Journal of JET Vo'ume 7 (2014) p.p. 17-34 Issue 3, August 2014 Energy Technology www.fe.um.si/en/jet.html TURBINE SEALING STEAM HEAT RECOVERY WITH DYNAMIC STIRLING ENGINES IZRABA TOPLOTE TESNILNE PARE TURBINE Z DINAMIČNIMI STIRLING MOTORJI Dušan StrušnikR, Milan Marčič, Jurij Avsec Keywords: analysis, Stirling engine, heat, working gas, sealing steam, steam turbine, regenerator, efficiency, isochoric compression Abstract This paper presents the possibilities of sealing steam heat recovery in a steam condensation turbine with the use of dynamic Stirling engines. The installation of dynamic Stirling engines into the turbine sealing steam system allows the recovery of sealing steam heat and the generation of electrical energy. The Stirling engine dynamics are expressed with a built-in working gas storage tank and working gas flow control to ensure adequate power output of the Stirling engine. The working gas quantity in the engine is controlled with regard to the available turbine sealing steam heat. Via the measurement of the turbine sealing steam quantity and quality, a model of the working gas pressure conditions in the Stirling engine is designed. The engine responsiveness at various working gas quantities and types is analysed. Povzetek V članku bomo predstavili možnosti izrabe toplote tesnilne pare parne kondenzacijske turbine z dinamičnim Stirling motorji. Z vgraditvijo dinamičnih Stirling motorjev v armaturo tesnilne pare tur- R Corresponding author: Dušan Strušnik, Energetika Ljubljana d.o.o. enota TE-TOL, Toplarniška ulica 19, 1000 Ljubljana, Slovenija. E-mail address: dusan.strusnik@gmail.com. m JET 17 Dušan Strušnik, Milan Marčič, Jurij Avsec JET Vol. 7 (2014) Issue 3 bine bomo izkoriščali toploto tesnilne pare in pridobivali električno energijo. Dinamika Stirling motorja se izraža z vgrajenim hranilnikom in regulacijo količine delovnega plina, ki poskrbi za ustrezno moč Stirling motorjev. Količina delovnega plina v motorju se uravnava glede na razpoložljivi toploto tesnilne pare turbine. S pomočjo opravljenih meritev količine in kvalitete tesnilne pare turbine, bomo izdelali model tlačnih razmer delovnega plina v Stirling motorju. Analizirali bomo odzivnost motorja pri različnih količinah in vrstah delovnih plinov. 1 INTRODUCTION Sealing steam is steam-generated process during a steam turbine operation. Due to the leakage of high-pressure labyrinth seals, a constant sealing steam flow is created, used for the sealing of the low-pressure labyrinth seals, whereby the remaining flow is directed to the sealing steam condenser. The steam condenses in the sealing steam condenser and emits heat to the network water used for district heating. With the installation of Stirling engines into the sealing steam system, it is possible to produce thermal energy, as well as electrical energy. Stirling engines have been widely applied in various energy-related solutions, [1-5]. A Stirling engine is a heat engine using hot air and is considered to be one of the simplest engines and is the only one to use external combustion, in contrast to other engines. Its design is simple; it requires no fuel injection system and can use various types of fuels (biomass, coal, methane, hydrogen, etc.). The advantages of a Stirling engine in comparison with other heat engines are higher efficiency, lower environmental impact, no explosions taking place in a cylinder, and lower levels of noise and vibrations, [6]. One particular feature of Stirling engines is their heat regenerator, located in the engine cylinder air passage. The regenerator increases the engine efficiency by accumulating a portion of the heat of the working gas passing between the Stirling engine cylinders. There are several types of engines, [7-10]. In practice, the alpha, beta, gamma and the combined configurations are most frequently used. The operating principle of all configurations is based on thermodynamic laws of working gas expansion and compression. In our case, a gamma configuration of the Stirling engine was chosen because the engine design allows the cylinders to be installed in separate locations. The Stirling engine gamma configuration has two separate cylinders (a hot and a cold cylinder), [9]. The hot cylinder is mounted into the sealing steam system, whereas the cold cylinder is mounted outside the system and is additionally cooled. In our case, demineralised water (18°C) will