DESIGN OPTIMIZATION OF WIND TURBINE BLADE
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The objective of this project is to design a wind turbine that is optimized for the constraints that come with residential use. The main tasks of this project are: > To study the design process and methodology of wind turbine > Derive the Blade Element Momentum (BEM) theory then use it to conduct a parametric study that will determine if the optimized values of blade pitch and chord length create the most efficient blade geometry > Analyse different air-foils to determine which one creates the most efficient wind turbine blade.
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DESIGN OPTIMIZATION OF WIND TURBINE BLADE
- 1. DESIGN OPTIMIZATION OF WIND TURBINE BLADE Presented By - Pawan Rama Mali BASP-002
- 2. OBJECTIVE OF THE STUDY The objective of this project is to design a wind turbine that is optimized for the constraints that come with residential use. The main tasks of this project are: • To study the design process and methodology of wind turbine • Derive the Blade Element Momentum (BEM) theory then use it to conduct a parametric study that will determine if the optimized values of blade pitch and chord length create the most efficient blade geometry • Analyze different airfoils to determine which one creates the most efficient wind turbine blade.
- 3. STATEMENT OF THE PROBLEM • Wind turbines are machines that remove energy from the wind by leveraging the aerodynamic principals of lift and drag. Lift and drag forces move the turbine blades which convert kinetic wind energy to rotational energy. • The objective of turbine blade design is also to maximize the lift force on the blade and reduce drag so that the force on the blade that acts in the tangential direction is maximized. • In air turbine design, it is crucial to reduce the thrust on the turbine blades because it wastes energy and it requires a stronger blade to withstand its loading.
- 4. INTRODUCTION • “Rotary engine in which the kinetic energy of a moving fluid is converted into mechanical energy by causing a bladed rotor to rotate” • Turbine blades spin from the wind and make energy, instead of using energy to make wind • Wind rotates the turbine blades • spins a shaft connected to a generator • The spinning of the shaft in the generator makes electricity
- 5. WHY ? o Clean, zero emissions - NOx, SO2, CO, CO2 - Air quality, water quality - Climate change o Reduce fossil fuel dependence - Energy independence - Domestic energy—national security o Renewable - No fuel-price volatility
- 7. ORIENTATION Turbines can be categorized into two overarching classes based on the orientation of the rotor Vertical Axis Horizontal Axis
- 8. VERTICAL AXIS TURBINES Advantages • Omnidirectional – Accepts wind from any angle • Components can be mounted at ground level – Ease of service – Lighter weight towers • Can theoretically use less materials to capture the same amount of wind Disadvantages • Rotors generally near ground where wind poorer • Centrifugal force stresses blades • Poor self-starting capabilities • Requires support at top of turbine rotor • Requires entire rotor to be removed to replace bearings • Overall poor performance and reliability • Have never been commercially successful
- 9. HORIZONTAL AXIS WIND TURBINES • Rotors are usually Up-wind of tower • Some machines have down- wind rotors, but only commercially available ones are small turbines
- 10. COEFFICIENT OF POWER FOR LIFT AND DRAG TYPE TURBINES
- 12. ACTIVE VS. PASSIVE YAW • Active Yaw (all medium & large turbines produced today, & some small turbines from Europe) • Anemometer on nacelle tells controller which way to point rotor into the wind • Yaw drive turns gears to point rotor into wind • Passive Yaw (Most small turbines) • Wind forces alone direct rotor • Tail vanes • Downwind turbines
