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However for wind turbines, to maximise power and therefore energy, it’s important to go as high as regulations and/or economics allow.Īnother factor that can be important when looking at average speeds is how constant the wind is.
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For smaller home and business systems, heights can be limited by local regulation, and of course economics. Power curve for a commercial 4 kW wind turbine.Ĭommercial wind farms with very large blades (80m plus) have their hubs more than 80m high and thus can reach the higher wind regions. At 40m high, the average wind speed increases to about 6-6.9 m/s. This is close to the edge of viability for a wind turbine. At 10m above the ground, the average wind speed is about 4-4.9 m/s. These maps show just how important height is. Three average wind speed maps of the United States are shown below, for heights of 10, 40 and 80m above the ground. This has a massive effect on average wind speed, with wind increasing substantially as height increases. There are a few things that need to be considered when using such maps. There are plenty of wind maps around showing average wind speeds at different locations, for example see the excellent NREL site at. So how do we figure out how much wind is at a particular site? Wind speed is obviously critical: the longer the wind blows at higher speeds the more energy the wind turbine produces. These things have all sorts of transport issues and are certainly not cheap!
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The world record length is currently set at a whopping 107m on the General Electric 12 MW Haliade-X turbine. Large wind farms have much less restrictions and there is a continual push to make larger turbine blades and thus more powerful turbines. Homes and commercial sites have space and regulatory considerations that limit the lengths of the blades. If you double the length of the blade, you will get four times the amount of power and energy. This equation is important because it shows just how much the power and energy a turbine produces is dependent on the length of the blade. In this case r is the length of the turbine blades. Remembering some basic high school maths, the area of a circle is pi x r 2. The spinning blades of the turbine define a circle, with wind passing through the area of the circle being converted to energy. They’re about 40-50% efficient at doing this. Wind turbines work by converting the wind that passes through the spinning turbines into energy. So what determines rated power? The biggest factor is the size of the turbine. A higher rated power will give you more energy, but you also need the wind to blow at a good speed for lots of time. For example, if a turbine runs for 1 hour at 1000W, it will generate 1000 watt-hours of energy. The units of power are watts, and units of energy watt-hours. Your electricity bill is based on how much energy you use: if you look at the bill you will be charged per kWh (short for kilowatt-hour) you use. However all this relates to power, not energy. Schematic Power Curve for a 1kW Wind Turbine This is 11 mph or 9.7 knots.Ĭommercial wind turbines have different power curves depending on whether they’re designed to operate at lower or higher wind speeds.
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At the lower end, a minimum wind speed of about 5 m/s is often considered necessary for a wind turbine to be viable. This shows that wind turbines have a wide operating window for stronger winds. A shut-down wind speed of 25 m/s is 56 mph or 48.6 knots. To put these speeds in perspective, maximum power is at about 11 m/s which is 24.6 mph or 21 knots. Typically shut-down speeds are about 25 m/s. The power then stays fairly constant with increasing wind speed until the turbine is shut down for safety reasons. The power increases with wind speed up to it’s rated power, which is at a defined wind speed (wind turbine specifications will state the rated power and the wind speed for the rated power). You’ll note that the power doesn’t start increasing at zero wind speed: each turbine has a ‘cut-in’ wind speed at which it starts to produce power. The curve below shows an example ‘power curve’ for a wind turbine rated at 1000W. Thus the rated power of a wind turbine is the power that the turbine will produce at a particular wind speed. The power output is fairly obviously dependent on how much wind is blowing. However, the turbine will not produce this rated power all the time. The most basic specification for a wind turbine is a power rating.Ī residential wind turbine might be rated at 5kW, and much bigger wind farm turbines might be rated at several MWs each.