Analysis and Dynamic Scaling of Tethered Wave Energy Converters in Irregular Waves Available to Purchase

Wave energy converters (WECs) are a technically and economically promising option for renewable electricity generation. This paper investigates the hydrodynamic characteristics of a tethered cylindrical wave energy absorber using analytical methods and derives the scaling relations for laboratory testing. The effects of the cylinder geometry, mooring system, and mass distribution on the idealized power takeoff and the pitch motions of a tethered point wave energy absorber in irregular seas are summarized. Analytical solutions for the hydrodynamic coefficients and wave forcing are based on potential flow formulations and eigenfunction expansions. The results show that a relatively light mooring system has little effect on the power takeoff, but introduces a low-frequency coupled pitch-surge resonance that can cause system failure in long period swells. While analytical solutions provide first-order estimates of the system response, laboratory experiments are required to evaluate the nonlinear, coupled system response. In order to design and interpret such experiments, appropriate scaling relationships are determined and validated using numerical simulations. The added mass, radiation damping, wave radiation and diffraction excitation forces, and mooring system mass and stiffness are found to be self-consistent using geometric and Froude number similarity. The effects of incomplete geometric similarity with a shallow wave tank and viscous forces are also discussed.