Thermodynamic evaluation of Denizli Kızıldere geothermal power plant and its performance improvement

Transcription

1 Energy Conversion and Management 46 (2005) Thermodynamic evaluation of Denizli Kızıldere geothermal power plant and its performance improvement Ahmet Dagdasß *, Recep Ozt urk, Sß ukr u Bekdemir Department of Mechanical Engineering, Yıldız Technical University, Besiktas, _Istanbul, Turkey Received 18 December 2003; accepted 21 February 2004 Available online 17 April 2004 Abstract A thermodynamic optimization of the Denizli Kızıldere power plant is performed using real data, and some important results are obtained. The optimum flashing pressure is found to be 200 kpa. According to the existing geothermal power plant, 18% power augmentation is provided when the plant operates at this state. In addition, a new flash-binary model is proposed, and the optimum operating pressure is found that makes the power output maximum. In this model, the maximum power is found to be 18,238 kw e. This means that 93.2% more power is obtained than that of the existing plant. Apart from that, the most suitable working fluid is investigated for the binary cycle. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Geothermal power plant; Power generation; Exergy analysis; Denizli 1. Introduction Turkey is one of the most important countries in terms of geothermal energy potential and occupies seventh place in the resource and fifth place in direct use. The main application areas of geothermal energy in Turkey are space and district heating, greenhouse heating, electricity production, dry ice production, process heating in the textile industry and balneology. However, Turkey has only one geothermal power plant that produces electricity with a nominal power 20.4 MW e. The power plant was built in Kızıldere-Denizli in In spite of the 20.4 MW e plant capacity, it runs at approximately 10 MW e because of some operation problems due to the high content of noncondensable gases and dissolved particles of considerable degree. * Corresponding author. Tel.: ; fax: address: dagdas@yildiz.edu.tr (A. Dagdasß) /$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi: /j.enconman

2 246 A. Dagdasß et al. / Energy Conversion and Management 46 (2005) Nomenclature h 0 enthalpy at dead state [kj/kg] h enthalpy [kj/kg] e n specific energy [kj/kg] _E n energy rate [kw] _m mass flow rate [kg/s] e k specific exergy [kj/kg] _E k exergy rate [kw] T 0 dead state temperature [ C] s entropy [kj/kg K] s 0 entropy at dead state [kj/kg K] _W net net power output [kw] _E kr exergy rate of geofluid at reservoir conditions [kw] _E nr energy rate of geofluid at reservoir conditions [kw] _E nwh energy rate of geofluid at wellhead conditions [kw] _E kwh exergy rate of geofluid at wellhead conditions [kw] g I overall first law efficiency of the power plant [%] g II overall second law efficiency of the power plant [%] UNDP United Nations Development Program With reference to year 1998, electricity production in the world was 14,411 billion kw h (15,342 billion kw h in 2000). The contribution of renewable energy in this total generation is 2826 billion kw h. Most of this contribution is from hydraulic energy and only 226 billion kw h is supplied from geothermal, solar, wind, biomass and tidal energy [1]. Total electricity production in Turkey is 28,332 MW of which 16,623 MW comes from fossil fuel power plants, and 11,673 MW comes from hydraulic power plants (2001 data). The contribution of renewable energies (except hydraulic) to total production is 0.12% (36.4 MW) [2]. Turkey has geothermal and wind power plants as renewable energy sources. The role of geothermal energy will be the most important to increase the contribution of renewable energies in the future. Denizli province, which is located in the southwest of Turkey, has a great amount of geothermal resources. Denizli also has many industrial plants, especially in the textile industry, which is famous. On the other hand, tourism has a remarkable share in the economical activities. Pamukkale (Hierapolis) and Laodicia are located in this area. Ancient thermal springs and Roman baths in Pamukkale prove that this region was an important hydrothermal area in the past. Kızıldere geothermal field is located 35 km west of Denizli and near the B. Menderes River. The Kızıldere power plant was first installed in 1974 as a pilot power plant of 0.5 MW e. Then, it was enhanced to 20.4 MW e. The estimated geothermal power potential of the field is approximately 30 MW e [3]. The noncondensable gases (NCG) and other chemical dissolved compounds in the geofluid give serious problems. The Kızıldere geofluid contains total dissolved solids of ppm and its NCG content in steam is 10 21% (by weight). Most of the NCG content is CO 2 (96 99%) [4]. For

