Design and Optimization of Kalina Cycle for Geothermal Energy in Kenya

GRC Transactions, Vol. 38, 2014 Design and Optimization of Kalina Cycle for Geothermal Energy in Kenya Wencheng Fu School of Electrical Engineering, Tianjin University of Technology, Tianjin, China fuwch@tju.edu.cn Keywords Kalina cycle, geotherma power plant, ammonia-water, cycle efficiency, exergy Abstract The Great Rift Valley, an area of Eastern Africa with strong tectonic activity, offers immense potential for large-scale geothermal 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 operating 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 6.9% and the largest exergy destruction occurs at the condenser. The utilization of the low-temperature 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 7000MW[1]. 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 promoted 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 1.5%[3]. The temperature of brine geothermal fluid below 150 belongs to medium-low temperature geothermal resource. Organic Rankine Cycle (ORC) and Kalina cycle are among the most feasible ways of using low-temperature 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 ammonia-water mixtures as working medium [6, 7], just with the aim of reducing the thermal irreversibility 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 El-Sayed [8], Kalina [9] and Thorolfsson[10]. The technology and economy research on Kalina cycle reveal the essence and direction of improvement by Lv [11,12] and Zhang[13], who analyzed the key parameters which influence the performance of Kalina cycle. According to the engineering practice of gas-steam combined cycle, and the highest temperature the Kalina cycle can reach is 300. The advantage of the Kalina cycle is to use the middle-low geothermal resources which are abundant in Kenya. The objective 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. 791

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 evaporator (state (1)). Then the two-phase ammonia-water 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 ammonia-water 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.. 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 /kg/s 89 Inlet temperature of cooling water / 5 Mass fraction of ammonia in the mixture /% 82 Pressure of the turbine inlet /bar 32.3 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 recuperator. 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: m w ( h 8 h 9 ) = Q con (1) Figure 1. Basic model of Kalina power cycle 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 ammonia-water temperature at the condenser outlet can be determined by condensing temperature [14]. 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: m l ( h 4 h 5 ) = m w ( h 11 h 12 ) (2) LT Recuperator: m l ( h 7 h 8 ) = m w ( h 10 h 11 ) (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 792

multiplied by the working fluid mass flow through the turbine. The equation is as follows: m g ( h 2 h 3 ) η g = W tur (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: Q geo = m w ( h 1 h 12 ) (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: m 1 x 1 = m 2 x 2 + m 4 x 4 (6) Where m is the mass flow (kg/s); 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, η g is the generator efficiency. Turbine isentropic efficiency: η turb = h 2 h 3 h 2 h 3s (7) Where h 3s is the enthalpy after isentropic expansion in the turbine (kj/kg). Thermal efficiency: η K = m 2 ( h 2 h 3 ) η g W pump Q geo (8) W pump is the power consumption of working fluid pump(kw), Q geo is the thermal power of geothermal water released (KW). The specific physical exergy of geothermal fluid at any state can be calculated from: e x E x = h h ) T ( s ) (9) ( 0 0 s0 = e = ( H H ) T ( S ) (10) m x 0 0 S0 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 efficiency, the losses should be minimized. The exergy destruction is defined as following: I E E (11) d xin = xout The formulas consist of a large series of nonlinear equations and the thermal properties of ammonia-water are the main difficulties in calculation. Nonlinear equations can be solved by programming through the software Engineering Equation Solver Figure 2. Structure of the computer code. (EES) which can easily obtain the ammonia-water 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 Ref.[15]. 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 parameters 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 mass flow rate of vapor /(kg/s) 16 14 12 10 8 100 115 130 145 Figure 3. Mass flow rate of vapor against turbine inlet pressure. 793

turbine work output /(KW) 4000 3500 3000 2500 2000 1500 1000 500 turbine inlet pressure /(bar) 100 115 130 145 turbine inlet pressure /(bar) Figure 4. Electricity generation of the turbine against turbine inlet pressure. power consumption of working pump /KW 2000 1500 1000 500 0 120 0.55 NH 3 0.65 NH 3 0.75 NH 3 0.85 NH 3 0.95 NH 3 20 40 60 80 turbine inlet pressure /bar Figure 7. Power consumption of working fluid pump against turbine inlet pressure. pump consumption of working pump /(KW) cycle efficiency 2500 2000 1500 1000 500 0 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 100 115 130 145 100 115 130 145 turbine inlet pressure Figure 5. Power consumption of working fluid pump against turbine inlet pressure. turbine inlet pressure Figure 6. Cycle efficiency against turbine inlet pressure. 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 Fig.3. 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 Fig.5. 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 pressure range under various temperatures is shown in Fig.6. 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 0.55, 0.65, 0.75, 0.85 and 0.95. 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. 794

