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Employing geothermal heat exchanger in natural gas pressure drop station in order to decrease fuel consumption
Energy 83 (2015) 164e176
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Employing geothermal heat exchanger in natural gas pressure drop station in order to decrease fuel consumption M. Farzaneh-Gord a, R. Ghezelbash a, A. Arabkoohsar b, *, L. Pilevari c, L. Machado d, R.N.N. Koury d a
Department of Mechanical Engineering, Shahrood University of Technology, Shahrood, Iran Department of Mechanical Engineering, Minoodasht Branch, Islamic Azad University, Minoodasht, Iran Lorestan Gas Company, Khoram Abad, Iran d Department of Mechanical Engineering, Universidade Federal de Minas Gerais (UFMG), Belo Horizonte, Brazil b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 July 2014 Received in revised form 15 December 2014 Accepted 4 February 2015 Available online 20 March 2015
Natural gas stream must be preheated before pressure reduction takes place at natural gas pressure drop station (CGS). It ensures that the natural gas stream remains above hydrate-formation zone. The heater which is employed to provide the required heat consumes a huge amount of fuel. In this work, the conventional configuration of the natural gas pressure drop station is amended by taking advantage of geothermal energy to provide either all (if applicable) or a considerable portion of the required heat. To evaluate the proposed system in terms of economic and thermal efficiency, Gonbad Kavoos station has been chosen as a case study. Comprehensive thermo-economic analysis has been carried out on the proposed system. The results show that a system comprising 8 boreholes with 150 m depth and 0.15 m diameter each is the most efficient configuration for Gonbad Kavoos station. The achievable providence for the case study was calculated by technical correlations over the standard life time of geothermal systems. Comparison between the proposed system and the previous studied systems proved that the current configuration outperforms all the prior propounded configurations, with IRR (internal rate of return) ¼ 0.155. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Natural gas pressure drop station Natural gas Hydrate forming zone Geothermal energy Thermo-economic analysis
1. Introduction Natural gas is transported with high pressure (5e7 MPa) to overcome pressure losses related to the long distance that it should pass from refinery to consumption points such as cities, factories, power plants and etc. At consumption points, natural gas pressure should decrease. This pressure reduction takes place in natural gas pressure drop stations (CGS). At CGSs, pressure is generally reduced to 1.5e2 MPa. This mission is currently accomplished by throttling valves [1]. Due to the positive JouleeThomson coefficient of natural gas, pressure reduction leads to natural gas temperature drop. There is a minimum value from which natural gas temperature should not be lower. This temperature, which is called hydrate forming temperature, depends on natural gas compositions and pressure [2]. In fact, the hydrate formation zone is where the water droplets suspended in the natural gas begin to freeze and
* Corresponding author. E-mail address: [email protected] (A. Arabkoohsar). http://dx.doi.org/10.1016/j.energy.2015.02.093 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
subsequently obstruct the transportation pipeline. To prevent hydrate forming, natural gas stream should be warmed up to a specific level before pressure reduction. Generally, line heaters are employed in the CGSs to preheat natural gas stream, consuming huge amount of fuel for providing the required heat. That's why the performance of a CGS is not at a satisfactory level. A few studies have already been done to improve the performance of the CGSs by employing turbo-expander and cogeneration [2,3]. Energy and exergy analysis on the systems suggested in these studies showed there is a considerable improvement potential in the first law and the second law efficiencies of the CGSs. On the other hand, some studies have been done to propose new configurations in order to decrease the amount of fuel consumption at the CGSs by employing renewable energy sources. In the first study, Farzaneh-Gord et al. [4] proposed employing flat plate solar collectors to provide a portion of required heat in the CGSs. In the second work, they modified their first proposed system by adding a storage tank. In this configuration, the storage tank could collect solar heat during day and give it back to the heater at night when the
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amount of required heat is much more [5]. As a result, fuel providence increased to 14% whereas it was only 11% in the first propounded system. In the third work, Arabkoohsar et al. [6] substitute the conventional line heater in the CGSs by an automatic controllable heater and compared the thermal behaviour of this system with the aforementioned systems. The comparison result showed the superiority of the last work done relative to the prior ones. Another source of renewable energy is geothermal heat. Geothermal energy is cost effective, reliable, sustainable, and environmentally friendly [7]. There are some disadvantages with geothermal heating systems such as releasing greenhouse gases from the geothermal boreholes and huge capital cost. However, it should be noted that, firstly, the amount of released greenhouse gases from the geothermal boreholes are much lower per energy unit than those of fossil fuels and, secondly, free obtainable energy from this source of heat after preliminary investment makes these system totally efficient and affordable. As a result, not only the geothermal power has the potential to help mitigate global warming, but also it is completely affordable in terms of economic [8,9]. In the current study, the application of geothermal energy in the CGSs is proposed in order to modify their conventional configuration and reducing the amount of fuel consumption. The detailed information about the proposed configuration is presented in the next sections. It is noteworthy that in order to evaluate the applicability and justifiably of the proposed system, Gonbad Kavoos station in Iran has been selected as a case study. Finally, comprehensive thermo-economic comparison among the current proposed system and the previous propounded systems (for the same objective) employing renewable energies is carried out.
2. Natural gas pressure drop station 2.1. The conventional configuration Based upon the primary information presented about a CGS, when a natural gas pipeline approaches a city, natural gas pressure must be reduced to the distribution level. A CGS is a pressure reduction point in which control and throttling valves accompanying with some other equipments are employed to reduce the natural gas pressure. A schematic diagram of a typical CGS is shown in Fig. 1. According to Fig. 1, high pressure natural gas (5 MPa < PNG1 < 7 MPa) enters the line heater to be warmed up before the throttling procedure. The inlet temperature (TNG-1) is basically equal to soil temperature because transportation pipeline is buried underground. In fact, natural gas stream should be preheated
165
before passing through the throttle valve to ensure that its temperature remains above the hydrate-formation temperature to prevent liquid and solid phase formation in natural gas stream. The desired value for the natural gas temperature after preheating procedure (TNG-2) depends on the inlet pressure. The higher natural gas pressure, the more the natural gas needs to be heated. As it was explained, line heaters are generally used in the CGSs to do preheating mission by burning natural gas. Line heaters are generally full of water. The water transfers heat from the fire tubes of heater to the natural gas stream flowing through a coil immersed in the water bath. Based on the standard outlet pressure of CGS as well as the natural gas compositions, the hydrate forming temperature (Thyd) could be calculated by thermodynamics models. The outlet natural gas stream temperature (TNG-3) is then selected 3e5 C above the hydrate forming temperature [6]. 2.2. The case study The proposed model was employed to study the thermal behaviour of Gonbad Kavoos CGS. Therefore, the effectiveness of the proposed system can be demonstrated in practice. Gonbad Kavoos station is located in Gonbad Kavoos city, one of the biggest cities in Golestan province in the north-east of Iran. For Gonbad Kavoos station, based on thermodynamic relations and the natural gas compositions, the hydrate temperature has been found to be 2 C [10]. Adopting the mean value of 4 C for safety factor, natural gas temperature should never be below 6 C in Gonbad Kavoos CGS. Gonbad Kavoos station is mainly fed by Torkman refinery for which natural gas compositions were reported in Table 1. The station inlet and outlet pressures are 5.9 MPa and 1.8 MPa, respectively. The reported value of temperature drop due to this range of pressure reduction is 15 C [10]. Therefore, it could be claimed that for such natural gas compositions, 100 kPa pressure reduction causes almost 0.366 C temperature drop. Consequently, the minimum temperature that natural gas stream should meet before pressure reduction procedure is the summation of the hydrate forming temperature, the safety factor value and the temperature drop through the throttling valve. This value for Gonbad Kavoos CGS is 21 C (2 þ 4 þ 15 ¼ 21 C). More technical information related to Gonbad Kavoos CGS was provided by Golestan Gas Company. Table 2 details this information. 2.3. The proposed configuration This section presents the detailed description of the proposed configuration. The proposed system takes advantage of geothermal energy to provide either the whole (if possible) or a portion of the required heat in CGS. Fig. 2 shows the proposed system schematic. According to the figure, if the inlet natural gas temperature (TNG 1) is above 21 C, natural gas goes directly toward the main throttling valve through a bypass line, because in this case, TNG-3 will remain above 6 C after pressure reduction process. Otherwise, natural gas should be preheated. In contrast with the conventional configuration in which there is only one throttling valve, in the proposed system the pressure reduction is supposed to be accomplished in two steps. The first step of pressure reduction is done by throttling valve 1. The second step is also carried out by
Table 1 Torkman natural gas compositions.
Fig. 1. The schematic configuration of a CGS.
