bluuebunny/crossref_metadata_2025 huggingface dataset:
Wave Excitation Tests on a Fixed Sphere
Authors: Andersen Jacob , Bech Kramer Morten
Published: September 2023

Wave excitation tests on a fixed sphere with the center at the still water level were carried out with three different physical wave basin setups. The tests were completed as a continued effort of the working group OES Wave Energy Converters Modelling Verification and Validation to increase confidence in numerical models of wave energy converters by generation of accurate benchmarks datasets for numerical model validation. An idealized testcase with wave excitation of a fixed sphere to be used with the benchmarks was formulated. The three investigated physical wave basin setups included: 1) a six degree-of-freedom load cell mounted to the top of the sphere, 2) a bending beam force transducer mounted to the top of the sphere, and 3) a system of six pretensioned wires mounted to the top and bottom of the sphere with force transducers attached to each wire. The aim of the present paper is to identify the best representation of the idealized testcase. To this end, the three experimental setups are inter-compared in terms of dynamic properties, sensitivity, and disturbances of the water phase from the presence of measurement equipment. Low inter-experiment variability was disclosed, i.e., 5-8% depending on wave-nonlinearity, indicating accurate representations of the idealized testcase across all setups. Setup 3 was found to be the more accurate representation and further work with this setup to release a public benchmark dataset was planned.
Investigation of Low Order Parameters Affecting Tidal Stream Energy Resource Assessments
Authors: Patel Misha , Smyth Amanda , Angeloudis Athanasios , Adcock Thomas
Published: September 2023

Tidal stream energy may have the potential to contribute to the baseline energy needs of the UK. There is a significant difference between the existing assessments of the UK’s resource, highlighting the need for a standardised model. The paper investigates some challenges of quantifying tidal stream energy. This study examines the sensitivity of resource assessments to low-order parameters, in order to inform higher order modelling and contribute to defining a standardised model. A combination of channel characteristics, turbine parameters and simulation settings formed the basis of 96+ cases to determine key parameters affecting resource assessments. Two 0-dimensional, idealised channels were modelled using 2-8 constituents from four UK sites spanning the semi-diurnal form factor range. Blockage-corrected blade element momentum theory was used to represent variable-speed, variable-pitch turbines. Three blockage designed rotors and one rotor designed for an unspecified blockage were simulated. Power capping and combined power and thrust capping were implemented.

 

The study indicates dynamic balance is a key parameter affecting tidal dynamics when turbines are present in a channel. Variability of annual energy production due to the nodal cycle was most significant in the drag dominated channel with increasing blockage. Constituents with large amplitude and phase differences led to greater variability of the resource across the nodal cycle. Results indicate that the maximum deviation from the average annual energy production over the 18.6-year nodal cycle is +/- 8.8%. At a low form factor site, the difference between using 2 or 4 constituents was insignificant. However, for a higher form factor site, that is still semi-diurnal, the peak velocity increases by 0.32 m/s with 8 constituents compared to 2. Random Forest modelling indicates that the most important characteristic point on a variable-speed, variable-pitch turbine performance curve is the rated speed. Comparison of capping strategies highlights that combined power and thrust capping leads to an increase in energy per swept area. Combined capping decreased the variability of the resource over the nodal cycle, which is helpful to maintain a steady supply to the grid and sizing energy storage systems.
A method for the growth inhibition of biofouling in Sihwa Tidal Power Plant
Authors: Lee SeoYeong , Kwak HeeJin , Jun ByungJoon , Kim Taekyun
Published: September 2023

Many facilities in contact with seawater are constantly affected by marine biofouling.In particular, marine biofouling cause many problems, such as reducing efficiency of water turbine, interfering with water barrier by stop log, and generating harmful gas (ammonia) due to the death of fouling organisms during Overhaul.For this reason, Sihwa Tidal Power Plant has been researching various fouling organism reduction technologies for the past 8 years, through this, we have increased tidal power generation and operated and maintained tidal power generation facilities smoothly.This paper presents an analysis of marine biofouling reduction technology of Sihwa Tidal Power Plant. It shows the status and characteristics of fouling organisms, problems and impacts, improvement plans and effects of the Sihwa Tidal Power Plant.
Wave Spectral Analysis for designing Wave Energy Converters
Authors: Portilla-Yandun Jesus , Guachamin-Acero Wilson
Published: September 2023

