Horizontal Axis Wind Turbine: Working Principle

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.

Outline:

• Discuss working of horizontal axis wind turbine
• How does a wind turbine produce an optimized power
• The power curve

Horizontal Axis 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 main components of wind turbine.

Wind Energy Conversion

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.

Angle of Attack

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.

Stall:

Stalling of turbine means increasing the angle of attack. As the angle of attack increases more surface area is available for aerodynamic forces.

Furl:

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 energy because wind turbines are designed to operate in particular wind speed (the rated speed for most turbines are 5m/s to 25m/s). Strong winds may damage the turbine, so mechanical and electrical brakes are provided, to stop the turbines.

Wind Energy conversion inside the Nacelle:

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.

How does a wind turbine generator produce power at rated frequency?

Just think about it, wind speed never remains constant, 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.

• Pitch and yaw adjustment (discussed above)
• Use frequency converters
• Use of modern DFIG

Frequency converters:

It is a simple and easy method, requires less complicated gearing mechanism. The block diagram of the frequency converter is given below.

• In this method, blades rotate freely within the specified range of wind speed
• The generator produces output according to the rotational speed of prime mover (blades). The output frequency is variable as we have discussed in the previous article
• This variable AC is then fed to frequency converters
• Electronic frequency converter produces the output frequency which is matched with the grid

Doubly Fed Induction Generator(DFIG):

It is a generator that can deal with unpredicted weather conditions and hence variable and uneven wind energy. It can work with variable wind speed and produce constant output frequency.

Power output of a wind turbine:

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 the wind turbine is:

P_in = ½ * ρAV³
P_out = C_P*½*ρAV³
C_P = P_out / P_in

Where
ρ = air density
A = blades swept area
V = velocity of the wind
C_P = power coefficient or efficiency of the wind turbine (C_P is always less 59.3%. In practice, this value wouldn’t achieve).

The Wind Turbine Power Curve

The power curve shows the relationship between wind speed and power output. Power output obtained at various wind speed is plotted.

Turbines are designed to work within a limited range of wind speed. The lower limit is called cut-in speed and the upper 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).

Wind Turbine Operating Speeds

Cut-in speed is the minimum speed required to generate electricity from the turbine. Cut-in speed is usually around 5 m/s.

 Cut-out speed is the maximum speed for turbines. Beyond this limit, there is a risk of damage. The braking system is there to stop the rotor. Cut-out speed is usually around 25 m/s.

Rated speed and rated output power: As the wind speed increases beyond the cut-in speed, the output power cubically increases with the 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.

Final Words

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. 

You may also want to read how electric energy is transmitted from generating stations to consumers and Load flow analysis of a power network.

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