Objective

• Measure the Power Output of the 500W Horizontal Axis Turbine within a specified operating range and load.
• Plot the Power produced at different wind speeds as a function of the wind speed

Introduction and Background

Wind power is an alternative clean energy source and can be relied on for the long-term future instead of the conventional fossil fuel energy. A wind turbine is a way of producing cost-effective, pollution free energy. A single household can be run on the energy generated from a wind turbine. Because wind is a source of energy which is non-polluting and renewable, wind turbines create power without using fossil fuels, without producing greenhouse gases or radioactive or toxic waste. Wind power reduces global warming.

Basic Theory of a Wind Turbine

Wind turbine is a mechanical device which converts wind energy to electricity. Electricity produced from a wind turbine is basically Alternating Current. When wind with sufficient speed hits the blade of the turbine, it creates a force on the blade causing the momentum from the moving wind to be transferred into rotating shaft. The rotating shaft will be part of the generator which produces electricity. Basically the kinetic energy of the wind is converted into electrical energy. Energy is generated when the wind speed reaches about 4 to 5 m/sec, and a speed of 11 to 12 m/sec allows the turbines to generate at their rated capacity. They shut down when the wind exceeds 24 to 25 m/sec. There are a number of safety systems that can turn off a turbine if wind speeds threaten the structure. There are two predominant types of wind turbines
• HAWT – Horizontal Axis Wind Turbine
• VAWT – Vertical Axis Wind Turbine
This experiment utlises a Horizontal Axis Wind Turbine, in which the axis of rotation is horizontal, or parallel to the Earth’s surface. A commercial grade horizontal wind turbine has the following subsystems

Turbine Subsystems

– Rotor
     • 1,2, or 3 blades
     • Aero braking systems
     • Pitch controls
– Drivetrain
     • Gearbox
     • Hydraulic Systems
     • Shafts
     • Braking system
     • Nose cover (nacelle)
– Yaw system
     • Direction control
     • Perp. to the wind stream
– Electrical and Electronic
     • Generator
     • Relays
     • Circuit Breakers
     • Wiring
    • Drop Cables
    • Controls
    • Sensors
    • Power Electronics
– Support Systems
    • Roads
    • Ground Support Equipment
    • Power Substation

Image source: Wikipedia

Probably the most commonly activated safety system in a turbine is the "braking" system, which is triggered by above-threshold wind speeds. There are many kinds of braking system namely
Pitch control mechanism
In this system,the turbine's electronic controller constantly monitors the turbine's power output. At wind speeds over 25 m/s, the power output will be too high, at which point the controller tells the blades to alter their pitch so that they become unaligned with the wind. This slows the blades' rotation. Pitch-controlled systems require the blades' mounting angle (on the rotor) to be adjustable.
Passive stall control
The blades are mounted to the rotor at a fixed angle but are designed so that the twists in the blades themselves will apply the brakes once the wind becomes too fast. The blades are angled so that winds above a certain speed will cause turbulence on the upwind side of the blade, inducing stall.
Active stall control
The blades in this type of power-control system are pitchable, like the blades in a pitch-controlled system. An active stall system reads the power output the way a pitch-controlled system does, but instead of pitching the blades out of alignment with the wind, it pitches them to produce stall.

Woking of the Generator inside the turbine

Generator consists of a rotor and stator.When the Rotor is permanent magnets in this case, the stator should be stationary coils of wire and vice versa. When these magnets move past the coils of wire or vice versa, AC electricity is produced in the coils. It is then converted into DC electricity using a rectifier.

Parameters in Wind energy production

In order to gain enough data to obtain meaningful results, three details would have to be logged in real time:
     1. The output power of the turbine. (watts)
     2. The rotation speed of the turbine (rev/s)
     3. The wind speed (m/s)
The analysis would require instrumentation to measure 4 parameters:
    1. System voltage
    2. System current
    3. Turbine rotational speed
    4. Wind speed

Wind Turbine Power output variation with steady wind speed

The figure below shows how the power output from a wind turbine varies with steady wind speed.

 

                       

Cut-in speed:
At very low wind speeds, there is insufficient torque exerted by the wind on the turbine blades to make them rotate. However, as the speed increases, the wind turbine will begin to rotate and generate electrical power. The speed at which the turbine first starts to rotate and generate power is called the cut-in speed and is typically between 3 and 4 metres per second.

Rated output power and rate output wind speed:

As the wind speed rises above the cut-in speed, the level of electrical ouput power rises rapidly as shown. However, typically somewhere between 12 and 17 metres per second, the power output reaches the limit that the electrical generator is capable of. This limit to the generator output is called the rated power output and the wind speed at which it is reached is called the rated output wind speed. At higher wind speeds, the design of the turbine is arranged to limit the power to this maximum level and there is no further rise in the output power. How this is done varies from design to design but typically with large turbines, it is done by adjusting the blade angles so as to to keep the power at the constant level.

Cut-out speed:
As the speed increases above the rate output 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, a braking system is employed to bring the rotor to a standstill. This is called the cut-out speed and is usually around 25 metres per second.

Wind turbine efficiency or power coefficient:

The coefficient of power of a wind turbine is a measurement of how efficiently the wind turbine converts the power in the wind into electricity.

The available power in a stream of wind of the same cross-sectional area as the wind turbine can easily be shown to be

                                   Available power in the wind,  

                                                                       

where v is wind speed in metres per second, ρ is density in kilograms per cubic metre and A is the swept area of the trubine, r is rotor radius in metres then the available power,P is in watts.

The efficiency, μ, or as it is more commonly called, the power coefficient, cp, of the wind turbine is simply defined as the actual power delivered divided by the available power.

                           Coefficient of power =( Powe produced by the wind turbine/Total power available in the wind)

The Betz limit on wind turbine efficiency:

There is a theoretical limit on the amount of power that can be extracted by a wind turbine from an airstream. It is called the Betz limit.

                                                                  Betz limit =16/27