Horizontal and Vertical Axis Wind Turbines: A Comparison

Wind energy is a clean and renewable power source that helps reduce greenhouse gas emissions and reliance on fossil fuels. Wind turbines convert wind’s kinetic energy into electrical energy. There are two main types of wind turbines: horizontal axis and vertical axis.

What is a Horizontal Axis Wind Turbine?

A horizontal axis wind turbine (HAWT) is defined as a wind turbine with a horizontal rotation axis parallel to the ground. HAWTs are the most common type used for large-scale electricity generation. They usually have three blades like airplane propellers, though some have two or one blade.

The main components of a HAWT are:

• The rotor, which consists of the blades and the hub that connects them to the shaft.
• The nacelle houses the generator, gearbox, brake, yaw system, and other mechanical and electrical components.
• The tower supports the nacelle and the rotor and elevates them above the ground to capture more wind.
• The foundation anchors the tower to the ground and transfers the loads from the wind turbine.

The working principle of a HAWT is based on lift, a force that pushes an object upward when air flows over its surface. The blades are shaped like airfoils, creating a pressure difference between their upper and lower surfaces when wind blows. This pressure difference makes the blades rotate around the horizontal axis, driving the shaft and generator to produce electricity.

The rotor plane of a HAWT must be aligned with the wind direction to maximize its efficiency. Therefore, a HAWT has a wind sensor and a yaw system that adjust the orientation of the nacelle according to the wind direction. A HAWT also has a pitch system that changes the angle of attack of the blades to control their rotational speed and power output.

The advantages of HAWTs are:

• They have higher efficiency than vertical axis wind turbines (VAWTs) because they can capture more wind energy with less drag.
• They have lower torque ripple and mechanical stress than VAWTs because they have fewer changes in aerodynamic forces during each rotation.
• They can be installed offshore on floating platforms or fixed foundations, where the wind speed is higher and more consistent.

The disadvantages of HAWTs are:

• They require a tall tower and a large land area to avoid turbulence and interference from nearby structures or terrain.
• They are more expensive and complex to install and maintain than VAWTs because they have more moving parts and electrical components.
• They are more susceptible to fatigue and damage from high winds, storms, lightning, birds, or ice.

What is a Vertical Axis Wind Turbine?

A vertical axis wind turbine (VAWT) is defined as a wind turbine with a vertical rotation axis perpendicular to the ground. VAWTs are less common than HAWTs but offer advantages for small-scale and urban use. They typically have two or three straight or curved blades.

The main components of a VAWT are:

• The rotor, which consists of the blades and the vertical shaft that connects them to the generator.
• The generator, which converts the mechanical energy of the rotor into electrical energy.
• The base, which supports the rotor and the generator and connects them to the ground.

The working principle of a VAWT is based on drag, a force that opposes an object’s motion when air flows over it. The blades, whether symmetrical or asymmetrical, create varying drag when facing or opposing the wind. This difference in drag makes the blades rotate around the vertical axis, driving the generator to produce electricity.

The rotor plane of a VAWT does not need to be aligned with the wind direction because it can capture wind from any direction. Therefore, a VAWT does not have a yaw system or a wind sensor. However, a VAWT may have a pitch system that changes the angle of attack of the blades to control their rotational speed and power output.

The advantages of VAWTs are:

• They have lower installation and maintenance costs than HAWTs because they have fewer moving parts and electrical components.
• They have lower noise levels than HAWTs because they rotate at slower speeds.
• They can be installed on rooftops or near buildings because they have lower heights and smaller footprints than HAWTs.

The disadvantages of VAWTs are:

• They have lower efficiency than HAWTs because they have more drag and less lift.
• They have higher torque ripple and mechanical stress than HAWTs because they have more changes in aerodynamic forces during each rotation.
• They cannot be installed offshore because they are less stable and durable than HAWTs.

Types of Vertical Axis Wind Turbines

There are two main types of VAWTs based on their blade design: Darrieus and Savonius.

Darrieus Turbines

Darrieus turbines are VAWTs that have curved blades that resemble an eggbeater or a trochoid. They were invented by French engineer Georges Darrieus in 1931. Darrieus turbines use lift as well as drag to rotate their blades. They can achieve high rotational speeds, but they require an external start-up mechanism, such as an electric motor or another turbine, because they cannot self-start.

The advantages of Darrieus turbines are:

• They have a higher power coefficient than Savonius turbines because they use lift as well as drag.
• They have a lower solidity ratio than Savonius turbines because they have fewer blades with larger gaps between them.

The disadvantages of Darrieus turbines are:

• They require an external start-up mechanism because they cannot self-start.
• They have higher centrifugal forces than Savonius turbines because they rotate at faster speeds.

Savonius Turbines

Savonius turbines are VAWTs that have straight blades that resemble an S-shape or a half-barrel. They were invented by Finnish engineer Sigurd Savonius in 1922. Savonius turbines use only drag to rotate their blades. They can self-start, but they have low rotational speeds and low power coefficients.

The advantages of Savonius turbines are:

• They can self-start because they use only drag.
• They have lower centrifugal forces than Darrieus turbines because they rotate at slower speeds.

The disadvantages of Savonius turbines are:

• They have a lower power coefficient than Darrieus turbines because they use only drag.
• They have a higher solidity ratio than Darrieus turbines because they have more blades with smaller gaps between them.

Comparison Between Horizontal and Vertical Axis Wind Turbines

The following table summarizes some of the main differences between horizontal and vertical-axis wind turbines:

Aspect	Horizontal Axis Wind Turbine	Vertical Axis Wind Turbine
Axis orientation	Parallel to the ground	Perpendicular to the ground
Wind direction	Must be aligned with it	Can capture it from any direction
Yaw system	Required	Not required
Pitch system	Required	Optional
Start-up mechanism	Not required	Required for some types
Efficiency	High	Low
Torque ripple	Low	High
Mechanical stress	Low	High
Installation cost	High	Low
Maintenance cost	High	Low
Noise level	High	Low
Height	High	Low
Land area	Large	Small
Offshore suitability	High	Low

Conclusion

Horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs) are two types of wind turbines that differ in their axis orientation, blade design, working principle, efficiency, performance, cost, noise level, installation location, and environmental impact. HAWTs are more suitable for large-scale electricity generation in open areas with consistent wind direction, while VAWTs are more suitable for small-scale electricity generation in urban areas with variable wind direction.

Both types of wind turbines have their advantages and disadvantages depending on their specific applications. Therefore, it is important to consider various factors such as wind speed, site conditions, power demand, budget, aesthetics, safety, reliability, durability, etc., before choosing between horizontal and vertical axis wind turbines.