TEMPUS ENERGY: TECHNICAL ASPECTS
1: Introduction
The basic principle of a wind turbine is quite straightforward. Due to the wind, rotor blades are rotating and these rotor blades drive an electrical generator which injects the generated power into an electrical grid.In reality, a lot of technical aspects are important in order to obtain a reliable wind turbine. It is impossible to discuss all technical aspects, but the present section discusses a selection of these technical aspects.
2: Yawing
When considering a horizontal axis wind turbine, it is important the wind turbine faces the wind. Using sensors, the wind direction is measured and the nacelle and the rotor blades are tuned accordingly i.e. yawing is needed. Figure 1 visualizes a wind turbine where the oncoming wind is flowing perpendicular with the plane of rotation of the rotor blades (due to the drag forces and the lift forces there is a useful force which drives the wind turbine and there is an undesired force which tries to tilt the tower).
Figure 1: Horizontal axis wind turbine facing the oncoming wind
In order to have a simple and cheap yawing mechanism, micro wind turbines use a passive yaw system based on a vane as visualized in Figure 2 (alternatively, the yaw mechanism relies on the aerodynamics of the rotor). Such a passive or free yaw system operates properly when the changes of the wind direction occur gradually. In case the changes in the wind direction are fast and unsteady, the generated power will decrease.
Large wind turbines use an active yaw system which means an electrical motor or a hydraulic motor is used to yaw the wind turbine. Figure 3 visualizesa yaw drive containing an electric drive motor. Using a gearbox (gear reducer) and a pinion gear, the yawing speed is reduced implying a slow yaw rate. Based on the speed reduction, even with a small motor a large torque is obtained.
A problem related with active yawing is the occurrence of continuous small yaw movements which causes wear and even breaking of the yaw drive. In order to avoid this problem, yaw brakes are frequently used (also in Figure 3). This brake is engaged when the turbine is not yawing and it is released just before yawing begins.
Figure 2: Micro wind turbine equipped with a vane
Figure 3: Yaw drive with brake (source: Manwell)
The active yawing mechanism is controlled by a control system. The wind direction is measured using a wind vane and compared with the orientation of the wind turbine. When the yaw error is larger than a predefined threshold, the yaw drive system is activated and the turbine will move in the appropriate direction.
Contrary to the horizontal axis wind turbines, vertical axis wind turbines generally do not need a yawing mechanism. For instance when considering a Savonius turbine, a Darrieusturbine or a Turby turbine, no yawing is needed i.e. the oncoming wind is accepted from all directions (and these wind directions are allowed to vary).
3: Solidity
In case of low solidity wind turbines, only a small part of the swept area of the rotor blades is covered by these rotor blades. For instance wind turbines having one single, two or three rotor blades have a low solidity. The optimum tip speed ratio (ratio between the speed of the rotor blades and the speed of the undisturbed wind) of a wind turbine strongly depends on the solidity. In order to extract a maximum amount of energy from the wind, the rotor blades have to interact as much as possible with the wind passing though the rotor swept area. In case of a low solidity wind turbine, this requires a high speed of rotation of the rotor blades i.e. a high tip speed ratio.
When the tip speed ratio is too slow, some of the wind goes through the rotor swept area without interacting with the rotor blades which reduces the efficiency. If the tip speed ratio is too high i.e. the speed of rotation of the rotor blades is too high, the turbine offers too much resistance to the wind implying that some wind goes around it which also reduces the efficiency.
Figure 4: The solidity of a wind turbine (source: Boyle)
Figure 5 visualizes the rotor efficiencies for a number of wind turbines having a different solidity. In case of a two bladed wind turbine, the tip speed ratio must be higher than the tip speed ratio of a three bladed wind turbine in order to obtain the maximum rotor efficiency. The higher the number of rotor blades, the smaller the required tip speed ratio. Notice that the horizontal axis wind turbines with three rotor blades are the most common ones and that they have the highest efficiency (closer to the Betz limit).
The two bladed wind turbines (Figure 6) have almost disappeared since they have a lower efficiency and their higher speed of rotation has a “nervous” impact on the landscape. Notice however, the higher speed when driving an electrical generator is an advantage. Since the blades of a wind turbine are expensive, a two bladed wind turbine can be cheaper than a three bladed wind turbine.
