Abstract
In this work, ferrite magnet linear generators for wave power applications are considered. These machines operate at unusually low speeds, around and even below 1 m/s, at which the optimal geometry differs from standard machines, since the copper loss and the force density become considerably more important. The focus is on translator design, and analytical two-dimensional (2D) expressions for the optimal 2D geometry are derived. Finite Element Analysis (FEA) is also applied to verify the analytical expressions and to determine effects from leakage fluxes and iron saturation. Demagnetization of ferrite magnets is also discussed and calculations are made to show the demagnetization situation for the magnets in different geometries. Finally, an example generator design is made to illustrate the findings. This generator is compared to three other generator concepts. It is concluded that ferrite magnet generators can have at least nearly the same shear stress as surface mounted neodymium magnet generators at low speed if the airgap is 3 mm or less, provided that a proper pole length is chosen, and that they can be economically competitive to neodymium magnet generators for wave power. It is also concluded that the demagnetization situation for the magnets can be severe, and that the choice of magnet grade and pole length is crucial in this respect.
Highlights
Wave power is a promising future alternative for renewable energy conversion, where the global resource has been estimated to about 2.11 TW [1] by Gunn and Stock-Williams, which corresponds to perhaps 5–15% of the world’s power demands today, depending on how large fraction of that resource that can be harvested in an economically viable way
This is in strong contrast to the corresponding standard case where surface mounted neodymium magnets are used, where performance does not have a strong dependence on the pole length. The reason for this difference is that the flux concentrating setup of the ferrite magnets gives a different magnetic circuit compared to the surface mounted neodymium case, which introduces strong dependencies on pole length in three different aspects; the flux density in the airgap that strongly affects the power density and/or the efficiency, the demagnetization situation for the magnets, which is very important since ferrite magnets are sensitive to demagnetization, and the iron losses, which depend on frequency and may become substantial if the pole length is too short. All these three aspects are very important for both the initial and operational costs of the generator per kWh, the focus of this paper is to describe the first two by deriving analytical expressions that are verified by Finite Element Analysis (FEA)
It is evident that the optimal pole length for typical slow speed ferrite magnet machines is in the range of 7–15 cm, unless the airgap is very small, and that the choice of pole length is very important for the performance of slow speed ferrite magnet generators the optimum is flat
Summary
Wave power is a promising future alternative for renewable energy conversion, where the global resource has been estimated to about 2.11 TW [1] by Gunn and Stock-Williams, which corresponds to perhaps 5–15% of the world’s power demands today, depending on how large fraction of that resource that can be harvested in an economically viable way. One of the key challenges is that wave energy is delivered with low speeds and large forces when compared to other renewable energy sources, such as wind power. Since the size and cost of the Power Take-Off (PTO) units, which convert mechanical power to electricity, and mechanical structures are related to force rather than power, this is unfavorable. This challenge is further complicated by the fact that maintenance is likely to be very expensive at sea, which may further increase the cost of wave power substantially. Direct driven linear generators for wave power are PTO solutions that in principle can be made maintenance free with a proper bearing arrangement, or at least can have long service intervals
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