| 1. |
- Ayob, Mohd Nasir, et al.
(författare)
-
Small-Scale Renewable Energy Converters for Battery Charging
- 2018
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Ingår i: Journal of Marine Science and Engineering. - : MDPI. - 2077-1312. ; 6:1
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents two wave energy concepts for small-scale electricity generation. In the presented case, these concepts are installed on the buoy of a heaving, point-absorbing wave energy converter (WEC) for large scale electricity production. In the studied WEC, developed by Uppsala University, small-scale electricity generation in the buoy is needed to power a tidal compensating system designed to increase the performance of the WEC in areas with high tides. The two considered and modeled concepts are an oscillating water column (OWC) and a heaving point absorber. The results indicate that the OWC is too small for the task and does not produce enough energy. On the other hand, the results show that a hybrid system composed of a small heaving point absorber combined with a solar energy system would be able to provide a requested minimum power of around 37.7W on average year around. The WEC and solar panel complement each other, as the WEC produces enough energy by itself during wintertime (but not in the summer), while the solar panel produces enough energy in the summer (but not in the winter).
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| 2. |
- Crespo, Alejandro J., et al.
(författare)
-
On the state-of-the-art of CFD simulations for wave energy converters within the open-source numerical framework of DualSPHysics
- 2023
-
Ingår i: Proceedings of the 15th European Wave and Tidal Energy Conference, Bilbao, 3-7 September 2023. - : European Wave and Tidal Energy Conference.
-
Konferensbidrag (refereegranskat)abstract
- There are currently several types of devices capable of harnessing wave energy, exploiting a broad variety of physical transformation processes. These devices – known as Wave Energy Converters (WECs) – are developed to maximize their power output. However, there are still uncertainties about their response and survivability to loads induced by adverse environmental conditions, with a consequent increase of the Levelized Cost of Energy (LCOE), which prevents in fact their commercial diffusion. As evidenced by a large body of research, marine renewable energy devices need to have more robust design practices. To address this issue, we propose the CFD-based DualSPHysics toolbox as a support in the design stages. DualSPHysics is high-fidelity software inherently suited to numerically address most challenges posed by multiphysics simulations, which are required to reliably predict WEC response in situations well beyond operational conditions. It should be noted that WECs, generally, may be connected to the seabed and comprise mechanical systems named Power Take-Offs (PTO) used to convert the energy from waves into electricity or other usable energies. To reproduce these features, DualSPHysics benefits from coupling with the multiphysics library Project Chrono and the dynamic mooring model Moordyn+. In this work, the augmented DualSPHysics framework is utilised to simulate a range of very different types of WECs with a variety of elements, such as catenary connections, taut mooring lines, or linear and nonlinear PTO actuators. Version 5.2 of the open-source licensed code was recently released, making the numerical framework publicly available as one unit. This work aims to provide a numerical review of past applications, and to demonstrate how the same open-source code is able to simulate very different technologies.Specifically, this paper proposes routine modeling and validation procedures using the SPH-based solver DualSPHysics applied to five different WEC types: i) a moored point absorber (PA); ii) an oscillating wave surge converter (OWSC); iii) a floating OWSC (so called FOSWEC); iv) a wave energy hyperbaric converter (WEHC); and v) a multi-body attenuator (so called Multi-float M4). For each device listed above, we provide validation proof against physical model data for various components of the floater(s) and PTO related quantities, performed under specific sea conditions that aim to challenge their survivability. Within the scope of this research, we present the WEC response with respect to the degrees of freedom that really matter for each of the floatings due to hydrodynamic interactions (i.e., heave, surge, and pitch), along with quantities more intimately connected to the anchoring systems (e.g., line tension) or the mechanical apparatus (e.g., end-stopper force). The quality of the results, the discussion built upon them and the demonstrated solver exploitability to a wide range of WECs show that one software model can run all cases using the exact same methodology, which is of great value for the marine energy R&D community. Finally, we discuss future research objectives, which include the implementation of automation to apply open control systems and possible applications to subsets of WEC farm arrays and other floating energy harnessing devices.
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| 3. |
- Engström, Jens, et al.
(författare)
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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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Konferensbidrag (refereegranskat)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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| 4. |
- Engström, Jens, et al.
(författare)
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Energy absorption from parks of point-absorbing wave energy converters in the Swedish exclusive economic zone
- 2020
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Ingår i: Energy Science & Engineering. - : John Wiley & Sons. - 2050-0505. ; 8:1, s. 38-49
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Tidskriftsartikel (refereegranskat)abstract
- In a future energy system based on renewable energy sources, wave energy will most likely play a role due to its high energy potential and low intermittency. The power production from parks of wave energy converters of point absorber type has been extensively studied. This is also the case for the wave energy resource at many coastal areas around the globe. Wave energy has not yet reached a commercial level, and a large variety of technologies exist; therefore, an established method to calculate the technical potential for wave energy has still not been established. To estimate the technical potential of wave energy conversion, some approximations inevitably need to be taken due to the systems high complexity. In this study, a detailed mapping of the wave climate and simulation of large arrays of hydrodynamically cross‐coupled wave energy converters are combined to calculate the technical potential for wave energy conversion in the Swedish exclusive economic zone. A 16‐year wave data set distributed in a 1.1 km × 1.1 km grid is used to calculate the absorbed energy from a park of 200 generic point absorbers. The areas with best potential have an average annual energy absorption of 16 GWh for the selected wave energy park adapted to 1 km2 when using a constant damping, while the theoretical upper bound is 63 GWh for the same area.
