Determination of optimum pipe diameter along with energetic and exergetic evaluation of geothermal district heating systems: Modeling and application

Energy and Buildings 40 (2008) 742–755 www.elsevier.com/locate/enbuild

Determination of optimum pipe diameter along with energetic and exergetic evaluation of geothermal district heating systems: Modeling and application Yildiz Kalinci a, Arif Hepbasli b,*, Ismail Tavman c a

Plumbing Technology, Department of Technical Programs, Izmir Vocational School, Dokuz Eylu¨l University, Education Campus Buca, Izmir, Turkey b Department of Mechanical Engineering, Faculty of Engineering, Ege University, 35100 Bornova, Izmir, Turkey c Department of Mechanical Engineering, Faculty of Engineering, Dokuz Eylu¨l University, 35100 Bornova, Izmir, Turkey Received 29 March 2007; received in revised form 10 May 2007; accepted 14 May 2007

Abstract This study deals with determination of optimum pipe diameters based on economic analysis and the performance analysis of geothermal district heating systems along with pipelines using energy and exergy analysis methods. In this regard, the Dikili geothermal district heating system (DGDHS) in Izmir, Turkey is taken as an application place, to which the methods presented here are applied with some assumptions. The system mainly consists of three cycles, namely (i) the transportation network, (ii) the Danistay region, and (iii) the Bariskent region. The thermal capacities of these regions are 21,025 and 7975 kW, respectively, while the supply (flow) and return temperature values of those are 80 and 50 8C, respectively. Based upon the assessment of the transportation network using the optimum diameter analysis method, minimum cost is calculated to be US$ 561856.906 year1 for a nominal diameter of DN 300. The exergy destructions in the overall DGDHS are quantified and illustrated using exergy flow diagram. Furthermore, both energy and exergy flow diagrams are exhibited for comparison purposes. It is observed through analysis that the exergy destructions in the system particularly take place due to the exergy of the thermal water (geothermal fluid) reinjected, the heat exchanger losses, and all pumps losses, accounting for 38.77%, 10.34%, 0.76% of the total exergy input to the DGDHS. Exergy losses are also found to be 201.12817 kW and 1.94% of the total exergy input to the DGDHS for the distribution network. For the system performance analysis and improvement, both energy and exergy efficiencies of the overall DGDHS are investigated, while they are determined to be 40.21% and 50.12%, respectively. # 2007 Elsevier B.V. All rights reserved. Keywords: Geothermal energy; Efficiency; Energy analysis; Exergy analysis; District heating system; Optimum pipe diameter

1. Introduction After the second half of the 20th century, the industrial revolution increased the utilization of the new technological products in our daily live. This caused more consumption of the energy and made it an inseparable part of the life. Moreover, the rate of energy consumption has become a criterion of success in the development for countries [1]. Fossil fuels provide the most part of the energy sources of the whole World. After 1973–1979 oil crises, people have resourced about alternative energy sources beside fossil fuels. One of them is geothermal energy. Geothermal systems are simple, safe and

* Corresponding author. Tel.: +90 232 3888562/5124; fax: +90 232 3888562. 0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2007.05.009

adaptable systems with modular plants (up to 50 MW) capable of providing continuous base load, load following or peaking capacity and benign environmental attributes (negligible emissions of CO2, SOx, NOx and particulates and modest land and water use). Because these features are compatible with sustainable growth of global energy supplies in both developed and developing countries, geothermal energy then becomes an attractive option to replace fossil fuels [2]. Geothermal energy is considered a renewable energy source and its projected life is about 30–50 years. There are electrical and non-electrical (direct utilization) utilizations of geothermal energy in Refs. [3,4]. Geothermal district heating has recently been given increasing attention in many countries, and numerous successful geothermal district heating projects have been

Y. Kalinci et al. / Energy and Buildings 40 (2008) 742–755

Nomenclature A b B c C D E˙ EN Ex ˙ Ex f F˙ h I˙ IP˙ k L m m ˙ n N Nu P P˙ Pr Re s S˙ SExI t T TEI TExI q0 ˙ Q V ˙ W z

section area (m2) investment cost factor characteristic running coefficient cost per unit (US$) cost (US$/kWh) diameter (m) energy rate (kW) element numbers (dimensionless) exergy (kJ) exergy rate (kW) friction factor (dimensionless) or exergetic factor (%) exergetic fuel rate (kW) specific enthalpy (kJ/kg) irreversibility (exergy destruction) rate (kW) improvement potential rate (kW) thermal conductivity (W/mK) length of pipe items (m) cost index (dimensionless) mass flow rate (kg/s) the number of existing pieces pumping power (kW) Nusselt number (dimensionless) pressure (N/m2) exergetic product rate (kW) Prandtl number (dimensionless) Reynolds number specific entropy (kJ/kgK) entropy rate (kW/K) specific exergy index (dimensionless) wall thickness (m) temperature (8C or K) total energy input (kW) total exergy input (kW) heat loss per one meter (W/m) heat transfer (thermal energy) rate (kW) characteristic loss coefficient (dimensionless) work rate, power (kW) annual running time (h/year)

Greek letters D interval x relative irreversibility (%) d fuel depletion rate (%) e exergy or exergetic or second law efficiency h energy or first law efficiency l fraction loss (dimensionless) r density (kg/m3) v velocity (m/s) j productivity lack (%) c flow exergy (kJ/kg) z particular resistance loss Subscripts A annual

d dest E EI gen HE i in k opt out r R Tot w 0 1 2

743

natural direct discharge destroyed energy equipment and installation generation heat exchanger each unit value inlet location optimum outlet reinjected geothermal fluid running total well-head reference index initial state final state