be used for cooling to compensate for the losses during the process. A heat flow is created between the hot and the cold cylinders. The Stirling engine's power may be changed through a temperature difference of the hot and cold ends of the engine, through a compression ratio and the type and mass of the working gas. Furthermore, the selection of working gas impacts the engine's power, because working gases have different specific heat ratios. Due to the structural properties of materials, the Stirling engine is designed for a particular gas, thus making any subsequent change in the engine working gas impossible. However, a change in the amount of the working gas during engine operation is possible. Such a change and a temperature change lead to a change in the pressure ratio in the engine, [11]. Everything indicated above must be taken into consideration in the engine design and construction. 18 JET Turbine sealing steam heat recovery with dynamic Stirling engines 2 MOUNTING AND CONTROL OF STIRLING ENGINES IN THE TURBINE SEALING STEAM SYSTEM Due to the large amount of available heat of the turbine sealing steam, two Stirling engines operating in parallel and sharing a common generator will be installed into the sealing steam system. Each engine is built into its own system, allowing easier engine handling and operation. Two regulation circles will ensure the rational production of the Stirling engine's output. The first regulation circle will control the sealing steam quantity in the system in accordance with the energy needs. The second regulation circle will control the amount of the working gas in the Stirling engine in accordance with the available quantity of sealing steam. The mounting of the Stirling engine in the sealing steam system is illustrated in Figure 1. In the system Figure 1: Mounting of Stirling engines into the turbine sealing steam system JET 19 Dušan Strušnik, Milan Marčič, Jurij Avsec JET Vol. 7 (2014) Issue 3 The first regulation circle will control the position of Throttle 1, 2 and 3 in accordance with the energy needs (Figure 1). In normal operation, the position control of Throttles 2 and 3 maintains the required sealing steam pressure, varying with the change in the turbine load. Throttle 1 is in the closed position, and Throttles 2 and 3 control the amount of the sealing steam for the Stirling engine operation. If the demand for district heating increases, the controller will start opening Throttle 1 and closing Throttles 2 and 3 in accordance with the required sealing steam pressure and the network water temperature in the condenser. This will lead to an increase in the amount of the sealing steam towards the condenser and a decrease in the amount of sealing steam towards the Stirling engines. In accordance with the amount of the sealing steam, the second regulation circle will change the working gas quantity and, therefore, the Stirling engine's power. A change in the amount of the Stirling engine's working gas will be carried out by placing an accumulation reservoir between the expansion and the compression space to control the amount of the working gas in the engine in accordance with the available sealing steam quantity. If less sealing steam is available, the working gas will be sent to the accumulation reservoir from the cylinder expansion area at a higher pressure. Moreover, conversely, with an increased amount of the sealing steam, the engine's power will be increased by feeding the working gas from the accumulation reservoir into the compression area of the engine. The control of engine working gas will be performed by means of two control valves, as illustrated in Figure 2. The accumulation reservoir has to be located on the hot cylinder in order to ensure that the accumulation reservoir pressure is always higher than the pressure in the compression cylinder. Figure 2: Gamma-type Stirling engine with the working gas control 20 JET Turbine sealing steam heat recovery with dynamic Stirling engines The working gas serves as a substance to which heat is added or removed, resulting in the working gas expansion or compression with the internal mechanism being in motion. The choice of the working gas has a direct impact on the Stirling engine's efficiency, power, safety and general operation. Working gases have been widely applied in various energy related issues, [11-14]. The influencing factors in the