Kalina Cycle para Energia Geotérmica no Quênia - Design e Otimização

GRC Transactions Vol 38 2014 791 Design and Optimization of Kalina Cycle for Geothermal Energy in Kenya Wencheng Fu School of Electrical Engineering Tianjin University of Technology Tianjin China fuwchtjueducn Keywords Kalina cycle geotherma power plant ammoniawater cycle efficiency exergy AbSTrACT The Great Rift Valley an area of Eastern Africa with strong tectonic activity offers immense potential for largescale geother mal projects The geothermal energy is also considered clean and renewable so this paper presents the exploitation of geothermal energy with Kalina cycle in Kenya The software Engineering Equation Solver EES is used to run the models for each op erating condition using the thermodynamic properties data of ammonia and water supplied with that software package Based on the good agreements with the actual operating parameters the thermodynamics analysis of Kalina geothermal power cycle was analyzed The optimum of the system is influenced by the condensation temperature ammonia mass fraction turbine inlet pressure and the temperature of heat source The cycle efficiency and the electricity generation for the Kalina cycle are illustrated with different conditions The ammonia content and the pressure of turbine is needed to be less than the optimum point The largest cycle efficiency is found 20 but the pressure is so high that the cost of components should be considered Thermodynamic analysis of an operational 1 MWe binary geothermal power plant in Kenya is performed Through energy and exergy the energy efficiency is about 69 and the largest exergy destruction occurs at the condenser The utilization of the lowtemperature energy will increase efficiency and reduce the consumption of the fossil fuels 1 Introduction Geothermal energy is abundant in Kenya of which the East African Rift may provide great geothermal resources about 7000MW1 Geothermal energy is the cleanest energy and is independent of the weather As a source of renewable energy geothermal energy is considered to be stable cheap and clean which has sparked more and more attention by countries all over the world Using it to generate electricity not only can effectively alleviate the pressure of electricity shortage but also reduce the emission of the carbon dioxide Due to the great geothermal resources in Kenya development of renewable energy is pro moted to achieve sustainability and is used to meet the electricity requirement The economic development is growing rapidly in Kenya but the problem between supply and demand of power electricity should be faced 2 The urbanization process increases the demand on energy resources Electricity shortages made the government take a series of intervention steps in Kenya The method makes the economy slows down Due to the shortage of reliable energy the annual revenue of companies in Kenya reduced by 7 the economic growth rate decreased by 153 The temperature of brine geothermal fluid below 150belongs to mediumlow temperature geothermal resource Organic Rankine Cycle ORC and Kalina cycle are among the most feasible ways of using lowtemperature sources The ORC has been developed for a long time and a pure working fluid is usually selected according to the heat source temperature 4 5 As a new kind of proposed cycle the Kalina cycle uses ammoniawater mixtures as working medium 6 7 just with the aim of reducing the thermal irrevers ibility in the heat conduction process especially between the heat source and the evaporating work fluids The Kalina cycle is higher in efficiency and also have more advantages than Rankine cycle by ElSayed 8 Kalina 9 and Thorolfsson10 The technology and economy research on Kalina cycle reveal the essence and direction of improvement by Lv 1112 and Zhang13 who analyzed the key parameters which influence the performance of Kalina cycle According to the engineering practice of gassteam combined cycle and the highest temperature the Kalina cycle can reach is 300 The advantage of the Kalina cycle is to use the middlelow geothermal resources which are abundant in Kenya The objec tive of this study is to evaluate the Kalina cycle which is fit for geothermal power generation in Kenya The thermodynamic cycle according to the actual power plant operation is analyzed This paper can also provide theoretical basis for Kalina system design and useful estimate for optimal operation 792 Fu 2 System Description The Kalina cycle uses a mixture of ammonia and water the evaporation and condensation will occur at variable temperatures the heat transfer process is very similar to the heat sources Fig 1 shows the schematic diagram of Kalina cycle which is suited to the environment conditions in