The length of the rotor blades of a modern 5MW wind turbine is more than 60m and weighs about 18 tonnes. The cost of production is about SEK 3.5 million representing 15-20% of the wind turbine's total cost. The dynamic forces created due to unsteady lift and drag forces are considerable. The time variation of the forces arises primarily because of the interaction between the flow around the rotor blades and time variations in the incoming wind. The characteristics of the incoming wind, i.e. its turbulence, depend on the surrounding environment (oceans, forests, mountains etc.) The turbulence of the incoming wind is dependent on the type of forest, such as northern or Småland coniferous forest, deciduous forest.
The cost per installed kWh for wind power has been halved between 1985 and 2000. Estimates show that 60% of this cost is due to upscaling, i.e. larger turbines. A limiting factor in building large turbines is the size of the rotor blades which are usually manufactured in GRP (Glass Fibre Reinforced Plastic). Today it is possible to manufacture rotor blades made of composite materials, CRP (Carbon Fibre Reinforced Plastic). Marströms Composite AB is a world leader in the use of CRP as a material in construction and manufacturing. Rotor blades manufactured in CRP are 40% lighter than comparable blades in GRP. A lighter blade has a major positive impact on the rest of the wind turbine: the loads on the tower, housing (with gearbox and bearings) and the foundation will be correspondingly less.
A major limiting factor for the expansion of wind power is the noise which often determines the location of both onshore and offshore wind farms. Flow-induced sound at the trailing edge of the rotor blade tip is the greatest source of noise with wind energy systems because of the rotor blade tip's high speed (210-250 km/h). A noise limit of 40 dB (A) applies. At the Department of Fluid Mechanics there is a lot of activity in aeroacoustics for aircraft. Large computer simulations are performed regarding how the sound is created by turbulence and how sound propagates in the air. Some calculations are also including sound-absorbing surfaces. This experience of sound calculations in the aviation field will be used to simulate the propagation of sound from wind turbines.
Icing is a problem in Sweden. ice build-up creates vulnerability in the form of falling ice. A major problem related to the rotor blades is production loss. Either in the form of stoppage or reduced effectiveness of the rotor blades. Even a small ice build-up has a major impact on the rotor blade as its shape is changed. This gives rise to altered flow around the rotor blades and the result is that the driving aerodynamic forces are reduced and thus also the energy produced. Ice build-up can also cause vibrations resulting in increased maintenance costs and reduced service life.
The wind creates the largest loads on a wind turbine. In the forest high turbulence and wind gradient are high compared to the conditions on the plains. Turbulence is also highly dependent on the height of the trees and the forest's condition. High turbulence and strong wind gradient give rise to large fluctuating forces and torque on the turbine. This leads to fatigue damage and shorter service life. Knowledge of the local wind conditions can be used to make a wise choice when placing wind turbines, which leads to a longer service life and higher reliability.
Issues within Turbine and Wind loads are:
How big are the dynamic loads? Amplitude and frequency?
How big are the forces on a wind farm in the forest? How decisive is the forest type? How do you choose an optimal location (siting) in a wooded area? What is the impact of the weather?
How do ice build-up forces affect the rotor blades and fatigue loads on a wind turbine? What is the impact on energy production? How does location (siting) and weather affect ice build-up?
How so the dynamic loads spread from the rotor blades to the gearbox and bearings?
What material should the rotor blades manufactured in? How does the choice of material and the rotor blade's weight affect the structure of the other parts of the wind turbine? New structures based on new materials can provide increased rigidity.
Smart rotor blades in composite: how much are the aerodynamic forces reduced by?
How is noise transported from wind turbines to the surroundings? How does the sound transmission affect the environment (meadows, forests, oceans, mountains)? How far is the sound transported in the marine environment?
The following projects within the Centre fall under Theme group 2: