| 11. |
- Carpman, Nicole
(author)
-
Resource characterization and variability studies for marine current power
- 2017
-
Doctoral thesis (other academic/artistic)abstract
- Producing electricity from marine renewable resources is a research area that develops continuously. The field of tidal energy is on the edge to progress from the prototype stage to the commercial stage. However, tidal resource characterization, and the effect of tidal turbines on the flow, is still an ongoing research area in which this thesis aims to contribute.In this thesis, measurements of flow velocities have been performed at three kinds of sites. Firstly, a tidal site has been investigated for its resource potential in a fjord in Norway. Measurements have been performed with an acoustic Doppler current profiler to map the spatial and temporal characteristics of the flow. Results show that currents are in the order of 2 m/s in the center of the channel. Furthermore, the flow is highly bi-directional between ebb and flood flows. The site thus has potential for in-stream energy conversion. Secondly, a river site serves as an experimental site for a marine current energy converter that has been designed at Uppsala University and deployed in Dalälven, Söderfors. The flow rate at the site is regulated by an upstream hydro power plant, making the site suitable for experiments on the performance of the vertical axis turbine in a natural environment. The turbine was run in steady discharge flows and measurements were performed to characterize the extent of the wake. Lastly, at an ocean current site, the effect that transiting ferries may have on submerged devices was investigated. Measurements were conducted with two sonar systems to obtain an underwater view of the wake caused by a propeller and a water jet thruster respectively.Furthermore, the variability of the intermittent renewable sources wind, solar, wave and tidal energy was investigated for the Nordic countries. All of the sources have distinctly different variability features, which is advantageous when combining power generated from them and introducing it on the electricity grid. Tidal variability is mainly due to four aspects: the tidal regime, the tidal cycle, local bathymetry causing turbulence, asymmetries etc. and weather effects. Models of power output from the four sources was set up and combined in different energy mixes for a “highly renewable” and a “fully renewable” scenario. By separating the resulting power time series into different frequency bands (long-, mid-, mid/short-, and short-term components) it was possible to minimize the variability on different time scales. It was concluded that a wise combination of intermittent renewable sources may lower the variability on short and long time scales, but increase the variability on mid and mid/short time scales.The tidal power variability in Norway was then investigated separately. The predictability of tidal currents has great advantages when planning electricity availability from tidal farms. However, the continuously varying tide from maximum power output to minimum output several times per day increases the demand for backup power or storage. The phase shift between tidal sites introduces a smoothing effect on hourly basis but the tidal cycle, with spring and neap tide simultaneously in large areas, will inevitably affect the power availability.
|
|
| 12. |
|
|
| 13. |
- Chatzigiannakou, Maria Angeliki
(author)
-
Offshore deployments of marine energy converters
- 2019
-
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.
|
|
| 14. |
- Chatzigiannakou, Maria Angiliki, et al.
(author)
-
Offshore Deployments of Wave Energy Converters by Seabased Industry AB
- 2017
-
In: Journal of Marine Science and Engineering. - : MDPI AG. - 2077-1312. ; 5:2
-
Journal article (peer-reviewed)abstract
- Since 2008, Seabased Industry AB (SIAB) has manufactured and deployed several units of wave energy converters (WECs) of different design. The WECs are linear generators with point absorber buoy systems that are placed on the seabed, mounted on a gravitation concrete foundation. These deployments have taken place in different areas, using different deployment vessels. Offshore deployments of WECs and underwater substations have so far been complicated procedures, that were both expensive and time-consuming. The focus of this paper is to discuss these deployments in terms of economy and time efficiency, as well as safety. Because seven vessels have been used to facilitate the deployments, an evaluation on the above basis is carried out for them. The main conclusions and certain solutions are presented for the various problems encountered during these deployments and the vessel choice is discussed. It is found that the offshore deployment process can be optimized in terms of cost, time efficiency and safety with a careful vessel choice, use of the latest available technologies and detailed planning and organizing.
|
|
| 15. |
- Chatzigiannakou, Maria Angeliki, et al.
(author)
-
Offshore deployments of wave energy converters by Uppsala University, Sweden
- 2019
-
In: Marine Systems and Ocean Technology. - : Springer Science and Business Media LLC. - 1679-396X .- 2199-4749. ; 14:2-3, s. 67-74
-
Journal article (peer-reviewed)abstract
- Ocean can provide an inexhaustible amount of energy. Many marine energy converters have been developed but most of them have not surpassed the experimental phase due to the high costs in installation, operation, and maintenance. Since 2002 Uppsala University has developed and deployed several units of wave energy converters of various designs. The Uppsala University wave energy converter concept consists of a linear generator directly connected to a point absorber buoy that is mounted on a concrete gravity foundation. Uppsala University deployments have been carried out using different deployment vessels and methods. Three main methods were utilized for these deployments that are discussed in terms of cost, manpower, and time efficiency. Depending on the desired outcome—multiple- or single-device deployment, low budget, etc.—one of the proposed methods can be used for the optimal outcome.
|
|
| 16. |
- Chen, WenChuang, et al.