Kenya The basic solution is heated to a high temperature by the energy provided from the brine geothermal fluid and partially vaporized in the evapora tor state 1 Then the twophase ammoniawater is separated to a saturated vapor state 2 and a saturated liquid solution state 4 through the separator The vapor in the turbine which contains most ammonia is expanded to a low pressure state 3 to produce power The weak ammoniawater solution from the separator is cooled to a low temperature state 5 through a high temperature recuperator and then decreased the pressure closed to the exhausted steam state 6 through a throttle valve After that the mixed fluid state 7 from the valve and the turbine is cooled down after the low temperature recuperator state 8 After the condenser the basic solution state 9 is pumped to a high pressure state 10 by the feed pump It is then heated by low state 11 and high state 12 temperature recuperator in turn Lastly the working medium enters the generator 3 Assumptions and basic Parameters The state properties of all processes in the Kalina cycle can be determined when some variables and assumptions are confirmed In order to get the solution more quickly the complex actual process should be simplified The assumptions used in the Kalina cycle are as follows a The resistances of pressure and heat along the piping are neglected b The fluid expansion in the throttling valve is considered as isenthalpic c The geofluid is in a liquid condition in the reservoir d The system operates at the stable state e The ammoniawater temperature at the condenser outlet can be determined by condensing temperature 14 In order to make the results more comparable some other necessary parameters 15 that are needed to determine in the operation are shown in table 1 Table 1 Parameters of the basic model Parameters Value Inlet temperature of geothermal fluid 122 Outlet temperature of geothermal fluid 80 Mass flow of geothermal fluid kgs 89 Inlet temperature of cooling water 5 Mass fraction of ammonia in the mixture 82 Pressure of the turbine inlet bar 323 Isentropic efficiency of turbine 87 Generator efficiency 96 Pump efficiency 98 Pressure losses after each device bar 1 Pinch point of evaporator 6 Pinch point of recuperator 5 Pinch point of condenser 3 4 Equation and Thermodynamic Simulation The components of the Kalina cycle are complex so the main equations and the main power plant components need to be discussed Condenser The condenser may be either water or air cooled The calculations for the condenser are roughly the same in both cases as the hot working fluid coming from the LT recupera tor The condensed fluid normally the cooling water enters the condenser to absorb the heat The condenser is nothing but a heat exchanger between the hot vapor and fluid from the recuperator and the cooling water from the cooling tower It has to be observed that the temperature of the hot fluid is higher than the one of the cold fluid throughout the condenser The equation is as followed mw h8 h9 Qcon 1 Recuperator The recuperator is a heat exchanger recovering the heat of the hot exit vapor from the turbine or the hot saturated liquid from separator The hot fluids from the seperator and turbine are on the hot side they will be condensed in the LT recuperator and in the condenser then pumped right away through the cold side of the HT recuperator towards the evaporator The fluid behavior is usually close to linear so it is normally not necessary to divide the regenerator into sections The equations are as follows HT Recuperator ml h4 h5 mw h11 h12 2 LT Recuperator ml h7 h8 mw h10 h11 3 Turbine The turbine converts a part of the vapor enthalpy to shaft work and then to electricity in the generator The ideal turbine is isentropic having no second law losses The turbine isentropic efficiency is given by the turbine manufacturer This efficiency is the ratio between the real enthalpy changes through the turbine to the largest possible isentropic enthalpy change The work output of the turbine is then the real enthalpy change Figure 1 Basic model of Kalina power cycle 793 Fu multiplied by the working fluid mass flow through the turbine The equation is as follows mg h2 h3 ηg Wtur 4 Evaporator The evaporator is the first component of a Kalina power plant The geothermal fluid is pumped to the evaporator and then injected into ground Obviously the heat removed from the source fluid has to equal the heat added to the working fluid The evaporator is nothing but a heat exchanger between the hot source fluid and the cold working fluid of the cycle As well it must be kept in mind that the relation between the power plant cycles and components design is the field enthalpy or energy content of the fluid The