bluuebunny/crossref_metadata_2025 huggingface dataset:
Intracycle Active Blade Pitch Control for Cross-Flow Tidal Turbines Using Embedded Electric Drive Systems
Authors: Zhao Zhao , Bennecke Timo , Hörner Stefan , Leidhold Roberto
Published: September 2023

Cross-flow tidal turbines (CFTTs) have proven advantages over horizontal axis turbines in terms of high power density per unit area, simplicity of design, and operation independence from inflow direction. However, they suffer from an unsteady flow regime which can comprise dynamic blade stall and thus problems of material fatigue or even failure. Active pitch control mechanisms on blade level have been shown to provide a potential solution, when continuously adjusting the pitching angle of each individual blade during the whole rotational cycle of the turbine.

As part of the research of the OPTIDE project, in this study, electric drive systems embedded in the blades of a CFTT flume model are proposed aiming to realize an active pitch control with high efficiency and fast response. The blade embedded actuation allows for reasonable flow conditions. For full scaled on-site applications this is required to reduce hydrodynamic losses and to protect the actuators and electronics from the harsh environmental and operation conditions. Based on the expected hydrodynamic loads from numerical flow simulations (CFD), several types of actuators are considered. The first type has brushless DC motors installed at both sides of each blade. Along with a gear box with proper reduction ratio, the actuators are able to provide required torque within expected cycle period. The second type of actuator drives the blade directly, which always results in faster pitching action, higher drive efficiency, more accurate positioning of blades, as well as simpler structure. Specifically, the shaft of each blade is designed as the primary of a limited angle torque motor, while the blades are used as the secondary with magnets inside. To mitigate the potential saturation effect on the iron of the primary­, the blade can also be used as the primary, where there always exists much larger space for windings. In this case, the magnets are now located at the shaft. By doing this, it is expected to output larger torque in a wider range of pitching angle as compared with the original one, while almost the same power is required. An experimental test bench for a single blade with both types of actuators is built to verify their ability of a fast and accurate pitching control. This also lays the foundation of identifying the optimal pitching angle for the control and inhibition of dynamic blade stall at various flow conditions and blade positions within the rotational cycle of the turbines. After successful optimization and testing of the model scaled mechatronical design, the actuators will be up scaled for realistic applications.
Experimental validation of rollout-based model predictive control for wave energy converters on a two-body, taut-moored point absorber prototype
Authors: Lin Zechuan , Huang Xuanrui , Xiao Xi
Published: September 2023

Model predictive control (MPC) has proven its effectiveness in improving the energy capture efficiency of wave energy converters (WECs) under physical constraints. Further application of MPC requires to speed up its online computation for an industrial controller. To this end, the rollout-based MPC (RMPC) for WECs has been developed. The idea of rollout is to decouple the optimization horizon and the prediction horizon, so as to achieve long-horizon performance with short-horizon optimization. In this paper, the RMPC that has previously only been validated by simulation is further put through wave tank testing. The experimental device is a realistic two-body, taut-moored point absorber prototype. The RMPC is based on a simplified model of the device and implemented in real time with wave force estimation and prediction. Experiment results confirm RMPC’s energy efficiency as well as constraint satisfaction, so its computational advantage against conventional MPC is highlighted.
Multi-PTO Wave Energy Converter for Low Energetic Seas: Ensenada Bay Case.
Authors: Meneses Gonzalez Paulino , Mendoza Baldwin Edgar , Stratigaki Vasiliki , Troch Peter , Carpintero Moreno Efrain
Published: September 2023

The paper presents a wave energy converter concept to harvest energy at Ensenada Bay, Mexico. The study area can be classified as a low energy sea due to the mean wave power being around 10 kW/m; the wave conditions are significant wave height of 0.5 to 2.5 and peak period of 5 to 20 s. The wave energy converter is formed by a torus buoy and anchored to the sea bottom by four connection structures distributed every 90°. The connected structures play a piston role, and their response is leveraged to run a power take-off system, for example, a linear or hydraulic system. This work does not focus on designing the power take-off system; therefore, it is simplified as linear damping, and an iterative process calculates its value. The hydrodynamic buoy study is made on the frequency domain with Nemoh BEM solver and the power absorption on the time domain with the WEC-Sim code. The capture width ratio is used to evaluate the wave energy converter performance and is a key factor in choosing the optimal size. The wave energy converter captures an average of 20% of the available energy and is well-fixed between periods 8 and 16 s. The present study achieves its target: providing a wave energy converter system to operate under site wave conditions.
Siting tidal energy projects through resource characterization and environmental considerations
Authors: Copping Andrea , Garavelli Lysel , Yang Zhaoqing , Wang Taiping , Deb Mithun , Briggs Candace
Published: September 2023

The development of tidal energy technologies has progressed to where devices can be deployed, operated, maintained, and recovered with some level of assurance that they will and produce adequate levels of power. Equally important to further the tidal energy industry is the ability to site and gain regulatory permission to deploy and operate these devices. This paper sets out a framework for reaching preliminary siting of tidal devices, drawing from case studies from three locations in the US where research studies have provided information in support of tidal deployments.

