A vertical-axis wind turbine (VAWT) is a type of wind turbine where the main rotor shaft is positioned perpendicular to the wind direction, and the main parts are placed at the base of the turbine. This setup places the generator and gearbox near the ground, making it easier to service and repair them. VAWTs do not need to face the wind directly, which eliminates the need for systems that sense wind direction or adjust the turbine's position. Early designs, such as Savonius, Darrieus, and giromill turbines, had problems like uneven force during each rotation and high stress on the blades. Later designs, like the Gorlov type, improved this by shaping the blades in a spiral pattern. Savonius VAWTs are not commonly used, but their simple design and better performance in areas with uneven wind patterns make them a useful option for power generation in cities.

A vertical-axis wind turbine has its axis perpendicular to the wind flow and vertical to the ground. A broader term for this type is "transverse axis wind turbine" or "cross-flow wind turbine." For example, the original Darrieus patent, US patent 1835018, covers both vertical and horizontal-axis designs.

Drag-type VAWTs, like the Savonius rotor, usually operate at lower tip speed ratios compared to lift-based VAWTs, such as Darrieus rotors and cycloturbines.

Computer models suggest that vertical-axis wind turbines placed in wind farms can produce more than 15% more power per turbine than when used alone. Some designs, like the Airfoil generator, create very little turbulence after the turbine, allowing them to be placed closer together for better use of land.

General aerodynamics

The forces and speeds acting on a Darrieus turbine are shown in Figure 1. The total air speed, W, is the sum of the air speed before it hits the turbine, U, and the speed of the blade moving forward, -ω×R.

The speed of the air hitting the blade changes as the blade moves through its cycle. The highest speed occurs when the blade is at θ = 0°, and the lowest speed occurs when the blade is at θ = 180°, where θ is the position of the blade as it moves around the turbine. The angle of attack, α, is the angle between the air moving toward the blade and the blade’s chord. The airflow creates a changing angle of attack on the blade. This angle becomes negative on the side of the turbine opposite the direction of the airflow.

From the geometry of the turbine’s motion, the relative speed of the air is calculated by combining the tangential and normal parts of the blade’s movement. Using the tip speed ratio, λ = (ωR)/U, the total air speed can be expressed as:

The angle of attack is calculated using the formula:

When this formula is applied, it shows how the angle of attack changes.

The forces acting on the blade are divided into two groups: lift (L) and drag (D), or normal (N) and tangential (T). These forces are assumed to act at the point one-quarter of the way along the blade’s chord. The pitching moment is calculated to analyze these forces. In aeronautics, lift is the force perpendicular to the airflow, and drag is the force parallel to the airflow. The tangential force moves the blade in the direction of its motion, while the normal force pushes against the turbine’s shaft. Lift and drag are useful for studying effects like dynamic stall and boundary layer behavior. For overall turbine performance and fatigue analysis, it is easier to use the normal-tangential force frame. Lift and drag forces are typically divided by the dynamic pressure of the airflow, while normal and tangential forces are divided by the dynamic pressure of the undisturbed air.

A = Blade Area (not to be confused with Swept Area, which is the area covered by the blade/rotor as it spins, calculated by multiplying the blade’s height by its diameter), R = Radius of the turbine.

The amount of power, P, that a wind turbine can capture is calculated using the formula:

Where C_p is the power coefficient, ρ is air density, A is the swept area of the turbine, and ν is the wind speed.

Types

There are two main types of vertical axis wind turbines: the Savonius wind turbine and the Darrieus wind turbine. The Darrieus rotor has several types, including helix-shaped, disc-like, and the H-rotor with straight blades. These turbines usually have three thin rotor blades that are moved by lift forces, which allow them to spin quickly. [1]

Many simple designs exist for vertical wind turbines, as described below. In real-world use, you may see many different versions and combinations of these turbines. Engineers often create new and varied designs for vertical wind turbines.

The Savonius wind turbine (SWT) is a drag-type VAWT. The most common design includes a rotating shaft with two or three scoops that capture the wind. Because of its simple and strong design, and its lower efficiency compared to other turbines, it is used when reliability is more important than efficiency. One reason for its lower efficiency is that only about half of the turbine creates useful force, while the other half moves against the wind and creates opposite force. A version of the SWT is the Harmony wind turbine, which has helix-shaped blades and an automatic system that adjusts the blades during strong winds.

The Darrieus wind turbine is a lift-type VAWT. The original design used curved aerofoil blades attached to a rotating shaft. However, some designs use straight vertical aerofoils, called H-rotor or Giromill Darrieus wind turbines. Additionally, the blades of the Darrieus turbine can be shaped into a helix to reduce uneven force by spreading the force evenly during each rotation.

As lift-type devices, Darrieus wind turbines can capture more energy from the wind than drag-type turbines, such as the Savonius wind turbine.

Revolving wing wind turbines, or rotating wing wind turbines, are a new category of lift-type VAWTs. These turbines use one vertically standing, non-helical aerofoil to create full 360-degree rotation around a vertical shaft that runs through the center of the aerofoil.

