles together with the second law ofthermodynamics for the analysis, design and improvement of energy and other systems. Theexergy method is a useful tool for furthering the goal of more ecient energy-resource use, for itenables the locations, types, and true magnitudes of wastes and losses to be determined [10].In this study, energy and exergy losses in the heat distribution network of Ataturk university

campus were analysed. In the University, heating and domestic hot water in building of suppliedheat is produced by district heating. Ataturk University is in the Erzurum which is one of thedonen processes, donated as energy and exergy losses. Energy and exergy losses can directly belated to an increase in primarily fuel consumption. Energy analysis is based on the rst law ofEnergy and exergy analyses are performed recently to show where energy eciencies occurby the pump for the circulation of hot water though pipelines, Qc, the heat provided for

1012 K. Comakli et al. / Applied Thermal Engineering 24 (2004) 10091017consuthe gtimechan

Table

Resist

Ri

Rc

rc

Rs mers and Qloss indicates heat losses in pipe network. Pipes are in the channel which is underround (Fig. 1). Below, heat loss formed in a channel with two pipes at a certain instant ofis indicated by using the model in Fig. 1 [2,11]. According to this model, heat loss formed innel of unit length is expressed as

_Qloss 2 Tw TaRi Rb Rc Rs

2

1

ance used in Eq. (2) [11,12]

1

2pkilnri=rp

Rb 1=2prhb

1

2pkc

lnrc tc=rc hb

kiDT

NuD

CC=2p

1

2pks

ln

hcrc

11 DT=hc2

q Nu 0:60 0:387Ra

1=6D

1 0:559=Pr9=168=27" #2

Fig. 1. (a) Illustration of district heating system and (b) canal cross-section.

Ta is daily average outer temperature and R is the thermal resistance of which types are shownTablanaly

Exergy balance may be written using Fig. 1

where E is hot water exergy, E is the electrical exergy given for the pump (E W ), E is the

_Ex;lossA 1 T0 _Qloss 4

network, through which is transformed into heat. The exergy of the heat at a temperature T is

total exergy losses comprise the losses from the irreversible heat transfer and the losses due to the

K. Comakli et al. / Applied Thermal Engineering 24 (2004) 10091017 1013friction of both follows. According to the ndings of Kotas [15], the exergy losses due to frictionfor the liquid ow (small specic volume) are relatively small, so the exergy losses during heattransfer are

_Ex;loss C T0 _QC 1Tw

1Tcw

6

The temperatures of the both water ows in the heat exchanger vary, and so, Eq. (6) is in a2.2.3. Exergy losses during heat transfer in heat exchanger Ex:loss CHeat transfer in a heat exchanger is an irreversible process, therefore exergy losses occur. Thedier_Ex;loss B _Wp Tw_Wp Tw

_Wp 5w

much lower than the exergy of the work Wp. As a result, the exergy losses [8] are

Tw T0 T02.2.2. Exergy losses due to hot water transportation Ex;loss BThe heat is transported from the heating to the consumers using hot water with a particular

enthalpy value. Electrically-driven pumps are used to create a ow of water. Electrical energyrepresents pure exergy and this exergy is used to overcome the ow resistance in the pipelineTwx;i x;w x;w p x;c

exergy provided for consumers. Exergy losses, Ex;lossA, Ex;loss B, Ex;loss C, are explained below.

2.2.1. Exergy losses formed from heat losses Ex;loss AHeat losses during the transport of hot water through pipes cause exergy losses. From the

known equation Ex;lossA can be dened as [14] _Ex;i _Ex;w _Ex;lossA _Ex;loss B _Ex;loss C _Ex;c 3_ _ference length CC, the channel thickness tc and thermal conductivity kc were taken as 5 m, 10 cm,2.5 W/mK, respectively. Thermal conductivity ks was taken as 0.91 W/mK for soil with moisture[13].

2.2. Exergy balancee 1. Glass wool (ki 0:65 W/mK) of average 8 mm thickness was used in the pipelinesed. Channel was manufactured from the mixture of stone and concrete. Channel circum-Temperature of pipeline is assumed to be the arithmetic average of Ts and Tr, Tw Ts Tr=2.ential form. The calculation of exergy losses for the heat exchanger can also be in an integral

form, with the temperatures of both uids (Tw and Tcw Tc;s Tc;r=2) determined by the ther-modynamic average temperature [8].

