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  • 1.
    Frost, Anna E.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Ulvgård, Liselotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Uppsala University.
    Sjökvist, Linnea
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Eriksson, Sandra
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Partial Stator Overlap in a Linear Generator for Wave Power: An Experimental Study2017In: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 5, no 4, article id 53Article in journal (Refereed)
    Abstract [en]

    This paper presents a study on how the power absorption and damping in a linear generator for wave energy conversion are affected by partial overlap between stator and translator. The theoretical study shows that the electrical power as well as the damping coefficient change quadratically with partial stator overlap, if inductance, friction and iron losses are assumed independent of partial stator overlap or can be neglected. Results from onshore experiments on a linear generator for wave energy conversion cannot reject the quadratic relationship. Measurements were done on the inductance of the linear generator and no dependence on partial stator overlap could be found. Simulations of the wave energy converter's operation in high waves show that entirely neglecting partial stator overlap will overestimate the energy yield and underestimate the peak forces in the line between the buoy and the generator. The difference between assuming a linear relationship instead of a quadratic relationship is visible but small in the energy yield in the simulation. Since the theoretical deduction suggests a quadratic relationship, this is advisable to use during modeling. However, a linear assumption could be seen as an acceptable simplification when modeling since other relationships can be computationally costly.

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  • 2.
    Hong, Yue
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hultman, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Castellucci, Valeria
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Ekergård, Boel
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Sjökvist, Linnea
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Elamalayil Soman, Deepak
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Krishna, Remya
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Haikonen, Kalle
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Baudoin, Antoine
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Lindblad, Liselotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Lejerskog, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Käller, Daniel
    Rahm, Magnus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Strömstedt, Erland
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Boström, Cecilia
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Waters, Rafael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Status Update of the Wave Energy Research at Uppsala University2013Conference paper (Refereed)
  • 3.
    Sjökvist, Linnea
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hydromechanical simulations of wave energy conversion: Linear aspects2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Hydrodynamic simulations of buoy movement is crucial when designing a point absorbing wave power plant. The wave energy converter (WEC) that is developed in the Lysekil project by Uppsala University is studied in this thesis. The buoy motion in response to an incoming wave can be simulated using potential linear wave theory by deriving a transfer function from the Fourier transformed equation of motion and solve it. The buoy response is found from the convolution between the incoming wave and the transfer function in time domain. It have been found that this approach shows good agreement with experimental results for normal operating conditions.The hydromechanical parameters exciting force, added mass and radiation damping have been simulated in WAMIT, using BIEM, and in COMSOL, using FEM. This was done to compare the simulation model built in COMSOL with the commercial software WAMIT. The results were comparable. The hydromechanical parameters were used to calculate buoy movement and line force in response to a 30 minutes sea state measured at the Lysekil research site. The simulated buoy movement and line force was compared to experimental results, and showed good agreement.A WEC is a complicated dynamical system. The buoy motion, and thereby the translator velocity, depends on the buoy geometry and dimensions, the mass of the moving parts of the system and on the damping force from the generator. The damping force from the generator is dependent on the translator velocity. The above mentioned approach for motion simulation can be used if the damping force from the generator is linear and described by the velocity multiplied with a constant damping coefficient. In this thesis have it been studied how the buoy draft and radius of cylinder buoys are influencing the performance of a WEC, assuming a constant generator damping coefficient. The results were compared to the experimental power from the WECs L2 and L3, two identical generators with cylinder buoys of different dimensions. It was concluded that the experimental difference in power absorption between L2 and L3 could be derived from the difference in draft and buoy mass, rather than buoy radius. The largest part of the difference have however been concluded to derive from the incorrect line lengths.

  • 4.
    Sjökvist, Linnea
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Wave Loads and Peak Forces on Moored Wave Energy Devices in Tsunamis and Extreme Waves2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Surface gravity waves carry enormous amounts of energy over our oceans, and if their energy could be harvested to generate electricity, it could make a significant contribution to the worlds power demand. But the survivability of wave energy devices in harsh operating conditions has proven challenging, and for wave energy to be a possibility, peak forces during storms and extreme waves must be studied and the devices behaviour understood. Although the wave power industry has benefited from research and development in traditional offshore industries, there are important differences. Traditional offshore structures are designed to minimize power absorption and to have small motion response, while wave power devices are designed to maximize power absorption and to have a high motion response. This increase the difficulty of the already challenging survivability issue. Further, nonlinear effects such as turbulence and overtopping can not be neglected in harsh operating conditions. In contrast to traditional offshore structures, it is also important to correctly account for the power take off system in a wave energy converter (WEC), as it is strongly coupled to the devices behaviour.

    The focus in this thesis is the wave loads and the peak forces that occur when a WEC with a limited stroke length is operated in waves higher than the maximum stroke length. The studied WEC is developed at Uppsala University, Sweden, and consists of a linear generator at the seabed that is directly driven by a surface buoy. A fully nonlinear CFD model is developed in the finite volume software OpenFOAM, and validated with physical wave tank experiments. It is then used to study the motion and the forces on the WEC in extreme waves; high regular waves and during tsunami events, and how the WECs behaviour is influenced by different generator parameters, such as generator damping, friction and the length of the connection line. Further, physical experiments are performed on full scale linear generators, measuring the total speed dependent damping force that can be expected for different loads. The OpenFOAM model is used to study how the measured generator behaviour affects the force in the connection line.

