bluuebunny/crossref_metadata_2025_split huggingface dataset:
Relevance of Robustness and Uncertainties Analysis in the Optimal Design of Wave Energy Converters
Authors: Giorcelli Filippo , Sirigu Sergej Antonello , Basile Dario 
Published: September 2023

The optimisation design of Wave Energy Converters (WEC) to reduce the cost of energy of the technology is a widely investigated topic. In literature classical optimisation strategies have been presented and applied to identify the optimal system parameters of WECs to optimise specific techno-economic metrics. The performance of the optimal identified devices relies on these nominal parameters and it can be strongly affected by construction and modelling uncertainties. In this context, the concept of robustness of the optimal solution plays a relevant role in the identification of a device whose performance is affected as little as possible by uncertainties of various kinds. In the first part of this paper different declinations of robustness concept are derived from other fields of application and described. The identified robustness indexes are then applied to optimal solutions obtained via classical optimisation to evaluate its importance in the design process of WECs.
Strictly related to this kind of methodology is the Sensitivity Analysis (SA) technique, it aims to investigate how the input variation (due to uncertainties or external noise or additional environmental parameters) influences the output results of a defined numerical model and highlight the relative input parameters relevance. Sensitivity Analysis, therefore, can be a valuable tool applicable in the uncertainty set estimation to identify the variables most subject to such uncertainties and their prominence.
The main objective of the work is to underline the importance of introduce the robustness evaluation of WECs during the optimisation process since classical optimisation techniques can lead to solutions that are affected by uncertainties.
A study on tidal rotors under the combined effects of currents and waves using actuator-line CFD simulations
Authors: Zilic de Arcos Federico , McNaughton James , Vogel Christopher , Pinon Grégory 
Published: September 2023

Determining loads and power on a tidal rotor under realistic flow conditions is challenging. Methods used by offshore industries for the design and deployment of marine devices rely on simulations of a significant number of cases that combine different environmental and operational conditions. Design studies of tidal rotors rely on engineering models of low to medium computational cost that often omit important hydrodynamic effects such as blockage, multi-rotor interactions, wave diffraction, etc., and have a questionable accuracy under conditions such as high-thrust regimes or highly unsteady flows. Alternatively, high-fidelity CFD simulations can implicitly capture most of the relevant physics, but their computational cost often limits their application in engineering practice.
This study will present a numerical study of a tidal rotor operating near the free surface and affected by combined currents and surface waves. The simulations will be performed with an intermediate fidelity actuator-line model implemented within OpenFOAM. The AL model is embedded within a simulated tank discretised with the finite volume method, that employs a RANS turbulence model with a Volume of Fluid (VoF) approach for the free surface. Relaxation zones, implemented with the Waves2Foam library, are used for the generation and absorption of surface waves and reflections. A preliminary render of the assembled model simulating a test condition can be seen in fig. 1.
The paper manuscript will present the results of an initial validation of the modelling strategy that will compare simulated cases with towing-tank experiments. The validation will be followed by a quantification of the interactions between the rotor and the free surface, as well as an analysis of the transient loads and power fluctuations for different operating conditions.
Tidal barrage operation optimization using moment-based control
Authors: Skiarski Agustina , Faedo Nicolás , Ringwood John 
Published: September 2023

As decarbonization of energy systems becomes an imperative and the deployment of intermittent renewables increases, the operation of electrical grids becomes challenging. In this sense, tidal barrage schemes can supply clean and predictable energy with more flexibility than the more traditional wind and solar plants. However, the high infrastructure costs associated with tidal barrage plants hinders its deployment. An optimal operation is key to maximize energy output and assure the project’s feasibility.
Recently, Ringwood and Faedo [3] introduced a novel approach to tidal barrage control by applying wave energy control techniques, with promising results. The model consists on a two-way tidal barrage scheme, where an analogy is made between latching/declutching algorithms applied in WEC control and holding/sluicing from tidal barrage. The optimization was done using the moment-based framework developed in [1] and [2], which can formulate linear and non-linear problems in a concave quadratic environment, thus allowing the application of efficient numerical solution algorithms. This preliminary study has a number of simplifying assumptions, leaving a pathway to further examine the optimal control problem.
This paper extends the results of the analysis from [3] by adding complexities to the model. Crucially, the basin is modeled such as the area is a polynomial function of the water level, instead of assuming constant surface area. This will affect the operational head of the optimal solution, as the volume of water at the top of the basin will be higher than at the bottom. Another added constraint will be the minimum head that the turbine requires to generate electricity. Furthermore, a cost function is introduced to penalize the energy consumption of the sluice gates, as shown in figures 1.a) and 1.b). This not only increases the accuracy of the model, but in many cases improves the convergence of the algorithm by adding a quadratic term. Figure 2 shows the value of the overall energy yield (as a fraction of the energy output without penalty) and simulation run time (as a fraction of the run time without penalty) with different penalty coefficients.
1.a) Sluice gate area without considering cost function
1.b) Sluice gate area with penalty factor of 3

2. Energy and run time variations with penalty

References:[1] N. Faedo, G. Scarciotti, A. Astolfi, and J. V. Ringwood. Energy-maximising control of wave energy converters using a moment-domain representation. Control Engineering Practice, 81:85–96, 2018.

