| 1. |
- Apelfrojd, Senad, et al.
(author)
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A Back-to-Back 2L-3L Grid Integration of a Marine Current Energy Converter
- 2015
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In: Energies. - : MDPI AG. - 1996-1073. ; 8:2, s. 808-820
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Journal article (peer-reviewed)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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| 2. |
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| 3. |
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| 4. |
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| 5. |
- Ekström Kelvinius, Filip, et al.
(author)
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Graph-based machine learning beyond stable materials and relaxed crystal structures
- 2022
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In: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 6:3
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Journal article (peer-reviewed)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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| 6. |
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| 7. |
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| 8. |
- Ekström, Rickard, et al.
(author)
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Electrical damping of linear generators for wave energy converters : A review
- 2015
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In: Renewable & sustainable energy reviews. - : Elsevier BV. - 1364-0321 .- 1879-0690. ; 42, s. 116-128
-
Research review (peer-reviewed)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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| 9. |
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| 10. |
- Ekström, Rickard, et al.
(author)
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Evaluating Constant DC-Link Operation of Wave Energy Converter
- 2012
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In: Proceedings of the 31st ASME International Conference on Ocean, Offshore and Arctic Engineering, vol. 7. - 9780791844946 ; , s. 269-276
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Conference paper (peer-reviewed)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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| 11. |
- Ekström, Rickard, et al.
(author)
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Evaluating Constant DC-Link Operation of Wave Energy Converter
- 2014
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In: Journal of Dynamic Systems Measurement, and Control. - : ASME International. - 0022-0434 .- 1528-9028. ; 136:1, s. 014501-
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Journal article (peer-reviewed)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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| 12. |
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| 13. |
- Ekström, Rickard, 1985-, et al.
(author)
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Fast solid-state tap-change using two current-controlled voltage-source inverters
- 2014
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In: IET Power Electronics. - : Institution of Engineering and Technology (IET). - 1755-4535 .- 1755-4543. ; 7:10, s. 2610-2617
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Journal article (peer-reviewed)abstract
- A new topology for on-load tap change using two current-controlled voltage-source inverters is demonstrated. With this topology, no additional tap change circuitry is required. This configuration can be useful when optimising the power extraction from renewable energy sources, and may replace a DC/DC converter. It is possible to run the inverters in parallel for higher current capacity, or run them in series across the transformer to improve the harmonic output. The topology can be extended to any number of taps, adding one mechanical contactor per tap. The procedure for the tap change, including the control implementations to keep active and reactive grid power flows constant throughout the tap change, is described. Experimental results are presented to show the viability of the proposed topology. Tap changes are conducted on all phases simultaneously, and individually at each phase current zero-crossing. Variations in grid power are investigated for the two strategies. Also, a dead-time in the tap change is introduced to evaluate how control system delays will appear as grid power dips.
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| 14. |
- Ekström, Rickard, et al.
(author)
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FPGA Control Implementation of a Grid-Connected Current-Controlled Voltage-Source Inverter
- 2013
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In: Journal of Control Science and Engineering. - : Hindawi Limited. - 1687-5249 .- 1687-5257. ; 2013, s. 10-
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Journal article (peer-reviewed)abstract
- The full control system of a grid-connected current-controlled voltage-source inverter (CC-VSI) has been designed and implemented on a field-programmable gate array (FPGA). Various control functions and implementation methods are described and discussed. The practical viability of the system is evaluated in an experimental setup, where a VSI supplies 30 kW into the local grid at 400 V. A phase-locked loop (PLL) is used for grid phase tracking and evaluated for simulated abnormal grid conditions. Power factor is kept at unity, and the implemented control system is stressed with step responses in the supplied active power. A moving-average filter is implemented to reduce the effects of noise and harmonics on the current control loops. A coupling between active and reactive power flow is observed for the step responses but may be ignored in this context. The proposed system is fully comparable with more conventional microprocessor-based control systems.
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| 15. |
- Ekström, Rickard, et al.
(author)
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Grid Connection of Wave Power Farm Using an N-Level Cascaded H-Bridge Multilevel Inverter
- 2013
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In: Journal of Electrical and Computer Engineering. - : Hindawi Limited. - 2090-0147 .- 2090-0155. ; 2013, s. 9-
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Journal article (peer-reviewed)abstract
- An N-level cascaded H-bridge multilevel inverter is proposed for grid connection of large wave power farms. The point-absorber wave energy converters are individually rectified and used as isolated DC-sources. The variable power characteristics of the wave energy converters are discussed, and a method of mitigating this issue is demonstrated. The complete power control system is given in detail and has been experimentally verified for a single-phase setup of the 9-level inverter. Theoretical expressions of the power sharing between multilevel cells are derived and show good correspondence with the experimental results.
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| 16. |
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| 17. |
- Ekström, Rickard, et al.
(author)
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Lower order grid current harmonics for a voltage-source inverter connected to a distorted grid
- 2014
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In: Electric power systems research. - : Elsevier BV. - 0378-7796 .- 1873-2046. ; 106, s. 226-231
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Journal article (peer-reviewed)abstract
- As a result of lower order grid voltage harmonics, the grid current injected by a voltage-source inverter will also be partially distorted. In large-scale applications, active harmonic filters or notch filters are used to reduce the grid current distortion. For small-scale units, this may not be economically viable. In this article, two different grid phase tracking methods are evaluated with respect to the grid current distortion. The first method uses the zero-crossing detection (ZCD) method together with a look-up table (LUT), to generate a perfectly sinusoidal voltage synchronized with the grid. The second method uses a single-phase phase-locked loop (PLL). This method will reflect the grid harmonics in the inverter output, resulting in either cancellation or superposition of the harmonics of the grid current. A theoretical expression for the grid current as a function of the grid voltage harmonics is derived. Individual grid current harmonics as well as the total harmonic distortion (THD) are experimentally evaluated for both ZCD and PLL, and compared with theory. Results are presented for different power flows into the grid and compared with grid codes.
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| 18. |
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| 19. |
- Ekström, Rickard, et al.
(author)
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Measurement system design and implementation for grid-connected marine substation
- 2013
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In: Renewable energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; 55, s. 338-346
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Journal article (peer-reviewed)abstract
- A marine substation for a grid-connected wave power plant has been designed and constructed at Uppsala University. The measurement system for the substation has been developed from basics, and the procedure is described in this article. Subsea electrical installations set high constraints on the control and data acquisition systems used, and traditional GSM networks and GPS time synchronization are difficult and inefficient. These circumstances exclude many well-proven methods to be utilized. A compactRIO-based system, with integrated real-time controller and Field-Programmable Gate Array chip, is chosen as most suitable for this task. The system is designed to meet the special requirements encountered in this unusual application, including large data sampling, grid connection control and protection systems. The system is communicating to shore via a subsea copper cable and single-pair high-speed digital subscriber line modems. The design was successful and full-fills all the system requirements. The aim of the article is to provide future researchers with a good design and implementation procedure for setting up large measurement systems.
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| 20. |
- Ekström, Rickard, 1985-
(author)
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Offshore Marine Substation for Grid-Connection of Wave Power Farms : An Experimental Approach
- 2014
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Doctoral thesis (other academic/artistic)abstract
- Wave power is a renewable energy source with great potential, which is why there are more than a hundred ongoing wave power projects around the world. At the Division of Electricity, Uppsala University, a point-absorber type wave energy converter (WEC) has been proposed and developed. The WEC consists of a linear synchronous generator placed on the seabed, connected to a buoy floating on the surface. Power is absorbed by heave motion of the buoy, and converted into electric energy by the generator.The point-absorber WEC must be physically much smaller than the wavelength of the incoming waves, and can therefore not be scaled to very high power levels. Instead, the total power output is boosted by increasing the number of WECs, connecting them in wave power farms. To transfer the electric energy to the grid, an intermediate marine substation is proposed, where an AC/DC/AC conversion step is performed.Within this PhD-work, a full-scale offshore marine substation has been designed, constructed and experimentally evaluated. The substation is rated for grid-connection of seven WECs to the local 1kV-grid, and is placed on the seabed 3km off the coast at a depth of 25m. Various aspects of the substation design have been considered, including the mechanical and electrical systems, the WEC electrical interface, offshore operations and the automatic grid connection control system. A tap change circuit and different multilevel topologies have also been proposed.This dissertation has an experimental approach, validating a major part of the work with lab results. The final substation electrical circuit has been tested at rated grid voltage with a fluctuating input power source. The efficiency has been measured and the implemented functions are verified. Offshore operations have been successfully carried out and offshore wave farm data is expected in the nearby future.