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| 5. |
- Engström, Jens, et al.
(författare)
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Offshore Measurements and Numerical Validation of the Mooring Forces on a 1:5 Scale Buoy
- 2023
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Ingår i: Journal of Marine Science and Engineering. - : MDPI. - 2077-1312. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- Wave energy conversion is a renewable energy technology with a promising potential. Although it has been developed for more than 200 years, the technology is still far from mature. The survivability in extreme weather conditions is a key parameter halting its development. We present here results from two weeks of measurement with a force measurement buoy deployed at Uppsala University’s test site for wave energy research at the west coast of Sweden. The collected data have been used to investigate the reliability for two typical numerical wave energy converter models: one low fidelity model based on linear wave theory and one high fidelity Reynolds-Averaged Navier–Stokes model. The line force data is also analysed by extreme value theory using the peak-over-threshold method to study the statistical distribution of extreme forces and to predict the return period. The high fidelity model shows rather good agreement for the smaller waves, but overestimates the forces for larger waves, which can be attributed to uncertainties related to field measurements and numerical modelling uncertainties. The peak-over-threshold method gives a rather satisfying result for this data set. A significant deviation is observed in the measured force for sea states with the same significant wave height. This indicates that it will be difficult to calculate the force based on the significant wave height only, which points out the importance of more offshore experiments. © 2023 by the authors.
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| 6. |
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| 7. |
- Engström, Jens, et al.
(författare)
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Performance of large arrays of point absorbing direct-driven wave energy converters
- 2013
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 114:20, s. 204502-
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Tidskriftsartikel (refereegranskat)abstract
- Future commercial installation of wave energy plants using point absorber technology will require clusters of tens up to several hundred devices, in order to reach a viable electricity production. Interconnected devices also serve the purpose of power smoothing, which is especially important for devices using direct-driven power take off. The scope of this paper is to evaluate a method to optimize wave energy farms in terms of power production, economic viability and resources. In particular, the paper deals with the power variation in a large array of point-absorbing direct-driven wave energy converters, and the smoothing effect due to the number of devices and their hydrodynamic interactions. A few array geometries are compared and 34 sea states measured at the Lysekil research site at the Swedish west coast are used in the simulations. Potential linear flow theory are used with full hydrodynamic interactions between the buoys. It is shown that the variance in power production depends crucially on the geometry of the array and the number of interacting devices, but not significantly on the energy period of the waves.
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| 8. |
- Forsberg, Samuel, et al.
(författare)
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Resilience to extreme storm conditions : A comparative study of two power systems with varying dependencies on offshore wind
- 2024
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Ingår i: Results in Engineering (RINENG). - : Elsevier. - 2590-1230. ; 23
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Tidskriftsartikel (refereegranskat)abstract
- In the next decades, the dependencies on power production from renewable energy sources are expected to increase dramatically. A transition towards large-scale offshore wind farms together with an increased electrification of the industry and transportation sectors introduces new vulnerabilities to society. Further, extreme weather events are expected to increase in intensity and frequency, driven by climate change. However, there are significant knowledge gaps concerning the impacts of severe weather conditions on the resilience of power systems with large dependencies on offshore wind. In the present study, a comparison between two different power systems’ resilience to historical extreme storm conditions has been conducted. The power systems are the IEEE39-bus New England model and the Great Britain model. The results show significant differences between the two power systems, which underlying reasons are analysed and explained. With an offshore wind penetration level of 30 %, the New England model stays intact in terms of connected load. When increasing the penetration level to 40 %, about 10 % of the total connected load gets disconnected, whereas about 33 % of the load gets disconnected with a penetration level of 50 %. The Great Britain model stays intact in terms of connected load with a penetration level of at least 49 %.
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| 9. |
- Forsberg, Samuel, et al.
(författare)
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Resilience to storm conditions of power systems with large dependencies on offshore wind
- 2023
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Ingår i: Journal of Physics, Conference Series. - : Institute of Physics Publishing (IOPP). - 1742-6588 .- 1742-6596. ; 2626
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Tidskriftsartikel (refereegranskat)abstract
- The ongoing transition towards large installations of offshore wind and the electrification of the transport sector and other critical infrastructures introduce new vulnerabilities to the society. Large dependencies of power production from offshore wind are expected in the next decades, but there are large knowledge gaps regarding the power production reliability under severe weather conditions. Simultaneously, weather extremes may increase in frequency and intensity, driven by climate change. In this paper we investigate the resilience of a power system subject to a hurricane event. The power system is based on the IEEE39-bus New England system but with different scenarios for increasing penetration of offshore wind. We find that an offshore wind penetration level of 30% or less results in a power system resilient to hurricane events, with no need for load disconnection. However, when increased to 40% offshore wind penetration, 650 MW corresponding to 10% of the total load demand gets disconnected during the storm peak. With a penetration of 50% offshore wind, the disconnected load ranges from 2.2 GW of load corresponding to 1/3 of the total load demand, to a total power system blackout.
