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Energy 60 (2013) 426e434

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Energy

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Exergoeconomic analysis of a district heating system for geothermalenergy using specific exergy cost method

Mehmet Ali Alkan a, Ali Keçebas b,*, Nurettin Yamankaradeniz c

aDepartment of Electricity and Energy, Ula Ali Koçman Vocational High School, Mu�gla Sıtkı Koçman University, 48640 Mu�gla, TurkeybDepartment of Energy Systems Engineering, Technology Faculty, Mu�gla Sıtkı Koçman University, 48000 Mu�gla, TurkeycDepartment of Air Conditioning and Cooling Technology, Vocational School of Technical Science, Uluda�g University, 16059 Bursa, Turkey

a r t i c l e i n f o

Article history:Received 17 April 2013Accepted 9 August 2013Available online 7 September 2013

Keywords:Geothermal energyDistrict heating systemThe SPECO methodExergoeconomic evaluation

* Corresponding author. Tel.: þ90 252 2115471; faxE-mail addresses: alikecebas@mu.edu.tr, alikeceba

0360-5442/$ e see front matter Crown Copyright �http://dx.doi.org/10.1016/j.energy.2013.08.017

a b s t r a c t

This study presents the exergoeconomic analysis and evaluation in order to provide cost based infor-mation and suggests possible locations/components in a GDHS (geothermal district heating system) forimproving the cost effectiveness. The analysis is based on the SPECO (specific exergy costing) method,and used to calculate exergy-related parameters and display cost flows for all streams and components.As a real case study, the Afyon GDHS in Turkey is considered based on actual operational data. Theobtained results show that the unit exergy cost of heat produced by the Afyon GDHS is calculated asaverage 5624 $/h. The HEX (heat exchanger)-III among all components should be improved quickly dueto the high total operating cost rate and relative cost difference. The HEX-I and PM (pump)-V have thehighest exergoeconomic factors among all other system components due to the high owning andoperating costs of these components. The heat production costs per exergy unit for all the HEXs decreasedue to the high exergy destruction cost rate of the system, while the well head temperature and ambienttemperature increase. The SPECO method may be used to improve the cost effectiveness according toexergy rates in GDHSs as a thermal system.

Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

GDHS (geothermal district heating system) has recently beengiven increasing attention in many countries. These systems aresimple, safe and adaptable systems, minimum negative environ-mental impact, low operating cost, decentralized production ad-vantages, and simplicity of their technologies. Numerous successfulGDHS projects have been reported. Experience by researchers andengineers still plays an important role in the system analysis,design and control [1,2]. Especially the heat economic losses inGDHSs cause the fast energy consumption, eventually environ-mental problems. Therefore, an optimization analysis is vital interms of exergetic and economic aspects.

Exergy is a way to sustainability while exergy analysis has beenrecently widely used as a very useful tool for performance assess-ment of energy-related systems as well as sustainable buildings [3].Exergy analysis helps to identify the inefficiencies caused by theirreversibilities within the system being. Therefore, exergy based

: þ90 232 2113150.s@gmail.com (A. Keçebas).

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methods reveal the location, the magnitude and the sources ofinefficiencies and costs. Exergoeconomic (or thermoeconomic)analysis also combines both exergy and economic analyses [4]. It isbased on the exergy costing principle, which assigns monetaryvalues to energy streams and to the thermodynamic inefficiencieswithin the system [5]. It also provides the designer or operator of anenergy conversion systemwith information crucial to the design ofa cost-effective system [6].

Two main groups of thermoeconomic methods have beendeveloped [7] as (i) cost accounting methods and (ii) optimizationmethods. The exergy cost theory [8], the average cost approach [9],the Last-in-First-out method [10] or the specific exergy costing[11e13] method have been used for the first method, while thethermoeconomic functional analysis [14] or engineering functionalanalysis [15] have been used for the other method.

One of the best developed and comprehensive methods is theSPECO (specific exergy costing) method presented by Lazzarettoand Tsatsaronis [13]. This tool provides simple and unambiguousprocedures for evaluating energy conversion systems and uses amatrix formulation which facilitates fast problem solving. Severalstudies have discussed the SPECO method, e.g. Refs. [4,13,16e20].Bejan et al. [4] and Tsatsaronis [16] discussed in detail the SPECO

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M.A. Alkan et al. / Energy 60 (2013) 426e434 427

method technique. Lazzaretto and Tsatsaronis [13] proposed amethodology to calculate exergetic efficiencies and exergy relatedcosts in thermal system to be used for SPECO method.

