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.

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.

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.

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.

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.

Initial laboratory experiments with coordinated phase control of cross-flow turbines in a dense array.

This is an exercise in optimizing the flow through a shrouded axial turbine to have the least resistance and to have optimal output and torque and energy. In this study, different variations of the original geometry of the current turbine designed by Hydrokinetic Energy Corp. (HEC) were evaluated for energy efficiency using Computational Fluid Dynamics (CFD). The objective was accomplished by a parametric study of the key geometric parameters for the shroud, the diffuser, and the hub. Project is part of the TEAMER RFTS 3 (request for technical support) program.

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.

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 submission contains several papers, a final report, descriptions of a theoretical framework for two types of control systems, and descriptions of eight real-time flap load control policies with the objective of assessing the potential improvement of annual average capture efficiency at a reference site on an MHK device developed by Resolute Marine Energy, Inc. (RME). The submission also contains an LCOE model that estimates the performance and related energy cost improvements that each advanced control system might provide and recommendations for improving DOE's LCOE model. The two types of control systems are for wave energy converters which transform data into commands that, in the case of RME's OWSC wave energy converter, provide real-time adjustments to damping forces applied to the prime mover via the power take-off system (PTO). The control theories developed were: 1) Model Predictive Control (MPC) or so-called "non-causal" control whereby sensors deployed seaward of a wave energy converter measure incoming wave characteristics and transmit that information to a data processor which issues commands to the PTO to adjust the damping force to an optimal value; and 2) "Causal" control which utilizes local sensors on the wave energy converter itself to transmit information to a data processor which then issues appropriate commands to the PTO. The two advanced control policies developed by Scruggs and Re Vision were then compared to a simple control policy, Coulomb damping, which was utilized by RME during the two rounds of ocean trials it had conducted prior to the commencement of this project. The project work plan initially included a provision for RME to conduct hardware-in-the-loop (HIL) testing of the data processors and configurations of valves, sensors and rectifiers needed to implement the two advanced control systems developed by Scruggs and Re Vision Consulting but the funding for that aspect of the project was cut at the conclusion of Budget Period 1. Accordingly, more work needs to be done to determine: a) means and feasibility of implementing real-time control; and b) added costs associated with such implementation taking into account estimated effects on system availability in addition to component costs.