Based on the highest energy density and constant availability, ocean waves are regarded as one of promising renewable energy resources and a number of permanent magnet linear generators (PMLGs) have been designed to generate electricity out of the ocean wave energy. The PMLGs generate from the relative motion of two bodies, an armature (coil part) and a permanent magnet. A challenge in design of the PMLG-based system is to maximize the relative motion to amplify power output, and resonance can be a solution.
In this study, a comparative study is carried out between numerical simulations and experiments for a PMLG system designed by Jeju National University. As a single phase alternator system, it consists of the permanent magnetic bar attached to an outer cylinder and the shorter armature fixed to an inner cylinder. Both cylinders are freely floating and moon pool exists between the cylinders. The two-body system has natural frequencies placed in the range of target main wave energy to amplify the power production. Both motions of the floating bodies and power outputs are investigated after confirmation of numerical scheme to calculate the power output without considering the coupled motions. After motion response amplitude operators (RAO) are compared in frequency domain with generator uncoupled, time-domain motion responses and power outputs to random waves are compared with the generator coupled.
Considering environmental importance, renewable energy such as wind, solar, and wave is regarded as increasingly more important alternative energy resources. In particular, ocean waves can be a promising renewable energy sources if conversion efficiency is greatly improved. It was estimated that the ocean waves have global power potential of 1-15TW (Falnes and Løvseth 1991, Duckers 1994).
A number of PMLGs have been proposed to generate electricity out of the ocean wave energy. It typically produces electricity from the relative translational motions of an armature (coil part) and a permanent magnet that can avoid turbine-type structures, and many PMLGs have been investigated for heave direction. Parthasarathy (2012) adopted a rectangular shape PMLG for regular waves. Stelzer (2012) used random waves from wave spectrums. Prudell et al. (2010) investigated a tubular type PMLG using real ocean wave elevation data. They assumed that heave motions of the PMLG exactly follow the given wave surface instead of solving the hydrodynamic interactions. Their main focus was developing generator systems with the ocean waves decoupled. To maximize the relative velocity resonance, Park et al. (2013) used a system with two masses and three springs to induce mechanical resonance effect.