The different studies have been conducted on this topic. Forexample; Abusoglu and Kanoglu [17] presented the thermoeco-nomic formulations using the SPECO method of an actual dieselengine powered cogeneration system installed in Gaziantep,Turkey. They then evaluated this analysis in Ref. [18]. Balli et al. [19]studied on the thermodynamic and thermoeconomic analyses of anactual trigeneration system with a rated output of 6.5 MW gasediesel engine installed in the Eskisehir Industry Estate Zone,Turkey. Kanoglu et al. [20] developed exergoeconomic formulationsand procedure including exergy flows and cost formation andallocation for a high temperature steam electrolysis system at threeenvironmental temperatures. Kalinci et al. [21] calculated exergy-related parameters of hydrogen production from plasma gasifica-tion of sewage sludge and display cost flows for all streams andcomponents. Yildirim and Gungor [22] conducted the exer-goeconomic analysis that combines exergy analysis with economicanalysis of a CHP (combined heat and power) system for the systemimprovement. Bagdanavicius et al. [7] conducted a thermoeco-nomic analysis of four different thermal systems operated bybiomass. They are biomass steam turbine combined heat and po-wer CHP, gas turbine CHP, biomass integrated gasification gas tur-bine CHP and biomass integrated gasification combined cycle CHPsystems. In a different study, Kalinci et al. [23] investigated threedifferent gasifiers, namely, downdraft gasifier, circulating fluidizedbed gasifier and plasma gasifier in cogeneration of hydrogen andpower for hydrogen production. Abusoglu et al. [24] presented thethermoeconomic analysis and assessment of a municipal waste-water treatment system. Cay et al. [25] developed the cost balancesand auxiliary thermoeconomic relations for direct gas heated andhot oil heated stenters in textile dryers and evaluated the exer-goeconomic aspect. Gungor et al. [26] analysed and evaluated theperformance of the drying system components and the dryingprocess in a gas engine-driven heat pump drying system based onthe experimental data from an exergoeconomic point of view.

As can be seen from the previously conducted studies reviewed,no studies on exergoeconomic analysis and assessment of GDHSsaccording to the SPECO method have appeared in the open litera-ture to the best of the authors’ knowledge. This study deals with theexergoeconomic analysis and evaluation for improving the costeffectiveness of the Afyon GDHS, based on an SPECO method. TheSPECO method is used in this analysis which is based on specificexergies, and costs per exergy unit, exergetic efficiencies, and theauxiliary costing equations for the system and its components. Inthis regard, the main objectives of this study are to (i) deriveexergoeconomic relations, (ii) evaluate the exergoeconomic per-formance of each component of the Afyon GDHS by using actualcost data, and (iii) conduct a parametric study on the effect of wellhead and ambient temperatures for the GDHS.

2. Description of the selected system

The Afyon geothermal district heating system (GDHS) wasinstalled in 1994 in the city of Afyonkarahisar/Turkey to provideresidential heating for buildings through geothermal water. Its heatsource originates from the Ömer-Gecek geothermal field, 15 kmnorth-west of the city of Afyonkarahisar. It was initially designedfor 10,000 residences. Nowadays, there are only 4613 residencesthat have been heated with a potential of 48.333 MWt. The averagereservoir temperature of wells is 105 �C. As can be seen in Fig. 1,modified from Refs. [27,28], the Afyon GDHS consists of three cy-cles: (i) the EPC (energy production cycle), (ii) the EDC (energydistribution cycle), and (iii) the ECC (energy consumption cycle).

For the EPC, the geothermal fluid collected from the productionwells is sent to the inlet of the mixing pool. The fluid at an averagetemperature of about 95 �C is then pumped through the mainpipeline to the Afyon GDHS, located in the centre of the Afyon-karahisar province. The geothermal fluid is sent to the six heat plateexchangers in the geo-heat mechanical room of the Afyon GDHSand is cooled to about 45e50 �C. For the EDC, the hot water ispumped to the six heat exchangers and then the supply (flow)water is sent to the heat exchangers installed under all the build-ings in the zones. The mean supply/return water temperatures ofthe building cycle are 60/45 �C. In this study, the ECC for the AfyonGDHS was not considered. The actual operational data on temper-ature, pressure and flow rate of the system have been hourlyrecorded since 2006 by the technical staff based on the statenumbers specified in Fig. 1. The pressure and temperature data onthe fluids (including hot water and geothermal fluid) have beenmeasured with Bourdon-tube pressure gauges and fluid-expansionthermometers, respectively. The volumetric flow rates of fluidshave also been measured by an ultrasonic flow meter.

3. The specific exergy cost (SPECO) method and its evaluation

The exergoeconomic is a unique combination of exergy analysisand cost analysis conducted at the component level, to provide thedesigner or operator of an energy conversion system with infor-mation crucial to the design of a cost-effective system [29]. Therehave been numerous published papers all around the world onexergoeconomic cost analysis, and its application and optimizationin thermal systems since the 1990s. Most of them have been pub-lished due to the improved structural formalism of the exer-goeconomic methodologies [30]. Among these methodologies, thespecific exergy costing (SPECO) method introduced by Lazzarettoand Tsatsaronis [13] has been largely and successfully used andapplied to energy intensive systems by the researchers in the fieldof thermoeconomics [30]. It is a systematic methodology forcalculating exergy related costs in thermal systems [13]. The SPECOmethod was also applied in this study.