When designing a Wave Energy Converter (WEC) it is inappropriate to consider overall bulk parameters, like for instance, the typical scatter diagrams Hs-Tp. The reason is that in most of the world wave conditions are complex and involve several long-term wave systems, each with different characteristics (Fig. 1a). This is true even for apparently simple basins like the Mediterranean or the North Sea, in the open ocean this is certainly the case with swells arriving from remote places. Naturally, each Wave System (WS) has different characteristics, like their seasonal variability, typical wave height, but most importantly, the wave period and the spectral width are fundamental design parameters. As WECs must resonate on the exciting force to effectively convert energy, that resonance depends on the wave period. On the other hand, the spectral width indicates how disperse is energy in frequency and direction, the broader the spectrum (more entropy) the more difficult to tap its energy. Therefore, a smart WEC’s design cannot aim to the full spectrum, but to the particular WS with the more favorable characteristics (high energy, narrow spectral band, low frequency). For the present application we focus on a site in the Galapagos Islands. Apart from the aforementioned spectral conditions, this tropical site offers a couple of additional advantages, namely a small seasonal variability, and the absence of high extremes (to be withstood by the device even if idle), with still an interesting average resource (~20 kW/m). At this site we find three main WSs (see Fig. 1, a and b). WS1 belongs to the southerly swells, from the Antarctic extra-tropical storms. WS2 corresponds to swells from the northern hemisphere. WS3 is due to the southern trade-winds, active locally in the area (wind-sea). Clearly, the target for energy conversion is WS1, but for a non-directional device (e.g., point absorber), WS2 is also interesting due to the similar peak frequency. In addition, WS1 and WS2 are seasonally complementary, as they are mainly active in the austral and boreal winters respectively. The identification and separation of these different WSs allows us to determine their characteristics and to obtain specific design parameters. The modulation strategy for the WEC design is the subject of a parallel paper.

Fig. 1. a) (left) Spectral wave conditions in the Galapagos Islands. WS1: Antarctic swells, WS2: northern swells, WS3: southern trade-winds, WS4: Central America wind jets. The colorbar indicates number of occurrences within 1979-2015. b) (right) Average one dimensional spectra for the WSs in panel a. Data obtained from GLOSWAC: https://modemat.epn.edu.ec/nereo/.
Tuning Wave Energy Converters to local wave conditions
Authors: Guachamin-Acero Wilson , Portilla-Yandun Jesus
Published: September 2023

A Wave Energy Converter (WEC) cannot operate efficiently if left alone floating to the mercy of the random sea surface. Moreover, high waves may impose destructive forces while contributing nil energy production in return. For a WEC to be efficient, its natural frequency should be close to the peak frequency of a wave spectrum. At the resonance or another lower natural frequency, dynamic responses are more regular, and thus, the power take-off system (PTO) can operate in a more efficient manner. This paper introduces a general systematic methodology for tuning the natural frequency of a resonant WEC for the desired degree of freedom and converting dynamic responses into power. A barge is used as a case study to tune its roll and heave natural period, which in general are much lower than the peak period of swells. The roll natural period is increased by hanging subsea structures to the sides of the vessel by means of mooring lines, see Fig. 1. The size of the structures and their entrapped water increase the roll mass moment of inertia to achieve the desired period. For the heave response, the natural period raises by adding skirts to the sides of the vessel, increasing the entrapped water. The resonant motions are significantly suppressed by means of the drag force acting on the subsea structures. Thus, the extreme drag forces acting on the structures act as a damper to the vessel response. These forces are transferred to the mooring lines, which can be converted into a rotary motion of a shaft of the PTO. Our application focuses on the Galapagos Islands, where narrow, low-frequency southerly swells with a typical 13 s peak period dominate the wave climate. This study allows for a systematic design of resonant-type WECs, which is necessary for the development of technical and economic feasibility studies of various novel and existing concepts.

Figure 1: Schematic representation of resonant WEC with tuned roll natural period
Multi-Actuator Full-Scale Fatigue Test of a Tidal Blade
Authors: Lopez Dubon Sergio , Vogel Christopher , Garcia Cava David , Cuthill Fergus , McCarthy Eddie , O Bradaigh Conchur
Published: September 2023

Fatigue testing for tidal turbine blades involves the application of cyclic loads without matching the blade's natural frequency, which is challenging due to their high stiffness and associated thermal issues of composite materials at those frequencies (typically around 18Hz cycles). An auxiliary system is required to load the blades to address this challenge. However, traditional hydraulic systems tend to be highly energy-demanding and inefficient.

To solve this problem, researchers utilized real on-site data to define a series of equivalent target loads and implemented them in FastBlade, which proved an efficient way to perform fatigue testing. They used a regenerative digital displacement hydraulic pump system and achieved a remarkable 75% energy savings compared to a standard hydraulic system. During the testing, they utilized a system of 3 actuators instead of the traditional single actuator system, which produced more realistic and complex loads. We also address such changes in temperature along large composite structures during the test and mechanisms to address these issues. 

