Exergetic and economic analysis of Kalina cycle for low temperature geothermal sources in Brazil

Transcription

1 PROCEEDINGS OF ECOS THE 25 TH INTERNATIONAL CONFERENCE ON EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS JUNE 26-29, 2012, PERUGIA, ITALY Exergetic and economic analysis of Kalina cycle for low temperature geothermal sources in Brazil Carlos Eymel Campos Rodríguez a, José Carlos Escobar Palacio a, César Rodríguez Sotomonte a, Marcio Leme a, Osvaldo J. Venturini a, Electo E. Silva Lora a, Vladimir Melián Cobas a, Daniel Marques dos Santos b, Fábio R. Lofrano Dotto c, Vernei Gialluca d a Federal University of Itajuba (UNIFEI). Mechanical Engineering Institute IEM. Excellence Group in Thermal Power and Distributed Generation (NEST). Minas Gerais. Brazil; eymelcampos@hotmail.com b AES Tietê; Bauru, São Paulo Brazil; danielmarques.santos@aes.com c FAROL Pesquisa, Desenvolvimento e Consultoria; fabio@farolconsultoria.com.br d Gênera Serviços e Comércio LTDA; vernei@generatech.com.br Abstract: This paper deals with the thermodynamic analysis (of both the first and second law of thermodynamic) of the Kalina cycle to find the optimum ammonia-water concentration and evaporation pressure at turbine inlet for different low temperatures geothermal sources on the basis of an exergy analysis. In this work, the Aspen- HYSYS software was used to simulate the Kalina cycle and to calculate the thermodynamic properties based on Soave-Redlich-Kwong (SRK) Equation of State (EoS). The influence of these parameters over the power generation and over the first and second efficiency laws, were calculated. The exergy losses of each component were also studied, pointing the ammonia-water concentration and pressure influence over the power generation and cycle efficiency. Finally the size of the component for the different configurations of the plant and the costs of heat exchanger, turbine and pump were evaluated for the condition of the real geothermal source in Brazil. Lower values of US$/kW (888 US$/kW) were obtained for the configuration of 84% of ammonia and 16% of water mass fraction in the composition of the working fluid at an evaporation pressure of 2500 kpa, producing kw with 5.86% of thermal efficiency. Keywords: Thermodynamic analysis, Kalina cycle, ammonia-water mixture, equation of state, exergy, costs of investment, energy output, geothermal energy. Introduction One of the pillars for sustainable development is based on the use of renewable energy sources. In Brazil, during the last years the demand for energy has increased significantly, and this growth trend will be maintained in the coming years. So it is essential that reliable sources of renewable energy are included in the national energy matrix. However, the use of alternative energy sources is always complementary to the use of traditional fossil sources. Currently some alternative renewable energy sources are already technical and economically feasible. The processes and equipment used have a significant degree of efficiency and reliability. The new renewable energy also increases the diversity of the energy supply, ensures the sustainability of the energy generation. In the long terms, reduces atmospheric emissions of greenhouse gases and air pollutants, creates new employment opportunities in remote rural areas and promotes stability and reliability of the energy system. More traditional renewable energy sources are: solar, wind, geothermal, hydropower and biomass. Renewable energy sources such as solar, low-enthalpy geothermal sources and large amounts of heat from the industrial wastes are potentially promising sources of energy, able to supply a large share of global electric energy demand. However, low and moderate temperatures of these sources cannot be efficiently converted into electricity through conventional power generation, which is why a lot of this energy is simply wasted

