Hydrodynamic performance of an elastic capsule oscillating water column type WEC: An experimental study

Abstract

Wave energy represents a promising and sustainable source of energy with substantial potential. To promote the utilization of wave energy resources as well as develop new wave energy converters (WECs) with application prospects, an innovative elastic capsule oscillating water column (EC-OWC) WEC is proposed, and an initial experimental investigation is conducted on the device. Besides, the expansion characteristics of the elastic capsule and the motion characteristics of the free bulge wave are investigated through bulge wave tests. Moreover, the hydrodynamic performances of the device on the condition that coupled with traveling regular waves are investigated through wave tests, as well as the damping characteristics of the impulse turbine model and the ring-opening as power take-off (PTO) systems in the EC-OWC. It indicates that the variation in the speed of the free bulge wave is consistent with the one-dimensional longitudinal wave theory, that the ring-opening model with an opening ratio of 0.76 percent has comparable operating characteristics to the impulse turbine, and that the impulse turbine model has outstanding operating characteristics in the EC-OWC system. The maximum primary hydrodynamic efficiency of the system is approximately 0.475. When the relative water depth is in the interval of approximately 0.07-0.09, the efficiency can reach 0.4. Furthermore, the efficiency increases and subsequently decreases with the variation of the relative hydraulic head, and the efficiency reaches the maximum value when the relative tube length is around 1.6. The results indicate that the one-dimensional longitudinal wave theory can provide support for the design of the elastic capsule. When the capsule length is close to half a wavelength and the internal and external oscillation periods are matched, the system reaches a near-resonant state, and the level of efficiency is optimized. The bulge wave contained within the EC-OWC is a half-wavelength standing wave.