Technical – Wind Turbine Magazine

Not surprisingly, higher-speed winds cause wind turbines to rotate very quickly. However, many people are shocked by how fast the tips of utility-scale wind turbine blades move, especially if they are viewing the wind turbines from a distance. Up close, it is more apparent how quickly turbines actually turn. In high winds, wind turbines with heavy blades can reach 290 kilometres per hour, or 180 miles per hour! Slightly smaller turbines may reach speeds of 161 km/h or 100 mph. There are various ways to measure the speed of the wind turbines as they rotate. There is both rotational speed and the velocity that the blades move through the air. Whereas blade speed is measured in kilometres or miles per hour, the rotation speed is measured in rotations per minute. The rotational speed of a large wind turbine is around 20 rotations per minute (rpm), but smaller turbines can rotate even more quickly. How do I calculate the speed that a wind turbine spins? First, you will need to know the length of the wind turbine blade and the time it takes for it to complete one rotation. Then, you can calculate the circumference of the circle, which is the distance that the tip of the wind turbine blade travels to make one rotation. C= 2πr Let’s say the turbine blade is 35 meters, then: C= 2 x π x 35 = 70π = 220 meters Next, you need to know the time it takes for the wind turbine blade to make one complete rotation. Let’s say it’s spinning at 15 rpm, so it takes 4 seconds to spin around one time. 220 meters / 4 seconds Thus, the wind turbine is spinning at 55 meters/second. Finally, you can multiply it by 3.6 to get kilometres per hour. 55 x 3.6 = 198 kilometers per hour (km/h) So, the wind turbine is moving at 198 km/h Another approach is to use a radar gun to measure the speed of the blades. Do Large Wind Turbines Move More Slowly than Smaller Ones? It depends on how you are measuring speed, be it rotational speed or the movement of the wind blades themselves. The rotational speed is the measurement of an object turning on an axis and is usually measured in rotations per minute or cycles per second. Often, smaller turbines make more rotations per minute than larger turbines. Although the rotational speed of smaller wind turbines is typically faster, the speed at which the tip of the blades moves through the air is typically slower because the blades are shorter. Of course, there are other factors at play as well, such as wind speed and turbulence. Using the formula from above highlights this point when using data from a smaller wind turbine. C= 2πr Let’s say the turbine blade is 15 metres, and it takes 3 seconds to complete one rotation (so its rotational speed is faster than the previous example). C= 2 x π x 15 = 30π = 94 meters 94 meters / 3 seconds So, it’s moving at 31.3 meters/second Finally, you can multiple 3.6 to get kilometres per hour. 31.3 x 3.6 = 113 km/h So, the wind turbine in question moves at only 113 km/hour even though it rotates more quickly than the larger turbine in the first example. Do Wind Turbines Generate More Energy When they Spin Quickly? The amount of power generated by the wind turbine is impacted largely by the wind speed, sweep area of the blades and air density. The sweep area is the area covered as a wind turbine rotates around in a circle. Thus, turbines with longer blades have a larger sweep area. Conversely, wind turbines with shorter blades have a smaller sweep area. A larger sweep area enables the wind machine to capture more kinetic energy. However, after a certain point, stronger winds and faster wind turbine rotation doesn’t result in more energy production. The power curve below helps highlight this point for a given model. For the wind turbine in question, it reaches its maximum power capacity at wind speeds of around 15 m/s. So, wind speeds of 20 m/s don’t result in more renewable power generation, even if the blades are rotating more quickly. Unfortunately, when the speed reaches 25 m/s, it reaches its cut-out speed. This means that the wind turbine shuts down and stops producing wind power. This is a safety feature to protect the wind energy equipment during severe weather events, such as hurricanes or tornadoes. What happens if a wind turbine spins too fast? Unfortunately, if a wind turbine rotates extremely quickly, it can cause significant equipment damage and potentially even people, animals, or property. For that reason, wind turbines have a cut-out speed as a safety feature. Remember, wind turbines are designed to last two decades or more. However, during that time, who knows what weather they may encounter. Therefore, wind turbines have an anemometer on them to measure wind speeds. Wind turbines have different cut-out speeds, depending on the make and model. For this reason, the power curve above abruptly drops at wind speeds of 25 m/s and renewable energy production drops from 3 MW to 0. If small turbines rotate more quickly, why don’t they produce more electricity? Smaller wind turbines have shorter blades and typically have a lower hub height (are closer to the ground). One critical factor in how much energy a wind turbine produces is the sweep area. Because utility-scale wind turbines have longer turbine blades, they have a larger sweep area and can capture more energy. Wind turbines rely on kinetic energy to produce electricity. According to Khan