| 1. |
- Boström, Cecilia, et al.
(författare)
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Design proposal of electrical system for linear generator wave power plants
- 2009
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Ingår i: 35TH ANNUAL CONFERENCE OF IEEE INDUSTRIAL ELECTRONICS. - : IEEE. - 9781424446483 - 9781424446506 ; , s. 4180-4185
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Konferensbidrag (refereegranskat)abstract
- This paper describes an electrical system layout for a wave power plant connecting linear generators to the grid. The electrical power out from the wave energy converters must be converted before they can be connected to the grid. The conversion is carried out in marine substations that will be placed on the seabed.The paper presents experimental power data from a wave energy converter that has been in operation at the Lysekil research site since March 2006. Moreover, results and analyses from experiments and simulations from tests with the generator connected to a rectifier and filter are presented. A simulation is made to show the difference between having the generator connected to a linear load and a nonlinear load, which would be the case when the generator is connected to the grid.
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| 2. |
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| 3. |
- Ekweoba, Chisom Miriam, 1992-
(författare)
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Hydro-mechanical optimization of a wave energy converter
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wave energy conversion technology has gained popularity due to its potential to be-come one of the most preferred energy sources. Its high energy density and low car-bon footprint have inspired the development of many wave energy converter (WEC) technologies, few of which have made their way to commercialisation, and many are progressing.The Floating Power Plant (FPP) device is a combined floating wind and wave converter. The company, Floating Power Plant, was established in 2004 and has developed and patented a floating device that consists of a semi-submersible that serves as a foundation for a single wind turbine and hosts four wave energy converters (WECs). Each WEC consists of a partially submerged wave absorber whose pitching motion generates energy from incoming waves. The wave absorbers are connected to an oil hydraulic power take-off system located in a dry “engine room” above the free water surface, where the mechanical energy in the absorber is converted to electricity. When undergoing pitching movements, there are interactions between individual wave absorbers and the surrounding platform. This thesis focuses on developing methods to improve the FPP WEC’s hydrodynamic interactions.The first part of this thesis optimises the wave absorber (WA) ballast. An ana-lytical model is developed to enable systematic selection of WA ballast combination with significantly less computational effort when compared with the more conven-tional means, such as using CAD software. The study suggests an algorithm with which the absorbed power and resonance frequency can be improved and adjusted by manipulating the ballasts’ mass, the position of its centre of gravity, placement and inclination of the WA. The proposed method is generic and can be applied to other WEC concepts or submerged bodies in general. The results show the feasibility of designing the absorber ballast to offer passive control for increased wave absorption. It demonstrates the effect of ballast on the WA inclination, resonance frequency and response amplitude operator (RAO).The second part focuses on the optimisation of the FPP platform geometry. The genetic algorithm optimisation technique is implemented to maximise the annual en-ergy produced by the relative pitch motion of the WA to the floating platform. The optimised variables are characteristic lengths of the floating platform, most of which are part of the immediate surrounding walls of the absorber. The objective function is a function of the WA’s annual energy production (AEP) and RAO. Results show the feasibility of improving the hydrodynamic interaction between the floating platform and its integrated wave absorbers for a given wave climate by using a heuristic search technique. The number of iterations to convergence tends towards increased values when considering more optimised variables. It is also observed that the computational time appears to be independent of the number of variables but is significantly impacted by the computational power of the machine used.
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| 4. |
- 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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| 5. |
- 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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| 6. |
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| 7. |
- 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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| 8. |
- Gravråkmo, Halvar, et al.
(författare)
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Power Production by Linear Wave Energy Converters of Point Absorber Type with Vertical Cylindrical Buoys during Various Sea States
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Tidskriftsartikel (refereegranskat)abstract
- Two wave energy converters (WEC) were deployed near the city of Lysekil as part of Uppsala University's wave energy project. The converters were electrically identical and mechanically very similar. The converters were installed on the seabed at 25 m depth at the same site and believed to be exposed to very similar wave conditions. They utilized permanent magnets oscillating vertically due to surface buoys actuating the generators. The buoys were vertical cylinders excitated by ocean waves, with different diameters but with equal volume. The power production from the converters and the sea states were measured and comparison of power production between the two converters during various sea states was conducted. Also wind and tidal oscillations were considered and were found to influence the power production directly or indirectly, as did also significant wave height and energy period. The only difference between the two converters were the buoys, as the generators and electric loads were equal. During a 4 day period of various wave climates, the WEC with the buoy with 78 % larger water plane area than the WEC with the more narrow buoy with a water plane area of 7.07 m2 was found to produce 11 % more power.
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| 9. |
- Gravråkmo, Halvar, et al.
(författare)
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Toroidal Buoy for Point Absorver WEC with Focus on Added Mass and Overloads
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
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