A combined concept consisting of a torus-type oscillating water column (OWC) device and an offshore wind turbine is proposed in this study for the multi-purpose utilization of offshore renewable energy resources. The wind turbine is supported by a monopile foundation, and the OWC is coaxial with the foundation. The OWC is of torus shape, and partly submerged with its bottom open to the sea. An air duct, which houses an air turbine, is installed on the roof of the chamber. The exterior shell of the OWC is connected rigidly to the monopile by four thin rigid stiffening plates. Correspondingly, the whole chamber of the OWC is divided into four fan-shaped sub-chambers by the plates. A numerical model is then developed to simulate the wave interactions with the system as well as the air-fluid interactions within the chamber by establishing an extended boundary integral equation and using a higher order boundary element method. In addition, the optimal pneumatic damping coefficient, which is expressed in terms of radiation susceptance and radiation conductance, is determined by solving a pressure-dependent wave radiation problem. Based on the developed model, a detailed numerical analysis is conducted, and the hydrodynamic characteristics related to the combined concept are explored.
The ocean is vast and powerful, enabling marine renewable energy potentially be a significant energy supply. Due to the high-power density and longtime availability, considerable efforts and advances have been made in exploiting the marine renewable energy. A variety of wave energy converters has been invented to harvest the wave energy. In the meantime, many offshore wind energy converters have been used to harvest the available enormous wind energy resources.
Among different classes of designs, the oscillating water column (OWC) device has been widely regarded as one of the most promising options (Falcão, 2010). A typical OWC device mainly consists of two key components: a collector chamber with an underwater bottom open to the sea and a power take-off (PTO) system, mostly an air turbine, on the roof of the chamber (Heath, 2012). The incident waves excite the water column inside the chamber to oscillate, and transfer energy to the air above the water column. The pneumatic power can then be converted into electricity when the air flows through the air turbine coupled with an electric generator. Due to the nature of simplicity, the OWC device can be flexibly adapted to the shoreline, nearshore and offshore through different forms.