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| 10. |
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| 11. |
- Giannini, Gianmaria, et al.
(författare)
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Wave Energy Converter Power Take-Off System Scaling and Physical Modelling
- 2020
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Ingår i: Journal of Marine Science and Engineering. - : MDPI. - 2077-1312. ; 8:9
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Tidskriftsartikel (refereegranskat)abstract
- Absorbing wave power from oceans for producing a usable form of energy represents an attractive challenge, which for the most part concerns the development and integration, in a wave energy device, of a reliable, efficient and cost-effective power take-off mechanism. During the various stages of progress, for assessing a wave energy device, it is convenient to carry out experimental testing that, opportunely, takes into account the realistic behaviour of the power take-off mechanism at a small scale. To successfully replicate and assess the power take-off, good practices need to be implemented aiming to correctly scale and evaluate the power take-off mechanism and its behaviour. The present paper aims to explore and propose solutions that can be applied for reproducing and assessing the power take-off element during experimental studies, namely experimental set-ups enhancements, calibration practices, and error estimation methods. A series of recommendations on how to practically organize and carry out experiments were identified and three case studies are briefly covered. It was found that, despite specific options that can be strictly technology-dependent, various recommendations could be universally applicable.
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| 12. |
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| 13. |
- Giassi, Marianna, et al.
(författare)
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Capturing the experimental behaviour of a point-absorber WEC by simplified numerical models
- 2020
-
Ingår i: Journal of Fluids and Structures. - : Elsevier BV. - 0889-9746 .- 1095-8622. ; 99
-
Tidskriftsartikel (refereegranskat)abstract
- The paper presents a wave basin experiment of a direct-driven point-absorber wave energy converter moving in six degrees of freedom. The goal of the work is to study the dynamics and energy absorption of the wave energy converter, and to verify under which conditions numerical models restricted to heave can capture the behaviour of a point-absorber moving in six degrees of freedom. Several regular and irregular long-crested waves and different damping values of the power take-off system have been tested. We collected data in terms of power output, device motion in six degrees of freedom and wave elevation at different points of the wave basin. A single-body numerical model in the frequency domain and a two-body model in the time domain are used in the study. Motion instabilities due to parametric resonance observed during the experiments are discussed and analysis of the buoy motion in terms of the Mathieu instability is also presented. Our results show that the simplified models can reproduce the body dynamics of the studied converter as long as the transverse non-linear instabilities are not excited, which typically is the case in irregular waves. The performance of the more complex time domain model is able to reproduce both the buoy and PTO dynamics, while the simpler frequency domain model can only reproduce the PTO dynamics for specific cases. Finally, we show that the two-body dynamics of the studied wave energy converter affects the power absorption significantly, and that common assumptions in the numerical models, such as stiff mooring line or that the float moves only in heave, may lead to incorrect predictions for certain sea states.
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| 14. |
- Giassi, Marianna, et al.
(författare)
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Comparison of wave energy park layouts by experimental and numerical methods
- 2020
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Ingår i: Journal of Marine Science and Engineering. - : MDPI AG. - 2077-1312. ; 8:10
-
Tidskriftsartikel (refereegranskat)abstract
- An experimental campaign of arrays with direct-driven wave energy converters of point-absorbing type is presented. The arrays consist of six identical floats, moving in six degrees of freedom, and six rotating power take-off systems, based on the design developed at Uppsala University. The goals of the work were to study and compare the performances of three different array layouts under several regular and irregular long-crested waves, and in addition, to determine whether the numerical predictions of the best performing array layouts were confirmed by experimental data. The simulations were executed with a frequency domain model restricted to heave, which is a computationally fast approach that was merged into a genetic algorithm optimization routine and used to find optimal array configurations. The results show that good agreement between experiments and simulations is achieved when the test conditions do not induce phenomena of parametric resonance, slack line and wave breaking. Specific non-linear dynamics or extensive sway motion are not captured by the used model, and in such cases the simulation predictions are not accurate, but can nevertheless be used to get an estimate of the power output.
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| 15. |
- Giassi, Marianna, et al.