Through the TEAMER funding opportunity in the US, tidal energy device and project developers were able to engage US Department of Energy national laboratory scientists and engineers to provide technical assistance for investigating potential tidal deployment sites within US waters. The bodies of water of interest had already been determined by the proponents at the start of the project and constraints and opportunities within those bodies of water were examined to optimize siting capabilities for the developers. Using numerical models and field observations, we characterized tidal resources at a scale that will allow for optimization of energy extraction. We examined the natural and human infrastructure constraints for deploying and operating tidal devices and arrays including channel widths, bathymetry, vessel traffic, ferry lanes, and grid interconnects, in order to narrow siting options. We also examined the biological resources in the water bodies of interest, with a focus on populations of endangered marine mammals and fish, and the critical habitats that support them. The biological resources were then related to the applicable regulatory requirements in place in US for federal and state statutes in areas where the tidal applicants wish to deploy. Based on these analyses, preferred deployment locations were delineated and processes for meeting regulatory requirements laid out, including post-installation monitoring plans that will be needed. This initial assessment of logistical, regulatory, and environmental conditions for the deployment of a tidal technology is a first step toward the achievement of regulatory compliance for tidal energy projects.

Three locations were considered for tidal energy development in the US. The first one included the area around an archipelago of islands in the northern portion of Washington State, near the US-Canada border, with the intent of installing one or more floating tidal devices to add energy resilience and independence for the single utility that services the isolated islands. The second location was in the coastal waters of Maine where tidal power would be added to the local electrical grid. The third location was in Cook Inlet, Alaska, where the applicant seeks to deploy multiple floating tidal devices to provide renewable energy in place of conventionally generated power for the city of Anchorage.
LUBRICATION OF OFFSHORE MECHANICAL COMPONENTS: TOWARDS SUSTAINABLE & RELIABLE POWER PRODUCTION
Authors: Zapata Tamayo Juan Guillermo , Glavatskih Sergei
Published: September 2023

As different wave and tidal energy generators advance towards the commercial deployment phase, addressing potential issues related to the lubrication of the machine components integrating the Power Take Off system (PTO) due to the harsh operating conditions encountered in the marine environment becomes even more essential. Environmentally Acceptable Lubricants (EALs) based on water-soluble polymers are proposed as a way to reduce friction and wear of the mechanical components in wave and tidal energy generators. Although these fluids have the advantage of being biodegradable and non-toxic, they have not shown to be as effective as other synthetic fluids, or mineral oils in preventing corrosion, severe friction, and wear, thus increasing the risk of moving parts experiencing premature failure.

This work explores the potential of different water-soluble polymers to be used in the formulation of EALs that can meet the strict environmental regulations while providing effective protection against wear, and corrosion in offshore operating conditions. To evaluate the potential of these lubricants as an alternative to replace conventional mineral oils, different polymers were analyzed from the point of view of their ability to form an effective full lubricant film that can keep separation between the contacting surfaces, mitigate wear, and prevent corrosion. The rheological properties of these polymers were also studied at different concentrations in order to optimize the performance in the application. The hydrodynamic film build-up properties of EALs formulated with water-soluble polymers with different molecular weight, concentration, and viscosity are reported. The corrosion resistance exhibited by steel components when exposed to the different formulations compared to seawater was an object of examination. The study also aimed to establish correlations between the lubricant film-build up properties, viscosity, and electrical impedance.

The results showed that high molecular weight polymers can form a separating film at relative high pressure in the low-speed region even at low polymer concentrations. While with the increasing speed, the fluid viscosity becomes more important to sustain a full film between contacting surfaces. With the increasing concentration of polymer in the aqueous solution the open circuit potential (OCP) becomes more negative indicating the deterioration of the steel corrosion resistance.

The results provide new insights into the design of EALs that can effectively protect the mechanical components of wave and tidal energy generators while minimizing environmental impact. The findings suggest that water-soluble polymers are a promising solution for offshore applications, as they can provide efficient full film lubrication, mitigate wear, and prevent corrosion. These polymers can help to improve the performance and lifespan of offshore power generators while minimizing the environmental impact.
Long term wave load trends against offshore monopile structures: A case study in the Bay of Biscay
Authors: Martinez-Iturricastillo Nahia , Ulazia Alain , Ringwood John
Published: September 2023

This study examines the trend in wave parameters over available annual records in the Bay of Biscay, where EWTEC2023 is being hosted. There have been various renewable offshore energy projects in the 21st century in this region,such as the OWC (Oscillating Water Column) plant in Mutriku [1], or BiMEP (Biscay Marine Energy Platform)[2],the infrastructure for testing ocean devices in Armintza, where better characterization of the wave resource could helpto provide design information to developing wave energy technology. Other studies [3] have shown that wave heighthas increased in recent decades, as well as the frequency at which extreme waves occur, which affects the fatigueexperienced by deployed devices. The aim is to study the effect of the maximal wave height, its frequency, and loadson monopile cylindrical structures, which are basic component structures for wave energy converters and offshorewind turbines. The effect of this evolution, against these generic structures in offshore wind and wave energy devices,is studied using the 20th century reanalysis ERA20 (1900-2010) of ECMWF (European Centre for Medium-RangeWeather Forecasts) [3]. A significant increase in two forces, drag forces and inertial forces, of up to 15%, is computedover 110 years at a gridpoint near the Western Basque coast, which constitutes a strong positive slope that can be alsoassociated with climate change, and has strong implications for the design of new marine renewable energy technology.
Numerical modelling of wave and tidal current interactions and their impact on wave parameters
Authors: Tan Tian , Venugopal Vengatesan
Published: September 2023

In regions where both waves and tidal currents coexist, tidal flow can significantly alter wave parameters and hence affects the estimation of wave energy resources.  This study uses a coupled wave-current numerical model to evaluate the influence of wave-current interactions on wave parameters. To achieve this, the simulation was performed in 3 stages. At first, a large-scale North Atlantic wave model was constructed using the spectral wave model TOMAWAC to generate wave conditions and boundary inputs. This wave model was calibrated and validated at four sites around the UK using field measurements. Secondly, a small-scale numerical model covering Pentland Firth and Orkney Waters, Scotland, UK, was chosen, and tidal flow and current speeds were simulated by the three-dimensional flow model TELEMAC 3D.  As with the wave model, the flow model was also calibrated and validated with site measurements from an ADCP; thus, both models were validated. In the third stage, a coupled TOMAWAC-TELEMAC 3D model was employed for the small-scale region, and the wave parameters generated by the large-scale model were input as boundary conditions. The TOMAWAC-TELEMAC 3D coupled model was validated with field measurements at two locations in Orkney Waters, where waves and currents coexist. Various wave and tidal currents parameters produced from the coupled model are presented in the paper.  To evaluate the wave-current impact on wave parameters, a qualitative and quantitative analysis of these parameters is carried out, and the results are presented and discussed in the paper. The large-scale and small-scale numerical models developed in this study are useful tools for generating wave boundary conditions and wave energy resource assessment, helping researchers and engineers better understand the characteristics of wave-current interactions.
Heterogeneous WEC array optimization using the Hidden Genes Genetic Algorithm
Authors: Abdulkadir Habeebullah , Ellithy Ahmed , Ossama Abdelkhalik
Published: September 2023

Wave Energy Converters (WEC) are deployed in arrays to improve the overall quality of the delivered power to the grid and reduce the cost of power production by minimizing the cost of design, deployments, mooring, maintenance, and other associated costs. WEC arrays often contain devices of identical dimensions and modes of operation. The devices are deployed in close proximity, usually having destructive inter-device hydrodynamic interactions. However, in this work, we explore optimizing the number of devices in the array and concurrently, the dimensions of the individual devices (heterogeneous) to achieve better performance compared to an array of identical devices (homogeneous) with comparable overall submerged volume. A  techno-economic objective function is formulated to measure the performance of the array while accounting for the volume of material used by the arrays. The power from the array is computed using a time-domain array dynamic model and an optimal constrained control. The hydrodynamic coefficients are computed using a semi-analytical method to enable computationally efficient optimization. The Hidden Gene Genetic Algorithm (HGGA) formulation is used in this optimization problem.  During the optimization, tags are assigned to genes to determine whether they are active or hidden. An active gene simulates an active WEC device in the heterogeneous array, while the hidden gene results in a reduction in the total number of devices in the array compared with the homogeneous array. The volume of the heterogeneous array is constrained to be close to that of the homogeneous array. These hidden tags do not exclude the associated devices from the optimization process; these devices keep evolving with the active devices as they might become active in subsequent generations. Heterogeneous arrays were found to perform better than homogeneous arrays.
SIMULATIONS OF EXTREME WAVE LOAD ON AN OSCILLATING WATER COLUMN WAVE ENERGY CONVERTER
Authors: Nguyen Nhu , Chartrand Chris , De Miguel Para Borja , Morgaetxebarria Aurrekoetxea Jon
Published: September 2023