[2] N. Faedo, G. Scarciotti, A. Astolfi, and J. V. Ringwood. Nonlinear energy-maximizing optimal control of wave energy systems: A moment-based approach. IEEE Transactions on Control Systems Technology, 29(6):2533–2547, 2021.[3] J. V. Ringwood and N. Faedo. Tidal barrage operational optimisation using wave energy control techniques. IFAC-PapersOnLine, 55(31):148–153, 2022. 14th IFAC Conference on Control Applications in Marine Systems, Robotics, and Vehicles CAMS 2022.
Hardware-in-the-loop testing framework for active accumulator wave energy converters
Authors: Zeng Chen , Gaspar José , Mendes Mario , Guedes Soares Carlos 
Published: September 2023

The hardware-in-the-loop technique is a hybrid co-simulation approach that joins real physical and virtual components. The real components may be the power take-off while the virtual ones the numerical models of sea wave and wave energy converter hydrodynamics. This simulation approach has been published with some regularity, however a review of the state-of-the-art has not been found, in particular in the research field dedicated to wave energy converters. Thus, the objective of this paper is to provide the ongoing results of such review, such as the generic testing framework and taxonomy, and their articulation on the simulation of a new oil-hydraulic power take-off concept. In this new concept, different power take-off nominal pressures may be regulated according to different sea states, by changing the hydraulic accumulator charging characteristics. Therefore, this active accumulator is intended to allow an increase on the range of possible Power Take-Off damping forces, thus, better wave energy harvesting performance. The paper results are also intended to support novice and advanced researchers to design simulation approaches in a clear and appropriate manner.
Limiting the available pneumatic power in a U-OWC
Authors: Henriques Joao , Gato Luís , Falcão António , Ferreira Diogo 
Published: September 2023

A fixed oscillating water column is a hollow structure open to the sea at its submerged part. The water column motion alternately compresses and decompresses the enclosed air above the inner free surface, which drives a self-rectifying air turbine. In 2003, Boccotti proposed the so-called U-OWC having in view the integration into breakwaters. In this configuration, the inner water is connected to the sea by a U-shaped duct whose opening faces upwards rather than sideways or downwards. This allows the OWC length to be extended (and the resonance frequency reduced) without placing the opening too deeply submerged where wave energy is attenuated by distance to the free-surface. An advantage of OWC converters is the capability to control or dissipate the excess energy available to the power take-off system in excessively energetic sea states. This is in general done by limiting the air turbine torque by controlling a by-pass air valve or a valve in series with the turbine. What is proposed here is to take advantage of the water column configuration to limit the energy absorbed from the waves in the more energetic sea states. This requires the tidal amplitude to be small as occurs in coastal areas like the Mediterranean Sea. The distance of the OWC opening to the mid-sea-level should be chosen such that, in the more energetic sea states, that opening is left uncovered by the troughs of the higher waves. This introduces a limitation to the energy absorbed in the more energetic sea states. This paper reports experiments with a 1:40th-scale U-OWC in a wave flume. The results show that limitations to the capture width ratio occurs in the more energetic wave systems.
The Impact of Uncertainty on the Control of a Multi-Axis Wave Energy Converter
Authors: Hall Carrie , Wu Yueqi , Rizaev Igor , Sheng Wanan , Dorrell Robert , Aggidis George 
Published: September 2023

