In this article we explore what wind energy is, how it’s obtained, the different types of wind turbines currently used and how those wind turbines work.
What is wind energy?
Wind energy is obtained from the conversion of the kinetic energy of masses of air into the mechanical energy of wind turbine blades, which consequently produces electrical energy through a generator. Wind is produced when masses of air move from areas of high atmospheric pressure to areas of low pressure, aiming to reach an equilibrium. These areas of different pressure exist as a result of uneven heating of the Earth’s surface by solar radiation. In this way, the sun is also responsible for wind power generation.
How does wind energy work?
Depending on the type of wind turbine used, the wind must be moving at a minimum speed before electrical energy can be generated. The minimum speed is referred to as the cut-in speed and the maximum speed which the turbine is capable of handling is referred to as the cut-out speed. The cut-in speed usually starts around 3-4 m/s (12.2 km/h), and the cut-out speed is usually no more than 25 m/s (90 km/h).
What are the parts which make up a wind turbine?
A wind turbine system consists of the wind turbine itself, which is located in a tower, and an electrical generator that works in similar fashion to a windmill. A standard horizontal axis wind turbine is composed of:
- A tubular steel tower that can reach up to 110 meters of altitude.
- Blades with lengths up to 100 meters.
- A rotor or turbine, formed by the blades and the shaft to which they are attached, alongside wind evaluation equipment.
- A nacelle within which the entire turbine mechanism (axle, gearbox, multiplier, bushing, electrical generator, control, guidance and brake systems, and refrigerator for generator) are located.
- A transformer located at the base of the tower.
What does the wind turbine do & how does it work?
The process of transforming wind energy into electrical energy begins when the wind turbine blades are moved by the wind, transmitting kinetic energy to the axis to which they are anchored. This axis, in turn, is connected to the multiplier, whose function is to increase the rotor shaft speed up to 60 times. It then transmits its movement to the generator which produces the electricity that will be taken to the transformer. In turn, the transformer adjusts the voltage to pass this electricity to the mains. Wind potential is the real capacity of a wind turbine to absorb wind energy. Due to the slowdown suffered by the wind through the turbine, currently only 59% of the original kinetic energy can be converted into mechanical energy.
Types of wind turbines
There are also different types of wind turbines, depending on their power, the layout of their axis of rotation, the type of generator, etc. When defining what wind energy is, we can distinguish two main types of wind turbines, those with a horizontal axis and those with a vertical axis; although recently we are also seeing remarkable bladeless and offshore wind turbines.
Horizontal Axis Wind Turbines (HAWT)
These are the predominant type of wind turbines, with an axis of rotation parallel to the ground, located at the top of the tower and oriented to the wind. Small wind turbines are oriented by a wind vane, while the larger ones use a steering sensor oriented by servomotors or geared motors.
There are two main types of electric generators: with and without a multiplier box. Wind turbines that do not require a multiplier are known as “direct-drive”, which are designed to work with wind speeds that vary between an average of 3 m/s (connection speed) to 25 m/s (cutting speed). When the wind speed exceeds the connection speed the wind turbine begins to produce electricity. As the wind speed increases, so does the power generated.
- All machinery and wind turbine controls must be located at high altitude and supported by a structure that resists weight.
- Lots of wiring is necessary for the signals sent to the control systems, and to drive the generated current.
- In the case of a breakdown, it is necessary to go up to the nacelle.
Types of Horizontal Axis Wind Turbines
This type has many blades, and usually has a wind vane as an orientation system whose function is to place the plane of the propeller perpendicular to the wind direction.
This type has only a few blades and their power per unit of weight is much higher with respect to slow wind turbines. In this way they have advantages such as the ability to take advantage of the effect of increasing wind speed with height.
Vertical Axis Wind Turbines (VAWT)
Vertical axis generators have a rotation axis perpendicular to the ground. Their advantages are as follows:
- They occupy less surface since they can be located closer to each other, as they do not produce the air braking effects of HAWTs.
- Their blades are omnidirectional and therefore don’t need a wind orientation mechanism.
- They can be placed closer to the ground and have a simpler maintenance, since they work with a lower wind speed.
- They are much quieter than HAWTs.
- They are suitable for small installations (less than 10 kW) due to the ease of installation and the smaller size.
Their disadvantages are:
- They are unable to take advantage of the faster air currents available at greater heights.
- They have a low efficiency.
- They are more expensive to produce by square meter of occupied surface.
- They require a connection to the main network to start generating electricity, and do not start automatically.
- They are less reliable and stable as their rotor blades are more prone to breaking or bending with strong winds.
In VAWTs, the shaft rotor can be differentially driven, with or without a screen (Savonius), cyclic variation of incidence (Darrieus), or a mixture of both (Darrieus-Savonius).
Types of Vertical Axis Wind Turbines:
This type’s rotation is based on the differential force exerted by the wind on the blades, which are in the form of hemispherical or semi-cylindrical cups. In this way, the action of the wind produces different forces in the concave and convex part of the blade. To eliminate the harmful effect of the force acting on the lower bowl, an adjustable screen can be incorporated into the rotor by means of a rudder, vane and a deflector system that facilitates the channelling of the air flow over the active blades.
The rotor of cyclic variation of incidence is formed by a set of blades joined together, which can rotate around a vertical axis and whose straight section is shaped like an aerodynamic profile. The blades have an arched shape, are biconvex and can have different shapes. Rotation of the rotor is caused by the aerodynamic action of the wind on the blades, resulting in aerodynamic forces that give rise to the torque.
In this case the starting torque is very small and in practice requires an auxiliary start. Therefore, in some prototypes a Savonius rotor is combined to facilitate the start of the first rotation. The main advantage of the Savonius over the Darrieus is the simplicity of its construction and better torque values at low speeds.
Vortex Bladeless Wind Turbine
This type of wind turbine eliminates the environmental impacts caused by blades and is cheaper to produce than turbines with blades. The Vortex Bladeless wind turbine is made of carbon fiber and produces electricity through a physical effect that takes place when the wind hits the structure. This causes eddies to be produced around it that cause it to begin to oscillate and sufficient mechanical energy is generated to obtain electricity through its transformation.
Its advantages over conventional wind turbines include:
- They occupy less surface.
- They do not endanger birds.
- Landscape impact is reduced.
- Noise pollution is reduced, which means they can be placed closer to houses.
- Their construction is 53% cheaper than conventional ones.
- They do not produce any lubricant residue as it does not have gears.
- They can reduce carbon footprint by 40%.
- They are ideal for offshore plants due to their ease of installation and maintenance.
However, their main disadvantage is that they produce 30% less energy than a traditional wind turbine.
3. Types of wind energy for homes
Micro or mini wind turbines are usually used for housing to carry out micro-generation or mini-generation of wind energy.
Micro wind generators are for personal use and range from 50 W to a few kW. These devices are placed on a mast without the need for anchor cables in a place exposed to the wind, such as on the roof. It is generally more efficient if electricity is generated near the place where it is consumed, since losses in transport are minimized and it is also possible to store energy in batteries.
Mini wind generators are used in isolated areas where there is a great cost to carry the power of the electricity grid, where they can also be accompanied by photovoltaic solar panels. They also serve for installations with a high index of electrical consumption that want to reduce it, with the aim to consume the energy they need and sell the surplus to the grid. Mini wind generators only differ from micro wind generators in that their turbines have a power that should not exceed 100 kW.
Do you know the advantages and disadvantages of wind energy in general? Read our article here to find out!