working gas selection include price, flammability, viscosity, thermal conductivity, diffusivity, specific heat and electrical efficiency. Electrical efficiency is the ratio between the generated electrical energy and the Stirling engine input energy. It is expected that electrical efficiency varies according to the gas constant, i.e. the highest at air and the lowest at hydrogen, [11]. The thermodynamic properties of the working gases used in Stirling engines are indicated in Table 1. Table 1: Working gas properties Name Symbol Density Gas constant Helium He 0.1785 kg/m3 2078 J/kgK Air 1.2 kg/m3 259.8 J/kgK Acetylene C^H, 1.097 kg/m3 319.6 J/kgK Ammonia nh3 0,73 kg/m3 488.3 J/kgK Hydrogen H2 0,08987 kg/m3 4122 J/kgK Freon CF2Cl2 5.11 kg/m3 68.8 J/kgK 3 MATHEMATICAL ANALYSIS AND CALCULATION OF STIRLING ENGINE'S POWER OUTPUT The Stirling engine can be used as a machine, a heat pump or a cooler, installed between a heat source (a heated body) and a heat sink (a cold body). The temperature difference between the bodies leads to a heat flow used by the heat engine. The regeneration that increases the efficiency of the engine will be taken into consideration in the calculation. Mathematical analysis has been widely applied in various energy related issues, [15-17]. Figure 3: Conversion of the working gas and generation of the Stirling engine work JET 21 Dušan Strušnik, Milan Marčič, Jurij Avsec JET Vol. 7 (2014) Issue 3 3.1 Stirling engine mathematical analysis States 1 to 2 Isothermal compression (Figure 3). The working fluid is in contact with the cold surface, where the working gas volume of the Stirling engine is reduced. The Boyle-Mariott law applies to the isothermal compression: p ■V = m ■ R ■ T = cons tan t (3.1) where p - working gas pressure (Pa), V - working gas volume (m3), m - working gas mass (kg), R - specific gas constant (J/kgK), T - gas temperature (K). The Stirling engine output heat (Q1-2) is calculated using the following equation: Ql2=p1-Vlla- = mRT1-\n- (3.2) n n where: Q1-2 - engine output heat (J), p1 - working gas pressure at point 1 (Pa), T1 - working gas temperature at point 1 (Pa), VI - working gas volume at point 1 (m3), n - compression ratio of working gas conversion. States 2 to 3 Isochoric compression (Figure 3). The working gas is heated by means of the accumulated regenerator of the Stirling engine, where the temperature is isochorically increased from State Tt to State T4 (Figure 3). At this point, the working gas pressure is the highest. The accumulated regenerator input heat (Q^) is calculated using the following equation: Q2-3 = V {Kpy m ■ cv ■(T3 - T) (3.3) where: V2 - working gas volume at point 2 (m3), p2 - working gas pressure at point 2 (Pa), p3 - working gas pressure at point 3 (Pa), K - ratio of specific heats, cv - working gas specific heat at a constant volume (J/(kgK), T2 - working gas temperature at point 2 (K), T3 - working gas temperature at point 3 (K), Regeneration entails a decrease in the input heat on account of the working gas regenerated heat. 22 JET Turbine sealing steam heat recovery with dynamic Stirling engines The regeneration rate (ct ) indicates how much heat has been regenerated in a particular circular process, and it is calculated using the following equation: ct= T2- T = T - T' (3'4) T - T T - T where: T2, - working gas temperature at point 2' (K), T4, - working gas temperature at point 4' (K). The thermal efficiency of the circular process, without a heat regenerator (rft) is calculated: n = (Q3-4 - Q1-2) (3.5) (23-4 + 02-3 ) where: Q34 - input heat of the process 3-4 (J), The thermal efficiency of the circular process with a heat regenerator (rt, ) is calculated: = 23-4 - Ql-2 ) = 1 - Tl (3.6) 03-4 T3 n = 3-4 States 3 to 4 (Figure 3) Isothermal expansion. The working gas is in contact with the heated surface, where the volume of the Stirling engine working gas is increased. The expansion piston performs the work, the volume increases, and the pressure drops at the working gas maximum temperature (T4). The Boyle-Mariott law applies to isothermal expansion (Equation 3.1). The input heat of the process (Q3 4) is calculated: 03_4 = plVl\a.7t = m-RTA-\a.n (3.7) where: V3 - working gas volume at point 4 (m3). The pressure at working point 4 (p4) is calculated: P3 • V _ P3 (3.8) P 4 =■ V4 n where: V4 - working gas volume at point 4 (m3) State 4-1 (Figure 3) Isochoric expansion. The