Wind Energy
What is Wind Energy?
People have known how to make use of energy from the wind for centuries. The kinetic energy of the wind can be changed into other forms of energy, either mechanical energy or electrical energy.
- When wind fills a boat’s sail, the boat is using wind energy to push it through the water.
- For many years, farmers have been using wind energy to pump water from wells using windmills.
- Windmills have been used in countries like Holland for centuries to pump water from low-lying areas.
- Wind is used to turn grinding stones to grind wheat.
- Today, people use wind turbines to generate electricity.
How is Wind Formed?
Theoretically, about 1 to 2% of the sun’s radiation turns into wind energy when it arrives at the earth, which is about a hundred times of all the energy consumed on the planet.
The following diagram illustrates how wind is formed:
- The sun heats the ground surface of the earth which then heats up the air above it. The degree of solar heating varies at different points on the earth’s surface.
- Hot air rises, creating low pressure. As hot air rises, it cools, moves horizontally and eventually falls down, creating high pressure at ground level.
- These variations in pressure push the air at ground level to move from high pressure areas to low pressure areas.
- This air movement is wind.
Nature of Wind
Broadly speaking, a 10% increase
in wind speed will lead to an
approximately 30% increase in
power output from a wind turbine. |
Broadly speaking, a 10% increase
in wind speed will lead to an
approximately 30% increase in
power output from a wind turbine. |
Wind speed is very important to
the operation and efficiency of a
wind turbine. A minimum wind
speed of 3 m/s is normally needed
to drive a wind turbine and
generate electricity. |
Wind speed is very important to
the operation and efficiency of a
wind turbine. A minimum wind
speed of 3 m/s is normally needed
to drive a wind turbine and
generate electricity. |
Wind is affected by many climatic and geographic factors, and its behaviour is difficult to predict.
- Wind is intermittent in nature.
- Wind occurs both on land and at sea.
- Wind speed is affected by topography.
- Wind speed increases with altitude.
How to Extract Wind Energy?
Wind farms:
Nowadays, onshore and
off shore wind farms, or
groups of wind turbines,
are growing in popularity
as a means of generating
electricity.
Photo Courtesy of Shell
Wind Energy |
Wind farms:
Nowadays, onshore and
off shore wind farms, or
groups of wind turbines,
are growing in popularity
as a means of generating
electricity.
Photo Courtesy of Shell
Wind Energy |
Wind turbines:
Once wind drives the
turbine, making the
rotor rotate faster than
the cut-in speed,
the generator will
produce electricity.
Courtesy: DOE/NREL
Credit: TVIG, Inc. |
Wind turbines:
Once wind drives the
turbine, making the
rotor rotate faster than
the cut-in speed,
the generator will
produce electricity.
Courtesy: DOE/NREL
Credit: TVIG, Inc. |
All moving objects contain kinetic energy. The kinetic energy contained in wind, which is air in motion, can be transferred to other objects, such as boat sails, or transformed into electrical energy through wind turbine generators.
How is electricity generated by wind turbine?
| Major Component |
Function |
| Nacelle |
This contains the key components of the wind turbine, the gearbox and the electrical generator. Service personnel can enter the nacelle from the wind turbine tower. |
| Rotor blades |
Rotor blades capture the wind and rotate the shaft, transferring power to the rotor hub. The built-in yaw control system senses wind direction using the wind vane and rotates the turbine assembly so that the blades face the wind. |
| Tower |
The tower holds the nacelle and the rotor. It is better to have a high tower since wind speed increases with altitude. |
| Hub |
The hub of the rotor is attached to the shaft of the wind turbine. |
| Shaft |
The shaft connects the rotor hub to the gearbox. The shaft rotates and drives the electrical generator. |
How does it work?
- Wind blowing over the angled blades creates pressure differences across the blade sections, resulting in a turning force.
- The turning force on the rotor will turn the shaft, gearbox and generator, which are all connected.
- The gearbox increases the rotational speed, enabling the generator to produce electricity.
- The yaw control enables the wind turbine to capture the maximum wind energy by turning the rotor and nacelle to face the wind.
Horizontal and Vertical Axis Wind Turbines
Horizontal axis wind turbine (HAWT)
Horizontal axis wind turbine
(HAWT)
Courtesy: DOE/NREL
Credit: Milligan, Michael |
Horizontal axis wind turbine
(HAWT)
Courtesy: DOE/NREL
Credit: Milligan, Michael |
- HAWT needs a yaw control system to keep its rotor facing the wind.
- HAWTs are more common than vertical axis wind turbines since they have better performance coefficient.
Vertical axis wind turbine (VAWT)
Vertical axis wind turbine
(VAWT)
Courtesy: DOE/NREL
Credit: Thresher, Bob |
Vertical axis wind turbine
(VAWT)
Courtesy: DOE/NREL
Credit: Thresher, Bob |
- VAWTs can accept wind from any direction. They are usually small in scale. The small tower and ground-mounted generators on VAWTs make them easier to maintain.
- VAWTs are not as efficient as HAWTs because they require a motor for start up and have a lower performance coefficient based on present technology.
Wind Farms
Wind farms can be built onshore or offshore.
Onshore wind farms
Onshore wind farms continue to make up the majority of wind farms around the world.
-
Advantages
Lower construction costs compared with offshore wind farms, easy access for maintenance, relatively convenient to connect to power grids.
