| 1. |
- Engström, Jens, et al.
(author)
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Buoy geometry and its influence on survivability for apoint absorbing wave energy converter : Scaleexperiment and CFD simulations
- 2017
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Conference paper (peer-reviewed)abstract
- For wave energy to be an economically viable energysource, the technology has to withstand power levelsduring storms that can be close to 50 times higher thanduring normal operating conditions, and withstandmany years of wear. The impact of high wave loads isstudied not only within the field of wave energy, buthas long been a subject of study for ships, platformsand other offshore structures.To model the force on the device under extreme and/orovertopping waves is a difficult task. Experiments areexpensive and difficult to implement, and numerical meth-ods are either very computationally demanding CFD-methods, or less accurate approximative methods. Inaddition, the performance and experienced forces during extreme waves are model dependent, and differentoffshore structures must be studied independently.Here, a 1:20 scale model of the Uppsala Universitypoint-absorber type wave energy converter (WEC) has been tested in extreme wave conditions at the COASTLaboratory Ocean Basin at Plymouth University. The WEC consists of a linear generator connected to a buoyat the sea surface, and performance of two differentbuoys is studied: a cylinder and cylinder with moon-pool. Two types of wave sets have been used: focusedwaves embedded into regular waves, and irregular waves. The focus of this paperis on comparing the performance of the two buoys, and on analysing the experimental data using a numerical model. A fully non-linear computational fluid dynamics(CFD) model based on OpenFOAM is presented and validated.
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| 2. |
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| 3. |
- Katsidoniotaki, Eirini, et al.
(author)
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Loads on a point-absorber wave energy converter in regular and focused extreme wave events
- 2020
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In: Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE. - : ASME Press. ; 9
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Conference paper (peer-reviewed)abstract
- Accurate modeling and prediction of extreme loads for survivability is of crucial importance if wave energy is to become commercially viable. The fundamental differences in scale and dynamics from traditional offshore structures, as well as the fact that wave energy has not converged around one or a few technologies, implies that it is still an open question how the extreme loads should be modeled. In recent years, several methods to model wave energy converters in extreme waves have been developed, but it is not yet clear how the different methods compare. The purpose of this work is the comparison of two widely used approaches when studying the response of a point-absorber wave energy converter in extreme waves, using the open-source CFD software OpenFOAM. The equivalent design-waves are generated both as equivalent regular waves and as focused waves defined using NewWave theory. Our results show that the different extreme wave modeling methods produce different dynamics and extreme forces acting on the system. It is concluded that for the investigation of point-absorber response in extreme wave conditions, the wave train dynamics and the motion history of the buoy are of high importance for the resulting buoy response and mooring forces.
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| 4. |
- Kramer, Morten, et al.
(author)
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Highly accurate experimental heave decay tests with a floating sphere : A public benchmark dataset for model validation of fluid–structure interaction
- 2021
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In: Energies. - : MDPI AG. - 1996-1073. ; 14:2
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Journal article (peer-reviewed)abstract
- Highly accurate and precise heave decay tests on a sphere with a diameter of 300 mm were completed in a meticulously designed test setup in the wave basin in the Ocean and Coastal Engineering Laboratory at Aalborg University, Denmark. The tests were dedicated to providing a rigorous benchmark dataset for numerical model validation. The sphere was ballasted to half submergence, thereby floating with the waterline at the equator when at rest in calm water. Heave decay tests were conducted, wherein the sphere was held stationary and dropped from three drop heights: a small drop height, which can be considered a linear case, a moderately nonlinear case, and a highly nonlinear case with a drop height from a position where the whole sphere was initially above the water. The precision of the heave decay time series was calculated from random and systematic standard uncertainties. At a 95% confidence level, uncertainties were found to be very low—on average only about 0.3% of the respective drop heights. Physical parameters of the test setup and associated uncertainties were quantified. A test case was formulated that closely represents the physical tests, enabling the reader to do his/her own numerical tests. The paper includes a comparison of the physical test results to the results from several independent numerical models based on linear potential flow, fully nonlinear potential flow, and the Reynolds-averaged Navier–Stokes (RANS) equations. A high correlation between physical and numerical test results is shown. The physical test results are very suitable for numerical model validation and are public as a benchmark dataset. © 2021 by the authors.
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| 5. |
- Sjökvist, Linnea, et al.