Tm _Qc_SQ

_QcZ T0

Ti

dQT

7

3. Results and discussion

Energy losses in pipelines which are used for the distribution of heat from the heating plant tothe exchanger in heating systems are very important. Heat losses in pipelines were calculated usingdaily average outer temperature values. Yearly heat loss in network was shown in Table 2. Heatlosses due to the network Qloss is about 8.62% of the energy provided by heating plant, Qi.Thickness of thermal insulation materials is the most eective factor which causes reduction of

heat losses in pipes. Heat losses decrease by increasing thickness of insulation. Especially, anincrease up to a thickness of 20 cm results in an important decrease in heat losses. Variation ofheat losses due to the insulation thickness can be seen in Fig. 2. It would be a decrease of about

Table 2

Heat and exergy losses of pipes (ke 1 W/mK, kc 2:5 W/mK, ki 0:065 W/mK (150 C), hc 1:2 m)Pipe diameter (mm) Pipe length (m) Heat loss (kJ) 109 Exergy loss (kJ) 109

250 3227 11.032 3.067

200 1870 5.825 1.619

220/m

1014 K. Comakli et al. / Applied Thermal Engineering 24 (2004) 100910175 10 15 20 25 30 35Insulation thickness (mm)100

140

180

Hea

t los

ses (

W150 984 2.701 0.751

125 2759 7.008 1.948

100 1495 3.457 0.961

80 1393 3.092 0.859

65 260 0.528 0.147

Total 33.643 9.352

260

)

CC=5 m CC=4.5 m CC=4 mFig. 2. Heat loss vs. insulation thickness (/250, L 1 m, Ta 15 C).

8.828.838.848.858.868.878.888.898.908.91

140 150 160 170 180Ts (0C)

Tr (0C)

Ex,lo

ssA

/Ex,i

(%)

105115125135

Fig. 3. Exergy loss due to heat loss.

K. Comakli et al. / Applied Thermal Engineering 24 (2004) 10091017 101525% in heat losses if an insulation thickness of 20 cm was used instead of 8 cm in pipes. Moreover,channel circumference length has a reduction eect of heat losses although it is small.The supply and return water temperatures are 140180 C and 105135 C, respectively, which

therefore have been taken in to account in the analysis in this paper. Exergy losses in hot waterdistribution pipes, in heat exchanger and from the circulation of hot water, change with thetemperature of hot water supplied and returning. In Fig. 3, variation of exergy losses in hot waterdistribution pipes due to the change of supplied and returning water temperature may be seen.Exergy losses increase with an increase of Ts while it decreases with an increase of Tr. A strongerdependence on the supply temperature rather than the return temperature is the case, whenlooking at Fig. 3.The exergy losses due to hot water transportation Ex;loss B is shown Fig. 4. Electrical energy is

supplied to the system to drive the circulating pumps. With a constant demand for the amount ofthermal energy to be supplied, the mass ow of the water varies and, therefore, so does theamount of supplied electrical energy. Fig. 4 shows that this is greater with lower temperaturedierences between the supplied and returning hot water.Fig. 5 shows that the exergy losses formed during heat transfer in exchangers are so high. With

an increase of 10 C in Ts causes an increase of about 1.6% in exergy losses. Besides, exergy losses1.00

1.10

1.20

1.30

1.40

1.50

1.60

140 150 160 170 180Ts (0C)

Ex,lo

ssB/

Ex,i

(%)

105115125135

Tr (0C)

Fig. 4. Exergy loss due to supplied electrical energy.

Fig. 5. Exergy loss in heat exchanger.

1016 K. Comakli et al. / Applied Thermal Engineering 24 (2004) 1009101712

13

14

15

16

Ex,to

t/Ex,fu

el (%

) . 105115125135

Tr (0C)35

38

41

44

47

50

140 150 160 170 180Ts (0C)

Ex,lo

ssC/E

x,i

(%)

105115125135

Tr (0C)increase with increasing Tr. Finally, exergy losses in exchanger are about 50% of the ones providedfor consumer.The heat supplied to consumers also delivers a certain amount of exergy, which can be cal-

culated using Eq. (3). If the total exergy loss is reduced to that of the amount of fuel exergy, thecurve shown in Fig. 6 is obtained. The maximum values reach a factor of almost 16%, whichmeans that in this case, almost 16% of the fuel exergy is lost during the transport and distributionof heat.