CH4
C2H6
C3H8
i-C4H10
i-C5H12
C6H14
C7H16
N2
CO2
94.21
2.25
0.53
0.36
0.26
0.17
0.18
1.90
0.14
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Table 2 Properties of Gonbad Kavoos CGS line heater. Natural gas source Natural gas density Lower heating value Natural gas inlet pressure Natural gas outlet pressure Natural gas maximum flow rate Surface area of fire tube heater Water capacity Diameter of coil Length of coil Heater maximum working temperature Heater maximum heating duty
Torkman 0.587 kg/m3 4.811 MJ/kg 5.9 MPa 1.8 MPa 80,000 Sm3/h 88.1 m2 36 m3 0.1 m 280 m 88 C 1750 kW
throttling valve 2. Between the first and the second steps of pressure reduction there are two stages of preheating (geothermal heater and line heater). The first throttling valve reduces the nat0 ural gas pressure down to that level in which TNG1 ¼ 6 C. Note that as TNG-1 is variable, therefore, the amount of pressure drop by 0 throttling valve 1 is also variable and as a result, PNG1 is change able. As the TNG1 < 21 C, therefore, the amount of temperature drop in this step is less than 15 C. Note that, the higher values of TNG-1, the higher portion of pressure reduction mission is carried out by throttling valve 1. In contrast, the lower values of TNG-1, the higher portion of pressure reduction is done by throttling valve 2 and the contribution of throttling valve 1 in pressure reduction procedure is less. As it was explained before, the value of 6 C is the minimum temperature that Torkman natural gas is allowed to meet so as to avoid hydrate forming [6,10]. The details of geothermal system designed for the proposed configuration at this work are presented in the next section. Regarding its characteristics, the geothermal heating unit is able to increase temperature up to 18.9 C [11], and even this temperature would rarely be achievable during the whole year. Therefore, nat0 ural gas pressure decreases down to that level at which TNG1 is equal to its minimum allowable value. In this way, maximum utilization of geothermal energy would be possible. Imagine that throttling valve 1 is omitted from the system, in this case, when TNG-1 > 12e13 C (mainly during spring and summer), the geothermal system could not take part in preheating process and it is the line heater, again, that should provide one hundred percent of the required heat. Therefore, throttling valve 1 is placed at the
station inlet, and after that the geothermal heater is set to provide geothermal heat. After passing through the geothermal heater, natural gas stream flows through the line heater to be heated up to higher temperatures, if necessary. It's noticeable that, depending on 00 00 the value of TNG1 and PNG1 , the line heater heating duty changes. It is also noteworthy that, the geothermal heat exchanger is a simple shell and tube heat exchanger with water as intermediate fluid. Cold water enters long vertical tubes in geothermal boreholes and receives geothermal heat while flowing through the boreholes. Warm water, then, comes back to the shell and tube heat exchanger and gives this heat back, increasing the natural gas temperature. In the design stage of geothermal heating unit in the proposed system, some important parameters (such as the number, the depth and the diameter of boreholes, distance between the boreholes, operating fluid mass flow rate and the optimal volume of shell and tube heat exchanger) should be determined. The investigations prove that the maximum achievable temperature in Earth's crust is just equal to the annual average weather temperature in the corresponding area. This value for Gonbad Kavoos is 18.9 C. This temperature is uniformly available for all days of the year after depth of 100 m in this city. After this depth, obviously, the more depths for the borehole, the more energy is obtainable from the borehole. However, deeper boreholes cause higher initial investment for the project. In Iran, boreholes up to 150 m cost 25 USD/m, including all the complementary equipments such as heat exchanger and piping costs [13]. Due to difficulties for delving boreholes with higher depths, the price increases significantly for such cases. As low temperature heat is required and economic considerations are the pivotal factors in this project, the depth 150 m was chosen as the ideal depth of boreholes in this work. On the other hand, BH factor for geothermal boreholes is defined as the ratio of distance between the boreholes to the depth of boreholes and it is recommended to be from 0.05 to 0.2 [12]. The value of BH for this work was considered 0.1. As a result, the distance between the boreholes should be 15 m. For determining boreholes diameters, it should be noted that, although the smaller the diameter of borehole, the greater the heat transfer efficiency; bigger diameters for boreholes lead to bigger heat transfer area and consequently higher heat transfer rate in unit borehole length. Therefore, trade off approach could be adopted for determining the most appropriate diameter for the boreholes. However, for the sake of simplification of designing a geothermal
Fig. 2. The proposed system schematic.
M. Farzaneh-Gord et al. / Energy 83 (2015) 164e176
system, it is recommended to select boreholes diameters in range of 90 mme190 mm [14]. The chosen value for the diameters of boreholes in this work was 150 mm. Naturally, the selection of appropriate number for boreholes depends on the amount of required heat as well as the amount of initial investment. Also, the volume of shell and tube heat exchanger depends on the number of boreholes. Obviously, heat exchanger with higher capacity and volume requires more investments. Therefore, the number of boreholes (consequently the volume of heat exchanger) should be defined on a basis of a thermo economic trade off. Fig. 3 shows the proposed array for the boreholes in an L shape 45 m 60 m field with regular 15 m gaps between the boreholes. According to the figure, the most appropriate number of boreholes for the case study is 8, resulted from a thermo-economic evaluation. It is worth mentioning that, the best configuration in terms of the amount of obtainable energy is an L shape array of boreholes [7]. Table 3 details the proposed geothermal system information for the case study. The results of economic analysis to select the most efficient layout of geothermal system will be explained thoroughly in conclusion section. The selective configuration will also be assessed economically by the IRR (internal rate of return) method. Through this assessment, the performance of the proposed system in this work is compared with the previous proposed systems employing renewable energies in the CGSs. The IRR is the rate of return used to measure and compare the profitability of different projects. The term internal refers to the fact that its calculation does not incorporate environmental factors such as the interest rate or inflation. Based on the definition, the IRR of a project is the rate of return that makes the NPV (net present value) of all cash flows from a particular investment equal to zero. N X
Cn NPV ¼ ¼0 ð1 þ rÞn n¼0
(1)
where, n, Cn and r refer to the number of year, cash flow in the project in the corresponding year and IRR respectively. Overall, the higher a project's IRR, the more desirable it is to undertake the project [15,16]. The critical parameter for calculating the IRR of a project is the number of year in which the NPV should be equal to zero. This totally depends on the investor of the project; however, for such industrial projects the duration of 8 years seems to be a good choice. 3. Energy demand In order to assess the proficiency of the proposed system, a comprehensive thermal analysis on the proposed system and the conventional system of the CGS should be done. The fuel consumption in the both systems must be calculated and compared.
Table 3 The geothermal system details for Gonbad Kavoos CGS. The numbers of boreholes Diameter of each borehole Depth of each borehole Distance between the boreholes Operating fluid Operating fluid mass flow rate Operating fluid velocity Pipe material Pipe thermal conductivity coefficient Pipe diameter Pipe thickness Grout thermal conductivity coefficient Heat exchanger type Earth's temperature Earth's thermal conductivity coefficient Earth's thermal diffusion coefficient Heat exchanger coil length Heat exchanger coil diameter Heat exchanger volume
8 0.15 m 150 m 15 m Water 0.14 kg/s 0.3 m/s High density polyethylene 0.42 W/K 0.026 m 0.006 m 1.25 W/K Shell and tube 18.9 C 1.5 W/K 0.0778 m2/day 112 m 0.1 m 12.5 m3
Considering the required investment for implementing such system and the fuel consumption comparison, it would be revealed that how efficient the proposed system is.
3.1. Energy demand in the conventional configuration Considering the information presented in Section 2 and the schematic diagram in Fig. 2, one could analyze the thermal behaviour of conventional configuration. According to Fig. 2, the natural gas temperature at the line heater exit can be calculated by:
TNG2 ¼ Thyd þ Tc þ DTtv
(2)
In which, Tc and DTtv represent the safety factor value (4 C) and the temperature drop due to pressure reduction through the throttling valve. Thyd also refer to the hydrate forming temperature of natural gas stream. For Torkman natural gas Thyd ¼ 2 C. The natural gas pressure and temperature at the station inlet are measured. Once the natural gas temperature and pressure at the heater exit are known, the rate of required heat could be calculated as below:
Q_ gh ¼ m_ NG ðhNG2 hNG1 Þzm_ NG $CPNG ðTNG2 TNG1 Þ
(3)
where, m_ NG , hNG, hNG and CPNG are the natural gas mass flow rate, enthalpy and thermal capacity, respectively. As the natural gas flows through a buried pipeline in depth of 1.2 m, the gas temperature is equal to the soil temperature in this depth which is a functional of ambient temperature (Tam) as follow [17]: 2 TNG1 ¼ Tsoil ¼ 0:0084 Tam þ 0:3182 Tam þ 11:403
(4)
The heater provides the required heat by burning natural gas as fuel. Considering the thermal efficiency of heater, hh, one could obtain the fuel mass flow rate, m_ fuel , as below:
m_ fuel ¼
Fig. 3. The proposed layout for arranging the boreholes.
167
. lþ1 l 3600 Twh Q_ gh þ mwh $Cpw Twh hh $LHV
(5)
In which, LHV, mw-h, Cp-w are the lowering heating value of fuel, the mass and thermal capacity of water in the heater, respectively. hh is the thermal efficiency of the heater which is in range of 0.35e0.5 [4]. In this work, this parameter was considered to be 0.40. Also the subscripts (l) and (l þ 1) count hourly periods. Since all measurements such as ambient temperature are done in hourly format,
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this equation is divided by 3600 s to present the instantaneous fuel mass flow rate of the heater. Tw-h also refers to the temperature of water in the heater. As the immersed coil in water bath could be considered as a pipe in constant temperature environment, based on the correlation presented by Incropera and DeWitt [18], Tw-h could be calculated by:
Twh TNG2 pDoc Lc Uc ¼ eY ; Y ¼ Twh TNG1 m_ NG CPNG
(7)
The previous studies suggest that the line heaters are so designed that in which Uc ¼ 568 W/m2 K [4]. The next step is presenting the formulation of energy analysis on the proposed configuration. Regarding Fig. 4, two distinct control volumes should be considered for doing energy analysis on the proposed configuration. As the figure shows, the first control volume consists of three items including the throttling valve, the shell and tube heat exchanger and the underground-vertical heat exchanger. The energy balance equation for this control volume could be as [19e22]:
mw Cw
dTw _P ¼ Q_ GHX Q_ NG1 W dt
(8)
where, mw, Cw and Tw are the mass, thermal capacity and temper_ P refer to ature of working fluid (water). Also, Q_ GHX ; Q_ NG1 and W the absorbed heat from the earth by the operating fluid, the gained heat by the natural gas stream and the work of pumps in the system, respectively. There are various types of geothermal heat exchangers for taking advantage of the Earth's heat such as horizontal and vertical heat exchangers. Regarding the fact that vertical heat exchangers have more efficiency, occupy less room and require less energy for pumping the working fluid in comparison with horizontal heat exchangers, vertical heat exchangers are considered to be used in this work. The rate of absorbed heat by the underground-vertical heat exchanger highly depends on the thermal and physical properties of soil, applied tube and material of borehole structure. Over the simulation process, all the aforementioned properties are considered to be constant over time, and the only variable parameter is the working fluid temperature. Note that the length of boreholes and the gap between the boreholes are identical. The total heat absorbed by the underground-vertical heat exchanger set could be given by the following equation:
Q_ GHX ¼ q$Nb $H
(9)
where, Nb and H are the number of boreholes and the effective length of each borehole, respectively. q is also the absorbable heat for the unit effective length of tube by one borehole and could be obtained by the following equation:
q¼
m_ f $Cf $ðTout Tin Þ H
Tout ¼ Tgr
(10)
Tn which, m_ f , Cf, Tout and Tin are the mass flow rate, thermal capacity, outlet temperature and inlet temperature of working fluid, respectively. Tout could be calculated by Refs. [19e22]:
13
0
6 B C7 6 B C7 Z∞ nt 7 6X ql ql1 B Ils ðH$S; D0 $SÞ C 6 B C7 þ6 $B dS Ie $ 7 C 6 C7 4$p$kgr B H$S2 6 l¼1 B pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C7 4 @ 4aðt1t Þffi A5 l
(6)
where, Doc, Uc and Lc are the external diameter, length and overall heat transfer coefficient of the coil, respectively. Rearranging the above equation, one could drive the following equation:
. 1 eY Twh ¼ TNG2 TNG1 eY
2
þ q$Rb
l1
q$H 2$m_ b $Cb (11)
where, l counts again the hourly time steps as all the calculation process is mainly done on a basis of hourly periods. Therefore, ql and ql-1 represent the absorbed heat in time steps l and l-1, respectively. Tgr, kgr, a also refer to ground temperature, thermal conductivity coefficient and diffusion coefficient of the borehole, respectively. Defining S as Laplace transform variable in the short-term response, h ¼ H.S, d ¼ D0 .S and D0 as the ineffective length of tube in the borehole, the other factors in the above equation could be given by the following equations.
