Wind Power

WHAT IS WIND ENERGY ?

wind turbine

Wind Power

Wind is air on the move. Each air molecule has kinetic energy  because it is moving. The energy of the wind is the combined kinetic energy of all of the molecules. The wind is a free and renewable source of energy, and it is tae world’s easiest-growing electricity resource.

 

HOW IS ELECTRICITY GENERATED FROM WIND?

Electricity companies catch the energy of the wind using wind turbines. A ‘wind farm’ will often contain many identical turbines. When the wind ows through the blades of a turbine, they rotate and spin, powering a rotor inside a generator and producing electricity.Each turbine works independently.

MODERN WIND MACHINES

Today, wind is harnessed and converted into electricity using machines called wind turbines. The amount of electricity that a turbine produces depends on its size and speed of the wind. Most large wind turbines have the same basic parts: blades, a tower, and a gearbox. These parts work together to convert the wind’s kinetic energy into mechanical energy that generates electricity.
1. The moving air spins the turbine blades.
2. The blades are connected to a low-speed shaft. When the blades spin, the shaft turns.
3. The low-speed shaft is connected to a gearbox. Inside the gearbox, a large slow-moving gear turns a small gear quickly.
4. The small gear turns another shaft at high speed.
5. The high-speed shaft is connected to a generator. As the high-speed shaft turns the generator, it produces electricity.
6. The electric current is sent through cables down the turbine tower to a transformer that changes the voltage of the current before it is sent out on transmission lines.
Wind turbines are most efficient when they are built where winds blow consistently at least 5.8 m/s (meters per second) (13 miles per hour). Faster winds generate more electricity. High above ground, winds are stronger and steadier. So wind turbines should be placed on top of towers that are at least 30 meters (100 ft) tall.
There are many different types of wind turbines with different blade shapes. Wind turbines can be designed to optimize output for specific ranges of wind speed. While one turbine might operate efficiently in winds as low as 2.5 m/s (5.6 mph), another may need winds up to 20 m/s (44.8 mph).
Wind turbines also come in different sizes, based on the amount of electrical power they can generate. Small turbines may produce only enough electricity to power a few appliances in one home. Large turbines are often called utility-scale because they generate enough power for utilities, or electric companies, to sell. The largest turbines in the U.S. produce 2.5-3.5 MW, enough electricity to power 750 to 1,750 homes. Large turbines are grouped together into wind farms, which provide bulk power to the electrical grid.
What a Drag—Aerodynamics
Efficient blades are a key part of generating power from a wind turbine. The blades are turned by the wind and spin the motor drive shaft while, at the same time, they experience drag. This mechanical force slows down the whole system, reducing the amount of power that is generated.
Drag is defined as the force on an object that resists its motion through a fluid. When the fluid is a gas such as air, the force is called aerodynamic drag, or air resistance. Aerodynamic drag is important when objects move rapidly through the air, such as the spinning blades on a wind turbine. Wind turbine engineers who design rotor blades are concerned with aerodynamic drag. Blades need fast tip speeds to work efficiently. Therefore, it is critical that the rotor blades have low aerodynamic drag.
There are many ways to reduce drag on wind turbine blades:
♦ Change the pitch: the angle of the blades dramatically affects the amount of drag.
♦ Use fewer blades: reduce drag by using fewer blades; each blade is affected by drag.
♦ Use light-weight materials: reduce the mass of the blades by using less material or lighter material.
♦ Use smooth surfaces: rough surfaces, especially on the edges, can increase drag.
♦ Optimize blade shape: the tip of a blade moves faster than the base; wide, heavy tips increase drag.

GEARING UP FOR MORE POWER

Another key part of generating power in a large wind turbine is the gears. Power output is directly related to the speed of the spinning drive shaft (revolutions per minute or rpm’s) and how forcefully it turns (torque).
A large wind turbine has a rotor with blades, a gearbox, and a generator. As the blades spin, the rotor rotates slowly with heavy torque. The generator has to spin much faster to generate power, but it cannot use all the turning force, or torque that is on the main shaft. This is why a large wind turbine has a gearbox.
Inside the gearbox, there is at least one pair of gears, one large and one small. The large gear, attached to the main shaft, rotates at about 20 rpm with a lot of torque. This large gear spins a smaller gear, with less torque, at about 1500 rpm. The small gear is attached to a small shaft that spins the generator at high speed, generating power. The relationship between the large and small gears is called the gear ratio. The gear ratio between a 1500-rpm gear and a 20-rpm gear is 75:1. Some small residential wind turbines spin much faster and do not have gears.
Wind Turbine Efficiency—Betz Limit
Wind turbines must convert as much of the available wind energy into electricity as possible to be efficient and economical. As turbines capture energy from the wind, the resultant wind has less energy and moves more slowly. If the blades were 100 percent efficient, they would extract all of the wind’s energy and the wind would be stopped. The maximum theoretical percentage of wind that can be captured has been calculated to be about 59 percent. This value is called the Betz Limit and modern turbines are designed to approach that efficiency. Most turbines today reach efficiencies of 35-45 percent. The total efficiency of a typical wind turbine system is 10-30 percent of the available wind energy, since there are conversion losses at every step in the system.

WIND FARMS

Wind power plants, or wind farms, are clusters of wind turbines grouped together to produce large amounts of electricity. These power plants are usually not owned by a public utility like other kinds of power plants are. Private companies own most wind farms and sell the electricity to electric utility companies.
Choosing the location of a wind farm is known as siting a wind farm. To build a wind farm, wind speed and direction must be studied to determine where to put the turbines. As a rule, wind speed increases with height and over open areas with no windbreaks. The site must have strong, steady winds. Scientists measure the wind in an area for one to three years before choosing a site.
The best sites for wind farms are on hilltops, the open plains, through mountain passes, and near the coasts of oceans or large lakes. Turbines are usually built in rows facing into the prevailing wind. Placing turbines too far apart wastes space. If turbines are too close together, they block each other’s wind.
There are other things to consider when siting a wind farm, such as:
What is the weather like? Do tornadoes, hurricanes, or ice storms affect the area? Any of these may cause expensive damage to the wind turbines and associated equipment.
Is the area accessible for workers? Will new roads need to be built? New roads are expensive.
Can the site be connected to the power grid? It is expensive to lay long-distance transmission lines to get electricity to where people live, so wind farms should be located near transmission lines with available capacity.
Will the wind farm impact wildlife in the area? Developers building a wind farm need to get permission from the local community and government before building. There are strict building regulations to follow.

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