Design and Characterization of a Small Wind Turbine Model equipped with a Pitching System

Transcription

1 Design and Characterization of a Small Wind Turbine Model equipped with a Pitching System In Partial Fulfilment of the Requirements for the degree Master of Science in Engineering Renewable Energy and Energy Efficiency for the MENA Region (REMENA) Done by Fadi Abdulhadi Supervised by Prof. Dr. sc.techn. Dirk Dahlhaus Dr. Michael Hölling Dr. Basman El Hadidi Kassel, 27 th Feb 2012

2 Table of Contents List of Figues.iii List of Tables.. v Nomenclature.... vi Acknowledgement.. vii Abstract.. viii 1. Introduction Statement of Purpose Literature Review Structure of the Thesis Theoritical Backgroud Wind Turbine Model Specifications and Main Components Blade Generator Stepper Motor Pitching System Pitching Mechanism Load Analysis Pitch Angle Calculation Wind Tunnel Testing Experimental Setup Test Procedure and Data Recording Characteristic Curves i

3 5. Results and Discussion Data Analysis Improvement Recommendations Conclusion References APPENDIX A: 2D Drawing and Dimensioning (CATIA V5) APPENDIX B. LabVIEW Program APPENDIX C. Parts Specifications ii

4 List of Figures Figure 2.1 Ideal wind turbine power curve... 5 Figure 2.2 Typical ( C - λ) curves at various pitch angles.6 p Figure 2.3 Velocities and aerodynamic forces on a blade airfoil... 6 Figure 2.4 Aerodynamic forces on a blade segment...7 Figure 2.5 Lift and drag coefficients vs. angle of attack..9 Figure 2.6 Basic variable-speed variable-pitch (pitch-to-feather) control strategy Figure 3.1 Wind turbine main components Figure 3.2 Nacelle assembly...12 Figure 3.3 Decomposition of the blade shape...13 Figure 3.4 Blade geometry...14 Figure 3.5 The generator housed in the nacelle...14 Figure 3.6 Stepper motor wiring Figure 3.7 Pitching system components Figure 3.8 Principle of operation of the traveling-nut linear actuator Figure 3.9 Changing the pitch angle manually...17 Figure 3.10 Pitching moment coefficient with respect to angle of attack...18 Figure 3.11 Local velocity vector at various blade segments...19 Figure 3.12 Branch Dimensions (Initial position) Figure 3.13 Member force analysis Figure 3.14 Screw Geometry and Force Analysis Figure 3.15 Friction force acting on the lead screw...22 Figure 3.16 Animation of linear and angular motion. 24 Figure 3.17 Pitch anlge function using curve fitting method Figure 4.1 Circuit diagram Figure 4.2 The wind turbine positioned within the wind tunnel Figure 4.3 Setting of the initial pitch angle Figure 4.4 ( C - λ) curve at various pitch angles at wind speed 9 m/s p iii

5 Figure 4.5 Wind turbine model power curve Figure 4.6 C p max locus Figure 4.7 Pitch angle vs. wind speed Figure 4.8 Feathering the blade Figure 5.1 C vs. wind speed curve P max Figure 5.2 Optimum λ vs. wind speed curve Figure 5.3 ( C - λ) curves at first and second setup p Figure 5.4 Original blade s dimensions (mm) Figure 5.5 Twist angle and chord length offset Figure 5.6 Recommended blade s dimensions Figure 5.7 Roller support iv

6 List of Tables Table 3.1 Wind turbine model specifications...11 Table 5.1 Characteristic data at rated wind speed 9 m/s and pitch angle Table 5.2 Characteristic details for the power controlled region...34 v

7 Nomenclature Tip Speed Ratio _ P Wind Power W W P Output Power W O 3 Air Density kg / m V Free Wind Speed m/ s Rotational Speed rad / s c Chord Length m/ s C Power Coefficient p C Lift Coefficient L C Drag Coefficient D C Pitching Moment Coefficient m r Radius m R Rotor Radius m M Pitching Moment D Drag Force N L Lift Force N Nm A Swept Area 2 m A Planform Area 2 p m Kinematic Viscosity 2 m / s Kinetic Viscosity kg/ ms Pitch Angle l Span Length m T Torque Nm F Force N vi

8 Acknowledgement It gives me a great pleasure to acknowledge the people who helped me, in one way or another throughout this project for their immense and continuous support. First and foremost, I would like to express my great gratitude to Engr. Agnieszka Parniak for her continuous and sincere support during the project design, manufacturing and construction phases. Special thanks go to my supervisor Dr. Michael Hölling for providing me the chance to work at his department, and helping me in building my knowledge base in experimental work. I would also like to thank my supervisor Dr. Basman El Hadidi for his valuable comments and scientific support through our regular Skype conferences. Of course I will not forget to thank my friend and my partner in this project Abdulkarim Abdulrazek for his moral and technical support throughout the project period. vii

