Wind turbine blade technology

Project leaders

dr.ir. Otto Bergsma
dr.ir. Harald Bersee
ir. Don van Delft (KC-WMC)

PhD. students

ir. Rogier Nijssen
M.Ing. Simon Joncas

MSc. student

vacant

Contactperson

M.Ing. Simon Joncas
ir. Rogier Nijssen

Summary

Background
During their lifetime, wind turbine rotor blades endure a large number of changes, in both wind speed and direction, extreme gusts, and loading reversals due to their own weight. Thus, modern blades experience a loading spectrum, which exceeds that of many aeronautical structures, both in terms of severity and in number of cycles. By contrast, the cost of the material and manufacturing processes need to be a fraction of common aerospace engineering prices, to keep the cost of energy to an acceptable level. Similarly, lifetime- and residual strength predictions need to be accurate in order to effectively assess the blades' lifetime and maximize energy yield for a given investment.

It is well known that the average size of wind turbine blades has been increasing steadily over the years from the first day wind turbines have been installed in wind farms. This desire to build bigger wind turbines is primarily driven by the fact that as the rated power of wind turbine increases, the cost per kW generated decreases. From the eighties to now, commercially sold wind turbine power rating passed from 300 kW to 3000 kW and some companies are now building prototypes able to generate 5000kW. Since the power output is in theory proportional to the square of the radius of the rotor, blades have also increased in size substantially, leading now blades that are 60m long.

At these lengths, wind turbine blade designers and manufacturers face problems never encountered when building smaller blades because of size and weight related issues. In the last few years, some manufacturers refined their processes and materials to try and improve both the manufacturing process and structural properties of these very large blades. Classical hand lay-up is now often giving place to resin infusion methods and more care is given to tolerances of the different components of the structure. As far as materials are concerned, new fibreglass fabrics have been introduced while carbon fibres are now starting to be used in some larger blades but extensive work needs to be done to characterize the laminates that are subjected to severe loads both in static and in fatigue.

While materials and processes have been improved over the year, the blade's overall structural lay-out was kept almost identical for the last 20 years. Many believe that at the rate the wind turbine blade size is increasing, a major change in the blade's overall structure will be needed to reduce in a significant way the weight generated stresses now present in these giant blades.

Structural layout optimization
The most efficient design for a specific structural element would be one which makes the best use of the structural lay-out to bear the load, uses the most adequate materials and can be manufactured with the most optimum process. This is often referred to as the trinity of shape, material and process. It is clear that the wind turbine blade industry has worked hard on the last two aspects when improving their product but left aside the optimisation through structural lay-out (blade's internal shape). It is understandable that they did so since changing the structural lay-out often implies major changes in the manufacturing process and poses uncertainties concerning the ability of the new structure to bear the load.

Undoubtedly, product improvement through structural lay-out optimisation has much more implications than product improvement through material or process optimisation, but on the other hand it can result in much greater improvements.

Research is presently being conducted on structural lay-out optimisation of MegaWatt size wind turbine blades. Through parametric modelling of the blade's structure, designs using optimum structural lay-outs will be investigated.

To this point, a parametric loading study was performed for blades of 20 to 90m in length to see the influence of the different kind of forces acting on the blades (lift, drag, centrifugal force, weight, etc..). From this study, different reference load cases (extreme gust, fatigue, etc..) were determined to guide the new designs by identifying the major design drivers. As part of the preliminary design process, topology optimization is currently being used on blade sections subjected to different load types (small blade vs. large blade) to see the differences between optimum designs dedicated to small blades compared to large blades. Also part of the preliminary design process, new concepts of power regulation and optimization are being investigated to see if other methods than currently used full span variable pitch system could be used. In that matter, flap control is being investigated at this point in the project. Once the preliminary design is completed, a more in-depth stress analysis will be performed to compare the traditionally used structural layout to the newly discovered design.

Fatigue of wind turbine materials
Most blades are made of glass-fibre reinforced plastics, which generally have superb fatigue properties compared to metallic materials. With the advent of cost-effective carbon blade materials, an increasing number of blades and substructures are designed in these materials to keep mass - and the associated weight-related stresses - and deformations down. These latter materials claim even better fatigue properties.

However, the effects of variations in mean and amplitude of the fatigue load cycles are not completely understood yet, and current design rules lack the ability to deal with them correctly. Thus, even when following existing guidelines, lifetime predictions can be dangerously non-conservative. This means that, in terms of fatigue, a rotor blade must be over-designed to warrant the technological lifetime.

Theoretical efforts to successfully investigate spectrum fatigue life prediction methods are mainly hampered by various competing failure modes, which prevents implementation of Finite-Element type crack modelling. Semi-empirical research is therefore most prevalent for this topic. In turn, the natural scatter in composite (fatigue) properties, duration of fatigue tests, and easily exploding number of test parameters can frustrate any spectrum load fatigue investigation. Test data are generally scarce, limited to a small number of materials and test types, and rarely available for spectrum loads.

Nevertheless, the present research aims at following the semi-experimental route towards improved design guidelines for wind turbine composite fatigue. A test programme is formulated that contains constant amplitude tests at various load types (covering both tensile, compressive fatigue, and tension-compression fatigue), simple two-block spectra, and complicated spectra, representative of wind turbine fatigue.
Test results are compared to predictions using the conventional methods and modifications, and to more advanced residual strength- or stiffness degradation models. The objective is, to find design guidelines that are capable of improving lifetime prediction accuracy and thus cost-effectiveness of wind turbine rotor blades. Results are expected to be generally applicable to other applications of similar composites.