3 A. Dagdasß et al. / Energy Conversion and Management 46 (2005) these reasons, the plant cannot run at full capacity. On the other hand, scaling is also an important problem. In this area, there are more than 20 research wells and 9 production wells. These wells are fairly shallow, their depths varying from 500 to 900 m. The flow rate of such wells ranges from 25 to 28 kg/s. Downhole temperatures vary from 200 to 210 C. The geofluid is in liquid condition in the reservoir. The downhole temperature and pressure of the new R1 well is 242 C and 3430 kpa, respectively. The depth of the R1 well is 2261 m, and its power capacity is 6 7 MW e. The well produces 63 kg/s geofluid and 15.5 kg/s steam [5]. Investigations have shown that the discharged brine from the separator to the B. Menderes River has considerably high energy and exergy. Hence, it must be used in a binary and cascaded system for better energy generation [3,6]. In this study, the use of waste geofluid in a binary cycle is investigated, and the performance analysis of a combined single flash-binary power plant is performed. It is the first detailed combined system analysis for the Kızıldere field. 2. Description of power plant operation The geofluid is in the liquid phase downhole. Its total flow rate and temperature are 264 kg/s and 200 C, respectively. The geofluid reaches the wellhead in two phase conditions (liquidsteam). There is not enough steam to produce electricity in the two phase flow, so it must be flashed. Flashing is a pressure drop process at constant enthalpy. The purpose of the flashing process is to supply more steam to the turbine even though at a lower enthalpy (Fig. 1). After the flashing process, the quality of the steam ðxþ is 0.108, and the steam is separated from the brine in the separator. After separation, saturated steam at 148 C and 28.5 kg/s is obtained. The steam is passed through a demister, and then, it is sent to a steam turbine generator group to produce Fig. 1. Flashing is a constant enthalpy process. After flashing more steam is produced.

4 248 A. Dagdasß et al. / Energy Conversion and Management 46 (2005) Fig. 2. A simplified scheme of Denizli-Kızıldere geothermal power plant. electricity. On the other hand, the liquid leaving the separator is directly discharged to the B. Menderes River. This process causes important environmental problems. The steam that exits from the turbine enters a mixing type condenser. The temperature and pressure of the steam at the outlet of the steam turbine are 48 C and 10 kpa, respectively. The condenser cooling water is geofluid, which is cooled through a cooling tower. The flow rate of cooling water is 2378 kg/s. The cooling tower is of the wet type with four fans to supply the cooling effect. Noncondensable gases (mainly CO 2 ) in the condenser are extracted by two gas compressors, then the CO 2 is sent to a dry ice production plant, property of Karbogaz Co., which produces 120,000 tonnes of dry ice a year [4]. The layout of the Denizli-Kızıldere geothermal power plant is shown in Fig Exergy and energy analysis In this study, exergy and energy analyses are performed in order to optimize the performance of the power plant. For the dead state, values of T 0 ¼ 16 C and P 0 ¼ 95:9 kpa are derived from local meteorological stations. The following equations are used in the exergy and energy analyses and efficiency evaluations [7,8]. Energy equations: e n ¼ h h 0 _E n ¼ _m e n ð1þ ð2þ Exergy equations: e k ¼ h h 0 T 0 ðs s 0 Þ _E k ¼ _m e k ð3þ ð4þ

5 A. Dagdasß et al. / Energy Conversion and Management 46 (2005) The first law efficiencies of the plants are g I ¼ _ W net _E nr ðat reservoir conditionsþ ð5þ g I ¼ _ W net _E nwh ðat wellhead conditionsþ ð6þ and second law efficiencies are g II ¼ _ W net _E kr ðat reservoir conditionsþ ð7þ g II ¼ _ W net _E kwh ðat wellhead conditionsþ ð8þ For performance analysis of geothermal power plants, first and second law efficiencies are usually used, but second law efficiencies are more suitable to assess their performance [9]. Some researchers indicate that the first law efficiencies of conventional plants compare directly with second law efficiencies of geothermal plants [10]. In this study, we prepared a mathematical model of the existing power plant and solved it with a computer. There are some basic assumptions considered in the analysis. For example, in the existing power plant, each well has its own separator, but we assumed that all streams are collected and flashed within the same pressure in a separator. With these assumptions, the computer simulation gave the results shown in Table 1. At the Denizli geothermal power plant, the flashing pressure was not optimized so far. For this reason, we determined the optimum flashing pressure for maximum power. Table 1 Characteristic values of the Denizli geothermal power plant at major locations State Temperature T ( C) Pressure P (kpa) Mass flow rate _m (kg/s) Enthalpy h (kj/kg) Entropy s (kj/kg K) Energy flow rate _E n (kw) Exergy flow rate _E k (kw) R ,198 47, ,198 47, ,198 45, ,261 21, ,226 21, , , , ) ) ,937 25,530