- 13. WIND TURBINES USE THE SAME AERODYNAMIC PRINCIPALS AS AIRCRAFT
- 14. EFFICIENCY OF WIND TURBINE
- 15. BETZ LIMIT
- 17. TIP-SPEED RATIO Tip-speed ratio is the ratio of the speed of the rotating blade tip to the speed of the free stream wind. There is an optimum angle of attack which creates the highest lift to drag ratio. Because angle of attack is dependant on wind speed, there is an optimum tip-speed ratio ΩR V TSR = ΩR R Where, Ω = rotational speed in radians /sec R = Rotor Radius V = Wind “Free Stream” Velocity ΩR R
- 18. Performance Over Range of Tip Speed Ratios • Power Coefficient Varies with Tip Speed Ratio • Characterized by Cp vs Tip Speed Ratio Curve 0.4 0.3 0.2 0.1 0.0 Cp 121086420 Tip Speed Ratio
- 19. TWIST & TAPER • Speed through the air of a point on the blade changes with distance from hub • Therefore, tip speed ratio varies as well • To optimize angle of attack all along blade, it must twist from root to tip
- 20. ROTOR SOLIDITY Solidity is the ratio of total rotor planform area to total swept area Low solidity (0.10) = high speed, low torque High solidity (>0.80) = low speed, high torque R A a Solidity = 3a/A
- 21. NUMBER OF BLADES – ONE • Rotor must move more rapidly to capture same amount of wind – Gearbox ratio reduced – Added weight of counterbalance negates some benefits of lighter design – Higher speed means more noise, visual, and wildlife impacts • Blades easier to install because entire rotor can be assembled on ground • Captures 10% less energy than two blade design • Ultimately provide no cost savings
- 22. NUMBER OF BLADES - TWO • Advantages & disadvantages similar to one blade • Need teetering hub and or shock absorbers because of gyroscopic imbalances • Capture 5% less energy than three blade designs
- 23. NUMBER OF BLADES - THREE • Balance of gyroscopic forces • Slower rotation – increases gearbox & transmission costs – More aesthetic, less noise, fewer bird strikes
- 25. BLADE ELEMENT MOMENTUM (BEM) THEORY • BEM theory is a compilation of both momentum theory and blade element theory. • Momentum theory, which is useful in predicted ideal efficiency and flow velocity, is the determination of forces acting on the rotor to produce the motion of the fluid. • Blade element theory determines the forces on the blade as a result of the motion of the fluid in terms of the blade geometry.
- 26. ASSUMPTIONS FOR MOMENTUM THEORY • Blades operate without frictional drag. • A slipstream that is well defined separates the flow passing through the rotor disc from that outside disc. • The static pressure in and out of the slipstream far ahead of and behind the rotor are equal to the undisturbed free-stream static pressure (p1=p3). • Thrust loading is uniform over the rotor disc. • No rotation is imparted to the flow by the disc.
- 27. ASSUMPTIONS FOR BLADE ELEMENT THEORY • There is no interference between successive blade elements along the blade. • Forces acting on the blade element are solely due to the lift and drag characteristics of the sectional profile of a blade element.
- 28. TIP LOSS FLOW
- 29. DESIGN CONSTRAINTS • SIZE OF THE WIND TURBINE • HEIGHT OF THE STRUCTURE • BLADE LENGTH • NOISE EMISSIONS
- 30. BEM RESULTS • The average wind speed at the maximum allowable height of 11.5 meters is about 5 m/s with a corresponding blade radius of 2.5 meters. • The tip speed ratio is initially defined as 7 to get a baseline value of performance and will be varied in the parametric study to determine the ideal ratio. • The coefficient of lift CL is initially defined as 0.88 based on the value of the coefficient of lift at the maximum glide ratio (CL/CD).
