Modern Geothermal Power: Binary Cycle Geothermal Power Plants

Transcription

1 ISSN , Thermal Engineering, 2017, Vol. 64, No. 4, pp Pleiades Publishing, Inc., Original Russian Text G.V. Tomarov, A.A. Shipkov, 2017, published in Teploenergetika. ENERGY CONSERVATION, NEW AND RENEWABLE ENERGY SOURCES Modern Geothermal Power: Binary Cycle Geothermal Power Plants G. V. Tomarov and A. A. Shipkov OOO Geoterm-EM, Moscow, Russia Received March 23, 2016; in final form, August 31, 2016 Abstract In the second part of the review of modern geothermal power plant technologies and equipment, a role, a usage scale, and features of application of binary cycle plants in the geothermal economy are considered. Data on the use of low-boiling fluids, their impact on thermal parameters and performance of geothermal binary power units are presented. A retrospective of the use of various low-boiling fluids in industrial binary power units in the world since 1965 is shown. It is noted that the current generating capacity of binary power units running on hydrocarbons is equal to approximately 82.7% of the total installed capacity of all the binary power units in the world. At the same time over the past 5 years, the total installed capacity of geothermal binary power units in 25 countries increased by more than 50%, reaching nearly 1800 MW (hereinafter electric power is indicated), by A vast majority of the existing binary power plants recovers heat of geothermal fluid in the range of C. Binary cycle power plants have an average unit capacity of 6.3 MW, 30.4 MW at single-flash power plants, 37.4 MW at double-flash plants, and 45.4 MW at power plants working on superheated steam. The largest binary cycle geothermal power plants (GeoPP) with an installed capacity of over 60 MW are in operation in the United States and the Philippines. In most cases, binary plants are involved in the production process together with a steam cycle. Requirements to the fluid ensuring safety, reliability, and efficiency of binary power plants using heat of geothermal fluid are determined, and differences and features of their technological processes are shown. Application of binary cycle plants in the technological process of combined GeoPPs makes it possible to recover geothermal fluid more efficiently. Features and advantages of binary cycle plants using multiple fluids, including a Kalina Cycle, are analyzed. Technical characteristics of binary cycle plants produced by various manufacturers are considered, and data on the Russian pilot binary geothermal power unit in the Pauzhetskaya GeoPP is provided. Expediency of the use of binary cycle plants for autonomous power supply and energy extension of existing GeoPPs without drilling extra wells and in flowsheets of newly designed combined GeoPPs are noted. Keywords: organic Rankine cycle, binary cycle power unit, fluid, flowsheets, installed capacity DOI: /S The evolution of global geothermal energy was initially based on the development of high-temperature geothermal resources, primarily superheated steam. Later on, the main primary source for GeoPPs became wet geothermal steam or a water-steam mixture. Presently, the development of geothermal energy in many countries is carried out by means of a lowtemperature fluid heat recovery as well as waste liquid phase in binary cycle GeoPP. Most of the world s energy potential of geothermal sources accounts for the deposits with a fluid temperature below 130 C. The GeoPPs transferring the fluid heat to another heat carrier, which is a working fluid of the secondary circuit, are called binary plants. In such plants, organic compounds are generally used as the working fluid. A low-temperature heating fluid requires the use of low-boiling organic substances with relatively low efficiency of energy conversion in binary plants. However, considering the fact that, in most cases, they are used for the recovery of waste geothermal fluid or any other industrial heating medium, in general, their operation is commercially advantageous, and their use in some remote areas may be particularly promising. Binary power units are ecologically clean, as a direct contact of the geothermal fluid containing harmful gas and contaminants and the environment is impossible. At the same time, low fluid temperatures help to reduce thermal pollution of the atmosphere. GEOTHERMAL BINARY PLANTS: ROLE AND FEATURES OF OPERATION An advantage of binary plants, consisting primarily in a possibility of producing electricity on the basis of a low-temperature heat source, to a large extent determined the following main areas of their application: (1) Energy supply (including autonomous) of the regions obtaining low-temperature geothermal resources; 243

