Wind turbines come in many varieties, but they can be broadly categorized into two types:
1. Horizontal axis wind turbine, that is, the rotation axis of the wind turbine is parallel to the wind direction;
2. Vertical axis wind turbine, that is, the rotation axis of the wind turbine is perpendicular to the ground or the direction of airflow.
Vertical axis wind turbine
This article mainly describes vertical axis wind turbines. Vertical axis wind turbines (VAWTs) are mainly classified into drag type and lift type.
Drag-type vertical axis wind turbines primarily utilize the drag generated by airflow over the blades as their driving force, while lift-type turbines utilize the lift generated by airflow over the blades as their driving force. Because drag decreases dramatically with increasing rotational speed while lift increases, lift-type vertical axis wind turbines are significantly more efficient than drag-type turbines.
There are several types of vertical axis wind turbines that utilize drag rotation. Among them are rotors made of flat plates and cups, which are purely drag-based devices; and S-shaped wind turbines, which have some lift but are primarily drag-based devices. These devices have large starting torques but low tip speed ratios, resulting in low power output for given rotor size, weight, and cost.
The Darrieux rotor was invented by G.M. Darrieux of France in the 1830s. In the 1970s, the National Scientific Research Agency of Canada conducted extensive research on it, making it a major competitor to horizontal axis wind turbines. The Darrieux rotor is a lift device with airfoil-shaped curved blades. It has a low starting torque but a high tip speed ratio, resulting in high power output for a given rotor weight and cost. Various Darrieux rotor types exist worldwide, such as Φ, Δ, Y, and H types. These rotors can be designed with single, double, triple, or multiple blades.
More than 180 years have passed since Darrieus invented the lift-type vertical axis wind turbine, but it has not been widely used, mainly due to some inherent shortcomings. Its inability to self-start is a significant drawback, but its primary weakness lies in its narrow requirements for varying wind speeds and loads, which also relates to its inability to adjust speed.
1. Fixed-blade lift-type vertical axis wind turbine
Figure 1. Arrangement of blades of the lift-type vertical axis wind turbine.
Traditional Darrieux wind turbines use ф-shaped blades, but nowadays more commonly use straight blades (H-type) structures. The blades of Darrieux wind turbines are fixed relative to the rotor, meaning the blade chord angle is not adjustable. Figure 1 shows the blade distribution of the rotor.
Lift-type wind turbines utilize the lift generated by the blades to drive the rotor to rotate and perform work. Under ideal conditions, most common airfoil blades can generate lift at an angle of attack of 0 to 15 degrees, while generating greater lift and less drag at 8 to 13 degrees. Figure 2 shows the airflow and force diagram when the wind turbine blades rotate to the windward side of the rotor (0-degree position).
Figure 2. Comparison of lift and drag of the blades under normal and stall conditions.
In the left-hand diagram of Figure 2, the blades generate lift (L) and drag (D) under the influence of relative wind speed (W). The angle between the relative wind speed (W) and the blade chord, i.e., the blade's angle of attack (α), is approximately 14 degrees. The relative wind speed (W) is the sum of the wind speed (V) and the blade's velocity (u). At this point, the blade's velocity is approximately four times the wind speed, meaning the tip speed ratio is 4. The resultant force of lift (L) and drag (D) is F, and the torque exerted by this force on the wind turbine is M, which drives the turbine's rotation. When the tip speed ratio is 4, the blades generate a torque that drives the turbine's rotation whether operating on the windward or leeward side. Only near the two sides (90 degrees and 180 degrees) is the lift very small, resulting in a small negative torque.
In the right-hand diagram of Figure 2, the wind speed doubles, the blade speed remains unchanged, the tip speed ratio is approximately 2, and the blade angle of attack α is approximately 27 degrees. The blade is operating in a stall state. At this time, the lift L generated by the blade decreases, the drag D increases significantly, and the torque M generated relative to the wind turbine is negative, which prevents the wind turbine from rotating. Under this wind speed and rotation speed, the blade is very likely to generate a negative torque.
