1. Basic structure of horizontal axis wind turbine
At present, horizontal axis wind turbines (Figure 1) are mostly used in wind turbines, and propeller-type blades are mostly used. For example, most of the wind turbines used in wind power generation are propeller-type horizontal axis wind turbines.
Common propeller wind turbines are mostly two-blade or three-blade, and there are also a small number of single-blade or four-blade or more. In order to improve the starting performance and reduce the aerodynamic loss as much as possible, the structure with high blade root strength, low blade tip strength and helical angle is mostly used. The blades of this wind turbine are similar to the propeller of an airplane, so they are also called blades.
The wind turbine mainly includes the wind wheel, tower, nacelle and other parts, as shown in Figure 2. The wind wheel is composed of a wheel and several blades (paddles) installed on the hub, which is the part of the wind turbine to capture wind energy; the tower is the support structure of the wind turbine, which ensures that the wind wheel can have a higher wind speed above the ground. Operation; in order to make the wind direction face the rotation plane of the wind rotor, the horizontal axis wind turbine needs to be equipped with a steering device for direction control. The steering device, control device, transmission mechanism and generator are all placed in the engine room.
Generally speaking, the larger the design power of the wind turbine, the larger the diameter of its rotor. For example, the rotor diameters of GE’s wind turbines with different power levels are shown in Figure 3.
2. The principle of horizontal axis wind turbine
1) Wing shape and force
The blades of modern wind turbines are similar in form to the wings of an airplane, called aerofoils. There are two main types of airfoils: symmetrical airfoils (which are symmetrical in cross-section) and asymmetrical airfoils. The shape characteristics of the airfoil include: a distinctly raised upper surface: a rounded head facing the direction of incoming flow, known as the leading edge of the wing; and a pointed or sharp tail, known as the trailing edge of the wing. A cross-section of a common asymmetric airfoil is shown in Figure 4.
Since most of the airfoils are not straight, but have a certain curvature or bulge, the chord line is usually used as the guideline for measurement. The angle between the airflow direction and the airfoil directrix is called the angle of attack (a in Figure 5). When the incoming flow is toward the underside of the airfoil, the angle of attack is positive.
The force on the blade in the airflow comes from the action of the air on it. The force from the airflow on the blade can be equivalently decomposed into two directions: the component caused by the airflow direction is called drag, and the component perpendicular to the airflow direction is called lift.
In Figure 5, the direction v refers to is the direction of the airflow, the shaded part represents the cross section of the blade, the blade will be subjected to the force F from the airflow, and if F is decomposed into two components as shown in the figure, it is the same as the airflow direction The component FD of is the drag force, and the component FL perpendicular to the direction of the airflow is the lift force.
In aerodynamics related to aircraft design, lift is the force that drives an aircraft off the ground and is therefore called lift. In practical applications, the lift may also be a lateral force (as on a sailboat) or a downward force (as on the spoiler of a racing car). When the angle of attack is 0″, the lift force is minimal. When the direction of the airflow is perpendicular to the surface of the object, the drag force on the object is the greatest.
The pressure of the air has a certain corresponding relationship with the speed of the airflow. The faster the flow rate, the lower the pressure. This phenomenon is called the Bernoulli effect. For the airfoil shown in Figure 6, the air flow on the convex part of the upper surface is faster, causing the air pressure on the upper surface to be significantly lower than that on the lower surface, thereby producing an upward “suction” effect on the airfoil object and increasing the lift.
The purpose of the airfoil design is to obtain the appropriate lift or drag to propel the wind turbine to rotate. Both lift and drag are proportional to wind strength. The blades of the wind turbine in the wind, under the combined action of lift, resistance or both, make the wind wheel rotate and output mechanical power on its shaft.
The angle of attack is related to the installation angle of the blade. The installation angle of the blade is called the pitch angle, sometimes also called the pitch, and is often represented by the letter 8. When the rotor rotates, the blades also move relative to the airflow in the direction perpendicular to the airflow, so the actual angle of attack a is different from the angle of attack when the blades are stationary.
As shown in Figure 7, the x-axis represents the direction of airflow movement, the rotating surface formed by the y-axis with the coordinate origin as the center represents the rotating surface of the wind rotor, and the angle between the direct line of the blade and the rotating surface of the wind rotor is called the blade installation angle , the pitch or pitch angle. ) axis direction represents the moving direction of a certain cross-section of the blade when the rotor rotates. If the rotating blade is used as the reference frame, there is a relative motion between the airflow and the blade that is opposite to the y-axis direction. Considering the actual movement of the airflow along the x-axis direction, the action direction of the airflow relative to the moving blade is shown in the figure Wr shown. Therefore, for the same horizontal wind, the angle of attack when the blade is rotating is different from the angle of attack when the blade is stationary.
The wind turbine can be a lift device (that is, the lift drives the wind wheel), or it can be a resistance device (the resistance drives the wind wheel). The designer generally likes to use the lift device because the lift force is much larger than the resistance force.
2) Wind energy utilization factor and wind turbine efficiency
If all the kinetic energy of the wind blowing to the rotor is absorbed by the blades, the air will stand still after passing through the rotor. As we all know, this is impossible. Even the wind energy passing vertically through the rotating surface of the wind rotor will not be absorbed by the wind rotor, so it is impossible for any type of wind turbine to convert all the contact wind energy into mechanical energy, and the wind energy capture efficiency is always less than 1.
The ratio of the energy that the wind turbine can absorb from the wind to the total wind energy in the area swept by the wind rotor (the energy when the airflow is not disturbed by the wind rotor) is called the wind energy utilization coefficient. The wind energy utilization coefficient Cp can be expressed by formula (1)
In the formula, P is the shaft power actually obtained by the wind turbine (W); ρ is the air density (kg/m3); s is the swept area of the wind turbine (m²); v is the upstream wind speed (m/s).
