Enhancing the relative capture width of submerged point absorbing wave energy converters

Abstract

Point absorbing wave energy converters account for 53% of the existing wave energy converter prototype designs. Generally, point absorbers are designed to operate on or just below the water surface, extracting wave power from the heaving motion. In recent years, an increasing amount of attention has been given to fully submerged point absorbers that demonstrate better survivability under storm conditions and capability of extracting wave power from motion in multiple degrees of freedom. This paper investigates three submerged point absorber designs operating in three degrees of freedom: a generic axisymmetric spherical buoy with a single tether power-take-off; and two modified types, one employing an asymmetric mass distribution buoy and the other employing a three tether power- take-off arrangement. Simulations in the frequency domain were used to study the behaviour of the three point absorber designs from the perspectives of dynamic response, power absorption principles and capabilities, and power-take-off requirements. Compared to the generic single tether spherical buoy design, both modified submerged point absorber designs demonstrate considerable improvements in their performance indices (e.g. the relative capture width and the power to PTO force ratio), while exhibit additional challenges in their implementations.