2 244 TOMAROV, SHIPKOV (46%) 18 (12%) 65 (43%) 4 10 (8%) 9 4 (3%) (a) 5 (b) 10 3 (2%) 14 (12%) 24 (16%) 38 (31%) 41 (27%) Fig. 1. Distribution of binary geothermal power units by (a) temperature t and (b) unit capacity N in the world. t, C: 1 less than 100; ; ; ; 5 more than 250; N, MW: 5 less than 0.5; ; ; ; 9 more than 50. (2) Energy extension of the existing GeoPPs operating on high-temperature geothermal fluid without drilling extra wells; (3) More efficient use of geothermal energy sources due to their utilization in the production process of newly designed combined cycle GeoPPs. For the first time in the world, a binary cycle GeoPP (Paratunskaya GeoPP) [1] was built in 1967 and tested in the Soviet Union in the Paratunskoe deposit, Kamchatka Peninsula. This was the first practical evidence for the efficiency of power generation by binary cycled plants based on the use of lowboiling organic compounds. The development of binary cycle energy technologies may be divided into three main periods. The first, from 1967 to 1984, when this technology was not widespread in the world. The second, from 1984 to 2000, was a period of the development of binary technologies to a limited extent. Since 2000, and, especially, in recent years (the third period), there is an active expansion of binary cycle power units. Only within 10 years (from 2000 to 2010) the total installed capacity of all binary cycle power plants (including power units utilizing nongeothermal heat) increased from 200 to 2000 MW in the world [2]. According to the World Geothermal Congress 2015, over the past 5 years, the total installed capacity of geothermal binary cycle power units in 25 countries has increased by more than 50% by 2015, reaching 1793 MW, including 873 MW in the United States, 265 MW in New Zealand, and 219 MW in the Philippines [3]. Currently, binary power technologies are returning to life in Russia. A pilot 2.5 MW geothermal binary cycle power unit has been designed and installed, which is meant for waste heat recovery of the geothermal fluid of the Pauzhetskaya GeoPP [4]. Binary power units operating in the world use geothermal fluid of different temperature level. Figure 1a provides information on the distribution of the binary power units operating on the fluids with different thermal potential, which vast majority (approximately 77%) recovers heat of geothermal fluid with a temperature of C. There are 286 binary cycle power units out of 613 power units existing in the world. Binary cycle power plants have an average unit capacity of 6.3 MW, single-flash power plants of 30.4 MW, double-flash plants of 37.4 MW, and power plants working on superheated steam of 45.4 MW. Data on the distribution of power units by the unit-installed capacity in the world are presented in Fig. 1b. The geography and scalability of application of geothermal binary power technologies are largely determined by the location and potential of the geothermal resource base. The largest GeoPPs with binary cycle power units with the installed capacity of over 60 MW are in the United States and the Philippines. It should be noted that, in most cases, binary cycle plants are involved in the production process together with a steam cycle. WORKING FLUIDS: PROPERTIES AND SELECTION FOR BINARY CYCLE PLANTS So far, there is no consensus about which organic matter is optimal for the use in binary cycle plants. Thermal, thermodynamic, and other properties of low-boiling organic matter have a significant impact on the pattern and efficiency of the thermal cycle, process parameters, structure and characteristics of the equipment, operation regimes, reliability, and environmental friendliness. In addition, it is necessary to take into account engineering design specification depending on the location of the binary cycle power unit. As an example, Fig. 2 shows results of the evaluated difference in heat content of the working fluid in the binary plant turbine Δh using various organic matters. This parameter substantially determines a capacity of the binary power unit and affects the height of the last stage blades.