working gas emits the heat to the regenerator that accumulates it, and it is cooled down from state T4 to state T4' (Figure 3). The regenerator output heat (Q44.) equals the regenerator input heat (Q22.), multiplied by the regeneration loss (), and is calculated: Q4-4 = V • = »• c • T - t4)=Q^_2 • n (3'9) where: p4, - working gas pressure at point 4' (Pa), p - working gas pressure at point 4 (Pa). JET 23 Dušan Strušnik, Milan Marčič, Jurij Avsec JET Vol. 7 (2014) Issue 3 Calculation of power The Stirling engine's power (P) is the difference between the engine input and output heat multiplied by a loss factor (TJem). The Stirling engine's power output is calculated: The Stirling engine's efficiency (T]) is the ratio between the work done and the input heat flow and is calculated using the following equation: m-R-\D.7v-{TA-T^)V _ ln^-TQ-Qr-l) (3.11) n m-V-c^-T^+R-T^tt (r4-7])+r4-ln^-(A:-l) 3.2 Stirling engine power calculation In order to calculate the Stirling engine's power, the quality and quantity of the sealing steam of a condensation turbine at a Slovenian district heating plant were analysed. The sealing steam pressure oscillation was established ranging between 1.7 bar to 2.4 bar or 2.1 bar on average in a two-month period. The pressures are absolute. The sealing steam temperature varies between 260 °C and 300 °C and amounts to 280 °C on average in a two-month period. The sealing steam quantity varies between 1.2 kg/s and 2 kg/s. After the establishment of the quality of the sealing steam, heat can be determined by means of the enthalpy differential to be used for the operation of the Stirling engines. The enthalpy differential (Points 1 and 2) is illustrated in Figure 4. 6,5 7 7,5 Entropy (kJ/kgK) Figure 4: Sealing steam enthalpy differential used for the operation of the Stirling engines If a drop in enthalpy of 92 kJ/kg is multiplied by the quantity of the sealing steam (1.6 kg/s), the power output to be used by the Stirling engine is obtained, and it amounts to 147 kW. If the sealing steam heat is used when a portion of the steam is directly led to a condenser via System 1 (Figure 1) 0.8 kg/s of the sealing steam or 74 kW of power may be used. A Stirling engine capable of using the sealing steam heat from 70 kW to 140 kW has to be designed. Considering that two Stirling engines operating in parallel will be mounted into the sealing steam system, the engine dimensions have to be such as to ensure the use of the sealing steam heat from 35 kW to 70 kW. Helium has been chosen as the Stirling engine working gas. The thermodynamic states of the working gas (helium) 24 JET Turbine sealing steam heat recovery with dynamic Stirling engines at points indicated in Figure 3 were calculated using the equations indicated in Chapter 3.1. The results are illustrated in Table 2. Table 2: Thermodynamic states of helium by point when using 35 kW and 70 kW Name Symbol At 35 kW At 70 kW Minimum temperature 291 K 291 K Maximum temperature T. 535 K 567 K Expanded gas pressure Pi 3.7093x105Pa 7x105 Pa Gas pressure, Pa Pi 14.837x105 Pa 28x105 Pa Compressed gas pressure P3 27.278x105Pa 54.557x105 Pa Gas pressure P. 6.8 196x105 Pa 13.639x105 Pa Expanded gas volume V 0.037022 m3 0.037022 m3 Compressed gas volume V 0.009255 m3 0.009255 m3 Compression rate n 4 4 Helium specific heat ratio K 1.66 1,66 Mass of the gas in the system m 0.02271 kg 0.042856 kg Once the thermodynamic states of the working gas (helium) are known, the heat levels of the process, the engine's power output and the Stirling engine's thermal efficiency can be calculated. The results are indicated in Table 3. Table 3: Heat, power output and efficiency at 35 kW and 70 kW Name Symbol At 35 kW At 70 kW Process input heat Q3-4 35 kW 70 kW Process output heat Q1-2 19.037 kW 35.926 kW Regeneration heat Q2-3 17.446 kW 37.241 kW Engine power output P 15.483 kW 33.051 kW Thermal efficiency with regeneration Vt ,reg 0.44 0.47 The results show that if using 70 kW of the sealing steam heat, the Stirling engine generates 33.051 kW of power and reaches 47% efficiency. If the Stirling engine uses 35 kW of the sealing steam heat, it generates 15.483 kW of power and reaches 44% efficiency. As two Stirling engines are mounted in the sealing steam system, the total maximum power is 66.102 kW and the minimum power 30.966 kW. JET 25 Dušan Strušnik, Milan Marčič, Jurij Avsec JET Vol. 7 (2014) Issue 3 4 