Effect of a negative stiffness mechanism on the performance of the WEPTOS rotors

The WEPTOS is a well-known wave energy converter (www.weptos.com), formed by several rotors, with a shape that draws upon the reputable Salters Duck geometry. The WEPTOS has a large efficiency under waves of one particular frequency, i.e. when resonance conditions occur. In order to extend the range of resonance conditions, the possible use of a negative stiffness in the rotor system is analysed. This note presents some considerations on the effectiveness of negative stiffness based on physical model experiments and on the numerical simulations used to interpret the test results. More specifically, the aim of the work is to evaluate how the WEPTOS performance is affected by the presence of a spring into the system that enhances the rotor oscillations applying a negative (un-stabilizing) torque proportional to the degree of rotation. Experiments were carried out in the deepwater wave basin of Aalborg University, on a 1:30 scale model of one rotor of the WEPTOS. Three different configurations were analysed, providing different values of the negative stiffness. A set of 16 regular and 5 irregular wave conditions were tested, with maximum heights up to approximately 6 m at prototype scale, periods ranging from 5 to 9 s. The incident wave characteristics, the device rotations and the power dissipated by a dummy power converter were accurately measured. The effect of the negative stiffness was partly hindered by the presence of friction, so that no conclusive evidence could be drawn. A simple numerical model, where the rotor was interpreted as an oscillating mass-damper- spring system, was set up and calibrated to the experimental results. Reasonably good agreement between predictions and measurements were found at model scale. The numerical simulations at prototype dimensions, where friction effects have a relatively lower importance, revealed that the springs responsible of the negative stiffness increase in fact the efficiency of the device.