Superscripts 0 for per one meter  rate

reported. Experience by researchers and engineers still plays an important role in the system analysis, design and control. Especially the installation costs of pipelines in geothermal district heating systems as well as in heating, ventilating and air-conditioning (HVAC) industry account for a significant part of the whole system costs. Therefore, an optimization calculation is vital in terms of economic [5–7] and exergetic [8] aspects. Exergy analysis is a method utilizing the conservation of mass and conservation of energy principles together with the second law of thermodynamics for the design and analysis of energy systems. The exergy method can be suitable for furthering the goal of more efficient energy-resource use, for it enables the locations, types, and true magnitudes of wastes and losses to be determined. Therefore, exergy analysis can be very useful whether or not and by how much it is possible to design more efficient energy systems by reducing the inefficiencies in existing systems [9,10]. Dincer [9] applied exergy analysis method to thermal energy storage systems in buildings. Gunerhan and Hepbasli [11] performed an exergetic assessment of solar water heating systems for buildings. Although energy and exergy analyses of some geothermal district heating systems in Turkey have also been reported in the companion works conducted by one of the co-authors [12–15], no studies on exergetic assessment of geothermal district heating systems along with their pipelines have appeared in the open literature to the best of the authors’ knowledge. In this regard, the present study differs from the previously performed ones due to the facts that (i) this study primarily performs an optimum diameter analysis and (ii) this includes energy and exergy analysis of a heat center along with three district heating networks, of which actual values may be obtained from Ref. [6].

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2. System description Dikili is at the North of Izmir, Turkey. It is located 120 km far from the city center and its surface measurement and population are 541 km2 and 30,115, respectively [6]. Geothermal investigations were started in 1939 after a big earthquake. Since then, many studies have been done in Dikili, while a lot of geothermal wells have been drilled, but haven not been utilized until the year of 2005. In the beginning of 2005, there were three wells drilled in the Kaynarca region. One of them has been used for greenhouses. The other two wells were not in operation at the date studied since the DGDHS has not been installed yet. The average values for depth, well head temperature and mass flow rate of the two wells are 356.5 m, 99.5 8C and 43.6 kg/s, respectively. Based on the preliminary studies conducted in the Dikili geothermal field, the Dikili municipality has decided to install the DGDHS with 4000 residences equivalence project. The outdoor and indoor design temperatures are 3 and 22 8C, respectively. The existing mass flow rate was not enough for the residences considered. Therefore, it is expected to have enough geothermal mass flow rates from the Kaynarca region, which is abundant due to geothermal resources. Fig. 1 illustrates a schematic of the DGDHS with the supply, return temperatures and mass flow rates. This consists mainly of four cycles, namely (i) energy production cycle (geothermal well and geothermal heating center loop), (ii) transportation cycle (energy transported from the Kaynarca region to the city center, it is 5.5 km), (iii) energy consumption cycle of 2900 residences equivalence the Danistay region and (iv) energy consumption cycle of 1100 residences equivalence the Bariskent region. 3. System analysis Large investments are needed to set up geothermal district heating systems. Therefore, the financial analysis has to be

made for every project. It is vitally important to have an optimum design to keep the setting up and running costs at optimum levels [5–7]. The optimum diameter selection method is one of such optimization methods. In this study, a transportation cycle is selected and optimum pipe diameter method is applied to this cycle. Exergy analysis is an effective thermodynamic method for using the conservation of mass and conservation of energy principles together with the second law of thermodynamics for the design and analysis of thermal systems, and is an efficient technique for revealing whether or not and by how much it is possible to design more efficient thermal systems by reducing the inefficiencies [13,16]. 3.1. Determination of optimum pipe diameter There are various calculation methods used to determine optimum pipe diameters in HVAC systems. In the following, one of them is given [5,7]. For every situation, additional pumping power (N) related to pressure loss is written as follows: N¼

DPm ˙ rh103

(1)

where DP is the pressure loss, m ˙ the mass flow rate, r the density and h is the efficiency. Annual running costs (or energy costs) can be calculated from: C R ¼ NzC E ;

(2)

where z is the annual running time and CE is the unit energy cost. Equipment-installation cost (CEI) can be found using:  mi X D C EI ¼ b ni c0i ; (3) D0

Fig. 1. A schematic diagram of the Dikili geothermal district heating system, where the values for the supply/return temperatures and mass flow rate are also included.

Y. Kalinci et al. / Energy and Buildings 40 (2008) 742–755

where b is the investment cost factor (interest, amortization and the other costs related with the investment), n the number of existing pieces (for example; pipe length, fittings, armatures, welded joints and others), c0i the cost per unit, D the pipe diameter and m is the cost index while the subscripts 0 and i stand for the reference and the each unit value, respectively. Annual cost is a sum of the equipment-installation costs and pumping energy cost, as given below: C A ¼ C EI þ C R ; where CR is written as follows:   DPmzC ˙ E CR ¼  103 rh

v¼



 lL X þ z ; D

DP ¼

0:81m ˙2 4 rD

 lL X þ z ; D

(8)

By inserting D0 into Eq. (9), the following becomes:   4  D zC E lL D0 X ˙3 3 m C R ¼ 0:81  10 þ z : D0 D h r2 D4 D0 (10) The optimum diameter (Dopt) obtained by minimizing the total cost is given as follows:

Dopt

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 3 3 m ˙ zC E ð5ðlL=DÞ þ 4 zÞ m0 þ4 0:81  10 P ¼ : hr2 D40 b ni mi c0i ðD=D0 Þmi m0

(11)

(12)

with zm ˙3 ; hr2

(13d)

