Influence of power take-off on flow field around oscillating water column wave energy converters: An experimental study based on particle image velocimetry

In oscillating water column (OWC) wave energy converters, the power take-off (PTO) system critically influences energy conversion. Although PTO configurations have been extensively optimized for energy conversion efficiency, their effects on water-column oscillation dynamics are not fully understood. In this study, particle image velocimetry (PIV) experiments were conducted under various regular wave conditions to investigate the effects of PTO damping on the OWC flow field. The results indicate that PTO damping affects the passage of wave energy entering and exiting the OWC chamber asymmetrically. Under high damping, the vicinity of the front wall is the primary pathway for both inflow and outflow. With reduced damping, the wave energy traverses through the whole entrance height of the OWC chamber more readily, and meanwhile, vortices near the front wall are markedly enhanced. These vortices reshape the flow structure by leading to different degrees of obstruction or acceleration of local flows, and produce pronounced differences between the inflow and outflow processes. It is imperative to recognise that excessive PTO damping impedes wave energy from entering the chamber, while insufficient PTO damping leads to inadequate energy conversion. Consequently, a pivotal challenge resides in the simultaneous admission of more wave energy into the OWC and the assurance of its efficient conversion. Based on the flow field investigation, a potential PTO control scheme can be proposed: applying low damping during the inflow process to admit more wave energy, then switching to higher damping during the outflow process to convert more of the oscillation into pneumatic power. This study provides new flow-field insights into the energy conversion mechanism of OWC systems and suggests potential pathways for improving their energy conversion efficiency.