6 250 A. Dagdasß et al. / Energy Conversion and Management 46 (2005) Performance evaluation of the Denizli geothermal power plant A mathematical model of the existing plant was prepared to compute the performance values. According to this analysis, the present plant gross power is evaluated to be 10,374 kw e and the net power reduces to 9440 kw e because of internal uses. The first and second law efficiencies of the plant are 4.556% and 19.97%, respectively, at wellhead conditions. These values become 4.556% and 19.78% at reservoir conditions. As seen from these values, the plant operates at low efficiencies with respect to similar geothermal plants Determination of the optimum flashing pressure The flashing pressure that makes the power output maximum is obtained to be 200 kpa with the computer simulation (Fig. 3). The net power rises to 11,140 kw e at this pressure, and this means that 18% more power could be generated with the existing plant. On the other hand, the first and second law efficiencies become 5.376% and 23.34%, respectively, at reservoir conditions. The comparison of net power outputs and efficiencies is given in Table 2. The geofluid of the Kızıldere field has considerably high amounts of dissolved mineral contents, so there is a lower limit of flashing pressure. Otherwise, there could be scaling problems but this phenomenon does not interest us here. In future, it can be solved Analysis of the proposed combined single flash-binary power plant During the planning period of the plant (1980s), a binary cycle power system was firstly suggested. It was calculated that a binary system would produce 30 MW e power with 2000 t/h geofluid flow rate. In spite of its lower plant cost, the UNDP (United Nations Development Program) could not dare to build a binary cycle because there were not enough experiences on it at that time. For this reason, a plant with a single flash cycle was built and put in operation in The power plant nominal capacity was 20.4 MW e at a cost of million $. In spite of that capacity, the plant runs at 10 MW e average power [3]. Fig. 3. Computed net power output vs. flashing pressure.

7 A. Dagdasß et al. / Energy Conversion and Management 46 (2005) Table 2 Comparison of existing and optimum values of Denizli single flash power plant (efficiencies are given at reservoir conditions) Present values Values at optimum flashing pressure Power output (kw e ) ,140 Overall first law efficiency (%) Overall second law efficiency (%) Since building a new plant is not feasible, modification of the existing plant will be more acceptable. It can be seen from Table 1 that the geofluid leaving the separator has a considerably high temperature. This fluid is discharged directly to the B. Menderes River without any other use in the existing plant. The exergy loss is approximately 53.48% of the exergy available at wellhead conditions (25,530 kw). However, we can use this exergy for increasing the power generation by adding a binary cycle. The layout of the proposed combined system is shown in Fig. 4. In this scheme, the geofluid leaving the separator is sent to a heat exchanger to evaporate the working fluid. After that, the superheated organic vapor goes to a turbine-generator group to generate electricity with the classic Rankine Cycle. Isobutane is firstly chosen as the working fluid because of its good thermodynamic characteristics. Since the saturated steam curve of isobutane has a positive slope, the organic vapor is still at a superheated state at the last stages of the turbine. For this reason, there will not be a deterioration of the turbine blades because of wet vapor, so the turbine life will be longer. Characteristic values of the binary cycle are shown in Table 3. On the other hand, calculations were also performed for different working fluids in the combined cycle in order to select the best working fluid. Geofluid heat is transferred to a binary system s working fluid by means of a heat exchanger. The inlet temperature of the geofluid to the heat exchanger is assumed as 148 C. The outlet temperature and pressure of the isobutane from the heat exchanger is chosen to be 135 C and Fig. 4. A simplified scheme of the proposed combined single flash-binary power plant in Denizli.

