6 WIND ENERGY

 WIND ENERGY

Wind is simple air in motion. 

It is caused by the uneven heating of the earth‘s surface by the sun.

Since the earth‘s surface is made of very different types of land and water, it absorbs the sun‘s heat at different rates. 

During the day, the air above the land heats up more quickly than the air over water. 

The warm air over the land expands and rises, and the heavier, cooler air rushes in to take its place, creating winds.

At night, the winds are reversed because the air cools more rapidly over land than over water. 

In the same way, the large atmospheric winds that circle the earth are created because the land near the earth's equator is heated more by the sun than the land near the North and South Poles. 

Today, wind energy is mainly used to generate electricity. 

Wind is called a renewable energy source because the wind will blow as long as the sun shines.

Wind Power System Components

The wind power system comprises one or more wind turbine units operating electrically in parallel. 

Each turbine is made of the following basic components

✓  Tower structure
✓Rotor with two or three blades attached to the hub
✓  Shaft with mechanical gear
✓  Electrical generator
✓ Yaw mechanism, such as the tail vane
✓  Sensors and control

Because of the large moment of inertia of the rotor, design challenges include starting, speed control during the power-producing operation, and stopping the turbine when required. 

The eddy current or another type of brake is used to halt the turbine when needed for emergency or for routine maintenance.

In a modern wind farm, each turbine must have its own control system to provide operational and safety functions from a remote location.

It also must have one or more of the following additional components

✓Anemometers, which measure the wind speed and transmit the data to the controller.

✓Numerous sensors to monitor and regulate various mechanical and electrical parameters.

A 1-MW turbine may have several hundred sensors.

✓Stall controller, which starts the machine at set wind speeds of 8 to 15 mph and shuts off at 50 to 70 mph to protect the blades from overstressing and the generator from overheating.

✓Power electronics to convert and condition power to the required standards.

✓Control electronics, usually incorporating a computer.

✓Battery for improving load availability in a stand-alone plant.

✓Transmission link for connecting the plant to the area grid.

The following are commonly used terms and terminology in the wind power industry

Low-speed shaft

The rotor turns the low-speed shaft at 30 to 60 rotations per minute (rpm).

High-speed shaft

It drives the generator via a speed step-up gear.

Brake

A disc brake, which stops the rotor in emergencies. It can be applied mechanically, electrically, or hydraulically.

Gearbox:

Gears connect the low-speed shaft to the high-speed shaft and increase the turbine speed from 30 to 60 rpm to the 1200 to 1800 rpm required by most generators to produce electricity in an efficient manner.

Because the gearbox is a costly and heavy part, design engineers are exploring slow speed, direct-drive

generators that need no gearbox.

Generator

It is usually an off-the-shelf induction generator that produces 50- or 60-Hz AC power.

Nacelle

The rotor attaches to the nacelle, which sits atop the tower and includes a gearbox, low- and highspeed shafts, generator, controller, and a brake. A cover protects the components inside the nacelle. Some nonacelles are large enough for technicians to stand inside while working.

Pitch:

Blades are turned, or pitched, out of the wind to keep the rotor from turning in winds that have speeds too high or too low to produce electricity.

Upwind and downwind

The upwind turbine operates facing into the wind in front of the tower, whereas the downwind runs facing away from the wind after the tower.

Vane:

It measures the wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.

Yaw drive:

It keeps the upwind turbine facing into the wind as the wind direction changes. A yaw motor powers the yaw drive. Downwind turbines do not require a yaw drive, as the wind blows the rotor

downwind.