9 Abstract A small pitch-controlled wind turbine with a 0.58 m rotor diameter has been designed, constructed and tested in the wind tunnel. The blade is a prototype of a larger blade that has been constructed in an earlier project. This thesis dissertation covers two main tasks. First, careful characterization of the small wind turbine is carried out at different wind velocities and pitch setting angles. Second, a pitch control system is implemented to control the power output of the turbine at wind speeds above the rated speed. Two control strategies have been adopted for this model; load control below the rated wind speed, and pitch to feather control above the rated wind speed. A novel lead screw mechanism is implemented for controlling the pitch angle by a stepper motor. This proved to be efficient and convenient for small wind turbines and worked perfectly. All measurements and control actions were achieved by utilizing a National Instruments data acquisition cards and a custom developed LabVIEW program. The wind tunnel measurements of the wind turbine characteristics were not as expected. The wind turbine maximum power coefficient was of the order of 0.02 and optimum tip speed ratio was around 3.2 at the rated wind speed of 9 m/s. viii

10 1. Introduction Variable-speed variable-pitch wind turbines are spreading widely nowadays, since they allow accurate control of the power output by either capturing the maximum energy at wind speeds below the rated wind speed (rated power), or by limiting the power generated when it exceeds the rated power (rated wind speed). 1.1 Statement of Purpose The project aims at designing, constructing and characterizing a pitch-controlled wind turbine model with a rotor diameter of 0.58 m. The long-term goal is providing the opportunity to simulate a real case and apply further investigations in fields such as wake effects and performing under turbulent wind conditions. Accordingly, the scope of work within this master thesis is considered as the backbone for a long-term project. The model is constructed, tested and characterized. And by characterized, it is meant that the wind speed power curve and the ( C - λ) curves are experimentally investigated and plotted at various pitch angles. Additionally, a pitch to feather control strategy is implemented and characterized. The pitching system including the pitching mechanism, design considerations and pitch angle calculation are illustrated in detail. It is worth-mentioning that small pitch-controlled wind turbines (up to 0.7 m rotor diameter) were not investigated intensively so far; consequently, this work will probably open a new horizon for further investigation in this field. Most previous small models at P least the published projects were not equipped with automatic pitch-control system [ 7], as it is the case in this project at ForWind Research Center of Wind Energy. 1

11 1.2 Literature Review This section gives an overview of some previous projects in the field of wind turbine modelling and testing. The projects tackled different issues such as characterizing and controlling various aerodynamical and structural loads, as well as improving the performance of wind turbines. Nevertheless, the overview highlights mainly the mode of operation for the tested models without focusing on the different outcomes of these projects. Wind turbines modelling and testing is a recent field of research. The research topic started to take place actively after the year 2000, when the growing threat of fossil fuel emissions has put the entire world in a dilemma, and considering clean alternative energy sources has become a necessity. Different small wind turbine models have been tested in wind tunnels in order to characterize the performance of wind turbines under various wind conditions. In the year 2005, a fixed-pitch fixed-speed wind turbine model of 0.7 m rotor diameter was built and [7] tested in a wind tunnel at the University of Stuttgart. In the same year, another wind turbine model with a rotor diameter of 0.54 m was built at the University of Applied Sciences in Lübeck [ 7], where the pitch angle is controlled automatically using a stepper motor and the model operates at a fixed-speed mode. In 2006, a new model was developed at the Technical University of Berlin [ 7]. The model has a rotor diameter of 0.7 m, however, the model is restricted by only four different pitch angles (0, 2.5, 5, 10 ) where each time the angle should be set manually before the test takes place. In 2008, the same model was modified and re-constructed at the Technical University of Berlin [ 8]. A major difference (compared to the previous model) is that the pitch angle can be set manually at eight different values (0, 1,25, 2.5, 3,75 5, 7.5, 10, 15 ), which allows the testing to be done at more angles of attack, and therefore, having better [8] characterization for the wind turbine model. Furthermore, the project wind turbine s efficiency by replacing the previous blades with new ones. has improved the In this project, a variable-speed small wind turbine model equipped with a pitching system is constructed and characterized. Testing under fluctuating and turbulent wind 2

12 conditions as well as further research on wake effects are planned to take place in the near future. 1.3 Structure of the thesis The thesis consists of six chapters. Chapter 2 includes the necessary theoretical background. In Chapter 3, the model s components, design considerations and the pitching system are discussed in details. The wind tunnel testing procedure starting from the experimental setup and ending up with the model s characteristic curves, are discussed and tackled in Chapter 4. In Chapter 5, the key results are presented and discussed analytically. Additionally, further improvements and recommendations are suggested and analyzed for future work. Finally, chapter 6 deals with the main conclusion. 3

