Thermodynamic performance evaluation of a geothermal ORC power plant

Thermodynamic performance evaluation of a geothermal ORC power plant

Journal Pre-proof Thermodynamic performance evaluation of a geothermal ORC power plant A.F. Altun, M. Kilic PII: S0960-1481(19)31906-8 DOI: https:/...

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Journal Pre-proof Thermodynamic performance evaluation of a geothermal ORC power plant A.F. Altun, M. Kilic PII:

S0960-1481(19)31906-8

DOI:

https://doi.org/10.1016/j.renene.2019.12.034

Reference:

RENE 12743

To appear in:

Renewable Energy

Received Date: 1 September 2019 Revised Date:

22 November 2019

Accepted Date: 8 December 2019

Please cite this article as: Altun AF, Kilic M, Thermodynamic performance evaluation of a geothermal ORC power plant, Renewable Energy (2020), doi: https://doi.org/10.1016/j.renene.2019.12.034. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

CRediT author statement Muhsin Kilic and Ayse Fidan Altun equally contributed to prepare this study.

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

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Thermodynamic performance evaluation of a geothermal ORC power plant A.F. Altun, M. Kilic*

2 3 4 5 6 7 8 9 10 11 12 13 14 15

This work presents a thermodynamic evaulation of an operating geothermal ORC power plant. The model is realized by using measured data of AFJET geothermal power plant which has 3MWe net power output capacity. Thermodynamic assessment of the system is conducted to see the energy and exergy efficiencies of each component, and the whole plant. Additionally, a parametric study is conducted to understand the effects of various operating conditions on the system performance. Different from previous studies, daily and annual net power output profile of the plant was investigated with considering ambient temperature fluctuations. Results revealed that net power output can drop as significant as 36% from winter to summer months. Also, between nighttime to daytime, the net power expectation may decrease by 5%. The exergy destruction rate of re-injection process constitutes the most significant part (38.1 %) of the total exergy destruction of the plant. The conversion and exergy efficiencies of the system are calculated as 11.24 % and 39.03 %, respectively. Also, to enhance the performance of the plant, an internal heat recovery system is recommended. The analyses show that the implementation of an internal heat recovery system improves the energy and exergy efficiencies of the plant by 15%.

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Keywords: ORC, geothermal energy, power plant, energy efficiency

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Bursa Uludag University, Department of Mechanical Engineering, 16059, Bursa, Turkey

1. INTRODUCTION Geothermal energy has great potential to save a country from the negative impact of energy price concerns. Since the heat generated by Earth’s core is limitless, the utilization of geothermal energy sources has been gaining increasing attention. Geothermal energy applications can be divided into two groups according to their temperature ranges: (i) electricity utilization and (ii) direct use applications [1]. Direct use applications include district heating and cooling systems, domestic hot water applications, hot spring bathing, etc. As far as direct use applications are concerned, various studies can be found in the literature [2–6]. Turkey is poor in fossil fuel resources; however, it is one of the top five countries globally in terms of geothermal energy applications. Most of the geothermal wells of Anatolia has a temperature limit from 90 to 125 °C [7]. As a result, those resources can be used for direct use applications, or they can be considered for electricity generation with the implementation of Organic Rankine Cycle (ORC). ORC is an electricity generation cycle that recovers thermal energy by using all different kinds of lowtemperature resources like solar and geothermal energy [8,9]. ORC and steam Rankine cycles are very similar in terms of working principle. However, instead of water, ORC uses organic fluid (R134a, R113, R123, etc.) [10]. Organic fluids recover energy from low-grade heat sources due to their lower boiling points [11]. Exergy is the evaluation of the useful work potential of a system, relative to given surroundings [12]. An exergy analysis is an essential and proven tool that should be used by decision and policy-makers and scientists who are working in the energy field [13]. Exergy analysis is mostly used to optimize an existing energy system by identifying elements that are most in need of redesign [14]. Thermodynamic modeling and performance assessment is crucial for geothermal power plants for finding optimum performance and operating conditions. There are some studies regarding the performance analysis of geothermal power plants. Yari [10], conducted a study to compare different geothermal power plant cycle concepts. Thermodynamic models were developed for each different system, and system efficiencies were compared. Yıldırım et al. [15] conducted an exergo-economic analysis for DORA 1 and DORA 2 geothermal power plants and presented improvement recommendations for each equipment of the plant. Kecebas et al. [16] conducted a

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conventional and advanced exergy analysis of a geothermal power plant. Throughout the study, the authors determined the order of the components that must be improved. Unverdi et al. [17] investigated the energy and exergy performance of Germencik geothermal power plant with 47.4 MWe capacity. They determined the exergy losses of each component of the plant and proposed alternative solutions for improving the overall plant efficiency. Luo et al. [18] investigated the potential use of geothermal energy for power generation in China. They compared different geothermal power systems, in terms of technical benefits and setbacks. Ozturk et al. [19] assessed the energy and exergy performance of Kizildere geothermal power plant. The authors of the study calculated exergy destructions of all components in the plant. Additionally, the influence of the reference temperature variation on the exergy efficiency of the plant was investigated. Ganjehsarabi et al. [20] examined the energy and exergy performance of DORA 2 binary geothermal plant with 9.5 MWe capacity. The energy efficiency of the plant was calculated as 10.7 %, and the exergy efficiency of the plant was found as 29.6 %. The authors of the study found as re-injection process has the most significant exergy destruction rate. Therefore, utilization of discharged waste heat should be used for some other processes to increase overall efficiency. Yekoladio et al. [21] conducted a thermodynamic optimization study for geothermal ORC power plants. The results of the study showed that the net power output of the plant increases when the temperature of the geo-fluid rises. The first law efficiency of the investigated geothermal binary cycles was found in the range between 8-15%, whereas, the second law efficiencies varied between 42 to 56%. DiPippo [22] investigated first and second law efficiencies of various binary plants working with low-temperature geothermal fluids. According to the results of the study, thermal efficiency of the geothermal binary plants mostly vary between 8-12 %; as a result, only 1-2 point increase in net power output can improve a plant’s thermal efficiency between 10-20%. The author also concluded that the second law efficiencies of the plants are much higher than the first law efficiencies. To increase the exergy efficiency, the design of the heat exchangers should be improved. DiPippo [23] conducted another study to investigate the performance of the various geothermal power plants located around the world. The author highlighted that the conversion efficiency of the plants is quite low compared to the conventional fossil-fuel plants. The main reason behind that trend is that geothermal plants are poor converters of heat into work, and they reject greater amount of waste heat than the conventional power plants. Kalinci et al. [24], assessed energy and exergy efficiency of a binary geothermal power plant. Results of the study showed that various parameters such as brine inlet temperature, upper cycle pressure, etc. significantly affect the power output of the plant. Usman et al. [25] investigated thermo-economic performance of air-cooled and cooling towerbased geothermal ORC systems for the different locations. The cooling mechanism (air-cooled condenser or cooling tower), working fluid selection, geothermal brine temperature are the parameters that were varied in the study, and systems were compared in terms of annual net power output, capital cost and the levelized cost of energy. Sun et al. [26] studied the influence of evaporator pinch point temperature of geothermal ORC systems on heat exchanger areas and the system cost. According to the results of the study, lower evaporator pinch point temperature results in higher power output, but it also results in greater heat transfer area requirement and higher investment cost. Although there are some studies regarding geothermal ORC plants, there are few studies that were conducted using actual plant data. AFJET (Afyon Jeotermal Turizm ve Ticaret A.Ş.- Afyon Geothermal Tourism and Trade Company) power plant is located in Afyonkarahisar province, which is one of the most crucial geothermal fields of Turkey. Yet, there are not any studies that concern the potential use of geothermal energy for electricity generation in Afyonkarahisar. In previous studies, the thermal performance of the existing geothermal plants was generally evaluated based on the constant ambient temperature. However, power output and thermal efficiency of the ORC power plants fluctuate throughout the year due to varying ambient temperatures. Ambient conditions affect the temperature of the cooling fluid entering the condenser, and the condensing pressure. In order to observe the effect of the ambient air temperature variations on the plant performance, the cooling tower of the system is modeled, and the thermodynamic performance of the cycle is analyzed under varying weather conditions. The main objectives of the study are given below: • To present energy, exergy assessment and thermodynamic modeling of a geothermal ORC power plant.

