The aerodynamic design of a wind turbine blade defines its width, thickness, direction and profile. It is a matter of finding the best compromise between air flow and strength (profile and structure). The purpose is to optimise performance while minimising loads.
At their very heart, wind turbine blades are very similar to airplane wings. By moving air across an airfoil surface, wind speed is higher across the curved surface, and lower across the flat surface. Adjusting the angle of attack a blade is attached at can optimize the distance the air travels across the blade without causing what is known as stall. Stall occurs when the lift induced drag caused by turbulence exceeds the lift created by the airflow.
The side of the blade across which the wind moves slower is the pressure side – this is usually the flatter side of the blade. The side of the blade across which the wind moves quickly is the suction side – this is the more curved portion of the blade. Bernoulli’s principle says that faster moving air has its air molecules spread further apart, causing lower air pressure, while slower air has its molecules closer together and higher pressure. This difference in high and low pressure against the blade is what causes it to rotate.
As the blades rotate the air flow across them becomes a combination of the air flow of the wind coming into the blades, as well as the air moving across the blade due to its rotational speed. For example, take a blade with a 10m length rotating at 100RPM.
The tip of the blade travels a distance of 2 x pi x r = 62.8 meters per revolution. At 100RPM, that’s 6,280 meters per minute, or 376 km/hr – about 100 meters per second. If the wind hitting the blade is coming in at 10 meters per second, it is only hitting the blade at roughly 10% the speed of the wind that the blade itself is encountering due to its rotational motion. There is significant drag in the motion of air across the blade as compared to the air rotating the blade.
This is why blades are pitched with a twist. At the inside of the blade, near the rotor, say 1 meter from the hub, the rotational motion at 100RPM is around 628 meters per minute (about 10 meters per second). In that case the wind speed is about equal to the rotational velocity at that section of blade – so an angle of attack near 45 degrees would give the blade its best lift characteristics. Around half way down the blade, the rotational velocity is 3,000 meters per minute (50 m/s) about 5 times the wind speed – so the blade is twisted to look flatter toward the oncoming wind and more into the air movement caused by rotation – somewhere over 80 degrees. At the tip of the blade the rotational air flow is 10 times the wind speed – so the blade will be very near 90 degrees rotated from the wind direction (or look almost flat when face-on), minimizing the drag encountered by the rotating blade.
As the blades rotate faster due to higher wind conditions, the specific twist and angle of the blade will cause it to enter a condition known as stall – these “Stall Profile” blades are designed that above a certain speed turbulence starts to be generated on the suction side of the blade, increasing the air pressure on that side of the blade causing it to lose that pressure differential that causes rotation. This, in turn, slows the blade back to near its stall speed.
Every blade is manufactured with an optimal wind speed and rotational speed profile that impact the point at which the blade makes its peak amount of rotational torque as well as the maximum speed the blade can be rotated at.
The formula for Power (in kW) = (Torque (N-m) x 2 x π x Speed (RPM)) / 60000
So, for example a 20kW wind turbine that is designed to rotate at a speed of 100RPM to give its rated power output, the required torque of the blade at 100RPM would be:
Torque = (Power x 60000) / (Speed x 2 x π) = (20 * 60000) / 628 = 1900 N-m
The best designed blade would be able to give this maximal torque and speed across a wide variety of wind speeds. In reality, in small wind turbine systems, most blades provide almost no torque at all until a wind speed around 3 meters per second (about 10km/hr or 6mph) is reached. From there to around 10 meters per second the torque grows exponentially until it reaches its maximum value just under 12 meters per second (about 40km/hr or 25mph). At that point the torque and speed remain constant until the wind speed starts to exceed the design capacity of the wind turbine, and the system either needs to have dynamic braking applied or be turned out of the wind, to prevent damage to the blades, the generator, and the electrical systems. This torque curve directly yields the power curve of the generator as seen here.
As can be seen in the graph above and the formulas relating torque and RPM to power output, the design of the blades and their aerodynamic performance is what will ultimately determine how much power can be developed on any machine. As the length of the blade is increased, the amount of swept area of the entire rotor and blade assembly increases, increasing the amount of torque and power that can be provided by the blades.