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Like solar panels and the sun, a wind turbine’s performance depends on the quality of the wind resource that it receives. There are several factors that determine the quality of the wind resource, with the key ingredient being ‘average wind speed’ - higher average wind speeds will always result in greater power output.
The graph below shows the relationship between wind speed and power output for our Hyland 920 small wind turbine. As can be seen, the relationship is not linear and is in fact cubic (up to a point). This means if the wind speed is doubled, the produced power increases by a factor of 8. Our Hyland 920 wind turbine is rated at 200W of power for a wind speed of 11m/s (39.6 km/hr) and as you can see from the figure below, the power output increases significantly for wind speeds beyond this. The wind distribution shows the average recorded wind speed at Australian airports. Siting our turbine in a location with a good wind resource (6 m/s +), will have a much higher total energy production than a site with a lower resource.
The location and surrounding terrain play a very influential role in the flow of air at a wind turbine site (more than one might think). As a result, the location and positioning of a small wind turbine is one of the most important factors in determining its ability to capture the wind’s energy and turn it into power.
Now that we understand how important the wind resource is to achieve optimal turbine performance. Let’s look at some of the key factors that determine wind quality.
Ideal sites are often located in coastal regions, in open fields or exposed hilltops. These sites generally have higher average wind speeds and are free from surrounding vegetation, buildings or other obstacles which could interrupt the wind resource.
It is quite easy to underestimate the effect that obstructions can have on the wind resource. Disturbances in air flow can be noticed well before and above the obstacle, and for an extremely long time after it – as shown in the diagram below. Consequently, it is very important to consider any obstacles that may disrupt the laminar flow of the wind resource at your potential wind turbine site. The below diagram can be used as a general rule of thumb.
Figure 1 - https://archive.knowledgepoint.org/en/answers/5351/revisions/
Wind speeds generally increase with height above ground level in a phenomenon known as ‘wind shear’ – this can be seen in the diagram below. Wind shear is partly caused by increased friction (obstructions) at the ground level as previously described. Other reasons include changes in pressure gradient and air density.
Figure 2 – sourced from: Recoskie, Steven & Lanteigne, Eric & Gueaieb, Wail. (2017). A High-Fidelity Energy Efficient Path Planner for Unmanned Airships. Robotics. 6. 28. 10.3390/robotics6040028.
As a result, a wind turbine mounted high above the ground would have a more favourable wind resource and hence produce more energy. In practice this is not always feasible due to increased costs of larger towers, with typical tower heights ranging from 12 - 30 m.
Installing wind turbines on tall pre-existing structures, such as a telecommunication towers, are a good way to harvest the wind at a site, but care must be taken to ensure that the structure can handle the added loading of the turbine. Counter intuitively, a wind turbine in operation has about 90% drag force of an equivalent sized flat plate.
While wind monitoring via an anemometer for at least 12 months is the “gold standard” for assessing a wind resource, people looking to install a small wind turbine rarely have the finances or time for a 12-month measuring campaign. Such a campaign is normally undertaken for large utility-scale wind farms, although some small wind customers may have access to data from local meteorological monitoring by the Bureau of Meteorology.
Several open databases exist which may assist with an initial site selection.
Determining the wind resource of a location can be a challenging endeavour as there are many factors that need to be taken into consideration. We have been refining our ability to predict site power output and are currently working on automating the process. If you are interested in checking the wind resource at your site and would like some assistance, all we need is a GPS co-ordinate. Please email us at sales@diffuse-energy.com.
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