For a general steady-state, steady-flow process, the three balance equations, namely mass, energy and exergy balance equations, are employed to find the heat input, the rate of exergy decrease, the rate of irreversibility, and the energy and exergy efficiencies. In general, the mass balance equation can be expressed in the rate form as: X X m m (14) ˙ in ¼ ˙ out ; where m ˙ is the mass flow rate, and the subscript in stands for inlet and out for outlet. The general energy balance can be expressed below as the total energy input equal to total energy output ðE˙ in ¼ E˙ out Þ, with all energy terms as follows: X X ˙ þ ˙ þ Q m m (15) ˙ in hin ¼ W ˙ out hout ; ˙ in  Q ˙ out is the rate of net heat input, ˙ ¼Q ˙ net;in ¼ Q where Q ˙ ¼W ˙ net;out ¼ W ˙ out  W ˙ in the rate of net work output, and h is W the specific enthalpy. Assuming no changes in kinetic and potential energies with no heat or work transfers, the energy balance given in Eq. (15) can be simplified to flow enthalpies only: X X m m (16) ˙ in hin ¼ ˙ out hout : The general exergy rate balance can be expressed as follows: ˙ work þ Ex ˙ mass;in  Ex ˙ mass;out ¼ Ex ˙ dest ˙ heat  Ex Ex

To simplify Eq. (12), the reference diameter DN 200 (D0 = 0.2 m) is replaced by the reference nominal diameter and the average cost index is taken to be m0 = 1.6 [5]. The optimum pipe diameter may be simplified as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P V 5:6 C E B P ; (13) Dopt ¼ 0:177 b EN

B¼

(13c)

3.2. General mass, energy and exergy balance equations

where v is the velocity, l the fraction losses, z the particular resistance losses and L is the pipe length. Pumping energy cost may be rearranged as follows:   m ˙ 3 zC E lL X C R ¼ 0:81  103 2 4 þ z : (9) D r D h

@C ¼ 0; @ðD=D0 Þ



(5)

(7) 

mi 1:6 D ENi ¼ ni mi c0i ; 0:2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 5:6 0:81  10 ; 0:177 ¼ 0:2 ð0:2Þ4

(13b)

where B is the characteristic running coefficient, V the characteristic loss coefficient and EN is element numbers.

(6)

4m ˙ ; prD2

745

(4)

with rv2 DP ¼ 2

X  X lL z ; V ¼ 5 þ4 D 

(13a)

(17)

and more explicitly,  X X X T0 ˙ ˙ dest ; ˙ þ m m Qk  W 1 ˙ in cin  ˙ out cout ¼ Ex Tk (18) ˙ k is the heat transfer rate crossing the boundary at where Q ˙ the work rate, C the flow exergy, temperature Tk at location k, W h the enthalpy, s the entropy, and the subscript 0 indicates properties at the restricted dead state of P0 and T0. The specific exergy and exergy rate equations for the geothermal fluid flow system can be defined as: c ¼ ðh  h0 Þ  T 0 ðs  s0 Þ; and ˙ ¼ m½ðh Ex  h0 Þ  T 0 ðs  s0 Þ: ˙

(19)

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Y. Kalinci et al. / Energy and Buildings 40 (2008) 742–755

For exergy destruction (or irreversibility), the following may be written: ˙ dest ¼ T 0 S˙ gen : I˙ ¼ Ex

(20)

3.3. Energy and exergy efficiencies Basically, the energy efficiency of the system can be defined as the ratio of total energy output to total energy input: E˙ out h¼ ; E˙ in

(21)

where in most cases output refers to useful one. Here, in a similar way we define exergy efficiency as the ratio of total exergy output to total exergy input,

xi ¼

I˙i ; I˙Tot

(22)

ji ¼

I˙i ; P˙ Tot

(29)

Exergetic factor :

fi ¼

F˙ i ; F˙ Tot

(30)

3.5. Irreversibilities due to heat transfer and fluid friction In geothermal district heating systems, heat distribution lines are very long. There are two types of losses due to the heat transfer and fluid friction irreversibilities [7]. Entropy generation is calculated as [22]:

Geothermal resources can be classified to reflect their ability to do thermodynamic work. In this regard, Lee [17] proposed a new parameter, namely specific exergy index (SExI) for better classification and evaluation as follows: hbrine  273:16sbrine SExI ¼ ; 1192

(23)

which is a straight line on an h–s plot of the Mollier diagram. Straight lines of SExI = 0.5 and SExI = 0.05 can therefore be drawn in this diagram and used as a map for classifying geothermal resources by taking into account the following criteria:  SExI < 0.05 for low-quality geothermal resources;  0.05  SExI < 0.5 for medium-quality geothermal resources;  SExI  0.5 for high-quality geothermal resources. In order to map any geothermal field on the Mollier diagram as well as to determine the energy and exergy values of the geothermal brine, the average values for the enthalpy and entropy are then calculated from the following equations: Pn ˙ wi hwi i¼1 m hbrine ¼ P ; (24) n ˙ wi i¼1 m Pn ˙ wi swi i¼1 m : (25) sbrine ¼ P n ˙ wi i¼1 m Van Gool [20] proposed the exergetic improvement potential rate by the relation: ˙ out Þ: ˙ in  Ex IP˙ ¼ ð1  eÞðEx

(26)

Some other thermodynamic parameters are given as follows [21]: I˙i di ¼ ; ˙FTot

(27)

(31)

with 0 S˙ gen;DT ¼

3.4. Specific exergy index (SExI)

(28)

Productivity lack :

0 0 0 S˙ gen ¼ S˙ gen;DT þ S˙ gen;DP

˙ out Ex e¼ ˙ in Ex

Fuel depletion ratio :

Relative irreversibility :

q0 2 ; pkT 2 NuðReD ; PrÞ

(31a)

and 32m ˙ 3 f ðReD Þ 0 : S˙ gen;DP ¼ 2 2 p r T D5

(31b)

where q0 is the heat loss per one meter length, f the friction factor and Nu is Nusselt number, which are calculated from, respectively, mðh ˙ 2  h1 Þ ; L

