Windtech: Rotor innovation: Unorthodox turbine rotor design

Norwegian turbine firm Norsetek has developed a new type of horizontal-axis wind turbine rotor for future 4-6MW onshore and 10-12MW offshore applications.

Founded in 2008, the wind industry newcomer claims that its Light Rotor technology offers superior rotor performance and other benefits compared with commonly applied cantilever-type blades. According to Norsetek co-founder Vidar Holmoy, these built-in advantages translate into substantial gains in cost effectiveness.

Another claim is that the new technology enables the design of longer blades and lighter towers. The rotor also divides into sections, enabling easy handling and transport. Common cantilever-type rotor blades are only attached to the rotor hub at the blade root. The longer the blades become, the higher bending stresses near the blade root and backward bending caused by wind loads. Under unfavourable circumstances, the blade tips can hit the tower. To avoid this, technology developers have tried to enhance blade stiffness by using thicker airfoils near the blade root and incorporate carbon fibre into highly stressed sections of the laminate.

Other options are to increase the distance between tower and blades with an inclined main shaft positioning. Norsetek chose a different approach. Each blade is structurally stiffened in the axial - or wind thrust - direction and in the rotor plane itself. Holmoy explains that major blade stiffening is achieved by a combination of airfoil-shaped struts and blade-support members. "A much-reduced bending strain and much smaller blade-tip deflection ... translate into significant materials input and cost savings," says Holmoy.

Also, tensioning cables interconnect individual blades, which counteracts flap-wise bending in the rotor plane. This is an infamous phenomenon caused by the hanging blade mass that changes direction twice during each rotor revolution. Holmoy says: "The Light Rotor system can be configured with either three or four rotor blades, but the latter is our preferred configuration for both structural and slightly superior aerodynamic efficiency reasons."

While a four-bladed version may ultimately not be the company's final solution, "it has several advantages for us to consider. One is in augmenting power through reducing tip loss and a second advantage is that it inherently smoothens loading and output power characteristics through increasing blade-passing frequency. Simultaneously the rotor mass penalty is only about 10% compared to a three-blade solution of similar size."

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