LECTURE No 6

Wind Power Generation

Introduction to wind energy More than 90 percent of the energy produced and consumed

in the world today is from non renewable sources. Such resources as coal, oil, natural gas, and the uranium used

for nuclear power cannot be replaced as they are used, or can only be replaced very slowly by natural processes.

Each of these sources has both benefits and drawbacks in terms of the ways it can be used, the jobs it provides, and the effects it has on the environment.

For example, today most fossil fuels are relatively plentiful and inexpensive.

But combustion of fossil fuels generates numerous air pollutants as well as gases that may contribute to global climate change.

Wind Farm Development

Wind Energy

Wind is moving air produced by uneven solar heating of the Earth's surface. Wind power has long been used for grinding grain and pumping groundwater.

Windmills' modern equivalent, tall wind turbines, use wind energy to generate electricity. Turbines catch the wind with blades mounted around a shaft to form a rotor.

On the downwind side of the blade, blowing wind forms a low-pressure pocket, which pulls the blade, turning the rotor to spin an electrical generator.

Wind power is now the fastest-growing energy source worldwide. However, land clearing for vast "wind farms" may produce environmental concerns. Many predict that wind energy will provide more U.S. electrical production as new turbine designs enhance economic and environmental viability.

Cont:

Wind power is converted to electricity by a wind turbine. In a typical, modern, large-scale wind turbine, the kinetic

energy in the wind (the energy of moving air molecules) is converted to rotational motion by the rotor – typically a three-bladed assembly at the front of the wind turbine.

The rotor turns a shaft which transfers the motion into the nacelle (the large housing at the top of a wind turbine tower).

Inside the nacelle, the slowly rotating shaft enters a gearbox that greatly increases the rotational shaft speed.

The output (high-speed) shaft is connected to a generator that converts the rotational movement into electricity at medium voltage (a few hundred volts).

The electricity flows down heavy electric cables inside the tower to a transformer, which increases

Cont:

the voltage of the electric power to the distribution voltage (a few thousand volts).

(Higher voltage electricity flows more easily through electric lines, generating less heat and fewer power losses.)

The distribution-voltage power flows through underground lines to a collection point where the power may be combined with other turbines.

In many cases, the electricity is sent to nearby farms, residences and towns where it is used.

Otherwise, the distribution-voltage power is sent to a substation where the voltage is increased dramatically to transmission-voltage power (a few hundred thousand volts) and sent through very tall transmission lines many miles to distant cities and factories.

Wind turbines come in a variety of sizes, depending upon the use of the electricity.

The large, utility-scale turbine described above may have blades over 40 meters long, meaning the diameter of the rotor is over 80 meters – nearly the length of a football field.

The turbines might be mounted on towers 80 meters tall (one blade would extend about half way down the tower), produce 1.8 megawatts of power (1.8 MW or 1800 kilowatts, 1800 kW), supply enough electricity for 600 homes, and cost over a million and a half dollars!

Inside the Nacelle

componentsAnemometer:Measures the wind speed and transmits wind speed data to the controller.Blades:Most turbines have either two or three blades. Wind blowing over the blades causes

the blades to "lift" and rotate.Brake:A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the

rotor in emergencies.Controller:The controller starts up the machine at wind speeds of about 8 to 16 miles per hour

(mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds.

Gear box: Gears connect the low-speed shaft to the high-speed shaft and increase the rotational

speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity.

The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gear boxes.

Generator: Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.High-speed shaft: Drives the generator.Low-speed shaft: The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.Nacelle: The nacelle is its atop the tower and contains the gear box, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on.Pitch: Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.Rotor: The blades and the hub together are called the rotor.

Tower: Towers are made from tubular steel (shown here), concrete, or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity.

Wind direction: This is an "upwind" turbine, so-called because it operates facing into the wind. Other turbines are designed to run "downwind," facing away from the wind.

Wind vane: Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.

Yaw drive: Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don't require a yaw drive, the wind blows the rotor downwind.

Yaw motor: Powers the yaw drive.