3 MODERN GEOTHERMAL POWER: BINARY CYCLE GEOTHERMAL POWER PLANTS 245 Δh, kj/kg R-218 C7F16 C6F12 RC-318 R-C316 R-227ea C3H3F5 R-124a R31-10 R-236fa R-113 R1318 R1113 R-12 R-143a R-114 R-134a R-22 R-245ca R-245fa R142b R-21 R141b R-152a R-600a R-32 R-601a R-290 C6H5F C2H5F R-601 R-600 RC270 C5H12O C4H10O C3H6O C3H8O Working fluid Fig. 2. Estimated difference in heat content of the working fluid in the 2.5 MW binary turbine at a geothermal fluid temperature of 120 C. C6H14 C7H16 C4H6 C8H18 C6H6 C4H8 C5H10 C7H8 C6H12 A problem of the working fluid selection for the binary power plant, which would have the optimum technical and performance characteristics is quite complicated, since the properties of working fluids cannot always meet all the customer s requirements. Therefore, its solution is to find a compromise with the priorities specified by the project originator. It is common practice to impose requirements on a working fluid that provide the following: (1) Incombustibility and explosion safety; nontoxity; (2) Thermodynamic cycle efficiency, which is determined by thermal and thermodynamic properties of the working fluid, including thermal capacity, thermal conductivity, etc.; (3) Overall operating efficiency of the power plant, which depends on characteristics of the power equipment; (4) Efficient use of the primary heat source; minimal impact on the greenhouse effect and the ozone layer of the Earth; wide practical application in power machines; (5) Insufficient corrosive effect on the applied structural materials; (6) Low cost and availability of the organic matter in the market. It is assumed that there are over 300 chemical compounds that theoretically can be used in a binary plant cycle. In practice, only approximately 15 organic substances and mixtures having a low boiling point are used. Currently, geothermal binary power units, which use various organic compounds as a working fluid is as follows (% of the total installed capacity of binary cycle power units in the world [5]): Hydrocarbons 82.7 Fluorocarbons 6.7 Chlorofluorocarbons 2.0 Ammonia water mixture 0.5 Data on the working fluid is n/a 8.2

4 TOMAROV, SHIPKOV Year C 2Cl 2F 4 C 2Cl 2F 4 Australia +CO2 CnHm +C 5F 12 Austria CnHm Argentina Zambia C nh m Guatemala Germany C 5F 12 +C 3H 3F 5 +CnHm + mixture (Н 2О+NН 3) +(H 2O + NH 3) Iceland Italy C 4H10 +C2H5Cl C 4H 10 China C 4H 10 +C nh m C nh n +N/A +N/A Kenya Mixture (Н2О+NН3) Costa Rica C nh m + i-c 4H 10 Mexico N/A i-c4h10 Nicaragua C5H12 New Zealand +i- C nh m Portugal CCl 2F 2 Russia C2H2F4 The United States N/A +C 2 Cl 2 F 4 +mixture (i-c 4H 10+ i-) +i-c 4H 10; C 2C l2f 4; i-c 4H 10; +i- i- +i-c 3H 8; +N/A i- El Salvador CnHm; C2Cl2F4; +; i-c3h8 +mixture mixture (i-c4h10 + i-c5h12); +C + +C nh m (Н2О+NН3) 2H 2F 4 +CHClF 2 +C 3H 3F 5 C4H10; C5H12; N/A; C5H12 C nh m Turkey The Philippines i-c 4H 10 France +C2H2F4 i-c i-c 4H 10; 5H 12 Ethiopia no available data C 2Cl 2F 4 НCFC-123 on the working fluid Japan Fig. 3. Use of working fluids in geothermal binary power units. Symbol + indicates the use of the working fluids in the GeoPP binary cycles since the beginning of their application until Relatively cheap hydrocarbons (pentane, isobutane, isopentane, etc.) characterized by good thermodynamic and thermal properties are explosive and flammable and can be used in open type power plants, which is not always acceptable to the areas with negative winter temperatures. Selecting the working fluid, it is necessary to consider performance properties of organic substances, especially their interaction with oils, water, and air. In the assimilation of binary cycle technologies, various low-boiling working fluids were applied. Figure 3 represents a retrospective diagram of the application of low-boiling working fluids in binary cycle plants in the world since BINARY CYCLE GeoPP FLOWSHEETS Binary cycle power units and combined binary cycle GeoPPs are used for a low temperature fluid heat recovery. Currently, organic substances and their mixtures are mainly used as working fluids. Along with this, binary technologies on the Kalina cycle have been developed in recent years that use an ammonia water mixture. For the binary cycle power units using organic substances, with saturated vapor curve x = 1 in T, s-coordinates has a negative slope or negative gradient of ds/dt < 0 (the substances of this group are water, ammonia, refrigerants R-12, R-21, R-22, R-134a, R-152, etc.), the turbine inlet steam superheating, as a rule, makes it possible to avoid the formation of moisture in the turbine wheelspace [1]. This has a positive effect on the reliability and efficiency of binary turbines. Significant steam superheating is possible in the presence of a high-temperature heat source and requires the turbine outlet heat recovery. This leads to the need for placement of additional heat exchangers and subsequent complication and appreciation of the binary system. Therefore, the selection of superheating in the superheater is determined under the condition of guaranteed absence of moisture after the last turbine stage with allowance for the efficiency. If using organic substances having a positive entropy gradient on the saturated vapor curve ds/dt > 0 (such as pentane R-601, isobutane R-600a, refrigerants R-236fa, R-318c, R-245fa, etc.), the initial turbine inlet steam parameters are located on the T, s-diagram

