| 1. |
- Eskilsson, Claes, et al.
(författare)
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Sensitivity analysis of extreme loads acting on a point-absorbing wave energy converter
- 2022
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Ingår i: International Marine Energy Journal. - : European Wave and Tidal Energy Conference. - 2631-5548. ; 5:1, s. 91-101
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Tidskriftsartikel (refereegranskat)abstract
- There are many uncertainties associated with the estimation of extreme loads acting on a wave energy converter (WEC). In this study we perform a sensitivity analysis of extreme loads acting on the Uppsala University (UU) WEC concept. The UU WEC consists of a bottom-mounted linear generator that is connected to a surface buoy with a taut mooring line. The maximum stroke length of the linear generator is enforced by end-stop springs. Initially, a Variation Mode and Effect Analysis (VMEA) was carried out in order to identify the largest input uncertainties. The system was then modelled in the time-domain solver WEC-SIM coupled to the dynamic mooring solver Moody. A sensitivity analysis was made by generating a surrogate model based on polynomial chaos expansions, which rapidly evaluates the maximum loads on the mooring line and the end-stops. The sensitivities are ranked using the Sobol index method. We investigated two sea states using equivalent regular waves (ERW) and irregular wave (IRW) trains. We found that the ERW approach significantly underestimate the maximum loads. Interestingly, the ERW predicted wave height and period as the most important parameters for the maximum mooring tension, whereas the tension in IRW was most sensitive to the drag coefficient of the surface buoy. The end-stop loads were most sensitive to the PTO damping coefficient.
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| 2. |
- Katsidoniotaki, Eirini, et al.
(författare)
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Midfidelity model verification for a point-absorbing wave energy converter with linear power take-off
- 2022
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Ingår i: International Marine Energy Journal. - : European Wave and Tidal Energy Conference. - 2631-5548. ; 5:1, s. 67-75
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Tidskriftsartikel (refereegranskat)abstract
- In the preliminary design stage of a wave energy converter (WEC), researchers need fast and reliable simulation tools. High-fidelity numerical models are usually employed to study the wave-structure interaction, but the computational cost is demanding. As an alternative, midfidelity models can provide simulations in the order of real time. In this study, we operate Uppsala University’s WEC in a relatively mild sea state and model it using WEC-Sim. The model is verified based on OpenFOAM simulations. To analyze the ability of the midfidelity model to capture WEC dynamics, we investigate the system separately with 1, 2, and 3 degrees of freedom. We examine the contribution of viscous phenomena, and study both linear and weakly nonlinear solutions provided by WEC-Sim. Our results indicate that the viscous effects can be neglected in heave and surge motion, but not for pitch. We also find that the weakly nonlinear WEC-Sim solution successfully agrees with the computational fluid dynamics, whereas the linear solution could suggest misleading results.
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| 3. |
- Palm, Johannes, et al.
(författare)
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Facilitating Large-Amplitude Motions of Wave Energy Converters in OpenFOAM by a modified Mesh Morphing Approach
- 2022
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Ingår i: International Marine Energy Journal. - : European Wave and Tidal Energy Conference. - 2631-5548. ; 5:3
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Tidskriftsartikel (refereegranskat)abstract
- <p>High-fidelity simulations using computational fluid dynamics (CFD) for wave-body interaction are becoming increasingly common and important for wave energy converter (WEC) design. The open source finite volume toolbox OpenFOAM is one of the most frequently used platforms for wave energy. There are currently two ways to account for moving bodies in OpenFOAM: (i) mesh morphing, where the mesh deforms around the body; and (ii) an overset mesh method where a separate body mesh moves on top of a background mesh. Mesh morphing is computationally efficient but may introduce highly deformed cells for combinations of large translational and rotational motions. The overset method allows for arbitrarily large body motions and retains the quality of the mesh. However, it comes with a substantial increase in computational cost and possible loss of energy conservation due to the interpolation. In this paper we present a straightforward extension of the spherical linear interpolation (SLERP) based mesh morphing algorithm that increase the stability range of the method. The mesh deformation is allowed to be interpolated independently for different modes of motion, which facilitates tailored mesh motion simulations. The paper details the implementation of the method and evaluates its performance with computational examples of a cylinder with a moonpool. The examples show that the modified mesh morphing approach handles large motions well and provides a cost effective alternative to overset mesh for survival conditions.
