Title-Wind as alternative energy source
Aerodynamics of Wind Turbines: Lift
|The rotor consisting of the rotor blades and the hub are placed upwind of the tower and the nacelle on most modern wind turbines. This is primarily done because the air current behind the tower is very irregular (turbulent).|
|What makes the rotor turn?|
The answer seems obvious - the wind.
But actually, it is a bit more complicated than just the air molecules hitting the front of the rotor blades. Modern wind turbines borrow technologies known from aeroplanes and helicopters, plus a few advanced tricks of their own, because wind turbines actually work in a very different environment with changing wind speeds and changing wind directions.
Have a look at the animation of the cut-off profile (cross section) of the wing of an aircraft. The reason why an aeroplane can fly is that the air sliding along the upper surface of the wing will move faster than on the lower surface. This means that the pressure will be lowest on the upper surface.
This creates the lift, i.e. the force pulling upwards that enables the plane to fly.
The lift is perpendicular to the direction of the wind. The lift phenomenon has been well known for centuries to people who do roofing work: They know from experience that roof material on the lee side of the roof (the side not facing the wind) is torn off quickly, if the roofing material is not properly attached to its substructure.
|Aerodynamics of Wind Turbines: Stall|
Now, what happens if an aircraft tilts backward in an attempt to climb higher into the sky quickly? The lift of the wing will indeed increase, as the wing is tilted backwards, but in the picture you can see that all of a sudden the air flow on the upper surface stops sticking to the surface of the wing. Instead the air whirls around in an irregular vortex (a condition which is also known as turbulence ). All of a sudden the lift from the low pressure on the upper surface of the wing disappears. This phenomenon is known as stall.
An aircraft wing will stall, if the shape of the wing tapers off too quickly as the air moves along its general direction of motion. (The wing itself, of course, does not change its shape, but the angle of the the wing in relation to the general direction of the airflow (also known as the angle of attack) has been increased in our picture above). Notice that the turbulence is created on the back side of the wing in relation to the air current.
Stall can be provoked if the surface of the aircraft wing - or the wind turbine rotor blade - is not completely even and smooth. A dent in the wing or rotor blade, or a piece of self-adhesive tape can be enough to start the turbulence on the backside, even if the angle of attack is fairly small. Aircraft designers obviously try to avoid stall at all costs, since an aeroplane without the lift from its wings will fall like a rock.
|Aerodynamics of Wind Turbines|
|Adding Wind Speeds and Directions (Wind Velocities)|
The wind which hits the rotor blades of a wind turbine will not come from the direction in which the wind is blowing in the landscape, i.e. from the front of the turbine. This is because the rotor blades themselves are moving.
To understand this, consider the picture of a bicycle which is equipped with a blue banner (or a wind vane) to indicate the direction of the wind: If we have completely calm weather, and the bicycles moves forwards, with, say, 7 metres per second (14 knots), the bicycle will be moving through the air at 7 metres per second. On the bicycle we can measure a wind speed of 7 metres per second relative to the bicycle. The banner will point straight backwards, because the wind will come directly from the front of the bicycle. Now, let us look at the bicycle again directly from above, and let us assume that the bicycle moves forward at a constant speed of, once again, 7 metres per second. If the wind is blowing directly from the right, also at 7 metres per second, the banner will clearly be blown partly to the left, at a 45 degree angle relative to the bicycle. With a bit less wind, e.g. 5 metres per second, the banner will be blown less to the left, and the angle will be some 35 degrees. As you can see from the picture, the direction of the wind, the resulting wind as measured from the bicycle, will change whenever the speed of the wind changes.
What about the wind speed measured from the bicycle?
The wind is, so to speak, blowing at a rate of 7 metres per second from the front and 5 to 7 metres per second from the right. If you know a bit of geometry or trigonometry you can work out that the wind speed measured on the bicycle will be between 8.6 and 9.9 metres per second.
Enough about changing wind directions, now what about the wind turbine rotor?
|Power Control of Wind Turbines|
Wind turbines are designed to produce electrical energy as cheaply as possible. Wind turbines are therefore generally designed so that they yield maximum output at wind speeds around 15 metres per second. (30 knots or 33 mph). Its does not pay to design turbines that maximise their output at stronger winds, because such strong winds are rare.
