Aerodynamics – a balance between performance and load

The aerodynamic design defines the width, thickness and twist of a wind turbine blade. It is a matter of finding the best compromise between optimised air flow and blade strength. From an aerodynamic viewpoint, the perfect blade would be wide, thick and long. However, such a blade would put too much strain on the wind turbine and would also strike the tower during rotation. In other words, it would be too heavy without being rigid enough.

The strongest blade would be one with a circular cross section. However, such a blade would have a minimal lift and be unsuitable for extracting energy from the wind. A round blade would also generate extreme loads on the blade itself as well as the other parts of the wind turbine. This would result in disproportionately high costs for the rest of the turbine and the price of wind energy would increase.

These considerations are oversimplified, but illustrate just how complicated it is to develop blades that can produce wind power at competitive prices. Based on our extensive experience and exhaustive research, LM Glasfiber has succeeded in finding different ways of improving the aerodynamics of wind turbine blades, coordinated with their structure. In addition to pre-bending of the blade, the Vortex generators and low noise tips are some of the more obvious examples of measures for improving aerodynamics. 

Vortex generators

A wind turbine blade does not have the same aerodynamic efficiency along its entire length. Towards the root, thick airfoils are used to enable the blade to bear its own weight. The blade is circular at this end, to permit mounting on the turbine. These factors have a negative effect on the blade’s performance.

Vortex generators consist of a number of small fins that protrude over the interface that occurs when the wind moves over the surface. These fins are mounted opposite each other at a certain angle that causes counter-current eddies in the air flow.
Vortex generators result in the flow of air subsequently separating from the surface at a later point, i.e. at higher wind speeds. This delays the moment when the blades stall and thus lose their capacity to produce power – the lowest part of the blade is thus efficient during a greater part of the turbine’s production time.

Vortex generators can boost blade performance by up to 4–6%.

Tip design

The very end of the blade is called its tip. This plays a very important part in the blade’s overall performance and the noise it makes during operation.

LM Glasfiber has carried out extensive research in this field, and LM blades have had different tip designs over the years. From an aerodynamic viewpoint, the best possible shape would feature a relatively wide blade tip. However, this would result in considerable noise and increased loading. A larger area at the tip would also affect the distance from the tower to the blade. A wide tip design would also entail using more materials to strengthen the blade and make it more rigid.

One way to reduce noise emissions from the tip is to slow the blade’s speed of rotation. However, this has a negative effect on production. An alternative solution is to optimise the blade tip as regards both noise and performance.

Low-noise tip

Targeted research carried out at LM Glasfiber has resulted in an optimal tip design with our low-noise tip.