(author)
-
Numerical modelling of a point-absorbing wave energy converter in irregular and extreme waves
- 2017
-
In: Applied Ocean Research. - : Elsevier BV. - 0141-1187 .- 1879-1549. ; 63, s. 90-105
-
Journal article (peer-reviewed)abstract
- Based on the Navier-Stokes (RANS) equations, a three-dimensional (3-D) mathematical model for the hydrodynamics and structural dynamics of a floating point-absorbing wave energy converter (WEC) with a stroke control system in irregular and extreme waves is presented. The model is validated by a comparison of the numerical results with the wave tank experiment results of other researchers. The validated model is then utilized to examine the effect of wave height on structure displacements and connection rope tension. In the examined cases, the differences in WEC’s performance exhibited by an inviscid fluid and a viscous fluid can be neglected. Our results also reveal that the differences in behavior predicted by boundary element method (BEM) and the RANS-based method can be significant and vary considerably, depending on wave height.
|
|
| 17. |
- Dolguntseva, Irina, et al.
(author)
-
Contour-Based Dead-Time Harmonic Analysis in a Three-Level Neutral-Point-Clamped Inverter
- 2015
-
In: IEEE Transactions on Industrial Electronics. - : IEEE. - 0278-0046 .- 1557-9948. ; 62:1, s. 203-210
-
Journal article (peer-reviewed)abstract
- The term dead time refers to a prime safety factor for most power electronic converter topologies, and it is included either in the control software or in the gate/base driver hardware, depending on the application as well as the control requirements. In this paper, the authors present a comprehensive numerical analysis of dead-time effects on the output voltage of a three-level neutral-pointclamped (NPC) inverter. To incorporate the dead-time effect in the output voltage, 3-D models of three-level carrier pulse width modulation (PWM) methods are modified for two dead-time implementations. Closed-form expressions of inverter phase voltage harmonics for phase opposition disposition (POD) PWM are derived based on the double Fourier series approach and modified contour plots. The harmonic spectra from numerical evaluations, simulations, and experiments for natural sampling (NS), symmetrical regular sampling (SRS), and asymmetrical regular sampling (ARS) are compared to validate the mathematical models. In addition, the fundamental voltage with respect to the dead time and the load phase angle is presented based on analytical results and simulation.
|
|
| 18. |
|
|
| 19. |
- Eklund, Petter, 1989-
(author)
-
Design of Rare Earth Free Permanent Magnet Generators
- 2018
-
Doctoral thesis (other academic/artistic)abstract
- Low speed permanent magnet (PM) synchronous generators (SGs) are commonly used in renewable energy. Rare earth (RE) PMs such as neodymium-iron-boron are a popular choice due to their high performance. In 2011 supply and cost issues were added to the previously existing environmental concerns regarding REPM raw materials as the world's major producer China imposed export restrictions. This thesis aims to investigate and propose design solutions for PMSGs that do not use REPMs. Two approaches are used: to design generators using the cheaper and more abundant ferrite PM materials, and to investigate how properties of new PM materials influence SG design.A ferrite PM rotor is designed to replace a REPM rotor in an experimental 12 kW wind power generator. The new design employs a flux concentrating spoke type rotor to achieve performance similar to the old REPM rotor while using ferrite PMs. The ferrite PM rotor design is built. The air gap length, magnetic flux density in the air gap, PM remanence, and voltage at both load and no load are measured. The generator has lower no load voltage than expected, which is mainly explained by lower than specified remanence of the ferrite PMs in the prototype. With the measured remanence inserted into the calculations some discrepancy remains. It is found that the discrepancy can be explained by the magnetic leakage flux in the end regions of the spoke type rotor, which is not modeled in the two dimensional simulations used for the design calculations.To investigate the influence of PM material properties three different PM rotor topologies are optimized for torque production using PM materials described by their remanence, recoil permeability, and demagnetization resistance. Demagnetization is considered using currents determined by a novel, winding design independent short circuit model. It is found that the spoke type rotor gives the highest torque of the three rotor topologies for low remanence materials as long as the PMs have sufficient demagnetization resistance. For high remanence materials the surface mounted PM rotor can give higher torque if the demagnetization resistance is high, but otherwise a capped PM rotor gives higher torque.
|
|
| 20. |
- Eklund, Petter, 1989-, et al.
(author)
-
Magnetic End LeakageFlux in a Spoke Type Rotor Permanent Magnet SynchronousGenerator
- 2017
-
Conference paper (other academic/artistic)abstract
- The spoke type rotor can be used to obtain magnetic flux concentration in permanent magnet machines. This allows the air gap magnetic flux density to exceed the remanent flux density of the permanent magnets but gives problems with leakage fluxes in the magnetic circuit. The end leakage flux of one spoke type permanent magnet rotor design is studied through measurements and finite element simulations. The measurements are performed in the end regions of a 12 kW prototype generator for a vertical axis wind turbine. The simulations are made using three dimensional finite elements to calculate the magnetic field distribution in the end regions of the machine. Also two dimensional finite element simulations are performed and the impact of the two dimensional approximation is studied. It is found that the magnetic leakage flux in the end regions of the machine is equal to about 20 % of the flux in the permanent magnets. The overestimation of the performance by the two dimensional approximation is quantified and a curve-fitted expression for its behavior is suggested.
|
|
