Exergy Analysis of Kalina Cycle System (KCS) 34 with Mass Fraction Ammonia–Water Mixture Variation
Nasruddin1, Maulana Rifaldi1, Agus Noor1, Rama Usvika2
1Department of Mechanical Engineering, Engineering Faculty, University of Indonesia
2PT. Rekayasa Industri, Jakarta, Indonesia
In order to solve the problem of primary energy consumption in the energy systems and to reduce environmental pollution, new thermodynamic cycles have been investigated and developed during the past 20 years .
Kalina Cycle® System 34 (KCS 34) was designed by Dr. Alexander Kalina specifically for generating electricity using low to medium temperature geothermal resources as a “topping” energy conversion prior to district heating. A flow diagram of KCS 34 is shown in Fig. 1. Basic flow parameters are shown on this diagram.
For applying a novel cycle a comprehensive view in thermodynamic is important, thermodynamic system is influence by surrounding environment such as pressure and temperature. This research is made for a study in Kalina cycle system that has been applied in Husavic, Iceland and the final conclusion is a validation model and the appropriate condition of working fluid composition that have improved efficiencies of thermal power production
The diagram below shows the Kalina Cycle for geothermal power plant, the hot water or brine water enters the evaporator and the heat is transferred to ammonia-water. And the ammonia water goes into turbine and the turbine generates the generator to produce electricity .
2.2.Exergy analysis method”]
The thermomechanical exergy associated with the state of a system is measured by the maximum reversible work that could be delivered by the system as it proceeds toward a state of equilibrium with the environment while any heat transfer occurs solely with the environment. This state of equilibrium is its dead state at T0 and P0, and the standard atmosphere or environment is recommended to be 298.15 K and 1.01325 bar (1 atm) 
under the same circumstances the irreversibility is
The total exergy of the system becomes the summation op physical exergy and chemical exergy
The physical exergy component calculated using the following relation:
The calculation procedure for the chemical exergy of various substances based on the standard chemical exergy values of respective species is widely discusses by Bejan et al., Ahrendts, and Szargut et al. for the ammonia-water fluids to be considered, the chemical exergy of the flow is calculated using the following relation:
Which the standard chemical exergy of ammonia and water, respectively, and their values are taken from ahrendts
First and second-law efficiencies are different in one other important respect. The first law is a conservation principle. On the other hand, entropy and exergy from a second-law viewpoint are non conserved properties. In the presence of irreversibilities, entropy is produced and exergy is destroyed. The former effect is measured by the entropy production , and the latter effect is measured by the irreversibility . Hence second-law efficiencies measure losses in exergy during a process. A general definition of a second-law effectiveness is
Total produced electric/mechanical power is Pgenerator or power from generator, Total own consumption is amount of energy use to run the plant auxiliaries, such as: the pump or total Ppump and Total energy input is total heat absorbed to evaporator.
2.1.Simulation and description
The schemes of Kalina cycle were built using cycle-tempo5.0, figure 3. For the input data were the factual data from Húsavík. No. 1 and 2 for the hot brine supply trough evaporator as a boiler transferring the heat to ammonia-water, ammonia-water is the pipe in close system and no. 9 and 11 for cooling water.
The input and parameters were made similar to the condition in Húsavík.
1. Results and discussion
3.1. Model validation
The cycle validation in calculating the data as the result shall similar or equal to factual output in the real Kalina power plant, the thermodynamic model was developed using Cycle Tempo in order to validate the reference took from geothermal power plant in husavic, Iceland.
From all references that show at table, Cycle-Tempo has calculated all input data. The result of generated electric power or gross electric power has a close accurate power which is has error around 0.46% or has difference power about 0.09 kW from the references. Therefore, the scheme and all input for each apparatus in the scheme could be concluded that could be use for further analysis.
3.1. Exergy Flow
The exergy calculation result below, is the Kalina cycle system model, see fig. 3. The exergy flow in the system influenced by environment, apparatus efficiencies, etc.
The Grassman diagram above show the exergy flow in the KCS 34 in the optimum condition. Apparatus denoted in Rome number, number I is Evaporator 1, number II is separator, number III is a turbine, number IV is generator, number V is LT Recuperator, number VI is drain tank, number VII is condenser, number VIII is a pump, number IX is a HT recuperator, all the unit is in kW. The losses in heat exchanging equipment such as apparatus number I, II, V, VII and IX are caused by pressure drop and other irreversibilities occurred. Turbine has the highest losses is because the turbine has a mechanical efficiency to the shaft rotation and manufacture.
By this condition the Kalina cycle would be analyze to get optimal performance by configuring the ammonia-water mass fraction and also by choosing a high efficiency apparatuses.
3.1. KCS 34 application in Indonesia
For applying KCS type 34 in Indonesia, there is a different condition to environment such as pressure and temperature for the dead state condition.
Geothermal site in Iceland has a different environment and climatic condition with Indonesia. In Indonesia, geothermal resources in normally located in the mountainous area, several hundred or thousands metres above sea level. However, the climatic condition in Indonesia is not as cold as Iceland. Typical air temperature at Indonesian Geothermal Site is around 18-24 oC. The cooling water source is normally taken from nearby river, with river water having average temperature around 22-24 oC. Meanwhile the cooling water supply temperature in Iceland is about 5oC. Hence, some thermodynamic parameters should be change in order to obtain the maximum electric power and efficiency using KCS-34.
Kalina cycle system 34 was then modified in several parameters to find the optimum power output. To obtain the optimum power output, ammonia-water composition is considered as the most influenting parameters, so it will be varied starting from 78% up to 85.5% mass fraction, and each mass fraction act as constraint, therefore the outlet pressure from turbine has lower and upper limit, here is the result :
The highest electric power produced can be achieved at 78% ammonia-water mass fraction and 7.4 bar outlet pressure, which can produced around 2.145 MW. Refer to fig.5, by increasing the electric power output and lowering the outlet pressure, the efficiency will increase and so does the exergy efficiency.
From each mass fraction, the graph below show us that increasing the outlet pressure from turbine will decreasing exergy efficiency because of the exergy absorb by the system from heat source is constant meanwhile the increasing of outlet pressure will lead to decreasing electrical power as useful work.
The net exergy efficiency:
For mass fraction 85.5% the highest exergy efficiency ηII = 69.00016%, For mass fraction 84% the highest exergy efficiency ηII = 68.62393%, For mass fraction 81% the highest exergy efficiency ηII = 67.06067%, For mass fraction 78% the highest exergy efficiency ηII = 66.88385%, and in a series the pressure are 8.3 bar ; 8.1 bar ; 7.8 bar ; and 7.4 bar, those are bottom line each mass fraction respectively.
This analysis refer to study Kalina cycle system type 34 application in Indonesia, the important things are how to choose the mass fraction and outlet pressure from turbine that maximize electric power output and also system efficiency.
- KCS-34 simulation model using Cycle Tempo has been proved to be a validate model for real installation in Husavic, Iceland. The model has produced calculated electrical output of 1.959 MW, which is very close to real condition in Husavic Site which currently produces electrical power output of 1.950 MW.
- If the outlet pressure from turbine is constant, by increasing the mass fraction of ammonia, the electrical power output will increase and so does the system efficiency. If the mass fraction is constant, by decreasing exit pressure from turbine, it will increase the electrical power output and the system efficiency.
- The highest electrical power output for KCS-34, if it is applied in Indonesia, shall be achievable using 78% mass fraction of ammonia-water and 7.4 bar exit pressure from turbine.
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