5 MODERN GEOTHERMAL POWER: BINARY CYCLE GEOTHERMAL POWER PLANTS T 2 3 WSM OWF a Water d g x = 0 b WSM x = 1 c Fig. 4. Process flow diagram of a combined binary cycle geothermal power plant. 1 producing well; 2 wellhead control valve; 3 separator; 4, 8 turbine operating on geothermal steam and OWF; 5 OWF superheater; 6 OWF condenser-evaporator; 7 heater (economizer); 9 condenser; 10 cooling tower; 11 cooling water pump; 12 make-up water; 13 feed water pump; 14 wast liquid phase injection well. h x = 0 b f a x = 0 c OWF e e d x = 1 f s H2O above the saturation line x = 1, and there is no moisture formation with the steam expansion and, on the contrary, its degree of superheating increases. Since the turbine outlet steam will be in the superheated state, it is possible in some cases to use a recovery heater to return the waste steam heat to the cycle. Along with this, studies show [6] that excessive steam superheating and, as a result, a substantial increase in the turbine heat drop are not always appropriate from the power engineering point of view. To justify the applied process solutions, their technical-and-economic efficiency should be also assessed [7]. Effectiveness of the heat exchange process is largely determined by a temperature drop, i.e., a temperature difference between the heating geothermal fluid and the heated working fluid of the binary power plant. The presence of a horizontal segment on the T, Q-diagram [1], which characterizes evaporation of the organic working fluid, running at a constant temperature, causes a minimum temperature difference ΔT min in most cases, limiting the heat exchange between the two media. A location of the so-called pinch-point on the T, s-diagram and the temperature difference at this point ΔT min in relation to the used heat exchange equipment to a large extent determine the design parameters of the technological flowsheet and the heat exchange equipment. A reduction in ΔT min increases the heat exchange surface area, massdimensional parameters, and appreciation of the equipment. In order to reduce pinch-point losses and make better use of the heat source production capacity, various processing solutions are implemented in practice, including a double-flash flowsheet [8]. This somewhat complicates the flowsheet and may require the installation of two turbines of different pressure, but s OWF Fig. 5. T, s-diagram of the heat conversion to electricity in a combined binary cycle GeoPP. a' geothermal fluid state in the productive stratum; a' b' downhole flashing; b' c', b' d' phase separation in the separator; c' e' geothermal steam expansion; e' f' steam condensation; g', h' geothermal fluid state in the waste liquid phase and condensate reinjection well; a" b" c" d" e" f" a" closed cycle of the organic working fluid; a" b" OWF compression by a feed water pump; b" c" d" e" OWF heating, evaporation, and superheating in heat exchangers due to the geothermal liquid phase heat; e" f" OWF stem expansion in the binary turbine; f'' a'' OWF steam condensation in the condenser of a binary power plant. this makes it possible to use the heat resource more efficiently and completely. Combined binary cycle GeoPPs are different in that the geothermal fluid of the primary circuit is not only a heat source for the secondary circuit but is used directly to convert thermal energy into mechanical work in the steam turbine. A vapor phase of the geothermal biphase fluid (a steam-water mixture) is used in the primary circuit to generate electricity due to the expansion in the backpressure turbine. Geothermal condensed steam heat, as well as liquid phase heat, is used in the secondary low temperature circuit, generating electricity by means of an organic working fluid (OWF). A combined binary cycle GepPP process flow diagram is shown in Fig. 4. Figure 5 conditionally combines two T, s- diagrams of the state of various substances: water and an organic compound. Actually, the primary circuit of the combined binary cycle GeoPP operates on geothermal steam, and its cycle on the T,s-diagram in fact corresponds to the single-flash technology.

