Numerical simulations of the power supply to a tidal compensation system for wave energy converters

Abstract: A wave energy converter (WEC) has been developed at the Division of Electricity at Uppsala University. The Uppsala WEC is a point absorber consisting of a buoy on the surface and a linear generator situated on the sea floor. The current WEC is optimised for the wave climate at a specific location, this is however problematic in areas with large tidal variations. A compensation system adjusting the cable length according to the sea level is under development. This system is powered by rechargeable batteries which require an independent power source. One way to do this is to install a small buoy inside the main buoy which utilises the relative height between the sea level and the main buoy elevation. The small buoy can then be connected to a generator. This project concerns the design, numerical simulation and small scale experiments of a generator for this purpose. The generator function has been modelled using the finite element software and show good agreement with the experimental tests done on a simplified version. The proposed design is a linear 3-phase permanent magnet generator. Ignoring losses other than resistive losses, it converts 86\% of the absorbed energy from the waves to electricity. Optimisation studies showed that the magnet and coil widths has the largest influence on the induced voltage and efficiency. There are several sources of uncertainty including other loss mechanisms, the coil inductance and the damping which depends on the exact specifications of the electrical circuit charging the batteries.