Throughout the test, a series of measurements were taken on the blade response and FastBlade itself, which revealed exciting results on the mechanical behaviour of the blade and best testing practices for FastBlade. Impressively, the blade withstood 40 years' worth of accelerated fatigue loading without catastrophic failure.
Performance Enhancement of Fluidic Diode for a Wave Energy System through Genetic Algorithm
Authors: Kerikous Emeel , Hithaish Doddamani , Samad Abdus , Hoerner Stefan , Thévenin Dominique
Published: September 2023

The oscillating water column (OWC) is an extensively studied wave energy converter that produces pneumatic power from the motion of the sea waves, which can be harvested using a pair of turbines without additional devices. However, its efficiency is hampered by poor flow blockage. Researchers have proposed a fluidic diode (FD) to improve flow blockage. Its performance is given by diodicity, which is the ratio of pressure drop in reverse to forward flow. A higher resistance in the reverse path signifies enhanced flow blockage, while a lower resistance in the forward flow minimises power loss at the turbine entry. In the present study, the numerical investigation was performed by solving three-dimensional unsteady Reynolds-Averaged Navier Stokes equations using ANSYS-Fluent 16.1 to simulate the flow behaviour inside the FD. Five geometrical parameters for FD were varied to obtain its optimal shape leading to a lower pressure drop in the forward direction and higher in reverse. The optimal shape was obtained through the genetic algorithm, showing a 12% improvement in performance compared to the base model. Detailed fluid flow and performance analysis of both base and optimum models are presented in this article.
Experimental passive and reactive control of a Laboratory Scale WEC Point Absorber
Authors: Bosma Bret , Beringer Courtney , Robertson Bryson
Published: September 2023

Scaled testing is an important and valuable step in the process of determining best practices of WEC development, validating numerical models of WEC systems, and/or preparing for larger scale testing. Validation and verification of numerical models are very important as there are physical phenomena that are hard to numerically model such as non-linear frictional effects. This paper builds on the 2021 EWTEC paper [1] in performing and evaluating experimental testing of the Laboratory Upgrade Point Absorber (LUPA). Since this last paper, LUPA has been fabricated and deployed and an initial characterization has been performed. Particularly passive (damping), and reactive (damping and stiffness) control methodologies are employed in regular waves to characterize and evaluate the mechanical power extracted from the waves. Reactive control allows us to invest energyin the system to get a greater average energy out as compared to damping control.

The LUPA project has just finished its first deployment in the Large Wave Flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University as shown in Figure 1. Three modes of operation were tested, namely one body heave only, two body heave only, and six degrees of freedom. The one body heave only mode restricts the motion to linear and vertical and fixes the spar body so that the float is the only body free to move. The two body heave only maintains the vertical linear motion, but unlocks the float such that the float and spar are free to heave. The six degrees of freedom case is a floating moored mode with no restrictions on motion.

This paper will focus on comparing passive and reactive control for the one body heave only case and the six degreesof freedom case with preliminary results shown in Figure 2. Regular waves were tested, focusing on a single wave heightand sweeping the wave period. Results will be presented in power (W) units and in capture width (m). Fig. 2. Top shows max power output vs input wave period for one body and two body configurations and just damping and damping and stiffness cases.Take note of differing input wave height. Bottom shows capture width.

The LUPA project will provide a valuable testing platform for students and researchers. It will also provide a publicly available open source design and dataset for the research community here: https://github.com/PMECOSU/LUPA. This paper will serve as documentation of the initial testing of the system, providing baseline control results to be compared in future testing.
Enhancing the efficiency of an axial impulse turbine with a diffuser
Authors: Saha Geetam , Ferreira Diogo Neves , Srikanth Narasimalu , Zuo Lei
Published: September 2023

In oscillating water column wave energy converters, the aerodynamic losses caused by flow misalignment between the rotor outlet and the outlet guide vane of self-rectifying impulse turbines are a significant design problem. The symmetrical positioning of guiding vanes on both sides of the rotor is the reason for it. In comparison to the equivalent Wells turbine, the efficiency of impulse turbines is lower because of these losses in the outlet guide vane. This paper presents a strategy to develop animproved design of an axial impulse turbine to reduce the losses in the outlet guide vane along with the residual kinetic energy loss by integrating a diffuser between the rotor and guide vanes. The proposed model was investigated numerically using RANS simulations. Multiple diffuser geometries were tested against the reference turbine for comparison of the performance characteristics. The results support the hypothesis behind the proposed design. The performance was compared extensively with the reference case in terms of the dimensionless loss coefficients for a better insight into the contribution of all the turbine sectors. The current work shows a possible design path for the performance improvement of the turbine. Results also support the fact that the guide vanes need to beredesigned for obtaining maximizing efficiency with the proposed design. The losses in the outlet guide vane were reduced by approximately 50% with an overall efficiency rise of around 2%.
Numerical and experimental studies of the effects of WEC motion on a combined wind-wave energy platform
Authors: Koo WeonCheol , Kim Hong-Bhin
Published: September 2023

The development of eco-friendly carbon-free marine energy is actively underway to alleviate the impact of global climate change due to excessive emission of carbon dioxide. To reduce the variability of energy production according to marine environment conditions and maximize energy production, the need for research on a suitable ocean energy platform is increasing. Therefore, various studies on motion control of ocean energy platforms are being conducted.