Types Types of Wind Turbines Modern wind turbines fall into two basic groups:1. horizontal-axis wind turbines (HAWTs) 2. vertical-axis wind turbines (VAWTs). As the name pertains, each turbine is distinguished by the

orientation of their rotor shafts. The former is the more conventional and common type everyone has come to know, while the latter due to its seldom usage and exploitation, is quiet unpopular. The HAWTs usually consist of two or three propeller-like blades attached to a horizontal and mounted on bearings the top of a support tower as seen in Figure.

When the wind blows, the blades of the turbine are set in motion which drives a generator that produces AC electricity. For optimal efficiency, these horizontal turbines are usually made to point into the wind with the aid of a sensor and a servo motor or a wind vane for smaller wind turbine applications.

Cont:

With the vertical axis wind turbines, the concept behind their operation is similar to that of

the horizontal designs. The major difference is the orientation of the rotors

and generator which are all vertically arranged and usually on a shaft for support and stability.

This also results in a different response of the turbine blades to the wind in relation to that of the horizontal configurations.

A typical vertical axis design is shown in Figure

Horizontal axis wind Turbines

Vertical axis wind Turbine

Sizes of Wind Turbines Utility-scale turbines range in size from 100 kilowatts to

as large as several megawatts. Larger turbines are grouped together into wind farms, which provide bulk power to the electrical grid.

Single small turbines, below 100 kilowatts, are used for homes, telecommunications dishes, or water pumping.

Small turbines are sometimes used in connection with diesel generators, batteries, and photovoltaic systems.

These systems are called hybrid wind systems and are typically used in remote, off-grid locations, where a connection to the utility grid is not available.

Electrical diagram

Small-scale wind power generation

A small Quiet revolution QR5 Gorlov type vertical axis wind turbine on the roof of Colston all in Bristol, England. Measuring 3 m in diameter and 5 m high, it has a nameplate rating of 6.5 kW.

Power in the Wind (W/m2)

= 1/2 x air density x swept rotor area x (wind speed)3

A V3

Density = P/(RxT) P - pressure (Pa) R - specific gas constant (287 J/kgK) T - air temperature (K)

Area = r2 Instantaneous Speed(not mean speed)

kg/m3 m2 m/s

Power in the Wind = ½ρAV3

– Effect of swept area, A– Effect of wind speed, V– Effect of air density,

Swept Area: A = πR2 Area of the circle swept by the rotor (m2).

Calculation of Wind Power

Wind velocity

212

kineticEnergy MV

M PAV

21 ( )2

kineticEnergy PAV V

312

kineticEnergy PAV

the effective functioning of a wind turbine is dictated by the windavailability in an area and if the amount of power it has is sufficient enough to keep the blades in constant rotation. The wind power increases as a function of the cube of the velocity of the wind and this power is calculable with respect to the area in which the wind is present as well as the wind velocity . When wind is blowing the energy available is kinetic due to the motion of the wind so the power of the wind is related to the kinetic energy.We know:

The volume of air passing in unit time through an area A, with speed V is AV and its mass M is equal to the Volume V multiplied by its density ρ so:

Substituting the value of M in 1st equation we get:

Cont:

To convert the energy to kilowatts, a non-dimensional proportionality constant k is

introduced where,

32.14 10k

3 3( ) 2.14 10power p PAV

3 3 3( ) 1.2 / / 2.33 10 /Airdesity p kg m slugs ft

( )Area A areasweptthebladesoftheturbine

( )velocity v windspeedinmph

US Wind Energy Capacity

0

2000

4000

6000

8000

10000

MW

2000 2001 2002 2003 2004 2005

U.S. Wind Energy Capacity

Large onshore wind farmsWind farm Current capacity (MW) CountryAlta (Oak Creek-Mojave)1,320 United States[19]

Capricorn Ridge Wind Farm662 United States[23][24]

Fowler Ridge Wind Farm600 United States[26]

Fântânele-Cogealac Wind Farm600 Romania[25]

Gansu Wind Farm6,000 China[17][18]

Horse Hollow Wind Energy Center736 United States[23][24]Jaisalmer Wind Park1,064 India[20]

Roscoe Wind Farm782 United States[22]

Shepherds Flat Wind Farm845 United States[21]

Whitelee Wind Farm539 United Kingdom[27]

Merits and Demerits• Environmental• Economic Development• Fuel Diversity & Conservation• Cost Stability• No air pollution• No greenhouse gasses • Does not pollute water with mercury• No water needed for operations