In fact, the blades are already on the verge of stalling when the tip speed ratio is 4 (α is 14 degrees). Below 4, lift L no longer increases, drag D increases significantly, and the torque M generated by the blades may be 0 or negative. Fortunately, the blades can generate positive torque in a region with a smaller angle of attack between 0 and 90 degrees, and there are also such regions between 90 and 180 degrees, 180 and 270 degrees, and 270 and 360 degrees. However, when the tip speed ratio is less than 3.5 (α is greater than 16 degrees), these regions become increasingly smaller.
Figure 3 shows the relationship between the power coefficient Cp and the tip speed ratio tsr of a lift-type vertical axis wind turbine. It can be seen that a higher power coefficient is achieved when the tip speed ratio is between 4 and 6, and the airflow is under ideal conditions.
Figure 3. Relationship between power coefficient and tip speed ratio of a fixed-blade vertical-axis wind turbine.
However, wind speed is not constant, and the wind turbine load is also not constant. When wind speed increases rapidly, the wind turbine speed cannot keep up immediately, and the tip speed ratio may drop below 3.5. The wind turbine may then be subjected to the impact of reverse torque and become unstable. This situation will also occur when the wind turbine speed decreases due to the increase in wind load, leading to a decrease in the tip speed ratio. When the wind speed decreases, the wind turbine speed will decrease even faster due to the load, which may also cause this situation. The requirement for a narrow range of wind speed or load variation is the main problem of fixed-blade lift vertical axis wind turbines. The inability to self-start is also a significant drawback of fixed-blade lift vertical axis wind turbines, all of which impose many limitations on their applications.
2. Airfoil adjustment method for lift-type vertical axis wind turbines
In horizontal axis wind turbines, variable pitch angle is used to adapt to changes in wind speed and adjust the relationship between wind speed and load. In vertical axis wind turbines, performance can also be improved by changing the airfoil. The following is a brief analysis of several methods for controlling airfoils and their advantages and disadvantages:
1) Change the blade angle according to the angle specified in the program.
Using a microprocessor to control the blade angle is the best method, but this article will not discuss microprocessor control methods, but only the simplest mechanical methods to control the blade angle.
The blade angle of attack is adjusted by using a cam push rod or eccentric wheel. There is a blade shaft along the blade length direction. The blade is mounted on the blade support of the wind turbine through the blade shaft. A connecting rod pulls the blade to rotate. The connecting rod is controlled by the cam or eccentric wheel. A wind-directing device is also installed so that the cam is controlled by the wind direction. The cam is designed according to a set control law so that the blade rotates to different positions and turns to a predetermined angle.
This type of wind turbine is self-starting and can operate over a wide range of wind speeds.
The disadvantage is that since the angle of attack of the blades at each position is fixed relative to the wind direction and is independent of the wind speed, it only has high conversion efficiency for the designed wind speed. At other wind speeds, the angle of attack of the blades may not be optimal. During normal operation, the blades should not swing at all. Therefore, this fixed swing pattern cannot achieve high conversion efficiency over a wide range of wind speeds.
Structural disadvantages: complex structure, high mechanical wear, unsuitable for operation in harsh environments, and also noisy.
2) Control the blade angle using wind power and baffles.
The blades are directly propelled by wind power, with a baffle limiting the swing angle. A blade pivot is located along the blade's length, forward of the blade's pressure center (in standard airfoils, the pressure center is typically located at 1/4 of the blade chord length from the leading edge during normal operation). The blades are mounted on the rotor's blade supports via this pivot, allowing them to rotate around it. Positioning the pivot 1/4 of the blade chord length from the leading edge ensures that the resultant force of the wind on the blades at any angle is applied behind the pivot, enabling the blades to swing with the wind. A baffle on the support further limits the blade's swing angle; Figure 4 shows a schematic diagram of this structure.
Figure 4 shows the control of the blade's swing angle using wind power and baffles.
Figure 5 is a schematic diagram of the blade oscillation under the action of wind force. The oscillation of the blade with the wind allows the wind turbine to work well even at lower wind speeds. When the blade rotates to the windward side of the wind turbine, the blade oscillates towards the inside of the wind turbine. When the blade rotates to the leeward side of the wind turbine, the blade oscillates towards the outside of the wind turbine. Both of these can generate a large torque force. The blade tip speed ratio in the figure is about 2. The left side shows the force situation when the blade rotates to the windward side of the wind turbine, and the right side shows the force situation when the blade rotates to the leeward side of the wind turbine.