Ils ðh; dÞ ¼ 2 ierf ðhÞ þ 2 ierf ðh þ 2dÞ ierf ð2h þ 2dÞ ierf ð2dÞ (12) where, the error function erf(z) (which is the normalized form of Gaussian function) is defined as:
2 erf ðzÞ ¼ pffiffiffi p
Zz
ez dz 2
(13)
0
Subsequently:
2 1 ierf ðzÞ ¼ z$erf ðzÞ pffiffiffi 1 ez p
(14)
The function Ie could also be given as below:
Ie ðsÞ ¼
N X N 2 1 X eri;j Nb i¼1 j¼1
S2
(15)
Here, ri,j is the radial distance between the boreholes i and j (i s j). The last important item in this equation is Rb which refers to the overall thermal resistance of borehole and may be calculated by:
Rb ¼
1 ðRconv þ Rcond Þ þ Rgr 2
(16)
where, the subscriptions conv, cond and gr refer to the convective thermal resistance coefficient of working fluid within the tubes, the conductive thermal resistance coefficient of tubes and the conductive thermal resistance coefficient of borehole wall. These parameters can be obtained by the following equations, respectively:
Rconv ¼
1 pdi hf
(17)
Rcond ¼
lnðdo =di Þ 2$p$kp
(18)
Rgr ¼
1 kgr $b0 $ðdb =do Þb1
(19)
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169
Fig. 4. The separate control volumes for analysing the proposed system.
In the above equations, di, do and db refer to the internal and external diameter of tube and the diameter of borehole, respectively. hf, kp and kgr also represent the convective heat transfer coefficients for working fluid, the conductive heat transfer coefficient of tube and the conductive heat transfer coefficient of borehole wall, respectively. b0 and b1 are also shape factors and are given in Ref. [23]. The work of pumps in a geothermal system is almost 2e3% of the total obtainable heat from thee geothermal boreholes. However, the following equation can give the exact value of pumps work in a geothermal system:
_ _ P ¼ mf $HT $DP $h W P rf
(20)
where, DP is the amount of pressure drop along the vertical pipes which is considered to 0:4 kPa m . rf is water density and equal to 1000 kg/m3 and hP is the pump efficiency which is considered to be 80%. HT is also the total effective length of pipes in the boreholes and can be given by: 0
HT ¼ 2Nb ðH þ D Þ ¼ 2$8$ð150 þ 3Þ ¼ 2448 m
(21)
Finally, considering the water mass flow rate in the system ðm_ NG ¼ 0:14 kg=sÞ as well as a safety factor equal to 1.5, the work of pumps in the system will be equal to 256.5 W. Employing the detailed formulation above, one could make a system of equation including Equations (10) and (11). This system of equation would be calculable employing computational and numerical methods. The employed computational method in this work is a modified version (by the authors) of the already available function quadgk in Matlab. The other unknown parameter in Equation (8) is the heat transfer rate to the natural gas stream which is given by the following equation:
00 0 Q_ NG1 ¼ m_ NG $CpNG $ TNG1 TNG1
(22)
0 As it was explained in the previous section, the value of TNG1 is 00 equal to 6 C whereas the value of TNG1 is variable and a functional
of the heat exchanger water temperature. This parameter is defined as below:
00 0 TNG1 ¼ Tw $ 1 eY þ TNG1 $eY
(23)
On the other hand, the energy balance for the second control volume shown in Fig. 4 could be written as:
mw Cw
dTw ¼ Q_ h Q_ NG2 dt
(24)
where, the heat transfer rate given to natural gas stream by the line heater could be given by:
00 Q_ NG2 ¼ m_ NG $CpNG $ TNG2 TNG1
(25)
Considering equation above, the heater heating duty in the proposed system could be calculated by:
Q_ h ¼ Q_ NG2 þ
lþ1 l mwh $Cw $ Twh Twh 3600
(26)
Therefore, the mass flow rate of consuming fuel to provide this amount of energy, considering the heater thermal efficiency, is computed by:
m_ fuel ¼
Q_ h hh $LHV
(27)
4. Results and discussion In this section the results of implementing the proposed system in Gonbad Kavoos CGS are presented. The key parameters required in simulation process are the ambient temperature, the natural gas inlet temperature and mass flow rate in Gonbad Kavoos CGS over a whole year. The database used for this work includes the aforementioned data in the year 2013.
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Fig. 5. The monthly-hourly average ambient temperature in Gonbad Kavoos in 2013.
Fig. 6. The monthly averaged mass flow rate of natural gas in Gonbad Kavoos station in 2013.
Fig. 5 shows the monthly-hourly averaged of ambient temperature and natural gas inlet temperature in Gonbad Kavoos. The monthly-hourly averaged temperature is much more appropriate than daily averaged temperature for this project because
temperature difference between daily and night hours is high in Gonbad Kavoos. Therefore, daily averaged temperature could not present acceptable estimation of actual temperatures and leads to considerable deviations in calculation process.
Fig. 7. Selecting the optimal capital cost for the project.
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171
Fig. 8. The natural gas stream pressure after the first throttling valve over a whole year.
Fig. 9. The natural gas stream hourly temperature outgoing from the shell and tube heat exchanger over a whole year.
As expected, the least entrance temperature belongs to January with a value below 5 C and the highest temperature in August with a value more than 31 C. Based on what explained about the system performance details, for the temperatures over the red line (21 C), the natural gas stream can directly go through the second throttling valve and there is no need to warm up the natural gas stream. For Gonbad Kavoos station, this situation is observed in almost four months of the year from early June to late September. Fig. 6 shows the monthly averaged mass flow rate of natural gas stream in Gonbad Kavoos CGS. As the figure shows, more mass flow rates occur during winter where the maximum natural gas consumption takes place in Gonbad Kavoos city in January as the
coldest months of the year. In contrast, the least consumption occurs in May. Fig. 7 shows how the amount of initial investment for the proposed system is chosen. For such system, naturally, increasing the number of boreholes leads to more obtainable energy and higher cost of capital simultaneously. Also, the amount of required energy is limited and known. Therefore, there should be a logical index for selecting the most appropriate amount of initial investment. For such cases, the capital cost value must be selected equal to the total annual cost of the system pivotal parameter (here, the annual price of fuel). As the figure shows, the capital cost of the system can be equal to the annual fuel cost if the numbers of boreholes are 8. Note
Fig. 10. The hourly averaged shell and tube heat exchanger water temperature over a whole year.
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Fig. 11. The hourly averaged line heater temperature over the whole year.
Fig. 12. Comparison between the hourly averaged heater heating duty in the conventional and proposed configuration in a whole year.
that the capital cost comprises all the required costs including heat exchanger price, boreholes delving cost, throttling valve price and etc. As it was explained before, in Iran, the total cost for such system is 25 USD/m. It should also be noted that the Fig. 7 has been prepared employing the average fuel consumption of the proposed system in 25 years of performance (14.39 m3/h). Geothermal heating systems are more efficient in early years of operation and the amount of annual achievable providence from the system reduces over time. Therefore, the average fuel consumption for the first year of operation is much less than this value and equal to 11.26 m3/h. The figures below are presented based on the data obtained for the first year of operation of a system taking advantage of an L shape array boreholes including 8 boreholes with regular distance of 15 m and 150 m depth each. Fig. 8 shows the natural gas stream 0 pressure right after the first throttle valve ðPNG1 Þ. Therefore, this figure also shows the contribution of throttling valve 1 in the pressure reduction mission over the year. As the figure shows, through warm months of the year the contribution of throttling valve 1 is zero as the natural gas temperature was more than 21 C. During these times, the natural gas pressure remains at 5.9 MPa. On the other hand, for the times in which TNG-1 is so close to 21 C, throttling valve 1 undertakes almost one hundred percent of the pressure reduction mission. At these points, the pressure of natural gas stream drops to values close to 1.8 MPa. It's recalled that 0 TNG1 ¼ 6 C. Fig. 9 shows the hourly averaged temperature of natural gas 00 stream outgoing from the shell and tube heat exchanger ðTNG1 Þ. According to the figure, the natural gas stream flowing through the heat exchanger could be warmed up to 10 C. The zone that is not
shown in the graph represents the times in which the natural gas doesn't enter the shell and tube heat exchanger as TNG-1 > 21 C. Fig. 10 presents the variation of the shell and tube water temperature in an hourly averaged format over a whole year. According to the figure, when TNG-1 > 21 C, the water temperature is equal to 7 C and the geothermal system is in standby state. Fig. 11 shows the hourly averaged line heater water temperature in the proposed system. The temperatures below 6 C (not shown in the figure) belong to the period during which the heating duty of the heater is zero and the heater is in standby state due to the high values of TNG-1. Fig. 12 depicts the hourly averaged heater heating duty in the both conventional and proposed systems in Gonbad Kavoos CGS over a whole year. It should be noted that as it was explained before, this graph is only valid for the first year of the proposed system application as in geothermal systems the obtainable energy from the Earth decreases over time. Therefore, the heater heating duty in the proposed system increases for the next years, though the amount of increase is considerable and the system can operate efficiently for at least 25 years. Fig. 13 also compares the amount of fuel consumption in the conventional and proposed systems. In this figure, obviously, the graphs have the same trend as Fig. 12 but the values are different. Fig. 14 illustrates the effect of long time operation on the performance of the geothermal heating unit by presenting the total annual obtainable benefits from the proposed system in Gonbad Kavoos CGS for each year. Evidently, for the first year of performance the annual benefit is over 7000 USD. Although, the annual benefit of the system decreases over time, the system still works impressively by the total annual benefit about 4500 USD in the 25th year of operation.