- 31. OPTIMIZED DIMENSIONLESS WIND TURBINE BLADE GEOMETRY Blade Segment - 1 2 3 4 5 6 7 8 9 Relative radius r/R 0.150 0.250 0.350 0.450 0.5500.6500.750 0.850 0.950 Speed ratio X 1.050 1.750 2.450 3.150 3.8504.5505.250 5.950 6.650 Angle, optimal phi 29.06919.83014.80211.7429.7078.2647.190 6.360 5.701 Pitch bet a 22.06912.830 7.802 4.742 2.7071.2640.190-0.640 - 1.299 Rel. chord length c/R 0.180 0.141 0.111 0.090 0.0750.0640.056 0.050 0.045
- 32. PERFORMANCE OF THE INITIALLY OPTIMIZED WIND TURBINE
- 33. VARYING TIP SPEED RATIOS
- 35. CONCLUSION • The tip speed ratio of the turbine should be designed for a tip speed ratio less than what it will be experiencing. • Blades designed for larger tip speed ratios have a larger range of efficient speed ratios. • Based on a tip speed ratio of 10 and the conclusions mentioned above, designing the blade for a tip speed ratio of 8 would create the optimal blade.
- 36. CONCLUSION • The allowable size of the turbine creates constraints that reduce the number of parameters required to maximize the efficiency of the turbine. • For a small wind turbine, the allowable size of the turbine creates constraints that reduce the number of parameters required to maximize the efficiency of the turbine. • The main parameters constrained due to the size requirement are the length of the blade and the height of the center of the hub. While it was shown that the coefficient of power is not affected by either wind velocity or blade length alone, power output will increase with an increase in both parameters.
- 37. FUTURE SCOPE • The structural modelling can be improved by using realistic models of composite blades where material properties and topology will be considered with greater importance. • The structural optimization method can be modified using more structural theory models like classical lamination theory, linear (eigenvalue) buckling theory and also some in depth finite- element model analysis. • Composite layup analysis can be extended for optimization for minimizing blade mass subjected to constraints like maximum allowable laminae stresses, blade tip deflection, panel buckling stresses and separation of blade natural frequencies.
- 38. THANK YOU
Notes de l'éditeur
- Wind energy is a renewable source of energy, and is considered renewable because it is derived from the sun and is capable of being replenished on a reasonable time scale. Although wind is a zero emissions electrical generation option, there are emissions in the construction and development of wind projects—concrete, transportation of components, etc.
- Prevailing winds are caused by the temperature differences between the Earth’s poles and its equatorial regions, as well as Earth’s rotation. The Earth’s atmosphere has several very large and steady prevailing patterns, such as the polar easterlies and the northeast trade winds. Winds are named based on the direction they originate from. In North America, one of the prevailing dominant wind paths track in an arc from the prairies to the Great Lakes and the eastern seaboard – this wind travels in a westerly direction. Wind energy is also affected by other factors. Air currents move faster and more consistently at higher altitudes–think of the blustery conditions at the tops of tall buildings or on mountain tops. Similarly, wind tends to gather energy when it moves unimpeded over longer distances, which is why very flat regions, such as the prairies, tend to be highly exposed to intense winds.
- Wind turbine efficiency is quantified by a non-dimensional value called the coefficient of power CP, which is the ratio of power extracted from the wind, P, to the total power in wind crossing the turbine area. Equation shows that the coefficient of power is a function of the air density ρ, the area inscribed by the turbine blade A, and the wind speed v1.
- Consider that if all of the energy coming from wind movement through a turbine was extracted as useful energy the wind speed afterwards would drop to zero. If the wind stopped moving at the exit of the turbine, then no more fresh wind could get in - it would be blocked. In order to keep the wind moving through the turbine there has to be some wind movement, however small, on the other side with a wind speed greater than zero. Betz' law shows that as air flows through a certain area, and when it slows from losing energy to extraction from a turbine, it must spread out to a wider area. As a result geometry limits any turbine efficiency to 59.3%.
- A designer of a wind turbine must find an ideal balance between these two extremes Under Betz Law an ideal wind turbine would slow down the wind by 2/3 of its original speed (the capture of 59.6% of the wind’s speed).
- Most common design is the three-bladed turbine. The most important reason is the stability of the turbine. A rotor with an odd number of rotor blades (and at least three blades) can be considered to be similar to a disc when calculating the dynamic properties of the machine. A rotor with an even number of blades will give stability problems for a machine with a stiff structure. The reason is that at the very moment when the uppermost blade bends backwards, because it gets the maximum power from the wind, the lowermost blade passes into the wind shade in front of the tower.