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Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151



To obtain a realistic model with the use of measured operating data of AFJET geothermal power plant. • To investigate the enhancement potentials of each component of the plant, and the energy and exergy efficiencies of the whole plant. • To perform a parametric study to see different operating conditions influence on the system performance. • To cover existing shortcomings in the literature by investigating the annual and daily power output profile of the plant by considering the influence of ambient temperature variation. • To investigate the influence of a new configuration with an internal heat recovery system to improve efficiency and work output of AFJET power plant. The results of the study would be an essential reference for designers, engineers, policy-makers who are interested in geothermal power plants. 2. SYSTEM DESCRIPTION AFJET was established in 1994. The geothermal water of the AFJET is supplied from Omer-Gecek geothermal field located in the northwestern part of Afyonkarahisar province [27]. Currently, there are 24 geothermal wells that belong to the company. The wells are being used for different intentions, such as space heating, thermal treatment, greenhouse heating, and electricity utilization. Company places emphasis on the re-injection process of geothermal water for keeping the geothermal source sustainable. AFJET district heating system was designed for 10,000 residences equally and has 48.3 MWt capacity [27]. AFJET ORC geothermal power plant has a net electricity production capacity of 3 MWe. The plant consists of three cycles: ORC cycle, heating cycle, and cooling cycle. There are two production wells; however, only one of the production wells (well no: 25) is under operation. The schematic diagram of the ORC system is illustrated in Fig.1a. Fig.2 depicts the image of AFJET ORC power plant. 2.1 ORC Loop The ORC loop is composed of five components which are an evaporator, a preheater, a condenser, an expander, and a pump. From point 5 to 6, the pump compresses the working fluid (R-134a) from the condensation pressure (537.5 kPa) to the maximum pressure of the cycle (2503 kPa). The pump transports the working fluid to the preheater where the working fluid is heated by the geothermal water supplied from the well. From state points 7 to 8, the preheated working fluid enters the evaporator and heated by the geothermal hot water and exits as a superheated vapor. From state point 8 to 4, the high-pressure vapour enters into the expander and is expanded to the condenser pressure. Its enthalpy is converted into useful work by the coupled expander-generator system. From state point 4 to 5 low pressure working fluid flows into the condenser and exits as saturated, then a new cycle begins. The AFJET plant does not have an internal heat recovery unit. However, to improve the efficiency of the plant, the system is also modeled with an internal heat recovery unit. This heat recovery process is shown in Fig.1b by the additional state points 4 and 6 . 2.2 Heating Loop The second cycle is the heat source loop, where the energy of hot water coming from the geothermal well is given to the working fluid in the evaporator and the preheater. Geothermal fluid is extracted from the production well (well no:25), at 121°C well-head temperature and 240 kPa pressure, with 81 kg/s mass flow rate (State Point 1). The geothermal fluid leaves the evaporator at 79.0°C and enters the preheater at state point 2. After most of the energy of the geo-fluid is transferred to the working fluid which circulates in the ORC loop, geo-fluid is sent back to the re-injection well (State Point 3) at 54.8°C. 2.3 Cooling Loop The third loop is the cooling loop where the cooling water, absorbs the heat from working fluid in the condenser and reject it in the cooling tower. In the cooling loop, cooling water with a total mass flow rate of 596.6 kg/s is used. Heat rejection is realized by heat transfer between the cooling water and ambient air [28].

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Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

152 153

154 155 156 157

158 159 160 161 162 163

a)

AFJET ORC power plant

b) Modified cycle with IHR Exchanger Figure 1 The schematic diagram of (a) AFJET ORC power plant, (b) Modified cycle with IHR Exchanger

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Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202

Figure 2 Image of AFJET Power Plant

3. RESEARCH METHODOLOGY AND MODELLING Exergy assessment is related to the first and second laws of thermodynamics, and it shows the maximum work amount that can be produced by a system [19]. The real operational data of temperature, pressure and flow rate of the plant were used which recorded by the SCADA (Supervisory Control System) on 21 February 2019. Some of the assumptions undertaken in this study are presented below [20]: - The system works under steady-state conditions. - Friction losses are ignored. - The dead state is accepted as T0=5˚C, P0=101.32 kPa. (Specified after the local environment) - In calculations, thermal water is considered the same as the water. In steady-state conditions, three balance equations are essential, namely, mass, energy and exergy balance [20]. The mass balance equation is shown below [29]: ∑



(1)

In Eq. (1), is mass flow rate, and stands for inlet and exit states [24].The overall energy balance can be shown as in Eq.(2): ∑ ∑ (2) and are the net heat input and net work output, respectively. and are the entering and exiting enthalpy. The net heat input and work output can be calculated as below [20]: (3) (4)

, ,

Exergy balance of the system can be expressed as below[20]: ∑ ∑ ∑ 1 ! 0 Where and represent net exergy input and output rate, respectively. temperature T, and ! is exergy destruction [30].