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Fig. 13. Comparison between the hourly fuel consumption in the conventional and proposed configuration in a whole year.
Fig. 14. The total benefit utilizing the proposed system in 25 years.
Based on the results presented above for the proposed geothermal system in Gonbad Kavoos CGS, Fig. 15 indicates the payback period of the system based on NPV method. In this assessment, the inflation rate has been considered 3% and the total annual O&M costs for the system has been considered 5% of the total capital cost. According to the figure, the payback period is
about 5 years. As the least period for the efficient performance of such systems is over 25 years, this value, by itself, can be a strong evidence for the high capability of the proposed system. It is not the first time that the configuration of CGS is proposed to be revised. Even, it's not the first time that employing renewable energies has been proposed for this aim. Therefore, the thermo
Fig. 15. NPV analysis on the system.
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Fig. 16. Determination of the appropriate capital cost for the first system.
Fig. 17. Determination of the appropriate capital cost for the second system.
economical performance of the proposed system in this work should be compared with the other already proposed systems aimed at demonstrating the superiority of this work relative to the other systems. In reference [4], flat plate solar collectors were suggested to be used to provide the required heat in the CGSs. The case study in this project was Akand station and through the whole simulation and calculation process the natural gas volume flow rate passing the station was assumed to be at the maximum level 1,00,000 m3/h. The simulation process addressed in this work was re-implemented for Gonbad Kavoos CGS by localizing and updating mass flow rate, weather data, solar irradiation and all other parameters effective in the simulation for this station. Fig. 16 shows the appropriate investment for this kind of system for Gonbad Kavoos station. Obviously from the figure, the optimum investment is 32,000 USD including 160 flat plate collectors. Detailed information about the flat plate solar collectors employed in the simulation process could be found in Ref. [4]. With this system the total annual benefit in Gonbad Kavoos CGS will be 4750 USD. In this system, as the collected solar heat was supposed to go through the line heater directly; therefore, when TNG-1 was high enough, the collected heat was wasted. In order for solving this problem, in the next study, the previous system was proposed to be equipped with a storage tank [5]. Therefore, this new system could store solar heat in a storage tank and employ it when required. This proposal also had its own problem. The line heater in this work was assumed to work similar to the actual available line heaters in the CGSs. These line heaters, in general, are uncontrollable i.e. they are
not capable to respond to the instant heating demand variations in the system. That's why the heater is set to provide the maximum required heat during a day. In this case, obviously, the heater fuel consumption is much more than necessity. Consequently, this system could not be as impressive as the other candidate systems. As equipping the line heaters with an automatic controller is not a difficult, the third system employing a controllable heater was proposed. This system was equipped with flat plat solar collectors, a capacious enough storage tank and a controllable line heater. The exergetic and energetic analysis implemented on the system proved the high efficiency of the system. Clearly, this system could outperform the other previous similar proposed systems. The case study for this system was also Akand station with a constant natural gas volume flow rate of 1,00,000 m3/h. The detailed information about this system is available in Ref. [6]. The simulation presented in this work was also localized and re-carried out for Gonbad Kavoos station. Fig. 17 shows the optimum initial investment on this system for Gonbad Kavoos station. Based on the figure, a solar thermal system including 80 flat plate solar collectors must be chosen for the case study. The investigations in the original paper showed that the volume of storage tank should be chosen 100 lit for each collector. Therefore, for this system the storage tank volume should be 8 m3. The total capital cost for this system will be 30400 USD and the total annual obtainable benefit will be 6350 USD. The above introduced systems as well as the current study are the only systems that have ever been proposed for providing the required heat in CGS by renewable energy sources. But, among these candidates, which one is the most efficient system?
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175
Fig. 18. IRR assessment results.
The answer is latent behind an IRR analysis. Fig. 18 shows the result of IRR assessment on the three candidates. As the figure shows, the system proposed in this work is much more efficient than others with IRR ¼ 0.155, whereas, the IRRs related to the first and second systems are 0.0395 and 0.131 respectively. This result proves that the current system is the best proposal ever presented for revising the CGS configuration aimed at reducing fuel consumption. 5. Conclusion Employing renewable energies (mainly solar energy) in order to provide the required heat in the CGSs had already been proposed by the authors. In this work, employing geothermal energy, as another source of renewable energy, in CGSs was proposed. Comprehensive energy analysis on the propounded system proved that it can significantly reduce the amount of fuel consumption in the CGSs. Therefore, the proposed configuration leads to greenhouse gases emission reduction and a considerable amount of economic providence. In order to select the best configuration of CGS accompanying with renewable energy systems proposed ever, all the suggested systems were compared thermo-economically. The results proved that the system proposed in this work is the best configuration suggested ever. Considering this fact that Iran is one the biggest natural gas producers of the world, and regarding the results of this research, the implementation of the proposed configuration in this work in all of the CGSs in Iran is strongly recommended. Acknowledgement The first and second author would like to thank the Lorestan Gas Company for financial support. References [1] Kargaran M, Arabkoohsar A, Hagighat-Hosini SJ, Farzaneh-Kord V. The second law analysis of natural gas behavior within a vortex tube. Therm Sci 2013;17(04):1079e92. [2] Farzaneh-Gord M, Hashemi S, Sadi M. Energy destruction in Iran's natural Gas pipe line network. Energy Explor Exploit 2009;25(6). [3] Farzaneh-Gord M, Sadi M. Enhancing energy output in Iran's natural gas pressure drop stations by cogeneration. J Energy Inst 01 December 2008;81(4):191e6.
[4] Farzaneh-Gord M, Arabkoohsar A, Rezaei M, Deymi Dasht-bayaz M. Feasibility of employing solar energy in natural gas pressure drop stations. J Energy Inst 2011;84(3):165e73. [5] Farzaneh-Gord M, Arabkoohsar A, Deymi Dasht-bayaz M, Farzaneh-Kord V. Feasibility of accompanying uncontrolled linear heater with solar system in natural gas pressure drop stations. Energy 2012;41(1):420e8. [6] Farzaneh-Gord M, Arabkoohsar A, Deymi Dasht-bayaz M, Machado L, Koury RNN. Energy and exergy analysis of natural gas pressure reduction points equipped with solar heat and controllable heaters. Renew Energy December 2014;72:258e70. [7] Bahadori Alireza, Zendehboudi Sohrab, Zahedi Gholamreza. A review of geothermal energy resources in Australia: current status and prospects. Renew Sustain Energy Rev May 2013;21:29e34. [8] Lund JW. Characteristics, development and utilization of geothermal resources. GHC Bull 2007;28:1e9. [9] Lund JW. Chena hot springs. GHC Quart Bull 2006;27:2e4. [10] www.nigc-golestan.ir/. [11] https://eosweb.larc.nasa.gov/cgi-bin/sse/retscreen.cgi?email¼[email protected]. gov. [12] Gultekin A, Aydın M, Sisman A, Determination of optimal distance between boreholes, thirty-ninth workshop on geothermal reservoir engineering, Stanford University, Stanford, California, February 24e26, 2014. [13] http://shaft-dig.blogfa.com/. [14] Sagia Zoi, Stegou Athina, Rakopoulos Constantinos. Borehole resistance and heat conduction around vertical ground heat exchangers. Open Chem Eng J 2012;6:32e40. [15] Farzaneh Gord M, Arabkoohsar A, Deymi Dashtebayaz M, Khoshnevis AB. New method of solar energy application in greenhouses in order to decrease fuel consumption. Int J Agric Biological Eng 2013;6(4):64e75. [16] Percoco Marco, Borgonovo Emanuele. A note on the sensitivity analysis of the internal rate of return. Int J Prod Econ January 2012;135(1):526e9. [17] Najafi-mod MH, Alizadeh A, Mohamadian A, Mousavi J. Investigation of relationship between air and soil temperature at different depths and estimation of the freezing depth (Case study: Khorasan Razavi). J Water Soil Ferdowsi Univ Mashhad 2008;22(2). [18] Incropera FP, DeWitt DP. Fundamentals of heat and mass transfer. 5th ed. New York: John Wiley; 2002. [19] McKibbin R. Mathematical models for heat and mass transport in geothermal systems. Transp Phenom Porous Media 1998:131e54. [20] Claesson J, Javed S. An analytical method to calculate borehole fluid temperatures for time-scales from minutes to decades. ASHRAE Trans 2011;117: 279e88. [21] Javed S. Thermal modelling and evaluation of borehole heat transfer. Chalmers University of Technology; 2012. €l. Simulation of [22] Philippe Pasquier DM, Bernier Michel, Kummert Michae ground coupled heat pump systems using a spectral approach. In: 13th Conference of International Building Performance; 2013. [23] Paul ND. The effect of grout thermal conductivity on vertical geothermal heat exchanger design and performance. Master of Science. South Dakota State University; 1996.