- The airfoils chosen for use in this turbine blade are NACA 23012 and NACA 4412. The NACA 23012 is a 5-digit series NACA cambered airfoil which is known for having a relatively high maximum coefficient of lift. The NACA 4412 is an airfoil used in older wind turbines such as the Windcruiser turbine made by Craftskills Enterprises. The lift and drag curves for these wind turbines are included in Appendix
- BEM theory is a compilation of both momentum theory and blade element theory. Momentum theory, which is useful in predicted ideal efficiency and flow velocity, is the determination of forces acting on the rotor to produce the motion of the fluid. This theory has no connection to the geometry of the blade, thus is not able to provide optimal blade parameters. Blade element theory determines the forces on the blade as a result of the motion of the fluid in terms of the blade geometry. By combining the two theories, BEM theory, also known as strip theory, relates rotor performance to rotor geometry.
- The following assumptions are made for momentum theory:
- BEM theory does not account for the interaction of shed vortices with the blade flow near the blade tip. While air is flowing over the blade, the pressure under the blade decreases relative to the pressure on the top of the blade. At the tip of the blade, the air will flow radially inward over the tip, reducing the circulation of the air, which reduces the torque and turbine efficiency, as shown in Figure
- The size of the wind turbine is the first constraint in designing a residential-sized wind turbine. Many towns have different zoning requirements for the maximum allowable height of an erected structure and the minimum required lot size that contains a wind turbine. Another parameter of the wind turbine design that is constrained by the allowable height of the structure is the size of the blades. Since the maximum theoretical power output of a wind turbine is proportional to the square of the blade length (Equation (19)), it is also important to maximize the blade length There is a slight trade-off between the height of the turbine and the blade length since the higher the blades are from the ground, the higher the wind speed is that they will encounter The final constraint regarding residential wind turbine use is the requirement that it cannot be overly loud when operating. According to Tangler, airfoil shape pure- tone noise can result from the presence of significant laminar separation bubbles interacting with the trailing edges, which is more prevalent in small turbines because of the lower Reynolds number. While the maximum sound level allowed for a wind turbine is defined to be 60 dB
- In order to start reducing the number of blade design variables, the constraints of a small wind turbine must first be translated to input values of the BEM analysis. The main constraint of a small wind turbine is the allowable height of the wind turbine which constrains both the wind speed and the blade length. Based on the assumptions made in the previous section, the average wind speed at the maximum allowable height of 11.5 meters is about 5 m/s with a corresponding blade radius of 2.5 meters.
- Table 1 below contains the pitch angle and relative chord length for each of the 9 blade segments (10 segments minus the inner-most segment for the hub). The values in the table are dimensionless so that the distributions of pitch and chord length can be applied to a blade of any size. Finally, using the spreadsheet shown in Appendix B, the power generated from the wind turbine is calculated and the coefficient of power is then determined by comparing the calculated power extracted by the wind turbine with the total power contained in the wind. Using a constant wind velocity of 5 m/s, which was determined to be the average wind speed for the southeast Connecticut shoreline at a height of 11.5 meters, the rotational velocity of the turbine was changed until it created a tip speed ratio of about 7
- From the data in Figure 15, it is evident that in terms of designing turbine blades, the blades should be optimized for tip speed ratios slightly less than what is anticipated. In addition to operating at peak efficiency, if the wind turbine is operating at tip speed ratios greater than what it was designed for, the decrease in performance for ratios greater than 12 is much more gradual than the decrease for ratios less than 7.