The larger the Cp value, the greater the proportion of energy that the wind turbine can obtain from the wind, and the higher the wind energy utilization rate of the wind turbine. The wind energy utilization factor mainly depends on the design (such as angle of attack, pitch angle, blade airfoil) and manufacturing level of the wind rotor blades, and is also related to the rotational speed of the wind turbine. The Cp value of high-performance propeller wind turbines is generally around 0.45.
The efficiency of the wind turbine also needs to consider the mechanical loss of the wind turbine itself, which is not a concept with the wind energy utilization coefficient.
3) Tip speed ratio and volume ratio
The ratio of the tip rotation rate of the blade to the undisturbed upstream wind speed is called the tip speed ratio, and is often represented by the letter n, that is,
In the formula, n is the rotational speed of the wind rotor (r/min): R is the radius of the blade tip (m); v is the upstream wind speed (m/s); ω is the rotational angular velocity of the wind rotor (rad/s).
The corresponding relationship between the wind energy utilization coefficient Cp and the wind turbine tip speed ratio is shown in Figure 8, where β is the pitch angle. It can be seen that for a given pitch angle, when the tip speed ratio a takes a certain value, the Cp value is the largest, and the tip speed ratio corresponding to the maximum value of Cp is called the optimal tip speed ratio.
In order to maintain the maximum value of Cp, when the wind speed changes, the speed of the wind turbine also needs to change accordingly, so that it runs at the optimum tip speed ratio. For any given wind turbine, the optimum tip speed ratio depends on the number of blades and the width of each blade. For modern low volume ratio wind turbines, the optimum tip speed ratio is between 6-20.
“Solidity” (sometimes called solidity) indicates the percentage of “solid” in the swept area. A multi-blade wind turbine has a high volume ratio, so it is called a high volume ratio wind turbine; a wind turbine with a few narrow blades is called a low volume ratio wind turbine.
To absorb energy efficiently, the blades must interact as much as possible with the wind passing through the swept area of the rotor. High volume ratio, multi-bladed wind turbine blades interact with almost all wind at very low tip speed ratios; low volume ratio wind turbine blades must operate at very high speeds in order to interact with all passing wind “Fill” the swept area. If the tip speed ratio is too low, some wind will blow directly across the swept area of the rotor without interacting with the blades: if the tip speed ratio is too high, the wind turbine will create too much resistance to the wind and some airflow will bypass the wind machine flows.
Multiple blades can interfere with each other, so wind turbines with high volume ratios are generally less efficient than wind turbines with low volume ratios. Among the low volume ratio wind turbines, the three-blade rotor is the most efficient, followed by the two-blade rotor, and finally the single-blade rotor. However, wind turbines with multiple blades generally produce less aerodynamic noise than wind turbines with fewer blades.
The mechanical energy absorbed by the wind turbine from the wind is equal in value to the product of the angular velocity of the blade and the moment the wind acts on the rotor. For a certain wind energy, if the angular velocity is small, the torque will be large; otherwise, if the angular velocity is large, the torque will be small. For example, the output power of low-speed wind turbines is small and the torque coefficient is large. Therefore, wind turbines used for grinding surfaces and lifting water often use multi-hour wind turbines. High-speed wind turbines have high efficiency and high output power, so wind power generation often uses low-volume ratio high-speed wind turbines with 2-3 blades.
4) Relationship between working wind speed and power
The expression for converting wind energy captured by wind turbines into mechanical power output is:
In the formula, Pm is the output power of the wind turbine (W): Pw is the power of the wind (W) Cp is the wind energy utilization coefficient: ρ is the air density (kg/m3): A=πR² is the wind turbine blade sweep area (m²) , where R is the radius of rotation of the rotor (m), and vw is the wind speed (m/s)
The output power of the wind turbine is related to the air density ρ, the wind speed vw, the blade radius R and the wind energy utilization coefficient Cp. Since air density, wind speed, blade radius, etc. cannot be controlled in real time, in order to maximize wind energy capture, the only control parameter is the wind energy utilization coefficient Cp.
In reality, wind turbines do not work well at all wind speeds. Various types of wind turbines usually have a design wind speed, or rated wind speed. At this wind speed, the working conditions of the wind turbine are the most ideal.
When the wind turbine is started, there is a minimum torque requirement. If the starting torque is less than this minimum torque, it cannot be started. The starting torque is mainly related to the installation angle of the impeller and the wind speed, so the wind turbine has a starting wind speed, which is called the cut-in wind speed.
The wind speed at which the wind turbine reaches the nominal power output is called the rated wind speed. The wind turbine provides rated or normal power at this wind speed. When the wind speed increases, the adjustment system can be used to keep the output power of the wind turbine constant.
When the wind speed exceeds the technically specified maximum allowable value, the wind turbine is in danger of being damaged. Based on safety considerations, mainly the tower safety and the strength of the wind turbine), the wind turbine should be stopped immediately. This shutdown wind speed is called the cut-out wind speed. Countries around the world have formulated different effective wind speed ranges and different cut-in wind speeds, rated wind speeds and cut-out wind speeds of wind turbines according to their own wind energy resources and operating experience of wind turbines.
For the wind energy conversion device, the available wind energy is within the effective wind speed range between the cut-in wind speed and the cut-out wind speed. The wind energy in this range is the effective wind energy, and the average wind power density within this wind speed range is called the effective wind power density. .
The relationship between wind turbine output power and wind speed is shown in Figure 9.