An experimental evaluation of the effects of relative draft and centre of gravity position on wave energy converter performance

Prior studies highlight that the Backward Bent Duct Buoy (BBDB) is one of the simplest and most robust wave energy converters (WECs). However, the impact of the relative draft and the location of the centre of gravity (COG) on the hydrodynamic performance of the BBDB remains inconsistent and underexplored in the field. Thus, this study investigates the optimal relative draft and centre of gravity (COG) location for maximizing the Backward Bent Duct Buoy (BBDB) hydrodynamic performance, a simple and robust wave energy converter. The research explores the influence of different drafts and COG locations across wave heights (0.03m, 0.04m, 0.05m) and periods (Is to 2s) using the OMEY's flume wave tank. A Froude scale of 1:13 was applied to the BBDB model. Verification and validation with previous studies yielded a percentage difference of 0.22%. Results show that a draft of T=0.3m (T/D=O.53) with an upright COG achieves the highest conversion efficiency of 56.7% at a wave height of 0.05m, outperforming other configurations. A lower efficiency of 16.1% was noted for the Port COG at H=O.03m. The T=O.3m draft with the exact COG maximizes heave and pitch oscillations, enhancing internal air pressure variations and overall energy conversion. Appropriate COG positioning and draft are key to achieving resonance, reducing drag, and improving performance. This study emphasizes the significance of draft and COG location in optimizing BBDB design and operation, particularly in low wave height conditions.