STIRLING ENGINE MODELLING The Stirling engine responsiveness to a change in the sealing steam quantity will be established through modelling. The model is designed using the Matlab-Simulink software tool and comprises the main programme and subprograms (the main program interconnects the subprograms). The subprograms compute the sought values at a specific order. Figure 5 illustrates the Stirling model. zInput Turbine steam mass flow Input NNET Output Neural Network □ Condenser power JZ 0- °C to K Mass of the gas Add1 Qin SM (1/2) Mass-gas Power SM(1/2) TempTS Qout SM (1/2) CTS^l Sealing steam temperature Power I [TS] ) ¡^ P1 Qin(1/2) P2 P3 Power P4 H TempTS at-ni Qin(1/2) Reg2-3Qdovedena P1 P2 Reg.4-1Qout P3 Ni reg. P4 Step reg. Stirling motor ~U" Regenerator Sealing steam temperature Constant To Workspace □ Regen D Efficiency Figure 5: Stirling engine model, [18] The neural network subprogram is a pre-trained network. The input data was given in a matrix format [1x2366]. The output data in a matrix format [3x2366] describe the sealing steam temperature, quantity and pressure. A neural network was designed on the basis of the input data of one group (amount of steam admitted to the turbine) and the results in three groups (sealing steam temperature, quantity and pressure). The neural network contains two groups of 90x10 hidden neurons. Other authors has been widely applied the neural network in various energy related issues, [19-22]. The neural network architecture is shown in Figure 6. Mass-gas Qsm Divide 26 JET Turbine sealing steam heat recovery with dynamic Stirling engines 4 Funcüön fitting Ntural Netwaik (*itw) [ = I 0.42 J 04 ^ 0.38 0.36 0.34 0.32 01 23456789 10 Time (minute) Figure 12: Stirling engine efficiency, [3] The heat flows, working gas pressure and volume variations at the Stirling engine sinusoidal power oscillation are shown in Figure 13. to ¡4 m m 2 2 Ol 2 ssurepl 50 y^Scy 40 Regeneration 30 heatin Time (minute) I20 Heat out 0035 0.03 0.025 Volume (m3) Pressure p2 Figure 13: Dynamic p - V diagram, [18] x 10 6 8 5 4 3 2 1 JET 31 Dušan Strušnik, Milan Marčič, Jurij Avsec JET Vol. 7 (2014) Issue 3 Furthermore, simulations of the Stirling engine operating with various working gases were carried out on the existing model. The simulations were carried out by using the gas constants in the model as indicated in Table 1. Figure 14 shows the variation of pressure p3, i.e. the maximum Stirling engine pressure. Figure 14: Variation of pressure p3 at various Stirling engine working gases, [18] Figure 14 shows that the highest pressure is achieved with hydrogen and the lowest with Freon. The choice of the engine working gas has to be anticipated prior to designing an engine, as the specification of the engine materials should be suitable for the thermal and pressure state. As the working gas pressure state is closely related to the engine's power, the variation in power at different working gases is equivalent to the pressure state. Figure 15 illustrates the Stirling engine's power output variations at different working gases. Figure 15: Stirling engine power output variations at different working gases, [18] 32 JET Turbine sealing steam heat recovery with dynamic Stirling engines 5 CONCLUSION This paper presents the possibility of using the sealing steam heat of a turbine for heating and power generation. The possibility of using the sealing steam heat that varies in accordance with the energy needs has been indicated. The use of the sealing steam heat for Stirling engines results in electrical energy generation and the application of the engine output heat for the heating of the demineralised supplementary water of a boiler. In the event of an increase in demand for district heating, the sealing steam control system reduces the Stirling engine's power output and increases the heating power. The Stirling engine adapts to the above changes by varying the working gas quantity and adjusting the power to the energy needs. The installation of Stirling engines into thermal plants is suitable for the use of a smaller amount of waste heat as the issue of overly large engine capacity emerges with larger amounts of waste heat. The above issue is addressed by mounting Stirling engines of a smaller capacity in a successive or a serial-parallel arrangement. References [1] S. Toghyani, A. 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