Wave-Powered AUV Recharging: A Feasibility Study

The aim of this study is to determine whether multiple U.S. Navy autonomous underwater vehicles (AUVs) could be supported using a small, heaving wave energy converter (WEC). The U.S. Navy operates numerous AUVs that need to be charged periodically onshore or onboard a support ship. Ocean waves provide a vast source of energy that can be converted into electricity using a wave energy converter and stored using a conventional battery. The Navy would benefit from the development of a wave energy converter that could store electrical power and autonomously charge its AUVs offshore. A feasibility analysis is required to ensure that the WEC could support the energy needs of multiple AUVs, remain covert, and offer a strategic military advantage. This paper investigates the Navy’s power demands for AUVs and decides whether or not these demands could be met utilizing various measures of WEC efficiency. Wave data from a potential geographic region is analyzed to determine optimal locations for the converter in order to meet the Navy’s power demands and mission set.

Design Methodology for a Floating Offshore Wind Turbine Large-Scale Outdoor Prototype

This paper discusses the methodology introduced by the authors to design a large-scale wind turbine model starting from the DTU 10MW RWT. The wind turbine will be coupled with the model of a multi-purpose floating structure, designed within the EU H2020 Blue Growth Farm project, and it will be deployed at the Natural Ocean Engineering Laboratory (NOEL). In this paper the different strategies used to design the wind turbine model rotor, tower and nacelle are discussed, focusing on how it has been possible to reproduce the full-scale system aero-elastic response while ensuring the same functionalities of a real wind turbine.

An Actuator Disc Analysis of a Ducted High-Solidity Tidal Turbine in Yawed Flow

This work elaborates a computational fluid dynamic model utilised in the investigation of the hydrodynamic performance concerning a ducted high-solidity tidal turbine in yawed inlet flows. Analysing the performance at distinct bearing angles with the axis of the turbine, increases in torque and mechanical rotational power were acknowledged to be induced within a limited angular range at distinct tip-speed ratio values. Through multiple yaw iterations, the peak attainment was found to fall between bearing angles of 15° and 30°, resulting in a maximum power increase of 3.22%, together with an extension of power development to higher tip-speed ratios. In confirmation, these outcomes were subsequently analysed by means of actuator disc theory, attaining a distinguishable relationship with blade-integrated outcomes.

The Effect of Operational Parameters on Vibration Signals of Wind Turbine Gearboxes

Vibration signals are widely used in wind turbine drivetrain condition monitoring with the aim of fault detection, optimization of maintenance actions and therefore reduction of operating costs. Signals are most commonly sampled by accelerometers at high frequency for a few seconds. The behavior of these signals varies significantly, even within the same turbine and depends on different parameters. The aim of this paper is to explore the effect of operational and environmental conditions on the vibration signals of wind turbine gearboxes. Parameters such as speed, power and yaw angle are taken into account and the change in vibration signals is examined. The study includes examples from real wind turbines of both normal operation and operation with known gearbox faults. The effects of varying operating conditions are removed using kalman filtering as a state observer. The findings of this paper will aid in understanding wind turbine gearbox vibration signals, making more informed decisions in the presence of faults and improving maintenance decisions.

A Critical Examination of the Hysteresis in Wells Turbines Using CFD and Lumped Parameter Models

The hysteretic behavior of OWC-installed Wells turbines has been known for decades. The common explanation invokes the presence of unsteady aerodynamics due to the continuously varying incidence of the flow on the turbine blades. This phenomenon is neither new nor unique to Wells turbines, as an aerodynamic hysteresis is present in rapidly oscillating airfoils and wings, as well as in different types of turbomachinery, such as wind turbines and helicopter rotors, which share significant similarities with a Wells turbine. An important difference is the non-dimensional frequency: the hysteresis appears in oscillating airfoils only at frequencies orders of magnitude larger than the ones Wells turbines operate at. This work contains a reexamination of the phenomenon, using both CFD and a lumped parameter model, and shows how the aerodynamic hysteresis in Wells turbines is negligible, and how the often measured differences in performance between acceleration and deceleration are caused by the capacitive behavior of the OWC system.

