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Wave Energy Converters: An experimental approach to onshore testing, deployments and offshore monitoring
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.ORCID iD: 0000-0001-6870-6729
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The wave energy converter (WEC) concept developed at Uppsala University consists of a point absorbing buoy, directly connected to a permanent magnet linear generator. Since 2006, over a dozen full scale WECs have been deployed at the Lysekil Research Site, on the west coast of Sweden. Beyond the development of the WEC concept itself, the full scale approach enables, and requires, experimental and multidisciplinary research within several peripheral areas, such as instrumentation, offshore operations, and wave power infrastructure.

This thesis addresses technical challenges of testing, deploying and monitoring full scale WECs. It is divided accordingly into three topics: offshore measurement systems, onshore WEC testing and deployments. Each topic presents new or improved technical solutions to enable offshore wave power research.

For the offshore measurement systems, a new portable data acquisition unit was developed, together with a new sensor system to be installed inside the WEC. The developed system offers a cheap and flexible option for short term offshore measurement ventures, when or where site infrastructure is not available. The system has been developed and tested during both onshore and offshore experiments, with promising results.

On the topic of onshore WEC testing, the thesis presents an experimental approach for assessing the power take-off (PTO) damping of the WEC. In previous experimental studies, it has been measured via the generated electrical power, which neglects both mechanical losses and iron losses. Consequently, the full PTO force acting on the WEC has been underestimated. The thesis presents experimentally attained trends for the speed dependence of the PTO damping at different resistive loads, as measured from both generated electric power and from measurements of the buoy line force. A study was also performed on how the generator damping is affected by partial stator overlap, which varies with the translator position. In order to assess how the characterized damping behavior will affect the WEC operation at sea, two simulation case studies were performed.

Finally, the thesis presents a new WEC deployment method, which has been developed through several deployment trials. By using only a tugboat, a WEC unit is transported and deployed, together with its buoy, in less than half a day. The procedure has proven to be faster, cheaper and safer than the previously used methods.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , p. 100
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1563
Keywords [en]
wave power, ocean energy, linear generator, measurements, sensors, point absorber, offshore, PTO, force
National Category
Energy Engineering Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-329856ISBN: 978-91-513-0077-1 (print)OAI: oai:DiVA.org:uu-329856DiVA, id: diva2:1143641
Public defence
2017-11-10, Siegbahnsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2017-10-18 Created: 2017-09-21 Last updated: 2018-03-08
List of papers
1. Offshore Measurement System for WavePower: Using Current Loop Feedback
Open this publication in new window or tab >>Offshore Measurement System for WavePower: Using Current Loop Feedback
2016 (English)In: Electronics, ISSN 2079-9292, Vol. 5, no 4, article id 86Article in journal (Refereed) Published
Abstract [en]

This paper presents the design and testing of a measurement system for wave powergenerators. The work is part of a project to build a robust and cheap measurement system for offshoremonitoring of wave power farms. Due to the harsh offshore environment, low accessibility andhigh cost for installation and maintenance, it is of key importance to minimize power consumption,complexity and cost of each measurement unit. For the first prototype, the objective was to measurevoltage, current and translator position inside the linear wave power generator. For this, twoprinted circuit boards (PCBs) were developed, using a two wire current loop transmitter setup.They were tested separately and in a three phase setup inside a wave power generator duringonshore tests. To ensure stability, speed and accuracy in the signal transfer, the PCBs were tested forlinearity, frequency response and step response. In addition, power consumption was measured,for operational time evaluation. Results show good agreement between expected and measuredperformance, with an input range of ±1560 V and ±420 A for alternating current measurements anda bandwidth of 10 kHz and 7 kHz, for voltage and current measurements, respectively. The powerconsumption was measured to 0.5 W for each measurement unit, at 24 V feed.

