Knowledge Development for the Design of Offshore Wind Energy Technology
a report by Michiel B Zaaijer Assistant Professor, Wind Energy Research Group, Delft University of Technology
Offshore wind energy deployment has clearly passed the demonstration phase. However, full-scale implementation is still facing a big challenge: costs. While offshore wind energy is not competitive with other sources in the energy market, public financial support will keep setting the pace.
One way that cost challenges can be tackled is through engineering design, which is driven by knowledge. The better the workings of wind turbines in the offshore environment are understood and the better the design process is mastered, the better the design solutions will be. Both academic and industrial activities have contributed to the knowledge base that is needed to design effective offshore wind farms and continue to do so. This article considers what has been learned so far and how the industry should keep learning.
Research in Academia
Academic research serves the understanding of how wind turbines work in the offshore environment along two lines. First, natural and behavioural sciences provide foundational knowledge with general applicability. Second, academic design case studies coherently integrate different types of knowledge to provide specific information about specific situations.
Foundational and Additional Knowledge for Accuracy A great deal of the foundational knowledge relating to onshore wind energy, other offshore applications and power management can be directly reused in the offshore wind energy industry. Offshore wind energy also requires the addition some application-specific knowledge. For instance, models for offshore wind conditions are being improved. They have never needed to be very accurate for the analysis of other offshore structures, but offshore wind turbine stability, wind shear, turbulence, the effects of land-sea transitions and wake mixing are important for the assessment of loads and electricity production. Hydrodynamic loading has been researched to determine the appropriate application and accuracy of available models to new types of structures.
Offshore wind energy also leads to the introduction of new concepts and scales for which existing models may be insufficient. For example, the effect of blade torsion on aerodynamic loading becomes more pronounced for large blades. This effect is being studied using load simulation tools.
Case Studies
Case studies are a very common modus operandi for design-related research in (offshore) wind energy. In the early days of this industry, several case studies covered all aspects relating to the design of offshore wind farms. These studies generated knowledge about various conceptual solutions and contributed to providing references for typical environmental conditions, overall requirements and expectations for system properties. Besides references, these case studies have helped identify critical issues such as the availability and costs of offshore activities.
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Currently, most case studies target a new or less common concept, analysing a separate aspect of an offshore wind farm. Such an aspect can be a component or a procedure.
Design Methodological Knowledge
Design methodologies are rarely addressed in literature about the design for offshore wind energy, let alone are they the subject of the research. In general, observations about the design methodology applied in case studies can only be made indirectly through the results presented. Concept selections are typically made after a qualitative assessment of the advantages and disadvantages of a design, although a generous dose of wishful thinking cannot be denied in several studies. Preliminary design optimisation often appears to be based on manual design iterations guided by the experience of the designer. For newer concepts such as suction buckets, this manual iteration may include experimentation in the loop.
Paradoxically, most research has a monodisciplinary focus on a single component or procedure, while ‘integrated design’ is considered essential for offshore wind energy. The largest contribution of research to improvement of the design synthesis process is possibly provided through tests of the effectiveness of optimisation routines. Such routines have been performed and proven successful for the parametric and topological optimisation of individual elements, such as airofoils, blades, generators, layout and structural components.
The Expanding Role of Tool Support Integration The scope for analysis and optimisation is currently expanding. First, more disciplinary integration can be observed in component design, such as simultaneous consideration of the active and structural material in generators or the aerodynamic and structural properties of blades. Second, simulation tools are increasing the possibilities for the analysis of different concepts, physical models and levels of detail. Examples include extensions of support structure topologies, various hydrodynamic and aerodynamic models and the inclusion of detailed gearbox dynamics in a full system simulation. Third, the optimisation of complete wind turbine systems is explored. Several approaches are tested to reduce the computational burden of such wide-scope optimisation.This can be carried out by using simplified physical models or cost models for the evaluation of intermediate solutions or through the separation of system- and component-level optimisation. Despite this expansion in tool support integration, there is limited guidance in the literature on how to organise the complex design process.
Learning by Doing
Since the installation of the offshore wind turbine ‘Svante 1’ in 1990 the experience with demonstration and later full-scale offshore wind farms has provided invaluable knowledge. There have been both disappointments and positive feedback about the effectiveness of the solutions in practice.
© TOUCH BRIEFINGS 2010
Wind
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