Date of Award

Summer 8-22-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science in Mechanical Engineering (MSME)

Department

Mechanical Engineering

First Committee Advisor

Krish Thiagarajan Sharman

Second Committee Member

Andrew Goupee

Third Committee Member

Wilhelm "Alex" Friess

Abstract

Wave power is abundantly available near coastal areas and far offshore. The power density is typically greater in offshore regions, yet the feasibility of harnessing the available energy becomes more costly and complex as the distance from shore increases. No matter the method of converting wave energy to electrical energy, the resource itself should not be ignored on a global level. Unlike the wind energy industry, wave energy has not yet reached the level of technology that allows for economically viable full-scale wave energy plants. Due to the complexity and variability of the wave environments around the world based on geography, seasonal weather patterns, and water depth, the industry remains primarily in the demonstration and pilot deployment stage of development. For these reasons the industry has not yet converged on an optimal technology. A major concern in the wave energy converters developed thus far is that they are exposed to extreme loads in storms or elevated sea states. These loads force designers to either build very robust structures that increase material costs or employ a survival mode that allows the device to shut down during extreme sea states resulting in periods of no power production. The work presented in this paper focuses on the experimental evaluation of a novel wave energy converter developed by the National Renewable Energy Laboratory that addresses these issues. The oscillating surge wave energy converter (OSWEC) studied here is a device that utilizes adjustable geometry to control hydrodynamic coefficients thereby structural loads. The body consists of a bottom-hinged solid rectangular frame with five horizontal flaps spanning the interior of the frame. The flaps can rotate independently about their centers of rotation to alter hydrodynamic coefficients. A 1:14 scale model was built for wave tank tests in two separate facilities that offer the ability of simulating a range of wave environments and water depths where the OSWEC’s unforced response to regular waves was measured. The first facility had a narrow wave tank where the OSWEC spanned the entire width of the tank restricting water particle motion to two-dimensional planar motion, thus allowing a two-dimensional analysis. The second facility contained a large wave basin where fluid could flow freely in three-dimensions around the model. Tests which fixed the flap vertically were performed to measure the moment induced by incident waves. Flap orientation was found to significantly change the device’s natural frequency which offers a valuable layer of control for tuning the device for optimal power capture efficiency and load control in future development. The experimental non-dimensional wave excitation moments and reflection coefficients were reduced by up to 54% in the two-dimensional tests, and the wave excitation moment was reduced by up to 59% in the three-dimensional tests. A brief numerical simulation was also compared to the two-dimensional tests for unforced response and load shedding and provided agreeable results. Overall, the flaps provide mechanical means to reduce loads on the structure and foundation of the device, thus improving the design life of a WEC system and potentially allowing it to operate in more extreme sea states when other devices would be forced to shut down. The experimental results presented in this paper warrant future development of this technology and provide confidence to move forward with the design of a control system to further optimize performance.

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