Ocean wave power

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.

This research focused on maximizing the power generated by an array of ocean
current turbines. To achieve this objective, the produced shaft power of an ocean current
turbine (OCT) has been quantified using CFD without adding a duct, as well as over a
range of duct geometries. For an upstream duct, having a diameter 1.6 times the rotor
diameter, the power increased by 8.35% for a duct that extends 1 diameter upstream.
This research also focused on turbine array optimization, providing a
mathematical basis for calculating the water velocity within an array of OCTs. After
developing this wake model, it was validated using experimental data. As the
downstream distance behind the turbine increases, the analytic results become closer to
the experimental results, with a difference of 3% for TI = 3% and difference of 4% for TI
= 15%, both at a downstream distance of 4 rotor diameters.

Several modifications have been implemented to numerical simulation codes based on
blade element momentum theory (BEMT), for application to the design of ocean current
turbine (OCT) blades. The modifications were applied in terms of section modulus and
include adjustments due to core inclusion, buoyancy, and added mass. Hydrodynamic loads
and mode shapes were calculated using the modified BEMT based analysis tools. A 3D
model of the blade was developed using SolidWorks. The model was integrated with
ANSYS and several loading scenarios, calculated from the modified simulation tools, were
applied. A complete stress and failure analysis was then performed. Additionally, the
rainflow counting method was used on ocean current velocity data to determine the loading
histogram for fatigue analysis. A constant life diagram and cumulative fatigue damage
model were used to predict the OCT blade life. Due to a critical area of fatigue failure being
found in the blade adhesive joint, a statistical analysis was performed on experimental
adhesive joint data.

The research presented in this thesis utilizes Blade Element Momentum (BEM) theory with a
dynamic wake model to customize the OrcaFlex numeric simulation platform in order to allow
modeling of moored Ocean Current Turbines (OCTs). This work merges the advanced cable modeling
tools available within OrcaFlex with well documented BEM rotor modeling approach creating a
combined tool that was not previously available for predicting the performance of moored ocean
current turbines. This tool allows ocean current turbine developers to predict and optimize the
performance of their devices and mooring systems before deploying these systems at sea. The BEM
rotor model was written in C++ to create a back-end tool that is fed continuously updated data on the
OCT’s orientation and velocities as the simulation is running. The custom designed code was written
specifically so that it could operate within the OrcaFlex environment. An approach for numerically
modeling the entire OCT system is presented, which accounts for the additional degree of freedom
(rotor rotational velocity) that is not accounted for in the OrcaFlex equations of motion. The properties
of the numerically modeled OCT were then set to match those of a previously numerically modeled
Southeast National Marine Renewable Energy Center (SNMREC) OCT system and comparisons were
made. Evaluated conditions include: uniform axial and off axis currents, as well as axial and off axis wave fields. For comparison purposes these conditions were applied to a geodetically fixed rotor, showing nearly identical results for the steady conditions but varied, in most cases still acceptable accuracy, for the wave environment. Finally, this entire moored OCT system was evaluated in a dynamic environment to help quantify the expected behavioral response of SNMREC’s turbine under uniform current.