In a conventional economic analysis, a cost balance is usuallyformulated for the overall system operating at steady state [21], asfollowing

_CP;tot ¼ _CF;tot þ _Z (1)

where _C is the cost rate and _Z denotes sum of the capital invest-ment and operatingemaintenance costs in this study.

For _Z value in the economic analysis of thermal systems, theannual values of carrying charges, fuel costs, raw water costs, andOM (operatingemaintenance) expenses supplied to the overallsystem are the necessary input data. However, these cost compo-nents may vary significantly within the economic life. Therefore,the levelized annual values must be used in the economic analysisof the overall system. The levelized cost is given by Abusoglu [18]

C _Asys ¼ P _WsysCRF (2)

where CRF is capital recovery factor which depends on the interestrate as well as estimated equipment lifetime. CRF is determinedusing the following relation

CRF ¼ �ið1þ iÞn=ð1þ iÞn � 1

�(3)

1þ i ¼ ð1þ inÞ=1þ r (4)

where in, r, i and n mean nominal interest, inflation, real interestrates and lifetime of processes as year, respectively.

Fig. 1. Schematic diagram of the Afyon GDHS modified from Refs. [27,28].

M.A. Alkan et al. / Energy 60 (2013) 426e434428

The present worth of the system is given as follows [31]

P _Wsys ¼ _Csys � _SsysPWFði;nÞ (5)

where _Ssys is salvage cost and PWF denotes the present value factoras stated below

PWF ¼ 1=ð1þ iÞn (6)

Note that the levelized cost ðC _AsysÞ should be calculated sepa-rately for the capital investment cost and operatingemaintenancecost. Thus, these costs are respectively as follow

_ZCIsys ¼ C _Asys

s(7)

and

_ZOMsys ¼ C _Asys

s(8)

where s is the annual operating hour of system.

The annual levelized capital investment cost and operatingemaintenance cost in the component level of the system arerespectively obtained from

_ZCIk ¼ _Z

CIsys

PECkPsys

PECk(9)

and

_ZOMk ¼ _Z

OMsys

PECkPsys

PECk(10)

where PECk is the purchased equipment cost for kth component inthe system.

For a system operating at steady state, there may be a number ofentering and exitingmaterial streams as well as both heat andworkinteractions with the surroundings. Therefore; in exergy costing asthermoeconomic analysis, for entering and exiting streams ofmatter with associated rates of exergy transfer _Ein and _Eout, power_W and the exergy transfer rate associated with heat transfer _Eq, the

M.A. Alkan et al. / Energy 60 (2013) 426e434 429

costs associated with each exergy stream can be written respec-tively [4]

_Cin ¼ cin _Ein (11)

_Cout ¼ cout _Eout (12)

_Cw ¼ cw _W (13)

_Cq ¼ cq _Eq (14)

where c and subscript w denote unit cost and work.Thus, the exergy cost equation for the component level of the

system can express the following [4]

X_Cout;k þ _Cw;k ¼

X_Cin;k þ _Cq;k þ _Zk (15a)

or more specifically

X�cout _Eout

�kþ cw;k

_Wk ¼X�

cin _Ein�kþ cq;k _Eq;k þ _Zk

(15b)

for

_Zk ¼ _ZCIk þ _Z

OMk (16)

Due to the total number of exergy streams exiting the compo-nent being considered, there are equal number of unknowns andonly one equation; the exergy cost balance. Thus, sufficient numberof auxiliary equations should be formulated. This is accomplishedwith the aid of the F and P principles of the SPECO method [13]. Inthis approach, the F principle states that the specific cost (cost perexergy unit) associated with this removal of exergy from a fuelstream must be equal to the average specific cost at which theremoved exergy was supplied to the same stream in upstreamcomponents. The P principle also states that each exergy unit issupplied to any stream associated with the “product” at the sameaverage cost. Since each exiting exergy stream is associated eitherwith fuel or with the product, the total number of exiting streams(n) is equal to the sum of the number of exiting exergy streamsassociated with the “fuel” definition of a component (nF) and thenumber of exiting streams included in the “product” definition (nP)[24]. Thus, the F and P principles together provide the required(n � 1) auxiliary equations.

By solving the system of the known and unknown equations asshown by Kotas [32], the cost of unknown streams of the system isobtained. These are the average unit cost of fuel (cF,k), average unitcost of product (cP,k), cost rate of exergy destruction ( _CD,k), andcost rate of exergy loss ( _CL,k). Mathematically, these are expressedas [33]

cF;k ¼ _CF;k=_EF;k (17)

cP;k ¼ _CP;k=_EP;k (18)

_CD;k ¼ cF;k _ED;k (19)

_CL;k ¼ cF;k _EL;k (20)

where the subscripts P, F, D and L are associated with the product(exergetic product), fuel (exergetic fuel), exergy destruction andexergy loss of the overall system, respectively.