(author)
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Numerical models for the motion and forces of point-absorbing wave energy converters in extreme waves
- 2017
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In: Ocean Engineering. - : Elsevier BV. - 0029-8018 .- 1873-5258. ; 145, s. 1-14
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Journal article (peer-reviewed)abstract
- Reliable simulation tools are necessary to study the performance and survivability of wave energy devices, since experiments are both expensive and difficult to implement. In particular, survivability in nonlinear, high waves is one of the largest challenges for wave energy, and since the wave loads and dynamics are largely model dependent, each device must be studied separately with validated tools. In this paper, two numerical methods based on fully nonlinear computational fluid dynamics (CFD) are presented and compared with a simpler linear method. All three methods are compared and validated against experimental data for a point-absorbing wave energy converter in nonlinear, high waves. The wave energy converter consists of a floating buoy attached to a linear generator situated on the seabed. The line forces and motion of the buoy are studied, and computational cost and accuracy are compared and discussed. Whereas the simpler linear method is very fast, its accuracy is not sufficient in high and extreme waves, where instead the computationally costly CFD methods are required. The OpenFOAM model showed the highest accuracy, but also a higher computational cost than the ANSYS Fluent model.
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| 6. |
- Thomas, Simon, et al.
(author)
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A model free control based on machine learning for energy converters in an array
- 2018
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In: Big Data and Cognitive Computing. - : MDPI. - 2504-2289. ; 4:2
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Journal article (peer-reviewed)abstract
- This paper introduces a model-free, "on-the-fly" learning control strategy for arrays of energy converters with adjustable generator damping. The devices are arranged so that they are affected simultaneously by the energy medium. Each device uses a different control strategy, of which at least one has to be the machine learning approach presented in this paper. During operation all energy converters record the absorbed power and control output; the machine learning device gets the data from the converter with the highest power absorption and so learns the best performing control strategy for each state. Consequently, the overall network has a better overall performance than each individual strategy. This concept is evaluated for wave energy converter (WEC) with numerical simulations and experiments with physical scale models in a wave tank. In the first of two numerical simulations, the learnable WEC works in an array with four WECs applying a constant damping factor. In the second simulation, two learnable WECs were learning with each other. It showed that in the first test the WEC was able to absorb as much as the best constant damping WEC, while in the second run it could absorb even slightly more. During the physical model test, the ANN showed its ability to select the better of two possible damping coefficients based on real world input data.
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| 7. |
- Thomas, Simon, et al.
(author)
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Performance of a Direct-Driven Wave Energy Point Absorber with High Inertia Rotatory Power Take-off
- 2018
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In: Energies. - : MDPI AG. - 1996-1073. ; 11:9
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Journal article (peer-reviewed)abstract
- An alternating rotatory generator using an eddy current break is developed as a physicalscale model of a direct-driven floating point absorber power take-off (PTO) for wave tank tests. It isshown that this design is a simple and cost-effective way to get an accurate linear damping PTO. Thedevice shows some beneficial characteristics, making it an interesting option for full scale devices:For similar weights the inertia can be significantly higher than for linear generators, allowing it tooperate with natural frequencies close to typical wave frequencies. The influence of the higher inertiaon the power absorption is tested using both a numerical simulation and physical wave tank tests.With the increased inertia the PTO is able to absorb more than double the energy of a comparabledirect-driven linear generator in some sea states. Moreover, the alternating rotatory generator allowsthe absorption characteristic to be tuned by changing the inertia and the generator damping.
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| 8. |
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| 9. |
- Wendt, Fabian, et al.
(author)
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Ocean energy systemswave energy modelling task: Modelling, verification and validation ofwave energy converters
- 2019
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In: Journal of Marine Science and Engineering. - : MDPI AG. - 2077-1312. ; 7:11
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Journal article (peer-reviewed)abstract
- The International Energy Agency Technology Collaboration Programme for Ocean Energy Systems (OES) initiated the OES Wave Energy Conversion Modelling Task, which focused on the verification and validation of numerical models for simulating wave energy converters (WECs). The long-term goal is to assess the accuracy of and establish confidence in the use of numerical models used in design as well as power performance assessment of WECs. To establish this confidence, the authors used different existing computational modelling tools to simulate given tasks to identify uncertainties related to simulation methodologies: (i) linear potential flow methods; (ii) weakly nonlinear Froude-Krylov methods; and (iii) fully nonlinear methods (fully nonlinear potential flow and Navier-Stokes models). This article summarizes the code-to-code task and code-to-experiment task that have been performed so far in this project, with a focus on investigating the impact of different levels of nonlinearities in the numerical models. Two different WECs were studied and simulated. The first was a heaving semi-submerged sphere, where free-decay tests and both regular and irregular wave cases were investigated in a code-to-code comparison. The second case was a heaving float corresponding to a physical model tested in a wave tank. We considered radiation, diffraction, and regular wave cases and compared quantities, such as the WEC motion, power output and hydrodynamic loading.
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