Wind energy has grown rapidly over the past ten years, though it’s been in use since the advent of agriculture – windmills being the first constructions to effectively harvest the power all around us. While we’ve become accustomed to the traditional windmill-type design, researchers have been hard at work engineering several different types of wind turbines. It turns out that there are many different ways to harness the wind, some more efficient than others, from the simple savonius turbine used in anemometers to the innovative bladeless turbine design. The future of wind energy is exciting, and this is just the beginning! Whether for your home, or in massive onshore and offshore farms, wind can take us to a green future.
In short, the main wind turbine types out there are:
- Horizontal Access Wind Turbines (HAWT)
- Vertical Access Wind Turbines (VAWT)
- Downwind Turbines
- Bladeless Wind Turbines
Horizontal Access Wind Turbines
When you think of a wind turbine, you probably think of a horizontal access wind turbine, called HAWT for short. These are the most common type of turbine and are easily recognizable by their propeller-like design. They usually have three blades, though some varieties have two. HAWTs generate electricity using lift with blades shaped like aerofoils, just like how an airplane gets off the ground. Although, the blades of a wind turbine have a lift-to-drag ratio several times higher than the wings of a commercial airliner. So far, these are the most efficient and thoroughly researched type of wind turbine.
HAWTs are set up high on large towers to take advantage of the wind speeds at high altitudes. While this provides better efficiency, it makes repairs difficult. The nacelle, or the small box behind the rotor, contains the gearbox, generator, speed brakes, yaw mechanism, and anemometer. The gearbox is meant to amplify the turbine speed, which allows the generator to create electricity.
HAWTs must remain perpendicular to the wind to maximize efficiency. The yaw mechanism rotates the turbines in the direction of the wind to ensure the turbine remains aligned with the wind direction. HAWTs also don’t do well in extreme wind speeds. When the anemometer picks up wind speeds that may damage the turbine, the speed brakes are activated to prevent the turbine from spinning too fast.
Vertical Access Wind Turbines
Vertical access wind turbines, or VAWTs – in contrast to HAWTs – spin on a vertical axis with the wind. They tend to be less common on an industrial scale. VAWTs are generally used on a smaller scale than HAWTs and are primarily used on rooftops as a supplemental energy source. They are usually close to the ground, making them less efficient due to the lower wind speeds but easier to service and maintain.
Not all VAWTs are the same. They come in two main designs, Darrieus and Savonius. Both are very different in the way they capture wind energy, and each has several offshoot designs that serve different purposes.
Darrieus Wind Turbines
Darrieus wind turbines are identified by two curved aerofoil blades attached to a central shaft, giving them an “eggbeater” shape. The blades attach at the top and bottom of the shaft and spin vertically around the shaft in response to the wind. Like HAWTs, Darrieus wind turbines generate electricity using lift.
Darrieus wind turbines have some advantages. They’re effective no matter the wind direction. The blades can also come in a variety of designs. While the eggbeater shape is the most common, there are also double-helix blades, where the blades twist around the central shaft. The gyromill design is also common, which uses straight blades where the ends are detached from the central shaft. Both of these designs are generally used on a smaller scale, like on rooftops. Save for a select few, Darrieus turbines usually have a capacity below 1 MW.
Still, Darrieus turbines have too many disadvantages. They tend to be less efficient than HAWTs because the blades are so close to the ground. The design also isn’t structurally sound at the MW scale. Guy wires are necessary to keep larger turbines from collapsing, dramatically increasing the space they need. Darrieus turbines have a low initial torque, meaning they require an external power source to get started. They will rarely ever self-start, even in high wind speeds.
Savonius Wind Turbines
Savonius wind turbines are the simplest of the wind turbine designs. They consist of up to four aerofoil scoops vertically attached to a central shaft. These turbines generate electricity using drag, making them less efficient. They tend to be used where cost and reliability are bigger factors than efficiency.
Thanks to their simple design, they require little maintenance and are completely self-starting. They are especially useful for low-power applications in remote locations. Deepwater buoys, water pumps, and cooling fans are common applications for savonius turbines. Many anemometers are savonius turbines, as the main goal is to measure wind speed, not generate power. In Europe, savonius turbines are sometimes used as advertisements, where a simple two-frame animation is placed inside the turbine, which is then activated by the turbine’s rotation.
Unconventional Wind Turbines
With the explosion in alternative renewable energy production over the past decade, engineers have put their efforts into designing new and experimental types of wind turbines. While some do show promise, they have yet to reach the efficiency of HAWTs.
The downwind turbine is almost identical in appearance to a normal HAWT, though with one major difference. Downwind turbines are essentially backward, where the turbines are placed downstream in relation to the wind. This puts the blades behind the nacelle.
The major advantage of downwind turbines is that they can passively rotate to face the wind, saving costs on a yaw mechanism. The blades can also be manufactured with more flexible material, which also saves on costs. Conventional HAWT blades can’t bend, as they’ll collide with the tower. The major downside for downwind turbines is the changes in load on the blades as they pass behind the tower, which over long periods of time can cause structural weakness, thus meaning they may not last as long as conventional turbines.
Bladeless Wind Turbines
Bladeless turbines are an innovative new design that relies on wind vortices as opposed to a constant breeze. At first glance, it’s hard to recognize a bladeless turbine as something that generates power from the wind. They have no blades and stand erect, resembling large white poles.
Bladeless turbines oscillate in response to wind vortices in a resonant phenomenon called vortex-induced vibration. When the wind hits a blunt object, vortices are created. The blunt object then begins to vibrate in response until the resonant frequency of the object matches with that of the wind. As the turbine vibrates, it generates electricity using an alternator system.
Bladeless turbines have fewer moving parts and take up less space. This gives them an opportunity to be more cost-effective. Still, the technology is in its infancy and is inefficient compared to other designs. The turbine also has to oscillate rather quickly to generate electricity. That places more stress on the turbine and can cause structural issues later on down the line.
Counter-rotating turbines are an interesting design, where an upwind and downwind HAWT are placed on the same tower. The two rotors then rotate against each other, with the downwind turbine capturing the leftover energy from the upwind turbine. Counter-rotating turbines have achieved an extra 40% boost in efficiency by capturing the downwind energy. They also don’t require a gearbox, and they passively align themselves with the wind direction.
While counter-rotating turbines seem advantageous, they’re difficult to engineer. The downwind turbines must be smaller than the upwind turbines and must be set to stall at higher wind speeds. They must also be timed to rotate at certain speed ratios to prevent vibrations that can lead to structural failures. As of today, very few, if any, counter-rotating turbines have been sold.
While windmills have existed for centuries, deep research into efficient designs for wind turbines are just taking off. Researchers are hard at work creating designs to harness wind energy in different ways. While the HAWT is currently still the most efficient design, engineers are quickly discovering new ways to improve on old methods. Unconventional designs like the bladeless turbine are an off-the-cuff concept that can change what wind power looks like. While the past ten years have seen an explosion in the growth of wind energy, the next decade can bring us new designs that put the traditional windmill to rest, and help to power the homes of the future.
Frequently Asked Questions
HAWTs are the most common type of wind turbine. As of today, the three-blade propeller-like design is the most efficient and widely researched type of wind turbine. When you think of wind energy, you most likely think of HAWTs.
HAWTs are the most common type of wind turbine, usually used for industrial-scale applications like wind farms. VAWTs are less common and are usually reserved for small-scale applications like rooftop wind generation or as anemometers.
Bladeless turbines do have their advantages. They have fewer moving parts and take up less space. Still, the technology is in its infancy. Efficiencies are low, and the design suffers from structural issues due to the high-speed oscillations necessary to generate electricity.
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