Wind Turbines

SkystreamWind is caused by the heating of the earth's surface by the sun. The heating of the surface is, however, uneven as time of day, water surfaces and cloud cover cause some areas to heat less quickly. As the heat rises it creates a low pressure area and air from adjacent high pressure areas flows toward this low pressure area thereby creating wind. Wind, like solar energy, is somewhat seasonal thus accurate overall wind measurement is useful.

It is generally recognized that an average annual wind speed of more than 18 km/h (5 m/s) is the required minimum for a stand alone wind-based system to be considered viable. For this reason the East and West coasts of North America, the far north and southern prairies offer the most promise for a primary wind power application. Outside of these areas wind turbines best complement solar systems or conventionally fueled generators to reduce fuel consumption in hybrid applications.

Wind Energy

Seldom is there a day without wind, though it seldom exceeds twice the annual average wind speed and the most frequent, or median, speed is only about 75% of average. Wind speed is an extremely important factor in determining the power output of a turbine. The energy content of the wind varies with the cube of the average wind speed, in other words a doubling of the wind speed will produce eight times as much energy.

There is a difference between the available power in the wind and the actual power produced by the wind turbine. While the maximum theoretical energy obtainable from a wind energy system is about 59%, which is referred to as the Betz limit, a wind turbine that is optimized for the site conditions where it will be used will only operate at about 25% efficiency.

The output of a wind turbine is typically expressed as its rated power (in Kw) at a predetermined, though not always the same, rated wind speed. Given the variances in manufacturers ratings, kWh per month at various wind speeds is the best comparison of turbine output.

The amount of power in the wind and a particular turbine may be calculated. The following table suggests the average power you may expect from a wind turbine given its swept area and various average wind speeds. The actual output is influenced by the area of the rotor and the wind speed.

Average Power Output (kW)

Swept Area(M2)
Diameter (meters)
4
6
8
10
1
1.1
0.019
0.063
0.150
0.292
4
2.3
0.075
0.253
0.599
1.170
10
3.6
0.187
0.632
1.520
2.920
40
7.1
0.749
2.530
5.990
11.700
100
11.83
1.870
6.322
15.011
29.211

What affects the performance of wind turbines?

Apart from the actual size swept area of the wind turbine and wind speed there are three other factors affecting performance.

Height: in unobstructed areas wind speed increases by 12% each time the distance between the turbine and ground is doubled. In Canada, all weather offices report wind speed at the standard height of 10 m above the ground.
Ground characteristics: wind near the earth's surface is slowed by trees, buildings, hills and mountains which all create their own form of friction which restricts free airflow. Because of the actual affect of local conditions it is best to measure the wind speed at a proposed site for at least one year to determine its suitability to a project. Seldom is the lee side of a hillor mountain an ideal location for a wind turbine.
Air temperature: the colder the air is the denser it is and denser air increases power output. The power from a wind turbine will increasenearly 16% as the temperature drops from + 20° C to - 20° C for any given wind speed.
Distance to point of use Seldom: is the location of larger wind turbines adjacent to the point of use, therefore the issue of power transmission needs to be addressed. Most wind turbines develop alternating current first which is then rectified to direct current for use in batteries, however, low voltage DC output from the turbinewill quickly translate into excessively large and expensive wire sizes. The first option should be the use of higher A C voltage, which may then be stepped down in voltage and rectified for battery charging.

Safety

Safety regulations for wind turbines varies amongst countries, Denmarkbeing the only one requiring that all blades be tested for both static and dynamic fatigue.

Overspeed protection is a critical component built into turbines that are designed to withstand winds of up to 125 MPH. Protection generally ensured via aerodynamic, mechanical or electric braking.

Aerodynamic braking rotates the entire turbine blade or blade tips up to 90° effectively stalling the turbine. Alternatively, pitch controlled protection is made by installing the spring loaded, tail assembly slightly off centre from the rotation of the blades to deflect the entire turbine away from the direction of the wind as wind speed increases.

Smaller wind turbines may incorporate a separate electric brake or stop switch that effectively short-circuits the turbine. Mechanical braking systems, similar to a parking brake, on larger turbines are used as a back-up to aerodynamic braking. It is interesting to note that Danish wind turbines are required by law to have two independent brake mechanisms.

Environmental concerns

While noise may be of concern for some applications, in North America attention should also be given to the flight path of migratory birds.

Useful information:
1 km/h = 0.28 m/s
1 m/s = 3.6 km/h

 

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