13 2. Theoretical Background Wind turbines convert wind kinetic energy into angular torque in order to drive an electric generator. The power available in the wind is represented by the following equation PW 1 AV 2 3 (2.1) where V is the wind velocity, A is the swept area of the rotor, and is the air density. The percentage of the extracted power is known as the power coefficient C ; and the physical performance limit (known as Betz s limit) imposes a maximum value of [4] CP equals to 59.3% of the available wind power. The maximum theoretical power extracted as a function of the available wind power is expressed by the following equation P P max AV C P max P W (2.2) 2 The power coefficient C P can be represented by the following comprehensive equation CP Ad Electric Mechanic (2.3) Where Ad is the aerodynamic efficiency, which depends on the blade design and tip losses, Electric is the electrical efficiency of the generator, Mechanic is the mechanical efficiency including mainly friction losses in the wind turbine assembly and drive train. So far C P value never reached it maximum limit of due to aerodynamic, mechanical and electrical losses. Figure (2.1) portrays the ideal characteristic power curve of a wind turbine. 4

14 Figure 2.1: Ideal wind turbine power curve [1] A very important characteristic design value is the tip speed ratio, which is a dimensionless variable described by the following equation R (2.4) V where R is the rotor radius, measured from rotor center to blade tip, and is the rotor rotational speed. The optimum tip speed ratio for 3-bladed grid-connected wind turbines is around =7 [ 2], which means that the blade tip tangential velocity is 7 times the free wind velocity. Moreover, operating at the optimum tip speed ratio means that we are extracting as much power as possible from the wind. One of the first experimental procedures for the characterization of wind turbines is plotting the turbine s power coefficient ( C ) against tip speed ratio ( ). Figure (2.2) illustrates typical ( C - ) curves at various pitch angles. P P 5

15 Figure 2.2: Typical (Cp - λ) curves at various pitch angles [3] Figure 2.3 shows the different angles, velocities and aerodynamic forces on a blade airfoil. Figure 2.3: Velocities and aerodynamic forces on a blade airfoil [3] 6

16 The pitch angle (also called the blade angle) is defined as the angle between the airfoil chord and the rotor plane of rotation, measured at the blade root. Changing the pitch angle will change the angle of attack, according to the following equation (2.5) 1 V tan (2.6) r where is the angle between the rotor plane and the local wind (also called relative wind or apparent wind or resultant wind), is the angle between the local wind and the airfoil chord, V is the free wind velocity, is the rotational speed, r is the radius at the point of computation measured from the rotor center. The local wind velocity vector varies along the blade s span according to the following equation V res 2 V 2 r (2.7) Figure 2.4 illustrates the aerodynamic forces acting on a blade element, the resultant of all pressure and friction forces can be characterized by two forces and a moment (lift, drag and pitching moment), which are all translated and calculated at the aerodynamic center (one fourth of the chord length). Figure 2.4: Aerodynamic forces on a blade segment [4] An important non-dimensional parameter for defining the characteristics of fluid flow conditions is the Reynolds number (Re) which is defined by 7

17 VZ VZ Re (2.8) where is the fluid density, is the kinematic viscosity, is fluid viscosity, V and Z are the fluid flow velocity and length that characterize the scale of the flow. The lift force emerges from the pressure difference between the top and the bottom surfaces of the blade, and it is defined to be perpendicular to the direction of the local wind [ 4]. The drag force is defined to be collinear with the direction of the local wind. And it arises from viscous friction forces at the surface of the airfoil and from unequal pressure on the airfoil surfaces facing toward and away from the local wind flow [ 4]. Pitching moment as shown in figure (2.4) is the moment that occurs about an axis perpendicular to the airfoil cross-section and at a distance of one fourth of the chord [4] (aerodynamic center). Lift and drag as well as the pitching moment can be characterized by their coefficients which are basically dependant on the angle of attack and slightly varies at different Reynold numbers. The lift coefficient for a blade segment is defined as the drag coefficient is defined as: and the pitching moment coefficient C C C L (2.9) 0.5 V A L 2 res D (2.10) 0.5 V A D 2 res M (2.11) 0.5 V Ac m 2 res Where L is the lift force, D is the drag force, M is the pitching moment, is the air density, V res is the local or resultant velocity which varies along the blade s span, A is the planform blade area ( c l ), c is the airfoil chord length, and l is the segment span length (see figure 2.4). 8