In an exergoeconomic evaluation, the relative cost differencebetween the average cost per exergy unit of product and fuel, andthe exergoeconomic factor used in making key decisions concern-ing the improvement of the system can be defined respectivelyas [4]

rk ¼ cP;k � cF;kcF;k

¼ cF;k _ED;k þ _ZkcF;k _EP;k

¼ 1� εk

εkþ

_ZkcF;k _EP;k

(21)

fk ¼_Zk

_Zk þ _CD;k¼

_Zk_Zk þ cF;k _ED;k

(22)

4. Results and discussion

In this study, an exergoeconomic approach is used to improvethe cost effectiveness according to exergy rates in a geothermaldistrict heating system (GDHS) as thermal system at the compo-nent level. This approach is the specific exergy costing (SPECO)method introduced by Lazzaretto and Tsatsaronis [13], and it can beused in this thermal system. Here, this approach is extended andapplied to a GDHS. As a real case study, the Afyon GDHS, located inthe city of Afyonkarahisar/Turkey, was considered. The actualoperational data obtained from the system on 8 February 2011werecollected at various state points of the system, as shown in Fig. 1.The reference environment conditions, in terms of P0 and T0 aretaken as 101.325 kPa and 275 K, respectively. For geothermal fluid,the thermodynamic properties of water are used.

First, it is defined input and output streams for all componentsof the conducted system. Therefore; the state temperatures andpressures, mass flow rates and total exergy rates of the varioussystem locations are listed in Table 1, based on the thermodynamicstates indicated in Fig. 1, for the chosen day. Exergy rates of thestreams are then calculated by using these values in Table 1. Theyare presented in Table 2 based on the data obtained from theexergetic analysis for all the components in the real Afyon GDHS. Ascan be seen from this table, the exergy destruction rate of2442.204 kW accounts for 18.46% of the total exergy input (exer-getic fuel) rate while the exergy loss rate of the system covers its52.25%, and the total exergetic product rate is 29.29%. Here, themost important components from the thermodynamic viewpointtake place in the HEXs (heat exchangers) and then PMs (pumps).

In the SPECO method, there are two main costs, which are CI(capital investment) and operatingemaintenance (OM) expenses.The levelized cost values of the carrying charges and expendituresof the Afyon GDHS are given in Table 3. Here, the cost data forpurchased-equipment and capital investment costs are obtainedfrom the company. The lifetime of the system and annual operationtime are basically considered as 20 years and 5040 h/year(24 h � 210 days during heating season for the Afyonkarahisarprovince) as the average capacity factor of %100 for the system,respectively. In the system, electricity is used for pumps andauxiliary devices. Thus, the interest (i) rate and the unit electricityprice are taken as 12% and 0.1363 $/kWh (as American Dollars)according to Turkey’s 2011 year status, respectively.

The system considered in this work consists of 15 componentsand has 48 streams as can be seen in Fig. 1. Therefore; the boundaryconditions and auxiliary equations for the cost flow rates and theunit exergetic costs are necessary. The equations corresponding toeach one of the components of the system are shown in Table 4,where the thermoeconomic balance given in Eq. (15a) is applied toeach component. The costs of input flows are known and consid-ered as constraints of the system. An additional set of 9 equations is

Table 2The exergetic variables for all the components in the real Afyon GDHS.

Components _EF,k (kW) _EP,k (kW) _ED,k (kW) _EL (kW)

Heat exchangersHEX-I 1371.946 1149.325 222.621 e

HEX-II 1470.526 1169.795 300.731 e

HEX-III 1498.104 1107.351 390.754 e

HEX-IV 1053.624 696.493 357.131 e

HEX-V 642.966 288.463 354.503 e

HEX-VI 414.733 216.347 198.386 e

Booster pumpPM-I 315.000 136.247 178.753 e

Circulation pumpsPM-II 90.000 54.962 35.038 e

PM-III 90.000 61.074 28.926 e

PM-IV 90.000 61.074 28.926 e

PM-V 50.000 45.394 4.606 e

PM-VI 50.000 26.133 23.867 e

PM-VII 40.000 19.600 20.400 e

Pump of pressurized water tankPM-VIII 40.000 0.000 40.000 e

Reinjection pumpPM-IX 315.000 57.436 257.564 e

Overall system 13,228.570 3874.924 2442.204 6911.443

Table 3The cost rates associated with the capital investment (CI) costs and the operatingand maintenance (OM) expenses for each component of the Afyon GDHS (asAmerican Dollars).

