Reducing the Structural Costs of a Wave Energy Converter Through Variable Geometry Design and Control

Our oceans contain a tremendous resource potential in the form of mechanical energy, sustainable food resources, and materials—concepts commonly grouped into the concept of a “blue economy”. With the ability to capture and convert energy carried in surface waves into usable electricity, wave energy converters (WEC) have long been an aspiration in ocean renewable energy. As of late, they are finding growing number of applications related to powering the blue economy: far offshore, wave energy converters can provide auxiliary power for ocean observation, marine aquaculture, and underwater remotely operated vehicles; nearshore, converters can vastly improve the resilience of coastal communities by serving as a primary source of electricity and powering desalination plants. Regardless of application, the commercial realization of any wave energy device ultimately hinges on its economic viability, commonly quantified by the levelized cost of energy (LCOE). The wave energy development community—university researchers, national laboratories, and industry leaders—identify structural costs as the primary driver of a WEC prototype’s LCOE. In a collaborative effort with the National Renewable Energy Laboratory, this work seeks to drastically reduce the structural costs of an oscillating surge wave energy converter, similar in concept to a bottom-hinged paddle, through the implementation of variable geometry “flaps” which open and close to alter the frontal surface of the paddle. Analogous to the blade-pitching capabilities of wind turbine blades, the rotating flaps enable load-shedding through reduction of wave excitation forces and moments. In combination with the device’s power take-off control system, the active control of the variable geometry flaps allows for the tuning of load profiles in response to sea state.

This work features two design iterations of the variable geometry oscillating surge wave energy converter. The first of these, the OSWEC, is a variable geometry paddle hinged at the seafloor. We characterize the performance of this device through a combination of wave basin experiments and hydrodynamic simulations. Next, we present the ongoing development of its successor, the VGOSWEC, which is raised from the seabed via a fixed platform to access the higher wave energy resource located in deeper waters. Hydrodynamic optimization, structural design, power take-off design, and adaptive control strategies are discussed, along with plans to validate the second-generation design through scaled wave tank experiments during the Fall of 2020. Through the continued exploration of variable geometry wave energy device design, as demonstrated here on an oscillating surge converter, we seek to carve an LCOE reduction pathway based on load shedding and active load profile control. These efforts, together with concurrent research into additional LCOE reduction pathways, will elevate wave energy conversion technology to better serve the renewable energy transformation and blue economy of the not-so-distant future.