Vertical axis wind turbines
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Vertical Axis Wind Turbines

Vertical axis wind turbines

Vertical wind turbines are a type of wind turbine that have a vertical rotor axis, unlike the traditional horizontal wind turbines. They have a futuristic design and often look fantastic, which may attract some people who want to have a vertical wind turbine for their commercial building or private house. They can generate electricity in an environmentally friendly way, but are they really recommended? In this post, we will answer some of the most common questions about vertical wind turbines, such as:

  • What are the different types of vertical wind turbines and how do they work?
  • What are the advantages and disadvantages of vertical wind turbines compared to horizontal wind turbines?
  • How efficient and economical are vertical wind turbines?

Working of Vertical Axis Wind Turbines

The overall look of a wind turbine depends on the orientation of its rotor axis. We categorize turbines as either horizontal or vertical, as shown in the figure below

Horizontal axis wind turbine
Horizontal axis wind turbine
Vertical Axis Wind Turbine
Vertical Axis Wind Turbine

The placement of the rotor axis plays a crucial role in shaping the rotor, determining its rotation plane, and influencing its capacity to convert wind energy into electricity.

Types of Vertical Axis Wind Turbines

There are different simple designs for small vertical wind turbines, explained below. In reality, you may encounter variations and combinations; developers often showcase their creativity in creating diverse forms of vertical wind turbines.

Video on Vertical Axis Wind Turbines

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Savonius Rotors

The Savonius rotor is a type of vertical axis wind turbines, characterized by its comparatively massive and drag-driven design.Savonius rotors are known as drag-type rotors because the entire rotor surface offers resistance to the wind and is essentially pushed away by the wind. However, drag also limits the speed and power of the rotor. Savonius rotors can only rotate as fast as the wind, and they have the lowest efficiency among all small wind turbines. This means that they produce much less electricity than other designs.

Savonius Rotor, a type of vertical axis wind turbine
Savonius Rotor, a type of vertical axis wind turbine

Darrieus Rotors

The Darrieus rotor is a type of vertical axis wind turbine (VAWT) known for its distinctive eggbeater shape. It consists of curved airfoil blades that are arranged in a vertical orientation around the central axis.

Darrieus rotors are named after Georges Jean Marie Darrieus, a French engineer who patented the design in 1926. Check out some historic and famous vetical axis wind turbines

Darrieus Rotor, a type of vertical axis wind turbine
Darrieus Rotor, a type of vertical axis wind turbine

Darrieus rotors belong to the category of lift-based rotors, which means that they use the aerodynamic force of lift to spin the blades. Unlike drag-based rotors, which have blades that face the wind directly, lift-based rotors have blades that are tilted at an angle to the wind. This allows the wind to flow smoothly over the blade surface, creating a pressure difference that generates lift.

Lift-based rotors can rotate much faster than the wind speed, which increases the power output and efficiency of the wind turbine. The power coefficient is a measure of how much of the available wind energy the rotor can capture. Most modern wind turbines, whether they have a vertical or a horizontal axis, are lift-based rotors.

The Darrieus rotor comes in various subforms, including helix-shaped, disc-like, and the H-rotor with straight blades. These turbines typically have three slim rotor blades driven by lift forces, allowing them to achieve high speeds. The aerodynamic properties, including wind pressure and lift effects, contribute to their efficiency, reaching a maximum of around 40% with the physically possible maximum for wind turbines being 59%.

H-Rotors

A common design type of the Darrieus rotor is the H-rotor. In this configuration, the blades of the turbine are not curved like in the classic model but rather straight. The H-rotor has a larger surface area opposing the wind, theoretically providing higher power output compared to models with curved blades.

H-Rotor, a type of vertical axis wind turbine
H-Rotor, a type of vertical axis wind turbine

Helical Rotors

Helical rotors are a type of vertical axis wind turbine (VAWT) that use lift to spin the blades. They have curved blades that form a helix or a spiral shape around a vertical shaft. The spiral design allows the rotor to capture wind from various directions without the need for complex mechanisms to turn and face the wind.