Glossary Cp and C: thermal capacity (kJ/kg C) Cn: cash flow in each year ($)
176 D and d: diameter (m) D0 : ineffective borehole length (m) h: enthalpy (kJ/kg) H: effective length of borehole (m) k: thermal conductivity (W/m C) LHV: lowering heating value (kJ/kg) L: length (m) m: water mass in heater (kg) _ mass flow rate (kg/s) m: n: number of year NPV: net present value ($) Nb: number of borehole q: heat transfer rate per unit length (kW/m) Q_ : heat transfer rate (kW) ri,j: radial distance between borehole i and j (m) r: internal rate of return R: thermal resistance (m2 C/W) S: Laplace transform variable t: time (s) T: temperature ( C) U: overall heat transfer coefficient (W/m2 C) Greek symbols
b: shape factor h: efficiency l: time step (1 h)
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a: thermal diffusion coefficient (m2/h) Subscriptions am: ambient b: borehole c: coil conv: convection cond: conduction f: fluid (in GHX) fuel: fuel gr: ground grt: grout GHX: ground heat exchanger h: heater hyd: hydrate or hydration i: internal in: inlet ls: line source NG: natural gas o: external oc: external oh heater out: outlet p: pipe soil: soil tv: throttling valve w: water w-h: water in heater
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Energy journal homepage: www.elsevier.com/locate/energy
Employing geothermal heat exchanger in natural gas pressure drop station in order to decrease fuel consumption M. Farzaneh-Gord a, R. Ghezelbash a, A. Arabkoohsar b, *, L. Pilevari c, L. Machado d, R.N.N. Koury d a
Department of Mechanical Engineering, Shahrood University of Technology, Shahrood, Iran Department of Mechanical Engineering, Minoodasht Branch, Islamic Azad University, Minoodasht, Iran Lorestan Gas Company, Khoram Abad, Iran d Department of Mechanical Engineering, Universidade Federal de Minas Gerais (UFMG), Belo Horizonte, Brazil b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 July 2014 Received in revised form 15 December 2014 Accepted 4 February 2015 Available online 20 March 2015
Natural gas stream must be preheated before pressure reduction takes place at natural gas pressure drop station (CGS). It ensures that the natural gas stream remains above hydrate-formation zone. The heater which is employed to provide the required heat consumes a huge amount of fuel. In this work, the conventional configuration of the natural gas pressure drop station is amended by taking advantage of geothermal energy to provide either all (if applicable) or a considerable portion of the required heat. To evaluate the proposed system in terms of economic and thermal efficiency, Gonbad Kavoos station has been chosen as a case study. Comprehensive thermo-economic analysis has been carried out on the proposed system. The results show that a system comprising 8 boreholes with 150 m depth and 0.15 m diameter each is the most efficient configuration for Gonbad Kavoos station. The achievable providence for the case study was calculated by technical correlations over the standard life time of geothermal systems. Comparison between the proposed system and the previous studied systems proved that the current configuration outperforms all the prior propounded configurations, with IRR (internal rate of return) ¼ 0.155. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Natural gas pressure drop station Natural gas Hydrate forming zone Geothermal energy Thermo-economic analysis
1. Introduction Natural gas is transported with high pressure (5e7 MPa) to overcome pressure losses related to the long distance that it should pass from refinery to consumption points such as cities, factories, power plants and etc. At consumption points, natural gas pressure should decrease. This pressure reduction takes place in natural gas pressure drop stations (CGS). At CGSs, pressure is generally reduced to 1.5e2 MPa. This mission is currently accomplished by throttling valves [1]. Due to the positive JouleeThomson coefficient of natural gas, pressure reduction leads to natural gas temperature drop. There is a minimum value from which natural gas temperature should not be lower. This temperature, which is called hydrate forming temperature, depends on natural gas compositions and pressure [2]. In fact, the hydrate formation zone is where the water droplets suspended in the natural gas begin to freeze and
* Corresponding author. E-mail address: [email protected] (A. Arabkoohsar). http://dx.doi.org/10.1016/j.energy.2015.02.093 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
subsequently obstruct the transportation pipeline. To prevent hydrate forming, natural gas stream should be warmed up to a specific level before pressure reduction. Generally, line heaters are employed in the CGSs to preheat natural gas stream, consuming huge amount of fuel for providing the required heat. That's why the performance of a CGS is not at a satisfactory level. A few studies have already been done to improve the performance of the CGSs by employing turbo-expander and cogeneration [2,3]. Energy and exergy analysis on the systems suggested in these studies showed there is a considerable improvement potential in the first law and the second law efficiencies of the CGSs. On the other hand, some studies have been done to propose new configurations in order to decrease the amount of fuel consumption at the CGSs by employing renewable energy sources. In the first study, Farzaneh-Gord et al. [4] proposed employing flat plate solar collectors to provide a portion of required heat in the CGSs. In the second work, they modified their first proposed system by adding a storage tank. In this configuration, the storage tank could collect solar heat during day and give it back to the heater at night when the
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amount of required heat is much more [5]. As a result, fuel providence increased to 14% whereas it was only 11% in the first propounded system. In the third work, Arabkoohsar et al. [6] substitute the conventional line heater in the CGSs by an automatic controllable heater and compared the thermal behaviour of this system with the aforementioned systems. The comparison result showed the superiority of the last work done relative to the prior ones. Another source of renewable energy is geothermal heat. Geothermal energy is cost effective, reliable, sustainable, and environmentally friendly [7]. There are some disadvantages with geothermal heating systems such as releasing greenhouse gases from the geothermal boreholes and huge capital cost. However, it should be noted that, firstly, the amount of released greenhouse gases from the geothermal boreholes are much lower per energy unit than those of fossil fuels and, secondly, free obtainable energy from this source of heat after preliminary investment makes these system totally efficient and affordable. As a result, not only the geothermal power has the potential to help mitigate global warming, but also it is completely affordable in terms of economic [8,9]. In the current study, the application of geothermal energy in the CGSs is proposed in order to modify their conventional configuration and reducing the amount of fuel consumption. The detailed information about the proposed configuration is presented in the next sections. It is noteworthy that in order to evaluate the applicability and justifiably of the proposed system, Gonbad Kavoos station in Iran has been selected as a case study. Finally, comprehensive thermo-economic comparison among the current proposed system and the previous propounded systems (for the same objective) employing renewable energies is carried out.
2. Natural gas pressure drop station 2.1. The conventional configuration Based upon the primary information presented about a CGS, when a natural gas pipeline approaches a city, natural gas pressure must be reduced to the distribution level. A CGS is a pressure reduction point in which control and throttling valves accompanying with some other equipments are employed to reduce the natural gas pressure. A schematic diagram of a typical CGS is shown in Fig. 1. According to Fig. 1, high pressure natural gas (5 MPa < PNG1 < 7 MPa) enters the line heater to be warmed up before the throttling procedure. The inlet temperature (TNG-1) is basically equal to soil temperature because transportation pipeline is buried underground. In fact, natural gas stream should be preheated
165
before passing through the throttle valve to ensure that its temperature remains above the hydrate-formation temperature to prevent liquid and solid phase formation in natural gas stream. The desired value for the natural gas temperature after preheating procedure (TNG-2) depends on the inlet pressure. The higher natural gas pressure, the more the natural gas needs to be heated. As it was explained, line heaters are generally used in the CGSs to do preheating mission by burning natural gas. Line heaters are generally full of water. The water transfers heat from the fire tubes of heater to the natural gas stream flowing through a coil immersed in the water bath. Based on the standard outlet pressure of CGS as well as the natural gas compositions, the hydrate forming temperature (Thyd) could be calculated by thermodynamics models. The outlet natural gas stream temperature (TNG-3) is then selected 3e5 C above the hydrate forming temperature [6]. 2.2. The case study The proposed model was employed to study the thermal behaviour of Gonbad Kavoos CGS. Therefore, the effectiveness of the proposed system can be demonstrated in practice. Gonbad Kavoos station is located in Gonbad Kavoos city, one of the biggest cities in Golestan province in the north-east of Iran. For Gonbad Kavoos station, based on thermodynamic relations and the natural gas compositions, the hydrate temperature has been found to be 2 C [10]. Adopting the mean value of 4 C for safety factor, natural gas temperature should never be below 6 C in Gonbad Kavoos CGS. Gonbad Kavoos station is mainly fed by Torkman refinery for which natural gas compositions were reported in Table 1. The station inlet and outlet pressures are 5.9 MPa and 1.8 MPa, respectively. The reported value of temperature drop due to this range of pressure reduction is 15 C [10]. Therefore, it could be claimed that for such natural gas compositions, 100 kPa pressure reduction causes almost 0.366 C temperature drop. Consequently, the minimum temperature that natural gas stream should meet before pressure reduction procedure is the summation of the hydrate forming temperature, the safety factor value and the temperature drop through the throttling valve. This value for Gonbad Kavoos CGS is 21 C (2 þ 4 þ 15 ¼ 21 C). More technical information related to Gonbad Kavoos CGS was provided by Golestan Gas Company. Table 2 details this information. 2.3. The proposed configuration This section presents the detailed description of the proposed configuration. The proposed system takes advantage of geothermal energy to provide either the whole (if possible) or a portion of the required heat in CGS. Fig. 2 shows the proposed system schematic. According to the figure, if the inlet natural gas temperature (TNG 1) is above 21 C, natural gas goes directly toward the main throttling valve through a bypass line, because in this case, TNG-3 will remain above 6 C after pressure reduction process. Otherwise, natural gas should be preheated. In contrast with the conventional configuration in which there is only one throttling valve, in the proposed system the pressure reduction is supposed to be accomplished in two steps. The first step of pressure reduction is done by throttling valve 1. The second step is also carried out by
Table 1 Torkman natural gas compositions.
Fig. 1. The schematic configuration of a CGS.