4. Conclusion

In this paper, the energy and exergy losses occurring in the district heating network system havebeen investigated; regarding the supply and return water temperatures. The analysis exergy losswhich occurs during the transport of thermal energy to consumers indicates that this loss is largeand primarily dependent on the temperature of the hot water. The total exergy losses increase0.75% with increasing the supply water temperature about 10 C, which is the case for the returnwater temperature. This analysis shows that this loss during heat transfer in district heatingnetwork can be reduce, by reducing the consumption of electrical energy during the transport ofhot water to the consumer and by reducing heat loss in pipelines.

10

11

140 150 160 170 180Ts (0C)

Fig. 6. Total exergy loss reduced to total fuel exergy.

These heat losses should be kept at a minimum, which is possible by lowering the supplytemperature from the plant and by increasing of thermal insulation thickness in pipes. However,lowering the supply temperature could result in unacceptably low temperature levels at the cus-tomers. Furthermore, if the supply temperature is reduced, the water ow in the system increases,resulting in higher pumping costs.

References

[1] D.K. Baker, S.A. Sherif, Heat transfer optimization of a district heating system using search methods, Int. J.

Energy Res. 21 (1997) 233252.

[2] T. Yilmaz, District heating, M.Sc. Thesis (in Turkish) _Istanbul Technical University, Turkey, 1988.[3] I. Adamo, G. Cammarata, A. Fichera, L. Marletta, Improvement of a district network through thermoeconomic

K. Comakli et al. / Applied Thermal Engineering 24 (2004) 10091017 1017approach, Renewable Energy 10 (23) (1997) 213216.

[4] A. Benonysson, B. Bohn, H.F. Ravn, Operational optimization in a district heating system, Energy. Convers.

Mgmt. 36 (5) (1995) 297314.

[5] M. Bojic, N. Trifunovic, S.I. Gustafsson, Mixed 01 sequential linear programming optimization of heat

distribution in a district-heating system, Energy Building 32 (2000) 309317.

[6] B. Bohn, On transient heat losses from buried district heating pipes, Int. J. Energy Res. 24 (2000) 13111334.

[7] L. Gustavsson, A. Karlsson, Heating detached houses in urban areas, Energy 28 (2003) 851875.

[8] A. Poredos, A. Kitanovski, Exergy loss as a basis for the price of thermal energy, Energy Convers. Mgmt. 43 (2002)

21632173.

[9] M.A. Rosen, W.H. Leong, M.N. Le, Modelling and analysis of building systems that integrate cogeneration and

district heating and cooling. www.esim.ca/2001/documents/.

[10] I. Dincer, The role of exergy in energy policy making, Energy Policy 30 (2002) 137149.

[11] K. Comakl, Energy and exergy analysis of district heating plant of Ataturk university, Ph.D. Thesis (in Turkish),Ataturk university, Erzurum, Turkey, 2003.

[12] F.P. Incropera, D.P. Dewitt, Fundamentals of Heat and Mass Transfer, second ed., John Wiley, New York, USA,

1985.

[13] A.A. Al-Temeemi, D.J. Harris, The generation of subsurface temperature proles for Kuwait, Energy Building 32

(2001) 837841.

[14] A. Bejan, Fundamentals of exergy analysis, entropy generation minimization, and the generation of ow

architecture, Int. J. Energy Res. 26 (2002) 545565.

[15] T.J. Kotas, The Exergy Method of Thermal Plant Analysis, second ed., Krieger Publishing Company, USA, 1995.

Evaluation of energy and exergy losses in district heating networkIntroductionSystem description and analysisEnergy balanceExergy balanceExergy losses formed from heat losses Ex,lossAExergy losses due to hot water transportation Ex,lossBExergy losses during heat transfer in heat exchanger Ex.lossC

Results and discussionConclusionReferences