In this study, the motion response of a combined wind-wave energy platform equipped with multiple wave energy converters (WEC) capable of controlling the motion of a floating offshore wind turbine (FOWT) was numerically analysed and verified through a wave tank experiment. The combined energy platform with multiple WECs attached to the FOWT not only produces stable energy by controlling the motion response of the FOWT with the movement of the WEC, but also produces wave energy by the relative motion of the WEC.

The FOWT is a spar platform with taut moorings, and WEC is a moving cylinder hinged to the FOWT. The motion reduction effect of the FOWT by the movement of the WEC was investigated. Numerical analysis used the ANSYS AQWA program, which is a potential flow-based hydrodynamic program. To consider the restraining force of the FOWT due to the motion of the WEC, the multi-body dynamics theory was applied. The accuracy of the numerical model was improved by applying mooring dynamics, weakly nonlinear Froude-Krylov force, and nonlinear hydrostatic force. The experimental model was fabricated on a scale of 1/100 of the numerical model and was performed in a two-dimensional mini wave tank. To prevent the sidewall effect of the tank, the experiment was conducted only for the incident waves with various periods under the head sea conditions.

The numerical analysis results were in good agreement with the experimental results. In particular, the pitch of FOWT decreased significantly near the pitch resonance period because of the motion of WEC. The change in motion characteristics of the combined energy platform according to the movement of the WEC under various wave conditions was analysed.
comparative study of power production using a generic empirical model in a tidal farm
Authors: Shariff Kabir Bashir , Guillou Sylvain
Published: September 2023

This study presents a generic model for estimating the velocity deficit and turbulence intensity ina tidal turbine farm. The proposed model considers a range of ambient turbulence intensity, the rotor diameter-to-depth ratio, and the rotor thrust coefficient in realistic applications. We evaluate the power generation of a large-scale tidal farm composed of 16 turbines in an in-line and staggered configuration in an ideal channel similar to the Alderney Race in the English Channel. The added turbulence effect is taken into account when assessing the velocity deficit in the farm. As supported by previous studies, the results show that the staggered array produces more power than the rectilinear array. The staggered arrangement benefits from flow acceleration and wide turbine spacing, which improves wake recovery. According to the results, the farm can be resized by decreasing the lateral spacing in the rectilinear array and decreasing the longitudinal spacing in the staggered array without affecting the farm’s efficiency. The reduction in farm size will reduce cable costs and provide an opportunity for future expansion. For the tidal turbines in shallow water regions, the ratio of rotor diameter to depth is shown to affect the power generated by the turbines. The power produced in the farm decreases with an increase in the rotor diameter-to-depth ratio due to the limited wake expansion along the vertical plane. This low-computational model can be useful in studying the wake interaction of tidal turbine parks in different configurations.
On the errors in annual energy yield estimation due to monodirectional wave spectra assumption
Authors: Cervelli Giulia , Giorgi Giuseppe
Published: September 2023

The wave energy sector has made significant progress, and its potential is tangible. However, in order to make wave energy production a key tool in the energy transition, it is crucial to minimize the uncertainty associated with the energy production assessment. Trustworthy evaluations are reachable only through detailed analyses based on reliable, accurate and representative wave data.The use of synthetic parameters exclusively derived from a frequency spectrum leads to the neglect of directional information and the combination of wind waves and swells is treated incorrectly. This study investigates errors associated with relying solely on the frequency spectrum and points out the relevance of directionality.The island of Pantelleria serves as a case study, and the results show that PeWEC (Pendulum Wave Energy Converter) performance is overestimated when the monodirectionality of the waves is assumed. Neglecting the directional information contained in the frequency-direction spectra results in an overestimation of energy production of 40%.
A comparison of AC and DC collection grids for marine current energy
Authors: Fjellstedt Christoffer , Forslund Johan , Thomas Karin
Published: September 2023

Important questions to enable the use of marine current energy are how the electrical system is designed, how multiple energy converters are interconnected offshore and how the power is transmitted to the shore. The Division of Electricity at Uppsala University have constructed and deployed a marine current energy converter in the river Daläven in Söderfors, Sweden. In the study presented in this article, a model of a near-shore low-voltage AC collection grid and a near-shore low-voltage DC collection grid is presented for the technology at the Söderfors test site. The models are implemented in MATLAB/Simulink. For collection grids of five turbines, it is shown that the proposed control schemes are able to deliver power to the distribution grid. The controllers are able to achieve this even when one turbine is suddenly disconnected from the grid. Furthermore, it is shown that the conduction losses of the DC system are higher than the losses of the AC system for nominal and high water speeds. However, in a qualitative comparison between the systems it is concluded that despite the higher losses, the DC system can be an interesting option. This is because fewer components need to be placed in the turbine, which is beneficial in offshore systems where space is a limiting factor. Furthermore, a DC system can be less expensive since fewer cables are needed.
Numerical optimisation of the active lift turbines using OpenFoam's overset method
Authors: Robin Ilan , Mouazé Dominique , Bennis Anne-Claire , Carpentier Guillaume
Published: September 2023

This work presents a preliminary study of the numerical optimization of "active lift turbine" using computational fluid dynamics (CFD). This active lift turbine is designed to improve efficiency of vertical axis energy recovery turbines (wind and tidal turbines). It uses a crank rod system to convert normal forces into momentum in order to improve the yield.

