Small guide through wind parameters

2008-11-16T03:58:00.000-08:00

Origin of Wind

The regions around equator, at 0° latitude are heated more by the sun than the rest of the globe . Hot air is lighter than cold air and will rise into the sky until it reaches approximately 10 km (6 miles) altitude and will spread to the North and the South. Since the globe is rotating, any movement on the Northern hemisphere is diverted to the right. In the Northern hemisphere the wind tends to rotate counterclockwise . In the Southern hemisphere the wind rotates clockwise around low pressure areas. As air has mass, its motion result in kinetic energy which can be used for transforming in another kind of useful energy. Things are simple, but are not so simple. This all happen on the troposphere level, around 10km altitude (up to), and are named Global Winds. At the altitude of around 1km, we have so named Geostrophic Winds, which are derivates of temperature and pressure difference, but are not yet influenced by the earth's relief. Winds which are influenced of earth surface, take place on the altitude of 100m, and are named Local Winds. Now lets see what are factors which influence local air movement. Difference in heating of land and see, or ocean is one of the reasons(breezes). Difference in heating on valleys, and mountains is another reason for moving air.

These are the winds we consider for converting their kinetic energy in electric and mechanic energy. An excellent manual to learn all about is on the site of Danish Wind Industry Association. This small guide is just for become familiar with parameters which define Wind Energy.

Factors affecting wind

Several forces affect the motion of the air, which make wind energy very useful.

Coriolis effect
Pressure gradient force
Topography and friction
Density

Lets see one by one:

Coriolis effect We've seen in the chapter before, the movement of the earth, causes deviations in the movement of air. This force is known as Coriolic effect. The Coriolis effect acts on all bodies on the earth. The Coriolis force is a visible phenomenon. Railroad tracks wear out faster on one side than the other. River beds are dug deeper on one side than the other. (Which side depends on which hemisphere we are in: In the Northern hemisphere moving particles are bent towards the right).

Pressure gradient force Air pressure is the pressure caused by the weight due to the effect of gravity.The force of gravity gives the air weight, causing the air closest to the surface to be compressed by the weight of air above it. Air tries to move in a straight line between an area of high pressure to an area of low pressure. The pressure gradient force predicts that wind should cross isobars at right angles. Once the wind is in motion, the Coriolis effect begins to act in the opposite direction to the pressure gradient force. When these forces become equal, the wind moves parallel to the isobars. These winds are so named Geostrophic Winds. The geostrophic wind is found at altitudes above 1000 meters (3300 ft.) above ground level. The geostrophic wind speed may be measured using weather balloons.

Topography and friction Local winds are always superimposed upon the larger scale wind systems, i.e. the wind direction is influenced by the sum of global and local effects. When larger scale winds are light, local winds may dominate the wind patterns.As the flow of air is always toward areas of low pressure, the wind tends to move slightly inward across isobars towards centers of low pressure. The opposite is true for high pressure systems as the movement of the air is outward across isobars. This factor is the major contributor to the circulation of air within atmosphere. The friction of the wind across the surface of the earth also creates an effect known as turbulence. It is caused when an obstruction interrupts the smooth laminar flow of wind, causing the wind to deviate from its preferred path and go around it. Obstructions may be hills, trees, mountains or buildings.

Density of air The kinetic energy of a moving body is proportional to its mass (or weight). The kinetic energy in the wind thus depends on the density of the air, i.e. its mass per unit of volume. In other words, the "heavier" the air, the more energy is received by the turbine. At normal atmospheric pressure and at 15° Celsius air weighs some 1.225 kilogram per cubic meter, but the density decreases slightly with increasing humidity. Also, the air is denser when it is cold than when it is warm. At high altitudes, (in mountains) the air pressure is lower, and the air is less dense.