As global energy demands and climate concerns continue to grow, the need for renewable energy is becoming increasingly clear and wave energy conversion (WEC) systems are receiving growing interest. Over the years, many WEC systems have emerged but most of these systems are designed to extract energy from a single direction of motion. In reality, there are six degrees of freedom that a conversion device can potentially harness and a device that can operate on multiple axes, should be able to more effectively and consistently produce power. However, the design and control of the power take off (PTO) system for a multi-axis device is challenging due to the system complexity and nonlinearity. WEC systems often utilize optimal control techniques for PTO operation and leverage a prediction of the upcoming wave force to ensure power optimization. Prior work has clearly demonstrated that high power production can be achieved when an exact system model is used and the upcoming wave conditions are known, but uncertainty in the underlying model or the wave prediction can degrade performance. PTO control on a multi-axis WEC must leverage predictions of forces in multiple directions and if model predictive strategies are used, must leverage a simplified model of the WEC dynamics to be able to optimize in real time. The uncertainty in these predictions and the model could severely degrade the WEC’s power output. This work examines the control of a multi-axis WEC system, TALOS, and leverages machine learning to predict wave forces over the upcoming time horizon. TALOS is a point-absorber type WEC with multi-axis PTO system. The PTO uses a heavy ball that is attached to the hull with springs and hydraulic cylinders. When the hull is pushed by the external waves, the relative motion between the ball and hull moves the hydraulic cylinders causing them to pump a fluid through a circuit, thereby driving a hydraulic motor to produce electricity. This design has shown promising results in energy output but is more challenging to control since the PTO can move over six degrees of freedom. This paper seeks to quantify wave prediction uncertainty and its seasonal variation and to examine the impact of the uncertainty of the prediction on a model predictive controller’s ability to optimize the power output of TALOS.
Experimental measurements of two elastic taut-slack mooring configurations for the multi-float M4 WEC
Authors: Draycott Samuel , Stansby Peter , Li Gangqiang 
Published: September 2023

Moorings are a vital and often problematic component of any floating offshore renewable energy system, whether for wind or wave energy conversion, and here we consider the multi-float WEC system M4 in a 122 configuration. Previous experimental work has shown elastic cables to reduce extreme snap loads by a factor of about 6 when considering single cables between the bow float and a mooring buoy, and between the mooring buoy and the bed [1]. Here we compare results for two alternative configurations designed to reduce the mooring footprint as well as extreme snap loads. The first system, shown in Figure 1 (a), consists of a large surface buoy connected to the seabed with three equi-spaced elastic cables in pre-tension, with a single elastic cable to the bow float. The second system, shown in Figure 1 (b) has a smaller submerged buoy, again connected to the seabed with three equi-spaced elastic cables in pre-tension, however, this is connected to a smaller surface buoy which is subsequently connected to the bow float. Both systems are taut-slack. In this paper we present experimental measurements taken at the COAST laboratory at the University of Plymouth. Both configurations are tested in irregular (JONSWAP) wave conditions up to limiting steepness, with run times of 35 mins at 1:40 scale or about 3.5 hours full scale. Peak, mean and rms bed loads for both configurations are presented along with rms and peak relative angular pitch response with drive train disengaged (free hinge between mid and stern floats).
References[1] P. Stansby et al., “Experimental study of mooring forces on the multi-float WEC M4 in large waves with buoy and elastic cables,” Ocean Eng., vol. 266, no. P4, p. 113049, 2022.
Experimental comparison of the flow-induced loading between a ducted bottom-mounted twin vertical axis tidal turbine at still and an unducted prototype
Authors: Saouli  , Coquet  , Facq  , Gaurier  , Germain Grégory , Gomez  , Marcon  , Maurice  , Moreau  
Published: September 2023

The FloWatt project aims to demonstrate the efficiency of the HydroQuest’s 2.5 MW-rated turbine for commercial farm deployment. The project is dedicated to develop an innovative design of a turbine for high energetic sites and to deploy seven vertical axis tidal turbines on the Raz Blanchard site, France. This technology has already been successfully developed in a previous project at a smaller scale on the Paimpol-Bréhat test site.
Along the past three years, an important amount of work has been achieved to widely characterise the behaviour of the 1/20 scale 2-VATT similar to the 1MW-rated demonstrator. The turbine was tested in many different operating conditions in the Ifremer’s wave and current flume tank to reproduce full-scale conditions and turbine response (wide range of operating points and incident velocities, facing aligned and misaligned currents, with and without vertical velocity shear, with and without bathymetry generated turbulence, with and without surface waves following and against the current). Based on that experience, we gain confidence in the design process by comparing those experimental results to numerical and in-situ ones.

After a quick presentation of the industrial development for the deployment of a pilot farm with the manufacture, installation and operation of 7 turbines on the Raz-Blanchard site, we will focus on the scientific development aiming to prepare large-scale deployments of the next generation of turbine. One important step is to compare the flow-induced loading between the 2 kinds of turbine (ducted and unducted) and to analyse environmental loads during survival conditions. To investigate those points, the behaviour of a 1/20 scale 2-VATT similar to the 2.5 MW-rated demonstrator is experimentally studied in a wide range of operating conditions: with and without vertical velocity shear, and with and without surface waves following and against the current.
In the paper, we will compare at 1/20 scale the behaviour of the two kinds of turbine: 1 MW-rated and 2.5 MW-rated turbines. The experiments have been done in the same basin, under the same experimental conditions. The databases are being processed before launching a fine physical analysis.
Integrated hydrodynamic-electrical hardware model for wave energy conversion with M4 ocean demonstrator
Authors: Apsley Judith , Zhang Xiaotao , Iacchetti Matteo , Damian Iñaki Erazo , Liao Zhijing , Li Gangqiang , Stansby Peter , Li Guang , Wolgamot Hugh , Gaudin Christophe , Kurniawan Adi , Zhang Xinan , Lin Zifan , Fernando Nuwantha , Shearer Chris , Saunders Brad 
Published: September 2023