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| 21. |
- Ekström, Rickard, et al.
(author)
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Transformer Magnetization Losses Using a Nonfiltered Voltage-Source Inverter
- 2013
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In: Advances in Power Electronics. - : Hindawi Limited. - 2090-181X .- 2090-1828. ; 2013, s. 7-
-
Journal article (peer-reviewed)abstract
- Results from the magnetization of an 80 kVA power transformer, using a directly coupled nonfiltered three-phase voltage-source inverter (VSI), are presented. The major benefits of this topology are reduction in switching filter size as well as filter losses. Drawbacks include higher stress on the transformer windings and higher transformer magnetization losses. In this paper, the total magnetization losses are presented for different voltage levels. The transformer has been magnetized with the rated frequency of 50 Hz at various voltage levels. The saturation characteristics as well as the magnetizing resistance are derived as functions of the voltage. These are used as inputs for the simulations. The magnetization losses have been experimentally measured and simulated for three different DC levels. Results from the simulations show good agreement with the experimental results. As expected, the pulsed voltage waveforms generate larger magnetization losses than the corresponding 50 Hz case. The losses are strongly dependent on the DC level.
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| 22. |
- Ekström, Rickard, et al.
(author)
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Transformer Magnetizing Inrush Currents Using a Directly Coupled Voltage-Source Inverter
- 2013
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In: ISRN Electronics. - : Hindawi Limited. - 2090-8679. ; 2013, s. 8-
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Journal article (peer-reviewed)abstract
- The connection of a power transformer to the grid is associated with magnetizing inrush currents that may result in power quality issues as well as faulty relay tripping. In distributed generation, the transformer may instead be premagnetized from the source to avoid this. In this paper, a VSI is directly coupled to a transformer. Three different strategies of premagnetization are implemented into the control system, and the inrush currents are measured for various values of the remanent flux in the core. The results show good reduction in the peak magnetizing inrush currents without using any external circuitry.
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| 23. |
- Hellstrom, Per Anders Rickard, 1975-, et al.
(author)
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Evaluation of the IngVaL Pedobarography System for Monitoring of Walking Speed
- 2018
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In: Healthcare Informatics Research. - Seoul : Korean Society of Medical Informatics (KOSMI). - 2093-3681 .- 2093-369X. ; 24:2, s. 118-124
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Journal article (peer-reviewed)abstract
- ObjectivesWalking speed is an important component of movement and is a predictor of health in the elderly. Pedobarography, the study of forces acting between the plantar surface of the foot and a supporting surface, is an approach to estimating walking speed even when no global positioning system signal is available. The developed portable system, Identifying Velocity and Load (IngVaL), is a cost effective alternative to commercially available pedobarography systems because it only uses three force sensing resistors. In this study, the IngVaL system was evaluated. The three variables investigated in this study were the sensor durability, the proportion of analyzable steps, and the linearity between the system output and the walking speed.MethodsData was collected from 40 participants, each of whom performed five walks at five different self-paced walking speeds. The linearity between the walking speed and step frequency measured with R2 values was compared for the walking speed obtained ‘A’ only using amplitude data from the force sensors, ‘B’ that obtained only using the step frequency, and ‘C’ that obtained by combining amplitude data for each of the 40 test participants.ResultsImprovement of the wireless data transmission increased the percentage of analyzable steps from 83.1% measured with a prototype to 96.6% for IngVaL. The linearity comparison showed that the methods A, B, and C were accurate for 2, 15, and 23 participants, respectively.ConclusionsIncreased sensor durability and a higher percentage of analyzed steps indicates that IngVaL is an improvement over the prototype system. The combined strategy of amplitude and step frequency was confirmed as the most accurate method.
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| 24. |
- Hellstrom, Per Anders Rickard, 1975-, et al.