(32a)

f ¼ 0:046Re0:2 D ;

(32b)

q0 ¼

0:4 Nu ¼ 0:023Re0:8 D Pr :

(32c)

3.6. Energy, exergy and efficiency relations for the system considered Energy, exergy and efficiency relations for the system components and whole system are presented below [12,13]. For the overall geothermal system, the mass balance equation is written as follows: n X

m ˙ w;Tot  m ˙r m ˙d ¼ 0

(33)

i¼1

where m ˙ w;Tot is the total mass flow rate at wellhead, m ˙ r the flow rate of the reinjected geo-fluid and m ˙ d is the mass flow rate of the natural direct discharge and in the DGDHS, m ˙ d is zero. The geothermal brine energy and exergy inputs from the production field of the DGDHS are calculated from the following equations: E˙ brine ¼ m ˙ w ðhbrine  h0 Þ

(34)

˙ brine ¼ m Ex ˙ w ½ðhbrine  h0 Þ  T 0 ðsbrine  s0 Þ:

(35)

Y. Kalinci et al. / Energy and Buildings 40 (2008) 742–755 Table 1 Main data for the transportation network Pipe equipment m ˙ r z CE l z for 908 bend z for 458 bend

St 37 isolated steel pipe 230.7 kg/s 977.6 kg/m3 4512 h/year 0.0886 US$/kWh [18] 0.01 0.51 [19] 0.4 [19]

The exergy destructions in the heat exchanger, pump and the system itself are calculated using: ˙ dest;HE ¼ Ex ˙ in  Ex ˙ out ¼ Ex ˙ dest ; Ex

(36)

˙ out  Ex ˙ in Þ ˙ dest;pump ¼ W ˙ pump  ðEx Ex

(37)

and ˙ dest;system ¼ Ex

X

˙ dest;HE þ Ex

X

˙ dest;pump : Ex

(38)

Based upon Eq. (21), the energy efficiency of the DGDHS is calculated from: hsystem ¼

E˙ useful;HE : E˙ brine

(39)

The exergy efficiency of a heat exchanger is determined by the increase in the exergy of the cold stream divided by the decrease in the exergy of the hot stream on a rate basis as follows: eHE ¼

m ˙ cold ðccold;out  ccold;in Þ : m ˙ hot ðchot;in  chot;out Þ

(40)

Using Eq. (22), the exergy efficiency of the DGDHS is calculated from one of the following equations: esystem ¼

˙ reinjected ˙ dest;system þ Ex ˙ useful;HE Ex Ex ¼1 : ˙ brine ˙ brine Ex Ex

(41)

4. Results and discussion 4.1. Determination of optimum pipe diameter In this study, the optimum diameter method is applied to the transportation network, which was already installed. The pipe

747

material was made of isolated steel pipe. The main data for this network are given in Table 1. When the transportation network is analyzed using economical velocity method, the values for the velocity, pressure loss and diameter are found to be 3.18 m/ s, 966.193 kPa and DN 300, respectively. For DN 350, the velocity is calculated to be 2.34 m/s and the district loss is obtained to be 461.019 kPa [6]. In calculating Dopt, the following assumptions are made: (a) The costs of pipe, fittings and their installation along with welding costs are taken into account, while those of other elements are ignored. (b) Pipe costs are taken from the relevant manufacturers’ specification, while installation and welding costs are estimated (pipe installation and bent installation are assumed to be 60% of pipe equipment and 22% of bent costs, respectively). (c) m0 = 1.6 is assumed as a cost index for the reference diameter D0 = 0.2 m [5]. (d) Investment cost factor b: US$ 0.24/US$-year [5]. The required data for the transportation network are calculated using Eqs. (13a)–(13c) and are listed in Table 2. Using this data and Eq. (13), the optimum diameter of the transportation network is calculated to be 0.298 m. The diameter calculated here is very close to the estimated diameter DN 300. If those two diameters (calculated and estimated) would be different, the value of Dopt could remain between the calculated and estimated diameters. In this case, an iteration should be made. In this study, the transportation network is designed for different pipe diameters. Effects of the changes in diameters on annual cost are also evaluated and the diameter with the lowest cost is chosen. Table 3 shows different pipe diameters of the transportation network. These diameters vary from DN 200 to DN 500. The equipment, fittings and welding costs increase with increasing the pipe diameters, whereas the pumping costs decrease. Table 3 illustrates equipment and installation costs, running cost and total cost for every pipe diameter. Total costs range from US$ 755530.01 year1 for DN 200 to US$ 2606444.10 year1 for DN 500. Figs. 2–4 show variations of nominal pipe diameters versus pipe, bent and welding costs, respectively. As seen in these figures, the costs for equipment, installation and welding increase with the increase in the nominal pipe diameter. Fig. 5

Table 2 P P A list of element numbers ( ENi ) and loss characteristic coefficient ( V i ) for the transportation network

Pipe equipment Pipe installation 458 bend 908 bend Bend installatiıon Welding

P

Quantity, ni

c0i (US$)

ni  c0i (US$)

mi

ðD=0:2Þðmi 1:6Þ

ENi

lL/D,

5500 5500 15 49 64 1045

49.18 29.54 151.69 151.69 33.38 107.24

270490 162470 2275.35 7432.81 2136.32 112065.8

1.9 0.95 2.6 2.6 2.02 1.35

1.13 0.77 1.50 1.50 1.19 0.90

580406.40 118587.11 8873.87 28987.96 5116.54 136704.89 P ENi = 878676.75

183.33 0 6 24.99 0 0

z

Vi 916.67 24.00 99.96

P

V i = 1040.63

748

Y. Kalinci et al. / Energy and Buildings 40 (2008) 742–755

Fig. 2. Pipe equipment and pipe installation cost factor (c0i) and cost index (mi).