Components PEC(�103 US$)

_ZCIk

(US$/h)

_ZOMk

(US$/h)

_Ztotk

(US$/h)

Heat exchangersHEX-I 186.85 5.21 6.78 11.990HEX-II 186.85 5.21 6.78 11.990HEX-III 186.85 5.21 6.78 11.990HEX-IV 127.32 3.55 4.62 8.170HEX-V 127.32 3.55 4.62 8.170HEX-VI 71.77 2.00 2.61 4.605

Booster pumpPM-I 15.32 0.43 0.56 0.983

Circulation pumpsPM-II 3.87 0.11 0.14 0.248PM-III 3.87 0.11 0.14 0.248PM-IV 3.87 0.11 0.14 0.248PM-V 3.35 0.09 0.12 0.215PM-VI 2.28 0.06 0.08 0.146PM-VII 2.28 0.06 0.08 0.146

Pump of pressurized water tankPM-VIII 0.47 0.01 0.02 0.030

Reinjection pumpPM-IX 15.32 0.43 0.56 0.983

Total purchased-equipmentcost (PEC)

937.59 26.13 34.04 60.164

Purchased-equipmentinstallation

187.52

Piping 692.09Electrical costs 8.62Office costs 74.85Engineering/supervision/staff costs

173.85

Other outlay costs 84.62Total capital investment 1221.54Total cost 2159.13

Table 1The recorded and calculated thermodynamics variables at various system locationsfor the real Afyon GDHS.

State no. Fluid type T (�C) P (kPa) _m (g/s) _E (kW)

0 W 0.2 101.32 e e

1 TW 99.0 183.34 100.0 6090.1522 TW 96.0 127.56 40.0 2302.5943 TW 98.0 212.21 40.0 2391.5724 TW 93.0 83.40 45.0 2444.2525 TW 95.0 94.85 175.0 9879.2126 TW 95.7 799.30 175.0 10,015.4597 TW 93.0 70.56 175.0 9505.4238 TW 93.0 70.56 37.5 2036.8769 TW 51.0 48.87 37.5 664.93010 TW 93.0 70.56 38.8 2107.48811 TW 49.0 52.05 38.8 636.96212 TW 93.0 70.56 41.7 2265.00613 TW 52.0 46.45 41.7 766.90214 TW 93.0 70.56 27.8 1510.00415 TW 49.0 47.91 27.8 456.38016 TW 93.0 70.56 16.7 907.08917 TW 48.0 48.54 16.7 264.12318 TW 93.0 70.56 12.5 678.95919 TW 56.0 50.50 12.5 264.22620 W 47.7 645.24 125.0 1953.41221 W 47.0 331.23 125.0 1898.44922 W 61.0 660.56 125.0 3102.73723 W 47.7 635.45 138.9 2170.63124 W 47.0 350.67 138.9 2109.55725 W 60.0 650.90 138.9 3340.42626 W 49.7 625.45 138.9 2341.33327 W 49.0 370.20 138.9 2280.25928 W 61.0 660.56 138.9 3448.68429 W 49.7 580.67 97.2 1641.08330 W 49.0 400.54 97.2 1595.68931 W 60.0 610.89 97.2 2337.57732 W 52.7 590.43 55.6 1048.66933 W 52.0 500.40 55.6 1022.53634 W 60.0 600.65 55.6 1337.13235 W 52.7 555.34 41.7 786.50236 W 52.0 510.90 41.7 766.90237 W 60.0 560.00 41.7 1002.84938 W 12.4 220.45 10.0 11.79039 e e e e e

40 e e e e e

41 W 12.4 410.45 1.9 2.24042 W 12.4 410.45 2.8 3.30143 W 12.4 410.45 2.8 3.30144 W 12.4 410.45 2.5 2.94845 TWR 50.0 70.56 175.0 2982.82146 TWNDD 50.0 70.56 52.8 899.96047 TWR 50.0 70.56 122.2 2082.86148 TWR 50.7 800.45 122.2 2140.298

M.A. Alkan et al. / Energy 60 (2013) 426e434430

required to solve the overall number of flows in the system. Thus, aset of auxiliary equations is formulated considering F and P prin-ciples [8,13]. These additional equations are found in the last col-umn of Table 4.

Solving the linear system consisting of related exergoeconomicequations obtained from the SPECOmethod, the cost flow rates andthe unit exergetic costs associated with each stream of the AfyonGDHS are obtained. The thermodynamic properties along with thecost flow rates and unit costs at various state points of the systemare summarized in Table 5 for the chosen day.

First in all; sensitivity analysis is a general concept which aims toquantify the variations of an output parameter of a systemregarding to changes imposed on some input important parameters[34]. A comprehensive sensitivity analysis is performed to examinethe impact of the variation of important factors on heat generationcosts. As shown in Fig. 2, increasing the system lifetime from 15 to35 years with 12% interest rate and keeping other economic andtechnical parameters constant will reduce heat generation cost by

1.36%. Fig. 3 presents variation of unit cost with operating loadwhich increases considerably the unit cost of heat produced byHEXs. By increasing operating load of the system from 25% to 100%,the unit cost of heat produced by HEXs increases about 19.75%.

Here, an important result of the exergoeconomic analysis isthe correlation of exergy destruction rate with costs. The exergy

Table 4Exergetic cost rate balances and auxiliary equations for the components of the Afyon GDHS based on the state numbers specified in Fig. 1.