(5) is the heat transfer rate at

The exergy rate is given by: Ψ

(6)

In Eq.(6), Ψ represents the specific exergy flow, and it is given by:

5

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235

Ψ

236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251

8

$h

h& '

(& $)

)& '

(7)

In Eq.(7), temperature, enthalpy, and entropy in the dead state are shown as T0, h0, s0, respectively. The energy and exergy balance equations are applied with considering each component as control volume [29]. In equations below, sub-indices with numbers represent corresponding cycle state points shown in Fig.1a. 3.1 Energy and exergy analysis for the evaporator The energy rate of the evaporator can be written as: *+,

-. . $ 0

1'

2. . $ 3

4'

(8)

In this equation, -. refers to the mass flow rate of the working fluid (kg/s), and 7 and 0 are the enthalpy of the working fluid at evaporator inlet and outlet, respectively. The heat transfer capacity of the evaporator can be also calculated based on the given energy rate of the geothermal fluid to the evaporator. In Eq.(8), 2. represents the mass flow rate of the geothermal fluid, and 1 and 4 are the enthalpy of the geothermal fluid at evaporator inlet and outlet, respectively. Exergy destruction rate of the evaporator [15]; ! *+, 6 3 $ 1 (9) 47 0' In Eq.(9), 3 and 4 are the exergy input and output rate of the geothermal fluid at the evaporator. Similarly, 1 and 0 refer to exergy input and output rate of the working fluid, respectively. The exergy efficiency of the evaporator can be shown as below [15]; $9 ;9 ' 8 *+, $9: <' (10) ;9 =

>

3.2 Energy and exergy analysis for the expander The isentropic efficiency of the expander is defined as the ratio of actual work delivered by the expander to the isentropic expander work ; 2 ;2 ? @, 2 :;2 A (11) :

AB

In Eq. (11), 0 is the enthalpy at the expander inlet, C is the enthalpy of the actual process at the exit, and CD is the enthalpy of the isentropic process at the exit. Exergy destruction rate of the expander is shown below [24]; ! @, 6 0 (12) C7 @, In Eq. (12), 0 and C are the exergy rate at the expander inlet and outlet, and the expander. The exergy efficiency of the expander [29]; @,

EFGH

$9: ;9A '



@,

is the power output of (13)

3.3 Energy and exergy analysis for the preheater Preheater energy rate can be written as; 2. . $ 4 K' ,I 2 -. . $ 1 J'

(14)

In Eq. (14), 1 and J are the enthalpy of the working fluid at the preheater exit and inlet. Also, 4 and K are the enthalpy of the geothermal fluid at the preheater inlet and outlet, respectively. Exergy destruction rate of the preheater; !,I 2 6 4 $ J (15) K7 1' In Eq. (15), 4 and K are the exergy rate of the geothermal fluid at the inlet and exit of the preheater; J and 1 are the exergy rate of the working fluid at the preheater inlet and outlet. The exergy efficiency of the pre-heater; 8,I

2

$9< ;9L ' $9> ;9M '

(16)

3.4 Energy and exergy analysis for the condenser

6

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277

Heat rejected from the condenser can be shown as; (17) N O -. . $ C P' N- . $ 3& Q' In Eq. (17), C and P are the enthalpy of the working fluid at the condenser inlet and outlet. Heat rejected from the condenser can be also calculated based on the heat transfer rate of the cooling fluid. In Eq.(17), N- is the mass flow rate of the cooling water, 3& and Q are the enthalpy at the condenser exit and inlet, respectively. Exergy destruction rate of the condenser [30]; !N O 6 4 (18) 97 5 7 6 10 In Eq. (18), C and P are the exergy rate of the working fluid at condenser inlet and outlet; Q and 3& are the exergy rate of the cooling water at condenser inlet and outlet, respectively.The exergy efficiency of the condenser [12,29]; 8N

O

$9=

9T ' $9A 9U '

(19)

3.5 Energy and exergy analysis for the pump Isentropic efficiency of the pump can be shown as; 2 ;2 ?, V, 2LB;2 U L

(20)

U

In Eq. (20), J and JD are the actual and isentropic enthalpy at the pump exit, and pump inlet. Pump work is given in Eq.(21). , V,

-. . $ J

P'

8,

V,

J

is the enthalpy at the

(21)

Exergy destruction rate of the pump [13]; !, V, $ J , V, P' In Eq.(22), P and the pump [15];

P

(22)

are the exergy rate at the pump inlet and exit, respectively. The exergy efficiency of

$9L ;9U ' EHWXH

(23)

278 279 280 281 282 283 284

3.6 First and second law efficiency of the system Conversion or first law efficiency is crucial for resource estimation. Compared to conventional thermal plants, geothermal power plants usually have lower conversion efficiency [14]. Thermal, conversion or first law efficiency of a geothermal plant can be calculated based on the formula of the net power output divided by the heat input [10,21];

285 286 287

And it becomes Eq.(25), after applying the state points;

?NYNZ

?NYNZ

$E[F\ ' V]F^ 62]F^ ;2_F` 7

$E[F\ ' Vdb $2= ;2M '

$E[F\ ' Vab 62ab,^W\ ;2ab,c[ 7

(24)

$E[F\ ' Vab $2: ;2L '

(25)

288 289 290

The exergy efficiency of the overall system can be written as below with respect to the reference state temperature [13,19,31];

291

?e@,3

E[F\ ∑ 9c[

?e@,4

E[F\ ∑$9c[ ;9^W\ '

292 293 294 295 296

$EFGH ;EHWXH '

(26)

Vdb f$2= ;2 '; $D= ;D 'g

?e@ is the exergy efficiency where the subscript “in” refers to “given” exergy to the system. Based on the exergy decrease of geothermal water, exergy efficiency of a binary cycle can be also defined as follows [10,21]; $EFGH ;EHWXH '

(27)

Vdb f$2= ;2M '; $D= ;DM 'g

7

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344

Coskun et al. [31,32] have determined some dimensionless energy and exergy parameters to investigate the performance of geothermal systems. The ratio of expander work to the total energy input (both geothermal and electrical) is defined as the energetic renewability ratio (hiej∈ ' and it is shown below: hiej∈

lmn ∑ oc[ pEHWXH

(28)

The ratio of useful exergy output of the system to the total exergy input into the system is expressed as the exergetic renewability ratio (hieje@ ': hieje@

lmn pEHWXH



(29)

The ratio of exergy destruction of any component 6 !, 7, to the total exergy destruction of the system, is defined as the dimensionless exergy destruction ratio (DExD) and it is shown as below: !qm!

!,

(30)

∑ !