(författare)
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Economical layout optimization of wave energy parks clustered in electrical subsystems
- 2020
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Ingår i: Applied Ocean Research. - : Elsevier. - 0141-1187 .- 1879-1549. ; 101
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Tidskriftsartikel (refereegranskat)abstract
- To obtain a maximum power output and minimized capital and operational costs, the layout of wave energyparks needs to be optimally designed. An economical model for large-scale wave energy systems is built andmerged into an evolutionary optimization routine for arrays of point-absorbing energy converters. The modelincludes all the parameters that affect the total system revenue such as electrical cable lengths, distance fromgrid connection point, number of substations and hydrodynamic interaction among the devices, with the goal tofind the optimal layout which minimizes the levelized cost of electricity. Converters inside the park are groupedin clusters via a k-means clustering algorithm, which allows to minimize the intra-array cable length under theinput of real wave climates. The results show that the hydrodynamical interaction has a large impact on theoptimal design of wave energy parks, and that the length of the intra-array cable does not play a significant rolein the economical layout optimization routine for the studied wave energy park system.
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| 16. |
- Giassi, Marianna, et al.
(författare)
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Layout design of wave energy parks by a genetic algorithm
- 2018
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Ingår i: Ocean Engineering. - : Elsevier BV. - 0029-8018 .- 1873-5258. ; 154, s. 252-261
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Tidskriftsartikel (refereegranskat)abstract
- If wave energy systems are to become a viable option competitive with more mature renewable energy sources, the systems must be optimized with respect to maximal electricity production and minimized costs. The number of parameters involved in large-scale wave energy systems is typically too large for traditional optimization methods to be feasible, and the solution space may contain many local minima. Here, an optimization tool for application in wave energy design based on a genetic algorithm is presented. The internal parameters of single point-absorbing wave energy converters (buoy radius, draft and generator damping) are optimized and the results validated against parameters sweep optimization. Further, since the individual devices in a park affect each other by scattered and radiated waves propagating in all directions, the tool is used to find the optimal spatial layout of parks. Arrays with different number of devices are studied and similar optimal layouts appear in all cases, which allows extrapolation of the results to even larger parks. The results show that the tool is effective in finding layouts that avoid destructive interactions and get a q-factor slightly above 1.
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| 17. |
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| 18. |
- Giassi, Marianna, 1988-
(författare)
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Numerical and experimental modelling for wave energy arrays optimization
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Many wave power conversion devices, especially point-absorbers, do not provide alone the necessary amount of converted electricity to be cost effective, instead they are designed to be deployed in arrays of many units. Such arrays, or parks, can satisfy a large-scale energy demand, reduce the costs of the produced electricity and improve the reliability of the system.The performance of a wave energy park is affected by multiple and mutually interacting parameters, and the complex problem that arises during its design is called array optimization.The scope of the present thesis is to study such systems and their design, by the development of an optimization routine able to predict the best layout of a wave energy park under fixed constraints. The wave energy converter considered is the point-absorber developed at Uppsala University, which consists of a linear electric generator located on the seabed and a floating buoy at the surface.An optimization routine based on a genetic algorithm was created, which allows simultaneous optimization of the geometry of the buoys, the damping coefficient of the linear generators and the geometrical layout of the park.Finally, an experimental campaign with a single device and three arrays of six devices was conducted in order to compare the theoretical results with experimentally acquired data.The results identify optimal configurations of wave energy arrays, and highlight the effect of optimizing upon different objective functions, including economical ones. In the experiments, standard models and common assumptions used for wave energy park optimizations were tested against realistic conditions.
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| 19. |
- Giassi, Marianna
(författare)
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Optimization of Point Absorber Wave Energy Parks
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Renewable energies are believed to play the key role in assuring a future of sustainable energy supply and low carbon emissions. Particularly, this thesis focus on wave energy, which is created by extracting the power stored in the waves of the oceans. In order for wave energy to become a commercialized form of energy, modular deployment of many wave energy converters (WECs) together will be required in the upcoming future. This design will thus allow to benefit, among others, from the modular construction, the shared electrical cables connections and moorings, the reduction in the power fluctuations and reduction of deployment and maintenance costs. When it comes to arrays, the complexity of the design process increase enormously compared with the single WEC, given the mutual influence of most of the design parameters (i.e. hydrodynamic and electrical interactions, dimensions, geometrical layout, wave climate etc.). Uppsala University has developed and tested WECs since 2001, with the first offshore deployment held in 2006. The device is classified as a point absorber and consists in a linear electric generator located on the seabed, driven in the vertical direction by the motion of a floating buoy at the surface. Nowadays, one of the difficulties of the sector is that the cost of electricity is still too high and not competitive, due to high capital and operational costs and low survivability. Therefore, one step to try to reduce these costs is the development of reliable and fast optimization tools for parks of many units. In this thesis, a first attempt of systematic optimization for arrays of the Uppsala University WEC has been proposed. A genetic algorithm (GA) has been used to optimize the geometry of the floater and the damping coefficient of the generator of a single device. Afterwards, the optimal layout of parks up to 14 devices has been studied using two different codes, a continuous and a discrete variables real coded GA. Moreover, the method has been extended to study arrays with devices of different dimensions. A deterministic evaluation of small array layouts in real wave climate has also been carried out. Finally, a physical scale test has been initiated which will allow the validation of the results. A multi--parameter optimization of wave power arrays of the Uppsala University WEC has been shown to be possible and represents a tool that could help to reduce the total cost of electricity, enhance the performance of wave power plants and improve the reliability.