cycle efficiency 0.00 20 40 60 80 turbine inlet pressure /bar Figure 8. Cycle efficiency against turbine inlet pressure with different ammonia content. cycle efficiency 0.20 0.16 0.12 0.08 0.04 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.55 NH 3 0.65 NH 3 0.75 NH 3 0.85 NH 3 0.95 NH 3 120 Cooling water 10 15 20 25 30 10 20 30 40 50 60 turbine inlet pressure /bar Figure 9. Cycle efficiency against turbine inlet pressure with different condensation temperature. 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 ammonia water 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 Fig.9. The solid lines indicate the stable field which is from 45Bar to 57 Bar, but the range is only appropriate for ammonia content of 0.82. 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 Temperature Pressure Ammonia mass No t ( C) P (Bar) fraction x Vapor content 1 116.2 32.3 0.82 0.671 2 116.2 32.3 0.9718 1 3 78.2 14.88 0.9718 0.9652 4 116.2 32.3 0.5104 0 5 67 31.3 0.5104 0 6 67.2 14.88 0.5104 0 7 76.8 14.88 0.82 0.6389 8 65.8 13.88 0.82 0.586 9 40.2 12.88 0.82 0 10 40.4 35.3 0.82 0 11 62 34.3 0.82 0 12 77.4 33.3 0.82 0 13 122 14 80 Table 3. Exergy rates and other properties for one representative unit. State numbers refer to Fig. 1. Enthalpy, h (kj/kg) Entropy, s (kj/kg C) Mass Flow m (kg/s) Specific Exergy e (kj/kg C) Exergy Rate E (kw) State No Fluid 0 geofluid 113.2 0.3949 0 0 ammonia-water 1031 4.413 0 1 ammonia-water 1093 3.438 18.53 354.5 6568.885 2 ammonia-water 1481 4.41 12.44 450.9 5609.196 3 ammonia-water 1378 4.454 12.44 334.7 4163.668 4 ammonia-water 299.8 1.454 6.097 156.5 954.1805 5 ammonia-water 63.7 0.8067 6.097 114.59 698.65523 6 ammonia-water 63.7 0.8128 6.097 112.76 687.49772 7 ammonia-water 945.7 3.257 18.53 261.5 4845.595 8 ammonia-water 840.7 2.972 18.53 242 4484.26 9 ammonia-water 59.49 0.5653 18.53 182.8 3387.284 10 ammonia-water 62.83 0.5653 18.53 186.14 3449.1742 11 ammonia-water 167.9 0.8898 18.53 193.86 3592.2258 12 ammonia-water 245.5 1.117 18.53 203.3 3767.149 13 geofluid 512.3 1.549 89 165.6751 14745.084 14 geofluid 335.1 1.075 89 130.6751 11630.084 Table 4. Exergy destruction for the representative unit. Components Exergy Destruction (kw) Heat transfer or power (kw) Evaporator 313.264 15704.18 Working Pump 61.8902 61.8902 Condenser 1096.976 14475.82 Turbine/Generator 312.528 1281.32 HT recuperator 80.60207 1439.502 LT recuperator 218.2834 1945.65 Separator 5.5085 0 Electricity Generation and Thermal Efficiency Wnet 1081 kw η κ 0.0688 795

The parameters of the representative unit are calculated completely. Because the condensation temperature is too high in Kenya, the thermal efficiency is only 6.9%. 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 gas-liquid two-phase flow are entered in the LT recuperator, 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 low-medium temperature and high condensation temperature were investigated. A simulation program on the basic physical properties of ammonia-water 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 accords 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, 197 208. [2] Pacifica F. Achieng Ogola, Brynhildur Davidsdottir., Ingvar Birgir Fridleifsson., 2011. 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