2 Nomenclature P pressure, (kpa) T (absolute) temperature, (K) t temperature, ( o C) R gas constant, (J/kg K) V specific volume, (m 3 /kg) w acentric factor of the working fluid a coefficient of the Soave-Redlich-Kwong equation of state (J/kg) b coefficient of the Soave-Redlich-Kwong equation of state (J/kg) U global heat transfer coefficient, (W/m 2 K) A area (m 2 ) C cost (US$) mass flow rate (kg/s) heat rate (kw) power rate (kw) h enthalpy (kj/kg) e specific exergy (kj/kg) s entropy (kj/kg K) exergy (kw) q efficiency (%) total heat transfer Subscripts c critical r reduced temperature in inlet out outlet 0 dead state t turbine p pump d destruction w geothermal fluid th thermal e exergy eq equipment h hot c cold Abbreviations HT high temperature LT low temperature For low-temperature geothermal reservoirs, the common type of plants used for the conversion of heat into electrical energy, are binary plants. Up to December 2010, there was not an installed capacity of electricity generation from geothermal energy reported in Brazil. Right now, the main use of geothermal resources in Brazil is in direct use as spas and heating. The Kalina cycle, originally conceived by Kalina [1] is potentially viable for efficiently generating energy from low temperature sources. The first geothermal plant of this type was built in Husavik, Iceland [2]. Currently, the Kalina cycle is of great interest in different applications [3-6]. Indeed, there are several different configurations of the Kalina cycle, depending, essentially, on the characteristics of the heat source. Various studies had been published about the thermodynamic properties of ammonia-water mixtures [7-10]. The design studies for the use of Kalina cycle, for electric generation from geothermal resources with low temperature indicate different compositions of ammonia-water mixtures, being the most common one about 70 % ammonia 30 % water [11-13]. Water-Ammonia mixture The ammonia-water mixture is non-azeotropic. The characteristic of nonazeotropic mixtures is that the composition and temperature changes during boiling for all possible compositions of the mixture. Ammonia-water mixture differ of pure water or pure ammonia, the four main differences of the mixture and pure water are listed below. 1. The ammonia-water mixtures have variable boiling and condensation temperatures. Conversely, pure ammonia and pure water have constant boiling and condensation temperatures. 2. The physical properties of the mixture can be altered by changing the concentration of ammonia. Since the thermophysical properties remain constants

3 3. These mixtures have thermophysical properties that lead to increases or decreases the fluid temperature without changing its energy content. 4. Another important characteristic is about the freezing point of the fluid. The pure water freezes at (0 C), pure at (-78 C). Solutions of ammonia and water have lower freezing temperature. Because of the above points, the ammonia-water solutions are appropriate to be used in Kalina cycle applications for low temperatures geothermal electric generation systems. Phase diagram Ammonia has a lower boiling temperature compared with water, and so is the most volatile component of the ammonia-water solution. This means that when the temperature of the mixture increases, the ammonia will boil first. Contrary, when the mixture is cooled, water will condense first. This unique feature is shown in Fig 1. This diagram plots the temperature versus the concentration of ammonia in the mixture at 550 kpa. Figure 1: Equilibrium temperatureconcentration curve for NH 3 - H 2 O at constant pressure. Figure 2. Evaporation process in a Kalina Cycle In figure 1, the lower curve is the curve of saturated liquid, which happen the start of boiling when the mixture is heated or complete condensation occurs when cooling. While, the upper curve shows the saturated vapour line, or point where there is the complete vaporization of the mixture or the onset of condensation. When a mixture of ammonia-water is vaporized, a phase diagram conveys much information about the process. For example, when the mixture begins to boil at the boiling point temperature, given by point 3, the % of ammonia in mass is 70% and 30% of water. As the mixture continues boiling, the temperature increases and point 4 is achieved, wherein the concentration of the remaining liquid and the vapour formed are given by points 6 and 5 respectively. Eventually, the line 7 is reached, where the mixture is saturated vapour at a temperature above the dew point and the vapour concentration is the same as the concentration of the liquid to the beginning of the evaporation process. In figure 2 is shown an evaporation process in a Kalina Cycle. This non-constant temperature in an evaporation/condensation process permits, if comparing whit pure working fluids cycles, lower values of irreversibilities and higher power output of the cycle. Kalina cycle modelling In the process flow diagram given in Fig 3, the main components of the Kalina cycle plant are: evaporator, separators, low and high temperature recuperator, circulation pump, condenser and 345-3

4 turbine-generator. The ammonia-water mixture is heated in the high-temperature recuperator and evaporator; ammonia-rich vapor is separated in the separator and sent to the turbine-generator. After passing through the turbine-generator, the expanded ammonia-rich liquid is mixed in the lowtemperature recuperator with the cool ammonia-poor liquid from the separator and sent to the condenser, whence it is recirculated to the evaporator to complete the cycle. Figure 3. Schematic representation of Kalina cycle with AspenPlus software Method The study of the Kalina cycle requires knowing the thermodynamic properties of the ammoniawater mixture which acts as the working fluid. The property packages available in Aspen-HYSYS allow you to predict properties of mixtures ranging from well defined light hydrocarbon systems to complex oil mixtures and highly non-ideal (non-electrolyte) chemical systems. These properties can be obtained from cubic equations of state (EoS). The cubic equations of estate SRK (Soave-Redlich-Kwong) packages contain enhanced binary interaction parameters for all library hydrocarbon-hydrocarbon pairs (a combination of fitted and generated interaction parameters), as well as for most hydrocarbon-nonhydrocarbon binaries. RT a P V b V ( V b ) (1) The parameters of the SRK EOS are calculated from the following equations: RTc a P 2 2 c (2) RTc b P c T 2 1 m 1 r (3) (4) m w w 2 (5) 345-4