After a validation work of CFD model in two dimensions, the numerical optimization of the tidal turbine system is carried out. In fact, due to the specific characteristics of the active lift turbine, the blades do not follow a circular trajectory (Fig. \ref{fig:combined}). The OpenFoam overset module is therefore chosen because it allows to control the motion of the blades easily \cite{Chalmers}. The Overset method allows a good performance evaluation and limits the risk of numerical divergence that could be caused by mesh deformation methods; Moreover it limits the computation time compared to remeshing methods. However, it is more expensive than a more traditional Arbitrary Mesh Interface method \cite{Openfoam}, which is not suitable for the active lift turbine simulations.

The impact of several parameters on the performances is studied. The simulations are performed for a range of operating conditions including different inflow velocities, angles of attack and active flow turbine settings (like the amplitude of the radius variation, see Fig \ref{fig:combined}). The results are evaluated in terms of the turbine power output and efficiency.

The results show the importance of numerical simulation for the development of new types of energy recovery systems. It allows a better understanding of the interactions between the turbine and the fluid, and of the operation of such systems.

Further studies are required for the active lift turbine: conventional attempts at optimisation are not perfectly suited to this system. Improving the lift/drag ratio makes less sense when the normal forces should also be taken into account to improves the performances. Furthermore, other aspects of operation should be analysed: variation of the turbulence rate, implementation of boundary layer re-attachment systems...
Critical Feature and Seawater Testing of Cross-Flow Rotor Components Fabricated with Additive Manufacturing
Authors: McVey James , Zaengle John , Cavagnaro Robert , Fenn Michelle , Lommers Brittnee , Rumple Chris
Published: September 2023

Cross-flow tidal turbines are an attractive option for powering remote or off-grid applications because of their simplicity as compared to axial-flow turbines. For instance, when oriented vertically, they harvest power from any current direction with a single degree of freedom and no yaw mechanism. Additive manufacturing (AM) offers an opportunity to print parts out of a wide variety of materials that can result in components that are lighter, stronger and/or less expensive to produce than with traditional manufacturing techniques. When coupled with cross-flow turbine rotors, which require critical features (blade-strut, strut-shaft connections) to be both structurally stiff and hydrodynamically shaped, which can be challenging for typical fabrication processes, AM offers the ability to do both well. This paper describes work on the feasibility of using advanced AM techniques to fabricate small cross-flow turbine rotors for applications at sea and near remote coastal communities.

AM materials were categorized into 3 classes – plastics, metals, and ceramics – and reviewed for suitability based on a set of engineering requirements and criteria related to turbine characteristics, material properties, and AM process capabilities. Two plastics and two metals were selected to undergo further testing: Essentium CF25, CarbonX Ult 9085, Titanium Ti-6Al-4V, and Inconel 718. Testing is conducted in three phases: the first is a long-term, 5-month submersion test in the seawater tanks at PNNL-Sequim to study corrosion, water uptake, and biofouling potential; in the second, materials are tensile tested on a load frame to find their failure parameters to compare to material standards; the third test is a fatigue test consisting of cyclically loading test parts with a known force on the order of that exerted on rotor blades in a 1.5 m/s current flow. These tests are designed to discern the suitability of AM materials since their properties from 3D printing processes are known to vary from published parameters. The test samples undergoing submersion testing will be tension tested and compared to control samples not subjected to extended seawater immersion. For fatigue life testing, a small rotor is expected to complete 100 million cycles over the course of a year-long lifespan, but for the case herein is restricted to 1 million for a preliminary performance evaluation. The first 10k cycles are run on an MTS 312.21 load frame at a rate of 0.2 Hz, with the remaining on a custom-built cyclic-deflection test rig at 0.8 Hz.
Experimental Investigation into the Air Compressibility Scaling Effect on OWC Performance and Wave Height
Authors: Cohen Nadav , Flocard Francois , Turner Ian L. , Sergiienko Nataliia , Cazzolato Ben
Published: September 2023

