Theoretical and experimental investigation of wave energy conversion by a phase-controlled heaving body

Two phase-control methods for maximising converted energy, optimum reactive control and sub-optimum latching control, are compared, and also numerically discussed for a heaving semi-submerged sphere. For a sphere of 10 m diameter and an incident sinusoidal wave of 9 s period and 0.5 m elevation amplitude, the maximum time-average absorbed power is 172 kW, 137 kW and 24 kW for the cases of reactive control, latching control and no phase control, respectively. With latching control, the need for reversing energy flow, during parts of each heave cycle, is avoided. For a more practically shaped axisymmetric body, a cylinder with hemispherical bottom, numerical results for the radiation resistance are quantitatively compared with experimental results, by application of approximate semi-empirical elementary-function formulas. Performed laboratory model experiments with such an axisymmetric body, which heave-resonates with a wave of frequency 1.1 Hz, demonstrate that, compared to no phase control, latching control results in appreciable increase of energy capture from waves of frequency 0.75 Hz and 0.5 Hz. Moreover, experiments indicate the need for developing a low-friction vertical guiding system for the heaving body.