Wind turbines with three blades are the norm. It’s unusual to imagine anything different. Even though traditional windmills have four blades, we’ve become accustomed to the three-bladed wind turbine. But did you know that some designs call for two blades, or even one? They’re uncommon and are usually reserved for experimental purposes. It turns out that there’s a good reason wind turbines have three blades instead of two or four. The three-blade design offers the perfect balance between stability and efficiency, hence why it’s the industry standard. It’s simply the most efficient and well-researched design to date.
Balancing Efficiency with Stability
The number of blades on a wind turbine is a result of a balance between efficiency and stability. Fewer blades equals higher efficiency, but less stability. The three bladed turbine achieves the perfect balance between the two.
Having one blade nets the highest efficiency in terms of blade count, but balancing a turbine with one blade is a challenge. The blade needs to have a counterweight, and even then, the rotation can cause structural instabilities.
Having two blades nets higher efficiency than three blades, but two-bladed turbines tend also to be a bit unstable. A phenomenon called gyroscopic procession causes the turbine to wobble as it rotates. This is due to the change in angular momentum from the blades being positioned at opposite ends to each other.
Turbines with three blades are rendered stable, as when one blade is at the top of its rotation, the other two blades provide balance. This allows for maximum stability with the lowest possible blade count.
The number of blades also has an effect on the gearbox. Fewer blades with higher rotational speeds reduce the peak torque on the gears. This lowers the stress on the drivetrain, which can decrease costs significantly. The gearbox and generator are usually the largest part of the operating and maintenance costs of a wind turbine, as the heavy loads require constant maintenance.
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Aerodynamics is the single most important aspect of blade design. The blades are shaped like an aerofoil, similar to an airplane wing. This allows the blade to generate lift, which then spins the rotor. The curve in the backside of the blade creates some drag, which causes air to move faster than at the front of the blade.
Engineers must ensure that the blades create the right amount of drag. If they create too much, it will take more force to spin the rotor, and the turbine will lose efficiency. If they don’t create enough drag, the blades will spin too easily, which can damage the components inside.
The relative direction of the wind also changes due to the rotation of the blades, so the wind strikes the blade at a slight angle. Engineers account for this by creating a twist along the blade to take advantage of this effect. This allows the blade to achieve higher efficiencies.
The blade must also be lightweight and durable. While most of the body of a wind turbine is made of steel and aluminum, the blades are composed of a composite material consisting of carbon fiber, glass fabric, and liquid plastic. This creates a material that has a low density and is lightweight and extremely tough.
While blade count does factor into aerodynamic efficiency, engineers can get the same efficiency with any number of blades by changing the width of the blade and/or the rotational speed. Theoretically, wind turbines with any number of blades can achieve the same efficiency, whether it be one, three, or even ten blades. In reality, this isn’t practical. Material costs and structural integrity place limits on the size, length, and velocity of the blades.
Single Blade Turbines
NASA and several aerospace companies conducted experiments using single-blade turbines in the 1980s. NASA built a one-bladed turbine with a 15-meter long rotor. The experiment observed some advantages. One blade meant fewer materials and lower costs. The single blade also saw reduced drag since there were no other blades to disturb the air in front of it. This meant higher rotational speeds and thus higher efficiency.
Still, the single-blade turbine saw more disadvantages. A single-blade turbine would sit in a six-o-clock position when left undisturbed. A counterweight is necessary to balance out the single blade, which adds unnecessary headaches. And even with the counterweight, the turbines are unstable. While the faster rotational speed adds efficiency, it can also cause structural issues, especially considering the lack of symmetry. They have also been described as unsightly and an eyesore due to their awkward appearance.
Double Blade Turbines
The pros and cons of double-bladed turbines are well documented, thanks to an experiment by NASA and the Department of Energy. NASA built seven two-bladed prototypes between 1975 and 1992. The experiment was spurred by the oil crisis, as the rapidly increasing price of oil drove the government to fund research into alternative energy. The subsequent drop in the price of oil in the mid-80s made wind energy uneconomical, and the research came to an end. Despite the apparent setback in the progression of wind energy, the joint NASA/DOE program set the stage for the wind turbines of today.