(author)
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Intelligent Wireless Body Area Network System for Human Motion Analysis
- 2015
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Conference paper (peer-reviewed)abstract
- Human motion analysis provides several important applications. Examples are fall risk assessment, sports biomechanics, physical activity monitoring and rehabilitation. This work in progress paper proposes an intelligent wireless body area network system for motion and gait symmetry analysis. A Bluetooth network with accelerometers, gyroscopes and in-shoe force sensing resistors gathers data and sends it to a web server after intelligent pre-processing and filtering. The system is flexible and adaptable for different use cases including combinations of gait analysis, gait symmetry and pressure measurements between foot and shoe.
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| 25. |
- Hellstrom, Per Anders Rickard, 1975-, et al.
(author)
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Walking Intensity Estimation with a Portable Pedobarography System
- 2016
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In: Studies in Health Technology and Informatics. - : IOS Press. ; , s. 27-32
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Conference paper (peer-reviewed)abstract
- The aim of this pilot study was to investigate the possibility to find a correlation between the output from a portable pedobarography system and the walking intensity expressed as walking speed. The system uses shoe insoles with force sensing resistors and wireless transmission of the data via Bluetooth. The force-time integral, at the toe-off phase of the step, for the force sensors in the forward part of the right foot was used to measure impulse data for 10 subjects performing walks in three different walking speeds. This data was then corrected by multiplication with the step frequency. This pilot study indicates that the portable pedobarography system output shows a linear relationship with the walking intensity expressed as walking speed on an individual level.
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| 26. |
- Hellstrom, Per Anders Rickard, 1975-, et al.
(author)
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Wearable Weight Estimation System
- 2015
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In: Procedia Computer Science, vol. 64. - : Elsevier BV. ; , s. 146-152
-
Conference paper (peer-reviewed)abstract
- Heavy working conditions, as well as sedentary behaviour, are risk factors for health. There is a lack of wearable measurement systems for monitoring carried loads while walking. Pedobarography, the study of force fields acting between the plantar surface of the foot and a supporting surface, is supposed to be useable for estimating carried loads. Purpose. The aim of this paper is to present a novel method for selecting appropriate measurement samples for weight estimation of carried load during walk and a wearable system, based on pedobarography, consisting of commercial off the shelf components. The main idea is to choose samples when half of the total weight is on the forward sensors and the other half is on the heel sensor “equipoise” in one foot while the other foot not touches the ground. Methods. The system consists of insoles with force sensing resistors, data acquisition with IOIO-OTG and analysis in Excel. Each subject was weighed on an electronic floor scale. Three walks were performed on level ground. The first walk without any added load and then with two increases of carried load. Equipoise was defined as having half the weight distributed on the heel and the other half over the metatarsal pad. An equipoise value of 0.5 represents equilibrium regarding the weight distribution on one foot, with the other foot in the air. Samples were chosen in the equipoise region of 0.5±0.1 and then the average of the samples collected during one minute estimated the total weight. Results. The system can detect increases in carried loads but has a tendency to overestimate them. The estimated value was always larger with increased weight but the system was not always linear. The average overestimation error was 16.7 kg. Discussion. This study shows that this type of wearable system is usable for estimating carried load during walk after calibration of the system to the body weight force distribution on the sensors. There is still need for future development to obtain real-time analysis and direct feedback. A smaller and lighter measurement system is also desirable. Conclusion. This study shows that the novel method, equipoise, is usable for selecting appropriate measurement samples for weight estimation of carried load during walk. This study also shows that the wearable system, consisting of commercial off the shelf components, can be used for these measurements. However, there is a tendency to overestimate the loads.