In case of stronger winds it is necessary to waste part of the excess energy of the wind in order to avoid damaging the wind turbine. All wind turbines are therefore designed with some sort of power control. There are two different ways of doing this safely on modern wind turbines.
|Pitch Controlled Wind Turbines|
On a pitch controlled wind turbine the turbine's electronic controller checks the power output of the turbine several times per second. When the power output becomes too high, it sends an order to the blade pitch mechanism which immediately pitches (turns) the rotor blades slightly out of the wind. Conversely, the blades are turned back into the wind whenever the wind drops again.
The rotor blades thus have to be able to turn around their longitudinal axis (to pitch) as shown in the picture. Note, that the picture is exaggerated:
During normal operation the blades will pitch a fraction of a degree at a time - and the rotor will be turning at the same time.
Designing a pitch controlled wind turbine requires some clever engineering to make sure that the rotor blades pitch exactly the amount required. On a pitch controlled wind turbine, the computer will generally pitch the blades a few degrees every time the wind changes in order to keep the rotor blades at the optimum angle in order to maximise output for all wind speeds.
The pitch mechanism is usually operated using hydraulics.
|Stall Controlled Wind Turbines|
Passive) stall controlled wind turbines have the rotor blades bolted onto the hub at a fixed angle.
The geometry of the rotor blade profile, however has been aerodynamically designed to ensure that the moment the wind speed becomes too high, it creates turbulence on the side of the rotor blade which is not facing the wind . This stall prevents the lifting force of the rotor blade from acting on the rotor.
If you have read the upper section on aerodynamics and aerodynamics and stall , you will realise that as the actual wind speed in the area increases, the angle of attack of the rotor blade will increase, until at some point it starts to stall.
If you look closely at a rotor blade for a stall controlled wind turbine you will notice that the blade is twisted slightly as you move along its longitudinal axis. This is partly done in order to ensure that the rotor blade stalls gradually rather than abruptly when the wind speed reaches its critical value. (Other reasons for twisting the blade are mentioned in the previous section on aerodynamics).
The basic advantage of stall control is that one avoids moving parts in the rotor itself, and a complex control system. On the other hand, stall control represents a very complex aerodynamic design problem, and related design challenges in the structural dynamics of the whole wind turbine, e.g. to avoid stall-induced vibrations. Around two thirds of the wind turbines currently being installed in the world are stall controlled machines.
|Active Stall Controlled Wind Turbines|
An increasing number of larger wind turbines (1 MW and up) are being developed with an active stall power control mechanism.
Technically the active stall machines resemble pitch controlled machines, since they have pitchable blades. In order to get a reasonably large torque (turning force) at low wind speeds, the machines will usually be programmed to pitch their blades much like a pitch controlled machine at low wind speeds. (Often they use only a few fixed steps depending upon the wind speed).
When the machine reaches its rated power , however, you will notice an important difference from the pitch controlled machines: If the generator is about to be overloaded, the machine will pitch its blades in the opposite direction from what a pitch controlled machine does. In other words, it will increase the angle of attack of the rotor blades in order to make the blades go into a deeper stall, thus wasting the excess energy in the wind.
One of the advantages of active stall is that one can control the power output more accurately than with passive stall, so as to avoid overshooting the rated power of the machine at the beginning of a gust of wind. Another advantage is that the machine can be run almost exactly at rated power at all high wind speeds. A normal passive stall controlled wind turbine will usually have a drop in the electrical power output for higher wind speeds, as the rotor blades go into deeper stall.
The pitch mechanism is usually operated using hydraulics or electric stepper motors.
As with pitch control it is largely an economic question whether it is worthwhile to pay for the added complexity of the machine, when the blade pitch mechanism is added.
|Other Power Control Methods|
Some older wind turbines use ailerons (flaps) to control the power of the rotor, just like aircraft use flaps to alter the geometry of the wings to provide extra lift at takeoff.
Another theoretical possibility is to yaw the rotor partly out of the wind to decrease power. This technique of yaw control is in practice used only for tiny wind turbines (1 kW or less), as it subjects the rotor to cyclically varying stress which may ultimately damage the entire structure.
|The Wind Turbine Yaw Mechanism|
|The wind turbine yaw mechanism is used to turn the wind turbine rotor against the wind.|
The wind turbine is said to have a yaw error, if the rotor is not perpendicular to the wind. A yaw error implies that a lower share of the energy in the wind will be running through the rotor area. (The share will drop to the cosine of the yaw error, for those of you who know math).