Extreme load analysis is an essential step in the design of structural and mooring systems for wave energy converters (WEC). The current study aims to evaluate the structural loads on the MARMOK Oscillating Water Column (OWC) WEC under both regular and irregular wave conditions, as well as to evaluate the effectiveness of its station keeping (mooring) systems. The project employs the open source computational fluid dynamics simulation software OpenFOAM to model the fully moored floating system. The WEC device hydrodynamics are validated against laboratory data for free decay in heave and pitch motions. In addition, the station keeping model is validated against mooring tension static offset tests and benchmarked against experimental data in irregular waves.  An elegant method of numerically recreating irregular wave inflow conditions from empirical measurements is shown which allows for consistent comparison between the model and experimental results. The calibrated numerical model is then employed to study the full-scale system’s responses. Preliminary results of structural loadings on the fixed and floating spar buoy (with one mooring configuration as sample) are presented in the paper.
On the survivability of WECs through submergence and passive controllers
Authors: Al Shami Elie , Pichard Alexandre , Cocho Mathieu , Santos Herran Miguel , Gough Ian
Published: September 2023

The survivability of WECs during extreme seas and heavy storms has proven to be challenging during the deployment of devices as they often fail during extreme storms. The survival mechanism is often inherited with the device design and mode of operation. It is not economically and logistically viable for devices to be taken back onshore in case of storms. Some devices lock their PTOs during heavy storms and others lock all the moving parts all together, in the case of CETO, its design has an advantage where it can be submergepad. CETO is a buoyant actuator WEC composed of a buoy submerged close to the surface. Three tethers connect CETO to the sea-bottom through rotary PTOs, allowing the device to be wound down, submerged, and therefore, less exposed to the extreme loads at the sea surface during large storms. This paper will study the survivability of the device during extreme sea-states and will examine the required depth to bring its response back to operational conditions. This work will also look at the alteration of some passive controllers, such as a conventional spring to minimize the response of the device instead of maximising the power capture. With the PTO objective altered in extreme sea-states to minimize the response instead of capturing power, the possibility of harvesting power during extreme sea-states with the device submerged will be checked. Finally, the survivability strategy of CETO through submergence will be showcased with wave tank experiments conducted at IHC as part of the Europewave program.
Estimation and characterisation of the wave-induced turbulent kinetic energy and turbulent dissipation from ADCP data
Authors: Calvino Clément , Bennis Anne-Claire , Furgerot Lucille , Pascal Bailly du Bois , Emmanuel Poizot
Published: September 2023

Turbulence in the water flow causes small-scale variations in the mechanical stress acting on submerged tidal turbines. As such it increases their fatigue loading and impacts greatly their lifetime. It is therefore essential for engineers to have an accurate knowledge and characterisation of turbulence at a given site as they design the structures to be deployed. The strength of the tidal currents is the main parameter influencing the intensity of turbulence through their friction with the sea bed. However, most potential tidal energy sites are located in a coastal environment with shallow enough water depths so that the direct impact of waves on turbulence can not be overlooked. Steepness-induced wave breaking is indeed observed to increase the turbulent mixing for such applications. In this context, we propose to estimate the contribution of surface processes to the total turbulence in Alderney Race, France, the most energetic tidal site in western Europe. The turbulent kinetic energy (TKE) specifically induced by waves and wind is characterised using measurements from a 5-beams ADCP deployed between 27/02/2018 and 06/07/2018.Analytical profiles are fitted to the data, the only fitting parameter of the model is an evaluation of the turbulence penetration depth, it determines how deep surface processes impact the water column. Its dependence towards mean wave and current parameters is studied. The results do not allow to conclude on the nature of turbulence observed in the mid water column.
Can Risk-Based Approaches benefit future Marine Renewable Energy deployment, planning and consenting processes?
Authors: Verling Emma , Menchaca Iratxe , Machado Inês , Soulard Thomas , O'Hagan Ann Marie , Bald Juan
Published: September 2023

The need to harness the vast energy resources of the oceans has led to a significant increase in the design, testing and deployment of novel technologies for Marine Renewable Energy (MRE). However, growth in this area has been slowed in part by several non-technological challenges, among them the ability to gain permissions to test and deploy installations. These consenting processes are often characterised by long bureaucratic procedures (with many authorities involved) and excessive environmental impact assessment studies, resulting in delays and additional costs to developers.

