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In-Situ Measurements of Relative Displacement between Piston Rod and Seal Housing with Estimations of Wear on a Sealing System in a Submerged Transmission of an Offshore Underwater Wave Energy Converter
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
(English)In: Journal of Renewable and Sustainable Energy, ISSN 1941-7012Article in journal (Other academic) Submitted
Abstract [en]

A type of submerged piston rod mechanical lead-through transmission has been designed and implemented inside the two first full scale wave energy converter (WEC) prototypes at the Lysekil wave energy research site on the Swedish west cost. The transmission constitutes a direct drive which transfers the mechanical force, absorbed from the waves by a heaving surface-floating buoy, through an encapsulation and into a permanent magnet linear generator bolted to a foundation on the seabed, while sealing off the corrosive seawater. A set-up of 7 laser triangulation sensors has been installed in the second wave energy converter prototype to measure the relative displacement of the piston rod and the seal housing in-situ. This paper focus on investigating the relative tilt angles of both objects, the differential tilt angle between the piston rod and the seal housing and how they move in the time and space as they tilt with the ocean waves. Estimations on wear of the sealing components in the dynamic sealing system are made. The measurements are performed for two sea states with 2.5 month separation. The paper describes the experimental set-up and the sensor measuring systems. Equations for calculating the relative differential tilt angle between piston rod and seal housing are presented. The calculated differential tilt angle is correlated with measurements of the piston stroke length and translator position performed with the draw-wire sensor. Tilt angles, azimuth angles and differential tilt angles are presented for one wave period in each measurement period. The wear is estimated by calculating the relative change in traversed clearance gap by the piston rod across the interface inside the seal housing. Results show the expected differential tilt angle early in the experiment. A substantial increase in differential tilt angle developed over 3 months time as a consequence of large wear between buoy line and guiding funnel causing a large tilt angle on the piston rod and unintentionally large normal forces between piston rod and the dynamic sealing system inside the seal housing. The results indicate the need of a more flexible mechanical lead-through design, which is presented at the end of the paper. The results are discussed and conclusions are drawn with specific emphasis on improving the sealing system with stronger guide elements and the patented mechanical lead-through design.

Keyword [en]
wave energy converter; linear generator; direct drive; piston rod; seal housing; mechanical lead-through; submerged transmission; relative displacement measurement; laser triangulation sensor; tilting; tilt angle; azimuth angle; estimation of wear; sealing component.
National Category
Energy Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-179737OAI: oai:DiVA.org:uu-179737DiVA: diva2:545960
Projects
The Lysekil Wave Power Project
Available from: 2012-08-21 Created: 2012-08-21 Last updated: 2012-09-07Bibliographically approved
In thesis
1. Submerged Transmission in Wave Energy Converters: Full Scale In-Situ Experimental Measurements
Open this publication in new window or tab >>Submerged Transmission in Wave Energy Converters: Full Scale In-Situ Experimental Measurements
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Different wave power technologies are in development around the world in different stages of prototype testing. So far only a few devices have been deployed offshore at full scale for extended periods of time. Little data is published about how these different devices perform.

This thesis presents results from experiments with the full-scale offshore wave energy converters at the Lysekil research site on the Swedish west coast. The theories, experiments, measurements, performance evaluations and developments of the submerged transmission in the direct driven permanent magnet linear generator are in focus. The reciprocating submerged transmission fulfills the purpose of transmitting the absorbed mechanical wave energy through the capsule wall into the generator, while preventing the seawater from entering the capsule and reducing the life time of the converter.

A measuring system with seven laser triangulation sensors has been developed to measure small relative displacements between piston rod and seal housing in the submerged transmission with excellent accuracy for the purpose of evaluating both functional behavior and successive wear in-situ. A method for calculating relative tilt angles, azimuth angles, differential tilt angles, and successive wear in the submerged transmission has been developed. Additional sensors systems have been installed in the converter enabling correlation and a thorough investigation into the operating conditions of the transmission and the converter. The thesis presents unique results from the measurements. A data acquisition system transmits the signals from the converter on the seabed to an onshore measuring station. Results are presented in time-, frequency- and the time-frequency domain.

The results have given important information for further development of the submerged transmission, which is important to the survivability of the system. The thesis describes the status of research, and is a step that may influence future designs of wave energy devices for reaching survivability and a cost-effective renewable energy system.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. 214 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 961
Keyword
Wave energy converter, direct drive, submerged transmission, piston rod, seal housing, sealing system, laser triangulation sensor, offshore measurements, relative displacement, vibrations, tilt angle, tilting, wear estimation.
National Category
Energy Systems Marine Engineering Ocean and River Engineering Energy Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-179740 (URN)978-91-554-8440-8 (ISBN)
Public defence
2012-09-14, Polhemssalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Opponent
Supervisors
Projects
The Lysekil Wave Power Project
Note

Published is a preprint version of the full text and should be combined by the errata.

Available from: 2012-08-24 Created: 2012-08-21 Last updated: 2013-01-22Bibliographically approved

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Strömstedt, Erland

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