6 248 TOMAROV, SHIPKOV Fig. 6. General view of the geothermal combined binary cycle power plant in Mokai, New Zealand with an installed capacity of 100 MW [11]. In a combined cycle GeoPP s geothermal circuit, it is advisable to maintain steam pressure in the condenser slightly above the atmospheric in order to remove noncondensable gases without using any special equipment. The use of such combined GeoPPs is especially advantageous in cases when the source geothermal fluid contains a lot of noncondensable gases, since energy costs of their removal from the condenser can be significant. Results of thermodynamic calculations [9] show that, the initial conditions being equal, due to the use of binary power units in the combined cycle GeoPPs, it is possible to increase the capacity of a single-flash GeoPP 15%, and that of a double-flash GeoPP by 5%. Currently, binary cycle power plants are produced at in the United States, Germany, Italy, Sweden, Russia, etc. Some data on the binary power plants' technical parameters, produced by different manufacturers, are presented in the table [10]. Combined binary cycle GeoPPs work in the Philippines, New Zealand, and other countries. Figure 6 represents a general view of one of the largest 100 MW GeoPP Mokai (New Zealand), built by an American Israeli company ORMAT, which uses a high-temperature geothermal fluid for electricity generation [11]. Presently, a combined binary cycle flowsheet is almost always used in the design of new GeoPPs in the fields of a biphase heat transfer fluid. In addition, waste fluid recovery in the liquid phase in binary power plants provides an opportunity to increase the installed capacity of GeoPPs without drilling additional wells. These technological solutions may become a basis for the modernization of the 50 MW Mutnovskaya GeoPP (Kamchatka Peninsula, Russia) in order to increase its capacity by 15 MW [12]. The improvement of the thermodynamic cycle efficiency of combined GeoPPs, as well as the reliability of steam turbines by means of reduction of the deposit formation rate and erosion wear of the turbine blades, is possible by the inlet steam superheating in a hydrogenoxygen steam generator [13]. The Kalina cycle geothermal binary power plants, which use an ammonia-water mixture instead of the organic working fluid, provide for a higher cycle efficiency with changes of operating terms and conditions. Zeotropic mixtures, unlike azeotropic ones and pure substances, boil at constant pressure with a variable temperature. This makes it possible to organize vaporization and condensation heat exchange more efficiently at a lower temperature difference. Concentration of the solution in various points of the cycle arrangement is different and can be adjusted to achieve the optimum thermal effectiveness. Characteristics of binary plants' equipment Manufacturer Unit capacity, kw Heat source temperature, C Technological features working fluid turbine ORMAT, United States n-pentane Two-stage axial Turboden, Italy OMTS As above GMK, Germany Multistage axial Turboden Pure Cycle, United States R245fa Radial Cryostare, France R-245fa, R-134а '' Infinity Turbine, United States 10 50, More than 250 Less than 90, R-134а, R-245fa Barber Nichols, United 700, 2000, 2700 More than 115 States Trans Pacific Energy, United States Mixture of organic compounds Kaluga Turbine Plant, Russia 2500 More than 100 R-134a Single-stage radialaxial

7 MODERN GEOTHERMAL POWER: BINARY CYCLE GEOTHERMAL POWER PLANTS 249 Evaporator Heater Condenser Heating Condenser Turbine plant Pumps Turbine plant Fig. 7. Kalina cycle geothermal 3.4 MW power unit (Siemens, Germany), GeoPP in Unterhaching, Germany. Fig. 8. Geothermal binary cycle power unit in the Pauzhetskaya GeoPP. In 1955, L.M. Rosenfeld first proposed to use zeotropic ammonia-water working fluid in a power plant steam cycle. Later, A. Kalina modified these steam turbine cycles by the introduction of advanced regeneration systems and patented thermal cycles developed by him [14]. For the first time, a technology for power generation based on the Kalina cycle, which has a number of advantages, was tested in For example, the condensation temperature in a steam Rankine cycle is constant and determined by the temperature of the cooling water at the condenser outlet, and the condensing temperature in an ammonia-water solution cycle is variable and determined by the temperature of the cooling water at the condenser inlet. This