3.7 Modeling of the cooling tower To understand the influence of the ambient air on the system performance, the cooling tower is modeled using the effectiveness-NTU method, which was developed by Kays and London [33]. This approach determines a dimensionless parameter called heat transfer effectiveness (ε), as shown below, [34]: r

o oXsG

tN +Z 2 + I+ D. I I+ u+@ V V , DD vZ 2 + I+ D. I I+

(31)

Actual heat transfer rate in the cooling tower can be found as [34]: w+ I . 6(+ I,

In Eq.(31), Eq.(33). V+@

V+@

(+ I, 7

wN- 6(N-,

(N-,

7

(32)

is the maximum possible heat transfer rate for the cooling tower, and it can be shown as in

wV . $(N-,

(+ I, '

(33)

In Eq. (32), (N-, is the temperature of the cooling water entering the cooling tower from the condenser and (+ I, is the temperature of the ambient air. Heat capacity rates of water and air can be shown as [34]: w+ I (34) + I x,,+ I wNx (35) N- ,,NIn the equations above, x, s the specific heat capacity of the fluids. + I and N- are the mass flow rates of the air and the cooling water, respectively. wV is the minimum of the heat capacity rates as shown in Eq.(36): wV = minimum( w+ I ; wN- ) (36) 3.8 Modeling of the internal heat recovery system An internal heat exchanger is added to the present cycle to investigate its effect on the improvement of the plant efficiency by means of internal heat recovery (IHR). In the AFJET plant, the temperature of the working fluid at the expander outlet (T4) is greater than that of the temperature in the outlet of the pump (T6). Therefore, it is possible to cool down the vapor and heat up the compressed fluid by transferring the heat with implementing an IHR heat exchanger into the cycle as shown in Fig.1b. The hot fluid inlet and outlet locations on the IHR heat exchanger are shown as the state points of 4 and 4’, respectively. Meanwhile, the compressed cold fluid inlet and outlet locations on the IHR heat exchanger are shown as the state points of 6 and 6’, respectively. Heat transfer rate of the IHR heat exchanger $ yzi ' can be written as below; yzi

ryzi . wV . $(C

(J '

-. . $ C

C{ '

-. . $ J

8

J

'

(37)

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374

In Eq.(37), ryzi is the heat transfer effectiveness of the internal heat recovery heat exchanger. wV is the minimum of the heat capacity rate of the compressed fluid at the pump exit, and the working fluid in the turbine outlet. Exergy destruction rate of the IHR heat exchanger; !yzi

6 4

4′ 7

6 6′

67

(38)

In Eq. (38), C and C are the exergy rates of the hot fluid at the IHR heat exchanger inlet and outlet, respectively; J and J are the exergy rates of the cold fluid at the IHR heat exchanger inlet and outlet, respectively. The exergy efficiency of the IHR heat exchanger; 8yzi

$9L{ 9L ' $9A 9A{ '

(39)

It should be noted that in Eqs. 14,15,16 and 25, the state point of 6 should be replaced with the state point of 6’ as representing the inlet of working fluid to the preheater. 3.9 Validation of the model To investigate the reliability of the present computations, the computed result of the present model is compared with another real geothermal binary power plant data given by Kanoglu and Bolatturk [35]. Although there are some differences between the present ORC system and the one in the study of Ref. [35], the current simulation model was modified according to the ORC system described by the Ref.[35]. In the study of the Ref. [35], there is no preheater, so a single heat exchanger is used as the evaporator. Also, there are two identical expanders, there is no internal heat exchanger, and there is an air-cooled condenser rather than a cooling tower. The operating parameters of the ORC system described in the study of Ref. [35] are presented in Table 1. Details of the plant and the cycle can be found in the study of Ref. [35]. Comparison of energy and exergy analysis results of the present study and the results of the Ref.[35] are given in Table 2. It can be seen that the agreement between the results is very well. The difference ratio (Dif.R. ) between the results are mainly less than 1%. The highest difference appears to be 3.24% at the reinjection heat rate values. It can be concluded that the reliability of the present model is proved and it can be used for further simulations with confidence. Table 1. Operating parameters of the ORC system used in the study of Kanoğlu et al. [35]. Parameters Working fluid Geo fluid temperature at heat exchanger inlet Mass flow rate of the geofluid Re-injection temperature Mass flow rate of the working fluid Evaporator Pressure Expander inlet temperature Expander outlet conditions Condenser exit temperature Cooling air and dead state conditions Air flow rate

375 376

377

Isobutane 158°C ; 609kPa 555.9 kg/s 90°C ; 423kPa 305.6 kg/s 3250 kPa 146.8°C 79.5°C ; 410 kPa 11.7°C 3°C ; 84 kPa 8580 kg/s

Table 2. Comparison of the present model calculation results with the study of Kanoğlu et al. [35]. Component Heat transfer rate or Power Effectiveness or isentropic Exergetic efficiency (%) (kW) efficiency (%) Present Study 209748

Dif.R. (%) 3.24

Kanoglu et al. [35] -

Present Study

Dif.R. (%)

Reinjection well

Kanoglu et al. [35] 202742

Heat exchanger Air-cooled condenser Expander I Expander II Circulation pump Parasitic power Cycle

160929 141271 10872 10872 2087 3262 16396

160905 141234 10876 10876 2081 3265 16406

0.014 0.026 0.036 0.036 0.287 0.091 0.061

47.1 88.6 78.2 78.2 73.4

47 88.6 78.2 78.2 73.4

0.212 0 0 0 0

80.5 28.2 81.8 81.8 74.1

80.6 28.1 81.9 81.9 73.5

0.124 0.355 0.122 0.122 0.810

10.2

10.2

0

33.511 - 41.722

33.511 - 41.722

0

1

ηEX,1 , ηEX,2 2

9

Kanoglu et al. [35] -

Present Study

Dif.R. (%)

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394

4. RESULTS AND DISCUSSION The study in this section is divided into two subsections as The realization of the model with the measured data of AFJET plant, and a parametric performance assessment of the plant. 4.1 Realization of the simulation model with the measured data of AFJET plant Using the formulations given in section 3, the cycle is simulated in Engineering Equation Solver (EES). The operating parameters of the plant are presented in Table 3. The performance assessment of the system is carried out by determining the mass, energy and exergy balance of the geothermal, cooling and working fluids at important cycle locations. At the date, the measured data from the several locations of the AFJET plant was recorded, the power production of 2.72 MW was observed.

Table 3. Operating parameters Evaporation Temperature

77.6 °C

Expander Inlet Temperature Condensation Temperature Expander Isentropic efficiency Pump Isentropic efficiency Internal Heat Exchanger effectiveness

97.5°C 18.0 °C 0.85 0.80 0.90

395

10

Table 4. Thermo-physical Properties and Exergy Rates for AFJET Geothermal ORC Power Plant

No 1 2 3 4 5 6 7 8 9 10

Fluid Thermal water Thermal water Thermal water R-134a R-134a R-134a R-134a R-134a Water Water

Fluid Phase Liquid Liquid Liquid Vapor Liquid Liquid Vapor Vapor Liquid Liquid

Temperature (˚C) 121.0 79.0 54.8 39.8 13.0 14.2 71.6 97.0 7.0 15.0

Pressure (kPa) 240.0 240.0 240.0 537.5 537.5 2503.0 2503.0 2503.0 220.0 220.0

Enthalpy (kJ/kg) 508.10 331.00 229.70 281.60 69.58 71.54 158.40 310.40 29.64 63.19

Entropy (kJ/kg.K) 1.539 1.064 0.766 0.993 0.267 0.268 0.543 0.974 0.106 0.224

Mass Flow Rate (kg/s) 81.00 81.00 81.00 94.38 94.38 94.38 94.38 94.38 596.6 596.6

