Numerical Study of a Weight-Adjustable Buoy for Efficient Wave Energy Conversion

Heaving motion of ocean wave is a promising renewable energy source but challenging to capture with consistent conversion efficiencies. Main issue of the varying energy extraction efficiency of wave energy converters (WEC’s) is primarily caused by the variation of incident wave frequency and amplitude. Traditional design of the WEC has to account for the extreme sea conditions, leaving the WEC to work at suboptimal regions for most of the time. Due to the loss characteristics of the fluid power components, the performance and efficiency drop rapidly when moved away from the optimal working condition. In order to improve the efficiency of WEC, the buoy needs to operate at maximum amplitude most of the time. To do so, a new buoy structure based on actively controlled fluid-air ratio is proposed. Contrast to the traditional buoy for WEC’s, which has fixed density and weight, the proposed structure is capable of weight manipulation, resulting adjustable system natural frequency. MATLAB/Simulink simulation analysis is carried out to verify the feasibility of adjusting the gas-liquid ratio inside the buoy with a water hydraulic system. To resonate with the unknown incident wave, maximum power point tracking (MPPT) algorithm is proposed to control the buoy mass with trial steps for maximizing the resonating amplitude of the buoy. Initial simulation results have shown that the proposed system is capable of adjusting the natural frequency and the MPPT algorithm can increase the amplitude of the buoy motion.