Wave energy converter arrays have the potential to provide coastal protection in addition to generating power from incoming waves. As part of a wider experimental study to investigate dual-use applications, this paper presents the results of wave flume testing conducted to analyse the performance characteristics of a single, generic Oscillating Water Column (OWC) device, in preparation for the next phase of study that will focus on multi-device arrays. The specific focus of this flume testing was to characterise parameters such as Response Amplitude Operator (RAO), Capture Width Ratio (CWR) and Phase Response, as well as the device’s effect on the local wave field.
A potential limitation when using scaled experimental results for OWCs are the differing scaling factors that should be applied to the device’s submerged volume (λ³) and air volume (λ²) which, together with the Power Take Off (PTO) damping, can greatly affect the air-spring stiffness experienced within the OWC.
A subset of 34 tests (out of a total 134) were conducted under monochromatic wave conditions with wave heights of 20 mm or 40 mm and wave periods ranging from 1.2 s to 2.2 s. In these tests the PTO damping was varied by adjusting the orifice diameter, while the air volume was varied via an adjustable auxiliary air chamber. Results show that for the smaller orifice diameters (i.e., higher damping) tested, air-spring stiffness played a significant role and counterintuitively increased with increased air volume.
Effects of the single OWC device on the wave field within the flume were also investigated. Results revealed that while there is a marked difference when comparing the OWC to an identically-shaped blockage, there was no significant measurable difference in the wave height change observed for all the damping and air volume parameter settings that were modelled, despite a general trend when comparing to the empty flume.
Objective Functions for the Blade Shape Optimisation of a Cross-Flow Tidal Turbine under Constraints
Authors: Ruiz-Hussmann Karla , Delafin Pierre-Luc , Bonamy Cyrille , Delannoy Yves , Thévenin Dominique , Hoerner Stefan
Published: September 2023

Hydro-kinetic cross-flow tidal turbines (CFTT) are omni-directional and offer higher area-based power density compared to horizontal-axis tidal turbines, making them very attractive for tidal energy exploitation. However, the rotating motion around the vertical axis results in continuously varying angles of attack, causing alternating loads, which may lead to fatigue failure and structural damage. The OPTIDE Project addresses these challenges by implementing intracycle blade pitching to individually control the angle of attack, increasing the power coefficient CP and reducing structural loads. For this purpose a Darrieus turbine is designed with embedded actuators in each blade. Firstly a blade shape optimization will be conducted to fit the actuator at the quarter-chord position while ensuring sufficient thickness. The optimization procedure couples Computational Fluid Dynamics (CFD) with a Genetic Algorithm. The employed optimizer OPAL++ sets ten variables for each individual, which describe the hydrofoil shape, length and tip speed ratio (TSR). A smooth hydrofoil shape is generated from the variables, followed by an automatic mesh generation. Subsequently, numerical simulations of each individual at the desired TSR are conducted, while keeping the blade pitch angle constant. Simulation results provide the CP and stress acting on the turbine blades, which are the two optimization objectives (maximize CP while minimizing stress). This process is repeated during the optimization, aiming to determine the most suitable blade shape, that fits the actuator, and operating point (TSR) in a trade-off between CP and structural loads. This will lead to the increase of efficiency and a longer turbine lifetime.
Numerical performance assessment of a new wave energy conversion system
Authors: Governo André F. L. , M.C.S. André José , Henriques João C. C. , Gato Luís M. C.
Published: September 2023

The greater wave energy content in deeper waters has initiated substantial research in offshore wave energy converters (WECs). However, the existing WECs are hindered by their high initial costs, primarily attributed to the demanding mechanical workloads they endure. This paper describes a novel wave energy conversion system developed at IST for deep waters (around 20-50 meters) based on a floater and a ballast, suspended by a set of cables. In the novel approach, the WEC is made up essentially by membranes, without a rigid structure, thus reducing the initial cost of investment comparing to other solutions.A simplified numerical model in the time-domain of the system based on linear theory is developed and a preliminary performance assessment under regular and irregular wave conditions (for conditions prevalent along the western coast of Portugal) is performed. Additionally, the effect of the ballast mass is considered. The results reveal the system's non-linear behavior during the floater's ascending phase and emphasize the significant influence of the ballast's mass on power production.
Review of TEAMER Awards for WEC-Sim Support
Authors: Keester Adam , Ogden David , Husain Salman , Ruehl Kelley , Pan Jack , Leon Quiroga Jorge Andres , Grasberger Jeff , Forbush Dominic , Tom Nathan , Housner Stein , Tran Toan
Published: September 2023

Testing Expertise and Access for Marine Energy Research (TEAMER) is a U.S. Department of Energy Water Power Technologies Office sponsored program, overseen by the Pacific Ocean Energy Trust, which aims to advance the state of marine energy technologies. The program connects technology developers with experts at U.S. facilities, including numerical modeling and analysis facilities, like WEC-Sim. The WEC-Sim facility is supported by the WEC-Sim development team at Sandia National Laboratories and the National Renewable Energy Laboratory. WEC-Sim (Wave Energy Converter SIMulator) is an open-source software for simulating wave energy converters. WEC-Sim can model the multi-body dynamics of devices comprised of bodies, joints, power take-off systems, and mooring systems. Since TEAMER’s first round of support in 2020, there have been eighteen TEAMER awards focused on numerical model development in WEC-Sim. TEAMER awards for WEC-Sim support have modeled a wide range of wave energy converter archetypes, including point absorbers, attenuators, oscillating water columns, and many other novel architectures. A wide variety of studies have been conducted, leading to important insights for TEAMER partners and software improvements for WECSim. This article highlights several successful WEC-Sim TEAMER awards. The awards described herein include TEAMER recipients Ocean Motion Technologies, AquaHarmonics, iProTech, East Carolina University, Virginia Tech, Maiden Wave Energy, and the University of Massachusetts Dartmouth. The awards of these seven partners contain a wide range of investigations and cover the creation of baseline hydrodynamic models, PTO modeling, geometry optimization in both boundary element methods and WECSim, and model tuning and validation.
Informing Early Design Decisions Through Functional Analysis of Maintenance Drivers
Authors: Algarra Nathan , Dong Andy
Published: September 2023

