Darrieus Vertical Axis Wind Turbines

This article aims to explain fundamental differences of vertical axis wind turbines (Darrieus type) from propeller-based turbines. It also aims to explain some of the inherent advantages of vertical wind turbine design. The most notable difference between vertical and horizontal types of turbines is that propeller turbine works based on aerodynamics of rotation (e.g. similar to a helicopters rotor), and a vertical turbine utilizes aerodynamics of linear movement (like a wing of an aircraft does). A rotating propeller creates a thin "air-permeable disc", while a rotating vertical axis turbine creates a shape of a cylinder (“ squirrel cage ”) which is also penetrated by air.

These are different aerodynamic mechanics of work of wind, and provide for different levels of efficiency - often represented as a coefficient of transformation of wind energy into mechanical or electrical energy. Darrieus-type turbines such as ANew-S1 can achieve up to 70% conversion efficiency of winds kinetic energy into electrical energy (after mechanical and electrical conversion losses). Whilst a propeller-based turbine would achieve 35-40% conversion efficiency under the same conditions of wind.

The other often overlooked factor of turbine operation and efficiency is the geometry (diameter, height, as well as swept area and area of the blades), direction of rotation, and ability of a turbine with vertical axis of rotation to accumulate and preserve energy similar to a flywheel. Because Darrieus-type wind turbines, such as ANew-S1, form a vertical cylinder during rotation, they are capable of extracting energy from the same flow of air twice, as shown in the diagrams below:

Darrieus Wind Turbines Aerodynamics Graph

When the blade is in position (1) the airflow W1 makes contact with the blade, giving it some of its own energy (about 60%) while losing about 27% of its speed up to W2 (slowed by the blade). When blade is moving through the position in diagram (2) air flow with original velocity W2 has the space and time to accelerate its speed up to W3. When the blade is in position shown on diagram 3, the airflow W3 has another contact with the blade, giving it some of its energy (about 60% of energy of airflow W3), which is equivalent to about 45% of the energy of primary airflow W1. It also loses about 27% of its speed in position W4.

As you can see, the blade in diagram 4 takes energy from the same airflow twice. The airflow itself had time to partially restore its speed (and energy) in the space between entering and exiting the turbine cylinder. In order for the above scenario to work, the blade must arrive into position 3 simultaneously with the flow of air which initially gave the blade part of its energy in diagram 1. This requires a specific linear velocity of the blade for each unique wind speed range.

If the turbine has two or more blades, the initial transfer of energy from the primary airflow is made to one of the blades, and the secondary energy transfer by the same airflow can occur with the second blade. That is, if the turbine has two or more blades, the linear velocity of the blades may be significantly less than linear velocity required by a turbine with a single blade.

In addition, Darrieus type of wind turbines allows for much quicker wind speed recovery than the traditional propeller based turbines. This means that the speed (and the energy) of the wind restore within shorter distance of the turbine. This advantage allows vertical turbines to be placed closer together. This topic is covered in greater detail in an article called Wind Speed Recovery.