One option which may help to release this block is to adopt a Risk-Based Approach (RBA) to energy consenting, whereby an assessment of risk is used in the decision-making process. The EU-funded SafeWAVE project (www.safewave-project.eu) has focussed on this possibility in France and Ireland, building on the work of an earlier EU-funded project, WESE (https://www.researchgate.net/project/Wave-Energy-in-the-Southern-Europe-WESE), in which similar work was undertaken in Spain and Portugal. Here we present some of the findings from these projects, in particular the process to work towards a set of guidance for the use of RBAs in MRE consenting processes.

RBAs have already been used in the context of Marine Spatial Planning and Ecosystem-Based Management and have been found to be useful for interpretation of data from experts, indicators and ecosystem models. Indeed, a number of RBAs have also been developed that are appropriate for Marine Renewable Energy (MRE) consenting processes. A thorough review was undertaken of several recognised RBAs and common components were identified across five of the most useful and relevant of these. From these common components, a ‘stepwise process’ was formulated, specifically designed to be embedded into MRE consenting systems. This stepwise process has been constructed such that it can be presented to regulatory stakeholders in France and Ireland with a view to determining the feasibility of its implementation. Ultimately the outcome of these discussions will form the basis for the development of a guidance document on risk based, adaptive management consenting processes with recommendations on how the process can be taken forward and utilised by regulators, planners and developers.  While the development of a prescriptive procedure is not feasible (due to the varying nature of the MRE installations devices themselves as well as differing environmental conditions and impacts where devices are deployed), there is scope for providing guidance to assist regulators in taking a robust and holistic risk-based approach. Such a set of guidelines could facilitate a broader understanding and thus the wider use of RBAs, which in turn has the potential to remove one significant barrier to progress in the field.
Analysis of the impact of floater interactions on the power extraction of a dense WEC array with adaptable nonlinear PTO
Authors: Bechlenberg Alva , Wei Yanji , Jayawardhana Bayu , Vakis Antonis I.
Published: September 2023

This research focuses on studying the interactions between a closely spaced wave energy converter (WEC) array with an adaptable hydraulic power take-off (PTO) system. The boundary element method is used to extract the hydrodynamic and hydrostatic coefficients, while the power extraction and hydrodynamic behaviour of the array are simulated in irregular waves with a mixed frequency-time-domain (MFT) model to include the nonlinear PTO forces of each array element. The interactions between floaters are assessed and the influence of adaptability on the behaviour of the total array and its elements in comparison to single floater performance (i.e., the q factor) is analysed. The differences in performance of the WEC array are assessed with three levels of adaptability: no adaptability, adaptability per sea state and adaptability per incoming wave.
Multiquery analysis of a PeWEC farm
Authors: Battisti Beatrice , Giorgi Giuseppe , Verao Fernandez Gael , Troch Peter
Published: September 2023

The maximization of the power output of an array of wave energy converters (WECs) and its impact on the surrounding area are two fundamental, strictly interconnected, aspects that must be considered in the design of a wave farm. The effect of those two elements combined, usually considered separately, is evaluated, in this study, for a small farm of floating devices. The numerical simulations are performed using a coupled model between the BEM solver Capytaine (near-field solution) and the wave propagation model MILDwave (far-field solution). The farm is located off the coast of Pantelleria Island and is composed of three PeWECs, WECs designed for the Mediterranean Sea. The power output and the perturbed wave field around the PeWEC farm are compared to the single device, for different distances among the WECs. Moreover, a Power Take-Off (PTO) is implemented for assessing the difference in the far-field effects between a controlled and an uncontrolled case. Results show that the wake produced by the PeWEC farm is more prominent and more complex with respect to the single device. The inclusion of the PTO contributes to further attenuate the wave field in the lee of the PeWEC farm.
Numerical investigation of a new hybrid floating wind turbine concept
Authors: Fenu Beatrice , Bonfanti Mauro , Mattiazzo Giuliana
Published: September 2023

In today’s energy scenario dominated by the need to develop new methods of generating electricity, energy-intensive infrastructures are a promising solution towards energy self-sufficiency and environmental sustainability. Floating offshore wind turbine represents one of the major solution to exploit renewable energies. Currently, the increase of offshore wind market opens up the possibility of integrating different technologies to take advantage of marine energy potential, in particular wave energy.