Academy, “Kinetic energy is the energy an object has because of its motion. If we want to accelerate an object, then we must apply a force. Applying a force requires us to do work. After work has been done, energy has been transferred to the object, and the object will be moving with a new constant speed. The energy transferred is known as kinetic energy, and it depends on the mass and speed achieved.” Smaller wind turbines have a diminished ability to harness the kinetic energy from the wind. As a result, it is usually more cost-effective to install one large wind turbine compared to several small ones. And utility-scale wind turbines continue to increase in size. To harness more kinetic energy, utility-scale wind turbines have grown considerably over the last couple of decades, and the hub height has also increased dramatically. As the hub height increases, wind turbines are able to access stronger and more consistent winds. The wind resource tends to be better higher above the ground because it is less impacted by surface roughness and topography. For example, trees, buildings, and topographic features can slow the wind when it is closer to the ground. Wind turbines with taller towers can reach these strong winds more readily. The hub height of a wind turbine is the distance from the ground to the middle of the rotor. This has increased 59% from 1998 to 1999 to about 90 meters (295 feet) in 2020 in the United States for onshore wind farms. However, the average hub height of off-shore wind turbines is even taller and trending upwards as well. Likewise, wind turbine diameter determines the width of the sweep area of the wind turbine blade. It has also steadily increased in recent years because turbines can capture more wind power and therefore generate more electricity, even in areas without a really strong wind resource. This means that wind turbines can be located in more places, increasing siting options for wind farms if the equipment can effectively produce wind electricity in low-speed winds. This is helpful as renewable energy becomes more and more widespread. However, large wind turbines are at a slight disadvantage to larger wind turbines because the blades are heavier, which slows down the speed at which the turbine blades spin and rotate. This is one of the reasons that wind turbines don’t have 5 or 7 blades. The more blades on the turbine, the more weight they add to the equipment, which can be counterproductive for energy production because it slows down the speed that it spins. Does wind farm layout impact how quickly the wind turbines spin? Yes, it does. Although modern wind farms are very efficient at generating power, the front row of turbines creates turbulence downwind. Unfortunately, the turbulence impacts turbines in subsequent rows and decreases how fast the wind turbines spin. This, in turn, reduces the amount of kinetic energy that is converted into electricity. This is one of the reasons why wind turbines need to be spaced out in wind farms, but it happens to some extent, even with ample spacing. Because wind turbines are expensive, wind farm developers must consider the financial advantages and disadvantages of spacing to optimize the farm energy yield and, therefore, the financial performance of a wind project. However, researchers from Oxford Brookes University are finding that vertical-axis wind turbines (VAWT) do not impact each other’s performance in the same way. Thus, they conclude that VAWTs are more efficient in large-scale off-shore wind farms than traditional horizontal-axis wind turbines. Another advantage to vertical-axis turbines is that they can generate wind power from any direction and don’t need to rotate the rotor to face the wind. Perhaps this compact technology will become more common in the future. Do wind turbines spin at the same speed throughout the year? Although many newer wind turbines are more effective at generating energy at lower speeds, there is seasonal variation in wind speeds. Different geographical areas have different seasonal wind resources, and this impacts the speed the turbines rotate. Just like how solar panels don’t produce electricity around the clock, wind turbines are not continuously producing power at their peak levels. The capacity factor of a wind farm measures how much of the turbines are producing at full capacity. It is calculated by taking the average power output divided by the maximum power capability of the turbine. Onshore wind farm capacity factors range from 0.26 to 0.52. However, the average power output is measured throughout the year and has significant seasonal variations. In the United States, California wind farms have the highest capacity factors in late spring and early summer and the lowest in December and January. By contrast, the Northeast and Midwest have very different seasonal wind patterns. They tend to peak in the winter months and dip to their lowest levels in late summer. Thus, how quickly wind turbines spin is a very complex topic that is impacted by many factors. Taller wind turbines can typically access a better wind resource compared to being closer to the ground. During very high wind speeds, wind turbines automatically shut down to prevent equipment damage. Even seasonal wind patterns have a big impact on how fast wind turbines spin. When wind turbines are really rotating quickly, many people are shocked at how fast they move.