- Figure contains the results of varying the blade pitch and chord length distributions based on optimizing for a range of tip speed ratios. The blades that were created with tip speed ratios less than 4 would not converge using the BEM solver. The blade design for a tip speed ratio of 4 was only able to converge for two data points, which did not include the supposed optimal conditions. However, the blades created for tip speed ratios of 5 through 8 were able to converge for a range of speed ratios, allowing a maximum coefficient of power for each blade to be calculated. The trend observed for a blade optimized for a tip speed ratio of 7, where the peak performance happens at a higher ratio, is common for all of the blades. One additional pattern that is observed in Figure 16 is that as the blades increase from values of X=4 to X=8, the peak performance occurs at increasingly greater ratios than the optimized ratio. For example, the blade made for X=5 has a peak coefficient of power at X=6, where the blade optimized for X=8 has a peak coefficient of power at X=10.5. The variable causing the separation between designed-for and actual peak tip speed ratios has a greater effect as the ratio increases. Another trend observed from Figure is that the blades with higher tip speed ratios have a more gradual slope of increasing coefficient of power, compared to the blades made for lower tip speed ratios. Based on this trend, designing for higher tip speed ratios is preferred because there is less of a penalty for having the tip speed ratio decrease below the desired value.
- The airfoil is another parameter that can be varied to optimize a blade designThe original airfoil used was NACA 23012, which is a standard cambered airfoil. The second airfoil that will be used for comparison is the NACA 4412. The NACA 4412 airfoil is different than the NACA 23012 in that the maximum glide ratio occurs at an angle of attack of 6 degrees, not 7 degrees like the NACA 23012. Another difference between the two that will reshape the blade is the coefficient of lift at the maximum glide ratio. The corresponding coefficient of lift for the NACA 4412 is about 1.05 instead of 0.88 Both airfoils seem to have beneficial characteristics that are highly dependent on the tip speed ratio which they encounter. However, since the ratios greater than 7 have the highest coefficient of power, it is apparent that the NACA 4412 airfoil is the more desirable of the two. The only detriment of the NACA 4412 airfoil is that if the ratio reduces to 6 or less, there is more of an abrupt decrease in efficiency than with the NACA 23012. . .
- The results of the variable tip speed ratio trade study have led to several conclusions when considering how exactly to shape the blade for optimal performance. First, the tip speed ratio of the turbine should be designed for a tip speed ratio less than what it will be experiencing. The second conclusion is that blades designed for larger tip speed ratios have a larger range of efficient speed ratios. While the average wind speed is known, the tip speed ratio that corresponds to this speed cannot be known without further in- depth analysis or testing. However, in order to obtain an approximate value of the average tip speed ratio experienced by this turbine, one can be approximated from the data shown in the paper by Vick. According to Vick’s paper, at 5 m/s, one can expect a tip speed of about 50 m/s, which means the average tip speed ratio is 10. Based on a tip speed ratio of 10 and the conclusions mentioned above, designing the blade for a tip speed ratio of 8 would create the optimal blade.
- By equating the thrust force on the rotor with the axial momentum force, one is able to solve for the axial interference factor . By equating the torque force with the angular momentum force on the rotor, one is also able to solve for the tangential interference factor . The assumption which was made without much prior knowledge was the value of tip speed ratio. Since the effect that the tip speed ratio would have on the turbine performance was not known, a parametric study was conducted which demonstrated that based on the methods of defining the pitch angle and chord length, the tip speed ratio that is chosen to shape the blade should be less than the expected value that the turbine encounters. Doing so will ensure the turbine operates at peak efficiency. Based on an approximate value of tip speed that corresponds to the wind speed, the average tip speed ratio was determined. From the average tip speed ratio calculated of about 10, and following several of the observations that were concluded from the parametric study, it was determined that the optimal blade should be designed for a tip speed ratio of 8.
- The genetic algorithm optimization method can be replaced with pattern search optimization algorithm in future studies since that are much faster and deterministic than genetic algorithm. Improvement in the design optimization code can be made possible by adding more design variables and constraint considerations in order to get the real time results.