Magnetic Circuit Generators for Wave Power Plants, ch. 1

1. Introduction to renewable and sustainable wave power

More than 80% of the world power production is generated by fossil fuels [1]. Meanwhile, questions have been raised about peak oil and greenhouse emissions and the demand for renewable energy is increasing. The discussions are found among top level leaders; President Obama stated, in March 2013, that “We produce more oil than we have in 15 years. We import less oil than we have in 20 years.

We’ve doubled the amount of renewable energy that we generate from sources like wind and solar” [2].

Moreover , according to the European Commission, “The EU aims to get 20% of its energy from renewable sources by 2020” [3]. In this pressing and contradictory situation, new energy sources have been investigated and the industry for renewable energy is growing. As thousands of birds are killed each year by wind power plants [4] and natural environments are destroyed by hydropower [5], one question remains: will all engineering solutions for renewable energy sources lead to a better world?

The future power should not only be generated from renewable energy sources – it should also be generated in a sustainable way. There are different definitions of sustainability; the most famous one is from the so called Brundtland Commission, 1987: “Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs” [6]. The idea of sustainability often incorporates economic awareness, environmental friendliness and social knowledge [6]. The world is in need of more sustainable systems and the goal of this project is to take a small step towards producing power from renewable energy sources in a sustainable way. This report will focus on power produced by the movements of the ocean waves.

1.1 Power from the ocean waves

The ocean waves are a renewable energy source with a high potential. The engineering challenge is to produce electric energy from the powerful movements of the ocean waves in a sustainable way [7]. The Division for Electricity at Uppsala University [8] has developed an energy conversion system for ocean waves, which is depicted in Figure 1. Sustainability has been of great importance in the work with the conversion system, for example, when choosing the material. The wave energy converter consists of a buoy, on the ocean surface, attached via a line to a linear generator. The linear generator consists of a moving part, the translator, and a stationary part, the stator. As the buoy moves with the ocean waves, power is generated in the linear generator [8].

The purpose of a generator is to generate electric energy from kinetic energy, and generators are used in almost all engineering solutions for power production. The generators can be of different types; however , all generators include a magnetic field, which can be created by using, for example, permanent magnets.

Unlike an electromagnet, a permanent magnet is permanently magnetized. One of the strongest permanent magnets is the Nd2Fe14B-magnet, which is often used in generators. Nd, neodymium, is a rare earth metal and it is mainly produced in China [7]. However , there are several environmental and health issues that could be related to the work with material.

Figure 1. Wave Energy Converter. The reference for Figure 1 is [7]. [Figure not shown]

Therefore , neodymium does not always fit in a sustainable system, and some clear examples as to why could be stated. Firstly, dust of neodymium can harm the eyes of the people who work with it [9] and its strength might cause crush injuries. Secondly, long transportations of the raw material might be included. Thirdly, the producers of neodymium have increased the price extensively. A weaker alternative to the Nd2Fe14B-magnets is the ferrite permanent magnets, called ferrites. Today, the price of Nd2Fe14B-magnets is around five times higher than the price of the ferrites [7].

Ferrites can be used to reach a sustainable energy system. They mainly consist of iron oxide [10], which can be produced in different countries, creating opportunities for new magnet developers and a more economically feasible and improved social situation. In the thesis of Boel Ekergård [7], it was shown that a linear generator for wave power production can be made of either ferrites or Nd2Fe14B, with similar power outcome. Thus , the most recently created linear generator from Uppsala University, called L12, installed in Lysekil in March 2013, was constructed entirely with ferrite permanent magnets of the type called Y40 [7]. The ferrites had to be mounted between so-called pole shoes in order to transfer the magnetic field in the desired direction and this is shown in Figure 2 [11]. There are different types of ferrites and pole shoes which can be used in a wave power plant.

Figure 2. The permanent magnets are mounted between pole shoes in the linear generator. The reference for Figure 2 is [11]. [Figure not shown]

1.2 Objective with the project is a more flexible design

This project within applied physics is part of the Wave Power Project at Uppsala University [8]. The project objective is to investigate the possibilities to create a linear generator for wave power plants with mixed types of ferrite permanent magnets, in order to increase the sustainability of the conversion system. Therefore , the goal of this report is to answer the question:

Is it 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 as previous designs?

If the answer is yes, this would lead to a more flexible design of the linear generator, which can generate a number of attractive features: lower costs, enhance the freedom of choosing environmental friendly retailers, shorten the transportation distances and provide a possibility to create magnets in-house. All of the positive effects would ultimately lead to a more sustainable wave power plant.

The magnetic circuit of the linear generator, with ferrite permanent magnets and pole shoes, will be simulated in a program called Ace with an interface called Kalk [12], which is based upon the theory explained in Section 2. The magnetic energy in the stator steel will be summed up as the ferrite permanent magnets are partly changed from a more expensive type, with higher magnetic properties, to a less expensive type, with lower magnetic properties. In order to increase the magnetic energy in the stator steel, while using mixed ferrite permanent magnets, the shape of the pole shoes is changed. Finally, the magnetic circuit will consist of a combination of differently shaped pole shoes and different types of ferrite permanent magnets.

1.3 Scope of the report and limitations

The project is restricted to the magnetic circuit in linear generators for wave power plants only. Moreover , the project is restricted to theoretical analysis and simulations. The shape, size and properties of the ferrite permanent magnets and the pole shoes of the translator are only discussed.

During the simulations, only two types of ferrites (Y30 and Y40) and two types of pole shoes (rectangular and T-shaped) are investigated. All resulting figures and values are limited in accuracy and resolution by the features of the simulation tool. The structure of the report starts with a theoretical understanding and then describes the practical work and its results. Firstly, the relevant background theory and analytical expressions will be stated and connected to the wave power converter. Secondly, a hypothesis will be presented.

Thirdly, the method used in the project will be presented by describing the simulation of mixed magnets and different pole shoes. Fourthly, the results of the project will be presented with, for example, tables and figures on the generated magnetic energy for the different investigated cases. Finally, the results will be analyzed, discussed and viewed in a greater perspective. Moreover, considerations important in industry will be made along with suggestions on future work.