Experimental investigation of breakwater-type WEC composed of both stationary and floating pontoons

Highlights: • Performance of the breakwater-type WEC composed of both stationary and floating pontoon is investigated experimentally. • Comparing with the single pontoon system, the energy conversion efficiency can be improved for the proposed system. • For the tested cases, the system with the smaller draft ratio gives a better energy conversion performance. Improving the energy conversion performance, especially in shorter waves, may enhance the competitiveness of wave energy converters (WECs) deployed at sea with medium wave conditions. In this paper, a novel system consisting of a front oscillating buoy type WEC and a rear fixed pontoon is proposed to improve the energy conversion performance of the original single pontoon breakwater -type WEC system by Ning et al. (2016). For the purpose of comparison, the experiments for both single pontoon system and two-pontoon system are conducted. Note that, to avoid the significant increase of the construction cost, the total pontoon volume (i.e., the displacement of the pontoon) of the proposed system remains the same with that of the single pontoon system. The two-pontoon system with different draft ratios d₁/d₂ (where d₁ and d₂ are the draft of the front pontoon and rear pontoon) are considered. The experiment shows that the system with smaller draft ratio gives more excellent energy conversion performance. The current controller-magnetic powder brake system is selected and used as the power take-off (PTO) system. The characteristics of the PTO system are investigated, which shows that the approximate Coulomb damping force can be simulated very well. The results also reveal that the proposed breakwater-type WEC with proper configuration improves the capture width ratio (CWR) effectively without compromising the coastal protection performance (i.e., the transmission coefficient K_T K_T η > 20%) can be broadened by improving the CWR in shorter waves.