Heave-enhanced, linear-sliding wave energy converter with end springs and controlled damping

Abstract

Our recent work on Wave Energy Converters (WEC) introduced the ideas of a Linear Sliding WEC (LS-WEC) and an electromagnetic spring, or eSpring, to dynamically control system forces and maintain resonance with the prevailing wave frequency. In that work, we assumed that the LS-WEC contained a sliding translator mass that moved on a linear track in response to buoy pitch and roll in a dynamic sea and that it was coupled to a spinning generator to produce power. In early work, we assumed that the LS-WEC was rigidly attached to a circular floating buoy and that the combined “LS-WEC-plus-buoy” system moved as a “wave rider” exactly with the local wave slope. More recently, we developed a new idea to increase power by exploiting wave heave motions to increase LS-WEC tilt angles. We showed the potential of achieving an order of magnitude increase in harvested energy under certain, but common, wave conditions. One consequence of larger tilt angles is that the translator mass could produce large pounding forces at the ends of the rail. In this paper, we replace hard end stops with mechanical coil springs and define two new quantities to characterize the pounding. One quantity is called the fractional Pounding Time (PTfr), which is the fraction of time when poundings occur. The other is called the average Pounding Force (PFavg), which is the integral of absolute pounding forces during pounding events. Our investigation of the pounding dynamics has concluded that allowing some pounding could increase the energy harvested by a significant amount, especially in random seas. In one example, we show a 5.9w increase in power, which is a 40% increase (from 14.7 to 20.6w). To prevent structural damage or fatigue, the pounding force magnitude and duration, as represented by the PTfr and PFavg, should be limited. Using these two measurable quantities, optimal damping control can be established for different LS-WEC designs.