A wind turbine is a generator for which the fuel is free. And what is that fuel? Clean and smoothly flowing wind. And, just like any generator, if you limit the fuel flow to the generator you’ll starve it and reduce its ability to produce electrical energy. So, selecting a good quality wind turbine is only part of the installation process—identifying the wind resource and the site are equally important. Proper site evaluations consider turbine location and its hub height which strives to place the turbine’s rotor in clear flowing wind. Below are some good general rules to follow when considering turbine location:
- Placing a wind turbine on a rooftop, above a billboard, by a large solar array, or near a cliff face can reduce the turbine’s energy production by as much as 50%. The upward flow of the wind over the building or up the cliff face, causes the wind to strike the turbine’s rotor disc at an angle. This reduces the apparent area of the rotor disc making turbine is capture less wind energy. To get a better image of what this looks like, hold a disc of paper or a paper plate up and tilt it to the side or upward. You’ll notice that the round shape changes to an ellipse, with a smaller apparent surface area. The actual area of the disc didn’t change, but to you (and to the wind flow) the area of that ellipse looks smaller. Capturing less wind energy directly translates into producing less electrical energy. In addition, this wind flow is now very turbulent and can shorten the life of the system. XZERES does not recommend rooftop installations.
- I’m sure you’ve read the common guideline that a wind turbine should be installed so that the rotor disc is at least 30 feet higher than anything else within a 300-500 foot radius. While this is a good rough guide, it doesn’t adequately cover all situations. The flow of wind over an obstacle (house, tree, etc.) is disrupted behind the obstacle for almost twice the height of the obstacle. In other words, wind flowing over a 60-foot tall structure will be disrupted up to a height of almost 120 feet above the ground (downwind of the structure). Keep this in mind when you evaluate a site.
- Remember that obstacles behind the turbine also have an effect. Placing a wind turbine on the beach, facing the prevailing wind from the water, may not work well if there’s a tall building right behind the turbine.
- Wind which flows through a wind turbine’s rotor is moving more slowly when it exits the blades. It’s also turbulent, since it’s swirling and no longer flowing smoothly. This slower moving, turbulent wind will eventually be dragged back into smooth flow by the wind passing by. But that doesn’t happen immediately, so any wind turbine which is installed downwind can be negatively impacted.A good guide is to space wind turbines downwind at a minimum distance equal to 7-9 times the rotor diameter. For the 442SR, this means spacing multiple turbines at least 50-65 meters apart. And for the Skystream, minimum spacing is 26-33 meters apart. Putting one turbine downwind of another, closer than this, can reduce the output of the downwind turbine by 30-50%.
- For coastal installations, or other sites with strong prevailing wind directions, turbines can be installed closer together. In these sites, placing the turbines in a line which is perpendicular to the prevailing wind direction, allows you to place the turbines as close as 2-3 rotor diameters apart. But it should be noted that if the wind shifts and blows in-line with your turbine row, the downwind turbines will be negatively impacted. Therefore, closer spacing is only suitable for specific sites with defined prevailing wind directions.
- Remember that trees grow. This may seem like common sense but it’s not uncommon to see trees which have grown up around an old wind turbine installation. Use a tree identification guide to determine the mature height of trees in the surrounding area, especially if they are recently planted.
Properly evaluating a wind turbine site involves the consideration of many other factors as well, such as site access, electrical connection locations, safety considerations (such as power lines nearby), sound emissions and the proximity of occupied structures. The above guides are only part of the total evaluation process. Please watch for announcements regarding our 2014 webinar schedule, where some of these topics will be covered in more depth. We at XZERES want to support our dealers in providing the best quality product and installations for our customers.