Wind Energy System for Electricity Generation: All You Need to Know

Wind energy is a renewable energy resource blessed by nature. It is one the most abundant form of renewable energy resource, which is available throughout the year. It is cost-effective and a reliable source of energy. Unlike fossil fuels, it is environment-friendly, clean, and free from greenhouse gases.  Wind energy system is more popular than a solar energy system because it can generate electricity 24 hours a day while solar systems can generate electricity only when the sun shines. The energy crisis faced by the world can be eradicated by the good utilization of renewable energy resource.

In this detailed post, you will get to know how wind energy is converted to electrical energy, various components a wind energy system and integration of a wind energy system into the conventional power system. Various parts of a typical Wind Energy Conversion System (WECS) are mentioned and elaborated.

Outline:

• Brief historical background
• What is a wind energy system?
• Explanation of important terminologies
• Conversion of wind energy into electrical energy
• Factors affecting wind energy system
• Small wind energy system (for households)
• Frequently asked questions

Historical Background:

The concept of wind energy system as a source of power is as old as human beings. Men used to sail boats with the power of wind. The people in past used wind energy in milling machines. Chinese used to pump water with long tower windmill. There were sawmills for cutting logs and lumbers. Men had to harness the nature, coming up with brilliant ideas to make his life full of comforts.

Today, scientists are using wind energy as a renewable source of energy to generate electricity.

What is a Wind Energy System?

Wind is movement different gasses or air in the horizontal direction flow from higher pressure to lower pressure region. Speed and direction are two parameters to specify wind. Have you ever thought, what causes the wind to blow?

Causes of Wind:

The wind blows because of the difference in air pressure and the rotation of the earth. Due to the uneven heating of the earth because of solar radiation some regions of the earth get warmer while the rest of the regions are colder. Warmer regions receive more light and heat from the sun while colder regions receive less light and heat from the sun. Warmer regions have warm air (lighter in weight) while colder regions have cold air (denser in weight). This causes a difference in air pressure and air temperature. Of course, higher the difference in atmospheric pressure, the higher the speed of the wind.

Wind energy or wind power is the process of generating electricity with the help of wind. As we discussed, air in motion is called wind. You know everything in motion possesses kinetic energy. So, wind also has kinetic energy. Men harness this kinetic energy of winds and used to generate mechanical energy which in turn converts into electrical energy.

Important terms:

Wind Turbines:

It is a power generating device, which converts kinetic energy of wind into mechanical energy, which is then converted into electricity and send to the grid. It is one of the most efficient way to convert wind energy into mechanical energy. Wind turbines can either be Horizontal Axis Wind Turbines (HAWT) or Vertical Axis Wind Turbines (VAWT). The difference between the two categories is the rotational axis.

Wind Farms

Wind farms are the dedicated area of land, where a large number of wind turbines are installed for generating electricity. The wind farms are designed in such a way that the land should not waste. For example,the two turbines are separated from each other by a calculated distance, tominimize mutual interference. Also, the land can be used to agriculturalpurpose and livestock rearing.

There is a calculated distance between the two turbines. The two turbines are apart from each other by 5 to 10 times the diameter of blades. For example, if the diameter of the turbine blades is approximately 15 meters. The two turbines are approximately 150 meters apart. The spacing between the turbines agricultural purposes visible in the figure below.

Offshore wind farms:

Offshore (on the sea) winds arise from the sea. Offshore farms generate electricity from sea breeze or install near the sea.

Onshore wind farms:

Onshore (on land) winds arise from land. Onshore farms generate electricity from offshore winds.

Types of Wind Turbines:

There are variety of designs available for a wind turbine. Turbines are classified according to the axis of rotation. In horizontal axis turbines, the axis of rotation (rotor’s rotation) is parallel to the ground or rotor shaft is pointed in the direction of the wind. In vertical axis turbines, the axis of rotation (rotor’s rotation) is perpendicular to the ground. In this article, we are going to discuss commonly used wind turbine that is Horizontal Axis Wind Turbines (HAWT).

Horizontal Axis Wind Turbines (HAWT):

HAWT is further classified according to the number of blades. That is single blade rotor, two blades rotor and three blades rotor.

Another classification of HAWT according to the position of a rotor. Upwind turbines and downwind turbines.

I am not going into the details of various kinds. You can easily understand the design and its properties here.

The article covers the design of 3 blades, upwind horizontal axis wind turbine.

So, let’s get started.

You have seen a wind turbine is mounted on a tower with a strong foundation. The connection to the grid is provided at the foundation. The nacelle and rotor are at the top of the tower. To access nacelle and rotor there are stairs inside the tower. The rotor consists of blades, blade pitch control and rotor hub.

