This submission includes two peer-reviewed papers from researchers at North Carolina State University presenting the modeling and lab-scale experimentation of the dynamics and control of a tethered tidal ocean kite. Below are the abstracts of each file included in the submission. Alvarez ECC: Flight and Tether Dynamics This paper models the dynamics of a marine tethered energy harvesting system focusing on exploring the sensitivity of the kite dynamics to tether parameters. These systems repetitively reels a kite out at high tension, then reels it in at low tension, in order to harvest energy. The kite?s high lift-to-drag ratio makes it possible to maximize net energy output through periodic cross-current flight. Significant modeling efforts exist in the literature supporting such energy maximization. The goal of this paper is to address the need for a simple model capturing the interplay between the system?s kite and tether dynamics. The authors pursue this goal by coupling a partial differential equation (PDE) model of tether dynamics with a point mass model of translational kite motion. Siddiqui JDSMC: Lab-scale closed-loop model and validation This paper presents a study wherein we experimentally characterize the dynamics and control system of a lab-scale ocean kite, and then refine, validate, and extrapolate this model for use in a full-scale system. Ocean kite systems, which harvest tidal and ocean current resources through high-efficiency cross-current motion, enable energy extraction with an order of magnitude less material (and cost) than stationary systems with the same rated power output. However, an ocean kite represents a nascent technology that is characterized by relatively complex dynamics and requires sophisticated control algorithms. In order to characterize the dynamics and control of ocean kite systems rapidly, at a relatively low cost, the authors have developed a lab-scale, closed-loop prototyping environment for characterizing tethered systems, whereby 3D printed systems are tethered and flown in a water channel environment.

This submission includes three peer-reviewed (under review) papers from the researchers at North Carolina State University presenting different control-based techniques to maximize the efficiency and robustness of a tethered energy-harvesting kite. Below are the abstracts of each file included in the submission. Naik ACC - Geometric Structural Control Co-Design.pdf Focusing on a marine hydrokinetic energy application, this paper presents a combined geometric, structural, and control co-design framework for optimizing the performance of energy-harvesting kites subject to structural constraints. While energy-harvesting kites can offer more than an order of magnitude more power per unit of mass than traditional fixed turbines, they represent complex flying devices that demand robust, efficient flight controllers and are presented with significant structural loads that are larger with more efficient flight. Daniels IFAC - Optimal Cyclic Spooling Control.pdf This paper presents a control strategy for optimizing the the spooling speeds of tethered energy harvesting systems that generate energy through cyclic spooling motions which consist of high-tension spool-out and low-tension spool-in. Specifically, we fuse continuous-time optimal control tools, including Pontryagin?s Maximum Principle, with an iteration domain costate correction, to develop an optimal spooling controller for energy extraction. In this work, we focus our simulation results specifically on an ocean kite system where the goal is to optimize the spooling profile while remaining at a consistent operating depth and corresponding average tether length. Reed IFAC - Kite Control in Turbulence.pdf This paper presents a hierarchical control framework for a kite-based MHK system that executes power-augmenting cross-current flight, along with simulation results based on a high-fidelity turbulent flow model that is representative of flow conditions in the Gulf Stream. The hierarchical controller is used to robustly regulate both the kite?s flight path and the intra-cycle spooling behavior, which is ultimately used to realize net positive energy production at a base station motor/generator system. Two configurations are examined in this paper: one in which the kite is suspended from a surface-mounted platform, and another in which the kite is deployed from the seabed.

Focusing on a marine hydrokinetic energy application, this paper presents a combined geometric, structural, and control co-design framework for optimizing the performance of energy-harvesting kites subject to structural constraints. While energy-harvesting kites can offer more than an order of magnitude more power per unit of mass than traditional fixed turbines, they represent complex flying devices that demand robust, efficient flight controllers and are presented with significant structural loads that are larger with more efficient flight.

Two papers submitted (and accepted) to the 2021 American Control Conference (ACC), both focused on different attributes of kite control in variable flow environments. Siddiqui et. al. focuses on tether elevation angle control in a spatiotemporally varying environment, and Reed et. al. focuses on spooling control in such an environment. The abstracts of each accepted paper are included below: Siddiqui - Gaussian Process-Based Receding Horizon Adaptive Control.pdf This work focuses on the development of an adaptive control strategy that fuses Gaussian process modeling and receding horizon control to ideally manage the tradeoff between exploration (i.e., maintaining an adequate map of the resource) and exploitation (i.e., carrying out a mission, which consists in this work of harvesting the resource). The use of a receding horizon formulation aids in the consideration of limited mobility, which is characteristic of dynamical systems. In this work, we focus on an airborne wind energy (AWE) system as a case study, where the system can vary its elevation angle (tether angle relative to the ground, which trades off higher efficiency with higher-altitude operation) and flight path parameters in order to maximize power output in a wind environment that is changing in space and time. We demonstrate the effectiveness of the proposed approach through a data-driven study on a rigid wing-based AWE system. Reed - Optimal Cyclic Control of an Ocean Kite System in a Spatiotemporally Varying Flow Environment.pdf This paper presents a technique for maximizing the power production of a tethered marine energy-harvesting kite performing cross-current figure-eight flight in a 3D spatiotemporally varying flow environment. To generate a net positive power output, the kite employs a cyclic spooling method, where the kite is spooled out while flying in high-tension crosscurrent figure-eight flight, then spooled in radially towards the base-station under low tension.

