Performance Predictions and Validation of a Two Coaxial-Cylinder System as a Wave-Energy Extractor

Experiments were conducted to investigate the performance of two newly designed coaxial cylinders as a wave-energy extractor system in regular waves. The coaxial-cylinder design as a point-absorber consists of a tension-tethered vertical inner cylinder and a heaving outer or toroidal cylinder moving in the vertical direction. The relative heave motion between the two cylinders is used to convert the wave-induced response to electrical energy. The first-order heave response of the outer cylinder is used as the mathematical model in the frequency domain and the predicted results are compared with experimental measurements taken in a wave basin. The analytical solutions for the hydrodynamic added mass, damping, and wave-exciting force of the heaving floater are obtained from Chau and Yeung (2012, OMAE2012-# 83987). Experimentally determined hydrodynamic coefficients from free-decay tests at the resonance frequency are obtained to account for the effects of viscosity. Experimental results first reported include the wave-exciting force on the outer cylinder and its free response induced by the incident waves. The permanent magnet linear generator (PMLG) developed in Tom and Yeung (2012, OMAE2012-# 83736) is next installed as a passive power take-off (PTO) system. The electrical power output from the linear generator is measured with the resistance load as a parameter. The measured performances of the coaxial cylinders with and without the PTO are compared with the theoretical predictions. Excellent agreement is found, confirming the effectiveness of guiding theoretical model and of the engineering design.