Operational expenditures dominate the cost of long field-life systems, with maintenance comprising a significant proportion for many systems. However, engineers lack the tools to assess maintenance during conceptual design. Familiar systems mitigate this problem by providing historical maintenance data from which empirical models can be derived. Emerging technologies, like marine renewables, lack operational maintenance data. As a result, engineers must make decisions with no historical data and minimal, if any, operational experience. The high operations cost incurred from basic maintenance tasks, and the loss of energy production during maintenance, further highlight maintenance as a critical cost driver. This paper develops a data-driven model to estimate maintenance intervals of long field-life systems during conceptual design. The model links the elementary functions of a component to maintenance requirements. Relative maintenance considerations were determined by mining function and maintenance data from manuals of long field-life systems. Machine learning was applied to generate a function-maintenance model from the maintenance data. The model consisted of functions grouped into buckets of increasing maintenance demand. The machine learning model was applied to an exemplary long field-life system, a wave energy converter, to explore possible redesigns to reduce maintenance costs. This paper shows that maintenance costs, actions and intervals can be confidently accounted. The function-maintenance model offers two beneficial impacts: it reduces life cycle cost uncertainty, and allows engineers to make informed decisions during conceptual design when redesign costs are at their lowest.
Optimal Design of a Submerged Tidal Device for Low Current Environment
Authors: Jo Chul-hee , Jeong Seung-won , Hwang Su-jin , Cho Sung-ho
Published: September 2023

The tidal current power is one of the reliable renewable ocean energy resources that can be deployed at the region having ocean current. The feasible current speed for the application of tidal device is known to be more than 2m/s. As there are many islands in Asia where the current speed is below than the feasible speed, the submerged device that can produce the power at lower current speed with easy deployment and retrieve is introduce. To simplify the installation and O&M operations, the 4-point taut mooring system is applied that can also secure the designated position. In this study, a HAT (horizontal axis turbine) of 5-kW submerged tidal device is designed and analyzed for a low current region that consist of two buoyancy tanks, a yaw control strut, and a diffuser-type brimmed duct. To analyze the most stable condition, 9 different cases of width and height of the device and 8 different mooring arrangements (total 72 different cases) are investigated both by frequency and time domains. The results show that the dynamic behavior that mainly affects the turbine performance decreases with lower strut height. However, it was observed that the device width has little effect on the dynamic response of the device. The device dynamic motion decreases with decreasing azimuth angle and increasing hang-off angle of the mooring configuration. The optimal design configuration that can produce the maximum power is found that of 4.42 m width, 2 m height, 30° azimuth angle and 60° hang-off angle. According to API RP 2K code, the mooring system is also designed in both intact and damaged conditions. The proposed tidal device could be applied for the low current speed areas with easy implementation and maintenance.
Material Characterization of Elastomeric Bearing Elements in Wave Energy Converters
Authors: Duraisamy Rimmie , Andersson Kjell
Published: September 2023

According to the European Council, 75% of the greenhouse emissions are caused by energy production and use. In order to decarbonize the energy sector, more research is focused towards the renewable energy sources. Among the many available resources, wave energy is one of the key resources to consider. As part of the research, KTH is developing a winch based point absorber wave energy converter. The winch consists of a chain link with elastomer bearings wound around a drum. The winding and unwinding motion of chain converts wave motion into electricity. A typical winch based wave energy converter undergoes around 80 million cycles in its lifetime. This calls for a durable system design requiring minimal service and maintenance. With an elastomer bearing, the winding and unwinding of the chain over the drum is realized as deformation of the elastomer thereby eliminating sliding. While the use of elastomer enables an efficient design, it also makes it more challenging due to its highly non-linear and viscoelastic behavior. A constitutive model is necessary to determine material characteristics of an elastomer for different loading conditions. In this work, a systematic design process is outlined and an attempt is made to determine a suitable hyperelastic material model for the elastomer. The study is focused on two materials – silicone and polyurethane. The test samples are compressed and followed by shear in deformation. For material model determination, the test data is curve fit and later verified using finite element method. The material is assumed incompressible.
Parametric resonance: a risk to be avoided or an opportunity to be exploited? A case for a 2:1 wave energy converter
Authors: Giorgi Giuseppe
Published: September 2023

