When people think of wind turbines, they often visualize the expansive rotors of a horizontal-axis system. A vertical axis wind turbine (VAWT) has blades mounted on the top of the main shaft structure, rather than in the front like an aircraft rotor. The generator is usually placed at the tower base.
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Used less often than their horizontal counterparts, VAWTs are more practical in residential areas. Two common designs include a turbine that resembles two halves of a 55-gallon drum, each mounted to the rotating element (Savonius rotor), and a smaller model that looks somewhat like an egg beater (Darrieus model). Savonius models are more commonly used and let air in through a hub to turn a generator; the turbine spins via rotational momentum when air passes through the blades.
The unit has two or three blades and can be shorter and closer to the ground than a horizontal system. A Giromill also features an egg beater design but has two or three straight blades on the vertical axis. Helical blades constitute another design, which resembles a structure like DNA. In general, vertical axis wind turbines come with their own advantages and disadvantages when compared to alternative configurations.
These turbines have fewer parts than those that orient the rotary mechanism and blades horizontally. That means fewer components to wear out and break down. Also, the supporting strength of the tower doesn’t need to be as much, because the gearbox and generator are near the ground. Parts for controlling pitch and yaw aren’t needed either.
The turbine doesn’t have to be facing the right wind direction either. In a vertical system, air flowing from any direction or speed can rotate the blades. Therefore, the system can be used to generate power in gusty winds and when they’re blowing steadily.
Other benefits include:
In addition, VAWTs are:
According to the Institution of Mechanical Engineers, vertical axis wind turbines are more suited for being installed in denser arrays. Up to 10 times shorter than horizontal models, they can be clustered into arrays that even create turbulence from one turbine to another, which helps increase the flow around them. Therefore, wind speeds up around each one, increasing the power-generated. A low center of gravity also makes these models more stable for floating in offshore installations.
The vertical design allows engineers to place the turbines closer together. Groups of them don’t have to be spaced far apart, so a wind farm does not have to take up as much ground area. The proximity of horizontal wind turbines to one another can create turbulence and reductions in wind speed that affects the output of neighboring units.
A report in the Journal of Renewable and Sustainable Energy, cited by Phys.org, noted that although vertical axis wind turbines produce less energy per tower, they have the potential to generate as much as 10 times more power over a comparative area of land when placed in arrays.
Not all of the blades produce torque at the same time, which limits the efficiency of vertical systems in producing energy. Other blades are simply pushed along. There is also more drag on the blades when they rotate. Although a turbine can work in gusty winds, that is not always the case; the low starting torque and dynamic stability problems can limit functionality in conditions the turbine wasn’t specifically designed for.
Since the wind turbines are lower to the ground, they do not harness the higher wind speeds often found at higher levels. If installers prefer to erect the structure on a tower, these are more difficult to install in such a way. However, it is more practical to install a vertical system on a level base, such as the ground or the top of a building.
Vibration can be an issue at times, and even increase the noise produced by the turbine. Air flow at ground level can increase turbulence, thereby increasing vibration. This can wear out the bearing. At times, this can result in more maintenance and therefore more costs associated with it. In earlier models, blades were prone to bending and cracking, causing the turbine to fail. Small units atop buildings or other structures may be subject to jostling forces, which add lateral stress that warrants ongoing maintenance and the use of stronger, more sturdy materials.
While they produce less energy than horizontal turbines, vertical axis wind turbines still produce power and can be a better option depending on the application. They’re more suitable where space is limited and come with fewer challenges and risks to maintain. This design has remained popular as engineers have addressed the challenges and have found applications in small-scale installations, particularly in urban areas. Over time, there is the potential for engineering innovations to improve the energy producing efficiency of VAWTs, and increase the advantages they can offer in various applications.
Vertical axis wind turbines are equipped with rotor blades that can pick up wind coming from any direction. Unlike so, horizontal axis wind turbines need to face the direction of the wind to operate. They rely on the yaw system to orientate the rotor in order to capture wind.
Due to this difference in operation mechanism, vertical axis wind turbines can be used to generate power even in unstable weather conditions such as turbulent, gusty wind. They also function well in mountain and coastal areas.
In a horizontal axis wind turbine power plant, the general rule-of-thumb for spacing is to place the turbines 5 diameters apart across the wind, and about 10 diameters apart extending downwind. This is to avoid disruption of air flow and reduction in wind speed caused by one turbine to another, which affects the power output of neighboring units.
Compared to horizontal axis wind turbines, vertical axis wind turbines can be grouped closer together in a wind power plant. This is because vertical axis wind turbines function well in turbulent wind. They are generally spaced 4 to 6 diameters apart.
Closer spacing would allow a wind power plant to capture more energy per square meter of land. Generally speaking, a single vertical axis wind turbine is not as energy-efficient as an individual horizontal axis wind turbine. However, a research led by then Stanford University mechanical engineering doctoral candidate, Anna Craig, had a different conclusion. The research suggested that a group of closely-spaced vertical axis wind turbines have the potential to generate as much as 10 times more power per unit of land compared to a group of widely-spaced horizontal axis wind turbines. The research is published in the Journal of Renewable and Sustainable Energy.
Vertical axis wind turbines have a lower starting wind speed compared to the horizontal axis models. The necessary starting wind speed for a typical vertical axis wind turbine is 2 to 3 m/s. This allows vertical axis wind turbines to generate electricity even when incoming wind is relatively weaker. Although the amount of electricity generated at lower wind speeds is much smaller, it is still better than having a wind turbine that is not able to harvest weaker incoming wind.
Generally smaller, the size of vertical axis wind turbines brings along a few advantages, one of which being the low environmental harm. Because they are built closely around the shaft, the rotor blades of vertical axis wind turbines do not create huge, extend drop shadows. The blades are also easier to spot for birds and other flying animals, decreasing the chance of animal casualty.
Furthermore, vertical axis wind turbines operate with quieter noise emission, so they do not disturb people in residential neighborhoods.
The second advantage thanks to the smaller size of vertical axis wind turbines is that they are easier to transport, set up, and maintain. For example, all parts of one LuvSide turbine can be delivered with a single truck with a 6-meter long storage space. Maintenance workers do not have to climb as high to reach parts of the turbine because the major components, such as the generators, are built closer to the ground.