VanZwieten, James

Marine and hydrokinetic (MHK) energy systems, including marine current turbines and wave energy converters, could contribute significantly to reducing reliance on fossil fuels and improving energy security while accelerating progress in the blue economy. However, technologies to capture them are nascent in development due to several technical and economic challenges. For example, for capturing ocean flows, the fluid velocity is low but density is high, resulting in early boundary layer separation and high torque. This dissertation addresses critical challenges in modeling, optimization, and control co-design of MHK energy systems, with specific case studies of a variable buoyancy-controlled marine current turbine (MCT). Specifically, this dissertation presents (a) comprehensive dynamic modeling of the MCT, where data recorded by an acoustic Doppler current profiler will be used as the real ocean environment; (b) vertical path planning of the MCT, where the problem is formulated as a novel spatial-temporal optimization problem to maximize the total harvested power of the system in an uncertain oceanic environment; (c) control co-design of the MCT, where the physical device geometry and turbine path control are optimized simultaneously. In a nutshell, the contributions are summarized as follows:

This research consists of the numerical model development and simulation of two prototype Wave Energy Convertor designs (WECs) across three simulation types. The first design is an oscillating body WEC called the Platypus designed to capture wave energy as three paddle arms actuate over the surface of the waves. The second design is an overtopping type WEC called the ROOWaC which captures and drains entrained water to generate power. Modeling of these systems was conducted using two techniques: the Morison load approach implemented using hydrodynamic response coefficients used to model the Platypus and a boundary element method (BEM) frequency-domain approach to model both WEC designs in the time domain. The BEM models included the development of hydrodynamic response coefficients using a discretized panel mesh of the system for calculation of added mass, excitation, and radiation forces. These three model families provided both performance predictions and power output information to WEC developers that supply important data for future full-scale designs. These models were used to predict power generation estimates for both WECs as follows: the Platypus WEC was predicted to have a maximum efficiency range between 14.5-35% and the ROOWaC WEC was predicted to generate a maximum peak average power of 19 W upon preliminary results.

Marine Hydrokinetic (MHK) energy is an alternative to address the demand for cleaner energy sources. This study advanced numerical modeling tools and uses these to evaluate the performance of both a Tidal Turbine (TT) and an Ocean Current Turbine (OCT) operating in a variety of conditions. Inflow models are derived with current speeds ranging from 1.5 to 3 m/s and Turbulence Intensities (TI) of 5-15% and integrated into a TT simulation. An OCT simulation representing a commercial scale 20 m diameter turbine moored to the seafloor via underwater cable is enhanced with the capability to ingest Acoustic Doppler Current Profiler (ADCP) data and simulate fault conditions. ADCP measurements collected off the coast of Ft. Lauderdale during Hurricanes Irma and Maria were post-processed and used to characterize the OCT performance. In addition, a set of common faults were integrated into the OCT model to assess the system response in fault-induced scenarios.