Figure 5. Schematic diagram of blade swaying under wind force.
The advantage of this method is that the wind turbine can start automatically and can work at wind speeds ranging from low to high.
The disadvantage is that when the blades operate in the area around 90 degrees or 270 degrees, they swing to the position between the two side baffles, in a downwind position, generating only drag and no lift. Furthermore, as the tip speed ratio increases, the downwind area expands. If the blade's swing range is ±15 degrees, the maximum tip speed ratio will not exceed 4. This is because if the tip speed ratio exceeds 4, the blades remain in a downwind position throughout the entire rotor rotation, generating only drag and no lift; even without load, the rotational speed will not increase. Therefore, if the blade's swing range exceeds ±15 degrees, the wind turbine's wind energy utilization efficiency will significantly decrease; if the blade's swing range is less than ±15 degrees, the wind turbine's self-starting capability will be very poor.
From a structural perspective, the advantages are: simple structure, fewest moving parts, and easy processing, installation, and maintenance. The disadvantages are: frequent impacts to the stop block can easily cause component damage, and it also generates more noise.
3) Scheme 1: Controlling the blade angle using wind power and centrifugal baffles
Reducing the blade yaw angle as the rotational speed increases allows for adaptation to a wider wind speed range and operation at a higher tip speed ratio. Below is one method for controlling the yaw angle:
The blade's rotation axis and installation are the same as in section 2. A pendulum is fixed to the blade, and a centrifugal sliding block can slide along the support axis and is connected to the support via a spring. Figure 6 (left) shows its structural schematic. When the blade swings, the pendulum swings within the V-shaped opening of the centrifugal sliding block, and the edge of the V-shaped opening limits the blade's swing angle.
When the wind turbine rotates, the centrifugal sliding block moves outwards under the action of centrifugal force, and the amount of movement increases with the increase of the rotational speed. When the wind turbine speed is low, the stop lever is at the upper end of the V-shaped opening, and the blades have a large swing amplitude, as shown in the middle of Figure 6; when the wind turbine speed increases, the stop lever is at the bottom of the V-shaped opening, and the blades can swing smaller, as shown in the right of Figure 6.
Figure 6 shows the use of a V-shaped centrifugal sliding stop to control the blade's swing angle.
The advantages of this swing control method are: the wind turbine can start automatically, and even when it has not reached the rated speed, such as when the tip speed ratio is 2, it will still have power output. When it reaches the rated speed, the blades will not swing and can operate at a higher tip speed ratio.
The disadvantage is that the blades still swing within a limited range, which decreases as the rotational speed increases. In other words, when the rotational speed remains constant, the angle at which the blades can swing is constant. When the blades reach a region around 90 degrees or 270 degrees, they swing with the wind within the limited range, generating only drag and no lift.
Structurally, it is more complex. The sliding parts have high requirements for machining, and sealing and lubrication are more troublesome. Moreover, there is still impact when the limit is applied at speeds below the rated speed, which affects the structural strength and also generates noise.
4) Scheme Two: Controlling Blade Angle Using Wind Power and Centrifugal Baffles
Figure 7 shows the control of the blade's swing angle using a centrifugal sliding stop.
Figure 7 illustrates another method of controlling the yaw angle using a centrifugal slider. A centrifugal slider with a baffle wheel is mounted on the rotor support, and the slider slides freely near the blade end. When the rotor rotates, the slider is pressed against the blade by centrifugal force, while the wind pushes the blade to oscillate. The oscillating blade then pushes the slider inwards towards the support, until the blade oscillates to a position where the wind force and centrifugal force are balanced. Because the baffle wheel on the slider remains pressed against the blade throughout the rotor's rotation, the slider's effect on the blade is continuous, and the blade's oscillation is also continuous, eliminating the downwind oscillation range.
The left image in Figure 7 shows the wind turbine not rotating, with the blades and baffles in any position; the middle image in Figure 7 shows the wind turbine not yet reaching its rated speed, with the blades in the upwind position and a large swing angle; the right image in Figure 7 shows the blades in the lateral positions or at the rated speed, with a very small swing angle.