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| 27. |
- Hellström, Per Anders Rickard, 1975-
(author)
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Wearable Pedobarography System for Monitoring of Walk Related Parameters
- 2019
-
Doctoral thesis (other academic/artistic)abstract
- Health care costs have increased over the last decades due to an ageing population. Therefore, research in personal health monitoring (PHM) has increased in response to this. PHM has advantages such as mobility (monitoring of health at work or at home), early detection of health problems enabling preventive health measures and a reduction of health care cost. Human motion analysis, using for example pedobarography (PBG), is an important subcategory of PHM. PBG is used to study the force fields acting between the plantar surface of the foot and a supporting surface. Gait and posture analysis, prosthetics evaluation and monitoring of recovery from injury or disease are examples of PBG applications. Portable PBG can be performed using force sensing resistors built into the insole inside the shoe. In accordance with this, the research aim for this thesis is to design, build and evaluate a wireless wearable measurement system based on PBG for monitoring of walk related parameters. Monitoring of carried weight and walking speed were chosen as the applications for validation of the system. Motivations for choosing these applications are that there is a lack of a wearable system for monitoring of weight while walking and a possible combination with accelerometers to improve the estimation of walking speed. Both walking speed and weight are important factors for estimating energy expenditure. A portable system, that estimates weight while walking, enables monitoring of heavy working conditions.The main research contributions include design of a PBG measurement system with a sensor implementation resulting in good sensor durability, several novel methods for weight estimation during walk and a novel method for analysing walking intensity and relating it to walking speed. The research results show that the new PBG system, in combination with the novel analysing methods, are suitable for use in wearable systems for monitoring of health related walk parameters.
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| 28. |
- Hellström, Per Anders Rickard, 1975-
(author)
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Wireless Wearable Measurement System Based on Pedobarography for Monitoring of Health
- 2016
-
Licentiate thesis (other academic/artistic)abstract
- Health care costs have increased over the last decades due to an ageing population. Therefore, research in personal health monitoring (PHM) has increased in response to this. PHM has advantages such as mobility (monitoring of health at work or at home), early detection of health problems enabling preventive health measures and a reduction of health care cost. Human motion analysis, using for example inertial measurement units and pedobarography, is an important subcategory of PHM. Pedobarography (PBG) is the study of pressure fields acting between the plantar surface of the foot and a supporting surface. Gait and posture analysis, prosthetics evaluation and monitoring of recovery from injury or disease are examples of PBG applications. Portable PBG can be performed using force sensing resistors built into the insole inside the shoe.In accordance with this, the research goal for this licentiate thesis is to design, build and evaluate a wireless wearable measurement system based on pedobarography for monitoring of health. In order to fulfil the objectives of the research, literature studies were done and problems with existing in-shoe system solutions were identified. Thus, it was found that further opportunities existed for new designs of PBG systems which take these problems into account. Cross-sectional test case studies were used for validation. The research area is multidisciplinary and encompasses biomedical measurements, electronics and computer science.The main research contributions include design and implementation of a PBG measurement system consisting of commercial off the shelf components, a novel method for selecting measurement samples for weight estimation of carried load during walk, and a novel method for analysing walking intensity using force-time integrals at the toe-off phase of the step. The research results suggest that the new PBG system, in combination with the two novel analysing methods, are suitable for use in wearable systems for monitoring of health. Personal health measurements are done to help decision making related to health. Thus, the future work will strive towards designing different decision support systems.
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| 29. |
- Hofmann, Robin, et al.
(author)
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Oxygen therapy in suspected acute myocardial infarction
- 2017
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In: New England Journal of Medicine. - : MASSACHUSETTS MEDICAL SOC. - 0028-4793 .- 1533-4406. ; 377:13, s. 1240-1249
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Journal article (peer-reviewed)abstract
- BACKGROUND: The clinical effect of routine oxygen therapy in patients with suspected acute myocardial infarction who do not have hypoxemia at baseline is uncertain. METHODS: In this registry-based randomized clinical trial, we used nationwide Swedish registries for patient enrollment and data collection. Patients with suspected myocardial infarction and an oxygen saturation of 90% or higher were randomly assigned to receive either supplemental oxygen (6 liters per minute for 6 to 12 hours, delivered through an open face mask) or ambient air. RESULTS: A total of 6629 patients were enrolled. The median duration of oxygen therapy was 11.6 hours, and the median oxygen saturation at the end of the treatment period was 99% among patients assigned to oxygen and 97% among patients assigned to ambient air. Hypoxemia developed in 62 patients (1.9%) in the oxygen group, as compared with 254 patients (7.7%) in the ambient-air group. The median of the highest troponin level during hospitalization was 946.5 ng per liter in the oxygen group and 983.0 ng per liter in the ambient-air group. The primary end point of death from any cause within 1 year after randomization occurred in 5.0% of patients (166 of 3311) assigned to oxygen and in 5.1% of patients (168 of 3318) assigned to ambient air (hazard ratio, 0.97; 95% confidence interval [CI], 0.79 to 1.21; P=0.80). Rehospitalization with myocardial infarction within 1 year occurred in 126 patients (3.8%) assigned to oxygen and in 111 patients (3.3%) assigned to ambient air (hazard ratio, 1.13; 95% CI, 0.88 to 1.46; P=0.33). The results were consistent across all predefined subgroups. CONCLUSIONS: Routine use of supplemental oxygen in patients with suspected myocardial infarction who did not have hypoxemia was not found to reduce 1-year all-cause mortality. (Funded by the Swedish Heart–Lung Foundation and others; DETO2X-AMI ClinicalTrials.gov number, NCT01787110.)