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| 20. |
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| 21. |
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| 22. |
- Göteman, Malin, 1980-, et al.
(författare)
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Advances and Challenges in Wave Energy Park Optimization : A Review
- 2020
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Ingår i: Frontiers in Energy Research. - : Frontiers Media SA. - 2296-598X. ; 8
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Forskningsöversikt (refereegranskat)abstract
- A commercial wave energy system will typically consist of many interacting wave energy converters installed in a park. The performance of the park depends on many parameters such as array layout and number of devices, and may be evaluated based on different measures such as energy absorption, electricity quality, or cost of the produced electricity. As wave energy is currently at the stage where several large-scale installations are being planned, optimizing the park performance is an active research area, with many important contributions in the past few years. Here, this research is reviewed, with a focus on identifying the current state of the art, analyzing how realistic, reliable, and relevant the methods and the results are, and outlining directions for future research.
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| 23. |
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| 24. |
- Göteman, Malin, 1980-, et al.
(författare)
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Arrays of Point-Absorbing Wave Energy Converters in Short-Crested Irregular Waves
- 2018
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Ingår i: Energies. - : MDPI AG. - 1996-1073. ; 11:4
-
Tidskriftsartikel (refereegranskat)abstract
- For most wave energy technology concepts, large-scale electricity production and cost-efficiency require that the devices are installed together in parks. The hydrodynamical interactions between the devices will affect the total performance of the park, and the optimization of the park layout and other park design parameters is a topic of active research. Most studies have considered wave energy parks in long-crested, unidirectional waves. However, real ocean waves can be short-crested, with waves propagating simultaneously in several directions, and some studies have indicated that the wave energy park performance might change in short-crested waves. Here, theory for short-crested waves is integrated in an analytical multiple scattering method, and used to evaluate wave energy park performance in irregular, short-crested waves with different number of wave directions and directional spreading parameters. The results show that the energy absorption is comparable to the situation in long-crested waves, but that the power fluctuations are significantly lower.
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| 25. |
- Göteman, Malin, 1980-, et al.
(författare)
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Fast modeling of large wave energy farms using interaction distance cut-off
- 2015
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Ingår i: Energies. - : MDPI AG. - 1996-1073. ; 8:12, s. 13741-13757
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Tidskriftsartikel (refereegranskat)abstract
- In many wave energy concepts, power output in the MW range requires the simultaneous operation of many wave energy converters. In particular, this is true for small point-absorbers, where a wave energy farm may contain several hundred devices. The total performance of the farm is affected by the hydrodynamic interactions between the individual devices, and reliable tools that can model full farms are needed to study power output and find optimal design parameters. This paper presents a novel method to model the hydrodynamic interactions and power output of very large wave energy farms. The method is based on analytical multiple scattering theory and uses time series of irregular wave amplitudes to compute the instantaneous power of each device. An interaction distance cut-off is introduced to improve the computational cost with acceptable accuracy. As an application of the method, wave energy farms with over 100 devices are studied in the MW range using one month of wave data measured at an off-shore site.
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| 26. |
- Göteman, Malin, 1980-, et al.
(författare)
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Fluid dynamics and wave-structure interactions
- 2022
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Ingår i: Modelling and Optimization of Wave Energy Converters. - Boca Raton; Abingdon : CRS Press. - 9781032057392 - 9781032057408 - 9781003198956 ; , s. 61-96
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 27. |
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| 28. |
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| 29. |
- Göteman, Malin, 1980-, et al.
(författare)
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Methods of reducing power fluctuations in wave energy parks
- 2014
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Ingår i: Journal of Renewable and Sustainable Energy. - : AIP Publishing. - 1941-7012. ; 6, s. 043103-
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Tidskriftsartikel (refereegranskat)abstract
- One of the major challenges in constructing effective and economically viable wave energy parks is to reduce the large fluctuations in power output. In this paper, we study different methods of reducing the fluctuations and improve the output power quality. The parameters studied include the number of devices, the separating distance between units, the global and local geometries of the array, sea state and incoming wave direction, and the impact of including buoys of different radii in an array. Our results show that, e. g., the fluctuations as well as power per device decrease strictly with the number of interacting units, when the separating distance is kept constant. However, including more devices in a park with fixed area will not necessarily result in lowered power fluctuations. We also show that varying the distance between units affects the power fluctuations to a much larger extent than it affects the magnitude of the absorbed power. The fluctuations are slightly lower in more realistic, randomized geometries where the buoys tend to drift slightly off their mean positions, and significantly lower in semi-circular geometries as opposed to rectangular geometries.