This work presents a new wave-wind hybrid floating platform with three Oscillating Water Columns (OWC) integrated in a floating offshore wind spar buoy. The design methodology is described showing the size of geometric characteristics of OWCs and the size of the energy conversion system. The hybrid system is investigated by a time-domain model that integrates the wind turbine model, the hydrodynamics of the floater, the thermodynamics of the OWC air chambers, and the damping effect induced by the OWC air turbine. The thermo-aero-hydro coupled numerical framework is described to highlight the implementation of the thermodynamic model in WEC-Sim/MATLAB environment. Specifically, the water column dynamics are solved as pistonrigid body representation, enabling the time-domain analysis of the OWC dynamic behaviour. Impulse turbines are considered as OWC Power Take-Offs (PTO). The resource scenario of Mediterranean Sea is considered as case study, in terms of both wind and wave conditions. The analysis focuses on the power extraction capabilities of the OWCs and on the impact of OWCs on the productivity of the wind turbine. Further developments on multi-objective design tools and on optimal PTO control design aiming power maximisation could be conducted for hybrid energy platforms based on this established numerical framework.
Performance Assessment of a Multi-Rotor Floating Tidal Energy System
Authors: Kaufmann Nicholas , Starzmann Ralf , Al-Kahli Nabil
Published: September 2023

Performance assessments of full-scale tidal turbines have been carried out for multiple machines deployed globally and reported in literature. The rotor number and diameter of the demonstrated tidal energy converters differs from single rotor devices in the MW class to scalable arrays of comparably small turbines. For multi-rotor systems (MRS), potential interaction, positive or negative, remains a key research question. Previous studies at Sustainable Marine prototype system (PLAT-I 4.63) have implied that variations in the incoming flow field are highly likely to cause deviations in rotor performance.

Sustainable Marine has developed and built the pre-commercial PLAT-I 6.40 platform. PLAT-I 6.40 has been connected to the Canadian grid in May 2022. Between May and end of September 2022 the system has been undergone comprehensive commissioning and performance trials.  PLAT-I 6.40, carries six 4m diameter SCHOTTEL Instream Turbines (SIT), each rated at 70 kW. This work presents the results of full-scale field tests that focussed on the assessment of differences in the performance between the individual turbines during summer 2022.

To evaluate the variances in the flow field across all rotors, a specific test campaign has been conducted, where a flow speed sensor was sequentially positioned upstream of each turbine while a second flow measurement device was stationary as reference. For all test configurations the power curves have been determined following the guidance of the IEC TS 62600-200 based on the flow speeds measured with both devices.

This paper presents the experimentally obtained power curves for each test configuration and compares it against the reference values. The results show that there are steady differences in the flow field resulting in varying power outputs across the different rotors (Figure 1). However, a comparison of the individual rotor power curves with the design predictions shows good agreement.
Preliminary design of an OWC wave energy converter battery charger
Authors: Ferreira D.N. , Gato L.M.C. , Henriques J.C.C. , Zuo L.
Published: September 2023

This paper introduces a low-power off-grid oscillating water column wave energy converter with an internal battery bank. The research aims at the preliminary design and devising of the control strategy of a power electronics interface between the turbo generator and the battery bank. The converter comprises a spar buoy, a biradial turbine, a permanent magnet generator, a full-wave bridge rectifier, a braking chopper, a DC-to-DC step-down converter, and a lead-acid battery bank. The power-take-off system was modelled in Simulink/MATLAB, and its performance was assessed with steady-state simulations, considering a wave climate characteristic of Leixões, Portugal. The chamber pressure, the turbine, generator and rectifier performance were taken from experimental data sets. A simple battery model was derived from the manufacturer's datasheet. An ideal step-down DC-to-DC converter operating in discontinuous conduction mode regulates the battery charging current. This converter, in parallel with the braking chopper, adjusts the generator counter torque by regulating the current through the rectifier. Twelve system variables were recorded for selected pairs of input pressure and step-down converter design coefficient. The power at the rectifier's output terminals was mapped for the rotational speed and input pressure. The results show a system rating of 1.4 kW with 400 W of electrical power at 200 rad/s for the most frequent sea states. The range of the duty cycle, the inductance and the braking resistance were derived. Two closed-loop controllers were proposed for managing the step-down converter and the braking chopper. Their set points and saturation limits were derived from the simulation results.
Numerical and Experimental Characterization of Rotational Floating Body Drag
Authors: Robertson Bryson
Published: September 2023

Hydrodynamic drag plays a significant role in the motions and response of floating bodies – whether it be a wave energy converter, floating wind structure, or offshore oil & gas platform. Existing literature provides significant overview of the methodologies (both experimental and numerical) to characterize translational drag, however, there is limited research on the contributions (and methods of application) for rotational drag.