8 252 A. Dagdasß et al. / Energy Conversion and Management 46 (2005) Table 3 Characteristic and assumed values of binary cycle (at optimum conditions with isobutane) No Phase Temperature ( C) Pressure (kpa) Mass flow rate (kg/s) 11 Sat. liquid (geofluid) Superheated working fluid Superheated working fluid Liquid working fluid Liquid working fluid Liquid (water) Liquid (water) kpa, respectively. The cooling tower is not used in the proposed plant and cooling water taken from the B. Menderes River is used for condenser cooling. The inlet temperature of the cooling water is taken as 20 C and the outlet temperature is 25 C. The flow rate of cooling water becomes 3935 kg/s at this condition. On the other hand, the isentropic efficiency of the binary cycle turbine is assumed to be 80% and the pump efficiency to be 72%. The pinch-point temperature between the geofluid and working fluid is suggested as 6 C. At these conditions, the net power of the binary cycle turbine is 6139 kw e, and the total power output of the combined cycle becomes 15,875 kw e. The pressure of the isobutane at the heat exchanger outlet is optimized for maximum net power output and the results of the calculations are given in Fig. 5. As can be seen from Fig. 5, the optimum isobutane pressure for maximum power is 1800 kpa Performance analysis of combined geothermal power plant at optimum flashing pressure and optimum isobutane pressure With the proposed single flash-binary power plant, the computer simulation of the combined plant is run again for optimum flashing pressure and optimum isobutane working pressure to obtain maximum power. We obtained the net power output of the binary turbine as 7098 kw e and isobutane Fig. 5. The optimum pressure of isobutane at the heat exchanger outlet for the proposed combined power plant. (Optimum pressure that makes net power output maximum in binary cycle.)

9 A. Dagdasß et al. / Energy Conversion and Management 46 (2005) Isobutane kpa kpa Fig. 6. Temperature entropy diagram of the Rankine cycle executed by isobutane based on optimum operating conditions. Table 4 The comparison of existing values of single flash power plant and the values of proposed combined power plant at optimum operating conditions Present values Combined cycle values at optimum conditions a Power output (kw e ) ,238 18,238 Overall first law efficiency (%) a Overall second law efficiency (%) a a At reservoir conditions. At wellhead conditions. Combined cycle values at optimum conditions b the total power output as 18,238 kw e. The T s diagram of the Rankine Cycle with isobutane is shown in Fig. 6. The power difference of the proposed combined cycle plant and existing plant is 8798 kw e. As can be seen from this result, the power output of the combined plant is 93, 2% higher than that of the existing plant (Table 4). The increase of power output comes from optimization of the single flash plant (1700 kw e ) and the addition of a binary cycle to the existing plant (7098 kw e ). According to the analysis, adding a binary system to the existing plant is suitable and feasible even from an economic point of view. It is estimated that 5.5 million $ annual revenue is possible, so the plant will be paid back within 3.5 years. On the other hand, the waste of geofluid, which has 75 C temperature, could be used in greenhouse and district heating. There are large and fruitful lands nearby the power plant. Although some greenhouses are present around, they are only used for testing. Also, the towns located nearby such as Buldan, Buharkent, Sarayk oy could be heated by waste fluid. In addition, the waste fluid heat could also be used as process heat in industrial plants. To do this, the industry area of Denizli should expand west instead of east. In this way, it will be possible to use the waste heat, reducing the pipeline and pumping cost. In the existing plant, the waste geofluid is directly discharged to the B. Menderes River, and this causes a serious chemical and thermal pollution. However, the spent brine contains boron at a high concentration

10 254 A. Dagdasß et al. / Energy Conversion and Management 46 (2005) (30 ppm). River water mixed with spent brine is used for irrigation purposes. As boron contents must be lower than 1 ppm for irrigation [11], the plant is switched off during irrigation time in the summer [5], causing important losses in electricity production and income. When the power plant is switched off, the boron concentration of the B. Menderes River was measured before and after the Kızıldere power plant and found as 0.38 and 1.4 mg/l, respectively [3]. It is clear that, when the plant is running, the concentration of boron is higher. Thus, waste geofluid must be reinjected to the reservoir via suitable wells as soon as possible. Net power output and first and second law efficiencies are evaluated in the proposed combined cycle power plant when using different working fluids. In addition to isobutane, isopentane and R 114 are also considered as working fluids. Power outputs and efficiencies are calculated for these fluids too Use of isopentane The proposed combined cycle computer model is operated for isopentane in this step. For the initial calculation, the temperature and pressure of the working fluid at the heat exchanger outlet are chosen at 135 C and 1000 kpa, respectively. According to these assumptions, the optimum isopentane pressure, which makes the power output maximum, is found to be 600 kpa (Fig. 7). The flow rate of the isopentane will be kg/s at the optimum situation. The efficiencies and power outputs obtained from this study are given in Table Use of R 114 When R 114 is used as the working fluid in the binary part of the combined power plant with the same initial values (135 C and 1000 kpa), the optimum pressure of R 114 at the heat exchanger outlet is found to be 1200 kpa (Fig. 8). The flow rate of R 114 is kg/s at optimum conditions. The efficiencies and power outputs obtained in this alternative at optimum pressure conditions are given in Table 5. The efficiencies and power outputs computed in this study are listed in Table 5. As can be seen by comparison of the results obtained with different working fluids, isobutane is the most convenient working fluid for the binary part of the combined power plant isopentane Fig. 7. Optimum isopentane pressure of the heat exchanger outlet that makes net power output maximum.