18 Figure 2.5 depicts the variation of lift and drag coefficients with the angle of attack. Figure 2.5: C and L C vs. angle of attack [5] D As shown in figure 2.5, when the angle of attack exceeds 14, the stall phenomenon takes place, which can be physically defined as the separation of flow that leads to dramatic drop in lift force and rise in drag force. The lift to drag ratio is an important design factor, on which the aerodynamic performance is based [ 4]. Changing the optimum angle of attack (13-14 in figure 2.5) will directly lower the maximum lift to drag ratio, and this will end up reducing the maximum aerodynamic input torque, hence reducing the power output. There are two strategies to limit the power output at high wind speeds, either by increasing the angle of attack via what s called active stall control (or pitch to stall), or by decreasing the angle of attack, to avoid the stall, through feathering the blades (pitch to feather). Accordingly, pitch control is used to limit the power output of wind turbines, in order not to exceed the rated output power, which might lead to the failure and damage of the generator. Figure (2.6) illustrates the pitch to feather control strategy for a variable-speed variable-pitch wind turbine. 9

19 Figure 2.6 Basic variable-speed variable-pitch (pitch-to-feather) control strategy [6] The strategy shown in figure 2.6 varies depending on the wind speed. At low wind speeds V V (where N N V is the wind speed correspondent to the generator rated rotational speed on the C P max locus), the locus of the maximum power coefficient is N tracked, in order to extract the maximum possible energy. As the wind speed increases, the rotational speed increases proportionally (assuming a constant tip speed ratio ). Point B represents the point at which the blade feathering gradually starts to take place in order not to exceed the generator s rated rotational speed N. As seen in figure 2.6, point C is not located on the C P max locus, which means that at that point, less wind energy is extracted and the wind turbine is operating at it s rated power P N. 10

20 3. Wind Turbine Model In this chapter, the specifications and main components as well as the pitching system of the wind turbine model are discussed in details. 3.1 Specifications and main components The following table 3.1 illustrates the model s main specifications Table 3.1 Wind turbine model specifications Figures 3.1 and 3.2 illustrate the wind turbine assembly and main components. In addition to the blades, pitching system and the generator, which will be discussed in details in the coming sections, there is the main shaft on which all parts are assembled, and the coupling which transmits the mechanical power to the generator. 11

21 Figure 3.1 Wind turbine main components Figure 3.2 Nacelle assembly As shown in figure 3.2, the nacelle consists of 5 segments (made of PVC), which are connected using two long screw nuts. 12

22 3.2 Blade [9] The blade is a prototype of a blade which was used in a previous project. It is developed based on three airfoil families as shown in figure 3.3, which are DU91-W2-250, RISØ A1-21, and NACA Figure 3.3: Decomposition of the blade shape [9] The original blade is designed to operate at a tip speed ratio =6.7 [ 10]. It is clear from figure 3.3 that both blades are different at their root ends. The original blade is gradually cambered at its first transitional zone in a way to match the tubular connection, whereas its prototype (which is used in this project) gets bigger in chord with sharp edge at its route. The blade is manufactured by a 3D printing machine, and is made of VeroBlue acrylic photopolymer. It has a span of 250 mm, and its chord varies from 35.8 mm at the root end to 10.3mm near the blade tip as shown in figure

23 Figure 3.4: Blade geometry (mm) 3.3 Generator The generator is a DC Micromotor which was purchased from the company FAULHABER. The mechanical power is transmitted through the coupling to the generator, and converted into electrical power which can be displayed and recorded as discussed later in chapter 4 (experimental setup). The generator rated power is W, and its maximum allowable speed is 8000 rpm at full load (see APPENDIX C). Figure 3.5 shows the generator main components. It is equipped with an encoder to detect the rotational speed. Figure 3.5: The generator housed in the nacelle 14

24 The selection criterion was based on a rated wind speed of 12 m/s and a power coefficient of 0.4 using equation (1.2) as follows: P max PW CP max AV CP max W W Stepper Motor The stepper motor is used to control the pitch angle using a leadscrew mechanism. The motor s holding torque is 22 mnm (see APPENDIX C). A special driver AD CM M1S is used (see APPENDIX C). AD CM M1S is a current mode driver, the current level can be controlled by on-board switches, which allows to apply a much higher voltage than needed to drive the current without risk of overheat. Additionally, it can increase torque output of the motor plus give the possibility to boost the current if necessary. The command side of the driver is connected to NI USB-6211 [11] Analog-Digital converter, which sends and receives digital signals to and from the computer. Figure 3.6 shows the stepper motor wiring. Figure 3.6: Stepper motor wiring 15

25 LabVIEW software is used to generate pulse trains and control the motion of the stepper motor including number of input steps and sense of rotation CW/CCW. On the other, LabVIEW can display the results detected from the encoder, such as the angular position. For further information about the program please refer to APPENDIX B. 3.5 Pitching System Pitching Mechanism The pitching system is shown in figure 3.7. It consists of L-shaped slider made of Teflon (with low coefficient of friction of 0.04), and is bored on both its upper and lower parts. The main shaft is fitted with a tube which is made of Teflon as well. A small tolerance was kept in order to allow the rotation of the main shaft through the lower hole of the slider. The screw nut is fitted in the upper hole of the slider. Figure 3.7: Pitching system components The lead screw translates the rotational motion into linear motion as shown in figure 3.8. Figure 3.8: Principle of operation of the traveling-nut linear actuator 16