equation is as followed Qgeo mw h1 h12 5 Separator Mass balance holds over the separator the sum of steam and saturated liquid mass flow equals the mass flow of the mixture in the cycle The steam fraction is then defined by the energy balance over the separator The separator is working in the thermodynamic wet area containing a mixture of steam and water in equilibrium All temperatures in the separator will thus be equal assuming that there are no significant pressure losses or pressure differences within the separator The equation is as follows m1x1 m2x2 m4x4 6 Where m is the mass flow kgs h is the specific enthalpy kJ kg W is the power kJ and Q is the quantity of heat kJ x is the mass fraction of ammonia ηg is the generator efficiency Turbine isentropic efficiency ηturb h2 h3 h2 h3s 7 Where h3s is the enthalpy after isentropic expansion in the turbine kJkg Thermal efficiency ηK m2 h2 h3 ηg Wpump Qgeo 8 Wpump is the power consumption of working fluid pumpKW Qgeo is the thermal power of geothermal water released KW The specific physical exergy of geothermal fluid at any state can be calculated from 0 0 0 s T s h h ex 9 m 0 0 0 S T S H H e E x x 10 The exergy destruction will be calculated for each element of the Kalina cycle system It will be recognized which element of the system causes most losses By optimizing the exergy ef ficiency the losses should be minimized The exergy destruction is defined as following xout xin d E E I 11 The formulas consist of a large series of nonlinear equa tions and the thermal properties of ammoniawater are the main difficulties in calculation Nonlinear equations can be solved by programming through the software Engineering Equation Solver EES which can easily obtain the ammoniawater correlations The main idea of EES is simultaneous modular approach and the solving process is shown in figure 2 5 results and Discussions The calculated thermodynamic parameters of each state has a good agreement with Ref15 The cycle efficiency is affected by many factors such as condensation temperature turbine inlet pressure the efficiency of heat exchanger and the mass fraction of ammonia etc On this basis of validated results some key param eters are analyzed in this paper For the design and operation of the Kalina cycle the optimum is influenced by the condensation temperature ammonia mass fraction turbine inlet pressure and the temperature of heat source Fig 3 shows that the mass flow rate of vapor produced in the separator has a peak value as the inlet pressure increases the Figure 2 Structure of the computer code 10 20 30 40 50 60 70 80 90 100 8 10 12 14 16 08 NH3 mass flow rate of vapor kgs 100 115 130 145 Figure 3 Mass flow rate of vapor against turbine inlet pressure 794 Fu peak value increase in both mass flow rate and pressure at higher temperature The power output of the turbine is shown in Fig 4 The trend of the power output is the same as Fig3 The mass flow rate and work output have the positive correlation with the temperature of geothermal fluid in Fig 3 and Fig 4 Furthermore there is an optimum in the Kalina cycle between generated electricity and power consumption for the working pump The pump consumption increases with increasing turbine inlet pressure as shown in Fig5 The enthalpy difference of the evaporator is decreased with the pressure because of the constant evaporation temperature Hence the mass flow of the circulating basic solution rises exponentially An increase in mass flow leads to an increase in power consumption of the feed pump The relation is the exponential function so the pressure should be kept at a lower level When the temperature of the heat source is relatively high the power consumption increases slowly The cycle efficiency versus the pressure within the pres sure range under various temperatures is shown in Fig6 The peak value increases with the temperature of the heat source because the heat sources influence the cycle efficiency The cycle efficiency has a peak point The peak points are formed because of two reasons Firstly the mass flow rate of vapor appears the best pressure at different temperature The mass flow decreases so the power generation of turbine reduces Secondly the power consumption of working pump increases exponentially the net power will be reduced at high pressure The mass fraction of ammonia also has directly influenced relationship with the mass flow rate of vapor which will further affect the turbine work output Power consumption of working fluid pump and cycle efficiency are calculated in different turbine inlet pressure when ammonia fraction is 055 065 075 085 and 095 The higher ammonia mass fraction in the mixture will increase the maximum generated electricity and increase the cost of the plant