 

This paper will detail both numerical modelling and a physical experimental campaign to assess how rotational drag impacts floating body dynamics, and best practices for numerical model inclusion. Specific focus will be on 1) the variety of methods used to input rotational drag into numerical models; 2) processes and lessons learnt from the experimental derivation of rotational drag coefficients; and 3) how does weakly non-linear wave stretching methods influence rotational drag.

 

The experimental campaign is currently underway to classify the significance of rotational drag coefficients in characterizing floating body behavior. Translational and rotational drag coefficients of a simplified, inertial property matched, 1:50 floating body is being determined through a series of calibration tests. Both traditional free decay tests and forced oscillation tests will be implemented to evaluate these coefficients across multiple degrees-of-freedom. The final paper will present an overview of the experimental campaign, the results and lesson learnt.

 

On the numerical side, the floating body will be modelled in the open-source wave energy converter modelling tool, WEC-Sim, and validated against the experimental results. Numerical results will be presented to review general body responses, with and without rotational drag, and generic wave conditions plus those expected at the PacWave wave energy test site in Oregon, USA.

 

The inclusion of rotational drag coefficients and weakly nonlinear hydrodynamics are expected to improve computational model results, especially in the nonlinear wave excitation range, providing a better understanding of floating body behavior.
Control of multiple PTOs in single OWC air chambers
Authors: PORTILLO JUAN , Gato Luís M. C. , Henriques João C. C.
Published: September 2023

The paper presents the study of multi-PTO interaction in single-air chamber oscillating-water-column (OWC) wave energy converters (WECs). This strategy seeks: i) tunning system's performance to sea states, ii) increasing reliability through modular designs that are easier to install and replace, and iii) increasing conversion efficiency through better matching between available power and electric conversion equipment. The strategy was assessed through a novel non-linear time-domain model for OWC WECs implemented in the multi-physics object-oriented language Modelica. The model considers multiple degrees of freedom associated with the various PTOs. Different case studies are presented to show the proposed approach's versatility, some of the constraints expected in real implementations, and potential pathways to overcome them. The wave-to-wire model considers air compressibility and non-linear power take-off systems, which is fundamental to assessing the damping level variation required for each sea state considered. The cases presented are for a fixed-structure coaxial-duct OWC WEC, but the results are generalisable for floating structures. Results show a significant increase in mean annual power, and control strategies are proposed for power maximisation. Furthermore, some critical points are shown in the operational conditions' envelop for the system under a selected Portuguese wave climate. The identification of these critical values is important for the control of the OWC WECs. This study represents an advance in the control strategies considering multiple PTOs for a single OWC air chamber to foster innovation actions.
Additive Manufacturing for Powering the Blue Economy Applications: A Tidal Turbine Blade Case Study
Authors: González-Montijo Miguel , Murdy Paul , Candon Charles , Beach Ryan , Nichols Casey , Barden Phil
Published: September 2023

As the marine renewable energy industry continues to expand, innovation in the manufacturing space must grow accordingly to reduce costs and ensure the economic feasibility of new technologies. Additive manufacturing, more commonly known as 3D printing, provides an alternative for rapid prototyping of marine hydrokinetic technologies, particularly supporting Powering the Blue EconomyTM initiatives of the U.S. Department of Energy Water Power Technologies Office. This study explores the application of additive manufacturing in the development of marine hydrokinetic structures, focusing on material and printing method selection, design, and analysis of a 3D-printed spar for an axial-flow tidal turbine blade. Corrosion-resistant metals were deemed ideal due to the loads and harsh marine environment the blade would experience. Laser metal deposition methods were determined to be the most effective and scalable for the considered scale. The designed spar adapts its geometry to the blade—a feature uniquely suited to additive manufacturing—and is intended to serve as the blade's primary structural component. A finite element model was used to study stresses and deformations under loading conditions. The spar was manufactured using 316L stainless steel through direct energy deposition, and defects were assessed and recorded. Future efforts will include mechanical testing of the spar. This research establishes a benchmark process for using additive manufacturing in developing marine hydrokinetic structures, paving the way for future optimization and techno-economic analysis.
Principles of ADCP deployment methodologies
Authors: Jeffcoate Penny , Hunt Andrew
Published: September 2023

