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Home » Renewable Energy » Horizontal-Axis Wind Turbine (HAWT) Working Principle | Single Blade, Two Blade, Three-Blade Wind Turbine
2. Least amount of vibration
4. Lowest energy cost when compared to other turbines with similar size blades1. Heavier than single- and two-blade turbines
2. Most capital-expensive of the three types
4. Requires the largest cranes to construct
6. Larger blades are more difficult to transport to the tower siteTwo-Blade Turbine1. Initial cost and weight are lower, and they are simpler to mount
2. Produces more energy than the single-blade turbine1. Noisier than the three-blade turbine
2. Produces less energy than the three-blade turbine (when blade size and speed are the same)Single-Blade Turbine1. Least expensive
2. Easiest to erect because of its lightweight and because the blade can be mounted while it is on the ground
3. Requires the smallest and lightest tower1. Noisier than the three-blade turbine
3. Most prone to vibration at the blade
Table 1 Advantages and Disadvantages of Single-, Two-, and Three-Blade Horizontal Axis Wind Turbines
In previous articles, you get to know about wind turbine and how it converts energy. We discussed important parts of a horizontal axis wind turbine. This article is intended to provide the function of each component in a wind turbine and the overall working of HAWT, control mechanism and control strategies, factors affecting the efficiency of the wind turbine.
We consider HAWT upwind turbines with three blades. This configuration is the most popular commercially.
The more the number of blades, the slower the rotor speed. So, turbines with 3 blades are relatively slower but will gain a high efficiency and a high torque. Wind turbines with a single blade are high-speed wind turbines.
As we discussed in a previous article, in upwind turbines rotor blades and nose face towards the wind. Wind vane detects the direction of air, while yaw mechanism is there to maintain the position of the rotor as the direction of wind changes.
The figure shows simplified diagram of maincomponents of wind turbine.
What happens when air strikes the wind turbine blades?
As the wind strikes the blades, it tends to rotate them due to aerodynamic forces. Blade pitch control is an electronic control for blades. The power output of the turbine is monitored every second. As the power output reaches the rated limit, then controller immediately adjust (pitch) the blades a few degrees.
Now it’s time to introduce angle of attack. The angle at which the blades adjusted, to get optimized wind energy, and hence maximize the power output.
Stalling of turbine means increasing the angle of attack. As the angle of attack increases more surface area is available for aerodynamic forces.
Furling of turbine means decreasing the angle of attack. Blades are adjusted in a way that edges are facing towards the wind. It is applicable when there is strong wind and less wind energy is enough to drive the turbine.
We don’t want to get maximum wind energybecause wind turbines are designed to operate in particular wind speed (therated speed for most turbines are 5m/s to 25m/s). Strong winds may damage theturbine, so mechanical and electrical brakes are provided, to stop theturbines.
Nacelle contains a low-speed shaft, a gear-box, a high-speed shaft, brakes and a generator and braking mechanism. The rotor is attached to the nacelle. As the rotor blades move due to aerodynamic forces, low-speed shaft attached to rotor hub moves as well. The gearbox transforms slow rotations of low-speed shaft into high-speed rotation.
The high speed shaft connects gearbox and generator. The high speed is required to derive the generator efficiently.
Braking system is there to limit over speed or it is used to stop turbine whenever it is needed.
Just think about it, wind speed never remainsconstant, so the output frequency changes whenever wind speed changes (Read electrical generator in the previous article). Of course, this is not going to happen. The electronic controller is there that keeps output within the limited range. The output frequency can be maintained by employing these ways.
It is a simpleand easy method, requires less complicated gearing mechanism. The block diagram of thefrequency converter is given below.
It is a generator that can deal withunpredicted weather conditions and hence variable and uneven wind energy. Itcan work with variable wind speed and produce constant output frequency.
You must know about Betz law. It is the law by which you can determine the amount of power you can generate, irrespective of the design. According to Betz law, maximum 59.3% of kinetic energy of wind, a wind turbine could capture. The factor 59.3% is called Betz coefficient. The output power of thewind turbine is:
Pin = ½ * ρAV3Pout = CP*½*ρAV3CP = Pout / Pin
Whereρ = air densityA = blades swept areaV = velocity of the windCP = power coefficient or efficiency of the wind turbine (CP is always less 59.3%. In practice, this value wouldn’t achieve).
The power curve shows the relationship betweenwind speed and power output. Power output obtained at various wind speed isplotted.
Turbines are designed to work within a limited range of wind speed. The lower limit is called cut-in speed and theupper limit is called cut-out speed. In between these limits, there’s a rated speed at which you can get rated output power (as shown in the graph).
Cut-in speed is the minimum speed required to generate electricity from the turbine. Cut-in speed is usually around 5 m/s.
Ratedspeed and rated output power: As the wind speedincreases beyond the cut-in speed, the output power cubically increases withthe wind speed (look at above equations).
The power output from the generator also has limits. There is a speed limit at which we can get maximum output from the generator. This limit is called rated power output. Beyond this speed limit, the output power is no more cubically increases because of the turbine design. Look at the straight part of the curve.
We hope you''ve found out about the basic functioning of a wind turbine and how they convert wind energy into electric energy. There are other energy resources that have been discussed in detail.
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