Fig. 3. Bend material and bend installation cost factor (c0i) and cost index (mi).

gives costs for energy, equipment-installation and annual based on the nominal pipe diameters. Parameters affecting the optimum diameter are illustrated in Fig. 6. These include the considered changes in the estimated pipe diameter, mass flow rate, density and running time, respectively. 4.2. Determination of energy and exergy values for the DGDHS Here, the balance equations are written for mass, energy and exergy flows in the system, which are treated as the steady-state steady-flow system and the respective energy and exergy efficiency equations are also written for the system and its components. The temperature and mass flow rate data for both geothermal fluid and hot water are given in accordance with their state numbers specified in Fig. 1. The exergy rates are calculated for each state and listed in Table 4. Note that state 0 indicates the restricted dead state for the geothermal fluid and hot water. In this study, the restricted dead state was taken to be the state of

Fig. 4. Welding cost factor (c0i) and cost index (mi).

environment, at which the temperature and the atmospheric pressure are 1.9 8C and 101.3 kPa, respectively, for geothermal fluid while the thermodynamic properties of water are used and are obtained from the general thermodynamic tables and software. The exergy efficiencies and destructions for the entire system and its major system components are calculated using the above equations and are listed in Table 5. It is important to note here that exergy is always evaluated with respect to a reference environment (i.e., dead state). When a system is in equilibrium with the environment, the state of the system is called the dead state due to the fact that the exergy is zero. At the dead state, the conditions of mechanical, thermal, and chemical equilibrium between the system and the environment are satisfied: the pressure, temperature, and chemical potentials of the system equal those of the environment, respectively. In addition, the system has no motion or elevation relative to coordinates in the environment [13]. The thermal data for energy and exergy analysis and performance assessment were taken at 0 8C, 1.9 8C (the coldest day), 4 8C, 8 8C, 12 8C, 16 8C and 20 8C. The respective

Fig. 5. Total pipe line cost depending on pipe diameter.

Table 3 Total cost for different diameters of the transportation network mi

1.9 5500 0.95 5500 2.6 15 2.6 49 2.02 64 1.35 1045

DN 300a

DN 350a

DN 400a

DN 450a

DN 500a

c0i (US$)

ni  c0i  ðD=D0 Þmi

c0i (US$)

ni  c0i  ðD=D0 Þmi

c0i (US$)

ni  c0i  ðD=D0 Þmi

c0i (US$)

ni  c0i  ðD=D0 Þmi

c0i (US$)

ni  c0i  ðD=D0 Þmi

c0i (US$)

49.19 29.54 151.70 151.70 33.38 107.24

270531.76 162481.54 2275.48 7433.23 2136.48 112063.52

94.24 56.87 434.49 434.49 95.57 160.19

1119871.24 459749.69 18702.89 61096.11 13873.94 289387.91

118.24 70.90 670.53 670.53 147.49 187.99

1883248.58 663592.40 43093.28 140771.38 29233.19 418169.55

148.75 89.36 965.58 965.58 212.41 222.38

3053388.36 949528.25 87813.05 286855.96 55135.43 592382.90

177.92 107.09 1319.65 1319.65 290.25 247.30

4568015.93 1272580.96 163013.32 532510.17 95579.01 772301.37

215.66 129.25 1732.64 1732.64 381.17 278.19

556922.01

1962681.78

3178108.38

5025103.96

7404000.76

ni  c0i  ðD=D0 Þmi 6764144.75 1697559.80 281478.11 919495.16 155286.60 1001559.64 10819524.07

Boru (Dd  t) Di (mm) Ai (m2) v (m/s) l  L/Di P l  L=Di þ z 2 Dp (N/m )

219.1  4.5 210.1 0.0347 6.8101 261.78 292.77 6637031.49

323  5.6 311.8 0.0763 3.0921 176.40 207.39 969224.80

355.6  5.6 344.4 0.0931 2.5344 159.698 190.688 598717.856

406.4  6.3 393.8 0.1217 1.9384 139.665 170.655 313449.1036

457.2  6.3 444.6 0.1552 1.5208 123.707 154.697 174885.8142

508  6.3 495.4 0.1927 1.2249 111.021 142.011 104147.9078

CR (US$/year) CEI (US$/year) CA (US$/year)

621868.725 133661.2821 755530.0071

90813.27897 471043.627 561856.906

56097.95739 762746.0115 818843.9689

29369.1833 1206024.951 1235394.134

16386.24413 1776960.182 1793346.426

9758.327461 2596685.776 2606444.103

Y. Kalinci et al. / Energy and Buildings 40 (2008) 742–755

Pipe equipment Pipe installation 458 bend 908 bend Bend installation Welding P ni  c0i  ðD=D0 Þmi

Quantity (ni) DN 200a

1US$ = 1.354 YTL for 24.12.2005. a Pipe diameter.

749

750

Y. Kalinci et al. / Energy and Buildings 40 (2008) 742–755

Fig. 6. Effects of optimization parameters.

thermodynamic properties were obtained based upon these data. There are two production wells in Dikili now. According to the wells, the specific exergy index (SExI) is found to be 0.0512 using Eq. (23). This represents that the Dikili geothermal field falls into the medium-quality geothermal resource according to the Lee’s classification [17]. Using the values given in Table 4 as well as Eqs. (39) and (41), the energy and exergy efficiencies of the DGDHS are determined to be 40.21% and 50.12%, respectively. The energy balance diagram is illustrated in Fig. 7. The thermal reinjection

accounts for 59.78% of the total energy input. The study of the exergy flow diagram given in Fig. 8 shows that 49.88% (corresponding to about 5153.38 kW) of the total exergy entering the system is lost, while the remaining 50.12% is utilized. The highest exergy loss (accounting for 38.77%) occurs from thermal reinjection. The second largest exergy destruction occurs from heat exchangers with 10.34% (corresponding to about 1068.37 kW) of the total exergy input. The smallest exergy destruction occurs from the pumps with 0.76% (corresponding to about 78.54 kW).

