| 1. |
|
|
| 2. |
- Göteman, Malin, 1980-, et al.
(author)
-
Methods of reducing power fluctuations in wave energy parks
- 2014
-
In: Journal of Renewable and Sustainable Energy. - : AIP Publishing. - 1941-7012. ; 6, s. 043103-
-
Journal article (peer-reviewed)abstract
- One of the major challenges in constructing effective and economically viable wave energy parks is to reduce the large fluctuations in power output. In this paper, we study different methods of reducing the fluctuations and improve the output power quality. The parameters studied include the number of devices, the separating distance between units, the global and local geometries of the array, sea state and incoming wave direction, and the impact of including buoys of different radii in an array. Our results show that, e. g., the fluctuations as well as power per device decrease strictly with the number of interacting units, when the separating distance is kept constant. However, including more devices in a park with fixed area will not necessarily result in lowered power fluctuations. We also show that varying the distance between units affects the power fluctuations to a much larger extent than it affects the magnitude of the absorbed power. The fluctuations are slightly lower in more realistic, randomized geometries where the buoys tend to drift slightly off their mean positions, and significantly lower in semi-circular geometries as opposed to rectangular geometries.
|
|
| 3. |
- Engström, Jens, 1977-
(author)
-
Hydrodynamic Modelling for a Point Absorbing Wave Energy Converter
- 2011
-
Doctoral thesis (other academic/artistic)abstract
- Surface gravity waves in the world’s oceans contain a renewable source of free power on the order of terawatts that has to this date not been commercially utilized. The division of Electricity at Uppsala University is developing a technology to harvest this energy. The technology is a point absorber type wave energy converter based on a direct-driven linear generator placed on the sea bed connected via a line to a buoy on the surface. The work in this thesis is focused mainly on the energy transport of ocean waves and on increasing the transfer of energy from the waves to the generator and load. Potential linear wave theory is used to describe the ocean waves and to derive the hydrodynamic forces that are exerted on the buoy. Expressions for the energy transport in polychromatic waves travelling over waters of finite depth are derived and extracted from measured time series of wave elevation collected at the Lysekil test site. The results are compared to existing solutions that uses the simpler deep water approximation. A Two-Body system wave energy converter model tuned to resonance in Swedish west coast sea states is developed based on the Lysekil project concept. The first indicative results are derived by using a linear resistive load. The concept is further extended by a coupled hydrodynamic and electromagnetic model with two more realistic non-linear load conditions. Results show that the use of the deep water approximation gives a too low energy transport in the time averaged as well as in the total instantaneous energy transport. Around the resonance frequency, a Two-Body System gives a power capture ratio of up to 80 percent. For more energetic sea states the power capture ratio decreases rapidly, indicating a smoother power output. The currents in the generator when using the Two-Body system is shown to be more evenly distributed compared to the conventional system, indicating a better utilization of the electrical equipment. Although the resonant nature of the system makes it sensitive to the shape of the wave spectrum, results indicate a threefold increase in annual power production compared to the conventional system.
|
|
| 4. |
|
|
| 5. |
|
|
| 6. |
- Giassi, Marianna, 1988-
(author)
-
Numerical and experimental modelling for wave energy arrays optimization
- 2020
-
Doctoral thesis (other academic/artistic)abstract
- Many wave power conversion devices, especially point-absorbers, do not provide alone the necessary amount of converted electricity to be cost effective, instead they are designed to be deployed in arrays of many units. Such arrays, or parks, can satisfy a large-scale energy demand, reduce the costs of the produced electricity and improve the reliability of the system.The performance of a wave energy park is affected by multiple and mutually interacting parameters, and the complex problem that arises during its design is called array optimization.The scope of the present thesis is to study such systems and their design, by the development of an optimization routine able to predict the best layout of a wave energy park under fixed constraints. The wave energy converter considered is the point-absorber developed at Uppsala University, which consists of a linear electric generator located on the seabed and a floating buoy at the surface.An optimization routine based on a genetic algorithm was created, which allows simultaneous optimization of the geometry of the buoys, the damping coefficient of the linear generators and the geometrical layout of the park.Finally, an experimental campaign with a single device and three arrays of six devices was conducted in order to compare the theoretical results with experimentally acquired data.The results identify optimal configurations of wave energy arrays, and highlight the effect of optimizing upon different objective functions, including economical ones. In the experiments, standard models and common assumptions used for wave energy park optimizations were tested against realistic conditions.
|
|
| 7. |
- Gravråkmo, Halvar, 1979-
(author)
-
Buoy Geometry, Size and Hydrodynamics for Power Take Off Device for Point Absorber Linear Wave Energy Converter
- 2014
-
Doctoral thesis (other academic/artistic)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.
|
|
| 8. |
- Göteman, Malin, 1980-, et al.
(author)
-
Advances and Challenges in Wave Energy Park Optimization : A Review
- 2020
-
In: Frontiers in Energy Research. - : Frontiers Media SA. - 2296-598X. ; 8
-
Research review (peer-reviewed)abstract
- A commercial wave energy system will typically consist of many interacting wave energy converters installed in a park. The performance of the park depends on many parameters such as array layout and number of devices, and may be evaluated based on different measures such as energy absorption, electricity quality, or cost of the produced electricity. As wave energy is currently at the stage where several large-scale installations are being planned, optimizing the park performance is an active research area, with many important contributions in the past few years. Here, this research is reviewed, with a focus on identifying the current state of the art, analyzing how realistic, reliable, and relevant the methods and the results are, and outlining directions for future research.
|
|
| 9. |
- Göteman, Malin, 1980-, et al.
(author)
-
Arrays of Point-Absorbing Wave Energy Converters in Short-Crested Irregular Waves
- 2018
-
In: Energies. - : MDPI AG. - 1996-1073. ; 11:4
-
Journal article (peer-reviewed)abstract
- For most wave energy technology concepts, large-scale electricity production and cost-efficiency require that the devices are installed together in parks. The hydrodynamical interactions between the devices will affect the total performance of the park, and the optimization of the park layout and other park design parameters is a topic of active research. Most studies have considered wave energy parks in long-crested, unidirectional waves. However, real ocean waves can be short-crested, with waves propagating simultaneously in several directions, and some studies have indicated that the wave energy park performance might change in short-crested waves. Here, theory for short-crested waves is integrated in an analytical multiple scattering method, and used to evaluate wave energy park performance in irregular, short-crested waves with different number of wave directions and directional spreading parameters. The results show that the energy absorption is comparable to the situation in long-crested waves, but that the power fluctuations are significantly lower.
|
|
| 10. |
- Göteman, Malin, 1980-, et al.
(author)
-
Fast modeling of large wave energy farms using interaction distance cut-off
- 2015
-
In: Energies. - : MDPI AG. - 1996-1073. ; 8:12, s. 13741-13757
-
Journal article (peer-reviewed)abstract
- In many wave energy concepts, power output in the MW range requires the simultaneous operation of many wave energy converters. In particular, this is true for small point-absorbers, where a wave energy farm may contain several hundred devices. The total performance of the farm is affected by the hydrodynamic interactions between the individual devices, and reliable tools that can model full farms are needed to study power output and find optimal design parameters. This paper presents a novel method to model the hydrodynamic interactions and power output of very large wave energy farms. The method is based on analytical multiple scattering theory and uses time series of irregular wave amplitudes to compute the instantaneous power of each device. An interaction distance cut-off is introduced to improve the computational cost with acceptable accuracy. As an application of the method, wave energy farms with over 100 devices are studied in the MW range using one month of wave data measured at an off-shore site.
|
|