Specific exergy (kJ/kg) 80.20 35.23 16.68 36.98 26.78 28.42 38.92 70.91 0.14 0.85

Table 5. Energy and Exergy efficiency of the system components

No 1 2 3 4 5 6

Name of the equipment Pre-Heater Evaporator Condenser Expander Pump Re-Injection

Exergy Efficiency

8

0.66 0.83 0.43 0.85 0.84 -

Exergy Destruction Rate ! (kW) 511.8 623.1 541.6 482.3 29.9 1351.0

Heat Transfer or Power - (kW) 8,203 14,346 20,014 2,720 184.9 18,605

1st Law Efficiency ? or effectiveness (ε) 0.85 0.80

Table 6. Overall Energy and Exergy Efficiency of the System First Law efficiency of the cycle ?CYCLE

%11.24

Second Law efficiency of the system ?qm,1

% 39.03

Second Law efficiency of the cycle

?qm,2

% 49.27

11

Energetic ratio hiej•

renewability

0.066

Exergetic renewability ratio hieje@

0.41

Rate of Exergy (kW) 6496.00 2853.63 1351.08 3490.00 2527.00 2682.00 3673.00 6693.00 88.74 509.9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

After subtracting the parasitic loads, the net power output obtained from AFJET power plant is found to be 2.53 MW. Properties at all state points are calculated, and results are given in Table 4. It should be highlighted that exergy calculations are conducted based on the accepted dead state of 5°C temperature and atmospheric pressure. For the parametric analysis, the reference temperature is accepted as the same as the ambient temperature. As can be seen from Table 4, the total exergy input to the plant is 6.5 MW. In Table 5, exergy destruction rates of each equipment (evaporator, condenser, pre-heater, expander, pump, re-injection) are estimated, and the exergy efficiency of the components is presented. According to Table 4, the total exergy destruction rate of the plant was estimated to be 3.54 MW. This means that 54.5% of the total input exergy is lost. By using dimensionless exergy destruction (DExD) parameter, the ratio of exergy destruction of each equipment to the total exergy destruction of the plant is found, and it is presented in Fig. 3. Within the system, the highest amount of exergy destruction rate occurs in re-injection (38.2 %) and it is followed by the evaporator (17.6 %) and the condenser (15.3 %). Since the performance of each component influences the overall efficiency of the plant, components with high DExD values should be improved. Results of the exergy analysis show that expander (13.6 %) and the pump (0.84 %) have the lowest amount of exergy destruction rates. Geothermal plants with low-temperature resources usually have lower energy efficiency values than their exergy efficiency values. The energy efficiency $ ?CYCLE' of AFJET power plant is determined as 11.24%. The exergy efficiency of the overall system 6?e@,3 7, and the cycle 6?e@,4 7 are found as 39.03%, and 49.27%, respectively. Compared to the energy and exergy evaluation study of Tuzla binary geothermal power plant [31], AFJET geothermal power plant has approximate but lower values of energetic (0.066), and exergetic renewability (0.41) ratios. Table 7 presents a comparison of the energy and the exergy efficiencies of several geothermal power plants reported in the current literature. It can be seen that the present study calculations about the AFJET power plant performance are in the range of the other plants' performance.

24 25

Table 7. A comparison of the energy and the exergy efficiencies of several geothermal power plants reported in the current literature. Related Study

Plant Information

Coskun et al. [32] Yekoladio et al. [21] Kalinci et al. [24] Ganjehsarabi al. [20] DiPippo [22]

Kanoglu et [35] Yari [10] DiPippo [23]

Present study

et

al.

7.5 MWe geothermal binary power plant Geothermal binary power plants with various configurations Geothermal binary power plant with 2.18 MWe capacity Geothermal binary power plant with 9.5 MWe capacity Heber binary power plant with 6.8 MWe Nigorikawa binary geothermal power plant with 1 MWe Geothermal binary power plant with 20 MWe capacity Geothermal binary power plants with various configurations Raft River Geothermal Binary Plant (4.6 MWe) Heber 2 Geothermal Binary Plant (33 MWe) Las Pailas Geothermal Binary Plant (35 MWe) Wabuska Geothermal Binary Plant (0.6 MWe) AFJET Geothermal Binary Plant with 3MWe capacity

Net Energy Efficiency of the plant $ ?‚ƒ‚„… ' 17.3%

Net Exergy Efficiency of the plant 6?e@,3 7 42.6%

Net Exergy Efficiency of the cycle 6?e@,4 7 69.1%

8-15%

37-47%

43-55%

11.0 %

30.8%

44.7%

10.7%

29.6%

-

13.2%

43.4%

50.7%

9.8%

21.6%

-

10.2%

33.5%

41.7%

12.6-15.3%

12

32.2-38.7%

46.8-52.7%

10.4%

39.7%

52.3%

10.6%

38.6%

56.1%

15.1%

33.2%

39.5%

8.0%

26.6%

37.3%

11.24%

39.03%

49.27%

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

26 27 28 29 30

In order to realize the simulation results, measured and simulated data of working fluid, cooling fluid and heating fluid at some important locations are compared. As it can be observed in Table 8, the comparisons between the simulation work and measured plant data are in good agreement, and there is only a slight difference.

Re-Injection 0.382

Pre-Heater Evaporator Condenser Expander Pump Re-Injection

Pre-Heater 0.145 Evaporator 0.176

Pump Condenser 0.008 Expander 0.153 0.136

31 32 33

Figure 3 Dimensionless exergy destruction (DExD) of the components Table 8. Comparison of the measured plant data and simulation work Measured

Simulation

Dif. R. (%)

Thermal Water Temperature at evaporator inlet (T1)

121 ˚C

121 ˚C

0.00

Thermal Water Temperature at evaporator outlet (T2)

79 ˚C

79 ˚C

0.00

Thermal Water Temperature at preheater outlet (T3)

54.7 ˚C

54.8 ˚C

0.18

Working Fluid Temperature at expander inlet (T8)

97.1 ˚C

97.0 ˚C

-0.10

2720 kW

2720 kW

0.00

Expander Work Output (Wexp)

34

13

35 36 37 38 39 40 41 42 43 44

In this part of the study, the influence of mass flow rate and well-head temperature variation of geothermal fluid on expander power output is investigated. The inlet temperature of the geothermal fluid to the evaporator and inlet temperature of the cooling fluid to the condenser are the parameters that were kept constant during the simulations of mass flow rate variation. As can be seen in Fig. 4, an increase in the mass flow rate of the geo-fluid leads to a significant increase in expander work. Results show that, as the mass flow rate of the geothermal fluid changes from 70 to 88 kg/s, expander power output increases from 2384 kW to 2918 kW, which is a 22.4 % increment. Only 2 kg/s change in mass flow rate might cause around 100 kW difference in power output of the plant.

