ABSTRACT

This study investigates the effect of design changes on the hydrodynamics of a novel oscillating surge wave energy converter being developed at the National Renewable Energy Laboratory. The design utilizes controllable geometry features to shed structural loads while maintaining a rated power over a greater number of sea states. The second-generation design will seek to provide a more refined control of performance because the first-generation design demonstrated performance reductions considered too large for smooth power output. Performance is evaluated using frequency domain analysis with consideration of a nonideal power-take-off system, with respect to power absorption, foundation loads, and power-take-off torque.

INTRODUCTION

The field of wave energy conversion is at an exciting point in development where many different kinds of technologies are being investigated, but the cost of energy from these wave energy converters (WECs) is too high to compete with other sources of energy generation. For wave energy to become competitive, WECs will need to adapt their performance so that maximum power is absorbed for all normal sea states, and so that loads can be shed in extreme conditions to reduce the structural and material costs for converters (Musial et al., 2013).

Recent work at the National Renewable Energy Laboratory (NREL) is focused on lowering the cost of energy of these devices by developing an oscillating surge wave energy converter (OSWEC) with controllable geometry. The novel OSWEC design utilizes the controllable geometry features to shed structural loads while maintaining a rated power over a greater number of sea states. This approach was first demonstrated with the first-generation design (Gen 1) that consists of a body comprised of four equal-size horizontal flaps spanning the length of the converter, see Fig. 1. The flaps are allowed to rotate, thereby altering the converter geometry and its hydrodynamic properties. The Gen 1 design demonstrated the ability to adapt its performance using the controllable geometry, but changes to the hydrodynamic properties with each open flap were considered too large for smooth power production in greater sea states (Tom et al., 2016a, 2016b). The second-generation (Gen 2) design will seek to address these large jumps and allow for finer control of performance using smaller controllable geometry sections.

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