Having two blades gives some of the same advantages as having one. They saw reduced drag, higher efficiency, and higher top speeds. They were also more aesthetically pleasing than their single-blade counterparts. Fewer blades also meant fewer materials and lower costs. Two blade turbines with the same tip speed as three-blade turbines produce less noise. Two-blade turbines also cost less to install, as the two blades can be manufactured as one large piece.
The default design of two-blade turbines creates inherent structural issues. The blades are at opposite ends of the rotor, creating an asymmetrical weight pattern as the blades rotate. This results in a phenomenon called gyroscopic procession, where the body of the turbine wobbles as it follows the rotation of the blades. This is similar to the wobble seen in a spinning top. The resulting wobbling can cause structural issues and eventual failure over long periods of time. This wobbling is especially pronounced when the turbine must turn to face the direction of the wind.
Despite having been seemingly phased out in the early 1990s, two-blade turbines made a short comeback in the early 2010s. A few companies, such as Nordic Windpower, manufactured two-blade turbines in 2010. A Chinese company called Ming Yang Wind Power experimented with a massive 6 MW double-bladed turbine in 2014. That same year, Hitachi and Fuji collaborated to build a few 2 MW downwind turbines with two blades.
Even today, some companies are eyeing the two-blade design for offshore wind farms. Offshore turbines are generally bigger, meaning they cost more to manufacture, transport, and install. The costs saved by reducing the blade count may outweigh the costs of the extra engineering challenges of having two blades.
Three Blades: The Magic Number
Engineers settled on three blades as a compromise between stability, cost, and efficiency. It’s the best choice to achieve the highest performance for the lowest cost, all while remaining stable. When one blade is at the top of its rotation, the other two act as a counterweight, preventing any wobbling in the structure of the turbine.
While we’re familiar with the four sails on a windmill, the drag created by the extra blade would cost the turbine some valuable performance points. This can be remedied by making thinner blades that spin slower, but that would create more fragile blades as thinner blades have difficulty maintaining stiffness. Engineers would have to either beef up the inner structure of the blade or swap out the materials altogether.
The extra blade would also cost more in manufacturing and installation costs. Transporting more blades costs more money and time. And now that wind companies are having a hard time disposing of old turbine blades, less is definitely more.
While the number of blades may be up for debate, what isn’t is the necessity of the blades themselves to generate wind energy. That was until the concept of the bladeless wind turbine started making waves. The bladeless turbine is almost completely alien in terms of its design. It resembles a large blunt object, or a giant baseball bat, rising from the ground. While it doesn’t look like something that one would expect to generate electricity from the wind, the bladeless turbine could potentially become the norm in the wind industry.
The bladeless wind turbine works via vibrations. When the wind strikes a blunt object, vortices are created. These vortices cause the turbine to vibrate. More wind vortices cause the turbine to have stronger oscillations until it reaches a resonant frequency with the wind. This is called vortex-induced vibration. The bladeless turbine capitalizes on these oscillations by using an alternator system that converts the vibrational energy into electricity.
Bladeless wind turbines have few moving parts, giving them an edge over bladed turbines in terms of maintenance and manufacturing costs. They are also lightweight, with a center of gravity closer to the ground, making them easier to install. They also are subject to much lower stress loads than bladed turbines.
While going bladeless sounds like a nice idea, the technology is still in its infancy. We still haven’t seen bladeless concept designs that rival the efficiency and cost-effectiveness of traditional turbines. Current designs require the turbine to vibrate at high speeds to generate electricity, which can cause major structural issues and instability.
The three-blade design creates stable, efficient, and aesthetically pleasing wind turbines. They’ve become the norm and trying to imagine two-bladed wind turbines dotting the landscape seems like an alien concept. Experiments using double and single-bladed turbines tell us that it simply isn’t feasible; the disadvantages outweigh the benefits. Although, some wind companies have tried to bring back two-blade turbines, though with mixed results. For now, having three blades is the standard, at least until the bladeless turbine is perfected.
Frequently Asked Questions
Three blades give the perfect balance between stability and efficiency. Turbines with two blades can become unstable, and turbines with four blades are less efficient.
Turbines with two blades undergo a phenomenon called gyroscopic procession, where the body of the turbine wobbles in response to the rotation of the blades. The lack of symmetry can eventually result in structural instabilities.
Two-bladed turbines are more efficient than three blades. This is because they are lightweight and have higher rotational speeds due to the reduced drag.
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