If this were the only thing that happened, then yaw control would be an excellent way of controlling the power input to the wind turbine rotor. That part of the rotor which is closest to the source direction of the wind, however, will be subject to a larger force (bending torque) than the rest of the rotor. On the one hand, this means that the rotor will have a tendency to yaw against the wind automatically, regardless of whether we are dealing with an upwind or a downwind turbine. On the other hand, it means that the blades will be bending back and forth in a flapwise direction for each turn of the rotor. Wind turbines which are running with a yaw error are therefore subject to larger fatigue loads than wind turbines which are yawed in a perpendicular direction against the wind.
|Cable Twist Counter|
Cables carry the current from the wind turbine generator down through the tower. The cables, however, will become more and more twisted if the turbine by accident keeps yawing in the same direction for a long time. The wind turbine is therefore equipped with a cable twist counter which tells the controller that it is time to untwist the cables.
Occasionally you may therefore see a wind turbine which looks like it has gone berserk, yawing continuously in one direction for five revolutions.
Like other safety equipment in the turbine there is redundancy in the system. In this case the turbine is also equipped with a pull switch which is activated if the cables become too twisted.
|Wind Turbine Safety|
Photograph Soren Krohn © 1998 DWIA
The components of a wind turbine are designed to last 20 years. This means that they will have to endure more than 120,000 operating hours, often under stormy weather conditions.
If you compare with an ordinary automobile engine, it usually only operates only some 5,000 hours during its lifetime. Large wind turbines are equipped with a number of safety devices to ensure safe operation during their lifetime.
One of the classical, and most simple safety devices in a wind turbine is the vibration sensor in the image above, which was first installed in the Gedser wind turbine. It simply consists of a ball resting on a ring. The ball is connected to a switch through a chain. If the turbine starts shaking, the ball will fall off the ring and switch the turbine off.
There are many other sensors in the nacelle, e.g. electronic thermometers which check the oil temperature in the gearbox and the temperature of the generator.
It is essential that wind turbines stop automatically in case of malfunction of a critical component. E.g. if the generator overheats or is disconnected from the electrical grid it will stop braking the rotation of the rotor, and the rotor will start accelerating rapidly within a matter of seconds.
In such a case it is essential to have an overspeed protection system. Danish wind turbines are requited by law to have two independent fail safe brake mechanisms to stop the turbine.
|Aerodynamic Braking System: Tip Brakes|
The primary braking system for most modern wind turbines is the aerodynamic braking system, which essentially consists in turning the rotor blades about 90 degrees along their longitudinal axis (in the case of a pitch controlled turbine or an active stall controlled turbine ), or in turning the rotor blade tips 90 degrees (in the case of a stall controlled turbine ).
These systems are usually spring operated, in order to work even in case of electrical power failure, and they are automatically activated if the hydraulic system in the turbine loses pressure. The hydraulic system in the turbine is used turn the blades or blade tips back in place once the dangerous situation is over.
Experience has proved that aerodynamic braking systems are extremely safe.
They will stop the turbine in a matter of a couple of rotations, at the most. In addition, they offer a very gentle way of braking the turbine without any major stress, tear and wear on the tower and the machinery.
The normal way of stopping a modern turbine (for any reason) is therefore to use the aerodynamic braking system.
|Mechanical Braking System|
|The mechanical brake is used as a backup system for the aerodynamic braking system, and as a parking brake, once the turbine is stopped in the case of a stall controlled turbine. Pitch controlled turbines rarely need to activate the mechanical brake (except for maintenance work), as the rotor cannot move very much once the rotor blades are pitched 90 degrees.|
|Wind Turbine Occupational Safety|
|Large, modern wind turbines normally use conical tubular steel towers. The primary advantage of this tower over a lattice tower is that it makes it safer and far more comfortable for service personnel to access the wind turbine for repair and maintenance. The disadvantage is cost.|
The primary danger in working with wind turbines is the height above ground during installation work and when doing maintenance work.
New Danish wind turbines are required to have fall protection devices, i.e. the person climbing the turbine has to wear a parachutist-like set of straps.
The straps are connected with a steel wire to an anchoring system that follows the person while climbing or descending the turbine.
The wire system has to include a shock absorber, so that persons are reasonably safe in case of a fall.
A Danish tradition (which has later been taken up by other manufacturers), is to place the access ladders at a certain distance from the wall. This enables service personnel to climb the tower while being able to rest the shoulders against the inside wall of the tower. In this image you see the editor of our Spanish web site verifying that this is actually a very practical solution.
Protection from the machinery, fire protection and electrical insulation protection is governed by a number of national and international standards. During servicing it is essential that the machinery can be stopped completely. In addition to a mechanical brake, the rotor can be locked in place with a pin, to prevent any movement of the mechanical parts whatsoever.