Wave power absorption by an oscillating water column (OWC) device of annular cross-section in a combined wind-wave energy system

This paper deals with a new combined concept consisting of an oscillating water column (OWC) device and an offshore wind turbine for the multi-purpose utilization of offshore renewable energy resources. The wind turbine is supported by a monopile foundation, and the attached OWC is coaxial with the foundation. Within the chamber, the exterior shell of the OWC and the monopile foundation are connected by four vertical stiffening plates. Correspondingly, the whole chamber is divided into four equivalent fan-shaped sub-chambers. A higher order boundary element method is then adopted to model the wave interaction with the combined system. Numerical models based on two different approaches, namely 'Direct' and 'Indirect', are both developed in this study. In addition, a self-adaptive Gauss integration method is developed to treat the nearly singular integration that occurs when the field and source points are very close to each other. A detailed numerical analysis is then conducted for the case of an OWC integrated into a NREL 5 MW wind turbine in both regular and irregular sea states. Numerical results illustrate that a significant energy extraction efficiency is attained when remarkable piston-like fluid motion is induced within each sub-chamber, and the wave power absorption by the OWC is not restricted by wave direction. The air compressibility makes a negative effect on the wave power absorption especially when the wave frequency is less than the resonance frequency of the piston-mode motion of the fluid in the chamber. In addition, the wave forces on the OWC and the monopile can balance each other at specific wave conditions, leading to a nearly zero net wave force on the whole system. The results also illustrate that by using an optimal turbine parameter, the wave power production by the OWC can be an important supplement to the combined system in operational sea states.