"Design of a direct drive wave energy conversion system for the Seaquest® concept"
Direct Drive Wave Energy Converter
The energy of oceans is re-emerging among the renewable sources with promising development chances.
The conversion of the very low speed reciprocating motion of waves through conventional devices usually requires a pneumatic or hydraulic interface, affecting efficiency and leading to possible wear problems.
A direct drive wave energy converter, on the other hand, presents an electrical generator directly coupled to the buoy or WEC concept. A direct drive WEC system, while allowing to reach a higher efficiency, leads to a variety of new problems such as output voltage varying both in frequency and amplitude, very high torque, low power factor, among others. The direct WEC system can however be controlled with flexibility by a power electronics interface to actuate an optimal control strategy needed to extract the maximum wave power from the ocean.
This work started from a feasibility study of the innovative energy pendulum generator presented in the Seaquest Project®, made by the Spanish Company Mecánica Industrial Buelna [1]. The characterization of marine environment along with a numerical modelling and frequency-domain analysis, allowed to choose the best floating buoy shape based on the maximum energy extraction by the system [2].An arch-shaped, flux-switching, permanent magnet generator has been chosen, with an outer stator layout (with the rotor on the pendulum tip), allowing this configuration a better stator cooling. The major advantages which lead to this choice are linked to the Permanent Magnets (absence of brushes, slip rings (both linked to wear problems), excitation coils, DC power supply and field winding copper loss), and to the Flux-Switching principle (rotor only made from iron – easier construction and heat dissipation, robustness, brush-less, less volume required, giving a higher torque density).
Despite all the benefits mentioned above, the design phase is still a challenging task as an explicit mathematical expression of this novel machine is not yet available.
The time-varying (assumed sinusoidal) pendulum angular position leads to a time-dependent induced voltage frequency and magnitude which should be handled by a specific power electronics converter.
The number of rotor poles Pr, plays the role of an amplifier of both the induced voltage frequency and amplitude, so increasing it has been investigated in order to allow a better energy conversion.
Design
Different design procedures are possible, depending on the project goal; the most important one is to reduce the very high torque, directly linked to dimensions, costs and a lower efficiency.
Respecting the initial propotype proportions, the small angular opening of the buoy limits the mechanical speed to a very low value leading to a very high torque (the magnitude, for a prototype rated at 10kW, is about 105 kN·m) and the aspect ratio of air-gap radius to active length of windings allowed in the prototype leads to high copper losses. Overcoming these geometrical constraints could lead to the reduction of torque and so the dimensions of the generator.
Optimization and Performance Analysis
Although the finite-element method is widely used to analyze the performance of electrical machines, a lumped parameter magnetic circuit model has been preferred for the first design stage, as proposed by [3], based on [4] researches on FSPM machines. In addition, this circuit, implemented in a numerical simulation, allowed to make a series of parametrical analyses, studying the influence of a wide range of geometrical parameters (split ratio, magnet width, teeth and bridges proportions).
The initial design assumptions for rotor and stator parts dimensions have been refined to save material and increase output torque, made on the results obtained in [5], based on 2D and 3D finite element analyses.
After this first attempt to identify an optimum machine design, the predicted air-gap field distribution, back-EMF waveform, winding inductances, and electromagnetic torque are validated by both two-dimensional (2-D, of a generator cross-section, neglecting end effects) and three-dimensional (3-D) finite-element analyses, using the Comsol Multiphysics® software.
Different simulations have been performed, in order to firstly refine the geometry, then optimize various machine parameters and finally obtain an equivalent electrical circuit, useful to predict performances.
Conclusion
The initial results obtained are promising: despite the simplifications in the study and all the difficulties related to this particular layout, a futher study and development could lead to a cost-effective machine.
References
[1] SEAQUEST Project: Development of a Wave Energy Converter, Leading Organisation: Mecánica Industrial Buelna, Meeting Eurogia+Madrid, February 2010
[2] SEAQUEST Project: Development of a Wave Energy Converter, Preliminary Analysis
[3] Investigation of Permanent Magnet Machines for Downhole Applications, A. Chen, NTNU –Trondheim
[4] Analysis of Electromagnetic Performance of Flux-Switching Permanent-Magnet Machines by Nonlinear Adaptive Lumped Parameter Magnetic Circuit Model, Z. Q. Zhu, Y. Pang, D. Howe,University ofSheffield, S. Iwasaki, R. Deodhar, A. Pride, University of Sussex, Brighton
[5] Influence of Design Parameters on Output Torque of Flux-Switching Permanent Magnet Machines, Z.Q. Zhu, Y. Pang, J. T. Chen, Z.P. Xia, D. Howe, University of Sheffield
