Solar Wind Canada

Easy RETS: Fundamentals: Renewable Energy: Wind Energy

Understanding the Wind

Wind is the motion of atmospheric air measured relative to the rotating Earth. It is the sun's energy that supplies the forces to move air. The sun radiates approximately 174,423,000,000,000 KWh [1] (Kilowatt-hour) of energy captured by the earth per hour or 1.74 x 10¹⁷ watts of power. However, only approximately 1 - 2% of this energy is converted into wind energy or air circulation.

This air circulation is caused by thermal (heat) and pressure contrasts or differences over the globe. There are secondary circulations caused by land and oceans, and tertiary circulations caused by local topography. The sun's heat raises the air (low pressure area) and the winds flow (from high pressure areas) to replace it. It is in response to the unequal rates of radiational heating and cooling within the Earth-atmosphere system that we have weather (air temperature, wind, cloudiness, and precipitation).

Animation depicting the seasonal variations
of temperature. Source: Department of
Geography, University of Oregon.

By virtue of air circulation, the energy in the wind is the kinetic energy present in the moving air. The amount of potential energy one is able to extract from the wind depends not only on wind speed but the volume of moving air from which the extraction occurs. The potential energy is also affected slightly by the density of the air, which is determined by the air temperature, barometric pressure, and altitude, however, most installations occur at sea level to within a few hundred feet of sea level resulting in negligable impact from the slightly less dense air at the higher altitude indicated.

Wind Power

For any wind turbine, the power and energy output increases dramatically as the wind speed increases and its swept area carved by its rotor (blades).

Image depicting the power generation capabilities as a
function of swept area. Source: Great Plains Windustry
Project.

Therefore, the most cost-effective wind turbines are located in the windiest areas. Wind speed is affected by the local terrain and increases with height above the ground, so wind turbines are usually mounted on tall towers in order to extract the most energy from the wind. The kinetic energy of a given mass varies with the square of its velocity. Because the mass flow increases linearly with the wind speed, the wind energy available to a wind turbine increases as the cube of the wind speed.

As the wind turbine extracts energy from the air flow, the air is slowed down, which causes it to spread out and diverts it around the wind turbine to some extent. A German physicist, Albert Betz, determined in 1919 that a wind turbine can extract at most 59% of the energy that would otherwise flow through the turbine's cross section. The Betz limit applies regardless of the design of the turbine. Today's wind turbines experience performance in the range of 20-30% efficiency. A consistent performance in the range of 25 - 30% is considered good performance.

Wind Speed and Direction

Wind doesn't blow at the same speed all the time. The ideal wind resource has relatively stable high speeds. To ensure the most effective use of a wind turbine, it should be exposed to the most energetic wind. Though the wind may blow more frequently from the West, more wind energy may come from a different direction if those winds are stronger. It is very important to find out which directions have the best winds for electricity production. An important tool for the measurement of wind speed and direction is called a "wind rose". A wind rose gives you information on the relative wind speeds in different directions, i.e. each of the three sets of data (frequency, mean wind speed, and mean cube of wind speed) has been multiplied by a number which ensures that the largest wedge in the set exactly matches the radius of the outermost circle in the diagram. A wind rose is readily available and dynamically calculated in the Canadian Wind Energy Atlas for any point selected in the maps available.

The Freshwater Society has created an entertaining and informative Wind Rose Animation which explains how to use such a tool.

Wind Shear, Obstacles and Terrain

The best form of wind to extract relatively continuous energy is "laminar" wind. That is, wind that moves at a relatively constant speed and direction. However, the world is not perfect, and a variety of geographical attributes can cause wind to be less than cooperative. Wind shear is the increase in wind speed at greater heights above ground, often at different directions as compared to wind just above the ground. In order to address this, one can increase energy production by installing a turbine on a taller tower. When wind flows around buildings, or a stand of trees, it slows down or becomes turbulent. A wind turbine should be placed in a location where the influence of obstacles is minimized, usually guided by the "rule of thumb" 10 x 100, where the turbine is positioned at least 10m above the height of the object causing the turbulence, and at least 100m in distance from the same object. The terrain can increase surface friction causing turbulence, eddies and wind shear with geologic structures such as hills, valleys, forests and cliffs.

Footnotes

KWh or Kilowatt-hour, a measure of energy equal to the use of one kilowatt in one hour.

References

Peterson, Denise. "Atmospheric Phenomenon and Air Temperature". Woodrow Wilson Leadership Program in Environmental Science
Climate Lab section of the Environmental Change Research Group © Department of Geography, University of Oregon . "Global Climate Animations". 2003
Great Plains Windustry Project. "Windustry...learn how to harvest the wind.". January 27, 2006.
Danish Wind Industry Association. "The Wind Rose". 19 September 2003.
US Department of Energy. Wind Power Animation. 2009. Last accessed: January 21, 2010.