1.  Foundation

Wind turbines have heavy, bulky and very tall structure, they need a strong foundation. Modern turbines are even taller, heavier and powerful. Onshore and offshore wind turbines need different types of foundations. Of course, the foundation should be strong and rigid enough to support such a huge structure. For example, GE-1.5 megawatt model whole assembly weighs 164 tons and a total height of 328 feet [5].

2.  Connection to grid

Connection to the grid is necessary because if the wind is not enough to generate electricity. Grid supplies your electricity needs. When the wind blows, the excess power is fed back to the grid.

3.  Tower

Tower height consists of 40m to 100m. Taller towers capture more energy and hence generating more electricity. If the tower height doubles then its diameter is also going to be doubled. Tubular steel, lattice steel or concrete towers are popular. For HAWT the tower height must be two to three times greater than blades length.

4.  Stairs to access top of tower

In order to access the top of the tower, for maintenance and repairs, stairs are provided inside the tower.

5.  Yaw mechanism

Yawning is the movement of the turbine along the vertical axis. Yaw mechanism consists of the yaw motor. Yaw mechanism orients turbines to move according to the wind direction and speed. It is a part of upwind turbines only. The yaw mechanism communicates with the wind vane (through the controller) and turns the nacelle with the help of motors.

6.  Nacelle

The Nacelle is nothing but the housing of generator, gearbox, braking system, shafts etc. It is a lightweight box made from glass fiber. It connects the tower to the rotor.

7.  Electrical generator

Of course, another most important part of wind turbines. Electrical generator gets input from high-speed shaft which rotates at the speed of (1000-1800 rpm). The generator converts rotational motion into electrical energy. The electrical generators used in turbines are either asynchronous generators or induction generators. Modern turbines can generate 600-3000 kilowatts (kW).

In a wind power system, we cannot alter wind speed and hence we cannot speed up the prime mover (rotor blades).

Induction Generator

The frequency of the generator is proportional to the speed of prime mover. So modern turbines utilize induction generators. Also, induction generators have a simpler structure, low investment costs, and low maintenance. Majorly, two types of induction generators are used in the wind energy system: Self Excited Induction Generators (SEIG) and Doubly Fed Induction Generators (DFIG).

The frequency of electricity generated is proportional to wind speed. The output of the generator is AC (having variable frequency or mismatched with transmission line) is converted to DC. The DC voltage is then converted to AC (frequency matched with the power grid).

8.  Anemometer and wind vane

As you know anemometer is used to measure the speed of the wind. And wind vane tells us the direction of the wind. Of course, this is an important part for controlling the turbines automatically.

Wind turbines are designed to work in moderate weather. If there is wind speed greater than 25m/s then a signal from anemometer to the controller and controller shuts off the turbine. Similarly, turbines don’t work if the wind speed is as low as 5m/s. This system automatically takes the decision and protects the turbine in extreme weather.

9.   Braking mechanism

Braking mechanism also provides safety of turbines in case of malfunctioning, over-speed (wind speed is unpredictable), overheating (generator).

Two types of braking mechanism available. These are aerodynamic and mechanical braking mechanism. Both of them work independently. Mechanical brakes are situated on between generator and gearbox. They are used as parking brakes

Aerodynamic brakes are used to turn the rotor blades perpendicular to the direction of the wind. All commercial HAWTs control pitch angle of their rotor blades, to minimize rotation speed.

10. Gearbox

The Low-speed shaft is connected to the left of gearbox. And the gearbox is connected to the braking mechanism and high-speed shaft. The gearbox transforms the low shaft speed (30-60 rpm) into faster speed (about 1000-1800 rpm). This speed is necessary to drive the electrical generator.

11. Rotor blades

The blades are foremost part of a turbine. Rotor blades are connected to the rotor hub. They face the wind, as the wind blows, blades try to rotate and lift according to the wind speed and power. The blades transmit power to the rotor hub.

12. Blade pitch control

Blade pitch control or blade angle adjustment. It is used to adjust rotation speed. Wind turbines have maximum and minimum speed limits. If the wind blows above or below these ranges turbines shut down. Pitch is an important part of large turbines. Wind turbines are designed to produce optimum output. But how this optimum output can be generated if the wind blows faster or slower than the optimum range. The speed and power of wind change frequently. The purpose of blade pitch is to adjust the blades (blades turn along the longitudinal axis) according to wind condition. For faster winds, it exposes less surface area to the wind.

13. Rotor hub

Rotor hub connects the rotor blades to the rest of the system. Rotor hub and rotor blades collectively called rotor. The rotor hub is connected to the nacelle.

