uu.seUppsala University Publications
Change search
ReferencesLink to record
Permanent link

Direct link
Toroidal Buoy for Point Absorver WEC with Focus on Added Mass and Overloads
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
Show others and affiliations
(English)In: Article in journal (Refereed) Submitted
Abstract [en]

In order for a wave energy converter (WEC) to produce electricity at competitive prices, the generator must not be over dimensioned in order to save money on production, transport and installation, but the WEC, in this case a point absorber, must also be dimensioned to withstand harsh sea states. High added mass will in some cases create severe inertia forces on the generator and might shorten the service life of the system. The magnitude of the overload forces can be reduced by choosing a buoy geometry with small added mass. Measurements of snap loads produced by a toroidal buoy and calculations of buoy intertia forces causing overloads on a generator with a toroidal buoy and an equal generator with a cylindrical buoy are investigated in order to compare the two shapes numerically. A toroidal buoy was found to have less added mass than a vertical cylindrical buoy with similar excitation force, which caused reduced overloads at short wave periods, thus reducing the overloads on the generator. Snap loads were found to not cause much greater loads than other forces found during operation.

Keyword [en]
WEC, point absorber, toroidal, torus; buoy, overload, survivability, wave, energy, converter
National Category
Energy Engineering
URN: urn:nbn:se:uu:diva-220341OAI: oai:DiVA.org:uu-220341DiVA: diva2:704664
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2014-09-18
In thesis
1. Buoy Geometry, Size and Hydrodynamics for Power Take Off Device for Point Absorber Linear Wave Energy Converter
Open this publication in new window or tab >>Buoy Geometry, Size and Hydrodynamics for Power Take Off Device for Point Absorber Linear Wave Energy Converter
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Wave energy converters of point absorber type have been developed and constructed. Full scale experiments have been carried out at sea and electricity has been successfully delivered. Linear permanent magnet generators together with a subsea substation and buoys of various geometric shapes have been investigated theoretically and experimentally. The design has in large extent an electronic approach, keeping the mechanical part of it as simple as possible, due to the long life span and reliability of electric components.

Because of the nature of a linear generator, the internal translator with permanent magnets has a limited stroke length which will be reached when the buoy is exposed to large wave heights. Internal springs at the top and bottom of the generator prevent the translator from hitting the generator hull. Inertial forces due to the mass and velocity of the translator and the buoy and its heave added mass compresses the spring. The added mass is a rather large part of the total moving mass. Simulations of a converter with a vertical cylindrical buoy and with a toroidal buoy were conducted, as well as real sea experiments with converters with cylindrical buoys of two different sizes and a toroidal buoy. The overloads are likely to affect the design and service life of the generator, the buoy and the wire which interconnects them.

Buoy shapes with as much excitation force as possible and as little heave added mass as possible were sought. A toroidal buoy caused less overloads on the generator at sea states with short wave periods and relatively large wave height, but for sea states with very long wave periods or extremely high waves, the magnitude of the overloads was mainly determined by the maximum displacement of the buoy.

Snap loads on the interconnecting wire, as the slack wire tensed up after a very deep wave trough, were found to be greater but of the same order of magnitude as forces during the rest of the wave cycle.

During a 4 day period at various wave conditions, two converters with cylindrical buoys proved efficiency between 11.1 % and 24.4 %. The larger buoy had 78 % larger water plane area than the other buoy which resulted in 11 % more power production. Short wave period was beneficial for the power production.

Infinite frequency heave added mass was measured for a cylindrical buoy at real sea and found to be greater than the linearly calculated theoretical added mass.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 71 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1130
point absorber, wave, energy, converter, ocean, wec, toroidal, buoy, torus, cylindrical, cylinder, experiment, full scale, trial, sea, energy, renewable
National Category
Energy Engineering Fluid Mechanics and Acoustics
Research subject
Engineering Science with specialization in Science of Electricity
urn:nbn:se:uu:diva-220344 (URN)978-91-554-8902-1 (ISBN)
Public defence
2014-04-28, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Available from: 2014-04-04 Created: 2014-03-13 Last updated: 2014-09-18Bibliographically approved

Open Access in DiVA

No full text

Search in DiVA

By author/editor
Gravråkmo, HalvarStrömstedt, ErlandEngström, JensSvensson, OlleLeijon, Mats
By organisation
Energy Engineering

Search outside of DiVA

GoogleGoogle Scholar
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Total: 265 hits
ReferencesLink to record
Permanent link

Direct link