26 By moving the slider back and forth (depending on the motor s sense of rotation), the base moves and three connected branches move accordingly (one branch consists of two connected links), which will end up changing the pitch angle as shown in figure 3.9. Figure 3.9: Changing the pitch angle manually Load Analysis The main criterion which was followed for the selection of the stepper motor is calculating the pitching moment at a maximum wind speed of 15 m/s and optimum tip [10] speed ratio of =6.7 (referring to the original blade ). V R rad / s 0.29 Referring to equation (2.11), the pitching moment at the aerodynamic center ( c /4) measured from the leading edge equals to M 0.5C V m 2 res ca Where velocity, C m is the pitching moment coefficient, c is the chord length, = 1.225kg/m 3, V res is the local wind C value depends basically on the angle of attack and varies slightly at different m Reynolds numbers as shown in figure (3.10) Ais the planform area of the blade. 17

27 Figure 3.10 Coefficient of pitching moment with respect to angle of attack [12] (The negative slope for positive α indicates stability in pitching) Figure 3.10 illustrates the variation of C m for a specific airfoil shape which does not belong to this project. However, the purpose of presenting this graph is just to point out that C m does not change considerably at different scales or Reynolds numbers. Therefore, using the same experimental values of the original blade is reasonable and convenient for the load calculations. To be on the safe side, the maximum value of C is considered (experimental values are not published, but obtained from m original source). In order to obtain more reasonable results, the blade is divided into 10 segments as shown in figure (3.11), each has an equal span length of m. The local pitching moment for each blade segment is calculated. Finally, all local pitching moments are summed up to find the total pitching moment acting on the whole blade. 18

28 Figure 3.11: Local velocity vector at various blade segments Following is a sample calculation for segment 1 M 10 M n n C V A c (3.1) m n 1 2 n( res) n n M Cm V1 res A1c 1 (3.2) V1 res V r1 (3.3) Assuming a linear variation of the chord length along the blade s span, the mean chord for each segment can be calculated following the formula c n n (3.4) 10 c m A m (Blade segment planform area) m 2 2 V1 res / 15 s 3 M N. m 1 3 The total pitching moment for one blade equals to M Nm. 19

29 Figure 3.12 shows the initial position and the dimensions of one branch in the pitching system. The initial position of the branch is the position which corresponds to the initial blade angle (before any pitching takes place). Figure 3.12 Branch dimensions (Initial position) In order to find the load acting on the lead screw, the forces acting on one branch are calculated, by simply performing a static force analysis as shown in figure 3.13 Figure 3.13 Member force analysis 20

30 Defining member BC as member 1, member AB as member 2, and the lead screw as member 3, the force applied from member 3 to member 2 (which is the lead screw force F 32) is calculated as follows Sum of moments at point C equals to zero (Member 1) M M C 0 F N (Values taken from figure 3.12) (3.5) sin Sum of forces in the y direction equals to zero (Member 2) F21 F 12 (3.6) F y 0 F12 F 32 F N (3.7) Total force acting on all branches = lead screw force = 3F N Figure 3.14 Screw geometry and force analysis The Torque needed to overcome the calculated lead screw force is found using the following power screw formulas: Torque Fd 2 2 sec b tan a 1 sec b tan a (3.8) Pitch tan a (3.9) d 2 21

31 Where a is the lead angle, b is half the thread angle which equals to 30, is the coefficient of friction between the screw and the nut which are made of brass and bronze respectively (it is considered =0.2 for safety), Pitch equals to 0.5 mm, d 2 is the nominal diameter (For M6 screw d 2 = 6 mm) tan a Torque N. m Additionally, the friction due to part of the weight acting on the screw nut can be roughly estimated. For safety considerations, the weight of the highlighted part (red dashed) in figure 3.15 is considered (which is more than the actual acting force since the two ball bearings fixed on the Nacelle stand part of the main shaft s weight). Figure 3.15 Friction force acting on the lead screw The total weight including the blades and the hub was measured on a digital scale and equals to 1.57N. Calculating for the frictional force F N N (3.10) This can be added to the total lead screw load, the recalculated torque including frictional forces equals to mnm. As mentioned in section 3.4, the holding torque of the stepper motor equals to 22 mnm. Accordingly, the factor of safety can be calculated as follows 22