CH4
C2H6
C3H8
i-C4H10
i-C5H12
C6H14
C7H16
N2
CO2
94.21
2.25
0.53
0.36
0.26
0.17
0.18
1.90
0.14
166
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Table 2 Properties of Gonbad Kavoos CGS line heater. Natural gas source Natural gas density Lower heating value Natural gas inlet pressure Natural gas outlet pressure Natural gas maximum flow rate Surface area of fire tube heater Water capacity Diameter of coil Length of coil Heater maximum working temperature Heater maximum heating duty
Torkman 0.587 kg/m3 4.811 MJ/kg 5.9 MPa 1.8 MPa 80,000 Sm3/h 88.1 m2 36 m3 0.1 m 280 m 88 C 1750 kW
throttling valve 2. Between the first and the second steps of pressure reduction there are two stages of preheating (geothermal heater and line heater). The first throttling valve reduces the nat0 ural gas pressure down to that level in which TNG1 ¼ 6 C. Note that as TNG-1 is variable, therefore, the amount of pressure drop by 0 throttling valve 1 is also variable and as a result, PNG1 is change able. As the TNG1 < 21 C, therefore, the amount of temperature drop in this step is less than 15 C. Note that, the higher values of TNG-1, the higher portion of pressure reduction mission is carried out by throttling valve 1. In contrast, the lower values of TNG-1, the higher portion of pressure reduction is done by throttling valve 2 and the contribution of throttling valve 1 in pressure reduction procedure is less. As it was explained before, the value of 6 C is the minimum temperature that Torkman natural gas is allowed to meet so as to avoid hydrate forming [6,10]. The details of geothermal system designed for the proposed configuration at this work are presented in the next section. Regarding its characteristics, the geothermal heating unit is able to increase temperature up to 18.9 C [11], and even this temperature would rarely be achievable during the whole year. Therefore, nat0 ural gas pressure decreases down to that level at which TNG1 is equal to its minimum allowable value. In this way, maximum utilization of geothermal energy would be possible. Imagine that throttling valve 1 is omitted from the system, in this case, when TNG-1 > 12e13 C (mainly during spring and summer), the geothermal system could not take part in preheating process and it is the line heater, again, that should provide one hundred percent of the required heat. Therefore, throttling valve 1 is placed at the
station inlet, and after that the geothermal heater is set to provide geothermal heat. After passing through the geothermal heater, natural gas stream flows through the line heater to be heated up to higher temperatures, if necessary. It's noticeable that, depending on 00 00 the value of TNG1 and PNG1 , the line heater heating duty changes. It is also noteworthy that, the geothermal heat exchanger is a simple shell and tube heat exchanger with water as intermediate fluid. Cold water enters long vertical tubes in geothermal boreholes and receives geothermal heat while flowing through the boreholes. Warm water, then, comes back to the shell and tube heat exchanger and gives this heat back, increasing the natural gas temperature. In the design stage of geothermal heating unit in the proposed system, some important parameters (such as the number, the depth and the diameter of boreholes, distance between the boreholes, operating fluid mass flow rate and the optimal volume of shell and tube heat exchanger) should be determined. The investigations prove that the maximum achievable temperature in Earth's crust is just equal to the annual average weather temperature in the corresponding area. This value for Gonbad Kavoos is 18.9 C. This temperature is uniformly available for all days of the year after depth of 100 m in this city. After this depth, obviously, the more depths for the borehole, the more energy is obtainable from the borehole. However, deeper boreholes cause higher initial investment for the project. In Iran, boreholes up to 150 m cost 25 USD/m, including all the complementary equipments such as heat exchanger and piping costs [13]. Due to difficulties for delving boreholes with higher depths, the price increases significantly for such cases. As low temperature heat is required and economic considerations are the pivotal factors in this project, the depth 150 m was chosen as the ideal depth of boreholes in this work. On the other hand, BH factor for geothermal boreholes is defined as the ratio of distance between the boreholes to the depth of boreholes and it is recommended to be from 0.05 to 0.2 [12]. The value of BH for this work was considered 0.1. As a result, the distance between the boreholes should be 15 m. For determining boreholes diameters, it should be noted that, although the smaller the diameter of borehole, the greater the heat transfer efficiency; bigger diameters for boreholes lead to bigger heat transfer area and consequently higher heat transfer rate in unit borehole length. Therefore, trade off approach could be adopted for determining the most appropriate diameter for the boreholes. However, for the sake of simplification of designing a geothermal
Fig. 2. The proposed system schematic.
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system, it is recommended to select boreholes diameters in range of 90 mme190 mm [14]. The chosen value for the diameters of boreholes in this work was 150 mm. Naturally, the selection of appropriate number for boreholes depends on the amount of required heat as well as the amount of initial investment. Also, the volume of shell and tube heat exchanger depends on the number of boreholes. Obviously, heat exchanger with higher capacity and volume requires more investments. Therefore, the number of boreholes (consequently the volume of heat exchanger) should be defined on a basis of a thermo economic trade off. Fig. 3 shows the proposed array for the boreholes in an L shape 45 m 60 m field with regular 15 m gaps between the boreholes. According to the figure, the most appropriate number of boreholes for the case study is 8, resulted from a thermo-economic evaluation. It is worth mentioning that, the best configuration in terms of the amount of obtainable energy is an L shape array of boreholes [7]. Table 3 details the proposed geothermal system information for the case study. The results of economic analysis to select the most efficient layout of geothermal system will be explained thoroughly in conclusion section. The selective configuration will also be assessed economically by the IRR (internal rate of return) method. Through this assessment, the performance of the proposed system in this work is compared with the previous proposed systems employing renewable energies in the CGSs. The IRR is the rate of return used to measure and compare the profitability of different projects. The term internal refers to the fact that its calculation does not incorporate environmental factors such as the interest rate or inflation. Based on the definition, the IRR of a project is the rate of return that makes the NPV (net present value) of all cash flows from a particular investment equal to zero. N X
Cn NPV ¼ ¼0 ð1 þ rÞn n¼0
(1)
where, n, Cn and r refer to the number of year, cash flow in the project in the corresponding year and IRR respectively. Overall, the higher a project's IRR, the more desirable it is to undertake the project [15,16]. The critical parameter for calculating the IRR of a project is the number of year in which the NPV should be equal to zero. This totally depends on the investor of the project; however, for such industrial projects the duration of 8 years seems to be a good choice. 3. Energy demand In order to assess the proficiency of the proposed system, a comprehensive thermal analysis on the proposed system and the conventional system of the CGS should be done. The fuel consumption in the both systems must be calculated and compared.
Table 3 The geothermal system details for Gonbad Kavoos CGS. The numbers of boreholes Diameter of each borehole Depth of each borehole Distance between the boreholes Operating fluid Operating fluid mass flow rate Operating fluid velocity Pipe material Pipe thermal conductivity coefficient Pipe diameter Pipe thickness Grout thermal conductivity coefficient Heat exchanger type Earth's temperature Earth's thermal conductivity coefficient Earth's thermal diffusion coefficient Heat exchanger coil length Heat exchanger coil diameter Heat exchanger volume
8 0.15 m 150 m 15 m Water 0.14 kg/s 0.3 m/s High density polyethylene 0.42 W/K 0.026 m 0.006 m 1.25 W/K Shell and tube 18.9 C 1.5 W/K 0.0778 m2/day 112 m 0.1 m 12.5 m3
Considering the required investment for implementing such system and the fuel consumption comparison, it would be revealed that how efficient the proposed system is.
3.1. Energy demand in the conventional configuration Considering the information presented in Section 2 and the schematic diagram in Fig. 2, one could analyze the thermal behaviour of conventional configuration. According to Fig. 2, the natural gas temperature at the line heater exit can be calculated by:
TNG2 ¼ Thyd þ Tc þ DTtv
(2)
In which, Tc and DTtv represent the safety factor value (4 C) and the temperature drop due to pressure reduction through the throttling valve. Thyd also refer to the hydrate forming temperature of natural gas stream. For Torkman natural gas Thyd ¼ 2 C. The natural gas pressure and temperature at the station inlet are measured. Once the natural gas temperature and pressure at the heater exit are known, the rate of required heat could be calculated as below:
Q_ gh ¼ m_ NG ðhNG2 hNG1 Þzm_ NG $CPNG ðTNG2 TNG1 Þ
(3)
where, m_ NG , hNG, hNG and CPNG are the natural gas mass flow rate, enthalpy and thermal capacity, respectively. As the natural gas flows through a buried pipeline in depth of 1.2 m, the gas temperature is equal to the soil temperature in this depth which is a functional of ambient temperature (Tam) as follow [17]: 2 TNG1 ¼ Tsoil ¼ 0:0084 Tam þ 0:3182 Tam þ 11:403
(4)
The heater provides the required heat by burning natural gas as fuel. Considering the thermal efficiency of heater, hh, one could obtain the fuel mass flow rate, m_ fuel , as below:
m_ fuel ¼
Fig. 3. The proposed layout for arranging the boreholes.
167
. lþ1 l 3600 Twh Q_ gh þ mwh $Cpw Twh hh $LHV
(5)
In which, LHV, mw-h, Cp-w are the lowering heating value of fuel, the mass and thermal capacity of water in the heater, respectively. hh is the thermal efficiency of the heater which is in range of 0.35e0.5 [4]. In this work, this parameter was considered to be 0.40. Also the subscripts (l) and (l þ 1) count hourly periods. Since all measurements such as ambient temperature are done in hourly format,
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this equation is divided by 3600 s to present the instantaneous fuel mass flow rate of the heater. Tw-h also refers to the temperature of water in the heater. As the immersed coil in water bath could be considered as a pipe in constant temperature environment, based on the correlation presented by Incropera and DeWitt [18], Tw-h could be calculated by:
Twh TNG2 pDoc Lc Uc ¼ eY ; Y ¼ Twh TNG1 m_ NG CPNG
(7)
The previous studies suggest that the line heaters are so designed that in which Uc ¼ 568 W/m2 K [4]. The next step is presenting the formulation of energy analysis on the proposed configuration. Regarding Fig. 4, two distinct control volumes should be considered for doing energy analysis on the proposed configuration. As the figure shows, the first control volume consists of three items including the throttling valve, the shell and tube heat exchanger and the underground-vertical heat exchanger. The energy balance equation for this control volume could be as [19e22]:
mw Cw
dTw _P ¼ Q_ GHX Q_ NG1 W dt
(8)
where, mw, Cw and Tw are the mass, thermal capacity and temper_ P refer to ature of working fluid (water). Also, Q_ GHX ; Q_ NG1 and W the absorbed heat from the earth by the operating fluid, the gained heat by the natural gas stream and the work of pumps in the system, respectively. There are various types of geothermal heat exchangers for taking advantage of the Earth's heat such as horizontal and vertical heat exchangers. Regarding the fact that vertical heat exchangers have more efficiency, occupy less room and require less energy for pumping the working fluid in comparison with horizontal heat exchangers, vertical heat exchangers are considered to be used in this work. The rate of absorbed heat by the underground-vertical heat exchanger highly depends on the thermal and physical properties of soil, applied tube and material of borehole structure. Over the simulation process, all the aforementioned properties are considered to be constant over time, and the only variable parameter is the working fluid temperature. Note that the length of boreholes and the gap between the boreholes are identical. The total heat absorbed by the underground-vertical heat exchanger set could be given by the following equation:
Q_ GHX ¼ q$Nb $H
(9)
where, Nb and H are the number of boreholes and the effective length of each borehole, respectively. q is also the absorbable heat for the unit effective length of tube by one borehole and could be obtained by the following equation:
q¼
m_ f $Cf $ðTout Tin Þ H
Tout ¼ Tgr
(10)
Tn which, m_ f , Cf, Tout and Tin are the mass flow rate, thermal capacity, outlet temperature and inlet temperature of working fluid, respectively. Tout could be calculated by Refs. [19e22]:
13
0
6 B C7 6 B C7 Z∞ nt 7 6X ql ql1 B Ils ðH$S; D0 $SÞ C 6 B C7 þ6 $B dS Ie $ 7 C 6 C7 4$p$kgr B H$S2 6 l¼1 B pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C7 4 @ 4aðt1t Þffi A5 l
(6)
where, Doc, Uc and Lc are the external diameter, length and overall heat transfer coefficient of the coil, respectively. Rearranging the above equation, one could drive the following equation:
. 1 eY Twh ¼ TNG2 TNG1 eY
2
þ q$Rb
l1
q$H 2$m_ b $Cb (11)
where, l counts again the hourly time steps as all the calculation process is mainly done on a basis of hourly periods. Therefore, ql and ql-1 represent the absorbed heat in time steps l and l-1, respectively. Tgr, kgr, a also refer to ground temperature, thermal conductivity coefficient and diffusion coefficient of the borehole, respectively. Defining S as Laplace transform variable in the short-term response, h ¼ H.S, d ¼ D0 .S and D0 as the ineffective length of tube in the borehole, the other factors in the above equation could be given by the following equations.
