| 1. |
- Chatzigiannakou, Maria Angeliki
(author)
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Offshore deployments of marine energy converters
- 2019
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Doctoral thesis (other academic/artistic)abstract
- The depletion warning of non-renewable resources, such as gas, coal and oil, and the imminent effects of climate change turned the attention to clean and fossil fuel-free generated electricity. University research groups worldwide are studying solar, wind, geothermal, biomass and ocean energy harvesting. The focus of this thesis is the wave and marine current energy researched at the division of Electricity at Uppsala University (UU). The main drawbacks that hinder the commercialization of marine energy converter devices is a high installation, operation, maintenance and decommissioning cost. Furthermore, these processes are highly weather dependent and thus, can be time consuming beyond planning. In this thesis, an evaluation of the cost, time and safety efficiency of the devices’ offshore deployment (both wave and marine current), and a comparative evaluation regarding the safety in the use of divers and remotely operated vehicles (ROVs) are conducted. Moreover, a risk analysis study for a common deployment barge while installing an UU wave energy converter (WEC) is presented with the aim to investigate the failure of the crane hoisting system.The UU wave energy project have been initiated in 2001, and since then 14 WECs of various designs have been developed and deployed offshore, at the Lysekil research site (LRS), on the Swedish west coast and in Åland, Finland. The UU device is a point absorber with a linear generator power take off. It is secured on the seabed by a concrete gravity foundation. The absorbed wave energy is transmitted to shore through the marine substation (MS) where all the generators are interconnected. In 2008 an UU spin-off company, Seabased AB (SAB), was established and so far has developed and installed several WECs and two MSs, after the UU devices main principle. SAB deployments were conducted in Sotenäs, Sweden, at the Maren test site (MTS) in Norway; and in Ada Foah, Ghana. The active participation and the thorough study of the above deployments led to a cost, time and safety evaluation of the methods followed. Four main methods were identified and the most suitable one can be chosen depending on the deployment type, for example, for single or mass device deployment.The first UU full scale marine current energy converter (MCEC) was constructed in 2007 at the Ångström Laboratory and deployed at Söderfors, in the river Dalälven in March 2013. The UU turbine is of a vertical axis type and is connected to a directly driven permanent magnet synchronous generator of a low-speed. With this deployment as an example, four MCEC installation methods were proposed and evaluated in terms of cost and time efficiency.A comparative study on the use of divers and ROVs for the deployment and maintenance of WECs at the LRS has been carried out, showing the potential time and costs saved when using ROVs instead of divers in underwater operations. The main restrictions when using divers and ROVs were presented. Most importantly, the modelling introduced is generalized for most types of wave energy technologies, since it does not depend on the structure size or type.Finally, a table of safe launch operation of a WEC is presented. In this table the safe, restrictive and prohibitive sea states are found for a single WEC deployment, using a barge and a crane placed on it. The table can be utilized as a guidance for offshore operations safety and can be extended for a variety of device types and vessels.
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| 2. |
- Fjellstedt, Christoffer
(author)
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Studies of the Grid Connection of Offshore Renewable Energy Sources : Technologies and Simulations
- 2024
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Doctoral thesis (other academic/artistic)abstract
- A substantial increase in renewable energy sources connected to the electrical grid is imperative to achieve net-zero emissions from the electricity sector. Marine energy sources, like marine current power and wave power, have the potential to significantly contribute to the increase of electricity from renewable energy sources. A crucial aspect of enabling marine energy utilization is the development of electrical systems for offshore renewable energy. Hence, this thesis addresses challenges regarding the grid connection of offshore renewable energy.Two important questions for offshore renewable energy are how to construct the offshore electrical grid and how to transmit the power to the shore. This thesis provides a review of AC and DC collection grid topologies and compares HVAC and HVDC transmission for offshore applications. It is concluded that HVDC is the preferred technology for transmission distances exceeding 50 to 100 km.Regardless of the configuration of the offshore collection grid, the energy converters must be connected to the collection and distribution grid. Uppsala University has deployed a marine current energy converter in the river Dalälven in Söderfors, Sweden. The grid connection system at the test site is based on a back-to-back converter technology. In the thesis, a simulation model of the grid connection system of the energy converter is presented. The simulation model is used to evaluate MPPT methods for marine current power. An advanced hydrodynamic model based on a two-dimensional free vortex method is utilized for this purpose. Additionally, a low-complexity hydrodynamic model is incorporated into the simulation model to assess electrical grids for marine current energy. One AC and one DC collection grid, each comprising five marine current energy converters, are compared. Furthermore, three DC collection grids, each with ten marine current energy converters, are assessed and compared.The grid connection system at the Söderfors test site includes an LC filter connected to a power transformer. A novel transfer function is derived for this system, and the transfer function is verified with simulations and experimental investigations. It is shown that the derived transfer function accurately captures the frequency response of the experimental system.
