Hydrogen Production and Storage from Wind Turbine Using Water Electrolysis Method

Transcription

1 Turbine Using Water Electrolysis Method Arash Khalilnejad, Hassan Gholami 2, Gholam H. Riahy 3 MSc, Amirkabir University of Technology, arash.khalilnejad@aut.ac.ir 2 MSc, Amirkabir University of Technology, gholami.hassan.kh@gmail.com 3 Associate Professor, Amirkabir University of Technology, gholam@aut.ac.ir Abstract The objective of this study is to produce maximum hydrogen from advanced alkaline electrolyzer. The input electricity of electrolyzer is generated with wind turbine. The wind turbine is utilized with a self-exited induction generator. The effect of variation of temperature in electrolyzer is investigated, considering both thermal and electrochemical equations of electrolyzer. The optimal sizing of wind turbine is performed to maximize the hydrogen production and minimize the excess power. Actual data for weekly wind speed of Sahand, Iran, is used. Results show that for a kw electrolyzer, the optimized nominal power of wind turbine is kw at wind speed of 2 m/s. The hydrogen production and average excess power during weekly performance of system are 8297 mol and.9 kw, respectively. Keywords: Hydrogen, Wind Turbine, Electrolyzer, Storage

2 Introduction The demand for environmentally friendly renewable energy resources is growing in recent decades because of decreasing of fossil fuels, and increasing the demand for power supply. Among various kinds of renewable power resources, Wind and solar power are the most well-known ones. Wind turbines can be utilized in places that wind blows to generate electricity. But the power generation from this method is dependent to climate situations. So, to get a reliable power an energy storage system is needed. Batteries are the most regular equipment for this purpose. However, using them has some issues. First, hourly power leakage of -5 percent during the operation of system makes them useless for long term applications. On the other hand, batteries usually don t have the capacity to store large amount of energy. Hydrogen is a suitable option and it can be used as a fuel to get a reliable power for almost every application that fossil fuels are used. Furthermore, hydrogen can be converted in other types of energy more efficiently than fossil fuels [-4]. For hydrogen production from electricity, an electrolyzer can be used which is able to operate in large range with high efficiency [5-6]. Various studies have been done about this issue including commercial, technological, techno-economical, and environmental aspects. Moreover, different conditions of energy management procedure about how to combine them to get the most efficient operation of system are performed [7-]. In this study hydrogen production with a stand-alone power system is evaluated. The optimal sizing of the wind turbine is done. The cost of components of this system is variable during time. So, another index for optimization is used which is independent of time. The introduced index relies on maximum hydrogen production through minimum excess power production of resources. The detailed components of WT, power electronic devices, electrolyzer, and storage tank are used. Stand-alone power system A wind turbine generates power needed for electrolysis process in an alkaline electrolyzer. The optimal size of wind power system should be used to get as much hydrogen as possible from a kw electrolyzer. Wind Turbine Based on local wind speed, mechanical power output of wind turbine is defined by [2-5]

3 () where ρ is the air density (Kgm -3 ), A is sweep area of the blades of WT (m 2 ), v is the wind speed (m/s), and C p is coefficient of performance of WT, which is given by (2) (3) λ is Tip speed ratio and can be described by (4) where r is radius of the rotor (m), and ω is angular velocity of rotor (rad s - ). The wind turbine power characteristic for different values of wind speed is shown in Figure. If the working point of WT lies on the peak of curves the maximum power extraction can be achieved. Electric Generator The wind turbine used in this work is equipped with self-exited induction generator (SEIG). The detailed model and related equations are of generator and capacitor is described by Seyoum et al. [6]. The operation of asynchronous generators can be analysed using d-q model. Electromagnetic torque and power output is given by (5) (6)

4 Turbine Output Power (pu) where P is the number of poles, L m is the magnetizing inductance (H), i sx (u sx ) and i sy (u sy ) are the stator currents (voltages) and i rx (u rx ), i ry (u ry ) are the rotor currents (voltages) in a general frame. The connection of electrical generator and WT results in electrical power production through turning the blades of wind turbine. Mechanical and electrical torque difference, is given by (7) where T wt is the aerodynamic mechanical torque (T wt =P wt ω); H is the total inertia constant. 2 m/s m/s 9.6 m/s 8.4 m/s 7.2 m/s 6 m/s Speed (pu) Figure. Wind Turbine characteristic Electrolyzer The electrolyzer used in this study is alkaline electrolyzer. The model of the electrolyzer is based on a combination of fundamental thermodynamics, heat transfer theory and empirical electrochemical relationships [7-8]. Electrochemical model The basic form of the U-I characteristic for a given temperature is expressed by ()

5 where r is the ohmic resistance parameter of electrolyte (Ωm 2 ), s and t are overvoltage coefficients which are temperature dependent (V), (A - K). In order to represent the temperature dependence of overvoltages, the equation () is expanded to get the more detailed model by (2) The parameters of U-I curve of electrolyzer are given in Table 2. Figure 2a shows U-I characteristic of typical alkaline electrolyzer at different temperatures. The difference between the curves is caused because of temperature dependence of overvoltages. The faraday efficiency is the ratio between actual and theoretical amount of maximum hydrogen production. An empirical expression for depiction of faraday efficiency is given by (3) where (ma 2 cm -4 ), and 2 are parameters in faraday efficiency calculation. Faraday efficiency parameters are given in Table 2. Figure 2b shows the faraday efficiency of electrolyzer in different temperatures. As it can be seen, at voltages above.8v this parameter is not dependent on temperature. Table : U-I curve parameters r r 2 t t 2 t 3 s s 2 s 3 A Ω m 2 Ω m 2 C - A - m 2 A - m 2 C A - m 2 C 2 V.38-3 V/C V/C 2 m 2 Table 2: Faraday efficiency parameters 2 25 ma 2 cm -4.96