5 Assumptions used in the analysis 1. Pressure drop and heat loss in pipe lines are neglected. 2. Ambient temperature 25 C. 3. The ammonia-water mixture at the turbine inlet is saturated vapour. 4. The condensing temperature, lower temperature of the system was set at 36 C. 5. The isentropic efficiency of the turbine is 85%. 6. The pump efficiency is assumed to be 55%. 7. The pinch point was set at 3 o C at the evaporation start, (see fig. 2) 8. The terminal temperature differential (TTD), between inlet geothermal source temperature and outlet working fluid temperature in evaporator was set in 10 C, (see fig. 2). Thermodynamics analysis Mass and energy balances for each component of the heating system can be calculated using equations (6-7). m in The objective of the exergy analysis is to determine the operating conditions of a system which destroys the least available work. The exergy of the ammonia-water mixture can be calculated from the following relation: Where the properties in the dead state are evaluated at T 0 and P 0. When the fluid is in the liquid phase at the dead-state conditions, it is sufficiently accurate to take the dead-state enthalpy and entropy values as if the fluid were a saturated liquid at the dead-state temperature, for this case, 25 o C. The exergy destruction rate can be calculated for each component of the cycle from the following exergy balance equation: Finally, the previous energy and exergy analysis makes it possible to calculate the respectively thermal and exergetic efficiency of the cycle from the followings equations: m out Q W m h m h (6) in in out out E m e m e W d in in out out (8) (7) (9) (10) Based on (6 9), the balance of first and second law of the thermodynamic of the most important cycle components has been developed. Results and discussion The following analysis was performed for 1 Kg/s and temperature between 90 C to 140 C of a geothermal source. In order to obtain the optimum performance for the Brazilian conditions of the cycle the concentrations of ammonia-water and the operation pressure were varied (11)

6 First, the optimum working pressure was evaluated, in order to obtain a maximum power output, at saturated conditions in the turbine inlet at different composition of the working fluid, that vary from ammonia 65% - water 35%, ammonia 75% - water 25%, to 84% ammonia water 16% as shown in Figure 4. It can be observed that the power output varies with the working pressure, temperature and composition of the ammonia-water mixture. At each concentration and temperature, we obtain a maximum power output under a determinate working pressure. For Brazilians condition, where the condensation temperature of the cycle is approximately 36 C due to the ambient temperature of 25 C, different condensation pressure is required for the different composition of the working fluid. At lower condensation pressures, which theoretically would render higher values of power output and efficiency, is not possible to work at, because the fluid will not get totally condensed at the end of the condenser and would cause damages to the circulation pump. Moreover, to vary the condensation pressure of the cycle by decreasing the concentration of ammonia in the working mixture was also analyzed and shown in Figure 4, where the evaporation pressure was plotted against the power output at different temperature and composition of the working mixture. Different condensation pressure was assumed for each ammonia-water composition in order to reach maximum power output and cycle efficiency. Condensation pressure of 800 kpa was used for 65%-35% of ammonia-water solution, 1000 kpa for 75%-35% and 1200 kpa for 84%-16%. (a) (b) 65% NH 3 75% NH 3 (c) 84% NH 3 Figure 4. Cycle maximum power achieved for different temperatures of the geothermal source and evaporation pressures. (a) 65% of ammonia fraction (b) 75% of ammonia fraction (c) 84% of ammonia fraction

7 (a) (b) 65% NH 3 75% NH 3 (c) 84% NH 3 Figure 5. Mass flow rate for different temperatures of the geothermal source and evaporation pressures. (a) 65% of ammonia fraction (b) 75% of ammonia fraction (c) 84% of ammonia fraction. (a) (b) 65% NH 3 75% NH 3 (c) 84% NH