Keywords
wave power; conditional monitoring; PCB; current loop; measurement system
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-313258 (URN)10.3390/electronics5040086 (DOI)000392387600008 ()
Available from: 2017-01-18 Created: 2017-01-18 Last updated: 2017-10-08Bibliographically approved
2. Portable Data Acquisition Systemfor Offshore Applications
Open this publication in new window or tab >>Portable Data Acquisition Systemfor Offshore Applications
(English)In: IEEE Journal of Oceanic Engineering, ISSN 0364-9059, E-ISSN 1558-1691Article in journal (Refereed) Submitted
Abstract [en]

In the development of ocean energy technologies, full scale sea trials has proven expensive and challenging. Asa contribution to the development of flexible, reliable and affordable measurement systems for such sea trials, thispaper presents and evaluates a portable data acquisition system. The system is battery powered and consists of asensor system and a logger unit, where the logger and battery is placed in a retrievable pressure vessel. Sensor outputis carried with 4-20 mA current signals between sensor system and logger. The custom logger is programmable andoffers many options to apply different logging schedules and to limit the sensor system power supply accordingly.Evaluation show that the system implemented offers 5-22 weeks of 1 kHz, 16 bit monitoring of 16 signals, dependingon chosen logging schedule. Suggestions are also given for how to raise the operation time to up to 50 weeks.Although lacking the sophistication of a full signal infrastructure, the system offers a cheap and flexible option forwhen and where such infrastructure is no available.

Keywords
Data Acquisition, Ocean Energy, Wave Power, Logger, Offshore Measurments
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-329827 (URN)
Available from: 2017-09-21 Created: 2017-09-21 Last updated: 2017-10-08
3. Measurement System For Wave Energy Converter - Design And Implementation
Open this publication in new window or tab >>Measurement System For Wave Energy Converter - Design And Implementation
2014 (English)In: 33Rd International Conference On Ocean, Offshore And Arctic Engineering, 2014, Vol 9A: Ocean Renewable Energy, AMER SOC MECHANICAL ENGINEERS , 2014Conference paper, Published paper (Refereed)
Abstract [en]

A Wave Energy Converter (WEC) measurement system has been constructed and installed with the purpose to measure, log and evaluate the WEC's performance during operation at sea. The WEC is to be deployed at Uppsala University's wave power research site in Lysekil on the west coast of Sweden. In designing such a system the key research objectives has been (1) to study the risk of overheating due to high currents in the stator windings, (2) to evaluate how the WEC's outer structure withstands drag and bending forces from the buoy line and (3) to construct a detection system which indicates if water leaks into the generator. The measurement system was designed to collect data essential to study these key objectives. Transducers were used to measure: buoy line force, translator position, phase currents, bending and tensile strain on the generator hull, water level inside generator and the temperature at multiple places inside the generator. The measurement system has been installed and calibrated in the WEC. Furthermore, the design has been evaluated in lab experiments in order to verify the function and accuracy of the different measurements. This paper presents the underlying research objectives for developing the WEC generator measurement system, together with a description of the technical implementation.

Place, publisher, year, edition, pages
AMER SOC MECHANICAL ENGINEERS, 2014
National Category
Energy Systems Ocean and River Engineering
Identifiers
urn:nbn:se:uu:diva-272118 (URN)000363499000005 ()978-0-7918-4553-0 (ISBN)
Conference
33Rd International Conference On Ocean, Offshore And Arctic Engineering
Note

Första författaren har bytt efternamn till Ulvgård

Available from: 2016-01-12 Created: 2016-01-12 Last updated: 2017-09-28
4. Measurement System for Evaluating Wanted and Unwanted Forces on a Point Absorbing Wave Energy Converter during Offshore Operation
Open this publication in new window or tab >>Measurement System for Evaluating Wanted and Unwanted Forces on a Point Absorbing Wave Energy Converter during Offshore Operation
2015 (English)In: Proceedings of the 25th International Ocean and Polar Engineering Conference,, 2015Conference paper, Published paper (Refereed)
Abstract [en]

A force measurement system has been developed and installed in awave energy converter at Uppsala University. The force in theconnection line as well as the vertical and horizontal strain in thegenerator hull is measured. With these measurements the forces actingon the generator during offshore operation can be monitored andanalyzed. This paper presented the principle, implementation and testresults of the measurement system. A brief discussion of error sourcesand possible improvements is also given.