6 Cell Voltage (V) Faraday Efficiency T=2 C T=8 C.6 T=8 C.6.5 T=2 C Current Density (ma/cm 2 ) Cell Voltage (V) (a) (b) Figure 2. Cell Voltage (a) and faraday efficiency (b) of an electrolyzer at low and high temperature The hydrogen production rate for several cells connected in series is expressed by (4) where F is faraday constant (C mol - ), and n c is number of cells in series. Thermal model The operating temperature of electrolyzer affects its performance. The temperature variation rate of the electrolyte is proportional of the difference of generated heat rate, less heat loss, and cooling heat. The thermal energy balance is given by (5) where C t is the overall thermal capacity of electrolyzer (J K - ). The heat rates is derived by (6) (7) ( ) (8)

7 Current (ma/cm 2 ) where U tn is the thermal voltage, R t is the overall thermal resistance (KW - ), C cw is thermal capacity of cooling water (J K - ), T cw,i and T cw,o are the inlet and outlet temperature of cooling water (K), respectively. Considering the interaction of U-I characteristics and thermal equations the temperature of electrolyzer changes during time. Figure 3 shows the U-I curve during time for a kw alkaline electrolyzer with 2 cells connected in series. In a desired working voltage of electrolyzer the current point changes during time because of temperature variation. Storage Physical hydrogen storage is a common technique to store hydrogen using tanks. The model of storage system which calculates tank pressure using the flow rate of produced hydrogen is expressed by [9] (9) Where M H2 is the molar mass of hydrogen (kg kmol - ), P b is the pressure of tank (Pa), P bi is the initial pressure of the storage tank (Pa), R is the universal gas constant (J kmol - K - ), T b is the operating temperature (K), V b is the volume of the tank (m 3 ), z is the compressibility factor as a function of temperature Voltage (V) Figure 3. U_I characteristics of electrolyzer during 24 hours period considering thermal model

8 Ambient Temprature ( C) Wind speed (m/s) Results and Discussion According to mentioned models, coupling of wind turbine and electrolyzer is simulated in this study. The simulation is based on actual data of weekly wind speed and temperature of region of Sahand in Iran which is shown in figure 4. The optimal size of the system is achieved through optimization algorithm of ICA. The various costs of the system are not included in optimization process. Because costs of this system are variable and strongly dependent in time, it doesn t lead a suitable cost function. So, another index to optimization is introduced, based on maximum hydrogen production and storage through minimum average excess power production, which in this case gives kw for the size of wind turbine for a kw electrolyzer. The result of simulation is given in table Figure 4. Data of meteorological conditions for one week Hydrogen production rate is shown in figure 5a. As it can be seen, the production rate at the times that wind speed is high is more. The maximum possible production rate is.27 mol/s. However, because it can t always work in that point, the average production of this system is decreased to.73 mol/s. One of the most effective parameters for this production is faraday efficiency. This factor is shown in figure 5b. The maximum possible point of it is.9. At low wind speeds the efficiency decreases to very low amounts which causes the lack of production. The average faraday efficiency of this system is.769. Table 3: Operation of simulated system WT Nominal Power Area of Turbine Rotor Average Power Production Average Unused Power Energy Production Unused Energy Hydrogen Production Rate Average Faraday Efficiency kw 42.3 m KW.9 kw 3.43 GJ 4.9 MJ.37 mol/s.769

9 Electrolyser Energy consumption (kj) Unused Energy (kj) Hydrogen Production Rate (mol/s) Faraday Efficiency (a) (b) Figure 5. Hydrogen production rate (a) and Faraday efficiency (b) The electrolyzer is not able to consume the power more than it s nominal amount. Therefore, the power production more than kw is named excess power. The system works efficiently when the average excess power is decreased to its minimum value while production is as much as possible. In other words, the excess energy should be minimum and energy consumption should be maximum. In this condition the amount of energy consumption and excess energy is shown in figure 6. As it can be seen at some of the hours the excess energy is produced. 3.5 x (a) (b) Figure 6. Energy consumption (a) and unused energy (b) of Wind/H2 system

10 Pressure (MPa) Hydrogen Production (mol) The total hydrogen production and storage pressure of tanks is shown in figure mol of hydrogen is produced during weekly operation of system. The produced hydrogen should be stored in storage tanks. The tanks used in this study are high pressure tanks which are switched at the end of the day. The maximum pressure of the tank is about 2.6 MPa. So, the uses tanks capacity should not be less than it (a) (b) Figure 7. Storage pressure (a) and Hydrogen production (b) Conclusion In this study, the operation of wind-hydrogen system is investigated and the optimal sizing of wind turbine is performed. The average power consumption of 5kW of alkaline electrolyzer produces 8297 mol of hydrogen during weekly operation of system. On the other hand, the average unused power production of wind turbine is.9 kw. So, an acceptable amount of hydrogen is produced without losing too much energy. Usually, the peak power generation of wind turbine is at the end of the day. For future research, the excess power can be used to other parts of the system to avoid loss of generated power, utilizing advanced equipment.

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