fact provides an opportunity to increase the efficiency by reducing the waste heat temperature. The use of a two-component working fluid (ammonia-water) makes it possible to effectively apply a recovery scheme, first of all, the outlet steam heat recovery (before the condenser). The use of an ammonia-water working fluid in a steam turbine cycle does not require the creation of new designs of steam turbines and heat exchange equipment. Water and ammonia have very similar molecular weight and isobaric heat capacity. This makes it possible to use standard power equipment designed to run on steam. The working fluid pressure is above the atmospheric at any point of the thermal system, which enables to get rid of steam or water ejectors and to facilitate the power plant operation. The use of the thermodynamic Kalina cycle provides an opportunity to increase the utilization efficiency of a geothermal fluid under favorable conditions. In 2000, a first commercial Kalina cycle GeoPP was built in Húsavík, Iceland, which has successfully operated demonstrating high thermodynamic efficiency [15]. A German company Siemens built a Kalina cycle 3.4 MW Kalina cycle GeoPP in Unterhaching in 2009 (Fig. 7) [16]. So far, there are several dozens of the Kalina cycle power units being in operation and in various stages of construction. Despite the advantages and energy efficiency of Kalina cycle binary power plants, their practical implementation is hampered by lack of sufficient experience in the design and operation as well as more complex equipment and control algorithms. PILOT BINARY CYCLE POWER UNIT IN THE PAUZHETSKAYA GeoPP Currently, the construction of a pilot 2.5 MW binary cycle power unit in the Pauzhetskaya GeoPP, Kamchatka, is being completed (Fig. 8). A heat source for heating and evaporation of the secondary circuit working fluid is a waste liquid phase of the Pauzhetskaya GeoPP at a temperature of 120 C. As a result of calculation and analytical studies with allowance for the factors determining efficiency, safety and environmental friendliness, an organic refrigerant compound R-134a was selected as the working fluid of a binary power plant [17]. A technological flowsheet of the Pauzhetskaya binary cycle plant includes an evaporator superheater, turbine, condenser, feed pumps, and other equipment. At the inlet of the evaporator superheater, the intake flow rate of the geothermal waste liquid phase is kg/s, the pressure is 0.2 MPa, and the temperature is 120 C. The Kaluga Turbine Works produced a binary axial-radial single-stage turbine with cantilevered mounting on a free end of the turbine-generator shaft [18]. The steam of the working fluid R-134a has the following parameters: Turbine inlet: pressure, MPa 2.19 temperature, C 78.5 flow rate, kg/s Turbine outlet: pressure, MPa 0.76 temperature, C 33

8 250 TOMAROV, SHIPKOV One of the features of a binary cycle turbine is efficacy of the gland steam system to operate under vacuum and at elevated pressure (up to 2.6 MPa) in case of emergency. A turbine condenser is designed as a shell and tube heat exchanger using cooling water at a temperature of 8 C. To feed the organic working fluid from the condenser to the evaporator superheater, a unit of three (two working and one standby) sealed feeding centrifugal AC magnetic-drive pumps are provided. The design and finalizing of binary cycle geothermal power technologies for arctic and hot-weather application are scheduled. Binary GeoPPs in arctic design (Kamchatka Peninsula, the Kuril Islands, etc.) based on the development of the pilot binary cycle geothermal power unit of the Pauzhetskaya GeoPP must meet the following requirements: (1) Resistance to harsh climatic conditions (low temperatures, wind, snow loads, etc.); (2) A heat source temperature is C; (3) Using steam and waste liquid phase as a heat source; (4) A low condenser temperature, which improves the cycle efficiency. Binary GeoPPs for hot-weather application (the North Caucasus, Krasnodar krai, etc.) are characterized as follows: (1) A heat source temperature is C; (2) Lightweight design of the building (a possibility of the on-the-site assembly); (3) Using thermal water as a heat source. The world practice shows that the attractiveness of modern geothermal power is determined by its commercial maturity, all-weather capability, a high installed capacity utilization factor, and environmental cleanliness. 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