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| 30. |
- Göteman, Malin, 1980-
(författare)
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Multiple cluster scattering with applications to wave energy park optimizations
- 2022
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Ingår i: Applied Ocean Research. - : Elsevier. - 0141-1187 .- 1879-1549. ; 125
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Tidskriftsartikel (refereegranskat)abstract
- Theory and methods to compute the wave field and hydrodynamic interaction in arrays of fixed or floating structures have many applications, ranging from offshore platforms to wave energy parks. Analytical iterative and non-iterative multiple scattering theory, and extensions thereof, are often used for this purpose. However, the computational cost grows with the number of interacting bodies, and studies of large arrays of independently moving bodies often require assumptions in terms of layout periodicity, expansion in different length scales, or neglected degrees of freedom. In optimization studies of wave energy parks, many simulations are usually required, implying that the existing methods are too slow. The current paper extends analytical multiple scattering theory by dividing the floats in clusters, and computes the hydrodynamic interaction within clusters exactly, but iteratively between clusters. The method constitutes a bridge between the iterative and non-iterative multiple scattering methods, and enables a faster modelling of parks of many floating bodies, to a retained accuracy. The proposed method is validated with numerical and analytical methods with excellent agreement, to a drastically reduced computational cost.
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| 31. |
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| 32. |
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| 33. |
- Göteman, Malin, et al.
(författare)
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Off-shell N = (4,4) supersymmetry for new (2,2) vector multiplets
- 2011
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Ingår i: Journal of High Energy Physics (JHEP). - 1126-6708 .- 1029-8479. ; :3, s. 088-
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the conditions for extra supersymmetry of the N = (2, 2) super-symmetric vector multiplets described in arXiv:0705.3201 [hep-th] and in arXiv:0808.1535 [hep-th]. We find (4, 4) supersymmetry for the semichiral vector multiplet but not for the Large Vector Multiplet.
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| 34. |
- Göteman, Malin, 1980-
(författare)
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Optimisation of wave farms
- 2022
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Ingår i: Modelling and Optimization of Wave Energy Converters. - Boca Raton; Abingdon : CRS Press. - 9781032057392 - 9781032057408 - 9781003198956 ; , s. 281-308
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 35. |
- Göteman, Malin, 1980-, et al.
(författare)
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Optimizing wave energy parks with over 1000 interacting point-absorbers using an approximate analytical method
- 2015
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Ingår i: International Journal of Marine Energy. - : Elsevier BV. - 2214-1669. ; 10, s. 113-126
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Tidskriftsartikel (refereegranskat)abstract
- Large arrays of wave energy converters of point-absorber type are studied using an approximate analytical model. The model is validated against a numerical method that takes into account full hydrodynamic interactions based on linear potential flow theory. The low computational cost of the analytical model enables parameter studies of parks in the MW range and includes up to over 1000 interacting devices. The model is actuated by irregular wave data obtained at the Swedish west coast. In particular, focus is on comparing park geometries and improving park configurations to minimize the power fluctuations.
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| 36. |
- Göteman, Malin, 1980-
(författare)
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Passive damping control in wave farms using cluster communication
- 2022
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Ingår i: Trends in Renewable Energies Offshore. - London : CRC Press. - 9781003360773 - 9781032420035 ; , s. 385-392
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Konferensbidrag (refereegranskat)abstract
- To reach a commercial potential comparable to offshore wind, the performance of waveenergy systems must be improved. At the same time, the costs must be reduced and the survivabilityimproved. To reduce costs and improve the power quality, wave energy converters can be installed together infarms. Advanced control methods have shown to be efficient in improving the energy absorption of waveenergy systems, but applying them in reality is difficult and costly. Here, a simple passive control approach isapplied to a wave energy farm and its effect on the energy absorption during one full year at the Wave Hubsite is investigated. The method identifies the current sea state peak period from the motion of some of thedevices in the farm. Based on this information, the generator damping of all the devices is then adjustedaccordingly. Based on this simple passive control strategy, the total energy absorbed by the park is improved.The energy increase is only moderate, which can be understood from the fact that the default damping hasalready been tuned to the average sea state at the site, and the absorbed power does not vary much when thegenerator damping is shifted slightly off the optimal value.
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| 37. |
- Göteman, Malin, et al.
(författare)
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Pseudo-Hyperkähler Geometry and Generalized Kähler Geometry
- 2011
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Ingår i: Letters in Mathematical Physics. - : Springer Science and Business Media LLC. - 0377-9017 .- 1573-0530. ; 95:3, s. 211-222
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the conditions for additional supersymmetry and twisted super-symmetry in N = (2, 2) supersymmetric nonlinear sigma models described by one left and one right semi-chiral superfield and carrying a pair of non-commuting complex structures. Focus is on linear non-manifest transformations of these fields that have an algebra that closes off-shell. We find that additional linear supersymmetry has no interesting solution, whereas additional linear twisted supersymmetry has solutions with interesting geometrical properties. We solve the conditions for invariance of the action and show that these solutions correspond to a bi-hermitian metric of signature (2, 2) and a pseudo-hyperkahler geometry of the target space.
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| 38. |
- Göteman, Malin, 1980-, et al.