-
Constraints
Height restrictions for hilltop wind turbines, unsteady wind conditions, concerns over noise and visual impact on the environment.
Offshore wind farms
An offshore wind farm located at Rodsand
of Denmark, with 72 wind turbines, total
installed capacity of 165.6 MW.
Source: Nysted Offshore Wind Farm |
An offshore wind farm located at Rodsand
of Denmark, with 72 wind turbines, total
installed capacity of 165.6 MW.
Source: Nysted Offshore Wind Farm |
Offshore wind farms are typically constructed in regions with high population densities and scarcity of suitable sites.
-
Advantages
Steadier and stronger supply of wind than onshore wind farms, less visual impact, less likely to be affected by height restrictions than hilltop wind turbines.
-
Constraints
Higher construction costs, subject to water depth restrictions (most existing off-shore installations are in waters shallower than 20 metres. Special consideration is needed for installation at 30-metre water depth and above), less convenient access for maintenance.
Site Selection
A wind turbine or wind farm can share the land it occupies with other ground-level activities such as farming. However, building structures or even tall trees obstruct wind flow, affecting the effectiveness of the wind turbine.
In general, wind farm developers usually favour sites with the following characteristics:
- Strong and consistent wind.
- Open land without obstacles to block wind flow. Distance between two wind turbines has to be at least five to seven times the rotor diameter.
- Proximity to a suitable transmission network.
- Suitable geology for transportation and turbine foundations.
- Minimum impact on the environment.
Installed Wind Power Capacity Worldwide
Germany, Spain and the US currently lead the world in terms of total installed wind power capacity.
| Rank |
Country |
Installed Capacity |
| 1 |
Germany |
16,629 MW |
| 2 |
Spain |
8,263 MW |
| 3 |
USA |
6,740 MW |
Germany
Schuby Wind Farm, near Schleswig,
with installed capacity of 18 MW.
Photo Courtesy of Nordex GmbH |
Spain
Leitza-Beruete Wind Farm, Navarre,
Spain, with installed capacity of
19.2 MW.
Photo Courtesy of Acciona Energía |
USA
White Deer Wind Farm, Texas, with
installed capacity of 80 MW.
Photo Courtesy of Shell Wind Energy |
Germany
Schuby Wind Farm, near Schleswig,
with installed capacity of 18 MW.
Photo Courtesy of Nordex GmbH |
Spain
Leitza-Beruete Wind Farm, Navarre,
Spain, with installed capacity of
19.2 MW.
Photo Courtesy of Acciona Energía |
USA
White Deer Wind Farm, Texas, with
installed capacity of 80 MW.
Photo Courtesy of Shell Wind Energy |
Case Study
Denmark
Rejsby Hede Wind Farm, in Denmark,
consists of 40 turbines with a total
installed capacity of 24 MW.
Source: Elsam Kraft |
Rejsby Hede Wind Farm, in Denmark,
consists of 40 turbines with a total
installed capacity of 24 MW.
Source: Elsam Kraft |
Although Denmark has the world’s 4th-largest total installed wind power capacity at 3,118 MW as of the end of 2004, it is widely recognized as the pioneer in modern wind energy development.
Wind energy made up over 18% of Danish electricity consumption in 2004 – the highest percentage in the world. Most modern wind turbines adopt a three-bladed machine designed by Denmark, which accounts for 40% of the world market in wind turbine manufacturing.
China
A wind farm in Huitengxile, Inner
Mongolia, consisting of 72 turbines
with a total installed capacity of
42.7 MW.
Photo Courtesy of IT Power |
A wind farm in Huitengxile, Inner
Mongolia, consisting of 72 turbines
with a total installed capacity of
42.7 MW.
Photo Courtesy of IT Power |
China ranks 10th in the world in terms of total installed wind power capacity with 764 MW as of the end of 2004. Given China’s growing demand for power, the government is promoting the development of wind energy.
Benefits of Wind Energy
- It is clean and does not pollute the air
Wind turbines do not emit greenhouse gases or contribute to global warming.
- It does not deplete resources
Every 1 million units of electricity generated by a wind turbine can offset approximately 350 tonnes of coal.
- It is more cost-effective than other forms of renewable energy
As wind energy technology matures, construction and operating costs continue to drop, providing greater cost effectiveness.
Challenges of Wind Energy
Courtesy: DOE/NREL
Credit: Spink, Todd |
Courtesy: DOE/NREL
Credit: Spink, Todd |
- It is intermittent and unpredictable
Electricity cannot be stored economically in large quantities. It has to be generated to match the demand. As wind is unpredictable in terms of speed and direction, wind turbine generator outputs are not controllable or predictable. Wind energy alone cannot be relied upon as the sole source of electricity.
- Wind farms occupy large areas
Places with high population densities and land limitation often have difficulty finding the necessary space for wind farms.
- Wind turbines can impose adverse impact on the environment
Turbines can impact migrating birds and create noise, and some may consider them a visual blight.
Wind Energy Through the Years
- People have been raising sails to capture wind energy and push their boats through the water.
- Wind has been used to produce electricity since the 19th century. One of the earliest wind turbines was built in Cleveland, Ohio, in 1888 by Charles F. Brush. It featured 144 rotor blades made of cedar, and had a 12 kW capacity.
- As at 2005, the largest wind turbine is a pilot installation in Hamburg, Germany, with a capacity of 5 MW.