Horizontal wind turbines dominate the landscape, but did you know that there are vertical wind turbines as well? You don’t see them as often, because they aren’t used on an industrial scale like horizontal turbines. Still, vertical turbines do have their advantages, which we will go into more depth in this article. But what exactly separates horizontal and vertical wind turbines?
In short, what truly classifies horizontal and vertical turbines is their orientation relative to the wind. Horizontal turbines spin on an axis that is parallel to the direction of the wind, while vertical turbines are oriented perpendicular to the direction of the wind.
Horizontal Wind Turbines
Horizontal access wind turbines, or HAWTs, are what you think of when you think of a wind turbine. They make up the majority of industrial-sized turbines and can be identified by their propeller-like design. They usually have three blades, though some have two, or even just one. Currently, horizontal turbines are the most efficient type of turbine, hence their use in large-scale wind farms.
Horizontal turbines use lift to rotate and generate power. The blades are shaped like aerofoils, similar to the wings of an airplane. The wind strikes the blades and creates a pressure differential which turns the rotor and generates electricity. The generator and gearbox are located right behind the rotor in the nacelle. The rotor plane must maintain a 180 degree angle with the wind direction to maintain maximum efficiency, and so HAWTs are equipped with a wind vane and yaw system which keep the turbine facing the right direction.
Vertical Wind Turbines
Vertical access wind turbines, or VAWTs, are identifiable by their eggbeater-like design as they rotate around a vertical shaft. Like horizontal turbines, they have a gearbox and generator, though they are located at ground level at the base of the turbine. Although horizontal turbines have become the norm, the very first recorded windmill was actually a vertical turbine. Vertical turbines are more commonly used on a smaller scale than HAWTs, usually used to provide supplemental power to buildings, homes, or boats. Though there are a few VAWTs that are used for industrial-scale wind power, they are few and far between. Vertical turbines are omnidirectional and can generate power no matter which way the wind is blowing.
There are two types of VAWTs; Darrieus and Savonius turbines. An easy way to remember the difference is that Darrieus turbines are lift-type turbines, while Savonius turbines are drag-type turbines. While the two may look similar, they’re actually very different in the way they operate and generate electricity.
Darrieus turbines use lift to generate electricity. Like HAWTs, they have blades shaped like aerofoils. The blades of a Darrieus turbine are connected to a central shaft that rotates in response to the wind. Some Darrieus turbines have curved blades which connect at the top and bottom of the shaft, while others have straight blades which connect at the center. Some even have a double-helix design, where the blades curve around the central shaft.
Darrieus turbines are usually used on a small scale, like on rooftops or parking lots to generate supplemental electricity. Some are used on an industrial scale, though they’re not the preferred design as they aren’t as efficient and suffer from structural instabilities at large sizes. Large Darrieus turbines usually need guy wires to maintain stability. The largest Darrieus turbine is rated at 4 MW. It’s located in Quebec and is affectionately named Éole.
Savonius turbines are similar in appearance to Darrieus turbines, but they use drag to generate electricity instead of lift. They can be identified by their use of scoops instead of blades, which are attached to a central shaft. Using drag to provide power is much less efficient, but the Savonius turbine does have plenty of practical uses.
Savonius turbines are cheap and easy to maintain, and are often used for low energy applications in remote locations. Deep ocean buoys, communications towers, or water pumps might all use a Savonius turbine for power. Their reliability and simplicity also make them useful as anemometers. Horizontal turbines typically use small Savonius turbines as anemometers to measure wind speed, which are typically attached to the top of the nacelle.
What’s the Difference?
While the differences between horizontal and vertical turbines may seem obvious to some, they vary in more than just looks. The contrasts between to two make for huge functional differences and decide where and how each turbine can be used.
Horizontal turbines are more suited to capture sustained high-quality winds. They’re placed high up on large towers to capture the stronger wind at higher altitudes. Since they have to rotate to face the breeze, horizontal turbines don’t do well in gusty winds with variable wind directions. Vertical turbines, on the other hand, are omnidirectional, so they are more efficient than HAWTs when it comes to gusty weather where the wind blows from all directions. That makes them well equipped to generate power in urban areas, where buildings and tall structures create variable wind patterns.
Horizontal and vertical turbines also differ in blade design. While both horizontal and Darrieus-type vertical turbines generate lift, the difference in their axis of rotation means the blades function differently. The rotor plane of a horizontal turbine is at an 180 degree angle to the wind flow, so the speed and angle of the wind vary along the length of the blade. To account for this phenomenon, the blade must be aerodynamically engineered with twists and turns to maximize efficiency. The blades on a vertical wind turbine see a uniform airspeed along the length of the blade, as well as the same angle of attack, so they can be much simpler in terms of design. The blades on a Darrieus turbine can be simple aerofoils and don’t have a need for complex additions to enhance the aerodynamics.
Ease of Maintenance
Horizontal and vertical turbines also differ in where the mechanical parts are places on the turbine. Horizontal turbines keep all of their working parts high up on the top of the tower. This makes maintenance more difficult as workers have to scale the tower to address mechanical issues. The working parts of a vertical turbine are all stored at the base, close to the ground, making repairs easier and less hazardous.
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The simple fact of efficiency is one of the largest differences between the two types. Savonius turbines in particular are limited by their use of drag. When one side of the turbine is propelled by the wind, the other side is then pushed into the oncoming wind, costing valuable power. This means Savonius turbines lose efficiency throughout half of their rotation.
While horizontal turbines are more efficient, some engineers say it’s due to the simple fact that more research has been dedicated towards perfecting horizontal turbines. Darrieus turbines, being driven by lift, have the potential to be as efficient as horizontal turbines given the right design. The major setbacks for Darrieus turbines are structural issues stemming from constant load changes, and the blade being at the optimal angle to the wind for a short period of time. Horizontal turbines are more stable, and the blades are always at the best possible angle to extract the wind’s energy. Engineers have tried to remedy this by modifying the design of the Darrieus turbine, but these designs add extra complexity and costs.
Horizontal turbines are the most common type of wind turbine, although vertical turbines have their place as well. Many wonder why vertical turbines aren’t as common, and the answer lies in their efficiency. They simply don’t produce as much power as horizontal turbines, though that may be because there have been fewer resources dedicated to researching newer and better designs. Engineers have been looking into ways to enhance the vertical turbine, but the horizontal turbine still reigns supreme. Vertical turbines do have their advantages, especially in urban environments. Their placement close to the ground also makes them easier to maintain. Vertical turbines also work well in unusual wind conditions, and they don’t need to rotate to face the breeze. Despite the advantages, it looks like we’ll be stuck with the horizontal turbine for decades to come, until the bladeless turbine is perfected, that is.
Frequently Asked Questions
Horizontal turbines spin on an axis parallel to the wind, while vertical turbines spin on an axis perpendicular to the wind.
Darrieus turbines, known as lift-type turbines, and Savonius turbines, known as drag-type turbines.
The blades of a horizontal turbine are always at an optimal angle to the wind. They also have had more resources allocated to perfecting their design.
Vertical turbines are best used for generating power in urban environments where wind speed and direction vary greatly. Savonius turbines are also great for generating power in remote areas, as they need little maintenance.
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