Use of Wind Energy

Converter of wind energy into a useful, mechanic or electric energy is wind pump, or wind turbine. Force of blowing wind is acting on the blades of turbine, converting kinetic energy of wind in a torque, acting on the rotor blades. Value of converted energy depend on air density and wind speed. As the kinetic energy is proportional to a body's mass, this mean the heavier air, the higher energy. Cold air is more dense than hot air. A surface where wind is acting is another element, which determine the quantity of obtained energy. This mean that a rotor diameter is very important element for wind efficiency. About size of wind turbines will be more discussion. Just keep in mind that by doubling a rotor area, you can get four times higher quantity of energy. Energy of wind is also proportional with the cube of wind speed. A formula which give us power which act on the swept area of rotor blades can be expressed:

P = 0.5 x rho x A x v³

Where:

P is power in watts

rho is density of air (1,22kg/m³ on sea level)

A exposed surface of rotor area

v speed of wind in m/sec This is ideal quantity a turbine can generate. In fact things are worse. Some elements like turbine efficiency, generator efficiency influence considerable amount of generated power. Turbine efficiency is maximum 60% in reality is around 35%. Generator efficiency is higher, above 80% for grid connected generators.

Lets see more detailed Wind Parameters, and Parameters of Wind Turbines on next pages.

Wind Parameters

Two parameter influence kinetic energy of wind. Density and speed.

Wind density

Density is by definition mass per volume unit. Density of wind is in fact density of air. This mean the heavier the air, more energy obtained by turbine. Density of air is 1,225 kg/meter cube. If there is higher humidity the air is less dense. Also on high altitudes, mountains, the air is less dense. Also should keep in mind that the cold air is more dense than warm air. Another characteristic of wind is:

Wind speed

Wind speed is extremely important, because the amount of energy delivered by wind turbine, is proportional with the cube of wind speed. For example if you double the wind speed, it will contain eight times more energy. This is because, in fact on wind turbines we brake the wind, and accordingly Newton's second law of motion there is delivered four times more power in braking the already doubled speed. Kinetic energy contained in wind is expressed in formula:

K_e = 0.5 x m x v²

Where m is mass of moving air, v speed of moving air. Must be mentioned some more characteristics which influence grossly energy obtained from wind.

Calculate your wind energy just in this moment.

Turbulence

Very irregular wind flows are influenced by obstacles like buildings or uneven terrains. Whirls are very bad for wind turbines functionality. That why height of tower, where turbine is mounted is very important to avoid these phenomena, which take place more on the surface. Turbines are specially designed for this kind of loads. In wind engineering there are two more parameters which influence the wind power. This is roughness and wind shear. More about this click here.

Wind Classes

Just to have an idea of quantity of power in different situations, bellow is the table of seven wind classes and respectively possible delivery power on 50 m height:

Class	Speed on 50m height [m/sec]	Power W/sqm
1	0-5.6	0-200
2	5.6-6.4	200-300
3	6.4-7	300-400
4	7-7.5	400-500
5	7.5-8	500-600
6	8-8.8	600-800
7	8.8-11.9	800-2000

At 1 m/s the wind has a power of 0.61 W/m 2 . At 17 m/s its power is 3009 W/m 2 ).

Turbine parameters

Wind turbines are built to catch the wind's kinetic (motion) energy. They are subject of fluctuating load, as well of fluctuating forces. Main parts of turbine are hub, rotor and rotor blades.
All these are mounted as one unit on the tower. Wind turbine generators are a bit unusual, compared to other generating units you ordinarily find attached to the electrical grid. One reason is that the generator has to work with a power source (the wind turbine rotor) which supplies very fluctuating mechanical power (torque).That why they have some unusual parameters, than other electrical devices. Mechanical fluctuations generate electrical fluctuations, and this is the reason wind turbines should be designed very carefully.
Wind turbine generators are a bit unusual, compared to other generating units you ordinarily find attached to the electrical grid. One reason is that the generator has to work with a power source (the wind turbine rotor) which supplies very fluctuating mechanical power (torque).

Turbine power

Is a little bit different than classic power choose. Efficiency and maximum speed of wind should be taken in consideration. Average of wind speed is a factor which influence efficiency of turbine. The rated output power (rated power, power rating, nominal output) of a wind turbine is declared for a particular rated wind speed (nominal wind speed). As power varies directly with the cube of wind speed, the rated power of a wind turbine varies significantly with the wind speed at which the rate power is stated.

Rotor diameter

It determine the surface where the wind catch and rotate turbine rotor. As power of wind is calculated in W/sm(square meter), a higher value of acting, on surface of rotor result in a higher delivery of power.

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