"WindPower for dummies"

WindPower for dummies

With courtesy of "Danish Wind Industry Association"
Anja Pedersen ap@windpower.org / tlf. +45 33 73 03 36

Vindmølleindustrien

Danish Wind Industry Association
Vester Voldgade 106
DK-1552 Copenhagen V, Denmark
Phone: +45 3373 0330
Fax: +45 3373 0333
E-mail: danish@windpower.org

http://www.windpower.org/

http://www.windpower.org/en/tour/wres/index.htm

Where does Wind Energy come From?

All renewable energy (except tidal and geothermal power), and even the energy in fossil fuels, ultimately comes from the sun. The sun radiates 174,423,000,000,000 kilowatt hours of energy to the earth per hour. About 1 to 2 per cent of the energy coming from the sun is converted into wind energy. That is about 50 to 100 times more than the energy converted into biomass by all plants on earth.

Temperature Differences Drive Air Circulation

The regions around equator, at 0° latitude are heated more by the sun than the rest of the globe. These hot areas are indicated in the warm colours, red, orange and yellow in this infrared picture of sea surface temperatures. 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. If the globe did not rotate, the air would simply arrive at the North Pole and the South Pole, sink down, and return to the equator.

The Coriolis Force

Since the globe is rotating, any movement on the Northern hemisphere is diverted to the right, if we look at it from our own position on the ground. (In the southern hemisphere it is bent to the left). This apparent bending force is known as the Coriolis force, named after the French mathematician Gustave Gaspard Coriolis 1792-1843.

In the Northern hemisphere the wind tends to rotate counterclockwise (as seen from above) as it approaches a low pressure area. In the Southern hemisphere the wind rotates clockwise around low pressure areas.

Wind Energy Resources: Global Winds

How the Coriolis Force Affects Global Winds

The wind rises from the equator and moves north and south in the higher layers of the atmosphere.

Around 30°; latitude in both hemispheres the Coriolis force prevents the air from moving much farther. At this latitude there is a high pressure area, as the air begins sinking down again.

As the wind rises from the equator there will be a low pressure area close to ground level attracting winds from the North and South.

At the Poles, there will be high pressure due to the cooling of the air.

Keeping in mind the bending force of the Coriolis force, we thus have the following general results for the prevailing wind direction:

Prevailing Wind Directions

Latitude	90-60°N	60-30°N	30-0°N	0-30°S	30-60°S	60-90°S
Direction	NE	SW	NE	SE	NW	SE

The size of the atmosphere is grossly exaggerated in the picture above (which was made on a photograph from the NASA GOES-8 satellite). In reality the atmosphere is only 10 km thick, i.e. 1/1200 of the diameter of the globe. That part of the atmosphere is more accurately known as the troposphere. This is where all of our weather (and the greenhouse effect) occurs.The prevailing wind directions are important when siting wind turbines, since we obviously want to place them in the areas with least obstacles from the prevailing wind directions. Local geography, however, may influence the general results in the table above, cf. the following pages.

The Atmosphere (Troposphere)
The atmosphere around the globe is a very thin layer. The globe has a diameter of 12,000 km. The troposphere, which extends to about 11 km (36,000 ft.) altitude, is where all of our weather, and the greenhouse effect occurs.

The Geostrophic Wind
The winds we have been considering on the previous pages on global winds are actually the geostrophic winds. The geostrophic winds are largely driven by temperature differences, and thus pressure differences, and are not very much influenced by the surface of the earth. The geostrophic wind is found at altitudes above 1000 metres (3300 ft.) above ground level.
The geostrophic wind speed may be measured using weather balloons.

Surface Winds
Winds are very much influenced by the ground surface at altitudes up to 100 metres. The wind will be slowed down by the earth's surface roughness and obstacles , as we will learn in a moment. Wind directions near the surface will be slightly different from the direction of the geostrophic wind because of the earth's rotation (cf. the Coriolis force ).
When dealing with wind energy, we are concerned with surface winds, and how to calculate the usable energy content of the wind.

Local Winds: Mountain Winds

Mountain regions display many interesting weather patterns. One example is the valley wind which originates on south-facing slopes (north-facing in the southern hemisphere). When the slopes and the neighbouring air are heated the density of the air decreases, and the air ascends towards the top following the surface of the slope. At night the wind direction is reversed, and turns into a downslope wind.

If the valley floor is sloped, the air may move down or up the valley, as a canyon wind. Winds flowing down the leeward sides of mountains can be quite powerful: Examples are the Foehn in the Alps in Europe, the Chinook in the Rocky Mountains, and the Zonda in the Andes. Examples of other local wind systems are the Mistral flowing down the Rhone valley into the Mediterranean Sea, the Scirocco, a southerly wind from Sahara blowing into the Mediterranean sea.