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| 3. |
- Frost, Anna E., 1988-
(author)
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In the Air Gap of Linear Generators for Wave Power
- 2021
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Doctoral thesis (other academic/artistic)abstract
- Wave power conversion is one type of renewable electricity generation. Within wave power, there are many different concepts, whereof some utilizes linear generators for converting the energy in the ocean waves into electricity. A linear generator consists of a translator, which is moving and have magnets of alternating polarity, and a stator, which have conductors sur-rounded by laminated steel. Between the translator and stator is an air gap, which is only a few millimeters wide. All linear generators for wave power, to the author’s knowledge, are permanent-magnet synchronous generators. This thesis looks into the forces and power flow in the air gap of linear generators for wave power, with the purpose of improving their future performance. The studies have focused on permanent magnet synchronous generators for wave power, but several of the results should also be applicable for other applications of linear elec-trical machines.Depending on the design of the linear generators, the translator can move so long that it only partially overlap the stator. This is common among several different wave power concepts with linear generators. When the stator is only partially overlapped by the stator it is denoted as partial stator overlap. It is studied how partial stator overlap affects the generated electric-ity, the absorbed energy, and the tangential and normal force in the air gap. The generated electricity and absorbed energy of a linear generator are quadratically dependent on the partial stator-translator overlap is shown through Faraday’s law and simulations. Experimental data showed that the absorbed energy is both linearly and quadratic depending on partial stator over-lap, where the linear dependence is at least partially due to frictional losses. Simulated results confirm that voltage is linearly dependent on partial stator overlap, which means quadratic de-pendence between generated electric and partial stator overlap. The simulated forces showed a linear dependence.Decades ago, the Poynting vector was used to derive an expression for the power flow in the air gap of rotating electrical machines. In this thesis the equivalent expressions for both flat and tubular linear electrical machines were derived. The analytical results were also compared with results from simulations. Both the analytical expressions and simulations showed that tubular and flat linear electrical machines have slightly different behavior.
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| 4. |
- Hettiarachchi, Pasan, 1977-
(author)
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Measurements of X-Ray Emission from Laboratory Sparks and Upward Initiated Lightning
- 2018
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Doctoral thesis (other academic/artistic)abstract
- In 1925 Nobel laureate R. C. Wilson predicted that high electric fields of thunderstorms could accelerate electrons to relativistic energies which are capable of generating high energetic radiation. The first detection of X-rays from lightning was made in 2001 and from long sparks in 2005. Still there are gaps in our knowledge concerning the production of X-rays from lightning and long sparks, and the motivation of this thesis was to rectify this situation by performing new experiments to gather data in this subject.The first problem that we addressed in this thesis was to understand how the electrode geometry influences the generation of X-rays. The results showed that the electrode geometry affects the X-ray generation and this dependency could be explained using a model developed previously by scientists at Uppsala University. The other missing information was the distribution of energy. Using a series of attenuators, we observed how the X-ray photons were attenuated as a function of barrier thickness and using a simple model we obtained the average and the maximum energy of X-rays. All the studies conducted previously was based on the lightning impulses, but in switching impulses, the voltage waveform rises very slowly compared to lightning impulses, and according to some scientists the rate of rise is an important parameter in X-ray development. Our study showed that the switching impulses were as efficient as lightning impulses in generating X-rays even though the rate of rise of voltage in switching impulses was hundreds of times slower.All the observations on X-ray generation from lightning by other scientists were based on either natural downward lightning flashes or triggered lightning in Florida. The first experiments to study the X-ray generation from upward lightning flashes systematically was conducted within this thesis work at Gaisberg Tower in Austria. The results showed that the X-ray emissions from these flashes were much weaker than the ones produced by either natural downward or triggered lightning. An attempt was made to explain this observation by invoking the possible differences in the charge distribution of leaders associated with the triggered lightning flashes in Florida and upward initiated lightning flashes at Gaisberg tower.
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