Components Exergetic cost rate balance equations Auxiliary equations

HEX-I _C8 þ _C20 þ _Zk;HEX�I ¼ _C9 þ _C22 c8 ¼ c9HEX-II _C10 þ _C23 þ _Zk;HEX�II ¼ _C11 þ _C25 c10 ¼ c11HEX-III _C12 þ _C26 þ _Zk;HEX�III ¼ _C13 þ _C28 c12 ¼ c13HEX-IV _C14 þ _C29 þ _Zk;HEX�IV ¼ _C15 þ _C31 c14 ¼ c15HEX-V _C16 þ _C32 þ _Zk;HEX�V ¼ _C17 þ _C34 c16 ¼ c17HEX-VI _C18 þ _C35 þ _Zk;HEX�VI ¼ _C19 þ _C37 c1 ¼ c2 ¼ c3 ¼ c4 ¼ c5PM-I _C5 þ _CW;PM�I þ _Zk;PM�I ¼ _C6 c6 ¼ c7PM-II _C21 þ _CW;PM�II þ _Zk;PM�II ¼ _C20PM-III _C24 þ _CW;PM�III þ _Zk;PM�III ¼ _C23 c38 ¼ c39 ¼ c40 ¼ c41 ¼ c42 ¼ c43 ¼ c44PM-IV _C27 þ _CW;PM�IV þ _Zk;PM�IV ¼ _C26 c9 ¼ c11 ¼ c13 ¼ c15 ¼ c17 ¼ c19 ¼ c45 ¼ c46 ¼ c47PM-V _C30 þ _CW;PM�V þ _Zk;PM�V ¼ _C29PM-VI _C33 þ _CW ;PM�VI þ _Zk;PM�VI ¼ _C32PM-VII _C36 þ _CW;PM�VII þ _Zk;PM�VII ¼ _C35PM-VIII _C38 þ _CW;PM�VIII þ _Zk;PM�VIII ¼ _C39PM-IX _C47 þ _CW;PM�IX þ _Zk;PM�IX ¼ _C48

Table 5The cost flow rates and the unit exergetic costs associated with each stream of thereal Afyon GDHS.

Stateno.

Fluidtype

T (�C) P (kPa) _m (kg/s) _E (kW) c ($/GJ) _C ($/h)

0 W 0.2 101.32 e e

1 TW 99.0 183.34 100.0 6090.152 46.39 1017.082 TW 96.0 127.56 40.0 2302.594 46.39 384.543 TW 98.0 212.21 40.0 2391.572 46.39 399.404 TW 93.0 83.40 45.0 2444.252 46.39 408.205 TW 95.0 94.85 175.0 9879.212 46.39 1649.876 TW 95.7 799.30 175.0 10,015.459 45.26 1631.887 TW 93.0 70.56 175.0 9505.423 45.26 1548.788 TW 93.0 70.56 37.5 2036.876 45.26 331.889 TW 51.0 48.87 37.5 664.930 45.26 108.3410 TW 93.0 70.56 38.8 2107.488 45.26 343.3911 TW 49.0 52.05 38.8 636.962 45.26 103.7812 TW 93.0 70.56 41.7 2265.006 45.26 369.0513 TW 52.0 46.45 41.7 766.902 45.26 124.9614 TW 93.0 70.56 27.8 1510.004 45.26 246.0315 TW 49.0 47.91 27.8 456.380 45.26 74.3616 TW 93.0 70.56 16.7 907.089 45.26 147.8017 TW 48.0 48.54 16.7 264.123 45.26 43.0418 TW 93.0 70.56 12.5 678.959 45.26 110.6319 TW 56.0 50.50 12.5 264.226 45.26 43.0520 W 47.7 645.24 125.0 1953.412 23.94 168.3521 W 47.0 331.23 125.0 1898.449 25.31 172.9822 W 61.0 660.56 125.0 3102.737 25.31 282.7123 W 47.7 635.45 138.9 2170.631 26.00 203.1724 W 47.0 350.67 138.9 2109.557 27.36 207.7825 W 60.0 650.90 138.9 3340.426 27.36 329.0226 W 49.7 625.45 138.9 2341.333 33.47 282.1127 W 49.0 370.20 138.9 2280.259 34.92 286.6628 W 61.0 660.56 138.9 3448.684 34.92 433.5429 W 49.7 580.67 97.2 1641.083 33.49 197.8630 W 49.0 400.54 97.2 1595.689 35.13 201.8031 W 60.0 610.89 97.2 2337.577 35.13 295.6332 W 52.7 590.43 55.6 1048.669 3.45 13.0233 W 52.0 500.40 55.6 1022.536 2.81 10.3434 W 60.0 600.65 55.6 1337.132 2.81 13.5335 W 52.7 555.34 41.7 786.502 26.97 76.3636 W 52.0 510.90 41.7 766.902 28.63 79.0437 W 60.0 560.00 41.7 1002.849 28.63 103.3638 W 12.4 220.45 10.0 11.790 18.56 0.7939 e e e e e e

