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- Gravråkmo, Halvar
(författare)
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Buoy for linear wave energy converter
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A wave energy converter (WEC) of point absorber type has been developed and tests have been conducted outside Lysekil. The project started in 2002 and linear permanent magnet generators together with a subsea substation and buoys of various geometric shapes have been built and tested. The system is based on a low number of mechanical moving parts and the power conversion from ocean waves to electricity maintaining the quality of the national grid is handled electrically, due to the long life span of electric components. Reliability is highly prioritized in this design. To monitor the test site, measurements of electric output are done on the generators and substation. Also measurements of acceleration in heave mode are done on the buoy itself together with measurements of force between the buoy and the generator. The measurements are transmitted through the public cellular network. Also a internet based camera is set up at the site to monitor the buoys of the WECs visually. The monitoring systems, both visual and quantitative have proven to work successfully. In order for a WEC to produce electricity at competitive prices, the generator must not be larger than necessary in order to save economically on production, transport and installation. However, the WEC must be dimensioned to withstand harsh sea states. High added mass will in some cases create harsh inertia forces on the generator and large inertia forces on the buoy which might shorten the life time of the system considerably. The magnitude of the unwanted forces can be reduced by taking account for added mass when choosing a buoy geometry. A toroidal buoy is found to have less added mass than a vertical cylindrical buoy with similar excitation force.
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| 2. |
- Gravråkmo, Halvar, 1979-
(författare)
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Buoy Geometry, Size and Hydrodynamics for Power Take Off Device for Point Absorber Linear Wave Energy Converter
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 3. |
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| 4. |
- Gravråkmo, Halvar, et al.
(författare)
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Measurements of Extreme Forces on a Wave Energy Converter of Point Absorber Type and Estimation of Added Mass of Cylindrical Buoy
- 2014
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Ingår i: 33Rd International Conference On Ocean, Offshore And Arctic Engineering, 2014, Vol 9B. - : ASME Press. - 9780791845547
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Konferensbidrag (refereegranskat)abstract
- A wave energy converter (WEC) of point absorber type is tested at the west coast of Sweden. The buoy is a vertical cylinder. The linear generator on the seabed has limited stroke length. Large waves cause the generator to reach its maximum stroke length. As this happen, a spring in the generator is compressed, causing the buoy to instantly come to rest. During this process the force between the buoy and the generator is measured. Also the acceleration of the buoy is measured. This process and the extreme forces on the generator hull is described and the study shows that the magnitude of this force is greatly influenced by the added mass of the buoy and thus the buoy geometry. The ratio between the extreme forces on the hull and the forces during normal operation will affect the dimensioning and economy of the WEC. Force acting between generator and buoy were measured during various events as the WEC was operating. Added mass was estimated from the measurements
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