Table 4 Exergy rates and other properties at various system location for representative unit (for state numbers refer to Fig. 1) No. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Fluid

Phase

Temperature, T (8C)

Specific enthalpy, h (kJ/kg)

Specific entropy, s (kJ/kg K)

Mass flow rate m ˙ (kg/s)

Specific energy c (kJ/kg)

Exergy rate ˙ ðkWÞ Ex

Energy rate E˙ ðkWÞ

Thermal water Thermal water (re-inj) Water Water Water Water Water Water Water Water Water Water Water Water Water Water

Liquid Liquid

1.9 99 60

8 414.8 251.2

0.029 1.296 0.831

177.2 177.2

58.31 22.61

10332.82 4006.47

72084.96 43095.04

Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid

84.7 85 85 85 55 55 55 55 79.7 80 50 79.7 80 50

354.7 355.9 355.9 355.9 230.23 230.23 230.23 230.23 333.7 334.9 209.33 333.7 334.9 209.33

1.131 1.134 1.134 1.134 0.768 0.768 0.768 0.768 1.072 1.075 0.704 1.072 1.075 0.704

230.7 230.7 63.4 167.3 63.4 167.3 230.7 230.7 63.5 63.5 63.5 167.4 167.4 167.4

43.59 43.97 43.97 43.97 18.97 18.97 18.97 18.97 38.82 39.20 15.67 38.82 39.20 15.67

10057.34 10143.82 2789.44 7354.38 1203.33 3172.60 4375.93 4375.93 2465.25 2489.05 995.12 6498.95 6561.69 2623.37

79983.69 80260.53 22070.78 58189.75 14098.27 37170.19 51268.46 51268.46 20681.95 20758.15 12784.46 54522.18 54723.06 33702.64

Y. Kalinci et al. / Energy and Buildings 40 (2008) 742–755

751

Table 5 Some exegetic, energetic and thermodynamics analysis data provided for one representative unit of the system Item no.

Component

1

Transportation network -HE Bariskent-HE Danistay-HE Transportation network-PUMP Bariskent-PUMP Danistay -PUMP HE and PUMP Reinjection Overall systema

2 3 4 5 6 7 8 9 a

Utilized power (kW)

Exergetic product rate P˙ ðkWÞ

Exergetic fuel rate F˙ ðkWÞ

646.21

28989.92

644.94

114.98 307.19 71.76

7972.51 21019.56 159.43

5.26 1.52 1146.92 4006.47 5153.39

29.39 65.13

Exergy destruction rate ˙ ðkWÞ Ex

Exergy (second law) efficiency (%)

Relative irreversibility x (%)

Fuel depletion rate d (%)

Productivity lack z (%)

Exergy factor f (%)

Energy (first law) efficiency (%)

6326.04

90

12.54

5.23

52.11

51.23

–

115.98 306.1 86.48

1586.11 4181.78 159.43

93 93 54.99

2.23 5.96 1.39

0.93 2.49 0.58

9.27 24.77 5.79

12.85 33.87 1.29

– – 77

23.8 62.74

29.39 65.13

82.1 97.67

0.10 0.03

0.04 0.01

0.42 0.12

0.24 0.53

67 70

1240.04

12347.88

50.12

40.21

Based on the exergy (or energy) input to thermal water.

Fig. 7. Energy flow diagram (given as the percentages of brine energy input).

Using Eq. (26), the exergetic improvement potential rate ˙ is determined for heat exchangers and circulation pumps in ðIPÞ the DGDHS, as shown in Fig. 9. As can be seen from this figure, transportation network-HE has the largest exergetic IP˙ rate as 65.88 kW, followed by the transportation network-PUMP, Danistay-HE, Bariskent-HE, Bariskent-PUMP and Danistay-

PUMP at 38.93, 22.48, 8.41, 4.26 and 1.46 kW capacities, respectively. In order to improve the system efficiency, system design temperatures should be optimized. Energy and exergy rate increase with decreasing dead state values. The total energy input values are obtained for a range from 73484.84 to 58635.48 kW for seven different environment

Fig. 8. Exergy flow diagram (given as the percentages of brine exergy input).

752

Y. Kalinci et al. / Energy and Buildings 40 (2008) 742–755

Fig. 9. Exergetic improvement potential rates for heat exchangers and pumps.

Fig. 10. Heat distribution cycles of the Dikili geothermal district heating system[6].

temperatures. The corresponding reference state (dead state) temperatures were 0, 1.9, 4, 8, 12, 16 and 20 8C, respectively, as the environment temperatures. In conjunction with this, the total exergy input values are obtained to vary between 10755.61 and 6689.3 kW for the same temperatures. As expected, the largest energy and exergy losses occur on the coldest day among the dead state values considered. Energy and exergy reinjection values decrease from 44494.92 to 29645.56 kW and from 4272.71 to 1854.35 kW, respectively. Pump exergy losses change from 78.54 to 106.24 kW. However, exergy rate of heat exchangers increase from 1064.45 to 1099.76 kW. In the geothermal district heating systems, the temperature difference between the geothermal resource and the supply temperature of the district heating distribution network plays a key role in terms of exergy loss. In fact, the district heating supply temperature is determined after the optimization calculation. In this calculation, it should be taken into account that increasing supply temperature will result in a reduction of