11 Table 5 Power output and efficiency values of the combined single flash-binary power plant for different working fluid (at optimum operating conditions) Working fluid A. Dagdasß et al. / Energy Conversion and Management 46 (2005) Binary unit power output (kw e ) Combined plant total power output (kw e ) Combined plant first law efficiency (%) a Combined plant second law efficiency (%) a Combined plant first law efficiency (%) b Isobutane , Isopentane , R , a At reservoir conditions. b At wellhead conditions. Combined plant second law efficiency (%) b R Fig. 8. Optimum R 114 pressure of the heat exchanger outlet that makes net power output maximum. 5. Conclusions The existing Denizli single flash geothermal power plant is running at lower efficiencies in comparison to similar plants. The main reasons for low efficiencies in the Denizli power plant are old technology and high chemical and noncondensable gas contents of the geofluid. More importantly, the spent brine of the plant is discharged at 148 C without using any cascaded system in the Denizli power plant, whereas this temperature of geofluid becomes the first step of geothermal utilisation in many countries. At present, it is possible to produce electricity easily with this fluid by using a binary cycle system. For this reason, we focused on the combined cycle constituted by adding a binary cycle to the present single flash plant. It is found that the optimum flashing pressure is 200 kpa. When the plant is operated at this state, it could be possible to gain approximately 18% of power production. Isobutane is found as the most convenient working fluid for the binary cycle, and the total maximum power is computed to be 18,238 kw e for this fluid. The first and second law efficiencies based on wellhead conditions are found to be 8.802% and 38.58%, respectively. These values point out that the proposed combined plant system can be operated at higher performance characteristics.

12 256 A. Dagdasß et al. / Energy Conversion and Management 46 (2005) On the other hand, adding the binary cycle to the existing plant can supply 5.5 million $ annual revenue, and the system will pay back in 3.5 years, which means that the combined plant will be economical too. Acknowledgements The authors would like to thank the plant manager, Halil Sarıkurt, and _ Ismail Ozel for providing technical data of the Kızıldere geothermal power plant. The authors thank also Bahri Sßahin, Arif Hepbasßlı, Ahmet Bay ulken and Mehmet Kanoglu for technical informations. References [1] Barbier E. Geothermal energy technology and current status: An overview. Renew Sustain Energy Rev 2001;6:3 65. [2] TEAS, Turkish Electricity Generation and Transmission Company Ankara, Turkey, [3] Serpen U. Technical and economical evaluations of Kızıldere geothermal reservoir, PhD dissertation prepared for Istanbul Technical University [in Turkish]. [4] Gokcen G, Ozturk H, Hepbasli A. Overview of Kızıldere geothermal power plant in Turkey. Energy Convers Manage 2004;45: [5] Sarikurt H. Personal communication, [6] Cß ercßi Y. Performance evaluation of a single flash geothermal power plant in Denizli Turkey. Energy 2003;28: [7] Cß engel Y, Boles M. Thermodynamics: an engineering approach. 2nd ed. Istanbul: Literature; [8] Kanoglu M, Cß engel Y, Turner R. Incorporating a district heating cooling system to an existing geothermal power plant. Trans ASME, J Energy Resour Technol 1998;120(2): [9] DiPippo R. Small geothermal power plants design, performance and economics. GHC Bull 1999;20(2):1 8. [10] Kanoglu M, Cß engel Y, Turner R. Thermodynamic evaluation of a single flash geothermal power plant in Nevada. ASME International Mechanical Engineering Congress and Exposition, November 17 22, Atlanta, Georgia, USA, Proceedings: AES, vol. 39, p [11] Arıt urk S, Parlaktuna M. Estimation of silica scaling temperatures of Kızıldere geothermal field effluent Turkey. Energy Sources 2001;23: