Abstract
The power generation of a wave energy converter (WEC) can be improved if the power take-off (PTO) generator is operated with a negative stiffness. The addition of the negative stiffness can lower the overall stiffness of a WEC thereby enabling a small WEC to be tuned to resonate with the low frequency ocean waves. By arranging mechanical or pneumatic springs into a special inverted-Y angled arrangement negative stiffness can be created and this approach is used by CorPower. However, magnets can also be arranged to create negative stiffness and while the energy density of a magnetic spring is lower the design is simpler and the magnet springs non-contact operating capability enables it to have high reliability and a long service life. If over-torque a magnetic spring can pole-slip rather than catastrophically failing.
Most fixed stiffness and variable stiffness magnetic torsion springs published to-date have a non-linear stiffness characteristic, this makes controlling them difficult. This presentation will provide a summary of recently funded DOE research into a new class of adjustable stiffness magnetic torsion spring, such as shown in Fig. 1 that can create a linear stroke length with a +/- 45 degrees. An example of the torque plot as a function of stroke length for the radial magnetic spring is shown in Fig. 2(a). This presentation will present the design of a new class of helical magnetic springs for WECs that can provide a stroke length of up to +/-150 degrees, an example plot of the torque vs. stroke length for a helical magnetic spring is shown in Fig. 2(b). This larger stroke length design is more suitable for WECs.