(författare)
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Resilience of wave energy farms using metocean dependent failure rates and repair operations
- 2023
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Ingår i: Ocean Engineering. - : Elsevier. - 0029-8018 .- 1873-5258. ; 280
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Tidskriftsartikel (refereegranskat)abstract
- Emerging offshore renewable energy technologies are expected to become an important part of the futureenergy system, and reliability for these new technologies in different metocean scenarios must be guaranteed.This poses a challenge in extreme weather scenarios like storms, in particular for less mature technologiessuch as wave energy. Not only the offshore survivability must be controlled; the restoration after disruptiveevents and failures should be addressed and optimized. Offshore operations are costly and cannot be carriedout if the weather is too harsh, and the resulting downtime after failures may be financially devastating forprojects. In this paper, the resilience of large wave energy systems is studied with respect to wave conditions,metocean dependent failure rates, and weather windows available for offshore repair operations. A metocean-and time-dependent failure rate is derived based on a Weibull distribution, which is a novelty of the paper.The performance of the farm is assessed using the varying failure rates and metocean data at different offshoresites. Critical metocean thresholds for different offshore vessels are considered, and the resilience is quantifiedusing relevant measures such as unavailability and expected energy not supplied. The resilience analysis iscoupled to an economic assessment of the wave farm and different repair strategies. Our results show thatthe commonly used assumption of constant failure rates is seen to overestimate the annual energy productionthan when a more realistic varying failure rate is used. Two offshore sites are compared, and the availabilityis found to be higher at the calmer site. Most of the evaluated repair strategies cannot be considered to beeconomically justified, when compared to the cost of the energy not supplied.
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| 39. |
- Göteman, Malin, et al.
(författare)
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Semichiral Sigma Models with 4D Hyperkähler Geometry
- 2013
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Ingår i: Journal of High Energy Physics (JHEP). - 1126-6708 .- 1029-8479. ; :1, s. 073-
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Tidskriftsartikel (refereegranskat)abstract
- Semichiral sigma models with a four-dimensional target space do not support, extended N = (4, 4) supersymmetries off-shell [1, 2]. We contribute towards the understanding of the non-manifest on-shell transformations in (2,2) superspace by analyzing the extended on-shell supersymmetry of such models and find that a rather general ansatz for the additional supersymmetry (not involving central charge transformations) leads to hyperkahler geometry. We give non-trivial examples of these models.
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| 40. |
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| 41. |
- Göteman, Malin, et al.
(författare)
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Sigma models with off-shell N = (4,4) supersymmetry and non-commuting complex structures
- 2010
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Ingår i: Journal of High Energy Physics (JHEP). - 1126-6708 .- 1029-8479. ; :9, s. 055-
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Tidskriftsartikel (refereegranskat)abstract
- We describe the conditions for extra supersymmetry in N = (2, 2) supersymmetric nonlinear sigma models written in terms of semichiral superfields. We find that some of these models have additional off-shell supersymmetry. The (4, 4) supersymmetry introduces geometrical structures on the target-space which are conveniently described in terms of Yano f-structures and Magri-Morosi concomitants. On-shell, we relate the new structures to the known bi-hypercomplex structures.
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| 42. |
- Göteman, Malin
(författare)
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The Complex World of Superstrings : On Semichiral Sigma Models and N=(4,4) Supersymmetry
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Non-linear sigma models with extended supersymmetry have constrained target space geometries, and can serve as effective tools for investigating and constructing new geometries. Analyzing the geometrical and topological properties of sigma models is necessary to understand the underlying structures of string theory.The most general two-dimensional sigma model with manifest N=(2,2) supersymmetry can be parametrized by chiral, twisted chiral and semichiral superfields. In the research presented in this thesis, N=(4,4) (twisted) supersymmetry is constructed for a semichiral sigma model. It is found that the model can only have additional supersymmetry off-shell if the target space has a dimension larger than four. For four-dimensional target manifolds, supersymmetry can be introduced on-shell, leading to a hyperkähler manifold, or pseudo-supersymmetry can be imposed off-shell, implying a target space which is neutral hyperkähler.Different sigma models and corresponding geometries can be related to each other by T-duality, obtained by gauging isometries of the Lagrangian. The semichiral vector multiplet and the large vector multiplet are needed for gauging isometries mixing semichiral superfields, and chiral and twisted chiral superfields, respectively. We find transformations that close off-shell to a N=(4,4) supersymmetry on the field strengths and gauge potentials of the semichiral vector multiplet, and show that this is not possible for the large vector multiplet.A sigma model parametrized by chiral and twisted chiral superfields can be related to a semichiral sigma model by T-duality. The N=(4,4) supersymmetry transformations of the former model are linear and close off-shell, whereas those of the latter are non-linear and close only on-shell. We show that this discrepancy can be understood from T-duality, and find the origin of the non-linear terms in the transformations.