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| 4. |
- Parwal, Arvind, et al.
(författare)
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A Comparative Analysis of Linear and Nonlinear Control of Wave Energy Converter for a Force Control Application
- 2024
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Ingår i: International Marine Energy Journal. - Southampton, UK : International Marine Energy Journal. - 2631-5548.
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Tidskriftsartikel (refereegranskat)abstract
- The aim of wave energy converters (WECs) is to harvest the energy from the ocean waves and convert into electricity. Optimizing the generator output is a vital point of research. A WEC behaves as a nonlinear system in real ocean waves and a control that approximates the behaviour of the system is required. In order to predict the behaviour of WEC, a controller is implemented with an aim to track the referenced trajectory for a force control application of the WEC. A neural model is implemented for the system identification and control of the nonlinear process with a neural nonlinear autoregressive moving average exogenous (NARMAX) model. The neural model updates the weights to reduce the error by using the Levenberg-Marquardt back-propagation algorithm for a single-input-single-output (SISO) nonlinear system. The performance of the system under the proposed scheme is compared to the same system under a PI-controller scheme, where the PI gains have been tuned accordingly, to verify the control capacity of the proposed controller. The results show a good tracking of dq (direct-quadrature) axes currents by regulating the stator currents, and hence a force control is achieved at different positions of the translator. The dynamic performance of the control is verified in a time domain analysis for the displacement of the translator.
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| 5. |
- Parwal, Arvind, et al.
(författare)
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A comparative analysis of linear and nonlinear control of wave energy converter for a force control application
- 2019
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Ingår i: International Marine Energy Journal. - : European Wave and Tidal Energy Conference. - 2631-5548. ; 2:1, s. 39-50
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Tidskriftsartikel (refereegranskat)abstract
- The aim of wave energy converters (WECs) is to harvest the energy from the ocean waves and convert into electricity. Optimizing the generator output is a vital point of research. A WEC behaves as a nonlinear system in real ocean waves and a control that approximates the behaviour of the system is required. In order to predict the behaviour of WEC, a controller is implemented with an aim to track the referenced trajectory for a force control application of the WEC. A neural model is implemented for the system identification and control of the nonlinear process with a neural nonlinear autoregressive moving average exogenous (NARMAX) model. The neural model updates the weights to reduce the error by using the Levenberg-Marquardt back-propagation algorithm for a single-input-single-output (SISO) nonlinear system. The performance of the system under the proposed scheme is compared to the same system under a PI-controller scheme, where the PI gains have been tuned accordingly, to verify the control capacity of the proposed controller. The results show a good tracking of dq (direct-quadrature) axes currents by regulating the stator currents, and hence a force control is achieved at different positions of the translator. The dynamic performance of the control is verified in a time domain analysis for the displacement of the translator.
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| 6. |
- Salar, Dana, 1985-, et al.
(författare)
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Low-RPM torque converter (LRTC) with Integrated direct shaft flywheel
- 2023
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Ingår i: International Marine Energy Journal. - 2631-5548. ; 6:1, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- The low-RPM Torque Converter (LRTC) is a rotating generator concept for use on the seabed with the driving force from sea waves motion on the sea surface. This concept is built up of two identical generators connected opposite each other via a spring drum with a built-in ball bearing clutch. The drum is connected to a buoy on the sea surface via a wire, the wire is rolled around the spring drum. With sea waves, the buoy moves either upwards or downwards and pulls the wire upwards or downwards. This movement causes the generators to spin.This article presents an upgrade of the LRTC generator concept and upgraded measurement system, both hardware and software.A flywheel system of the thin-disc type has been designed for direct connection to the generator's rotor shaft and an electronic measuring system has also been developed for more accurate measurements and minor disturbances.More detailed tests have been performed both for the purpose of comparing the systems and to explore the performance of the generator concept in more detail.Three different experiments have been done in this article. The first two experiments were performed to investigate the performance of the flywheel and to see the performance of the LRTC system with and without flywheel.The third experiment investigated the optimization of the flywheel mass by increasing the mass of the flywheel with the addition of more thin discs.All movements are simulated with a six-joint industrial robot. Several sinusoidal types of wave motions have been simulated with different time periods and also several real wave climate motions (data taken from fields) have been simulated with the robot. The experiments show that the addition of the flywheel in the LRTC system provides advantages in increasing both peak power, average output power and also softens the output power oscillation.
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