Let’s look now at the development and design of rotor blades. What we must bear in mind is that they absorb the energy of the wind and therefore also much of the forces that impact the wind turbine.
The main goal of the planning and manufacturing of the rotor blades is to keep the costs of power generation as low as possible. Three key aspects have to be fulfilled:
  • Optimization of the aerodynamic characteristics and maximization of the power generation
  • Minimization of the costs of wind turbines
  • Reduction of the loads caused by the rotor blades on the remainder of the construction and their impact on the costs of the system.

Before aerodynamic optimization of the rotor blades, the turbine data is reviewed: rated capacity, rotor diameter and the corresponding local wind class. Analyzing this data permits initial conclusions on the fundamental properties of the rotor blades, e.g. length, shape, cable routing, diameter at the root of the rotor blade and the selected aerodynamic profile properties and thus the relative thickness and torsional ratio of the rotor blades.

Once the aerodynamic shape of the rotor blades has been determined, the internal structure can be planned. This is a gradual process, which must incorporate the forces impacting the rotor blades. In turn, they depend on the structure of the rotor blades, especially their mass and stiffness.

The internal structure of the rotor blades must be designed to ensure compliance with the legally required safety limits in the following scenarios:
Collision of rotor blades with the tower
Fiber breakage etc.
General instability
Local instability
Weakening of adhesion points
Fatigue breaks.

The internal structure of the rotor blades consists largely of thin-walled support structures like those used in aviation applications. They form a largely hollow structure consisting of adjacent longitudinal chambers divided by partitions known as spars. LEITWIND wind turbines are produced as pre-assembled units, have an extremely low error rate and can be tested efficiently.
The rotor blades are produced from composite material via vacuum infusion processes. The structure of the rotor blades depends largely on the materials used, the lay-up of the individual components (up to 100 layers are processed) and the layout of the individual chambers, which is determined by the center spar and number of shear webs. The materials used are chosen based on the intended use. Standard or high-performance fiberglass cloth, resin and epoxy resin adhesives and filling materials like balsa wood and PVC foam are used.

The rotor blades are manufactured in heated molds, which consist partially of composite material, containing the heating system and a steel frame which ensures that the mold remains rigid. As soon as the configuration has been selected, prototypes of the new rotor blades are produced. The manufacturing process work flows are monitored thoroughly and the finished rotor blades are subjected to structural certification testing if suitable.

Testing the rotor blades
The main purpose of testing is to confirm suitability and test whether the static and dynamic properties of the finished rotor blades can withstand the heavy loads in practice.

The following result is from a test in November 2017. Rotor blade LS44 for turbine LTW90 was loaded with 28,000 kg along its entire width (equivalent to the weight of 20 passenger cars, and therefore almost twice the normal operating load). During the test, the blade bent more than 12 m without suffering damage.
After passing the test, production of an initial set of rotor blades for a prototype turbine is generally the first step. It is used to test the aerodynamic performance (i.e. the performance curve) and the expected aero-elastic properties (loads and vibrations).
After that, volume production of rotor blades, and thus the product update management phase begins, which prioritizes three goals:
  • Optimizing the manufacturing process
  • Reducing the costs
  • Meeting highest quality standards
The work flows for manufacturing and quality assurance of rotor blades include full traceability of the materials used, documentation of the duration and nature of the individual production phases and various other controls.