In the figure below, the blades are listed in eight positions from 0 degrees to 315 degrees. The wind enters from the left. The light blue vector v is the wind speed, and the green vector u is the linear velocity inverse of the blade's circular motion. direction (that is, the airflow speed felt by the blade when there is no wind), the blue vector w is the resultant airflow speed felt by the blade (that is, the relative wind speed), and the purple vector L is the lift force experienced by the blade.
Let’s analyze the stress on the blades at these eight angles. At the positions of 90 degrees and 270 degrees, the relative wind speed does not produce lift. At the other six positions, the lift received by the blades can all move. The direction produces a torque force, which is why the Darieu wind turbine can rotate under the wind.
In fact, the situation is much more complicated. The previous analysis diagram is an ideal state, which is the state when the ideal tip speed ratio and no resistance from the blades are present. The torque force used by the blades to push the rotor to rotate is the component of the combined force of lift and drag in the forward direction of the blades. Let’s take the situation at 315 degrees to analyze the situation with resistance. The black vector D in the figure is the resistance experienced by the blades. The brown vector F is the combined force of lift L and resistance D. The component force M of this force in the forward direction of the blades is M. This is the actual torque force. Obviously, the torque force at this time is significantly less than the ideal condition.
And at angles near 180 degrees and 270 degrees, the combined force of lift and drag produces a reverse torque force.
The Darieux wind turbine has a greater output force only when the blades are near 360 degrees and 180 degrees. Even so, it can only operate when the tip speed ratio is above 3.5. This can be illustrated by the following figure.
In the figure on the left, the blade is affected by the relative wind speed W to generate lift L and drag. D. The angle between the relative wind speed W and the blade chord line, that is, the angle of attack α of the blade, is about 14 degrees. The relative wind speed W is composed of the wind speed V and the blade movement speed u. At this time, the blade movement speed is about 4 times the wind speed, that is The tip speed ratio is 4. The resultant force of lift L and resistance D is F, and the moment force exerted by this force on the wind wheel is M, which is the force that drives the wind wheel to rotate. When the tip speed ratio is 4, the blade can generate a moment to push the rotor when running on the windward or leeward side. Only near the two sides (90 degrees and 180 degrees), the lift force is very small and there will be a small negative impact. directional moment.
The wind speed on the right side of the figure has doubled, but the speed of the blade movement has not changed. The tip speed ratio is about 2, and the angle of attack α of the blade is about 27 degrees. At this time, the blade is working in a stall state. , the lift L generated by the blades decreases significantly, but the resistance D increases greatly. The moment force M generated by the wind wheel is negative, which prevents the rotation of the wind wheel. At this time, the blades generate negative moments in most positions. For most common airfoils, when the tip speed ratio is less than 3.5, the blades basically generate no force to push the wind wheel to rotate.
It is difficult for the Darrieux wind turbine to operate under low wind speeds. Only under higher wind speeds can the wind turbine rotate at a tip speed ratio of 3.5 or above to operate normally
, higher power output can be obtained when the tip speed ratio is 4-6. In order to reduce drag and increase lift, the cross-sectional shape of the wind turbine blades isShape (airfoil) selection and surface finish requirements are relatively high. Since the Darieu wind turbine cannot rely on lift when the tip speed ratio is below 3.5, can it operate on drag? Since each fin is evenly fixed on the circumference of the wind wheel, the resistance moment generated by each fin due to the wind is not large and the total moment combined by each fin is very small. Even if a certain moment can be generated at a certain angle, it may not be generated at another angle. Reverse torque is generated, so the Darieu wind turbine cannot start automatically by wind power alone. It must be started by external force to make the blade tip speed ratio reach more than 3.5 before it can operate with lift. The typical Darieu wind turbine wing is not straight, but curved, and the two wings form a φ shape. The picture below shows a Darieu wind turbine model.
Today’s Darieu wind turbines mostly use straight blades, which some people call H-shaped wind turbines. The number of blades of H-type wind turbines is generally 2 to 6.
The blades of Darieu wind turbines are fixed on the rotating shaft through both ends or the middle, which is beneficial to increasing the mechanical strength and can be made very lightweight; The Darieux wind turbine is not top-heavy, has lower requirements on the tower, is suitable for fixing with cables, is easy to install, and is convenient for maintenance. These are its advantages. Regarding the problem that the Darieu wind turbine cannot start automatically, the general method is to use a generator as a motor to drive the wind turbine to rotate during starting, so that the blade tip speed ratio reaches more than 3.5. Due to the strict requirements on wind speed changes and load changes, it is difficult to operate smoothly and efficiently, and due to the disadvantages of not being able to start automatically, the development of Darieu wind turbines was slow. It was not until recent years that after technical improvements, it began to develop significantly. .
