Performance of an Attenuator Type Wave Energy Converter in Multi-directional Waves

Ocean waves are irregular, nonlinear and directionally spread (short-crested). The irregularity of waves can be considered by superposing components at different frequencies. In many frequency-domain analyses, second-order contributions can be calculated to take account of the nonlinearity in wave-structure interactions. The directional spreading of the waves is usually described by introducing a directional spectrum. Several directional spectra have been suggested by researchers. In short-crested waves, the wave energy propagates in different directions around principal wave directions, which will affect the performance of Wave Energy Converters (WEC). Especially, less energy may be harnessed for some directionally sensitive WEC.
In recent work, we have examined the performance of an attenuator type WEC (Fig.1) by calculating the relative rotations between floating modules in uni-directional and multi-directional waves. The emphases have been put on operational sea states and only linear incoming waves have been considered. In this presentation, the description of multi-directional waves in a numerical model will be introduced briefly. Then the motions of an attenuator type WEC and shear forces acting on power take-off system (PTO) will be investigated. Different wave spreadings have been considered. The results of relative pitch motions have shown the significance of wave directional spreading on performance of attenuator type WECs, in particular when wave spreading factor is small. Larger relative pitch motions are obtained in uni-directional waves. Similar to the relative pitch motions, vertical shear forces acting on hinges for small wave spreading factor have been reduced by up to 26% of those in uni-directional waves. However with the increase of spreading factor, wave directional spreading has less effect on vertical shear forces acting on ideal hinges.