32 3 Tcritical f safety 1.66 (3.11) 3 T max Pitch Angle Calculation The main goal is to detect and control the pitch angle position, and for that purpose it is required to know exactly the number of steps done by the stepper motor. The stepper motor is designed to move 24 steps per revolution, with a 15 change in shaft s angle per step, and each step consists of 5 pulses (3 per pulse). Since the step ( S ) is the smallest unit that can be entered as a variable for the motion control, and the shaft angle is the only variable that can be detected from the stepper motor encoder, the screw linear distance ( X ) as a function of the shaft angle in degrees can be expressed as follows Pitch(screw) 0.5 X (3.12) S (3.13) It is important to mention that M6 standard screw has a pitch of 1 mm, whereas the screw which is used in this pitching system is specially designed and manufactured to have 0.5 mm. The reason for that is to have less pitch angle difference corresponding to the linear motion, and therefore higher precision in controlling the pitch angle. Now that the linear motion is detected, it is still required to find the pitch angle as a function of the linear motion. Considering one branch as shown in figure (3.16) 23

33 Figure 3.16 Animation of linear and angular motion The system can be mathematically presented as follows X d 25cos 23cos (3.14) 23sin 25sin 16 (3.15) From the initial position ( X =0): d 25cos0 23cos the equation can be rephrased as: X cos 23cos (3.16) From equation sin 16 sin 1 (3.17) 25 Substituting in equation

34 1 23sin 16 X cos sin 23cos (3.18) 25 X is required in order to define the function in LabVIEW program. Since it is too complicated to mathematically solve for the inverse function, X can be solved for using curve fitting method. Using Excel, an approximate function can be expressed as a polynomial of the power 6 as follows ( X ) X 7 10 X 3 10 X X X X 2 (3.19) Figure 3.17 Pitch angle function using curve fitting method 25

35 4. Wind Tunnel Testing The purpose of this project is to design and characterize a small wind turbine model. Now that the design issues and considerations are discussed in details, the experimental set up and the characteristic curves of the wind turbine are summarized in this chapter. 4.1 Experimental Setup Figure 4.1 portrays the system setup Figure 4.1 Circuit Diagram The circuit diagram illustrates the connections and the information flow in the system, basically the inputs and the outputs of the stepper motor and the generator. From the generator, it is required to detect the rotational speed, voltage and current values which 26

36 are necessary for the characterization of the wind turbine (( C - λ) curves). Simultaneously, a voltmeter and an ammeter are used, in order to check that the readings displayed on LabVIEW program comply with the measured values. The electric output power is simply calculated by the multiplication of both variables P=VI. For further details regarding the electrical control circuit which was used for the load characterization, please refer to [ 13]. For the stepper motor, it is required to track the pitch angle position following the correlations explained in section This happens by sending and receiving digital signals using LabVIEW pitch control program as shown in the circuit diagram. The wind turbine model is fixed facing the incoming wind as shown in figure (4.2). The wind speed can be controlled using the control panel over the range (1 m/s - 50 m/s), but for safety considerations concerning the blade durability, a maximum wind speed of 14 m/s was considered in this project. P Figure 4.2 The wind turbine positioned within the wind tunnel 27

37 Additionally, two computers were used for the characterization process, one for controlling the stepper motor (pitch angle control) and the other for detecting the rotational speed and the power output of the generator. Before starting the test run, the initial pitch angle 20 is set (measured at the blade root) as shown in figure 4.3. The figure represents the smallest pitch angle which can be reached in the system (angle between the chord the rotor plane). Figure 4.3 Setting of the initial pitch angle 4.2 Test Procedure and Data Recording After setting up the system, the experiment started by operating the wind tunnel at low wind speeds to find out the cut-in speed. The cut-in speed was 3.5 m/s. However, experimental measurements were first conducted at wind speed 4 m/s to avoid high fluctuation in frequency readings. At each wind speed, the pitch angle is changed (starting at 20 ) with a steady increment of 1. At each pitch angle setting, the rotational speed is changed by changing the load from 5 Ω to 50 Ω with a steady increment of 5 Ω. At each load setting the power output is measured, and finally the ( C - λ) curves are plotted. For further details regarding the load characterization, please refer to [ 13]. P 28

38 It was found out after repeating the previous procedure over the wind speed range 4 m/s 9 m/s, that the optimum pitch angle is 21 for all wind speeds. 4.3 Characteristic Curves Figure (4.4) illustrates the ( C - λ) curves at different pitch angles at wind speed 9 m/s P Figure 4.4 (Cp- λ) curve at various pitch angles at wind speed 9 m/s As shown in figure 4.4, the maximum C occurred at pitch angle 21 ( C (max) =0.022, P maximum power output = 2.52 Watt). The optimum tip speed ratio was =3.21. The maximum power output values at wind speeds up to 9 m/s were used to plot the first part of the characteristic power curve as shown in figure (4.5). Figure 4.5 depicts the wind turbine power characteristic curve. It is important to mention that the wind speed 9 m/s is considered as the presumed rated wind speed. Therefore, the power characteristic curve does not represent the actual system s limitations, but the P 29