The Energy in the Wind: Air Density and Rotor Area

A wind turbine obtains its power input by converting the force of the wind into a torque (turning force) acting on the rotor blades. The amount of energy which the wind transfers to the rotor depends on the density of the air, the rotor area, and the wind speed.

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 kilogrammes per cubic metre, 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.

Rotor Area

A typical 1,000 kW wind turbine has a rotor diameter of 54 metres, i.e. a rotor area of some 2,300 square metres. The rotor area determines how much energy a wind turbine is able to harvest from the wind. Since the rotor area increases with the square of the rotor diameter, a turbine which is twice as large will receive 2 2 = 2 x 2 = four times as much energy.

Power Output Increases with the Swept Rotor Area

When a farmer tells you how much land he is farming, he will usually state an area in terms of hectares or acres. With a wind turbine it is much the same story, though doing wind farming we farm a vertical area instead of a horizontal one. The area of the disc covered by the rotor, (and wind speeds, of course), determines how much energy we can harvest in a year. The picture gives you an idea of the normal rotor sizes of wind turbines: A typical turbine with a 600 kW electrical generator will typically have a rotor diameter of some 44 metres (144 ft.). If you double the rotor diameter, you get an area which is four times larger (two squared). This means that you also get four times as much power output from the rotor.

Rotor diameters may vary somewhat from the figures given above, because many manufacturers optimise their machines to local wind conditions: A larger generator, of course, requires more power (i.e. strong winds) to turn at all. So if you install a wind turbine in a low wind area you will actually maximise annual output by using a fairly smallgenerator for a given rotor size (or a larger rotor size for a given generator) For a 600 kW machine rotor diameters may vary from 39 to 48 m (128 to 157 ft.) The reason why you may get more output from a relatively smaller generator in a low wind area is that the turbine will be running more hours during the year.

Reasons for Choosing Large Turbines

1. There are economies of scale in wind turbines, i.e. larger machines are usually able to deliver electricity at a lower cost than smaller machines. The reason is that the cost of foundations, road building, electrical grid connection, plus a number of components in the turbine (the electronic control system etc.), are somewhat independent of the size of the machine.
2. Larger machines are particularly well suited for offshore wind power. The cost of foundations does not rise in proportion to the size of the machine, and maintenance costs are largely independent of the size of the machine.
3. In areas where it is difficult to find sites for more than a single turbine, a large turbine with a tall tower uses the existing wind resource more efficiently.

Reasons for Choosing Smaller Turbines

1. The local electrical grid may be too weak to handle the electricity output from a large machine. This may be the case in remote parts of the electrical grid with low population density and little electricity consumption in the area.
2. There is less fluctuation in the electricity output from a wind park consisting of a number of smaller machines, since wind fluctuations occur randomly, and therefore tend to cancel out. Again, smaller machines may be an advantage in a weak electrical grid.
3. The cost of using large cranes, and building a road strong enough to carry the turbine components may make smaller machines more economic in some areas.
4. Several smaller machines spread the risk in case of temporary machine failure, e.g. due to lightning strikes.
5. aesthetical landscape considerations may sometimes dictate the use of smaller machines. Large machines, however, will usually have a much lower rotational speed, which means that one large machine really does not attract as much attention as many small, fast moving rotors. (See the section on wind turbines in the landscape ).

The Future for Megawatt-Sized Turbines

The megawatt market really took off in 1998. Since then, it has been clear that the market trend is towards bigger projects with bigger wind turbines.

Megawatt-sized machines will be ideal for offshore applications, and for areas where space for siting is scarce, so that a megawatt machine will exploit the local wind resources better.

Grid Connection of Offshore Wind Parks

The Grid

The picture shows the Danish electrical transmission grid. Major power stations are shown in yellow.

Total generating capacity was some 10,000 MW in 1998.

Present and future offshore wind parks with a total of some 4,100 MW are shown in white and blue.

The western and eastern part of the country are not directly connected, but are connected to the German and Swedish electrical transmission systems using AC (alternating currency transmission lines). The rest of the connections to Sweden, Norway, and Germany are DC (direct current) connections.

Grid connection of offshore wind parks is not a major technical problem per se , in the sense that the technologies which are involved are well known. Optimising these technologies for remote offshore sites will be important, however, to ensure reasonable economics.

The first commercial-sized offshore wind farms in Denmark - Horns Rev and Nysted - are located 10-20 km (6-12 miles) from shore, at water depths from 6 to 14 metres. The wind farms are 160 MW (Horns Rev) and 158 MW (Nysted) and they consist of respectively 2 MW and 2.2 MW wind turbines.

© Copyright 1997-2007 Danish Wind Industry Association
http://www.windpower.org/en/tour/wres/globwin.htm