To analyze the aerodynamic performance of the wind wheel, it is necessary to understand the flow field at the wind wheel in order to analyze the aerodynamic force, torque and power generated. For this purpose, an aerodynamic model of the lift-type wind wheel must be established.
What is the difference between horizontal axis and vertical axis wind turbines?
Wind generator
[Abstract]
The utility model discloses a wind generator, which is composed of a fan and a generator. The blades of the fan and The shaft of the generator is connected, and the front part of the fan is equipped with a wind collecting funnel. The wind collecting funnel is connected to the universal wheel through the bracket, and the universal wheel is fixed on the ground. The utility model has a simple structure, is easy to install and use, has a small floor area, high efficiency and large power generation capacity, and can meet the needs of people's daily life. It is an ideal wind power generator.
The differences between horizontal axis and vertical axis wind turbines are in the following aspects:
1. Design method
The blade design of horizontal axis wind turbines, Momentum-leaf element theory is commonly used, and the main methods include Glauert method, Wilson method, etc. However, because the blade element theory ignores the flow interference between each blade element, and at the same time ignores the airfoil resistance when designing blades using the blade element theory, this simplification inevitably leads to inaccuracy in the results. This simplification It has little impact on the blade shape design, but has a greater impact on the wind energy utilization rate of the wind wheel. At the same time, the interference between the blades of the wind wheel is also very strong, and the entire flow is veryIt is very complex, and there is absolutely no way to get accurate results if we only rely on leaf element theory.
The blade design of vertical axis wind turbines used to be based on the horizontal axis design method and relied on blade element theory. Since the flow of the vertical axis wind turbine is more complex than that of the horizontal axis, it is a typical large separation unsteady flow and is not suitable for analysis and design using blade element theory. This is also an important reason why the vertical axis wind turbine has not been developed for a long time.
2. Wind energy utilization rate
The wind energy utilization rate of large horizontal axis wind turbines is mostly calculated by the blade designers and is generally above 40%. As mentioned before, due to the flaws in the design method itself, the accuracy of the wind energy utilization calculated in this way is very questionable. Of course, wind turbines in wind power plants will draw wind power curves based on measured wind speed and output power. However, the wind speed at this time is the wind speed measured by the anemometer at the rear of the wind wheel. It is smaller than the incoming wind speed and wind power. The curve is too high and must be corrected. After applying the correction method, the wind energy utilization rate of the horizontal axis will be reduced by 30% to 50%. Regarding the wind energy utilization rate of small horizontal axis wind turbines, the China Aerodynamics Research and Development Center has conducted relevant wind tunnel experiments, and the measured utilization rate is between 23% and 29%.
3. Structural characteristics
During one rotation, the blades of the horizontal axis wind turbine are affected by the combined effects of inertial force and gravity. The direction of the inertial force changes at any time. The direction of gravity remains unchanged, so the blade is subjected to an alternating load, which is very detrimental to the fatigue strength of the blade. In addition, the horizontal axis generators are placed at an altitude of tens of meters, which brings a lot of inconvenience to the installation, maintenance and inspection of the generators.
The blades of the vertical axis wind turbine are much better stressed during rotation than those of the horizontal axis. Since the direction of the inertial force and gravity always remains unchanged, the blades are subjected to a constant load. The fatigue life is longer than that of horizontal axis wind rotors. At the same time, the vertical axis generator can be placed under the wind wheel or on the ground for easy installation and maintenance.
4. Starting wind speed
It is a consensus that the starting performance of horizontal axis wind turbines is good. However, according to the wind speed of small horizontal axis wind turbines conducted by the China Aerodynamics Research and Development Center According to tunnel experiments, the starting wind speed is generally between 4 and 5m/s, and the maximum reaches 5.9m/s. Such starting performance is obviously unsatisfactory. It is also a consensus in the industry that the starting performance of vertical axis wind turbines is poor, especially for Darrieus type Ф type wind turbines, which have no self-starting ability at all, which is also a reason that limits the application of vertical axis wind turbines. However, for the Darrieus type H-shaped wind wheel, there is the opposite conclusion. According to the author's research, as long as the airfoil and installation angle are appropriately selected, quite good starting performance can be obtained. According to the hole experiment, the Darrieus type H-shaped wind wheel isThe starting wind speed only needs 2m/s, which is better than the above-mentioned horizontal axis wind turbine.