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- Apelfrojd, Senad, et al.
(författare)
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A Back-to-Back 2L-3L Grid Integration of a Marine Current Energy Converter
- 2015
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Ingår i: Energies. - : MDPI AG. - 1996-1073. ; 8:2, s. 808-820
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Tidskriftsartikel (refereegranskat)abstract
- The paper proposes a back-to-back 2L-3L grid connection topology for a marine current energy converter. A prototype marine current energy converter has been deployed by a research group at Uppsala University. The concept behind the prototype revolves around a fixed pitch vertical axis turbine directly connected to a permanent magnet synchronous generator (PMSG). The proposed grid connection system utilizes a well known and proven two level voltage source converter generator-side combined with a three-level cascaded H-bridge (CHB) multilevel converter grid-side. The multilevel converter brings benefits in terms of efficiency, power quality and DC-link utilization. The system is here presented for a single marine current energy converter but can easily be scaled up for clusters of marine current energy converters. Control schemes for both grid-side and generator-side voltage source converters are presented. The start-up, steady state and dynamic performance of the marine current energy converter are investigated and simulation results are presented in this paper.
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| 5. |
- Ekström Kelvinius, Filip, et al.
(författare)
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Graph-based machine learning beyond stable materials and relaxed crystal structures
- 2022
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Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 6:3
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Tidskriftsartikel (refereegranskat)abstract
- There has been a recent surge of interest in using machine learning to approximate density functional theory in materials science. However, many of the most performant models are evaluated on large databases of computed properties of, primarily, materials with precise atomic coordinates available, and which have been experimentally synthesized, i.e., which are thermodynamically stable or metastable. These aspects provide challenges when applying such models on theoretical candidate materials, for example for materials discovery, where the coordinates are not known. To extend the scope of this methodology, we investigate the performance of the crystal graph convolutional neural network on a data set of theoretical structures in three related ternary phase diagrams (Ti,Zr,Hf)-Zn-N, which thus include many highly unstable structures. We then investigate the impact on the performance of using atomic positions that are only partially relaxed into local energy minima We also explore options for improving the performance in these scenarios by transfer learning, either from models trained on a large database of mostly stable systems, or a different but related phase diagram. Models pretrained on stable materials do not significantly improve performance, but models trained on similar data transfer very well. We demonstrate how our findings can be utilized to generate phase diagrams with a major reduction in computational effort.
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| 8. |
- Ekström, Rickard, et al.
(författare)
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Electrical damping of linear generators for wave energy converters : A review
- 2015
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Ingår i: Renewable & sustainable energy reviews. - : Elsevier BV. - 1364-0321 .- 1879-0690. ; 42, s. 116-128
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Forskningsöversikt (refereegranskat)abstract
- The electrical damping of point-absorber wave energy converters is crucial to optimize the power output. Many circuit topologies have been proposed, but the possible increase in power absorption must be weighed against parameters such as cost, reliability and control system complexity. In this paper, the known electrical damping circuits are categorized, described and compared. The hydrodynamic damping of the buoy is covered, and how a linear generator can be used as a power take-off unit to apply a damping force. A qualitative comparison of the circuits is presented in the end. A more complex and costly power electronics system may be viable for wave energy converters (WECs) of large-scale power rating. However, for farm operation with small-scale WECs, a simpler and passive damping may be more suitable.
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| 10. |
- Ekström, Rickard, et al.
(författare)
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Evaluating Constant DC-Link Operation of Wave Energy Converter
- 2012
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Ingår i: Proceedings of the 31st ASME International Conference on Ocean, Offshore and Arctic Engineering, vol. 7. - 9780791844946 ; , s. 269-276
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Konferensbidrag (refereegranskat)abstract
- A Wave Energy Converter (WEC) based on a linear generator and a point-absorbing buoy has been developed at Uppsala University. Interconnecting an array of WECs in parallel requires a point of common coupling, such as a common DC-bus. The DC voltage level seen by the generator is directly linked to the electromagnetic damping of the generator A lower DC-level results in a higher damping factor and is important for increased absorption of the wave power The drawback is increased losses in generator windings and cable resistance. There will be an optimal DC-level for maximum power output. This is a function of not only generator and buoy characteristics, but the current sea state. Experimental results of the full-scale system have been carried out, and used as validation of a simulation model of the system. The model is then used to evaluate how the DC-level seen by the generator influence the power output. The results indicate that higher DC-levels should be used at higher sea states, and power output may vary by up to a factor five depending on which DC-level is chosen.
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