Numerical and Experimental Modeling of Direct-Drive Wave Energy Extraction Devices

The solutions to today’s energy challenges need to be explored through alternative, renewable and clean energy sources to enable a diverse national energy resource plan. An extremely abundant and promising source of energy exists in the world’s oceans in the forms of wave, tidal, marine current, thermal (temperature gradient) and salinity. Among these forms, significant opportunities and benefits have been identified in the area of wave energy extraction. Waves have several advantages over other forms of renewable energy such as wind and solar, in that the waves are more available (seasonal, but more constant) and more predictable, thus enabling more straightforward and reliable integration into the electric utility grid. Wave energy also offers higher energy densities, enabling devices to extract more power from a smaller volume at consequent lower costs. However, many engineering challenges need to be overcome to ensure wave energy device survivability, reliability and maintainability, in addition to efficient and high quality power take-off systems. Optimizing wave energy technologies requires a multi-disciplinary team from areas such as Electrical, Chemical, Ocean, Civil and Mechanical Engineering, to enable innovative systems-level research and development. This paper presents some recent research developments on experimental and numerical modeling on direct-drive approaches and the associated devices designed to convert the motion of the ocean waves into electrical energy using point absorber wave energy converters. This research is focused on a simplification of processes, i.e., replacing systems using intermediate hydraulics or pneumatics with direct-drive approaches to allow generators to respond directly to the movement of the ocean by employing magnetic fields for contact-less mechanical energy transmission, and power electronics for efficient electrical energy extraction. The term “direct” drive describes the direct coupling of the buoy’s velocity and force to the generator without the use of hydraulic fluid or air. The wave energy buoy and spar are designed to efficiently capture ocean wave energy and transfer it to the generator. These buoys have been tested at the Oregon State University O.H. Hinsdale Wave Research Laboratory, with planned testing off the coast of Oregon. The paper will examine several direct-drive approaches, including electrical and mechanical design characteristics, describe the numerical modeling of the associated conceptual devices, prototype testing, and some ongoing research on the dynamics of buoy generator systems for design optimization.