A study of the effects of increasing the degrees of freedom of a point-absorber wave energy converter on its harvesting performance

In this paper, the effects of increasing the number of degrees of freedom through adding submerged bodies to a point absorber wave energy converter (WEC) were studied in an attempt to capture more power at a lower resonant frequency while keeping the same total mass and volume of the system, which is similar to those of the piezoelectric vibration energy harvesters. Novel multiple degrees of freedom wave energy harvesters were proposed, starting from a typical two-body point absorber and increasing the number of degrees of freedom through adding submerged bodies from 2 degrees of freedom (DOF) to 5 DOF while keeping the same total mass and volume of the system. Both spherical and cylindrical submerged bodies were studied. The dynamic model of a multiple degree of freedom wave energy converter system was formulated with linearized viscous damping drag force effect. The hydrodynamic properties were simulated using the commercial software ANSYS AQWA. It was found that increasing the number of degrees of freedom through adding spherical submerged bodies increases the average captured power by 26% for the WECs going from 2 DOF to 4 DOF and decreases the resonant frequency by 19% going from 2 DOF to 5 DOF. These results are important for point absorbers, as they translate to capturing power more efficiently at a lower resonant frequency, which is closer to the realistic ocean wave excitation frequencies. As for the wave energy converters with cylindrical submerged bodies, even though the resonant frequency reduces with the increase of the number of degrees of freedom, the power massively decreases as well because of the dominant effect of the viscous damping drag force on the system's dynamics. (C) 2019 Elsevier Ltd. All rights reserved.