Magnetic Circuit Generators for Wave Power Plants, ch. 6-7

6 Conclusions and discussion of the possibility to create a more flexible design of the linear generator

In this project, the magnetic circuit of a linear generator used for wave power conversion was investigated. The magnetic circuit for a segment of a linear generator was simulated in Kalk and Ace. The magnetic energy in the stator steel was calculated as the magnetic properties of the generator were changed. First, the ferrite permanent magnets in the generator were divided and different configurations of Y30 and Y40 were studied. Second, the shape of the pole shoes was changed and studied. Third, mixed ferrites and differently designed pole shoes were combined and investigated. This investigation of mixed ferrite permanent magnets and differently designed pole shoes in a linear generator has never previously been performed, to the knowledge of the author and supervisors, and could have a great impact on future research.

This study set out to determine whether it is possible to create a more flexible design of the magnetic circuit in a linear generator for wave power. A more flexibly designed linear generator would increase the sustainability of the wave power converter, as it generates a possible cost reduction, increases the opportunities to choose environmentally friendly materials, and improves the situation for the workers (see Section 1).

This study has shown that a linear generator can be made of different types of ferrite permanent magnets and pole shoes. The magnetic energy in the stator steel can decrease if parts of the permanent magnet are changed from Y40 to Y30. However , for mixed magnets, the magnetic energy in the stator steel can increase if the pole shoes are redesigned and Y40 is placed closer to the stator steel than Y30. It is possible to create a linear generator for wave power production, with different types of ferrite permanent magnets and pole shoes, but with a similar amount of magnetic energy in the stator steel.

The results were based on simulations and no real experiments were performed. However , the calculations of the simulation tool were verified in different ways, and previous studies had verified the simulation results with experiments.

In the creation of a real wave power plant , the tolerances would be of great importance and different sensitivity analyses would be needed. The losses that would be generated are not discussed here, as the investigation is to determine whether the magnetic energy in the stator steel changes in simulations in comparison with a more standard type of linear generator.

In this project, the magnetic energy in the stator steel has been increased in different ways by changing the pole shoes. However , there is an upper limit on how high the magnetic field in the stator steel should be to work at its best (see Section 2.6). It is, therefore, not always beneficial to increase the magnetic energy in the stator even further, as the stator steel is saturated at about 1.8 T.

It is concluded that the shape and size of the pole shoes affect the properties of the magnetic circuit. There is a trade-off between the cost of the ferrite permanent magnets and the cost of the pole shoes, and it could be determined in more detail whether the changes lead to lower costs for the same amount of magnetic energy in the stator steel.

7 Outlook and future work with a wave power converter with mixed types of magnets and differently shaped pole shoes

The investigations in this project have never previously been performed, to the knowledge of the author and supervisors. All findings presented in this report are therefore new and have raised many more questions. One important finding from this project is that it is possible to create a more flexible and sustainable magnetic circuit of a linear generator for wave power production. Further work needs to be done to establish whether it is feasible to create a linear generator with mixed types of magnets and differently shaped pole shoes, but this study has taken the first step towards a more sustainable wave power plant.

Additional future work could include study of the losses that could occur in a real power plant. Also, the magnetic field in other parts of the generator could be studied, such as in the small air gap between the stator and the translator. The FEM analysis could be improved by creating a finer grid; this would generate a longer run time. The electrical equivalent of the magnets could be outlined in more detail, to gain a further understanding of how the magnetic circuit is affected if the magnets are divided. These ideas have not yet been tested.

Many questions were raised about the properties of the simulation tools, Kalk and Ace. A future study could look more closely into the simulation tools used for linear generators and compare simulated results with the results from a real linear generator for wave power.

The finding from Simulation 2 (see Section 4.2) suggests further new designs of pole shoes for linear generators can be made and tested, possibly generating an even higher magnetic energy in the stator steel. In a similar way, the shape and the size of the ferrite permanent magnets could be changed. One example of future work could be to investigate what happens if the ferrite permanent magnets of different types are positioned in series relative to each other (as opposed to parallel relative to each other, as investigated in this project). With Kalk and Ace, it is possible to create a great variety of pole shoes and magnets, and many more magnetic circuits could be tested.

In a future study, a real linear generator in a wave power plant could be built with divided ferrite permanent magnets of different types, and with new types of pole shoes. Experiments would show if the simulations are valid. Research is also needed to determine what such a change in material, design etc. could cost and whether companies are interested in this type of linear generator.

Moreoverit would be interesting to learn how ferrites are made and to try to create ferrites in-house. What knowledge, equipment and materials are necessary, and what would it cost to create magnets in comparison to buying the magnets? Would the sustainability of the wave power plant improve if the ferrites were produced in-house?

As the overall goal is to increase sustainability, it is very important that research shows that the changes made reduce the costs, increase the awareness of the environment, and are socially responsible (see Section 1). Therefore ,  future work with wave power conversion combines many different research fields. In a wider view, the Wave Power Project at Uppsala University shows that it is possible to generate power from renewable energy sources with a focus on sustainability. Hopefully, this project could inspire others to work hard with sustainability as well.