Tuning Wave Energy Converters to local wave conditions

Abstract

A Wave Energy Converter (WEC) cannot operate efficiently if left alone floating to the mercy of the random sea surface. Moreover, high waves may impose destructive forces while contributing nil energy production in return. For a WEC to be efficient, its natural frequency should be close to the peak frequency of a wave spectrum. At the resonance or another lower natural frequency, dynamic responses are more regular, and thus, the power take-off system (PTO) can operate in a more efficient manner. This paper introduces a general systematic methodology for tuning the natural frequency of a resonant WEC for the desired degree of freedom and converting dynamic responses into power. A barge is used as a case study to tune its roll and heave natural period, which in general are much lower than the peak period of swells. The roll natural period is increased by hanging subsea structures to the sides of the vessel by means of mooring lines, see Fig. 1. The size of the structures and their entrapped water increase the roll mass moment of inertia to achieve the desired period. For the heave response, the natural period raises by adding skirts to the sides of the vessel, increasing the entrapped water. The resonant motions are significantly suppressed by means of the drag force acting on the subsea structures. Thus, the extreme drag forces acting on the structures act as a damper to the vessel response. These forces are transferred to the mooring lines, which can be converted into a rotary motion of a shaft of the PTO. Our application focuses on the Galapagos Islands, where narrow, low-frequency southerly swells with a typical 13 s peak period dominate the wave climate. This study allows for a systematic design of resonant-type WECs, which is necessary for the development of technical and economic feasibility studies of various novel and existing concepts.

Figure 1: Schematic representation of resonant WEC with tuned roll natural period