investment cost for the distribution system and the electrical energy required for pumping stations, while it causes an increase of heat losses in the distribution network [13]. This study evaluates the heat distribution network of the DGDHS from the exergetic point of view. There are three cycles indicated in Figs. 1 and 10. In the calculations, we assumed a heat loss of 0.5 8C per 1 km pipe length [6]. In geothermal district heating systems, there are two types of losses, which are due the heat transfer and the fluid friction irreversibilities [7]. For each item of network, exergy, entropy generation and exergy destruction rates are shown in Table 6. Entropy generation due to the heat transfer is calculated to be 0.0001943, 0.0004693 and 0.0009982 W/mK for the transportation network, the Bariskent and the Danistay regions, respectively, while entropy generation due to pressure loss is found to be 0.095255, 0.17990991 and 0.332315 W/mK, respectively. The biggest exergy loss is obtained to be 153.376 kW in the transportation network, as shown in Figs. 11 and 12. The results presented here show that exergy analysis is a potential tool in determining locations, types and true magnitudes of wastes and losses. In addition, exergy analysis can help optimize systems.

Fig. 11. Entropy generation values for the whole network.

Fig. 12. Exergy rates for the whole network.

Table 6 Entropy generation and exergy loss rates for each item of the network considered Length (m)

T (K)

ReD

f

Nu

q0 (kW/m)

0 S˙ gen; DT ðW=mKÞ

0 S˙ gen;DP ðW=mKÞ

0 S˙ gen ðW=mKÞ

˙ dest ¼ Ex T 0 S€ure ðkWÞ

Transportation network 1–67 230.70

0.3118

5842.14

356.69

2800357.502

0.002362

4449.81

0.481765244

0.000194286

0.095255476

0.095449762

153.376

Bariskent region 67–92 92–94 94–96 96–107 107–110 110–124 124–129 129–136 136–141 141–156 156–163 163–176 176–185 185–187 187–198 198–199 199–210 210–215 215–228 228–233

0.2101 0.2101 0.2101 0.2101 0.2101 0.2101 0.2101 0.2101 0.2101 0.2101 0.2101 0.2101 0.2101 0.2101 0.2101 0.1603 0.1603 0.1603 0.1603 0.1603

2389.13 24.92 9.7 13.5 25.49 42.57 2.5 78.06 6.94 30.97 16.69 35.2 35.88 14.02 66.03 30.09 19.49 18.84 32.13 9.56

352.55 351.95 351.94 351.93 351.93 351.91 351.90 351.88 351.86 351.85 351.83 351.82 351.80 351.79 351.77 351.75 351.73 351.72 351.71 351.70

1093468.736 1075455.523 1065475.118 1025939.627 1015952.594 956570.0236 936716.7033 906910.8405 886962.9204 827735.3273 798056.1156 738822.6981 699265.6666 659768.9947 610382.0012 606278.1089 541697.7568 515841.7657 438396.7297 412555.7605

0.002851 0.002860 0.002866 0.002888 0.002893 0.002928 0.002941 0.002960 0.002973 0.003014 0.003036 0.003084 0.003118 0.003154 0.003204 0.003208 0.003281 0.003313 0.003423 0.003465

2141.90 2120.52 2104.86 2042.21 2026.39 1931.25 1899.23 1850.93 1818.49 1720.77 1671.34 1571.45 1503.91 1435.66 1349.17 1342.08 1226.52 1179.52 1035.66 986.58

0.133521407 0.253691814 0 0 0 0.132158797 0 0.136715347 0 0.157313529 0 0.123579545 0.11477146 0 0.054460094 0 0.124987173 0.123142251 0.061375661 0

3.18533E05 0.000116612 0 0 0 3.47573E05 0 3.88174E05 0 5.52941E05 0 3.73709E05 3.3685E05 0 8.45619E06 0 4.90054E05 4.94681E05 1.39968E05 0

0.017612885 0.017211262 0.016773381 0.01509169 0.014689476 0.012418053 0.011714912 0.010709152 0.010070856 0.008302071 0.007498664 0.00604534 0.005185665 0.004408725 0.00354843 0.00598716 0.004369856 0.003812013 0.002418523 0.002041023

0.017644738 0.017327874 0.016773381 0.01509169 0.014689476 0.012452811 0.011714912 0.010747969 0.010070856 0.008357365 0.007498664 0.006082711 0.00521935 0.004408725 0.003556886 0.00598716 0.004418862 0.003861481 0.00243252 0.002041023

11.595 0.119 0.045 0.056 0.103 0.146 0.008 0.231 0.019 0.071 0.034 0.059 0.052 0.017 0.065 0.050 0.024 0.020 0.021 0.005

Total

0.000469317

0.179909136

0.180378453

12.7390

0.351265558 0.278683461 0.261 0.223701127 0.108972899 0 0.261436417 0.148109347 0.383223993 0 0.135324312 0.081043316 0.142341562 0.05394125 0.123273114 0 0.06780872 0.046888513 0.043328491

0.000121009 7.8608E05 9.82946E05 7.56671E05 1.84079E05 0 0.000109761 3.56082E05 0.000258532 0 4.24677E05 1.65818E05 5.35648E05 7.92012E06 4.44533E05 0 1.60767E05 8.39949E06 6.66269E06

0.090335052 0.081987683 0.023804264 0.020291871 0.018671489 0.017131768 0.01654139 0.015965102 0.012030217 0.009583581 0.004598122 0.003424252 0.002918232 0.002639347 0.002056328 0.000863018 0.001732599 0.001273613 0.002838103

0.090456061 0.082066291 0.023902558 0.020367538 0.018689897 0.017131768 0.016651151 0.01600071 0.012288749 0.009583581 0.00464059 0.003440834 0.002971797 0.002647267 0.002100781 0.000863018 0.001748676 0.001282013 0.002844765