Figure 5: The rotor efficiency for different types of wind turbines
Figure 6: A two bladed wind turbine
Multi-bladed wind turbines have lower speeds of rotation (Figure 5) and associated with these lower speeds of rotation, the developed torques are higher. This makes them less appropriate to drive an electrical generator but they are suited to drive mechanical loads. For instance the so-called Western Wheel is used to pump water, to irrigate the land…
Figure 7: Western Wheel used to pump water in India
4: The rotor blades
4.1: Up-wind and down-wind rotor
When considering the rotor of a horizontal axis wind turbine, a distinction can be made based on the solidity of the rotor. A distinction can also be made between an up-wind and a down-wind wind turbine as visualized in Figure 8. Up-wind turbines are more common than down-wind turbines. Large up-wind turbines generally have an active yaw mechanism and small up-wind turbines have a passive yaw mechanism based on a vane (as visualized in Figure 8).
Figure 8: Up-wind and down-wind rotor (source: Boyle)
In case of an up-wind turbine, the effect of the tower shading is smaller. In case of a down-wind turbine, the effect of the tower shading is larger implying the drop in the generated torque is larger each time a rotor blade passes the tower. Notice the relative drop in the developed torque decreases as the number of rotor blades increases. For instance a three bladed wind turbine has each revolution tree relatively small torque drops (see Figure 9) whereas a two bladed wind turbine has each revolution two relatively largetorque drops.
Figure 9: Torque variations due to tower shading
Due to the wind, the blades tend to bent in the direction of the tower in case of an up-wind turbine. An up-wind turbine needs an extended nacelle in order to position the rotor sufficiently far away from the tower in order to avoid any problems with a blade strike. Moreover, these blades must be sufficiently stiff in order to avoid them to strike the tower.
A down-wind turbine has a rotor on the back side of the turbine as visualized in Figure 8. The rotor blades can be used to yaw the wind turbine. Moreover, the rotor blades are allowed to be more flexible since there is no danger they hit the tower. In general, these flexible rotor blades are less expensive.The effect of the tower shading is more pronounced when considering a down-wind turbine i.e. the relative drop in the torque when passing the tower is larger.
4.2: Construction of the rotor blades
Very often, the rotor blades are made of composites. Composites are materials containing at least two dissimilar materials. Very often, a binder (polyester, epoxy, vinyl ester) contains glass fibers or carbon fibers. Carbon fibers are more expensive than glass fibers but they are stronger and stiffer. Sometimes carbon fibers and glass fibers are combined in one composite material. In general, these composites are easy to fabricate in the desired aerodynamic shape, they have a high strength and they have a high stiffness to weight ratio. Moreover, they are corrosion resistant.
As visualized in Figure 11, the rotor blades of a large wind turbine have an appropriate shape. Consider the rotor blade visualized in Figure 10. The pitch angle is the angle between the chord line of the cross section of the rotor blade and the plane of rotation. The incoming wind speed equals in the axial direction. In Figure 10, there is a radius between the cross section of the rotor blade and the axis of rotation. When the rotor blade has a pulsation , the speed of the rotor blade at the cross section equals . Based on and the speed of the rotor blade, the relative wind speed with respect to the rotor blade is obtained.
Figure 10: Airflow on a rotor blade segment (source: Heier)
Figure 11: Twisted rotor blade
The angle between the resulting wind speed and the chord line of the airfoil equals the angle of attack . The sum of the angle of attack and the pitch angle equals
Notice the wind speed in the axial direction is (in a first approximation) independent of the radius but of course depends on . This implies the relative wind speed with respect to the rotor blade and also the angle depends on . As increases, decreases and in order to keep constant the pitch angle must decrease. This means the rotor blades need a twist as visualized in Figure 11.
5: Gearbox
In case of a direct drive wind turbine, there is no gearbox between the rotor blades and the rotor of the generator. This implies the generator has a low speed of rotation (e.g. 30 revolutions per minute). In case a synchronous generator is driven, this generator contains a large number of pole pairs and Figure 12 visualizes such a synchronous generator in a direct drive Enercon wind turbine.
A lot of wind turbines contain a gearbox which increases the speed of the input shaft to the speed of the generator. For instance the rotor blades and the input shaft rotate at a speed of 30 revolutions per minute where the generator rotates at 1500 revolutions per minute.
