Basic knowledge about rotor blades

Rotor Blades

Rotor blades are one of the main components of modern wind turbines.Due to their dimension, weight, the used materials and complex manufacturing, they have one of the highest component costs in the entire system.

Today the largest rotor blades have got a length around 130 meters and a weight about 60 tons (2023) [1].

Rotor blades convert kinetic energy of the wind into the rotation of the rotor. The movement of the rotor drives a generator, which produces electrical energy [2].

Structure and Manufacturing

Modern rotor blades are made of fiber-reinforced plastics. They offer very good mechanical properties at a relatively low weight. Mostly glass fibers (and sometimes carbon fibers) are used as fiber materials. The plastic matrix forms a thermoset, usually epoxy resin [3].

The most common design of rotor blades for wind turbines is described here: Two separately manufactured half-shells are bonded together. These half-shells form the aerodynamic shape of the blade and counteract torsion.

The shells of the rotor blade are longitudinally reinforced by fiber-reinforced spar caps made of numerous layers (around 30 to 60) of unidirectional fiber fabrics. These spar caps absorb tensile forces and counteract bending of the rotor blade. They are positioned around the area of the greatest profile thickness and on the trailing edge (see Figure 1) [3].

This blade-design contains a shear web perpendicular to the surfaces of the spar caps. It is used to absorb shear forces and, together with the spar caps, forms a rigid spar, like an I-beam. This is the primary, supporting structure of the rotor blade [1].

The shells of a rotor blade and the shear webs are built as a sandwich structure (see the explanation below). They are bonded together at the leading and the trailing edges. In Figure 1 you can see the longitudinal profile of a rotor blade on top. An exemplary cross section is shown below.

Operating Principle

Rotor blades have an aerodynamic profile. Their function is similar to the wings of an airplane.

When air flows around the rotor blade, there are different pressure conditions on its “top and bottom”. That is why the two half shells of the rotor blade are called “suction side” and “pressure side”. These pressure conditions around the blade create an aerodynamic lift that is perpendicular to the incoming air.

Due to the design of current wind turbines, it is not possible to use the whole aerodynamic lift to turn the rotor. The lift primarily causes the rotor blades to bend in the direction of the turbine tower. Only a small part of the lift force can be used to drive the rotor [2].

Aerodynamic power coefficient (cp)

The aerodynamic power coefficient describes the efficiency of the rotor. It expresses which part of the kinetic energy of the wind can be converted into the rotation of the rotor.

This topic was first described scientifically in the 1920s by the German physicist Albert Betz.

The basics of his research can be easily understood through a simple thought experiment: If a wind turbine could extract 100 % of the kinetic energy from the wind, the air behind the rotor would stand still. In this case no fresh wind could blow through the rotor because the air behind the rotor is not transported away. The wind turbine would be aerodynamically blocked. This means: if a wind turbine tries to extract too much energy from the wind, then the system sabotages itself.

Which amount of the wind's energy a rotor can convert is expressed by the rotor's "power coefficient". In scientific context it is declared with the letters “cp”.

Albert Betz was the first to calculate the maximum of energy that can be extracted from the wind using today’s common wind turbines. It is about 59 %. That means from an aerodynamic perspective, an ideal wind turbine can convert a maximum of 59 % of wind energy.

And this maximum of the aerodynamic efficiency is used as the title of our company: cp.max. And behind these incomprehensible letters lies our company's maxim: the optimal adjustment of the rotor blades to maximize yield.

Sandwich construction

In sandwich construction, materials with different properties are put together in layers. Often there is a core in the middle, which is covered by top-layers on both sides [6].

Almost the entire rotor blade is build as a sandwich construction. The cover layers consist of fiber-reinforced plastic. The core is made of light balsa wood or foam. In this way, a stable but relatively light component is achieved.

The fiber-reinforced layers in the rotor blade are very tensile, but due to their slimness they are also susceptible to bending. By introducing the core material, the wall thickness is increased and thus the walls of the rotor blade are stabilized.