39 assumed limitations. A comprehensive accurate characterization of the wind turbine model can be recommended for future work. Figure 4.5 Wind turbine model power curve 30

40 Below the rated power region (4 m/s - 9 m/s), the pitch angle is fixed at 21 and the optimum load [ 13] (corresponding to the optimum rpm) is set at each wind speed, in order to track the C P max locus as shown in figure 4.6 Figure 4.6 C locus P max At speeds above 9 m/s, the load is fixed, and the power output is controlled aerodynamically by increasing the pitch angle. In this project, pitch to feather control strategy was adopted in order to avoid high thrust load caused by the abrupt drop in lift coefficient and increase in drag coefficient (refer figure 2.5). Figures 4.7 and 4.8 portray the pitch angle vs. wind speed, and the blades feathering respectively. 31

41 Figure 4.7 Pitch angle vs. wind speed Figure 4.8 Feathering the blade 32

42 5. Results and discussion In this chapter, the results of the characterization procedure are discussed and analyzed. Additionally, some future improvements and recommendations are suggested for future work. 5.1 Data Analysis Table 5.1 illustrates the characteristic data at the rated wind speed 9 m/s and pitch angle 21. Resistor (Ω) Speed (rpm) Generator Voltage (V) Current (A) Power (W) 2,5 385, , , , , , , , ,5 875, , , , , , , , , , , , ,1374 6, , , , , , , , , , , , , , , , , , , , , , , , , , , Table 5.1 Characteristic data at rated wind speed 9 m/s and pitch angle 21 As shown in the table, the maximum power output takes place at a resistance of 10 Ω (corresponding speed of 950 rpm). The wind power available at wind speed 9 m/s can be calculated using equation 1.1 as follows * Watt P W *1.2 * * 56 2 C p

43 R V 2 nr 60V Table 5.2 shows the characteristic details for the power controlled region (9 m/s 14 m/s) Table 5.2 Characteristic details for the power controlled region Figure 5.1 illustrates the C P values at all wind speeds. CP max varies in both below and above rated wind speed (9 m/s). At wind speeds above 9 m/s, C P max decreases due to pitch control and this is desired to maintain constant power. At lower speeds such as 4 and 5 m/s, there is very little wind energy available and most [14] of it is needed to overcome frictional and electrical losses in the wind turbine. 34

44 Figure 5.1 C P max vs. Wind speed Figure 5.2 illustrates the variation of optimum tip speed ratio under different wind speeds. Figure 5.2 λ vs. Wind speed 35

45 As shown in the previous tables and figures, the values of C P are too low, and does not reach half of the expected value ( =6.7 for the original blade of MEXICO [10] project ). Following are some possible reasons for the unexpected results 1. The discrepancy in the initial blade angle setting between the three blades affects the aerodynamic efficiency. The initial pitch angle (20 ) is not exactly the same for all three blades, which means that each blade operates at a different angle of attack, and therefore different lift and drag coefficients. The reason of this discrepancy is that no precision measurement tool was used to set exactly the same angle for all blades and it was measured with the naked eye. As an example for the blade s angle discrepancy effect, the following figure 5.3 depicts the difference in power coefficient and tip speed ratio for two different setups. In the first setup, there was one blade which had about 10 discrepancy (measured with the naked eye) compared to the other two blades. In the second setup (which was the setup adopted for the characterization), the three blades were set at nearly the same angle (20 ). Figure 5.3 (Cp - λ) curves at first and second setup 36

46 The figure depicts the sensitivity to discrepancies and the necessity to use a precision measurement tool to ensure exactly the same pitch angle for all blades. 2. Any inaccuracy in the blade manufacturing or any discrepancies in the blade s curvature, twist angle or chord length will affect the aerodynamic efficiency of the wind turbine. 3. The inaccurate scaling of MEXICO wind turbine model (which will be handled thoroughly in section 5.2). 4. The system has high friction and electrical losses, and this can be observed clearly at lower wind speeds. 5. The components along the main shaft might be misaligned with each other, and one of the possible reasons is the absence of a fixation point or bearing close to the hub (see figure 3.15). The front part of the main shaft could be fixed to the nacelle to ensure a better alignment. 6. Further design considerations for the stability of the tower structure are desired, since high vibration of the machine at high wind speeds was observed. 7. There might be high electrical losses (wasted power) due to using a variable [15] resistor for the speed control. 37

