Permanent Magnet Linear Generators for Marine Wave Energy Converters
Thesis or dissertation
University of Exeter
Abstract: Direct drive Permanent Magnet Linear Generators (PMLGs) are used in energy converters for energy harvesting from marine waves. Greater reliability and simplicity can be achieved for Wave Energy Converters (WECs), by using direct drive machines linked to the power take-off device, in comparison with WECs using rotational generators combined with hydraulic or mechanical interfaces to convert linear to rotational torque. However, owing to the relatively low velocities of marine waves and the desire for significant energy harvesting by each individual unit, direct drive PMLGs share large permanent magnet volumes and hence, high magnetic forces. Such forces can generate vibrations and reduce the lifetime of the bearings significantly, which is leading to an increase in maintenance costs of WECs. Additionally, a power electronics converter is required to integrate the generator‘s electrical output to meet the requirements for connection to the national grid. This thesis is concerned mainly with the fundamental investigation into the use PMLGs for direct drive WECs. Attention is focused on developing several new designs based on tubular long stator windings topologies and optimisation for flat PMLGs. The designs are simulated as air- and iron-cored machines by means of Finite Element Analysis (FEA). Furthermore, a new power electronics control system is proposed to convert the electrical output of the long stator generators. Various wave energy-harvesting technologies have been reviewed and it has been found that permanent magnet linear machines demonstrate great potential for integration in WECs. The main reason is the strong exaltation flux provided by the high number of permanent magnets. Such flux, combined with design simplicity, can deliver high induced voltage as well as structural integrity. In the thesis, a flat single and double structured iron-cored PMLG is studied and optimised. Several magnetic force mitigation techniques are investigated and an optimisation is conducted. The optimisation is concerned mainly with increasing electrical output power and reducing the magnetic forces in the generators. As a result, an optimal design introducing the idea of separated magnetic cores has been proposed. The FEA simulations reveal that magnetic separation in the yoke can increase significantly the energy-harvesting capability of PMLGs. Furthermore, the concept of the design of long stator windings for tubular PMLGs is studied. Two long stator generators having different magnetisation topologies and similar sizes to existing machine are modelled and compared to the existing machine. The similar-sized existing and proposed PMLGs are simulated by FEA. In this way, settings such as different boundary conditions, symmetry boundaries and material properties are used to gain confidence in the simulated results of the proposed machines. Moreover, the simulated results for the existing PMLG are verified against previously performed numerical simulations and practical tests delivered and published as part of other research. The outcome for the proposed PMLGs reveals several advantages for the long stator design, such as lower cogging forces and higher energy harvesting and a lower price of the raw structural materials. Additionally, the thesis proposes and simulates a new design for an air-cored PMLG. To boost the output power, the proposed design is based on a long stator topology adopting two sets of permanent magnet rings sandwiching copper windings in a tubular structure. The design is compared with a current machine in FEA and the results show significant reduction in radial forces and an increase in energy harvesting. Finally, a novel power electronics control system, bypassing inactive coils is suggested and simulated as part of the grid integration system for the long stator PMLGs. The new system achieves a reduction in the thermal losses in the power electronics switches in comparison with existing systems. The power electronics system and the generator have been simulated in Matlab coupled externally with FEA (JMAG Designer).
PRIMaRE/University of Exeter
PhD in Renewable Energy