Wind turbines are devices that convert the wind’s kinetic energy into electrical energy.
We can simply describe the wind as moving air, caused by differences in atmospheric pressure between different regions of our planet.
Differences in atmospheric pressure arise as a result of temperature differences.
When the air moves from an high pressure region into a low-pressure region, we feel the moving air as wind.
On the other hand, we can describe wind energy as the process in which wind is used to generate mechanical energy or electricity.
Since the beginnings of civilization man has always took advantage of wind energy: from the simple sail boats, to the grinding grain with the windmills, up to the actual production of electricity thanks to the wind energy converters, also known as WIND TURBINES.
There are different types and different sizes of wind turbines.
In this video we will focus on the most common horizontal axis turbines:
• We’ll find out which are the main components of this particular type of wind turbine
• We will understand how these devices are able to transform wind energy into electric current.
• We’ll see why don’t wind turbines have more than 3 blades, through the observation of their airfoil shape.
• And we will also see what are the main causes of accidents that can occur during the operation of these devices.
The first wind turbines were built towards the end of the nineteenth century with the aim of supplying electricity to small isolated villages
In the following years, the gradual structural and technological improvements of these devices, allowed the creation of real wind farms entirely dedicated to the production of electricity
Nowadays there are about 26 companies in the world specialized in the production of wind turbines
JAES, besides being a qualified partner for some of the most important wind turbine manufacturers, is constantly engaged in the supply of all those spare parts necessary for the production, assembly, repair and maintenance of wind turbines.
As we already mentioned at the beginning of this video, wind turbines transform mechanical energy into electrical energy. The wind energy caught by the wind turbines is in fact transferred thanks to a mechanical shaft to a generator.
The generator transforms the rotation energy into electrical energy, which in turn is transferred to a transformer, usually placed at the base of the wind tower. The transformer acts as a link between wind turbines and distribution grid. It steps up the low output voltage from the generator to higher distribution voltage level.
Let’s take a look at the various components of a wind turbine:
This is the TOWER, one of the most important parts. It represents the supporting structure of the wind turbine and has the function of absorbing the vibrations generated by the rotary motion of the blades.
These are the BLADES which are aerodynamically optimized to capture the maximum power from the wind and transfer it to the rotor hub. Each blade is usually 20 m or more in length, depending on the power level.
The ROTOR is the rotating part of the wind turbine. The rotor hub holds the wind turbine blades while connected to the gearbox via the low-speed shaft. Here we can find the pitch control system. Thanks to the pitch control, blades are turned in their longitudinal axis to change the angle of attack according to the wind directions. This mechanism also blocks the rotation of the rotor when the wind is too strong, or too weak.
The NACELLE is instead the enclosure of the wind turbine that houses all of the generating components, including: the gear box, the braking system and the generator, but also the pitch control system and the yaw system, which is responsible for the orientation of the wind turbine rotor towards the wind.
An anemometer places at the rear of the nacelle, in fact, constantly detects the wind direction. Whenever there’s a change in wind direction, the anenometer sends a signal to the yaw system, that provides to rotate the entire nacelle and consequently the rotor and the blades , to meet the wind at the best angle.
But now let’s see in detail what happens when the wind turbine goes into action:
The blades starts to turn when the wind reaches the so called “cut-in wind speed”, that is the point at which the turbine starts generating electricity from turning.
The rotor is connected to the drive shaft, which rotates inside the necelle.
The rotational energy of the drive shaft is transformed into electrical energy by the generator.
A generator is in fact a device that uses electromagnetic induction to produce electrical voltage.
One of the main elements of the power transmission of the wind turbine is the speed multiplier. Since the rotor blade rotation is low, the speed multiplier inside the gear box is used with the aim to rich higher values of the rotational speed, needed by the electric generator in order to produce the electrical power.
Most wind farms are also equipped with inverters, that convert direct current into alternating current at 220 volts, making it suitable for grid feeding or self-consumption
If the wind speed increases, there will be a gradual increase in the instantaneous power of the turbine, until reaching the rated wind speed, that is the wind speed in which the generator reaches its rated power output.
As the speed increases above the rated wind speed, the forces on the turbine structure continue to rise and, at some point, there is a risk of damage to the rotor. As a result, the braking system is employed to bring the rotor to a standstill. This is called the cut-out speed and is usually around 25 meters per second.
We’ve already said that the turbine’s blades start to turn when they’re hit by the wind. If we look closely we can notice that the blade has a lot of airfoil cross-sections consisting of different sizes and shapes from the root to tip.
The teardrop shape of the blade is very similar to the aerodynamics of the wing of the aircraft, which we’ve already discussed in our previous video. In that case, the air flow hitting the wing followed precise trajectories, allowing the molecules of the lower surface to have a higher density than those of the upper surface. The difference in pressure between the two parts allowed the wing, and consequently the whole aircraft, to lift up.
Also in this case, the lifting process of the wind turbine blades is very similar.
Thanks to the pitch control mechanism, the blade is tilted in order to be always aligned with the air flow. When the blade speed is increased to the tip, the relative wind speed becomes more inclined towards the tip. This means that a continuous twist is given to the blade from the root to tip
By observing the operating principle of wind turbines, we may be led to believe that a turbine with many more blades is more powerful than one with three, two or even a single blade.
This conclusion has some truth, more blades allow to achieve more power. But, the more blades there are on a wind turbine, the higher will be the torque, which is the force that creates rotation, and the slower the rotational speed, because of the increased drag caused by wind flow resistance.
So a wind turbine with many blades will be MORE POWERFUL but LESS EFFICIENT than one with less blades
A wind turbine with 3 blades placed at 120 degrees from each other, represents a good compromise between POWER and EFFICIENCY. A turbine with 4 blades means more weight and therefore more costs. Also a wind turbine with 2 blades will obtain similar performances but would require bigger dimensions and therefore higher costs.
We have seen so far that the design and implementation of a wind turbine involve a series of structural choices that are not directly linked to the POWER of the wind turbine, but take into account factors such as: costs, maintenance, noise pollution, landscape assessment and many others.
MAINTENANCE still represents a fundamental element for the correct functioning of these devices. Unfortunately, many accidents still occur if regular maintenance activities are not carried out.
The causes of these accidents can be mainly attributed to:
- bad weather conditions or thunderbolts.
- fires related to the malfunctioning of internal components or lubrication system.
- damage caused when foreign objects make impact with the blades.