Wave energy is well known to be a renewable energy resource with worldwide capacity similar to wind. However there is to date negligible generation of electricity from wave. Many devices have been proposed without convergence on a particular design as there has been for wind. We are here concerned with a multi-float attenuator type M4 which has been widely tested in wave basins and modelled by linear diffraction/radiation methods. Potential of MW capacity for grid supply has been demonstrated at many sites. To advance development, small scale ocean tests are being planned for Albany, Western Australia where summer wind-wave conditions in King George Sound will excite the device giving principal absorption with mean periods in the range 2 - 3.5 seconds (or peak periods of 2.5 – 4.5 s). The aim is to learn about most aspects of ocean deployment from wave climate and environment planning to realistic electricity generation, albeit at kW scale. In this paper the emphasis is on the specification of electrical drive train (power take off) which requires the input of torque time variation for the wave conditions on the site, as described by a scatter diagram. First a linear time domain wave multi-float model (Fortran) is set up for the particular 121 configuration, shown in Fig. 1. Such models have been used and validated against wave basin tests for similar configurations. This is then converted into state-space form in Matlab. This is highly efficient and suited for real time PTO control in Simulink. Fig. 2 shows the main components of the electrical drive train, including the gearbox, generator, super-capacitors, power electronic converters and resistor bank to dissipate electricity. Bespoke Matlab models will be run for the wave conditions in the scatter diagram to check that components are suitably rated for normal sea-states, and are safely protected through electrical power-limiting control in high sea states. Simulated electrical generator results will be shown for typical sea states, with some power-limiting. Instrumentation will be specified. Only uni-directional waves are considered in this paper. Ultimately the efficacy of the system will be demonstrated in ocean conditions.
Large-eddy simulations of interaction between surface waves and a tidal turbine wake in a turbulent channel
Authors: Stallard Tim , Mullings Hannah , Draycott Sam , Ouro Pablo 
Published: September 2023

Tidal stream turbines are now being developed for array deployments, largely at sites with relatively shallow water depths on either bed-supported, or floating support structures. Proximity to the free-surface presents design challenges with increased exposure to wave-induced kinematics leading to potential for increased peak- and fatigue-loads. Free surface proximity can also alter wake recovery rates which can influence the siting, and operation, of further turbines. To-date the impact of waves on turbine loading and on wake recovery has receivedlimited attention, generally for specific combinations of conditions reproducible in experimen tal facilities [1-2] or numerical models [3]. Improved understanding of how waves affect both turbine loading, and wake dynamics is necessary to inform the development of appropriate load prediction and mitigation methods and to further parameterise wake recovery to inform array siting.
This work presents CFD analysis of the loading and wake of a three-bladed horizontal axis tidal turbine within a wide, shallow channel with a turbulent inflow developed using a synthetic eddy method and free surface waves modelled using a Level-Set Method (LSM). The turbine and wave conditions considered are based on prior experimental studies [1] within a channel depth h = 0.45 m and mean flow U = 0.47 m/s. The simulated channel is width 3.67h and length 88h and long-crested regular waves are imposed, with the focus of this study on waves with non-dimensional wavenumber kh = 2.8, comparable with experimental conditions. Simulations are accomplished using the in-house large-eddy simulation (LES) code DOFAS(Digital Offshore Farms Simulator) [4], which adopts an Actuator Line Method (ALM) to resolve the turbine blades. LSM is used to model the air-water interface in an accurate manner [5] with waves can be generated in DOFAS using linear or second-order Stokes theories with an absorption layer placed at the outlet of the domain to avoid wave reflections. At the inlet a mean logarithmic velocity profile is imposed over which artificial turbulence is added. Simulations were run for 400 s of physical time on 8,000 cores using ARCHER2.
The waves studied are shown to alter the rate of wake recovery, increasing the rate in the near-wake of the turbine and slightly reducing the rate beyond eight diameters downstream. Analysis of turbulence characteristics indicates a significant variation across the water column due to wave action. Propagation over the turbine wake also introduces directionality to the wave field, which is associated with the change of wave-speed over the wake region. Modal decomposition analysis of the velocity fluctuations with Proper Orthogonal Decomposition(POD) reveals that the wake dynamics behind the turbine change due to the waves.