40 e e e e e e

41 W 12.4 410.45 1.9 2.240 33.49 0.2742 W 12.4 410.45 2.8 3.301 33.47 0.4043 W 12.4 410.45 2.8 3.301 26.00 0.3144 W 12.4 410.45 2.5 2.948 23.94 0.2545 TWR 50.0 70.56 175.0 2982.821 45.26 486.0146 TWNDD 50.0 70.56 52.8 899.960 45.26 146.6447 TWR 50.0 70.56 122.2 2082.861 45.26 339.3748 TWR 50.7 800.45 122.2 2140.298 46.39 357.44

M.A. Alkan et al. / Energy 60 (2013) 426e434 431

destruction cost rate is calculated at the component level, andcompared to the respective investment cost rates. The componentsare then conducted depending on their total operating cost rate( _CD,k þ _Zk), which consists of their capital investment and exergydestruction cost rates. The higher this total operating cost rate, thehigher the influence of the component on the overall system andthus, the more significant the component is considered. As can beseen in Table 6, the largest sum of exergy destruction and capitalcost rate ( _CD,kþ _Zk) are observed in the HEX-III (75.66 $/h), followedby the HEX-II, HEX-I, HEX-IV and HEX-V, respectively. This is relatedto the large exergy destruction in these components. The high totaloperating cost rate ( _CD,k þ _Zk) and relative cost difference (rk) of theHEX-III suggest that this component should be improved byreducing the exergy being destroyed within it. Large relative costdifference is also observed in the circulation pumps (see Table 6).For example; the total operating cost rate in the PM-V is very low.The contribution of the pump to the total cost of the system isinsignificant. In addition, the low exergoeconomic factor (fk) ofthe HEX-III indicates that the increase of capital costs of thesecomponents would be justified if better efficiency was achieved.

Table 6The unit exergy costs of fuels and products, relative exergetic cost difference,exergoeconomic factor, cost rate of exergy destruction, and total investment costrate for the plant components.

Components cF,k($/GJ)

cP,k($/GJ)

_CD,k

($/h)

_Zk ($/h) _CD,k þ _Zk($/h)

rk (%) fk (%)

Heat exchangersHEX-I 39.06 49.52 31.30 11.99 43.29 26.79 27.69HEX-II 41.68 55.24 45.12 11.99 57.11 32.54 20.99HEX-III 45.26 64.24 63.67 11.99 75.66 81.93 15.85HEX-IV 25.88 42.41 33.27 8.17 41.44 63.87 19.71HEX-V 23.27 59.73 29.70 8.17 37.87 61.04 21.58HEX-VI 19.38 43.06 13.84 4.61 18.45 55.00 24.97

Booster pumpPM-I 37.86 89.54 24.36 0.98 25.34 71.72 3.88

Circulation pumpsPM-II 37.86 63.25 4.78 0.25 5.03 67.06 4.94PM-III 37.86 56.92 3.94 0.25 4.19 50.34 5.92PM-IV 37.86 56.92 3.94 0.25 4.19 50.34 5.92PM-V 37.86 43.02 0.63 0.22 0.85 13.62 25.52PM-VI 37.86 73.99 3.25 0.15 3.40 65.44 4.31PM-VII 37.86 79.34 2.78 0.15 2.93 52.28 5.00

Pump of pressurized water tankPM-VIII 37.86 0.00 5.45 0.03 5.48 58.29 0.55

Reinjection pumpPM-IX 37.86 212.39 35.10 0.98 36.08 72.17 2.72

Overall system 116.84 403.19 1027.25 60.16 1087.41 71.02 5.53

_CL,tot ¼ 2907.12 $/h for the overall system.

Fig. 2. Relation between average unit cost of heat produced by HEXs and systemlifetime, for various interest rate.

Fig. 4. Effect of the average well head temperature on the unit cost rate of the heatproduced by HEXs.

M.A. Alkan et al. / Energy 60 (2013) 426e434432

In addition, an increase in its economic efficiencies will improve theeffectiveness of a thermal system. For the Afyon GDHS, this can bedone by reducing the pressure drop taking place during its opera-tion and increasing the temperature difference in all the HEXs.

The cost per exergy unit of some components of the Afyon GDHS(especially HEXs) for parameter such as well head and ambienttemperatures are given in Figs. 4 and 5. These parameters indicatethe economic performance of the system on a rational exergeticcost basis. One is noted the followings from the exergoeconomicresults of the Afyon GDHS as shown through Figs. 4 and 5.

The effect of the average well head temperature on the unit costrate of the heat produced by HEXs is presented in Fig. 4. It can beobserved that the unit cost rates of the heat production of all HEXsdecrease as this temperature increases. Considering that the costrate of the heat production is a direct function of the well heattemperature, therefore, as the exergy input increases as well theexergy losses decrease, the cost rate of the heat production de-creases. This figure shows that the cost rate of the HEX-III is slightlyhigher than the other HEXs. The reason for that is the high exergydestruction cost rate of the system, as well as the cost of thegeothermal fluid for the Afyon GDHS.

