Rezultaty
The objective of the NOI COMHUB (Innovative Composite Hub for Wind Turbines) was the design and development of an innovative lightweight composite hub capable of withstanding the external wind loadings as well as the loads provoked by the wind turbine operation.
The development of hubs traditionally made of cast iron is limited by the weight as well as production quality aspects for large rotors. The introduction of composite materials and an innovative geometric design leads to a weight reduction in the order of 40-50% compared to the metal solution. The stiffness of the composite hub can be adapted in such a way that the flexibility of this component reduced peak loading on the complete rotor, including blades.
The structural design of such a composite part has involved different steps. The constraints for the composite hub design from the wind turbine and rotor blades side have been taken into account. Attention has been paid to the standardisation of hub flanges in order to ensure the adaptability to standard blades. The structural design has been developed by means of the Finite Element methods using different software codes. A major issue has been the modelling and data analysis of thick shell composite materials and the fracture analysis.
From this design analysis of the rotor hub made of glass fibres reinforced composites, the areas of maximum loading of the structure have been determined. The design has always obeyed the compromise between low weight, high structural stiffness and the maximization of material property usage. With this purpose, various load conditions in extreme static as well as fatigue loading were analysed.
The wind turbine chosen for the design and subsequent installation is a 800 kW wind turbine. The control system is with pitch regulation and variable speed generator, and the design is made according to wind class IEC II a. The Composite hub diameter is of 4 m, and the blades that are used are NOI 27.4. Later on the Comhub will be available for all common types of wind turbines.
The Composite hub was designed with a 4 m diameter and the corresponding interfaces to the three blade flanges with 1.400 mm pitch circle diameter for the pitch bearing of a pitch/variable speed wind turbine. The further interface was designed for the connection to the main shaft of the wind turbine with respect to the criteria of interchange ability of the composite hub to a traditional cast iron hub.
The production of the first composite hub has been successfully finished within the present project. The production was based on experience in-house coming from rotor blade production. The foam models, the mould and the two halves of the hub, the joint of the two halves were produced within the project. Finally, the prototype hub was finished with a PU surface coating.
The Resin Injection Mould technology that had been chosen for the prototype production can be considered to be adequate also for large series productions. Thus, the production of the first composite hub for wind turbine is an innovative step for the further development of lightweight rotor systems.
The results of the successful production of the first composite hub are of different character. The commercial success of the present R&D project will be depending on the demonstration of the functioning of the prototype that has been manufactured in the COMHUB project. In the frame of dissemination of project results, the progress of the work has been continuously presented to Fairs and Expositions, such as the DEWEK Conference in October 2002 in Wilhelmshaven (Germany) and the EWEA Conference in June 2003 in Madrid (Spain). Furthermore, the COMHUB prototype has been exposed on the HUSUM Wind-Messe in Germany in September 2003. The user potential is widespread. On one hand side, turbine manufacturers are seeking for a technical and economical solution for the weight reduction of the rotor for new turbine designs. On the other hand, wind farm operators are looking for an option in order to increase energy capture with a larger swept area using a composite hub by substituting a cast iron hub.
The benefits are therefore as follows:
- Reduction in loads on main shaft and drive train by lighter rotor with composite hub.
- Increase of energy capture with larger hub diameter while maintaining the maximum allowable loads. - Incorporation of rotor blade root into the composite hub structure leads to additional cost and weight reduction. The expected benefits are the start a demonstration project for a large wind turbine and the subsequent start of series production.
In the sense of diversification of business activities, the blade manufacturer NOI can now offer the supply of a complete rotor system, consisting of a composite blade and composite hub.
Continuous-fibre reinforced composite materials are typically used in light structures, which involve small thicknesses of the material (a few millimetres). However, in very large structural components of wind turbines such as the blade or the hub, it is necessary to achieve laminates with a very large number of plies and thicknesses of several centimetres. This fact poses several difficulties to the finite element modelling (FEM) of these structural components and, in consequence, to their design procedures. First, most of the finite element codes have a limited number of layers to be introduced in the definition of a layered element. Second, the hypothesis of a plane stress state (that exists in a thin laminate) cannot be admitted without high concern.
During the development of the COMHUB project, a methodology for the finite element modelling of thick composite materials has been introduced. This methodology involves the use of an equivalent material which properties results from the combination of the constitutive equations of each layer according to their volume fraction. By means of this equivalent material, the laminate of the composite hub (consisting on more than 300 layers) can be introduced in the finite element modelling through 5 to 10 layers of selected equivalent material.
As the application of failure criteria to each UD layer is no longer possible using this method, the following procedure has been applied to the equivalent material. First, the principal strains in each layer are calculated. Secondly, these principal strains are compared to the failure strain of the UD layer. This method is conservative as it implicitly assumes that there is a layer of the equivalent material with an orientation that coincides with the principal strain.
Moreover, in the framework of this methodology, a procedure for the analysis of the fatigue behaviour of the material has been also developed. This method takes into consideration the use of the equivalent material and the kind of load information that the software employed for wind turbine design is able to produce. The methodology permits the calculation of the load state in the hub when the 3 blades are charged by means of a phase angle shift between blades. Then, classical damage accumulation methods are used.
Dissemination: Due to the basic research nature of this result, it is foreseen to be disseminated by means of scientific publications in specialised journals. It is expected that the corresponding papers will be published during 2004.
This result can be applied in the design step of structural components of thick composite materials and, in particular, to wind turbine components. Indeed, at present, the blades are the only component of wind turbines commonly made of composite materials. However, blades are mainly unidirectional components and they are usually designed as beams. For that reason, the use of finite element modelling of beams is scarce. However, the presumable use of composite materials in other structural components of wind turbines (like the hub, the tower or the nacelle) with complex geometries and 2D or 3D stress states, will certainly require the use of numerical tools to assist the design. Specific tools for very thick composites will be required. The present result provides a methodology for that purpose which is computationally efficient.
This result can contribute to a more efficient design procedure of structural components of composite materials and enables the development of new wind turbine components in composite materials. These factors lead to a more efficient wind energy production.
The design of a fibre reinforced Epoxy Hub to 800 kW wind turbine was checked according to its conformity with the relevant regulations. For standard commercial calculation codes for composites are developed for the design of thin walled laminates, special components of the calculation program had to be added. Especially transitions between regions of different wall thickness lead to implausible results. Furthermore standard programs were not able to manage considerable thick laminate of more than 300 single layers.
On the whole the design fulfilled the requirements of the regulations taken into consideration. However a buckling analysis has not been checked yet. Therefore only a preliminary Design Approval was released.
There were a few details having elongations somewhat above the limit. Condition for a final Design Approval is some alterations of a few details of the construction and some final calculation runs of the altered parts. Fatigue loads were not critical at all.
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