The gearbox is one of the heaviest components in the nacelle and they are expensive. An appropriate design is needed, this design needs knowledge about gearboxes but also knowledge about wind, wind speeds, variations in the wind speed and wind turbines is needed. A badly designed gearbox is generally one of the major sources of operational problems. In general, gearboxes are a source of noise and sufficient lubrication can be a challenge (e.g. due to low temperatures the viscosity of the oil may be too high).
Figure 12: Synchronous generator in an Enercon wind turbine
Parallel shaft gearboxes are an option but also planetary gearboxes are used. Gears of the spur type are used but also helical gears are found in wind turbines. A lot of gearboxes contain more than one stage. Generally, one single stage will not give a speed-up of more than 6:1. In case a speed-up of for instance 30:1 is needed (for instance the generator has a speed of 1500 rpm and the rotor blades have a speed of 50 rpm), it is possible to place a stage with a speed-up of 5:1 in series with a stage with a speed-up of 6:1.
6: Foundations and towers
6.1: Small scale wind turbines
The nacelle of a horizontal axis wind turbine is mounted on a tower. In order have a higher wind speed, it is important the height of this tower is sufficiently large. There exists a rule of thumb which advises that the lowest blade tip is at least 9 meters above any obstacle within a distance of 150 meters. Suppose at a distance of 100 meters there is a tree having a height of 20 meters which implies the lowest blade tip needs a height of 29 meters. In case the rotor blades have a radius of 3 meters, this implies a tower of 32 meters is needed. This approach avoids too much turbulences in the wind.
Not only the height of the tower must be chosen, also the type of tower must be determined. On the right, Figure 13 visualizes a guyed tower and on the left a non-guyed tower is visualized. In general, guyed towers are cheaper since their foundations are less demanding and less expensive (especially when the soil is marshy, expensive foundations can be required). In general, guy wires are assumed to be ugly and they require a lot of space. It is also important the guy wires are not damaged by for instance a falling tree.
Figure 13: Non-guyed tower and guyed tower
Quite often, the tilt-up towers are used using a hinge. This approach allows to mount the nacelle on the tower while the tower is lying on the ground. Lowering the tower to the ground is also useful when performing maintenance and to protect the installation in case of a (heavy) storm. Tilting the tower can be performed by means of a crane, but sometimes it is also possible to tilt the tower using a car or a jeep. Figure 14 visualizes the tilting-up of a tubular tower. Notice not only tubular towers are used, for instance also lattice structures are used (as visualized on the background on Figure 14).
Figure 14: Mounting a wind turbine using a tilt-up tower
6.2: Large wind turbines
Very often, the towers of large wind turbines are tubular tapered steel structures. Steel plates are rolled into cylinders and seam is then longitudinally welded. At the ends of the tower sections, flanges are welded to the inside of the cylinders for onsite bolding during erection. In general, three tower sections are constructed and transported to the site (see Figure 15). In case the tower becomes too large (the rated powers of the wind turbines are increasing) to transport these tower sections,
-it is an option to transport smaller sections which are welded on site,
-it is an option to cast a concrete tower as the base and then install steel tower sections above the concrete base.
In general, towers of large wind turbines contain an access door at the bottom and they also contain a “man-lift” system to transport the crew to the nacelle.
Figure 15: Transportation of a tower section of a wind turbine
Figure 16: Foundations of a large wind turbine
Large wind turbines have demanding foundations and Figure 16 visualizes the foundations of a wind turbine in the MW-range. Together with the tower, the foundations must withstand all forces and moments acting on the wind turbine construction. Especially when the soil is marshy, the requirements on the foundations are demanding. Also when considering offshore wind turbine farms, the realization of the foundations is a challenge. Figure 17 visualizes an offshore wind turbine of the Thornton bank realized by C-Power (Belgium). Notice the gravity based foundation having a height of approximately 44 meters which allows to mount the tower above the sea level. Figure 18 visualizes the construction of these gravity based constructions on land.
Figure 17: Offshore wind turbine on the Thornton bank (source: C-Power)
Figure 18: Construction of the gravity based foundations by C-Power
References
Boyle G., Renewable Energy: Power for a Sustainable Future, Oxford University Press, 2004.
Heier S., Grid Integration of Wind Energy Conversion Systems, Wiley and Sons, 2006.
Jain P., Wind Energy Engineering, Mc Graw Hill, 2011.
Manwell J., Mc. Gowan J. and Rogers A., Wind Energy explained: theory, design and application, Wiley and Sons, 2009.
Woofenden I., Wind Power for Dummies, Wiley Publishing, 2009.