A sensitivity study on the effect of mass distribution of a single-tether spherical point absorber

Abstract

In previous work, a single-tether submerged spherical point absorber with asymmetric mass distribution(SPAMD) was proposed to enable harvesting wave energy induced by the surge and heave motion of thebuoy. By taking advantage of the motion coupling arising from the asymmetric mass distribution of thebuoy, the SPAMD was found to be up to 3 times more efficient than a generic single-tether point absorber(with uniform mass distribution buoy) under regular waves. For motion-coupled systems like theSPAMD, the mass distribution is a significant factor that governs the kinematics and the efficiency of thedevice. Therefore, in this paper, a 3DOF (surge, heave and pitch) spectral-domain model consideringviscous drag was developed, to investigate the sensitivity of the mass distribution on the power output ofthe SPAMD in irregular waves. The aim of the sensitivity study is to provide a guideline for the waveenergy industry when designing such devices. At the end of this paper, the yearly mean power output ofthe SPAMD with optimal mass distribution was assessed at three test sites near Yeu Island (France), Perth(Australia) and Sydney (Australia). It was found that in such sea sites, the SPAMD was at least 1.5 timesmore efficient than the generic single-tether point absorber, indicating that the SPAMD might have asignificant commercial potential. Furthermore, for the three sites investigated, the performance of theSPAMD was found to be relatively tolerant to non-optimal PTO configuration, implying that sea-statespecific tuning is unnecessary.