8 Figure 6. Vapor mass flow rate for different temperatures of the geothermal source and evaporation pressures. (a) 65% of ammonia fraction (b) 75% of ammonia fraction (c) 84% of ammonia fraction. From figure 4, 5 and 6, it can be conclude that exist a value of pressure in which power output is greatest, pressure below this optimal value the cycle is able to evaporate more working fluid but the variation of the enthalpy in the turbine is lower, that result in lower power output of the cycle. By other hand, higher pressure than optimal produce higher variation of enthalpy in the turbine, but the cycle is able to evaporate less mass flow rate, that result in lower power output too. Then, is possible to determine, what is the evaporation pressure of the Kalina Cycle for a given mass flow rate and temperature of the geothermal source in which is obtained higher cycle performance. The energy efficiencies of the Kalina cycle plants vary between 5.5 and 10.6% while the exergetic efficiency vary from 32.3 to 46.7%. The variations of thermal and exergetic efficiency are shown in Fig. 7. The maximum thermal efficiency is 10.6%, when geothermal fluid temperature is 140 C and 84% of ammonia mass fraction in the composition of the working fluid mixture. Higher values of exergy efficiency were obtained under the same working conditions. In practice, 90% ammonia fraction is the break point of this curve beyond which efficiency start to decrease sharply as the plant approaches a standard binary cycle [2]. Figure 7. Influence of the temperature of the geothermal source and ammonia-water composition over the thermal and exergetic efficiency of the cycle. In figure 8 is shown the exergy destruction of each component of the cycle for the different ammonia-water composition as a working fluid. The exergy destruction in the cycle behaves as fallow: the condenser is responsible for the biggest irreversibility, following by the evaporator, turbine HT and LT recuperator and finally the pump. Figure 8. Exergy destruction by components of the Kalina Cycle

9 Economic Evaluation Table 1 is shown some operation parameters for a Kalina Cycle of the geothermal source found in Brazil [17], (100 C and 100 kg/s of geothermal water), working at different composition of the working fluid. An economic evaluation to find a lower cost per kw was carried out for the different percent of ammonia in mass in the composition of the working fluid, taking to account the costs of the components of the cycle. Table 1. Operation parameters of the cycle at different ammonia-water compositions of the working fluid. Operation parameters % of ammonia mass fraction Power output (kw) Evaporator heat consumption (kw) Circulation pump (kw) Thermal efficiency (%) Exergetic efficiency (%) Evaporation pressure (kpa) Condensation pressure (kpa) Mass flow rate (kg/s) Vapor mass flow rate (kg/s) Cooling water temperature ( o C) Turbine efficiency (%) Pump efficiency (%) Size of the Components and Operations Cost Estimation Can be consider that the total cost of the equipments of the cycle (evaporator, pump, generator and condenser) contributes largely to the total system cost in a low-temperature geothermal power plant and is assumed to be representative of the complete system cost. Heat exchanger sizing The size of the main components (heat exchangers, pumps and turbine) can be estimated for the four different options fixed for the Kalina cycle. Basically, the size of the heat exchanger can be calculated using the LMTD methods (log Mean Temperature Difference), [20] The total heat transfer rate per unit of time (q), can be expressed in the following equations: The determination of the overall heat transfer coefficient (U) is often tedious and needs data not yet available at the preliminary stages of the design. As a first approximation, for preliminary (12)

10 calculations, the values shown below were used. Since the heat exchangers can be built according to varies geometrical design, there are corrections factors that must be used with the equations (12) depending on the configuration [22]. Therefore, typical values of U are useful for quickly estimating the required exchange area [18, 21, 22]. Thus, U is given as a conventional value of 1 kw/m 2 K to estimate the sizes of the HT and LT recuperators. Values of 0.9 and 1.1 kw/m 2 K were used for size estimation of the vaporizer and the condenser. The value for the vaporizer is the lowest one, for a steam in the shell and liquid in the tubes of the heat exchanger, running under forced circulation. The value for the condenser is based on ammonia in the shell and cooling water in the tubes. Estimated heat transfer areas of the heat exchangers in the Kalina cycle are listed in Table 2 for the different configurations. Table 2: Estimated sizes of the heat exchangers for different ammonia fractions in Kalina Cycle. % of ammonia mass fraction Components Size (m 2 ) Vaporizer Condenser HT recuperator LT recuperator Total Costs estimation of the equipments To determine the purchase costs of the equipments an approach estimation of the costs was used, based on costs values from past purchase orders, and quotations from experienced professional of cost estimations [18-19]. In the estimation of the purchase cost of the heat exchangers, the base cost C o =588 US$ per square meter of heat transfer surface area was used. Thus: Where the exponent n is a constant decimal number, in this case 0.8, and stands for the heat transfer surface area of the heat exchanger, available in table 2 for the different configurations. Using these data the costs of the heat exchangers have been estimated by the relationship in Equation 13 and are presented in Table 3. Table 3: Costs estimation of the heat exchangers for different ammonia fractions in Kalina Cycle Components % of ammonia mass fraction Cost, US$ Vaporizer 83, , , Condenser 130, , , HT recuperator 24, , , LT recuperator 44, , , Total 283, , , The cost for the other main components (turbine and pump) has been estimated. Considering in these cases, the power capacity of each component: (13)