Accurate models for wave energy converters are paramount for appropriate and effective assessment of the system behaviour under operational conditions; if such models are also computationally efficient, they can be used for control and design applications. Crucially, the underlying design and working principle can be substantially modified only at early development stages and typically based on fast mathematical models with several iterations; since such models are often linear or weakly-nonlinear, complex nonlinear phenomena are rarely embedded into holistic design tools or optimization schemes. A secondary consequence is that nonlinearities are dealt with at a later verification and assessment stage and are typically a burden or an issue to limit. The aim of this paper is to highlight how nonlinearities can actually be a beneficial resource to be exploited and leveraged, rather than a bother. To do so, a computationally efficient nonlinear model is used, able to articulate relevant phenomena at a convenient computational time. The system under analysis is a prismatic floating wave energy converter and the nonlinear model implements an analytical formulation of Froude-Krylov forces computed on the instantaneous wetted surface; this modelling approach is able to articulate a 2:1 parametric resonance which, when activated under certain conditions, is a type of instability able to amplify the amplitude of motion. Note that the 2:1 proportionality refers to the ratio between wave and natural frequencies of the system. Therefore, this paper embeds parametric resonance into the notional design of a wave energy converter purposely designed to experience and take advantage of such an instability. Results are promising, since a substantial amplification is achieved in the 2:1 region, whereas similar or higher oscillation amplitude is obtained in the 1:1 region.
Maximizing the surge amplitude of a floater through an adaptable mooring tightening technique
Authors: Asiikkis Andreas , Grigoriadis Dimokratis , Vakis Antonis
Published: September 2023

Abstract

A technique to optimize the response of Wave Energy Converters (WECs) by maximizing the amplitude of motion along the surge direction is presented. This is achieved by utilizing an adaptable mooring tightening technique for a floater moored with tension legs (tendons). To gain a deeper understanding of the effect of sea states and mooring cable lengths on the surge response of the system, a series of numerical simulations were conducted for various wave conditions while varying the length of the mooring cables. WEC-Sim [1] was used to solve the multi-body dynamics of a rectangular cuboid floater by solving the equations of motion using a time-domain formulation. The dynamics of the mooring cables were simulated using the MoorDyn model with a lumped-mass formulation [2]. To validate the accuracy of the numerical methods, a series of experimental tests were performed in a small-scale wave tank. It was observed that tightening the mooring cables by decreasing their length amplifies the surge motion of the floater while the mooring forces in the heave direction rise due to the increased tension in the cables. Stretching the cables further was found to (i) decrease the surge amplitude and (ii) drastically increase the mooring forces, threatening the integrity of the cables. Therefore, there is an optimum value of the length of the cables that maximizes the surge amplitude of the floater while ensuring that the cables will not break. The impact of other mooring cable parameters such as diameter and material properties were also evaluated. More specifically, increasing the stiffness by increasing the diameter or the tensile modulus of elasticity was found to reduce the floater’s surge amplitude. For the geometry used in this study, the optimum length, diameter and properties of the cables are provided for several sea states. The current results lay the foundations for the design of new types of WECs that harness the surge motion of a floater rather than the heave which is the most common approach for floating WECs.

References

[1]: Kelley Ruehl, David Ogden, Yi-Hsiang Yu, Adam Keester, Nathan Tom, Dominic Forbush, Jorge Leon, Jeff Grasberger, and Salman Husain. (2022, September), WEC-Sim (Version v5.0.1), DOI: 10.5281/zenodo.7121186.

[2]: Hall, M., & Goupee, A. (2015). Validation of a lumped-mass mooring line model with DeepCwind semisubmersible model test data. Ocean Engineering, 104, 590–603. https://doi.org/10.1016/j.oceaneng.2015.05.035
A new type of wave tank: prototype and proof of concept
Authors: Berque Joannes , Zarate I`ñigo , Blanco Jesús , Bidaguren Iñigo , Touzon Imanol , Fernández Luisa
Published: September 2023

The prototype and proof of concept of a new type of wave tank are described. If successful, this innovation would enable many wave tank experiments for only a few percent of current costs. Tank testing remains indispensable for new designs, and its high cost is a significant inhibitor of offshore innovation. To date, all wave tanks generate and propagate scaled but real water waves towards a physical model. This new type of tank instead reproduces the velocity field that such waves would have, but only near the model. In practice, this can be approximately achieved only for cases where the marine structure is much shorter than the wavelength, and its draft shallow compared to the wavelength and depth. In such cases the wavefield is time-varying but approximately spatially uniform near the structure. Therefore, it can be reproduced by moving a container of water like wave orbitals; and ensuring slosh modes are minimised. A prototype was built, and after initial challenges in controlling slosh excitation, a proof of concept was successfully achieved: a cylinder was excited by surge oscillations and its response appears consistent with rough calculations. Range of potential applications of the new type of wave tank, lessons learnt by prototyping, and next developments are discussed.