6.4.1 Aerofoils and harnessing aerodynamic forces
There are two main types of aerofoil section:
1. Asymmetrical aerofoils – optimised to produce most lift when the underside of the aerofoil is closest to the direction from which the air is flowing.
2. Symmetrical aerofoils – able to induce lift equally well (although in opposite directions) when the air flow is approaching from either side of the ‘chord line’ (shown in Figure 7).
The angle which an aerofoil (or flat or cambered plate profile) makes with the direction of an airflow, measured against a reference line (usually the chord line), is called the angle of attack α (alpha). When airflow is directed towards the underside of the aerofoil, the angle of attack is positive.
The lift and drag characteristics of many different aerofoil shapes have been determined by measurements in wind tunnels, and catalogued (e.g. in Abbott and von Doenhoff, 1958). The lift and drag characteristics measured at each angle of attack can be described using non-dimensional lift and drag coefficients (CL and CD) or as lift to drag ratios (CL/CD). Knowledge of these coefficients is essential when selecting appropriate aerofoil sections in wind turbine blade design. Lift and drag forces are both proportional to the energy in the wind.
Harnessing aerodynamic forces
Modern horizontal and vertical axis wind turbines harness aerodynamic forces in a different way.
In a HAWT with fixed-pitch blades, with its rotor axis in constant alignment with the wind direction, for a given wind speed and constant rotation speed the angle of attack at a given position on the rotor blade stays constant throughout its rotation cycle.
In most horizontal axis wind turbines the rotation axis is maintained in line with wind direction by a ‘yawing’ mechanism, which constantly realigns the turbine.
In addition to its swept area and rotor diameter, the performance (power output, torque and rotation speed) of a HAWT rotor depends on other factors, including the number and shape of the blades, the choice of aerofoil section, the length of the blade chord,, the blade pitch angle, the angle of attack at positions along the blade, and the amount of blade twist between the hub and tip.
By contrast, in a VAWT with fixed pitch blades, under the same conditions the angle of attack at a given position on the rotor blade constantly varies throughout its rotation cycle. This means that the ‘suction’ side reverses during each cycle, so a symmetrical aerofoil has to be employed to ensure that power can be produced irrespective of whether the angle of attack is positive or negative.
Let’s now concentrate on the power and energy that wind turbines can deliver.