Ils ðh; dÞ ¼ 2 ierf ðhÞ þ 2 ierf ðh þ 2dÞ ierf ð2h þ 2dÞ ierf ð2dÞ (12) where, the error function erf(z) (which is the normalized form of Gaussian function) is defined as:
2 erf ðzÞ ¼ pffiffiffi p
Zz
ez dz 2
(13)
0
Subsequently:
2 1 ierf ðzÞ ¼ z$erf ðzÞ pffiffiffi 1 ez p
(14)
The function Ie could also be given as below:
Ie ðsÞ ¼
N X N 2 1 X eri;j Nb i¼1 j¼1
S2
(15)
Here, ri,j is the radial distance between the boreholes i and j (i s j). The last important item in this equation is Rb which refers to the overall thermal resistance of borehole and may be calculated by:
Rb ¼
1 ðRconv þ Rcond Þ þ Rgr 2
(16)
where, the subscriptions conv, cond and gr refer to the convective thermal resistance coefficient of working fluid within the tubes, the conductive thermal resistance coefficient of tubes and the conductive thermal resistance coefficient of borehole wall. These parameters can be obtained by the following equations, respectively:
Rconv ¼
1 pdi hf
(17)
Rcond ¼
lnðdo =di Þ 2$p$kp
(18)
Rgr ¼
1 kgr $b0 $ðdb =do Þb1
(19)
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169
Fig. 4. The separate control volumes for analysing the proposed system.
In the above equations, di, do and db refer to the internal and external diameter of tube and the diameter of borehole, respectively. hf, kp and kgr also represent the convective heat transfer coefficients for working fluid, the conductive heat transfer coefficient of tube and the conductive heat transfer coefficient of borehole wall, respectively. b0 and b1 are also shape factors and are given in Ref. [23]. The work of pumps in a geothermal system is almost 2e3% of the total obtainable heat from thee geothermal boreholes. However, the following equation can give the exact value of pumps work in a geothermal system:
_ _ P ¼ mf $HT $DP $h W P rf
(20)
where, DP is the amount of pressure drop along the vertical pipes which is considered to 0:4 kPa m . rf is water density and equal to 1000 kg/m3 and hP is the pump efficiency which is considered to be 80%. HT is also the total effective length of pipes in the boreholes and can be given by: 0
HT ¼ 2Nb ðH þ D Þ ¼ 2$8$ð150 þ 3Þ ¼ 2448 m
(21)
Finally, considering the water mass flow rate in the system ðm_ NG ¼ 0:14 kg=sÞ as well as a safety factor equal to 1.5, the work of pumps in the system will be equal to 256.5 W. Employing the detailed formulation above, one could make a system of equation including Equations (10) and (11). This system of equation would be calculable employing computational and numerical methods. The employed computational method in this work is a modified version (by the authors) of the already available function quadgk in Matlab. The other unknown parameter in Equation (8) is the heat transfer rate to the natural gas stream which is given by the following equation:
00 0 Q_ NG1 ¼ m_ NG $CpNG $ TNG1 TNG1
(22)
0 As it was explained in the previous section, the value of TNG1 is 00 equal to 6 C whereas the value of TNG1 is variable and a functional
of the heat exchanger water temperature. This parameter is defined as below:
00 0 TNG1 ¼ Tw $ 1 eY þ TNG1 $eY
(23)
On the other hand, the energy balance for the second control volume shown in Fig. 4 could be written as:
mw Cw
dTw ¼ Q_ h Q_ NG2 dt
(24)
where, the heat transfer rate given to natural gas stream by the line heater could be given by:
00 Q_ NG2 ¼ m_ NG $CpNG $ TNG2 TNG1
(25)
Considering equation above, the heater heating duty in the proposed system could be calculated by:
Q_ h ¼ Q_ NG2 þ
lþ1 l mwh $Cw $ Twh Twh 3600
(26)
Therefore, the mass flow rate of consuming fuel to provide this amount of energy, considering the heater thermal efficiency, is computed by:
m_ fuel ¼
Q_ h hh $LHV
(27)
4. Results and discussion In this section the results of implementing the proposed system in Gonbad Kavoos CGS are presented. The key parameters required in simulation process are the ambient temperature, the natural gas inlet temperature and mass flow rate in Gonbad Kavoos CGS over a whole year. The database used for this work includes the aforementioned data in the year 2013.
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Fig. 5. The monthly-hourly average ambient temperature in Gonbad Kavoos in 2013.
Fig. 6. The monthly averaged mass flow rate of natural gas in Gonbad Kavoos station in 2013.
Fig. 5 shows the monthly-hourly averaged of ambient temperature and natural gas inlet temperature in Gonbad Kavoos. The monthly-hourly averaged temperature is much more appropriate than daily averaged temperature for this project because
temperature difference between daily and night hours is high in Gonbad Kavoos. Therefore, daily averaged temperature could not present acceptable estimation of actual temperatures and leads to considerable deviations in calculation process.
Fig. 7. Selecting the optimal capital cost for the project.
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171
Fig. 8. The natural gas stream pressure after the first throttling valve over a whole year.
Fig. 9. The natural gas stream hourly temperature outgoing from the shell and tube heat exchanger over a whole year.
As expected, the least entrance temperature belongs to January with a value below 5 C and the highest temperature in August with a value more than 31 C. Based on what explained about the system performance details, for the temperatures over the red line (21 C), the natural gas stream can directly go through the second throttling valve and there is no need to warm up the natural gas stream. For Gonbad Kavoos station, this situation is observed in almost four months of the year from early June to late September. Fig. 6 shows the monthly averaged mass flow rate of natural gas stream in Gonbad Kavoos CGS. As the figure shows, more mass flow rates occur during winter where the maximum natural gas consumption takes place in Gonbad Kavoos city in January as the
coldest months of the year. In contrast, the least consumption occurs in May. Fig. 7 shows how the amount of initial investment for the proposed system is chosen. For such system, naturally, increasing the number of boreholes leads to more obtainable energy and higher cost of capital simultaneously. Also, the amount of required energy is limited and known. Therefore, there should be a logical index for selecting the most appropriate amount of initial investment. For such cases, the capital cost value must be selected equal to the total annual cost of the system pivotal parameter (here, the annual price of fuel). As the figure shows, the capital cost of the system can be equal to the annual fuel cost if the numbers of boreholes are 8. Note
Fig. 10. The hourly averaged shell and tube heat exchanger water temperature over a whole year.
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Fig. 11. The hourly averaged line heater temperature over the whole year.
Fig. 12. Comparison between the hourly averaged heater heating duty in the conventional and proposed configuration in a whole year.
that the capital cost comprises all the required costs including heat exchanger price, boreholes delving cost, throttling valve price and etc. As it was explained before, in Iran, the total cost for such system is 25 USD/m. It should also be noted that the Fig. 7 has been prepared employing the average fuel consumption of the proposed system in 25 years of performance (14.39 m3/h). Geothermal heating systems are more efficient in early years of operation and the amount of annual achievable providence from the system reduces over time. Therefore, the average fuel consumption for the first year of operation is much less than this value and equal to 11.26 m3/h. The figures below are presented based on the data obtained for the first year of operation of a system taking advantage of an L shape array boreholes including 8 boreholes with regular distance of 15 m and 150 m depth each. Fig. 8 shows the natural gas stream 0 pressure right after the first throttle valve ðPNG1 Þ. Therefore, this figure also shows the contribution of throttling valve 1 in the pressure reduction mission over the year. As the figure shows, through warm months of the year the contribution of throttling valve 1 is zero as the natural gas temperature was more than 21 C. During these times, the natural gas pressure remains at 5.9 MPa. On the other hand, for the times in which TNG-1 is so close to 21 C, throttling valve 1 undertakes almost one hundred percent of the pressure reduction mission. At these points, the pressure of natural gas stream drops to values close to 1.8 MPa. It's recalled that 0 TNG1 ¼ 6 C. Fig. 9 shows the hourly averaged temperature of natural gas 00 stream outgoing from the shell and tube heat exchanger ðTNG1 Þ. According to the figure, the natural gas stream flowing through the heat exchanger could be warmed up to 10 C. The zone that is not
shown in the graph represents the times in which the natural gas doesn't enter the shell and tube heat exchanger as TNG-1 > 21 C. Fig. 10 presents the variation of the shell and tube water temperature in an hourly averaged format over a whole year. According to the figure, when TNG-1 > 21 C, the water temperature is equal to 7 C and the geothermal system is in standby state. Fig. 11 shows the hourly averaged line heater water temperature in the proposed system. The temperatures below 6 C (not shown in the figure) belong to the period during which the heating duty of the heater is zero and the heater is in standby state due to the high values of TNG-1. Fig. 12 depicts the hourly averaged heater heating duty in the both conventional and proposed systems in Gonbad Kavoos CGS over a whole year. It should be noted that as it was explained before, this graph is only valid for the first year of the proposed system application as in geothermal systems the obtainable energy from the Earth decreases over time. Therefore, the heater heating duty in the proposed system increases for the next years, though the amount of increase is considerable and the system can operate efficiently for at least 25 years. Fig. 13 also compares the amount of fuel consumption in the conventional and proposed systems. In this figure, obviously, the graphs have the same trend as Fig. 12 but the values are different. Fig. 14 illustrates the effect of long time operation on the performance of the geothermal heating unit by presenting the total annual obtainable benefits from the proposed system in Gonbad Kavoos CGS for each year. Evidently, for the first year of performance the annual benefit is over 7000 USD. Although, the annual benefit of the system decreases over time, the system still works impressively by the total annual benefit about 4500 USD in the 25th year of operation.