23.827 6.577 0.789 1.482 0.451 0.029 0.642 0.269 0.072 0.099 0.219 0.061 0.057 0.064 0.082 0.011 0.059 0.038 0.075

Mass flow rate (kg/s)

63.8 63.22 62.64 60.32 59.74 56.26 55.1 53.36 52.2 48.72 46.98 43.5 41.18 38.86 35.96 27.26 24.36 23.2 19.72 18.56

Danistay region 67–282 168.2 282–286 162.4 286–289 104.4 289–294 98.6 294–296 95.7 296–298 92.8 298–300 91.64 300–302 90.48 302–304 81.78 304–307 75.4 307–311 58 311–313 52.2 313–316 49.3 316–319 47.56 319–322 43.5 322–325 31.9 325–329 27.84 329–331 24.94 331–334 20.88

0.263 0.263 0.263 0.263 0.263 0.263 0.263 0.263 0.263 0.263 0.263 0.263 0.263 0.263 0.263 0.263 0.2101 0.2101 0.1603

957.68 291.37 120 264.46 87.82 6.09 140.21 61.09 21.34 37.51 171.44 64.41 69.27 88.17 141.15 45.02 123.17 106.38 96.38

352.91 352.60 352.50 352.40 352.31 352.29 352.25 352.20 352.18 352.17 352.11 352.05 352.02 351.98 351.92 351.88 351.84 351.78 351.73

2313232.652 2224786.81 1428391.99 1347427.266 1306368.815 1266413.193 1250016.26 1233424.131 1114541.237 1027404.076 789800.1495 710302.0671 670563.83 646582.4223 590967.9481 433127.7135 472932.3742 423369.073 464274.25

0.002454 0.002473 0.002703 0.002734 0.002751 0.002768 0.002776 0.002783 0.002840 0.002887 0.003043 0.003108 0.003144 0.003167 0.003224 0.003431 0.003371 0.003447 0.003384

3893.00 3779.87 2653.18 2533.48 2472.71 2412.33 2387.78 2363.03 2179.24 2042.00 1655.00 1520.82 1452.62 1411.21 1313.64 1024.77 1099.70 1006.80 1084.19

Y. Kalinci et al. / Energy and Buildings 40 (2008) 742–755

Internal diameter (m)

Item no

753

35.0132 0.333313256 0.000998237 Total

0.332315019

0.010 0.000456409 0.000456239 1.69466E07 150055.7086 79.13 3.48 338–340

0.0825

351.57

0.004241

439.60

0.004397826

0.001144648 0.002034153 0.001141301 0.002031448 335094.3808 308425.1764 111.89 118.77 15.08 9.28 334–336

0.1603 0.1071

351.68 351.62

0.003612 0.003672

835.49 782.10

0.026955045 0.023440263

3.34731E06 2.70512E06

0 S˙ gen ðW=mKÞ 0 S˙ gen;DP ðW=mKÞ 0 S˙ gen; DT ðW=mKÞ

q0 (kW/m) Nu f ReD T (K) Length (m) Internal diameter (m) Mass flow rate (kg/s) Item no

Table 6 (Continued )

0.035 0.066

Y. Kalinci et al. / Energy and Buildings 40 (2008) 742–755 ˙ dest ¼ Ex T 0 S€ure ðkWÞ

754

5. Conclusions In this study, we have studied on two important aspects of geothermal district heating systems. These included determination of optimum pipe diameters in the network as well as energetic and exergetic assessment of a geothermal district heating system in Turkey. Exergy destructions (representing the losses) in the overall system are quantified and illustrated using an exergy flow diagram along with an energy flow diagram. We utilized actual project data taken from the Dikili municipality. We can extract some concluding remarks from this study as follows: (a) Determination of optimum pipe diameter is very essential in terms of running and investment costs. The optimum pipe diameter of the transportation network was calculated to be 0.298 m, corresponding to DN 300. (b) Energy and exergy efficiency values for the DGDHS were estimated to be 40.21% and 50.12%, respectively. (c) The largest exergy destruction occurred in the reinjection with 38.77%. (d) Based upon the SExI parameter, the current geothermal field falls into the category of medium-quality geothermal resources. (e) The entropy generation rates due to the heat transfer and pressure loss were obtained to be 0.00166 and 0.60745 kW/ mK, respectively. (f) The exergy loss in the transportation network was calculated to be 153.38 kW. (g) Exergy analysis is more useful tool than energy analysis for system performance assessment and evaluation since it allows true magnitudes of the losses to be determined. (h) This study can be used as source for future studies, such as exergoeconomic analysis, which is a combination of exergy and economics. (i) The results are expected to be beneficial to the researchers, government administration, and engineers working in the area of geothermal district heating systems. Acknowledgements The authors gratefully acknowledge the support provided for the present work by Mr. Osman Guven, the Mayor of Dikili. They also would like to thank the Scientific & Technological Research Council of Turkey (TUBITAK) for the incentive support given, while the valuable comments of the reviewers are gratefully acknowledged. References [1] Z. Utlu, A. Hepbasli, A review on analyzing and evaluating the energy utilization efficiency of countries, Renewable and Sustainable Energy Reviews 11 (1) (2007) 1–29. [2] L. Ozgener, A. Hepbasli, I. Dincer, Exergy analysis of two geothermal district heating systems for building applications, Energy Conversion and Management 48 (4) (2007) 1185–1192. [3] L. Ozgener, A. Hepbasli, I. Dincer, Energy and exergy analysis of Salihli geothermal district heating system in Manisa, Turkey, International Journal of Energy Research 29 (2005) 393–408.

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