Keywords
Wave energy converter; strain gauges; offshore measurement system; offshore monitoring; force, strain
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-265217 (URN)
Conference
ISOPE 2015, Kona Island, Hawaii
Available from: 2015-10-25 Created: 2015-10-25 Last updated: 2017-09-21
5. Line Force and Damping at Full and Partial Stator Overlap in a Linear Generator for Wave Power
Open this publication in new window or tab >>Line Force and Damping at Full and Partial Stator Overlap in a Linear Generator for Wave Power
2016 (English)In: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 4, no 4, article id 81Article in journal (Refereed) Published
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.

Keywords
wave power, force measurement, line force, power take-off, damping
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-313254 (URN)10.3390/jmse4040081 (DOI)
Note

Liselotte Ulvgård and Linnea Sjökvist contributed equally to this work.

Available from: 2017-01-18 Created: 2017-01-18 Last updated: 2018-03-19Bibliographically approved
6. Speed Dependent PTO Damping in a Linear Generator for Wave Power - Measured Damping and Simulated WEC Behaviour
Open this publication in new window or tab >>Speed Dependent PTO Damping in a Linear Generator for Wave Power - Measured Damping and Simulated WEC Behaviour
(English)Manuscript (preprint) (Other academic)
National Category
Marine Engineering
Identifiers
urn:nbn:se:uu:diva-328494 (URN)
Available from: 2017-08-24 Created: 2017-08-24 Last updated: 2017-09-21
7. Partial Stator Overlap in a Linear Generator for Wave Power: An Experimental Study
Open this publication in new window or tab >>Partial Stator Overlap in a Linear Generator for Wave Power: An Experimental Study
Show others...
2017 (English)In: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 5, no 4, article id 53Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
MDPI AG, 2017
Keywords
Wave Power, linear generator, stator overlap, active area, end-stop forces
National Category
Energy Engineering Marine Engineering
Identifiers
urn:nbn:se:uu:diva-329833 (URN)10.3390/jmse5040053 (DOI)000423694200009 ()
Available from: 2017-09-21 Created: 2017-09-21 Last updated: 2018-03-19Bibliographically approved
8. Experimental study of generator damping at partial stator overlap in a linear generator for wave power
Open this publication in new window or tab >>Experimental study of generator damping at partial stator overlap in a linear generator for wave power
Show others...
2017 (English)In: 12th European Wave and Tidal Energy Conference Series, Cork, Ireland, 27 August - 1 September, 2017, 2017Conference paper, Oral presentation only (Refereed)
National Category
Marine Engineering
Identifiers
urn:nbn:se:uu:diva-328496 (URN)
Conference
EWTEC2017
Available from: 2017-08-24 Created: 2017-08-24 Last updated: 2017-09-21
9. Wave Energy Research at Uppsala University and The Lysekil Research Site, Sweden: A Status Update
Open this publication in new window or tab >>Wave Energy Research at Uppsala University and The Lysekil Research Site, Sweden: A Status Update
Show others...
2015 (English)Conference paper, Published paper (Refereed)
Abstract [en]

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.

Keywords
Wave energy, point absorber, experiments, arrays, generators, ROVs
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Ocean and River Engineering
Identifiers
urn:nbn:se:uu:diva-265218 (URN)
Conference
Proceedings of the 11th European Wave and Tidal Energy Conference. Nantes, France, September 2015
Available from: 2015-10-26 Created: 2015-10-26 Last updated: 2018-08-07Bibliographically approved
10. Status Update of the Wave Energy Research at Uppsala University
Open this publication in new window or tab >>Status Update of the Wave Energy Research at Uppsala University
Show others...
2013 (English)Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Aalborg, Denmark: , 2013
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-212701 (URN)
Conference
10th European Wave and Tidal Conference (EWTEC)
Available from: 2013-12-13 Created: 2013-12-13 Last updated: 2017-12-07
11. Offshore Deployments of Wave Energy Converters by Uppsala University
Open this publication in new window or tab >>Offshore Deployments of Wave Energy Converters by Uppsala University
2017 (English)In: Article in journal (Refereed) Submitted
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-329835 (URN)
Available from: 2017-09-21 Created: 2017-09-21 Last updated: 2017-09-27

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