11 where the base cost, C o, for the pump is 1120 US$/kW, and for the turbine 4405 US$/kW. The exponents 0.8 and 0.7 were used for size estimation of the pump and turbine. Estimated values of purchased equipment cost for the turbine and the pump are listed in Table 4. Table 4: Capacities and costs of the turbine and pump for different ammonia fractions in Kalina Cycle % of ammonia mass fraction Components Capacity (kw) (14) Cost, US$ Capacity (kw) Cost, US$ Capacity (kw) Cost, US$ Turbine , , , Pump , , , So, in figure 9, the increase of the total costs of the equipment was plotted with the power output of the turbine and the ammonia-water solution of the working fluid. Just taking to account the capital cost of equipment of the cycle, was obtain a rate of US$/kW of power output of the plant as shown in figure 10. Figure 9. Increase of the total purchase costs of equipment with the power output for different ammonia-water solution of the working fluid. Figure 10. Variation of the costs of a kw with the total purchase cost of equipment for different ammonia-water solution of the working fluid

12 Conclusions The present study analyze the Kalina cycle from the point of view of the first and second law of thermodynamic and the costs of the kw produced, taking to account the equipment costs of the system. Considering the key parameters, which affect the cycle as a whole, and they were identified for this analyze as: the ammonia-water composition, the evaporation/condensation pressures and the temperature of the heat source, identifying the operation point where the plant provides higher power output with the best efficiency. 1. The maximum performance of the cycle, for the analyzed conditions, is reached with 84% of ammonia mass fraction. 2. When increasing the percent of ammonia mass fraction in the composition of the working fluid, the mass flow rate decrease and increases the percentage of mass that can be evaporated. It impact in lower size of heat exchangers and higher power output of the cycle for this composition of the mixture. 3. The lower value of 888 US$/kW of the investment was obtained at 84% of ammonia mass fraction and a power output of kw. Acknowledgments The authors want to thanks the Coordination of Improvement of Higher Education (CAPES), The National Council of Technological and Scientific Development (CNPq) and The Foundation for Research Support of Minas Gerais State (FAPEMIG) for their collaboration and financial support in the development of the research work. Also want to thanks AES Tietê Company for funding the Project: Technological Alternatives for the Implantation of Hybrid Geothermal Energy in Brazil from Low-Temperature Sources. References [1] I. Kalina, Combined cycle system with novel bottoming cycle. ASME Journal of Engineering for Gas Turbine and Power 106 (1984) 737 e 742A. [2] H. Leibowitz, H. Micak, Design of 2 MW cycle binary module for installation in Husavik, Iceland, Geothermal Resourses Council Transaction 23 (1999) [3] E. Olsson, U. Desideri, S. S. Stecco, G. Svedberg, An Integrated Gas Turbine Kaline Cycle for Cogeneration. ASME Paper 91-GT-202, [4] E. Thorin, Power cycles with ammonia-water mixture as working fluid. Analysis of different applications and the Influence of thermophysical properties. Doctoral thesis. Department of Chemical Engineering and Technology. Energy Processes. Royal Institute of Technology. Stockholm. Sweden. (2000). [5] D. A. Jones, A Study of the Kalina Cycle System 11 for the Recovery of Industrial Waste Heat with Heat Pump Augmentation. MSc Thesis. Faculty of Auburn University. Alabama, [6] P. Bombarda, C. M. Invernizzi, C. Pietra, Heat recovery from Diesel engines: A thermodynamic comparison between Kalina and ORC cycles. Applied Thermal Engineering 30 (2010) [7] B. Ziegler, C. H. Trepp, Equation of State for Ammonia-Water mixtures. Refrig., 7 (1984) [8] M. Barhoumi, A. Snoussi, E. N. Ben, K. Mejbri, A Bellagi, Modeling of the Thermodynamic properties of the Ammonia/Water mixture. Int. J. Refrig., 27 (2004) [9] R. Senthil, P. M. V. Murugan, Subbarao, Thermodinamic Analysis of Rankine-Kalina Combined Cycle. Int. J. Thermodynamic. 11 (2008)

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