47 5.2 Improvement As mentioned in the previous section, the highest optimum tip speed ratio as shown in [14] figure 5.2 is about half the optimum tip speed ratio of MEXICO project blade. One of the reasons for this considerable difference can be the wrong model scaling. Wrong scaling means that for the same pitch angle and at a specific point of computation, there are different chord lengths and twist angles. As a result, there are different aerodynamic forces and coefficients (lift and drag) compared to the original model. Figure 5.4 shows the original blade s span length and distances measured from rotor center. Figure 5.4 original blade s dimensions (mm) The dashed extension represents the design difference between the MEXICO project blade and its prototype, while the rest remains the same for both blades (see also figure 3.3). The comparison starts at the distance r DU 91, which is the distance from the rotor center to the first section of the airfoil family DU91 as illustrated in the decomposition of the blade (figure 3.3). The point of computation r DU 91 was selected since it is the first common section to both blades. Figure 5.5 illustrates the offset in both the chord length and the twist angle due to the wrong scaling. 38

48 Figure 5.5 Twist angle and chord length offset (experimental values are not published, but obtained from original source) The figure reveals an offset of 2 in twist angle and about 7% deviation in the scaled chord length at the point of computation r DU91, which means that both blades are facing different aerodynamic conditions. Compared to the prototype blade (figure 3.11), the ratios of the distances between the rotor center to blade root ( r root ) over the rotor center to the blade tip ( R ) in both blades differ considerably as follow 39

49 r root R (MEXICO model) r root R (Our model) And since the chord length and the twist angle are functions of the radius at the point of computation (measured from the rotor center), the same function for both blades should be maintained using the correct scaling in order to maintain similar aerodynamic conditions. And this can be done by either decreasing the rotor center to blade root distance ( r root ) by reducing the hub size, or by increasing the dimensions of the blade to match the same twist angle and chord length functions along the blade s span. As the internal space of the hub is already used to house the pitching mechanism, a recommendation could be to increase the blade size proportional to the original blade. The new distance from the rotor center to the blade tip can be calculated as follows: rroot R mm New blade length equals to mm 2020 Scale (Scale 5.75:1) All other dimensions including the chord lengths and blade twisting should follow the same scale. Figure 5.6 shows the recommended blade s dimensions. Figure 5.6 recommended model blade s dimensions (mm) 40

50 5.3 Recommendations Following are some future recommendations 1. Using a precision measurement tool to accurately set the initial blade angle 2. Setting of the initial blade angle below 20 (preferably at 0 ), in order to apply pitch to stall control method. 3. Ensure good lubrication for the bearings 4. Ensure a better alignment of the main shaft components by adding an extra bearing close to the hub and fix it to the nacelle, or by adding a roller support as shown in figure 5.7. This will also transfer the extra load on the screw nut to the nacelle body (see figure 3.15). Figure 5.7 Roller support 5. Further design considerations for the stability of the tower structure are recommended in order to eliminate vibrations as mentioned in section Using a more efficient method to control the rotational speed rather than a variable resistor or a rheostat. 41

51 6. Conclusion Small wind turbines can be very sensitive to small design mistakes or discrepancies in prototyping. Consequently, special attention to details and precise design issues should be taken into consideration when it comes to characterization of small wind turbines. Linear actuators or lead screws are convenient for pitch control systems, since they need small torque to overcome high pitching moment, which makes it more feasible and attractive compared to other automatic pitch control systems. Pitch to feather control is a convenient way to limit the wind turbine power output, since the high thrust load can be avoided. The disadvantage of pitch to feather technique is the need of the continuous and intensive pitch angle control in order to assure a constant power output. 42

52 References Optimal Rotor Tip Speed Ratio, M. Ragheb, 4/15/ Wind turbine control algorithms. Technical Report ECN-C , E.L. van der Hooft; P. Schaak; T.G. van Engelen, December Wind Energy Explained: Theory, Design and Application. James F. Manwell, Jon G. McGowan, Anthony L. Rogers, Fernando D. Bianchi, Hernán De Battista and Ricardo J. Mantz, April 2006, Wind Turbine Control Systems Principles, Modelling and Gain Scheduling Design 7. Aufbau und Inbetriebnahme einer Messeinrichtung für Modelluntersuchungen an Rotorblättern von Windkraftanlagen, Daniel Schütze, Berlin, Konstruktion und Aufbau einer Messeinrichtung für Modelluntersuchungen an WEA- Rotoren mit dem Hochlaufversuch, Staffan Wallmann, David Janke, Berlin, The MEXICO project (Model Experiments in Controlled Conditions): The database and first results of data processing and interpretation, H Snel, J G Schepers and B Montgomerie, Clancy, L.J. (1975), Aerodynamics, Pitman Publishing Limited, London 13. Design and Power Characterization of a Small Wind Turbine Model in Partial Load Region, Abdulkarim Abdulrazek, Oldenburg,

53 APPENDIX A 44

54 APPENDIX B 45

55 46

56 APPENDIX C 47

57 48

58 49

59 50

60 51

61 52

62 53

63 54

64 55

65 56

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67 58