ADCPs are regularly used for resource characterization in wave and tidal energy sites; however, methodologies and best practices are entirely dependent on the site characteristics and the assets available. EMEC have used their widely varied experience at multiple sites and locations to develop a review of methodologies and the pros and cons of different deployment styles for data acquisition.When deploying ADCPs there are a number of site criteria that must be assessed, such as seabed type, bed flow speeds, surface flow speeds, recovery windows (slack and weather), and water depth and range. These criteria can significantly affect the success or failure of marine operations, not necessarily for deployment but certainly for recovery. When recovery operations fail then projects lose assets (ADCP units and bed frames), data, and money (in hire/unit costs, and vessel hire for example). When the data is not recovered then this can jeopardise project success, wasting time, effort and funding.Different deployment configurations and recovery techniques have been used for ADCP deployments. Equipment such as ground lines, acoustic releases, USBLs, and surface buoys are used to mark, locate, and/or recover devices. These are varied depending on a number of factors. Ground type, for example, can affect systems used since a ground line cannot be effectively used in delicate ecosystems since the dredging required to recover may disturb local flora and fauna. Also, for example, flow speed and vessel traffic will determine whether a surface buoy can be used.Based on multiple ADCP deployments to characterize resource for wave and tidal energy projects in Scotland, England, France and other sites, EMEC have reviewed different deployment methods and recovery techniques. There is a proposed set-up for deployments suitable for a number of active marine energy sites and suggestions for edits based on site characteristics and target data requirements.
Laboratory Tests Assessment of a Mechanical Sensor-less MPPT Control Strategy for Tidal Turbines
Authors: Rafiei Mohammad , Salvatore Francesco , Frost Carwyn , Benson Ian
Published: September 2023

The purpose of this study is to demonstrate through towing tank experiments the effectiveness of a novel sensor-less Maximum Power Point Tracking (MPPT) control for Tidal Stream Turbines (TST) under fluctuations of the onset flow.
 
Ocean energies will play a crucial role in the renewable energy sector over the next decades. Instream turbines for tidal currents are a rapidly maturing technology to exploit a highly predictable energy source. However, short-term fluctuations on the inflow velocity caused by waves or turbulence determine fatigue loads that affect system reliability. Power control strategies to maximize the energy yield can be also used to mitigate the effects of transient loading on drivetrain components.
Aim of this paper is to present a straightforward and robust MPPT control method based on the linear relationship between the current and voltage squared of the generator's DC outputs. The method requires pre-determined turbine characteristics to establish the control reference that is effective across operating conditions. The proposed MPPT model was derived mathematically through linearization and simplifications of the turbine power conversion system and validated by model tests carried out in the wave-towing tank facility of CNR-INM in Rome, Italy, using the 1.5 m diameter Tidal Turbine Testing (TTT) device developed at the Queen’s University Belfast (QUB).
 
In the study, a conventional TSR control method was also considered in order to perform a comparative analysis of system response to inflow speed fluctuations with time scales comparable to turbine revolution periods. TSR control was tested using two control references: TSR = 5 (design point) and TSR = 6 (over-speed zone) to verify the operation of the turbine under different loading conditions. The tests were conducted in two scenarios: calm water (steady state) and unsteady inflow with a regular (sinusoidal or monochromatic) waveform, with amplitude chosen to simulate an extreme wave case.
The power output was measured from the turbine during regular wave conditions and compared to results from steady flow to assess the impact of wave-induced velocity on turbine performance (Fig. 1). Test results showed that by using the proposed MPPT control strategy, the algorithms converged to the maximum power coefficient (Fig. 2), which validates the proposed methodology. Results also demonstrated the capability of the proposed MPPT to significantly reduce mechanical loads fluctuations as compared to the TSR control.
In the full-length paper, the proposed MPPT control strategy is outlined, the test methodology, set-up and conditions are described, and main results are presented and discussed.
Characterisation of turbulent flow and the wake of a tidal stream turbine in proximity to a ridge
Authors: Hurubi Sulaiman , Stallard Tim , Stansby Peter , Mullings Hannah , Ouro Pablo
Published: September 2023

Fast tidal currents are generally found in shallow water depths where tidal turbines can be deployed to operate. In complex environments in which there is an irregular bathymetry, seabed shape changes can induce pressure gradients that accelerate or decelerate the flow depending on the slope and relative depth, affecting turbine wake recovery. In this study, a laboratory scale turbine, represented numerically using an Actuator Line Method (ALM), is computed using Large-Eddy Simulations (LES) over a flat-bed and in presence of three short ridges with varying streamwise lengths. Turbines are positioned at several locations, namely at ridge-centre (0D) and at 2D upstream and downstream from the foot of the ridge to establish the influence of location on wake recovery rate. Turbine operating point is selected to yield a constant tip-speed ratio based on the disc-averaged velocity at each location obtained from a precursor simulation run without turbines. Results show that, relative to the same turbine in a flat channel, there is a noticeable enhancement of the rate of wake recovery when turbine is sited upstream of the ridge and it experiences high fatigue loads when sited downstream of the ridge. Implications for array design regarding performance losses due to turbine- and bathymetry-induced wakes are drawn.