Wind turbine blades, whether horizontally or vertically oriented, convert the energy of the air into shaft power known as torque.
Wind power generator efficiency can be increased through optimal blade design that captures and decelerates wind speed. Other important factors to consider include angle of attack, pitch angle and number of blades.
1. Angle of Attack
When designing a wind turbine blade design, angle of attack is an important factor as it influences aerodynamic properties such as lift and power. The relative airflow arriving at the blade is determined by wind radial velocity and angle of attack (Betz theory Section 5.1) .
Ideal tip speed ratio for rotors should be nine to ten, with minimal mechanical movement when pitching the blade. Modern horizontal axis rotors typically employ tip speeds ratios between nine and ten for two-bladed and six to nine for three-bladed models (Section 1).
Blades are optimized using computer analysis software such as ANSYS or SOLIDWORKS (Figures 4 and 5). A moving reference frame approach is employed, which involves varying translation and rotation velocities between cell zones in the mesh.
The optimal pitch and twist angle is determined by several factors, including design tip speed ratio, desired aerofoil angle of attack and site conditions. As the blade moves from its hub toward its tip, twist angle decreases from zero until zero at the tip; this reduces aerodynamic drag on the blade and improves power output efficiency for rotors.
Stall is an aerodynamic effect that occurs at high angles of attack when the boundary layer separates from the aerofoil surface. To avoid it, select a blade design which limits its maximum pitch angle while maintaining adequate lift levels.
You can reduce a turbine’s dynamic loads by decreasing its pitch angle during high wind speeds. This technique, known as feathering, can be done either by pitching the blade or decreasing its height.
In addition to aerodynamic loads discussed above, design loads on a turbine blade also include gravitational, centrifugal and gyroscopic forces as well as operational demands like breaking, connecting the generator to the blade and pitching. The magnitude of each load depends on the operational scenario under consideration; further details about these are presented in Chapter 6.
When designing the blades for a wind turbine, there are two primary methods to reduce power output at high operational wind speeds: stall regulation or pitch control. Stall regulation limits rotor speed by stopping the blade from rotating, while pitch control adjusts blade angle of attack in accordance with wind speed.
Both methods utilize a single control system to adjust the blades’ pitch angle, but they do so differently. Pitch control relies on hydraulic pressure for pitch control, usually with a spring acting as a failsafe should there be any loss in pressure. Hydraulic systems offer longer life, greater driving power for higher speed response, and low-maintenance backup in case of malfunction.
The main disadvantage of the pitch system is that it requires constant monitoring to make sure blades are pitched at an appropriate angle for current loads. This can be challenging in turbulent wind conditions when blades are constantly changing direction and position.
Pitch control is being explored with various technologies, such as a patented design that incorporates active pitching and stall regulation. A number of research institutes are studying these approaches.
According to some experts, the ideal blade designs feature a fixed bending moment of inertia that allows the turbine to convert most energy into rotational motion. This results in greater power for smaller turbine sizes and an improved capacity factor.
However, smart blades that change shape according to wind condition are being explored. These are based on similar concepts used in helicopter control and could reduce ultimate loads and fatigue loads as well as enhance dynamic energy capture.
This research is important as it shows that even a slight deviation in blade angle can result in significant power losses. Therefore, finding the optimal blade angle for each wind speed and load is critical to maximize energy production and guarantee wind turbines operate safely no matter what the weather conditions may be.
3. Number of Blades
Wind turbines are an effective way to produce electricity, but they come with some drawbacks. Not only are they expensive and in some places the wind might be too strong for them to function effectively, but their presence also causes harm to local wildlife.
When selecting a blade design for land, the number of blades should be tailored to maximize energy conversion efficiency.
Blade types range from flat to curved and rotor. Flat blades have been around since ancient times and remain popular today due to their broad shape that captures wind energy for rotation. Although not as efficient as curved or rotor blades, flat blades require less effort to build and use than their more complex counterparts.
Curved blades are another popular design, featuring a curved shape at the tip. These blades tend to be more efficient than flat ones due to increased air speed over a curved blade; this boost in speed helps it turn faster and thus increases energy production potential.
Most manufacturers are currently opting for three-blade designs, as these offer the highest efficiency of all options. However, some are experimenting with two-bladed options as these have less than 5% lower efficiency than their three-bladed counterparts and may be more cost effective for those seeking to utilize renewable energy sources.
In addition to these advantages, blades can be designed for reduced loads such as gyroscopic or operational ones by altering system parameters. These changes will affect both aerodynamic and gravitational properties of the rotor as well as its structure.
These changes can be achieved through optimization techniques such as aerofoil bending and multiple iterations to create optimal blade plans. The ideal blade plan will depend on the design tip speed ratio and number of blades (Figure 3).
Engineers can test different designs for wind turbine blades to find the optimal shape, size and length. This can be done through online computational fluid dynamic (CFD) simulation.
When designing a wind turbine blade design, material selection is critical for performance and longevity. It should be durable and resistant to wear, damage, and corrosion as well as capable of withstanding repeated loading. Common materials used in rotor blades include E-glass, carbon fiber, and epoxy resins.
When designing a rotor blade, the material used depends on its structural requirements and how power generation systems use it. E-glass is typically employed for the main body of the blade while carbon fibers have become increasingly popular due to their superior stiffness and strength. Furthermore, aramid (Kevlar) offers greater durability at higher costs when mechanical properties need improving.
E-glass is a reliable material for most applications, however its shortcomings must be taken into account. For instance, it can become vulnerable to fatigue loading which could significantly decrease its capacity to withstand dynamic stresses.
Furthermore, it can be brittle and challenging to weld, making it challenging to create a high-performance blade design.
To combat this issue, manufacturers often utilize thick aerofoil sections at the root region of rotor blades. These sections bear heavy loads and require high structural integrity to avoid deformation during service.
Another advantage of thick sections is their low drag, meaning they are less vulnerable to stalling when exposed to high wind speeds. This is essential because a stalling rotor may cause fluttering.
Finally, thick section aerofoils are less fragile than thin aerofoils and can be designed for higher efficiency by increasing their spar cap area. This reduces overall load bearing material requirements as well as allows significant weight reductions.
To prevent rotor stalling during high wind speeds, manufacturers can adjust the angle of attack by feathering or increasing the pitch angle. Feathing increases the blade’s angle of attack and reduces generated lift force; pitching decreases this angle of attack but increases lift force generated.
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