The Airfoil generator is a new type of vertical axis wind generator. The rotor is made of several helical blades attached to a central axis, similar to a Savonius wind turbine. This rotor is placed inside an airfoil that uses Bernoulli's principle to increase wind speed before energy is captured. This process allows the turbine to produce more power at all wind speeds. The airfoil also protects the blades from drag that other VAWT designs experience when the blades move against the wind. Because of this reduced drag, the Airfoil generator can produce more energy than other unshrouded designs.

Advantages

Vertical-axis wind turbines (VAWTs) have several benefits compared to traditional horizontal-axis wind turbines (HAWTs):

• VAWTs can capture wind from any direction without needing to turn to face it. This avoids the need for complicated parts and motors that adjust the turbine’s position.
• Repairs and maintenance of the gearbox are easier because the gearbox is located on the ground, so workers do not need to climb high into the air. Gearbox problems are often major challenges in operating and maintaining wind turbines.
• Some VAWT designs use screw pile foundations, which reduce the need to transport large amounts of concrete and lower the environmental impact during installation. These screw piles can be fully recycled at the end of their useful life.
• VAWTs can be placed below existing HAWT wind farms to increase overall power production.
• VAWTs may work in conditions where HAWTs cannot. For example, the Savonius rotor, a type of VAWT, functions in slow or uneven winds near the ground. It is often used in remote areas, even though it is less efficient than other VAWT designs.
• VAWTs produce less noise than HAWTs.
• VAWTs pose less risk to birds compared to HAWTs.

Disadvantages

As the speed of a VAWT wind turbine increases, the power it produces also increases. However, after reaching a peak speed, the power begins to decrease until it stops completely, even though the turbine continues to spin rapidly. To manage this, disc brakes are used to slow the turbine during very strong winds. In some cases, the brakes can overheat, leading to fires.

Some VAWT designs experience dynamic stall, which occurs when the angle of the blades changes quickly during operation.

Darrieus-type VAWTs are more likely to experience blade wear because the forces acting on the blades change significantly during each rotation. The upright blades can twist or bend with every turn, reducing their lifespan. In contrast, Airfoil and Savonius types have blades that are supported along their entire length, making them less likely to flex and wear out.

Compared to HAWTs, most VAWTs (except drag-type models) have been less reliable in the past. However, newer designs have improved and solved many of these early problems.

Research

A 2021 study tested a setup for VAWTs that showed they could outperform a similar HAWT setup by 15%. A 11,500-hour test showed this increased efficiency, partly by arranging the VAWTs in a grid pattern. One benefit of this arrangement is reducing air movement problems that lower efficiency in HAWT setups. Other improvements included adjusting the angle of the turbine array, the direction of rotation, the space between turbines, and the number of rotors.

In 2022, Norway’s World Wide Wind introduced floating VAWTs with two sets of blades that spin in opposite directions. These sets are attached to two shafts that are centered on each other. Each set has a turbine connected to it—one to the spinning part and the other to the stationary part. This design allows the blades to move faster compared to a fixed stator. The company claimed these VAWTs can produce more than twice the electricity of the largest HAWTs. HAWTs need heavy parts like drivetrains, gearboxes, generators, and blades at the top of the tower, which require heavy underwater weights to balance. VAWTs put most of these heavy parts at the bottom of the tower, reducing the need for underwater weights. The blades move in a cone shape, which helps reduce air movement problems behind each tower, allowing more towers to be placed closely together. The company plans to build a 400-meter (1,300-foot) unit that produces 40 megawatts of electricity.

In 2024, a study on the Airfoil generator design was done by the National Renewable Energy Laboratory (NREL). This led to a collaboration between three Midwestern companies to build the first large-scale model of an Airfoil Generator. Building will continue until 2026, with the first use planned for September 2027.

Applications

The Windspire is a small VAWT designed for use at homes or offices. It was created by the US company Mariah Power in the early 2000s. Mariah Power said that many units were installed in the United States by June 2008.

Arborwind, a company from Ann Arbor, Michigan, makes a small VAWT with special rights. It has been placed in several locations in the US since 2013.

In 2011, wind-energy researchers at Sandia National Laboratories began a five-year study on using VAWT design technology for offshore wind farms. The researchers explained that offshore wind power has different costs than land-based turbines because of challenges during installation and operation. VAWTs have three advantages that could lower wind energy costs: a lower center of gravity, simpler machines, and the ability to grow larger. A lower center of gravity improves stability on water and reduces stress from gravity. The drivetrain is near the surface, which might make repairs easier and faster. Fewer parts, less stress, and easier repairs can lower maintenance costs.

In the early 2010s, Caltech professor John Dabiri set up a demonstration with 24 VAWTs in southern California. His design was used in a 10-unit wind farm in the Alaskan village of Igiugig in 2013.

In March 2014, Dulas in Anglesey got permission to test a new VAWT prototype on the breakwater at Port Talbot. The turbine, made by C-FEC in Swansea, Wales, will be tested for two years. This VAWT has a wind shield that blocks wind from the moving blades, so it needs a wind direction sensor and a positioning system. This is different from the "egg-beater" VAWTs mentioned earlier.

StrongWind, a Canadian company, makes a patented urban VAWT. It has been placed in several locations in Canada and other countries by 2023.

Architect Michael Reynolds, famous for Earthship homes, created a fourth-generation VAWT called Dynasphere. It has two 1.5 kW generators and can generate electricity even at low wind speeds.