45 46 47 48

Re-injection plays a vital role in the sustainable utilization of geothermal systems and increases production capacity in most cases [36]. As a result, re-injection process should be kept maintaining carefully for the sustainability of the geothermal source and for supplying a sufficient amount of geothermal fluid to the plant in long-term.

49 50 51 52 53 54 55 56 57 58

In Fig. 5, the impact of the geothermal well-head temperature $(3 ' variation on the expander work is presented. It is important to highlight that the simulations were conducted with keeping the mass flow rate of the geo-fluid, and inlet temperature of the cooling fluid as constant. Similar to the findings of Coskun et al. [37], our results show that the power output of the expander increases as the resource temperature increases. 2°C change of well-head temperature might cause a 75 kW difference in expander power output. Power output increases from 2455 kW to 2895 kW, when the resource temperature changes from 114 to 126˚C. The results of the analysis show that a 12˚C temperature increase in the resource temperature improves the power output by 18%. As a result, it is crucial to consider cooling predictions associated with re-injection process. Sending geo-fluid at very low temperatures to the re-injection well is not recommended, and the temperature of the re-injection fluid should be controlled carefully to avoid cooling of production wells.

59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83

4.2 Parametric performance assessment of the plant

In Fig.6, the variation of energy and exergy efficiencies versus the expander inlet temperature $(0 ' is presented. To conduct the simulations, geothermal resource temperature, geothermal mass flow rate, evaporation temperature, cooling water inlet temperature is selected as constant parameters. The mass flow rate of the working fluid and the expander work output are unknown parameters. Results show that the exergy efficiency of the plant and the cycle 6ƞe@,3 , ƞe@,4 7 , and the energy efficiency of the cycle ƞx‡xˆl increases with increasing expander inlet temperature. The reason behind this trend is increasing power output. The increase in the expander temperature resulted in an increase in the working fluid’s enthalpy. As a result, the power output of the expander increases at higher expander inlet temperatures. With varying the expander inlet temperature from 82 to 98˚C, exergy efficiency of the system 6ƞe@,3 7 changes from 37.64% to 38.99 %, exergy efficiency of the cycle 6ƞe@,4 7 increases from 47.67% to 49.38%, and energy efficiency of the cycle increases from 10.86% to 11.25%. The net power output of the plant increases from 2440 kW to 2533 kW with the variation of the expander inlet temperature from 82 to 98 ˚C. It can be concluded that with increasing expander inlet temperature by 16˚C, the conversion efficiency of the cycle can be improved by 3.59%, and the exergy efficiency of the plant and the cycle increases by 3.58%. Figs. 7 and 8 show the effect of evaporation and condensation temperature on the energy and exergy efficiency of the plant, respectively. Geothermal water mass flow rate, resource temperature and the inlet temperature of the cooling fluid are the constant parameters. Similar to the findings of Imran et al. [38], our results show that the increase in the evaporation temperature results in an increase in both energy ƞx‡xˆl and exergy efficiencies $ƞe@,3 ' , $ƞe@,4 ' of the plant and the cycle. When the evaporator temperature varies from 60 to 85˚C, the energy efficiency of the cycle changes from 8.87 to 11.88 %, exergy efficiency of the plant $ƞe@,3 ' increases from 30.76 % to 41.21 %, exergy efficiency of the cycle $ƞe@,4 ' increases from 38.95 % to 52.19 %, and the net power output of the plant increases from 1998 kW to 2677 kW, respectively. Higher evaporation temperature results in more enthalpy difference across the expander, and this enables greater power output.

14

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

84 85 86 87 88 89 90 91 92 93 94

For ORC geothermal power plants using air as their cooling medium, condenser temperature fluctuates throughout the year, and this affects the power output and the plant’s efficiency [35]. Usually, at lower ambient temperatures, the plant has higher work output than at higher ambient temperatures. In Fig. 8, the effect of condenser temperature on the energy 6ƞNYNZ 7 and exergy efficiencies $ƞe@,3 ' , $ƞe@,4 ' of the plant and the cycle is presented. Similar to the findings of Kanoglu et al. [35], Imran et al. [38] and Yang et al. [39], as the condenser temperature increases, energy 6ƞNYNZ 7 , and exergy efficiencies $ƞe@,3 , ƞe@,4 ' of the plant and the cycle decreases. When the condenser temperature varies from 18 to 30˚C, the energy efficiency of the cycle 6ƞNYNZ 7 decreases from 11.24% to 9.11%, the exergy efficiency of the plant $ƞe@,3 ' decreases from 38.99% to 31.61%, and the exergy efficiency of the cycle $ƞe@,4 ' decreases from 49.37% to 40.03%, respectively. 3000 2900

Wexp [kW]

2800 2700 2600 2500 2400 2300 70

72

74

76

78

80

82

84

86

88

mhf [kg/s] 95 96 97

Figure 4 Impact of geothermal fluid mass flow rate (

2. )

on the power output of the expander (

@, '

3000 2900

Wexp [kW]

2800 2700 2600 2500 2400 2300 114

116

118

120

122

124

126

T1 [C] 98 99 100 101

Figure 5 Impact of geothermal fluid temperature at evaporator inlet $(3 ' on the power output of the expander ( @, '

15

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

Fig. 9 shows the variation of the net power output of the plant with regard to the monthly mean ambient air temperature of Afyon province. It can be observed that at lower ambient temperatures as in wintertime, the net power output of the plant is greater. The net power output of the plant is expected to be the greatest during January when the mean ambient temperature is 1˚C. During wintertime, the net power output of the plant varies from 2500 kW to 2800 kW. On the other hand, during summer months, the monthly expected net power output of the plant drops significantly. For instance, when the mean monthly ambient temperature is 21.6˚C in July, net power output decreases 36 % compared to the power output in January. As a result, it was concluded that variant ambient conditions affect the net power output of the plant significantly. Varying ambient temperature changes the cooling water temperature entering the condenser. Since cooling water temperature is related to the condenser pressure, the net power output of the plant is considerably affected by the ambient temperature fluctuations. 0.116

0.52 ƞcycle

0.5

0.112

0.48

0.11

0.46

0.108

0.44

0.106

0.42

0.104

0.4

0.102

0.38

0.1

0.36 80

114 115 116 117

ƞEx2

ƞEx

ƞcycle

0.114

ƞEx1

82

84

86

88

90

92

94

96

98 100

T8 (°°C) Figure 6 Impact of expander inlet temperature $(0 ' on energy (ƞNYNZ ' and exergy efficiency (ƞe@,3 ' of the plant and the cycle 6ƞe@,4 7

ƞcycle

0.120

ƞcycle

ƞEx1

ƞEx2

0.580

0.114

0.545

0.108

0.510

0.101

0.475

0.095

0.440

0.089

0.405

0.083

0.370

0.076

0.335

0.070

0.300

ƞEx

102 103 104 105 106 107 108 109 110 111 112 113

60 62 64 66 68 70 72 74 76 78 80 82 84

118 119 120

Tevap (°°C) Figure 7 The impact of evaporation temperature $( *+, ' on energy (ƞNYNZ ' and exergy efficiencies $ƞe@,3 ' , $ƞe@,4 ' of the plant.