because of increasing the irreversible losses The power consumption increases with the turbine inlet pressure at every fraction Fig 7 and the theoretical efficiency can reach about 20 Fig 8 when the fluid temperature is 120 turbine inlet pressure bar 10 20 30 40 50 60 70 80 90 100 500 1000 1500 2000 2500 3000 3500 4000 08 NH3 turbine work output KW turbine inlet pressure bar 100 115 130 145 10 20 30 40 50 60 70 80 90 100 0 500 1000 1500 2000 2500 08 NH3 pump consumption of working pump KW turbine inlet pressure 100 115 130 145 10 20 30 40 50 60 70 80 90 100 004 006 008 010 012 014 016 018 020 022 08 NH3 cycle efficiency turbine inlet pressure 100 115 130 145 20 40 60 80 0 500 1000 1500 2000 120 power consumption of working pump KW turbine inlet pressure bar 055 NH3 065 NH3 075 NH3 085 NH3 095 NH3 Figure 6 Cycle efficiency against turbine inlet pressure Figure 5 Power consumption of working fluid pump against turbine inlet pressure Figure 4 Electricity generation of the turbine against turbine inlet pres sure Figure 7 Power consumption of working fluid pump against turbine inlet pressure 795 Fu Also the condenser pressure has an influence on the generated electricity The condenser pressure can be dropped down with low cooling water temperatures It is known that the temperature of cooling water is changed during the year so it is not practical to change the ammoniawater mixture for different cooling water temperatures During the operation of the plant the mixture ratio can be used in a specific range for optimization The range is shown in Fig9 The solid lines indicate the stable field which is from 45Bar to 57 Bar but the range is only appropriate for ammonia content of 082 The pressure level and ammonia content give the designer additional flexibility in the design of the cycle The exergy rates and the exergy destruction of each part of the system is calculated for one representative unit in Table 3 and Table 4 The boundary conditions are the same except the condensation temperature so the temperature of cooling water is assumed to be 37 This is because the average temperature is too high all the year in Kenya The parameters for the representative unit are listed in Table 2 The temperature of the dead state is 27 It is necessary to point out that the condenser is different with other components The condensing heat of cooling water can not be used repeatedly and discharge to the environment The exergy destruction of condenser is equal to the sum of the condensing heat loss and heat transfer loss Table 2 Parameters for the representative unit State numbers refer to Fig 1 State No Temperature t C Pressure P bar Ammonia mass fraction x Vapor content 1 1162 323 082 0671 2 1162 323 09718 1 3 782 1488 09718 09652 4 1162 323 05104 0 5 67 313 05104 0 6 672 1488 05104 0 7 768 1488 082 06389 8 658 1388 082 0586 9 402 1288 082 0 10 404 353 082 0 11 62 343 082 0 12 774 333 082 0 13 122 14 80 Table 3 Exergy rates and other properties for one representative unit State numbers refer to Fig 1 State No Fluid Enthalpy h kJkg Entropy s kJkgC Mass Flow m kgs Specific Exergy e kJkgC Exergy rate E kW 0 geofluid 1132 03949 0 0 ammoniawater 1031 4413 0 1 ammoniawater 1093 3438 1853 3545 6568885 2 ammoniawater 1481 441 1244 4509 5609196 3 ammoniawater 1378 4454 1244 3347 4163668 4 ammoniawater 2998 1454 6097 1565 9541805 5 ammoniawater 637 08067 6097 11459 69865523 6 ammoniawater 637 08128 6097 11276 68749772 7 ammoniawater 9457 3257 1853 2615 4845595 8 ammoniawater 8407 2972 1853 242 448426 9 ammoniawater 5949 05653 1853 1828 3387284 10 ammoniawater 6283 05653 1853 18614 34491742 11 ammoniawater 1679 08898 1853 19386 35922258 12 ammoniawater 2455 1117 1853 2033 3767149 13 geofluid 5123 1549 89 1656751 14745084 14 geofluid 3351 1075 89 1306751 11630084 Table 4 Exergy destruction for the representative unit Components Exergy Destruction kW Heat transfer or power kW Evaporator 313264 1570418 Working Pump 618902 618902 Condenser 1096976 1447582 TurbineGenerator 312528 128132 HT recuperator 8060207 1439502 LT recuperator 2182834 194565 Separator 55085 0 Electricity Generation and Thermal Efficiency Wnet 1081 kW ηκ 00688 20 40 60 80 000 004 008 012 016 020 120 cycle efficiency turbine inlet pressure bar 055 NH3 065 NH3 075 NH3 085 NH3 095 NH3 10 20 30 40 50 60 000 002 004 006 008 010 012 014 016 018 Cooling water turbine inlet pressure bar cycle efficiency 10 15 20 25 30 Figure 8 Cycle efficiency against turbine inlet pressure with different ammonia content Figure 9 Cycle efficiency against turbine inlet pressure with different condensation temperature 796 Fu The parameters of the representative unit are calculated completely Because the condensation temperature is too high in Kenya the thermal efficiency