AeroDyn is a time-domain wind and MHK turbine aerodynamics module that can be coupled into the FAST version 8 multi-physics engineering tool to enable aero-elastic simulation of horizontal-axis wind turbines. AeroDyn V15.04 has been updated to include a cavitation check for MHK turbines, and can be driven as a standalone code to compute wind turbine aerodynamic response uncoupled from FAST. Note that while AeroDyn has been updated to v15.04, FAST v8.16 has not yet been updated and still uses AeroDyn v15.03.

This dataset contains data recordings generated during Year 3 of a DOE-funded project focused on the co-design of marine energy converters and autonomous underwater vehicle (AUV) docking and recharging systems. Data was collected during experimental testing at the O.H. Hinsdale Wave Research Laboratory and support foundational research aimed at advancing coupled Wave Energy Converter (WEC)-AUV systems for marine energy applications. This release builds on and supplements data provided in the previously submitted Year 3 project software and data submission from this project, linked below. This dataset includes measurements of wave elevation, water pressure, dock motion, load on a dock, and load on a fixed Autonomous Underwater Vehicles (AUV). Additionally, a testing log is provided including testing logs and summary of the five conditions tested: -(1) regular and random waves -(2) waves with dock motions -(3) multi-sine waves -(4) multi-sine dock motions -(5) multi-sine waves with dock motions.

This data is from measurements at Admiralty Head, in Admiralty Inlet (Puget Sound) in June of 2014. The measurements were made using Inertial Motion Unit (IMU) equipped ADVs mounted on Tidal Turbulence Mooring's (TTMs). The TTM positions the ADV head above the seafloor to make mid-depth turbulence measurements. The inertial measurements from the IMU allows for removal of mooring motion in post processing. The mooring motion has been removed from the stream-wise and vertical velocity signals (u, w). The lateral (v) velocity has some 'persistent motion contamination' due to mooring sway. Each ttm was deployed with two ADVs. The 'top' ADV head was positioned 0.5m above the 'bottom' ADV head. The TTMs were placed in 58m of water. The position of the TTMs were: ttm01 : (48.1525, -122.6867) ttm01b : (48.15256666, -122.68678333) ttm02b : (48.152783333, -122.686316666) Deployments TTM01b and TTM02b occurred simultaneously and were spaced approximately 50m apart in the cross-stream direction. Units ----- - Velocity data (_u, urot, uacc) is in m/s. - Acceleration (Accel) data is in m/s^2. - Angular rate (AngRt) data is in rad/s. - The components of all vectors are in 'ENU' orientation. That is, the first index is True East, the second is True North, and the third is Up (vertical). - All other quantities are in the units defined in the Nortek Manual. Motion correction and rotation into the ENU earth reference frame was performed using the Python-based open source DOLfYN library (http://lkilcher.github.io/dolfyn/). Details on motion correction can be found there. Additional details on TTM measurements at this site can be found in the included Marine Energy Technology Symposium paper.

Aquantis 2.5 MW Ocean Current Generation Device, Tow Tank Dynamic Rig Structural Analysis Results. This is the detailed documentation for scaled device testing in a tow tank, including models, drawings, presentations, cost of energy analysis, and structural analysis. This dataset also includes specific information on drivetrain, roller bearing, blade fabrication, mooring, and rotor characteristics.

Dataset contains MHK Hydrofoils Design and Optimization and CFD Analysis Report for the Aquantis 2.5 MW Ocean Current Generation Device, as well as MHK Hydrofoils Wind Tunnel Test Plan and Checkout Test Report.

Final report on a TEAMER RFTS 2 (request for technical support) study undertaken by Alden Research Laboratory for the Mono-radial turbine invented by John Clark Hanna DBA: Hanna Wave Energy Primary Drives. The study is a predictive numerical and CFD (computational fluid dynamics) report of the mentioned Hanna Mono-Radial Turbine. The device is an impulse-type mono-radial air turbine PTO for wave energy conversion. The turbine is self-rectified, meaning that it spins in one direction only while capturing the bi-directional air flows developed within an OWC (Oscillating Water Column) system.