Fig. 3. Variation of average unit cost of heat produced by HEXs with operating load.

The cost per exergy unit of the heat production of HEXsaccording to ambient temperature is illustrated in Fig. 5. It can benoticed that the heat production costs per exergy unit for all theHEXs decrease as ambient temperature increases. This decrease isattributed to the decrease in the exergy efficiency. The cost perexergy unit of the Afyon GDHS is around 52.37 $/GJ. Alternatively,the average costs per exergy unit of the heat production are around60.67 $/GJ for the HEX-III, 56.16 $/GJ for the HEX-V, and 51.67 $/GJfor the HEX-II. It should be noted that if the HEXs in the Afyon GDHSwere designed better, they would have higher effectiveness andmore energy would be recovered. This would increase the capitalcost, decrease the exergy destruction in the HEXs, and decrease theheat production cost as a result.

5. Conclusions

The developed thermoeconomic analysis procedure and for-mulations based on the specific exergy costing (SPECO) method areapplied to an existing geothermal district heating system (GDHS)(namely, the Afyon GDHS) using actual operational data. The results

Fig. 5. Variation with ambient temperature of the unit cost rate of the produced heatfor HEXs.

M.A. Alkan et al. / Energy 60 (2013) 426e434 433

from exergoeconomic analysis provide important cost-basedinformation, suggesting possible locations/components for thesystem improvement. Thus, this study can be reached the followingconclusions:

� The cost accounting results show that the unit cost of heatingfrom geothermal water in the Afyon GDHS is average 5624 $/hat 100% load conditions.

� Increasing the system lifetime will reduce unit cost of heatproduction by HEXs about 1.36%.

� Increasing operating load of the system from 25% to 100% re-duces the unit cost of heat produced by HEXs about 19.75%.

� The SPECO method is used to improve the cost effectivenessaccording to exergy rates. The specific exergetic fuel andproduct unit costs for the HEX-III are 244 $/h and 256 $/h,respectively. This difference is mainly due to the fact that it hasthe lowest exergetic efficiency among all HEXs. It has also oneof the lowest exergoeconomic factors in here.

� Due to the high total operating cost rate and relative cost dif-ference of the HEX-III, this component should be improved byreducing the exergy being destroyed within it. The maximumexergy destruction rate is due to the HEX-III as 390.754 kW. Thecost per exergy unit of heat produced by HEX-III is 64.24 $/GJ atthe outlet of the HEX-III.

� The HEX-I and PM-V have the highest exergoeconomic factorsamong all other plant components. This is mainly due to thehigh owning and operating costs of these components.

� The unit cost rates of the heat production of all HEXs decreaseas average 11.86% due to the high exergy destruction cost rateof the system, when well head temperature increases.

� The heat production costs per exergy unit for all the HEXsdecrease due to the decreased exergy efficiency, when ambienttemperature increases.

� The HEXs in the Afyon GDHS would have higher effectivenessand more energy would be recovered, if they were designedbetter.

This study should be given an opinion on future improvementsfor a thermal system (for a GDHS in this study) by decreasing theunit exergy cost of the investigated systems and checking theexergy consumption locations within the system.

Acknowledgements

The authors would like to thank for the support provided by theAfyon Geothermal Inc., Turkey.

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Glossary

c: cost per exergy unit ($/GJ)_C: unit cost rate ($/h)C _A: annual levelized cost ($/yr)_E: exergy rate (kJ/s or kW)f: exergoeconomic factor (%)i: interest (%)

M.A. Alkan et al. / Energy 60 (2013) 426e434434

_m: mass flow rate (kg/s)n: number of componentP: pressure (kPa)P _W: present worth ($)r: relative cost difference (%)_S: salvage cost ($)T: temperature (�C or K)_W: work rate or power (kJ/s or kW)_Z: the sum of the CI and OM costs ($/h)

Greek symbolsε: exergy/exergetic or second law efficiency (%)s: annual operating hour (h)

SubscriptsD: destructionF: fuelin: inletk: componentL: lossout: outletP: productq: heatsys: systemtot: total/overall

w: work0: reference state

SuperscriptsOver dot: quantity per unit timeCI: capital investmentOM: operatingemaintenance

AbbreviationsCHP: combined heat and powerCI: capital investmentCRF: capital recovery factorECC: energy consumption cycleEDC: energy distribution cycleEPC: energy production cycleGDHS: geothermal district heating systemHEX: heat exchangerOM: operatingemaintenancePEC: purchased equipment costPM: pumpPWF: present worth factorSPECO: specific exergy costingTW: thermal waterTWNDD: thermal water natural direct dischargeTWR: thermal water reinjectionW: water