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 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.

This submission includes three peer-reviewed (under review) papers from the researchers at North Carolina State University presenting control-based techniques to maximize effectiveness of a tethered tidal kite. Below are the abstracts of each file included in the submission. Cobb TCST - Iterative learning for kite path optimization.pdf This paper presents an iterative learning control-based approach for optimizing the flight path geometry of a tethered MHK system. Tethered MHK systems, which replace the tower and turbine of a conventional system with a tether and a lifting body, capture energy by driving a generator with the tension in the tether. By spooling out tether during the high tension portions of cross-current flight and spooling in during low tension portions, net positive energy is generated over one cycle. Because the net energy generation is sensitive to the shape of the flown path, we employ an iterative learning update law to adapt the path shape from one lap to the next. Additionally, we present a realistic system model, along with lower-level path-following and power take-off (PTO) controllers. We then demonstrate the efficacy of our algorithm on this model in both uniform and realistic flow environments. Siddiqui ACC - Optimal spooling control of kites in variable flow.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 ACC - Spatial optimization of kite paths.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.

This paper examines the control of an autonomous underwater vehicle (AUV) with a deployable energy-harvesting kite for oceanographic observation and surveillance. The proposed design and control strategies specifically address objectives of achieving high-payload, long-endurance AUV operation through the deployment of an energy-harvesting kite while the AUV is anchored to the seabed, followed by the retraction of the kite for continued operation of the AUV. While deployed, the kite executes power-augmenting cross-current flight motions, using a hierarchical controller. When the AUV is in motion and the kite is retracted, a dynamic programming-based controller is used to select charging locations that minimize total charging time when traversing a prescribed mission path. Focusing on oceanographic observation along a Gulf Stream transect, using a hindcast model of the Gulf Stream current resource, the paper demonstrates the efficacy of the proposed control approach, as compared to several non-optimized alternatives.

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.

The Ocean Renewable Power Company's (ORPC's) goal is to design, develop, and test hydrofoils with large deflections. The effects of the deflections on cross-flow turbine performance would be evaluated in order to inform design considerations for full-scale water turbines and other marine hydrokinetic devices. CFD models of helical model scale turbines tested at UNH OpenFOAM v1912 Tip Speed Ratio (TSR) = 3.00 Different strut configurations

The Ocean Renewable Power Company's (ORPC's) goal is to design, develop, and test hydrofoils with large deflections. The effects of the deflections on cross-flow turbine performance would be evaluated in order to inform design considerations for full-scale water turbines and other marine hydrokinetic devices. FEA models - NASTRAN Helical foil turbines tested at UNH tow tank Glass and carbon composite material properties Loads derived from CFD models

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

The Ocean Renewable Power Company's (ORPC's) goal is to design, develop, and test hydrofoils with large deflections. The effects of the deflections on cross-flow turbine performance would be evaluated in order to inform design considerations for full-scale water turbines and other marine hydrokinetic devices. OpenFOAM V1912 files for straight foil model scale turbines in the University of New Hampshire tow tank. Strut Locations = (0.13, 0.225, 0.450, 0.675, 0.900) [m] Tip speed ratio = 2.40

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.