The advantage of this swing control method over the previous one is that it allows for continuous swing control without a downwind swing range, which helps to further improve the operating efficiency of the wind turbine.
The structural advantages are the elimination of impact and reduced noise. However, the slider moves very frequently, which still presents problems such as high requirements for the machining of sliding parts, and difficulties in sealing and lubrication. At the same time, dust prevention for the small guide rollers is also quite troublesome.
3. Directly control the blade angle using wind power and centrifugal force.
Without the need for sliding blocks, a new structural design directly controls the blade's oscillation angle using centrifugal force and wind power, minimizing kinematic pairs. The structure is as follows:
Figure 8 shows the control of the blade's swing angle using wind power and a centrifugal pendulum.
The blade's rotation axis and installation are the same as in section 2. A pendulum is fixed on the side of the blade facing the outside of the wind turbine, pointing outwards from the wind turbine. Its axis passes through the blade's rotation axis and the wind turbine's rotation axis. There is a centrifugal hammer at the outer end of the pendulum, which rotates together with the blade around the blade's rotation axis. Figure 8 is a schematic diagram of the structure and oscillation.
Figure 9 Schematic diagram of the blades subjected to centrifugal force and wind force.
Figure 9 analyzes the forces acting on the blades during controlled yaw when the wind turbine rotates under wind power. The figures only show the blades, centrifugal hammer, and the main force vectors. Arrow W represents the direction of the wind. The blades are operating normally at a linear velocity u, and are subjected to an aerodynamic force F2, primarily lift, at the blade's pressure center. Due to the turbine's rotation, the centrifugal hammer is subjected to a centrifugal force F1. The moments of F1 and F2 relative to the blade's axis of rotation are opposite. Under the action of these two moments, the blades yaw towards the position where the two moments are balanced. This position is the controlled yaw angle of the blades as they rotate with the turbine. The left figure shows the force state when the blades rotate to the windward side of the turbine, yawing inwards towards the turbine. The right figure shows the force state when the blades rotate to the leeward side of the turbine, yawing outwards towards the turbine. Higher rotational speeds result in greater centrifugal force and smaller blade yaw angles, allowing for higher tip speed ratios at higher wind speeds.
The magnitude of centrifugal force can be adjusted by changing the mass of the centrifugal hammer or its position on the centrifugal pendulum, so that the blades can operate in a more suitable state.
Limit bars are still installed on the wind turbine support. The limit bars are only used to limit the blade swing angle when the wind turbine starts. After the wind turbine starts, as the speed increases, the centrifugal force increases, which reduces the blade swing angle. The blades will not hit the limit bars. After the wind speed reaches the rated wind speed, the wind turbine works in the lift state, and the blades only swing at a small angle.
Wind turbines using this oscillation control method can start automatically. When the blade tip speed ratio is below 1, they operate in a drag state. When the blade tip speed ratio is above 1, they enter a lift-drag mixed state. When the blade tip speed ratio is above 1.5, they enter a lift working state. The blade oscillation is continuous and there is no downwind oscillation range, which is conducive to improving wind energy utilization efficiency.
Structurally, the mechanism is simple, with the only moving part being the connection between the blades and the rotor support via bearings. It operates reliably, is easy to manufacture and install, easy to lubricate and seal, inexpensive, and requires virtually no maintenance. Once the wind turbine is running, the blade oscillation angle is controlled by the balance of wind force and centrifugal force, preventing impact with the baffle and eliminating noise.
However, this scheme requires the blades to have a very small moment of inertia, which places high demands on material selection and structural design.
Summarize
The aforementioned simple airfoil control methods improve the performance of lift-type vertical axis wind turbines, enabling them to start at lower wind speeds and output power when the blade tip speed ratio exceeds 1. Two schemes are more suitable: Scheme 2, which uses wind power and centrifugal blocks to control the blade angle, and Scheme 2, which uses wind power and centrifugal force to directly control the blade angle. The former has the problem of being more complex to manufacture and requiring more maintenance, while the latter has the problem of high cost due to the use of lightweight blades.
However, none of these methods can solve the problem of limiting the speed increase of wind turbines at high wind speeds. For medium and large-sized wind turbines, the ultimate solution for lift-type vertical axis wind turbines is to be able to control the speed of the wind turbine by changing the airfoil.