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| 43. |
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| 44. |
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| 45. |
- Göteman, Malin, 1980-
(författare)
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Wave energy parks with point-absorbers of different dimensions
- 2017
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Ingår i: Journal of Fluids and Structures. - : Elsevier BV. - 0889-9746 .- 1095-8622. ; 74, s. 142-157
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Tidskriftsartikel (refereegranskat)abstract
- An analytical model for point-absorbing wave energy converters connected to floats of different geometries and topologies is presented. The floats can be truncated cylinder or cylinder with moonpool buoys and have different outer radius, inner radius, draft, mass and can be connected to linear generators of different power take-off constants. The model is implemented into a numerical code where the input is measured time-series of irregular waves. After validation against benchmark software, the model is used to study optimal configurations of wave energy arrays consisting of different wave energy devices. It is shown that the total power absorption can be improved if the wave energy array consists of devices of different dimensions, and that a higher power-to-mass ratio can be achieved.
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| 46. |
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| 47. |
- Hai, Ling, et al.
(författare)
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A Methodology of Modelling a Wave Power System via an Equivalent RLC Circuit
- 2016
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Ingår i: IEEE Transactions on Sustainable Energy. - 1949-3029 .- 1949-3037. ; 7:4, s. 1362-1370
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Tidskriftsartikel (refereegranskat)abstract
- The equivalent circuit method can be an effective modelling technique for system studies of point absorbing wave energy converters (WECs). For the continuously evolving design and study of WEC systems, an instruction on how to draw the corresponding equivalent RLC circuit model is needed. It is not only vital to make sure the model is correct, but to allow the model to be easily adapted for different cases and implemented by different researchers. This paper presents a force analysis oriented methodology based on a typical WEC unit composed of a heaving buoy and a linear generator. Three cases are studied in order to demonstrate the procedures: the generator with a retracting spring, the connection line with a rubber damper, and buoy motion in both heave and surge directions. The presented methodology serves as a guide to produce non-linear circuit models that give a reliable description of the dynamics of real wave energy systems.
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| 48. |
- Isberg, Jan, 1964-, et al.
(författare)
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Control of rapid phase oscillations in the modelling of large wave energy arrays
- 2015
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Ingår i: International Journal of Marine Energy. - : Elsevier. - 2214-1669. ; 11, s. 1-8
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Tidskriftsartikel (refereegranskat)abstract
- Several recently developed concepts for economically viable conversion of ocean wave energy are based on large arrays of point absorbers. Simulations of the hydrodynamic interactions between devices in wave energy parks provide guidelines for optimal configurations with regard to maximizing produced electricity while minimizing fluctuations and costs. Parameters that influence the performance include the geometrical lay-out of the park, the number of wave energy converters and their dimensions and separating distance, as well as the wave climate and the incoming wave spectral characteristics. However, the complexity of the simulations increases rapidly with growing number of interacting units, and simulations become a severe challenge that calls for new methods. Here we address the problem of rapid phase oscillations appearing in the simulation of large arrays of point absorbers using potential theory for the structure–fluid interaction. We do this by analytically integrating out the factors that are causing the oscillations. Our group has successfully utilized this method to model parks with up to 1000 point absorbers.
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| 49. |
- Jin, Peng, et al.
(författare)
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Performance optimization of a coaxial-cylinder wave energy converter
- 2019
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Ingår i: Energy. - : Elsevier. - 0360-5442 .- 1873-6785. ; 174, s. 450-459
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Tidskriftsartikel (refereegranskat)abstract
- To achieve a wider frequency range where the device has a larger capture width ratio, the performance of a heaving coaxial-cylinder wave energy converter is optimized through actively controlled generator damping and stiffness using a linear frequency domain model. The generator power take-off system is modeled as a damping-spring system, and the numerical model is validated against published results. The coupled dynamics of a two-body model is analyzed to search for the optimal generator damping and stiffness leading to maximal capture width ratio. The optimization process, which can be decoupled into two independent steps, leads to an improved performance of the device, with increased frequency bandwidth and better capture width ratio. The effects of water depth, mooring stiffness, and the dimensions of the WEC on the capture width ratio are also studied, and parameter values are identified which correspond to optimal performance of the device.
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| 50. |
- Katsidoniotaki, Eirini, et al.
(författare)
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Comparison of dynamic mesh methods in OpenFOAM for a WEC in extreme waves
- 2020
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Ingår i: Developments in Renewable Energies Offshore. - 9781003134572
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Konferensbidrag (refereegranskat)abstract
- One of the big challenges for offshore wave energy systems is how to guarantee the survivability in harsh environmental conditions. Nonlinear and complex phenomena in steep and high amplitude waves can be captured by CFD methods. Mesh morphing is the most common used dynamic mesh method, yet the high and steep waves are a challenge since the large amplitude body motion leads to deterioration of the computational mesh quality. Advanced mesh methods have been developed to overcome this issue, such as overset method. The goal of the present paper is to compare morphing and overset methods implemented in OpenFOAM by modelling the response of a point-absorbing wave energy converter in extreme waves, identified along the 100-year extreme wave contour at the Humboldt Bay site, California. Simulations of two extreme sea states are conducted and the results from two different methods are compared and discussed.
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