Assessment of Latching Control for the Hemispheric Heaving Buoy Type Point Absorber With and Without Nonlinear Froude-Krylov Force Acting on the Buoy

In this study, a latching control strategy was utilized to increase the efficiency of a heaving buoy-type point absorber with a hydraulic Power take-off (PTO) system. For this purpose, the hydrodynamic performance of a floating buoy was analyzed based on the potential flow theory and Cummins equation. Nonlinear Froude-Krylov (FK) force according to instantaneous wetted surface of a buoy was calculated by a theoretical solution. The effect of the latching control on a point absorber was evaluated by considering PTO performance with hydrodynamic coefficients including nonlinear FK force. The hydraulic PTO system was modeled as an approximate coulomb damping force.

Oscillating Water Column Motion Inside Circular Cylindrical Structures

A non-linear mathematical model is presented for the Equation of Motion of the Water Column inside circular cylindrical structures in different cases, comparing to previous models in literature. Experimental model tests were carried out investigating the water column decay under given initial conditions, and an analysis is performed for each cycle showing the dynamic behaviour of OWC evolving in time. The results show asymmetric pattern in the time series acquired in the decay tests as a consequence of variations of the Added Length and quadratic viscous damping as the direction of the flow changes, as observed in previous studies. A general procedure is proposed to assess the unknown parameters including the quadratic damping viscous coefficients through the concept of “equivalent linear harmonic” as a linearisation of such terms, enlightening its dependence on the motion amplitude as well as the water column draft. Experimental data for the OWC response under a set of incoming regular waves is also presented, comparing the results to numerical simulation through a solver based on the estimation of the damping coefficients obtained in the decay tests.

Dynamic Response of Spar-Type Floating Offshore Wind Turbine in Freak Wave

Simulations are conducted in time domain to investigate the dynamic response of a SPAR-type floating offshore wind turbine under the scenarios with freak wave. Towards this end, a coupled aero-hydro numerical model is developed. The methodology includes a blade-element-momentum model for aerodynamics, a nonlinear model for hydrodynamics, a nonlinear restoring model of SPAR buoy, and a nonlinear algorithm for mooring cables. The OC3 Hywind SPAR-type FOWT is chosen as an example to study the dynamic response under the freak conditions, while the time series of freak wave is generated by the Random Frequency Components Selection Phase Modulation Method. The motions of platform, the tensions in the mooring lines and the power generation performance are documented in different cases. According to the simulations, it shows that the power coefficient of wind turbine decreased rapidly at the moment when freak wave acted on the floating structure.

Development of a Model Predictive Controller for the Wave Energy Converter Control Competition

The University of Maynooth is hosting a competition to develop a control strategy for a heaving point absorber wave energy converter (WEC). A linear model predictive control (MPC) design for the competition is presented. The state space model used in the MPC was derived numerically from the provided WEC-Sim model using linear system identification methods. A Kalman filter was used as the estimator, while also serving as an unknown input estimator to provide estimates of the excitation force on the WEC. The required excitation force predictions were made using an autoregressive linear prediction model. The inputs to the prediction model included estimated wave excitation forces and measured water surface elevation values from an up-field wave probe. Simulation results of the final control system design are also presented for each of the six wave cases specified by the competition organizers.

The Impact of Modelling Air Compressibility in the Selection of Optimal OWC Design Parameters in Site Specific Wave Conditions

The importance of properly modelling the effects of air compressibility in the selection of the optimal design parameters for an Oscillating Water Column wave energy converter is investigated. For this purpose, a wide dataset of capture width ratios, obtained from both experimental tests and Computational Fluid Dynamic simulations, is used to formulate an empirical model able to predict the performance of the device as a function of its basic design parameters (chamber width and draught, turbine damping) and of the wave conditions (wave period, wave height). A multiple non-linear regression approach is used to determine the model numerical coefficients. The data used to formulate the model include the effects of air compressibility. The impact of considering such effects on the selection of the optimal geometry of the device is evaluated and discussed by means of the model application for the optimization of a device to be installed in a site located in the Mediterranean Sea (in front of the coast of Tuscany, Italy).