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| 30. |
- Kurupath, Venugopalan, et al.
(author)
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Optimal Constant DC Link Voltage Operation of aWave Energy Converter
- 2013
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In: Energies. - : MDPI AG. - 1996-1073. ; 6:4, s. 1993-2006
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Journal article (peer-reviewed)abstract
- This article proposes a simple and reliable damping strategy for wave powerfarm operation of small-scale point-absorber converters. The strategy is based on passiverectification onto a constant DC-link, making it very suitable for grid integration of the farm.A complete model of the system has been developed in Matlab Simulink, and uses real sitedata as input. The optimal constant DC-voltage is evaluated as a function of the significantwave height and energy period of the waves. The total energy output of the WEC is derivedfor one year of experimental site data. The energy output is compared for two cases, onewhere the optimal DC-voltage is determined and held constant at half-hour basis throughoutthe year, and one where a selected value of the DC-voltage is kept constant throughout theyear regardless of sea state.
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| 31. |
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| 32. |
- Parwal, Arvind, et al.
(author)
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Wave Energy Research at Uppsala University and The Lysekil Research Site, Sweden : A Status Update
- 2015
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Conference paper (peer-reviewed)abstract
- This paper provides a summarized status update ofthe Lysekil wave power project. The Lysekil project is coordinatedby the Div. of Electricity, Uppsala University since 2002, with theobjective to develop full-scale wave power converters (WEC). Theconcept is based on a linear synchronous generator (anchored tothe seabed) driven by a heaving point absorber. This WEC has nogearbox or other mechanical or hydraulic conversion systems,resulting in a simpler and robust power plant. Since 2006, 12 suchWECs have been build and tested at the research site located atthe west coast of Sweden. The last update includes a new andextended project permit, deployment of a new marine substation,tests of several concepts of heaving buoys, grid connection,improved measuring station, improved modelling of wave powerfarms, implementation of remote operated vehicles forunderwater cable connection, and comprehensive environmentalmonitoring studies.
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| 33. |
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| 34. |
- Vickhoff, Björn, 1946, et al.
(author)
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Music structure determines heart rate variability of singers
- 2013
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In: Frontiers in Psychology. - : Frontiers Media SA. - 1664-1078. ; 4
-
Journal article (peer-reviewed)abstract
- Choir singing is known to promote wellbeing. One reason for this may be that singing demands a slower than normal respiration, which may in turn affect heart activity. Coupling of heart rate variability (HRV) to respiration is called Respiratory sinus arrhythmia (RSA). This coupling has a subjective as well as a biologically soothing effect, and it is beneficial for cardiovascular function. RSA is seen to be more marked during slow-paced breathing and at lower respiration rates (0.1 Hz and below). In this study, we investigate how singing, which is a form of guided breathing, affects HRV and RSA. The study comprises a group of healthy 18 year olds of mixed gender. The subjects are asked to; (1) hum a single tone and breathe whenever they need to; (2) sing a hymn with free, unguided breathing; and (3) sing a slow mantra and breathe solely between phrases. Heart rate (HR) is measured continuously during the study. The study design makes it possible to compare above three levels of song structure. In a separate case study, we examine five individuals performing singing tasks (1–3). We collect data with more advanced equipment, simultaneously recording HR, respiration, skin conductance and finger temperature. We show how song structure, respiration and HR are connected. Unison singing of regular song structures makes the hearts of the singers accelerate and decelerate simultaneously. Implications concerning the effect on wellbeing and health are discussed as well as the question how this inner entrainment may affect perception and behavior.
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