16

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

ƞcycle

0.580

ƞEx2

0.114

0.545

0.108

0.510

0.101

0.475

0.095

0.440

0.089

0.405

0.083

0.370

0.076

0.335

0.070

0.300 18

121 122 123 124 125

ƞEx1

20

22

24

26

28

ηEx

ƞcycle

0.120

30

Tcond (°°C) Figure 8 The impact of condensation temperature $(N $ƞe@,3 ' , $ƞe@,4 ' of the plant

3000

O ' on

energy (ƞNYNZ ' and exergy efficiencies

25

Wnet Tair

2500

20 15

1500 10 1000 5

500 0

0 1

126 127 128 129 130 131 132 133 134 135 136 137 138 139 140

Tair [°C]

Wnet (kW)

2000

2

3

4

5

6

7

8

9

10

11

12

Months

Figure 9 Effect of ambient temperature $(+ I ' on the annual net power output $

' of AFJET power plant

Figure 10 demonstrates the influence of ambient temperature on the thermal efficiency ƞx‡xˆl and exergy efficiencies of the plant $ƞe@,3 ', and the cycle $ƞe@,4 '. It should be noted that, for the exergy calculations, reference state temperature is accepted as the same with the ambient temperature. It can be seen from the results that both the conversion efficiency ƞx‡xˆl and the exergy efficiency of the cycle $ƞe@,4 ' decreases with increasing ambient temperature. From 5˚C to 22.5 ˚C, energy efficiency of the cycle ƞx‡xˆl changes from 11.23% to 8.05%, the exergy efficiency of the cycle $ƞe@,4 ' decreases from 49.31 % to 44.98 %, respectively. Greatest energy and exergy efficiencies are achieved at 0˚C ambient temperature. However, the exergy efficiency of the plant $ƞe@,3 ' shows a different trend due to it is reliance on dead state temperature. Since the dead state temperature is accepted as equal to the ambient temperature, the exergy efficiency of the plant $ƞe@,3 ' is not influenced by the ambient temperature fluctuation and changes slightly. Between 5 to 22.5 ˚C, the exergy efficiency of the plant 6ƞe@,3 7 stays almost constant at 39%.

17

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

In Fig.11, the daily net power output profile for a typical winter day (24th February 2019) versus ambient temperature is presented. Results of the study revealed that during nighttime to early morning when the ambient temperature varies from 0.8 to 2.5 ˚C, net power output changes between 2630 to 2696 kW. On the other hand, during the daytime, when the ambient temperature is between 2-4.2 ˚C, the net power output of the plant decreases slightly. From 19:00 to 24:00, when the ambient temperature starts falling, net power output expectation has an increasing tendency. The net power output of the plant is at the highest value, which is 2696 kW, at 05:00, when the ambient temperature is at the lowest value. At 19:00, when the ambient temperature is 4.2 ˚C, net output expectation of the plant drops to 2565 kW. It can be observed that the net power output of the plant decreases by 5 % from 5:00 to 19:00. 0.510

ƞcycle

0.139

ƞcycle

ƞEx1

ƞEx2

0.127

0.491

0.116

0.473

0.104

0.454

0.092

0.435

0.080

0.416

0.069

0.398

0.057

0.379

0.045

ηEx

141 142 143 144 145 146 147 148 149 150

0.360 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Tair (°°C)

151 152 153 154 155 156

Figure 10 Impact of ambient temperature $(+ I ' on energy (ƞNYNZ ', and exergy efficiencies $ƞe@,3 ' , $ƞe@,4 ' of the plant

2750

4.5 Wnet

T_air

4

2700 3.5 3 2.5 2600 2 2550

Tair [C]

Wnet [kW]

2650

1.5 1

2500 0.5

157 158 159 160 161 162

24.00

23.00

22.00

21.00

20.00

19.00

18.00

17.00

16.00

15.00

Hours

14.00

13.00

12.00

11.00

9.00

10.00

8.00

7.00

6.00

5.00

4.00

3.00

2.00

0 1.00

2450

Figure 11 Daily power output profile of the plant for a typical winter day (24th February 2019) Measured data revealed that the mass flow rate of the geo-fluid and the ambient temperature fluctuates throughout the year. To understand the influence of simultaneous fluctuation of mass flow rate and ambient temperature simulations were carried out, and results are presented in Fig.12. As can be observed from the

18

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180

results, the greatest power output of the plant is achieved when the ambient temperature is 0˚C, and the mass flow rate of the geothermal brine is at the highest value. When the ambient temperature is 20 ˚C, and the mass flow rate of the geothermal fluid is 65 kg/s, the net power output of the plant decreases by 41%. In this part of the study, in order to increase the thermal efficiency 6ƞNYNZ 7, exergy efficiency$ƞe@,4 ' and the net power output of the cycle 6 7, a new configuration of ORC that includes an internal heat recovery system (IHR) is investigated and compared with the system at present. The new system is composed of a pump, a preheater, an evaporator, an internal heat exchanger, an expander, a condenser, and a cooling tower. Table 9 shows the power output, energy, and exergy efficiency of the plant with and without the implementation of an internal heat recovery system. Results show that both energy and exergy efficiency of the plant increases with the application of the heat recovery system. Therefore, it can be concluded that adding a heat recovery unit to the present system can be considered, and it is a better choice in terms of the thermal performance of the plant. The work output of the plant, the conversion efficiency 6?NYNZ 7, and the exergy efficiency of the cycle 6?qm,2 7 increase by 15 % with the addition of the internal heat recovery unit. The net power output of the plant rises from 2529 kW to 2921 kW at 5°C, after the implementation of an internal heat recovery unit. Also, conversion and exergy efficiencies of the cycle increase from 11.23% and 49.31% to 12.97% and 56.95%, respectively.