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173
Fig. 13. Comparison between the hourly fuel consumption in the conventional and proposed configuration in a whole year.
Fig. 14. The total benefit utilizing the proposed system in 25 years.
Based on the results presented above for the proposed geothermal system in Gonbad Kavoos CGS, Fig. 15 indicates the payback period of the system based on NPV method. In this assessment, the inflation rate has been considered 3% and the total annual O&M costs for the system has been considered 5% of the total capital cost. According to the figure, the payback period is
about 5 years. As the least period for the efficient performance of such systems is over 25 years, this value, by itself, can be a strong evidence for the high capability of the proposed system. It is not the first time that the configuration of CGS is proposed to be revised. Even, it's not the first time that employing renewable energies has been proposed for this aim. Therefore, the thermo
Fig. 15. NPV analysis on the system.
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Fig. 16. Determination of the appropriate capital cost for the first system.
Fig. 17. Determination of the appropriate capital cost for the second system.
economical performance of the proposed system in this work should be compared with the other already proposed systems aimed at demonstrating the superiority of this work relative to the other systems. In reference [4], flat plate solar collectors were suggested to be used to provide the required heat in the CGSs. The case study in this project was Akand station and through the whole simulation and calculation process the natural gas volume flow rate passing the station was assumed to be at the maximum level 1,00,000 m3/h. The simulation process addressed in this work was re-implemented for Gonbad Kavoos CGS by localizing and updating mass flow rate, weather data, solar irradiation and all other parameters effective in the simulation for this station. Fig. 16 shows the appropriate investment for this kind of system for Gonbad Kavoos station. Obviously from the figure, the optimum investment is 32,000 USD including 160 flat plate collectors. Detailed information about the flat plate solar collectors employed in the simulation process could be found in Ref. [4]. With this system the total annual benefit in Gonbad Kavoos CGS will be 4750 USD. In this system, as the collected solar heat was supposed to go through the line heater directly; therefore, when TNG-1 was high enough, the collected heat was wasted. In order for solving this problem, in the next study, the previous system was proposed to be equipped with a storage tank [5]. Therefore, this new system could store solar heat in a storage tank and employ it when required. This proposal also had its own problem. The line heater in this work was assumed to work similar to the actual available line heaters in the CGSs. These line heaters, in general, are uncontrollable i.e. they are
not capable to respond to the instant heating demand variations in the system. That's why the heater is set to provide the maximum required heat during a day. In this case, obviously, the heater fuel consumption is much more than necessity. Consequently, this system could not be as impressive as the other candidate systems. As equipping the line heaters with an automatic controller is not a difficult, the third system employing a controllable heater was proposed. This system was equipped with flat plat solar collectors, a capacious enough storage tank and a controllable line heater. The exergetic and energetic analysis implemented on the system proved the high efficiency of the system. Clearly, this system could outperform the other previous similar proposed systems. The case study for this system was also Akand station with a constant natural gas volume flow rate of 1,00,000 m3/h. The detailed information about this system is available in Ref. [6]. The simulation presented in this work was also localized and re-carried out for Gonbad Kavoos station. Fig. 17 shows the optimum initial investment on this system for Gonbad Kavoos station. Based on the figure, a solar thermal system including 80 flat plate solar collectors must be chosen for the case study. The investigations in the original paper showed that the volume of storage tank should be chosen 100 lit for each collector. Therefore, for this system the storage tank volume should be 8 m3. The total capital cost for this system will be 30400 USD and the total annual obtainable benefit will be 6350 USD. The above introduced systems as well as the current study are the only systems that have ever been proposed for providing the required heat in CGS by renewable energy sources. But, among these candidates, which one is the most efficient system?
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175
Fig. 18. IRR assessment results.
The answer is latent behind an IRR analysis. Fig. 18 shows the result of IRR assessment on the three candidates. As the figure shows, the system proposed in this work is much more efficient than others with IRR ¼ 0.155, whereas, the IRRs related to the first and second systems are 0.0395 and 0.131 respectively. This result proves that the current system is the best proposal ever presented for revising the CGS configuration aimed at reducing fuel consumption. 5. Conclusion Employing renewable energies (mainly solar energy) in order to provide the required heat in the CGSs had already been proposed by the authors. In this work, employing geothermal energy, as another source of renewable energy, in CGSs was proposed. Comprehensive energy analysis on the propounded system proved that it can significantly reduce the amount of fuel consumption in the CGSs. Therefore, the proposed configuration leads to greenhouse gases emission reduction and a considerable amount of economic providence. In order to select the best configuration of CGS accompanying with renewable energy systems proposed ever, all the suggested systems were compared thermo-economically. The results proved that the system proposed in this work is the best configuration suggested ever. Considering this fact that Iran is one the biggest natural gas producers of the world, and regarding the results of this research, the implementation of the proposed configuration in this work in all of the CGSs in Iran is strongly recommended. Acknowledgement The first and second author would like to thank the Lorestan Gas Company for financial support. References [1] Kargaran M, Arabkoohsar A, Hagighat-Hosini SJ, Farzaneh-Kord V. The second law analysis of natural gas behavior within a vortex tube. Therm Sci 2013;17(04):1079e92. [2] Farzaneh-Gord M, Hashemi S, Sadi M. Energy destruction in Iran's natural Gas pipe line network. Energy Explor Exploit 2009;25(6). [3] Farzaneh-Gord M, Sadi M. Enhancing energy output in Iran's natural gas pressure drop stations by cogeneration. J Energy Inst 01 December 2008;81(4):191e6.
[4] Farzaneh-Gord M, Arabkoohsar A, Rezaei M, Deymi Dasht-bayaz M. Feasibility of employing solar energy in natural gas pressure drop stations. J Energy Inst 2011;84(3):165e73. [5] Farzaneh-Gord M, Arabkoohsar A, Deymi Dasht-bayaz M, Farzaneh-Kord V. Feasibility of accompanying uncontrolled linear heater with solar system in natural gas pressure drop stations. Energy 2012;41(1):420e8. [6] Farzaneh-Gord M, Arabkoohsar A, Deymi Dasht-bayaz M, Machado L, Koury RNN. Energy and exergy analysis of natural gas pressure reduction points equipped with solar heat and controllable heaters. Renew Energy December 2014;72:258e70. [7] Bahadori Alireza, Zendehboudi Sohrab, Zahedi Gholamreza. A review of geothermal energy resources in Australia: current status and prospects. Renew Sustain Energy Rev May 2013;21:29e34. [8] Lund JW. Characteristics, development and utilization of geothermal resources. GHC Bull 2007;28:1e9. [9] Lund JW. Chena hot springs. GHC Quart Bull 2006;27:2e4. [10] www.nigc-golestan.ir/. [11] https://eosweb.larc.nasa.gov/cgi-bin/sse/retscreen.cgi?email¼[email protected]. gov. [12] Gultekin A, Aydın M, Sisman A, Determination of optimal distance between boreholes, thirty-ninth workshop on geothermal reservoir engineering, Stanford University, Stanford, California, February 24e26, 2014. [13] http://shaft-dig.blogfa.com/. [14] Sagia Zoi, Stegou Athina, Rakopoulos Constantinos. Borehole resistance and heat conduction around vertical ground heat exchangers. Open Chem Eng J 2012;6:32e40. [15] Farzaneh Gord M, Arabkoohsar A, Deymi Dashtebayaz M, Khoshnevis AB. New method of solar energy application in greenhouses in order to decrease fuel consumption. Int J Agric Biological Eng 2013;6(4):64e75. [16] Percoco Marco, Borgonovo Emanuele. A note on the sensitivity analysis of the internal rate of return. Int J Prod Econ January 2012;135(1):526e9. [17] Najafi-mod MH, Alizadeh A, Mohamadian A, Mousavi J. Investigation of relationship between air and soil temperature at different depths and estimation of the freezing depth (Case study: Khorasan Razavi). J Water Soil Ferdowsi Univ Mashhad 2008;22(2). [18] Incropera FP, DeWitt DP. Fundamentals of heat and mass transfer. 5th ed. New York: John Wiley; 2002. [19] McKibbin R. Mathematical models for heat and mass transport in geothermal systems. Transp Phenom Porous Media 1998:131e54. [20] Claesson J, Javed S. An analytical method to calculate borehole fluid temperatures for time-scales from minutes to decades. ASHRAE Trans 2011;117: 279e88. [21] Javed S. Thermal modelling and evaluation of borehole heat transfer. Chalmers University of Technology; 2012. €l. Simulation of [22] Philippe Pasquier DM, Bernier Michel, Kummert Michae ground coupled heat pump systems using a spectral approach. In: 13th Conference of International Building Performance; 2013. [23] Paul ND. The effect of grout thermal conductivity on vertical geothermal heat exchanger design and performance. Master of Science. South Dakota State University; 1996.
Glossary Cp and C: thermal capacity (kJ/kg C) Cn: cash flow in each year ($)
176 D and d: diameter (m) D0 : ineffective borehole length (m) h: enthalpy (kJ/kg) H: effective length of borehole (m) k: thermal conductivity (W/m C) LHV: lowering heating value (kJ/kg) L: length (m) m: water mass in heater (kg) _ mass flow rate (kg/s) m: n: number of year NPV: net present value ($) Nb: number of borehole q: heat transfer rate per unit length (kW/m) Q_ : heat transfer rate (kW) ri,j: radial distance between borehole i and j (m) r: internal rate of return R: thermal resistance (m2 C/W) S: Laplace transform variable t: time (s) T: temperature ( C) U: overall heat transfer coefficient (W/m2 C) Greek symbols
b: shape factor h: efficiency l: time step (1 h)
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a: thermal diffusion coefficient (m2/h) Subscriptions am: ambient b: borehole c: coil conv: convection cond: conduction f: fluid (in GHX) fuel: fuel gr: ground grt: grout GHX: ground heat exchanger h: heater hyd: hydrate or hydration i: internal in: inlet ls: line source NG: natural gas o: external oc: external oh heater out: outlet p: pipe soil: soil tv: throttling valve w: water w-h: water in heater