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  • 5.
    Sjökvist, Linnea
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Engström, Jens
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Larsson, Stefan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahm, Magnus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Isberg, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Simulation of Hydrodynamical Forces on a Buoy - a Comparison Between Two Computational Approaches2012Conference paper (Refereed)
  • 6.
    Sjökvist, Linnea
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Center for Natural Disaster Science (CNDS), Uppsala, Sweden.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Peak forces on a point absorbing wave energy converter impacted by tsunami waves2019In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 133, p. 1024-1033Article in journal (Refereed)
    Abstract [en]

    Although a tsunami wave in deep sea can be simulated using linear shallow water theory, the wave dynamics of a tsunami running up a continental shelf is very complex, and different phenomena may occur, depending on the width and profile of the shelf, the topography of the coast, incident angle of the tsunami and other factors. How to simulate tsunami waves at an intermediate depth is studied in this paper by using three different simulation approaches for tsunamis, a soliton, a simulated high incident current and a dam-break approach. The surface wave profiles as well as the velocity- and pressure profiles for the undisturbed waves are compared. A regular Stokes 5th wave of the same amplitude is simulated for comparison. A wave energy converter model, previously validated with wave tank experiment, is then used to study the survivability of the Uppsala University wave energy device for the different waves. The force in the mooring line is studied together with the resulting force on a bottom mounted column, corresponding to the linear generator on the seabed.

  • 7.
    Sjökvist, Linnea
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Peak Forces on Wave Energy Linear Generators in Tsunami and Extreme Waves2017In: Energies, E-ISSN 1996-1073, Vol. 10, no 9, article id 1323Article in journal (Refereed)
    Abstract [en]

    The focus of this paper is the survivability of wave energy converters (WECs) in extreme waves and tsunamis, using realistic WEC parameters. The impact of a generator damping factor has been studied, and the peak forces plotted as a function of wave height. The paper shows that an increased damping decreases the force in the endstop hit, which is in agreement with earlier studies. However, when analyzing this in more detail, we can show that friction damping and velocity dependent generator damping affect the performance of the device differently, and that friction can have a latching effect on devices in tsunami waves, leading to higher peak forces. In addition, we study the impact of different line lengths, and find that longer line lengths reduce the endstop forces in extreme regular waves, but on the contrary increase the forces in tsunami waves due to the different fluid velocity fields.

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  • 8.
    Sjökvist, Linnea
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    The Effect of Overtopping Waves on Peak Forceson a Point Absorbing WEC2016Conference paper (Refereed)
  • 9.
    Sjökvist, Linnea
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahm, Magnus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Waters, Rafael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Svensson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Strömstedt, Erland
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Calculating Buoy Response for a Wave Energy Converter - a Comparsion Between Two Computational Methods and Experimental Results2017In: Theoretical and Applied Mechanics Letters, ISSN 2095-0349, Vol. 7, no 3, p. 164-168Article in journal (Refereed)
    Abstract [en]

    When designing a wave power plant, reliable and fast simulation tools are required. Computational fluid dynamics (CFD) software provides high accuracy but with a very high computational cost, and in operational, moderate sea states, linear potential flow theories may be sufficient to model the hydrodynamics. In this paper, a model is built in COMSOL Multiphysics to solve for the hydrodynamic parameters of a point-absorbing wave energy device. The results are compared with a linear model where the hydrodynamical parameters are computed using WAMIT, and to experimental results from the Lysekil research site. The agreement with experimental data is good for both numerical models.

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  • 10.
    Sjökvist, Linnea
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
    Krishna, Remya
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
    Rahm, Magnus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
    Castellucci, Valeria
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
    Hagnestål, Anders
    On the Optimization of Point Absorber BuoysArticle in journal (Refereed)
  • 11.
    Sjökvist, Linnea
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Center for Natural Disaster Science (CNDS), Villavägen 16, SE-752 36 Uppsala, Sweden.
    Wu, Jinming
    Department of Mechanical Engineering, Harbin Institute of Technology, Harbin, China.
    Ransley, Edward
    School of Engineering, Plymouth University, UK.
    Engström, Jens
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Eriksson, Mikael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Numerical models for the motion and forces of point-absorbing wave energy converters in extreme waves2017In: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 145, p. 1-14Article in journal (Refereed)
    Abstract [en]

    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.

  • 12.
    Ulvgård, Liselotte
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Sjökvist, Linnea
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Line Force and Damping at Full and Partial Stator Overlap in a Linear Generator for Wave Power2016In: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 4, no 4, article id 81Article in journal (Refereed)
    Abstract [en]

    A full scale linear generator for wave power has been experimentally evaluated bymeasuring the line force and translator position throughout the full translator stroke. The measuredline force, in relation to translator speed, generator damping and stator overlap, has been studied bycomparing the line force and the damping coefficient, γ, for multiple load cases along the translatorstroke length. The study also compares the generator ’s behavior during upward and downwardmotion, studies oscillations and determines the no load losses at two different speeds. The generatordamping factor, γ, was determined for five different load cases during both upward and downwardmotion. The γ value was found to be constant for full stator overlap and to decrease linearly witha decreasing overlap, as the translator moved towards the endstops. The decline varied with theexternal load case, as previously suggested but not shown. In addition, during partial stator overlap,a higher γ value was noted as the translator was leaving the stator, compared to when it was enteringthe stator. Finally, new insights were gained regarding how translator weight and generator dampingwill affect the translator downward motion during offshore operation. This is important for powerproduction and for avoiding damaging forces acting on the wave energy converter during operation.

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