Broad-banded wave power absorption with submerged balloons

It is well-known that to optimally absorb energy from oceanwaves, a wave energy device needs to oscillate with optimum amplitude and phase. A wave energy device in a real sea needs to operate as close as possible to this optimum condition fora range of wave periods, wherein lies the challenge. This challenge is particularly pertinent for point absorbers, which, by definition, are much smaller than the incident wavelengths. When oscillating in the conventional rigid-body heave, they are inherently narrow-banded. In this paper, we show that point absorbers in the form of submerged air-filled balloons could have a surprisingly broad wave absorption bandwidth. These devices oscillate not in the conventional rigid-body heave, but in a pulsating mode, whose natural period of oscillations is determined not by the submerged volume of the oscillating body, but by the volume of air provided in the system, which acts as an air spring.