is only 69 The exergy destruction of condenser is the most this is because the condensation energy could not be recycled The vapor could not be absorbed by the liquid the gasliquid twophase flow are entered in the LT recu perator so the exergy destruction of LT recuperator is not high The working pump is driven by the electric energy and the exergy efficiency of electric energy is 100 so the exergy destruction of the working pump is the same with the power consumption In the design of Kalina cycle many factors should be taken into consideration for example 10 or more liquid content will have an effect on the safe operation of turbine 16 6 Conclusions Kalina cycle for heat recovery applications of lowmedium temperature and high condensation temperature were investigated A simulation program on the basic physical properties of ammo niawater was completed and verified effectively in the literature The efficiency can reach 20 when the geothermal fluid temperature is 120 but the pressure is too high The costs and stability should be considered For the Kalina cycle in operation the high pressure will increase the costs of the components so the pressure of turbine would need to be less This parameter could avoid a complete loss in the thermal efficiency of the cycle do to a mixing problem or leak When given heat source temperature the maximum generated energy can be found the mass fraction of ammonia in the binary mixture turbine inlet pressure and the temperature of cooling water can be used in a specific range for optimization The temperature of the tailing geothermal fluid is assumed to be 80 because the energy can be used further such as drying or heating Besides the condensation energy is not used efficiently The Kalina cycle process becomes an interesting option for power plants to use heat at low temperatures conversion In addition this process contributes to increasing efficiency reducing the consumption of the fossil fuels and protecting resources It ac cords with the new energy policy so as to cut tax The electricity generation first fulfills system requirements and the remainder is merged into the grid The economic benefit is high and the payoff period is short Acknowledgement The authors gratefully acknowledge financial support provided by National High Technology Research and Development Program of China 863 Program No SQ2011AAJY3014 references 1 Silas M Simiyu G Randy Keller 2000 Seismic monitoring of the Olkaria Geothermal area Kenya Rift valley Journal of Volcanology and Geothermal Research 95 197208 2 Pacifica F Achieng Ogola Brynhildur Davidsdottir Ingvar Birgir Fridleifsson 2011 Lighting villages at the end of the line with geo thermal energy in eastern Baringo lowlands Kenya Steps towards reaching the millennium development goals MDGs Renewable and Sustainable Energy Reviews 3 JK Kiplaga RZ Wang 2011 Renewable energy in Kenya Resource potential and status of exploitation Renewable and Sustainable Energy Reviews 15 29602973 4 BM Burnside 1976 The immiscible liquid binary Rankine cycle Journal of Mechanical Engineering Science 18 2 7986 5 G Angelino and P Colonna 1998 Multicomponent working fluids for Organic Rankine Cycles ORCs Energy 23 6 449463 6 Kalina AI 1984 Combined cycle system with novel bottoming cycle Eng Gas Turbines Power 106 73742 7 Kalina AI and Leibowitz HM 1989 Application of the Kalina cycle technology to geothermal power generation Geothermal Resources Council Transactions 13 60511 8 ElSayed and YM Tribus M 1985 A theoretical comparison of the Rankine and Kalina cycles ASME Special Publications AES1 97102 9 Kalex Kalina Cycle Power Systems For Use as a Bottoming Cycle for Combined Cycle Applications httpwwwkalexsystemscomKalex 20Bottoming20Cycle20Brochure201010pdf 10 Thorolfsson G 2002 Optimization of low temperature heat utilization for production of electricity MSc thesis University of Iceland Dept of Mechanical Engineering 11 Lv Canren Yan Jinyue Ma Yitai 1991 The research and development of Kalina cycle and an analysis of its efficiency enhacement potentiality Power Engineering 61 17 12 Lu Ling Yan Jinyue and Ma Yitai 1989 Thermodynamic analysis of heatreleasing process of Kalina cycle Journal of Engineering Ther mophysics 103 249251 13 Zhang Ying He Maogang and Jia Zhen 2005 First Law Based Thermodynamic Analysis of Kalina Cycle Journal of Power Engineer ing 252 708712 14 Yuan Weixing Wang Hai Hua Yanhong 2008 Study on current simulation method of ammoniawater absorption heat pump system Acta Energiae Solaris Sinica 29 4 454458 15 Sirko Ogriseck 2009 Integration of Kalina cycle in a combined heat and power plant a case study Applied Thermal Engineering 29 28432848 16 HD Baehr and S Kabelac 2006 Thermodynamic SpringerVerlag GmbH Berlin