Helix-Rotor, a type of vertical axis wind turbine
Helix-Rotor, a type of vertical axis wind turbine

Key Technological Aspects of Vertical Wind Turbines

The aerodynamics of vertical wind turbines

Vertical wind turbines face an aerodynamic challenge, as part of the rotor moves against the wind, resulting in energy loss due to wind resistance. This is a significant disadvantage compared to horizontal wind turbines, which operate on lift and achieve high speeds with minimal drag. Vertical wind turbines also suffer from vibrations and resonances, which limit their use of higher masts and require additional precautions.

Aerodynamic challenge on Vertical Axis Wind Turbine

The wind tracking of vertical wind turbines

Vertical wind turbines do not need wind tracking, as they can capture wind from any direction. This seems advantageous, but it also means that they cannot turn the entire rotor away from the wind, which is a crucial feature for storm protection. Horizontal wind turbines have mechanisms to align themselves with the wind direction and to pitch and yaw the blades to control the power output and avoid damage.

Vertical wind turbines noise

Vertical wind turbines are generally quieter than horizontal wind turbines, due to their slower rotational speeds and lower tip speeds. This makes them more suitable for residential areas, where noise pollution is a concern. However, the sound level of vertical wind turbines also depends on the type of rotor, the wind speed, and the distance from the source.

Advantages of Vertical Wind Turbines

  • They are more practical in residential areas and can be used to generate power in gusty winds and when they are blowing steadily.
  • They have fewer parts than horizontal wind turbines, which means fewer components to wear out and break down.
  • The supporting strength of the tower does not need to be as much, because the gearbox and generator are near the ground. Parts for controlling pitch and yaw are not needed either.
  • The turbine does not have to be facing the right wind direction either, as it can capture wind from any direction.
  • They are cheaper to produce than horizontal wind turbines, more easily installed, transportable from one location to another, and permissible where taller structures are prohibited.
  • They are also quieter to operate, so they do not disturb people in residential neighborhoods.

Disadvantages of Vertical Wind Turbines

Some of the main disadvantages are:

  • They produce less energy than horizontal wind turbines, have lower efficiency rates, limited scalability, more turbulence, and higher costs.
  • They require a higher wind speed to start rotating, which means they may not work well in low-wind areas.
  • They are more prone to fatigue and stress due to the varying torque on the blades.
  • They may pose a threat to birds and bats that fly close to the ground.

Efficiency and Economy of Vertical Wind Turbines

The cost of buying vertical wind turbines is usually similar to or higher than that of horizontal wind turbines. In the small wind power market, prices and costs per kilowatt can vary widely based on the manufacturer and turbine model, ranging from 3,000 to 10,000 euros per installed kilowatt.

Vertical wind turbines face challenges in aerodynamics, system regulation, and storm protection. Unlike horizontal turbines, vertical ones can’t be turned out of the wind for storm protection. This makes system regulation more complex, leading to additional construction costs for special technical solutions.

Summary

Small horizontal wind turbines emerge as the more economical choice due to their capacity for generating higher electricity, benefiting from aerodynamic advantages and regulatory simplicity. These turbines, prevalent in the market, are widely accepted and often certified under the rigorous standard. Their efficiency and ease of regulation make them the preferred option for electricity generation.

On the other hand, vertical Savonius rotors, while exhibiting lower efficiency, find niche applications in specific contexts, such as pumping water. However, a major drawback is the scarcity of manufacturer-independent references, leaving potential buyers with limited information. This lack of transparency impedes decision-making regarding the effectiveness and economic viability of these rotors for widespread electricity generation in inland areas.

Similarly, Darrieus rotors, distinguished by lift efficiency, face challenges related to pronounced vibrations and difficulties in storm protection. The limited ability to turn a vertical wind turbine out of the wind, a feature found in horizontal counterparts, further complicates their use.
Despite these challenges, both vertical designs share the advantage of easy maintenance, with machine components conveniently located on the ground. The aesthetically pleasing designs of many Darrieus rotors contribute to their positive reception in certain settings.

What innovations will emerge to tackle these technical challenges? One thing is clear: the enthusiasm for vertical wind turbines is immense and the demand is growing. We would be happy to see vertical small wind turbines become even more popular and we will make sure to keep you informed, especially about the exciting developments in vertical wind power.

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