14. Low speed shaft

The rotor is connected to the low-speed shaft. And low-speed shaft is connected to the left of the gearbox. As the rotor spins, the low-speed shaft spins as well. In this way, the rotor transfers its mechanical energy (which is in the form of rotation) to the low-speed shaft. It can rotate up to low speed at 30-60 rpm.

15. High speed shaft

Gearbox low-speed to a high-speed shaft. And high-speed shaft drives the electrical generator. The high-speed shaft rotates at 1500-1800 rpm. Most of the generators require to speed up to 1800 rpm. It is equipped with an emergency braking system. Mechanical as well as an aerodynamic braking system. The Mechanical brake disc low-speed when there is the failure of the aerodynamic braking system.

 16. Controller:

This is an electronic controller effectively controls the turbines. You can optimize or control the power output of turbines by these control methods.

• Generator torque and speed
• Blade pitch control
• Rotation of the turbine (yaw mechanism)

Factors Affecting Wind Energy System:

There are certain parameters that can affectthe performance of the wind energy system

• The shape and the length of turbine blades
• Height
• The speed of the wind
• Air density

1.   The shape and the length of blades

The output of a wind turbine is directly proportional to the size of blades. Larger the size of blades, the higher the output power. Of course, large blades can capture more wind energy than smaller once. Or you can say that the larger blades can translate more kinetic energy of wind into mechanical energy and hence increase electrical power output.

2.   Height

At higher altitudes, there are less obstruction like tree, buildings etc. At higher altitudes wind is more consistent. Doubling the height of tower doubles the output power. It doesn’t mean you can erect a wind turbine on the mountain. Because at higher altitudes air density decreases.

3.   The direction and speed of wind

The output of the wind turbine is dependent on both the speed and density of the wind. Of course, higher wind speeds tend to rotate blades faster. Due to the faster rotation, more mechanical energy converts into electrical energy.

P = 0.593*½*ρAV³

Where P = wind power
V = wind speed
A = rotor area
ρ = density of air

Betz Limit on Efficiency of a Wind Energy System

The constant 0.593 in the above equation is the Betz Limit. It was introduced by Albert Betz, a German physicist, in 1919. According to Betz’s law, no turbine can capture more than 16/27 (59.3%) of the kinetic energy in wind. The factor 16/27 (0.593) is known as Betz’s coefficient. So, the efficiency of a wind energy system can not surpass the theoretical limit of 59.3%. Practical utility-scale wind turbines achieve at peak 75% to 80% of the Betz limit.

The faster the wind speed, the greater the output of the turbine. It means wind speed is a major factor that can affect the efficiency of the whole system. Whenever someone decides to erect wind turbines, it is advisable to look into wind conditions at that place, for at least one year.

4.   Air density

The third factor is air density, which is dependent on temperature, pressure and altitude. Colder air is denser and it exerts more pressure on rotor blades and hence increases output power.

Frequently Asked Questions:

Difference between a windmill and a wind turbine/Wind Energy Sytstem

In essense, there is no difference in between the windmill and wind turbine. The windmill is an ancient term when wind energy used to drive mills (that is why it is named as windmill) and pump water. While a wind turbine (being a part a Wind Energy System) is used to generate electricity by converting wind energy into electrical power. Wind turbines have fewer blades than windmills. Windmills have to do more physical work, that’s why they are provided with more blades. Windmill converts wind energy into mechanical energy. While the wind turbines convert wind energy into electricity.

Do wind turbines produce noise?

HAWT are noisy and are popular in areas away from downtown. Commercial wind turbines produce noise just like a small jet plane. It causes health problems and noise pollution. In urban areas or home-based wind turbine systems utilize vertical axis wind turbines (VAWT). Vertical axis turbines are the first choice for homes because it requires less space and is less noisy. But the efficiency of a VAWT may be less that of a HAWT in a wind energy system.

How do wind turbines work when there is no wind?

If there is no wind, no electricity is produced. To derive a turbine the wind speed should be greater than the cut-in speed (that is approximately 4m/s). The anemometer is there to measure the speed of the wind.

Although, the Capacity Factor of a wind energy system is higher than many other renewable energy resources; but still it is much less than that of a conventional power plant. So, typically, a wind energy system is backed up with other renewable or conventional energy resources to keep a continuous supply of electrical power to the consumers.

Bibliography:

[1] Wikimedia online
[2] Small wind electric system
[3] Wind energy foundation
[4] Energy education
[5] Windturbine FAQs
[6] Electrical Academia

Final Words

We hope you’ve found out about the basic functioning of a wind energy system and how they convert wind energy into electric energy. There are other energy resources that need to be 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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