3000

65 kg/s 70 kg/s

WNET [kW]

2500

75 kg/s 80 kg/s

2000

1500

1000 0

181 182 183 184 185

5

10

15

Tair (˚C)

20

25

30

Figure 12 Influence of the variation of mass flow rate and ambient temperature $(+ I ' on the net power output of the plant $ ' Table 9. Performance comparison of ORC cycles with and without an internal heat exchanger Tair

0˚C 5˚C 10˚C 15 ˚C 20 ˚C 25˚C 30 ˚C

?NYNZ $%' Present Sys. With IHR 12.13 13.94 11.23 12.97 10.33 11.99 9.42 11.00 8.51 9.99 7.50 8.87 6.67 7.94

?e@,4 $%' Present Sys. With IHR 50.19 57.71 49.31 56.95 48.30 56.06 47.13 55.00 45.76 53.70 43.94 51.95 42.14 50.17

@,

Present Sys. 2916 kW 2714 kW 2511 kW 2305 kW 2097 kW 1864 kW 1669 kW

186 19

With IHR 3352 kW 3134 kW 2914 kW 2689 kW 2460 kW 2203 kW 1987 kW

Present Sys. 2731 kW 2529 kW 2327 kW 2123 kW 1918 kW 1691 kW 1502 kW

With IHR 3140 kW 2921 kW 2700 kW 2477 kW 2251 kW 1999 kW 1789 kW

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238

Geothermal ORC power plants are among the most environmentally-friendly power generation systems. The only possible form of pollution might be the thermal pollution which is the heat that must be rejected from the cycle [40]. According to the data provided by Enerdata [41] 490 grams of CO2 was emitted per kWh power generated in Turkey for the year 2009. Since geothermal plants have no emissions to the surroundings, AFJET ORC power plant saves 7350 tons of CO2 emissions annually when compared to a conventional power plant. 5. CONCLUSIONS In this study, thermodynamic modeling and evaluation of geothermal ORC power plants are presented. The proposed model is validated with measured plant data (pressure, temperature, mass flow rate) of AFJET power plant. In addition to identifying the energy and exergy efficiency of the entire plant, exergy destruction rates and exergy efficiencies of each component is calculated. Furthermore, a parametric analysis is conducted to investigate different operating conditions' influence on energy and exergy efficiencies of the plant. Previous studies in the literature did not consider the influence of the ambient temperature variations on the performance of the geothermal ORC power plants. Current research covers existing shortcomings in the literature by considering the impact of the ambient temperature fluctuation on the net power output and system efficiency. Furthermore, the implementation of an internal heat recovery system is investigated, and it is proposed to improve the performance of the plant. The main results are as follows: (1) Conversion6?NYNZ 7 and exergy efficiencies 6?e@,3 7, 6?e@,4 7of the plant and the cycle were found as 11.24%, 39.03%, 49.27% respectively. Total exergy input to the plant is found as 6.5 MW, and 3.54 MW of it is the exergy losses. The greatest exergy destruction was found in the re-injection process (38.1%). (2) Geothermal mass flow rate has a significant impact on expander power output. With varying mass flow rate of the geo-fluid from 70 to 88 kg/s, the power output of the expander changes from 2384 kW to 2918 kW. The results highlight the importance of re-injection process. To keep the geothermal resource sustainable re-injection process should be kept maintaining carefully. (3) The temperature of the geothermal fluid also influences the power output of the expander, vitally. Results show that with varying temperatures of the geo-fluid from 114 to 126˚C, the expander power output changes from 2455 kW to 2895 kW. These findings highlight the importance of the cooling predictions associated with re-injection process as cooling of the geothermal resource decreases the power production of the plant dramatically. (4) Increasing the expander inlet temperature increases energy efficiency and the exergy efficiency of the plant. This is mainly because of increasing power output. (5) Increasing the evaporation temperature has positive impacts on both energy and exergy efficiencies. This is mainly because of the more significant enthalpy difference across the expander and more power output. (6) Condenser temperature has a significant influence on the power production of the ORC power plants since the performance of the cooling towers is dependent on ambient conditions. The results of the study show that, with increasing condenser temperature, energy and exergy efficiency of the plant decreases simultaneously. (7) The results of the study revealed that the net power output of AFJET power plant is much higher during the winter season when compared to the summer season due to lower cooling water temperature entering the condenser. In July, the power output reduction can be as high as 36% of its expectation in January. Fluctuation in ambient temperature also has impacts on first and second law efficiencies of the plant and the cycle. From 5 to 22.5˚C ambient temperature, the conversion efficiency6?NYNZ 7and the exergy efficiency 6?e@,4 7 of the cycle decreases by 28 % and 9 %, respectively. However, the exergy efficiency of the plant $ƞe@,3 ' shows a different behavior due to it

20

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257

is dependence on dead state temperature, and it stays almost constant at 0.39, under varying temperatures. (8) Considering daily ambient temperature fluctuation, the results of the study showed that the net power output of the plant might decrease by 5 % between night and day time. (9) To increase the efficiency of the plant, a new cycle configuration is recommended with an internal heat recovery system. It was determined that the plant with an internal heat recovery system has 15 % greater conversion efficiency 6?NYNZ 7, exergy efficiencies 6?e@,3 , ?e@,4 7 and power output 6 7 when compared to the present system. An alternative heat rejection system will be studied in future work to keep the power output of the plant more stable throughout the year. These analyses are expected to be a useful reference for the researchers who are interested in geothermal based ORC power plants. ACKNOWLEDGEMENT We would like to thank the general manager of AFJET Dr Yusuf Uluturk for his contribution to this study. Also, the authors acknowledge the financial support given by TUBITAK (Project no: 218M805), gratefully. NOMENCLATURE w x,

h

P ) T

258

259 260 261 262



!

Heat capacity rate (kW/°C) Specific heat capacity [kJ/(kg.°C)] Enthalpy (kJ/kg) Mass flow rate (kg/s) Pressure (kPa) Heat transfer rate (kW) Entropy [kJ/(kg.°C)] Temperature (°C) Power (kW) Exergy rate (kW) Exergy destruction rate

Greek letters 8 ? r Œ

Exergy efficiency Efficiency Heat exchanger effectiveness Specific exergy (kJ/kg)

Subscripts and abbreviations air AFJET cond cw !qm! Dif.R. e evap exp Ex geo hf IHR in max min ORC out 0 preh rej hiej∈ hieje@ SCADA

Ambient air Afyon Jeotermal Turizm ve Ticaret A.Ş.- Afyon Geothermal Tourism and Trade Company condenser Cooling water Dimensionless exergy destruction ratio Difference Ratio Electricity evaporator expander exergy geothermal Hot fluid Internal Heat Recovery inlet maximum minimum Organic Rankine Cycle outlet Reference state preheater reinjection energetic renewability ratio exergetic renewability ratio Supervisory Control System

21

Altun et al.: Thermodynamic performance evaluation of a geothermal ORC power plant

263 264 265 266 267 268

t wf

[1]

thermal Working fluid

REFERENCES M. Tan, A. Keçebaş, Thermodynamic and economic evaluations of a geothermal district heating system using advanced exergy-based methods, Energy Convers. Manag. 77 (2014) 504–513. https://doi.org/10.1016/j.enconman.2013.10.006.

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[2]

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1–6.

HIGHLIGHTS •

Simulation model is realized with the data of the AFJET geothermal ORC power plant.



The thermodynamic performance assessment of the geothermal ORC plant is performed.



Different operating conditions impact on the performance of the system is examined.



Ambient temperature variation affects the performance of the plant significantly.



An internal heat recovery system can increase the power output of the plant by 15%.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: