Institutionen för teknikvetenskaper

Nya doktorsavhandlingar

Nattakarn Suntornwipat, "Diamond devices based on valley polarization", Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214; 1670


Diamond is a wide bandgap semiconductor with extreme properties such as high thermal conductivity, high breakdown field, high carrier mobilities and chemical inertness. These properties together with the possibility to synthesize high purity Single-Crystalline (SC) diamond by Chemical Vapor Deposition (CVD), make it a very interesting material and a candidate for use in power electronics and in hazardous environments. The low impurity concentration achieved when fabricating diamond by CVD allows for a detailed study of the intrinsic electronic properties.

Diamond has six equivalent conduction band valleys oriented along the {100} axes with a uniquely low scattering rate between them. At low temperatures, the intervalley phonon scattering rate in diamond becomes negligible, which leads to a stable valley polarization state. We have observed non-equilibrium valley populations (valley-polarized electron ensembles), which in turn have been found to result in a Negative Differential Mobility (NDM).[...] 

Victor Mendoza, "Aerodynamic studies of vertical axis turbines using the actuator line model", Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1671


This thesis addresses the unsteady aerodynamics involved in the operation of vertical axis wind turbines (VAWTs). The main focus is to represent and understand the most relevant phenomena within the resulting flow pattern as the wake structure, loads on the different turbine components and the performance of the rotor. An actuator line model has been used for this purpose.

This model has been validated against experimental measurements from diverse cases with different operating conditions in both confined wind tunnels and open site locations. Numerical works were carried out considering a wide range of tip speed ratios (TSRs), and therefore covering from the no stall to the deep stall regime. The latter requires the implementation of a dynamic stall model for the proper representation of the unsteady forces on the blades. Also, different inlet conditions such as a uniform flow, a logarithmic wind shear and an atmospheric boundary layer (ABL) have been tested. The so-called recycling method technique was used to produce the fully developed ABL flow. Additionally, the resulting wake and performance of interacting turbines has been studied.[...] 

Kaspars Siliņš, "Plasma Enhanced Chemical- and Physical- Vapor Depositions Using Hollow Cathodes", Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1629


Development of coating deposition technologies, in terms of performance and costs, is an ongoing process. A promising class of deposition technologies are based on hollow cathode discharges.

This thesis investigates performance of selected hollow cathode plasma sources developed at the Plasma group, at Uppsala University for coating deposition at moderate pressures. Amorphous carbon film deposition was investigated by Radio frequency (RF) Hollow Cathode Plasma Jet (RHCPJ) and Magnets-in-Motion (M-M) linear hollow cathode plasma sources. Titanium nitride (TiN) films were deposited by a magnetized Hollow Cathode Enhanced magnetron Target (HoCET). Aluminium nitride (AlN) deposition by RHCPJ was compared with High Power Impulse Magnetron Sputtering (HiPIMS).[...] 

Pasan Hettiarachchi, "Measurements of X-ray emission from laboratory sparks and upward initiated lightning", Digital  Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1618


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.[...] 

Erik Hultman, "Robotized Production Methods for Special Electric Machines", Digital  Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1608


A research project on renewable energy conversion from ocean waves to electricity was started at the Division of Electricity at Uppsala University (UU) in 2001. The Wave Energy Converter (WEC) unit developed in this project is intended to be used in large offshore WEC farms and has therefore been designed with large-scale production in mind. The concept has now also been commercialized by the spin-off company Seabased Industry AB.

An essential part of the UU WEC is the linear direct-drive generator. This thesis presents the pilot work on developing robotized production methods for this special electric machine. The generator design is here investigated and four different backbreaking, monotone, potentially hazardous and time consuming manual production tasks are selected for automation. A robot cell with special automation equipment is then developed and constructed for each task. Simplicity, reliability and flexibility are prioritized and older model pre-owned industrial robots are used throughout the work. The robot cells are evaluated both analytically and experimentally, with focus on full scale experiments. It is likely that the developed production methods can be applied also for other similar electric machines. [...] 

Dalina Johari,  "Features of the Electric Fields Generated by Lightning with Special Attention to Positive Ground Flashes", Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1588


This thesis focuses on the main processes in positive ground flashes and the distant lightning environment for both positive and negative ground flashes. It presents the characteristics of the preliminary breakdown pulses (PBPs), the characteristics of the electric field pulses observed during leader propagation, and the characteristics of the electric fields produced by the first and the subsequent return strokes. It also features the observations of distant positive and negative ground flashes at distances up to 1000 km. The results were based on electric field measurements conducted remotely during summer thunderstorms in Sweden in 2014.

We found that the majority of the positive ground flashes were preceded by PBPs. Some were preceded by more than one PBP train, and the parameter values for the subsequent PBP trains were found to be smaller than the values for the first PBP train. Three types of PBPs were also identified. The results suggest that the PBPs in positive ground flashes during summer thunderstorms in Sweden are weak, and the inverted dipole charge cloud configuration is consistent with our observation. [...] 

Morgan Rossander,  "Electromechanics of vertical axis wind turbines", Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1581


Wind power is an established mean of clean energy production and the modern horizontal axis wind turbine has become a common sight. The need for maintenance is high and future wind turbines may need to be improved to enable more remote and offshore locations. Vertical axis wind turbines have possible benefits, such as higher reliability, less noise and lower centre of gravity. This thesis focuses on electromechanical interaction in the straight bladed Darrieus rotor (H-rotor) concept studied at Uppsala University.

One of the challenges with vertical axis technology is the oscillating aerodynamic forces. A force measurement setup has been implemented to capture the forces on a three-bladed 12 kW open site prototype. The normal force showed good agreement with simulations. An aerodynamic torque could be estimated from the system. The total electrical torque in the generator was determined from electrical measurements. Both torque estimations lacked the expected aerodynamic ripple at three times per revolution. The even torque detected is an important result and more studies are required to confirm and understand it. [...] 

Deepak Elamalayil Soman, Dissertation,  "Multilevel Power Converters with Smart Control for Wave Energy Conversion", Digital  Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1597


The main focus of this thesis is on the power electronic converter system challenges associated with the grid integration of variable-renewable-energy (VRE) sources like wave, marine current, tidal, wind, solar etc. Wave energy conversion with grid integration is used as the key reference, considering its high energy potential to support the future clean energy requirements and due the availability of a test facility at Uppsala University. The emphasis is on the DC-link power conditioning and grid coupling of direct driven wave energy converters (DDWECs). The DDWEC reflects the random nature of its input energy to its output voltage wave shape. Thereby, it demands for intelligent power conversion techniques to facilitate the grid connection. [...] 

Birger Markusson, Dissertation: "Magnetic leakage fields and Eddy current power losses in synchronous generators", Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1575


The conversion of mechanical energy to electrical energy is done mainly with synchronous generators. They are used in hydropower generators and nuclear plants that presently account for about 80% of the electric energy production in Sweden. Because of the dominating role of the synchronous generators, it is important to minimize the power losses for efficient use of natural resources and for the economies of the electric power companies and their customers. For a synchronous machine, power loss means undesired heat production. In electric machines, there are power losses due to windage, friction in bearings, resistance in windings, remagnetization of ferromagnetic materials, and induced voltages in windings, shields and parts that are conductive but ideally should be non-conductive.

The subject of this thesis is prediction of end region magnetic leakage fields in synchronous generators and the eddy current power losses they cause. [...] 

Tobias Kamf, "Automated Production Technologies and Measurement Systems for Ferrite Magnetized Linear Generators",Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1573


This thesis will mainly focus on the development and evaluation of technologies used to automate the manufacturing of the translator, a central part of the linear generator, using industrial robotics. The translator is a 3 m high and 0.8 m wide three sided structure with an aluminum pipe at its center. The structure consists of alternating layers of steel plates (pole-shoes) and ferrite magnets, with a total of 72 layers per side. To perform experiments on translator assembly and production, a robot cell (centered on an IRB6650S industrial robot) complimented with relevant tools, equipment and security measures, has been designed and constructed. The mounting of the pole-shoes on the central pipe, using the industrial robot, proved to be the most challenging task to solve. However, by implementing a precise work-piece orientation calibration system, combined with selective compliance robot tools, the task could be performed with mounting speeds of up to 50 mm/s. Although progress has been made, much work still remains before fully automated translator assembly is a reality. [...] 

Liselotte Ulvgård, "Wave Energy Converters: An experimental approach to onshore testing, deployments and offshore monitoring", Digital  Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1563


The wave energy converter (WEC) concept developed at Uppsala University consists of a point absorbing buoy, directly connected to a permanent magnet linear generator. Since 2006, over a dozen full scale WECs have been deployed at the Lysekil Research Site, on the west coast of Sweden. Beyond the development of the WEC concept itself, the full scale approach enables, and requires, experimental and multidisciplinary research within several peripheral areas, such as instrumentation, offshore operations, and wave power infrastructure.

This thesis addresses technical challenges of testing, deploying and monitoring full scale WECs. It is divided accordingly into three topics: offshore measurement systems, onshore WEC testing and deployments. Each topic presents new or improved technical solutions to enable offshore wave power research.[...]

Linnea Sjökvist,  "Wave Loads and Peak Forces on Moored Wave Energy Devices in Tsunamis and Extreme Waves", Digital  Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1551


Surface gravity waves carry enormous amounts of energy over our oceans, and if their energy could be harvested to generate electricity, it could make a significant contribution to the worlds power demand. But the survivability of wave energy devices in harsh operating conditions has proven challenging, and for wave energy to be a possibility, peak forces during storms and extreme waves must be studied and the devices behaviour understood. Although the wave power industry has benefited from research and development in traditional offshore industries, there are important differences. Traditional offshore structures are designed to minimize power absorption and to have small motion response, while wave power devices are designed to maximize power absorption and to have a high motion response. This increase the difficulty of the already challenging survivability issue. Further, nonlinear effects such as turbulence and overtopping can not be neglected in harsh operating conditions. In contrast to traditional offshore structures, it is also important to correctly account for the power take off system in a wave energy converter (WEC), as it is strongly coupled to the devices behaviour.

The focus in this thesis is the wave loads and the peak forces that occur when a WEC with a limited stroke length is operated in waves higher than the maximum stroke length.

José Pérez-Loya, "Analysis and control of magnetic forces in synchronous machines", Digital  Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1542


In a synchronous machine, radial, tangential, and axial forces are generated. In this thesis, three different technologies to control them are proposed. The first one, involves the utilization of the radial forces that arise between the rotor and the stator. This is achieved by segmenting the rotor field winding into groups of poles and controlling their corresponding magnetization individually. This technology is particularly useful to achieve magnetic balance and to create controllable radial forces. The second technology, involves the control of the rotor field in order to influence the tangential forces that produce torque. This is achieved by inverting the rotor field winding polarity with respect to the stator field. With this technique, breaking and accelerating torques can be created. It is particularly useful to start a synchronous machine. Finally, the application of axial forces with a magnetic thrust bearing is discussed. The main benefits of this technology are higher efficiency and increased reliability.[...] 

Mohd Muzafar Ismail,  "Features and origin of electromagnetic fields generated by lightning flashes", Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1535


Negative cloud-to-ground (CG) lightning flashes transport negative charge from cloud to ground. Negative ground flashes typically involve various processes identified as preliminary breakdown, stepped leader, return stroke, dart leader, dart-stepped leader, subsequent return stroke, and cloud activity between strokes, such as regular pulse trains and chaotic pulse trains. These processes can be identified through their electromagnetic field signatures. The main focus of this thesis is to document the features and understand the origin of electromagnetic fields, especially the chaotic pulse trains, generated by lightning flashes.

Electric field measurements have been used to study lightning flashes in Sweden. The equipment was a parallel flat plate antenna with an analog filter buffer circuit, connected to a digital high speed oscilloscope. Four simultaneous measurements were made: wideband measurement of the E-field (the vertical component) and its time derivative dE/dt, and two narrowband measurements of the E-field, centred around 3 MHz and 30 MHz. Fourier and wavelet transforms were used in the analysis of the measured data.[...] 

LiGuo Wang, "Modelling and advanced control of fully coupled wave energy converters subject to constraints: the wave-to-wire approach", Digital  Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1528


Ocean wave energy is a promising renewable source to contribute to supplying the world’s energy demand. The Division of Electricity at Uppsala University is developing a technology to capture energy from ocean waves with a wave energy converter (WEC) consisting of a linear permanent magnet generator and a point absorber. The linear generator is placed on sea bed and is driven directly by the floating absorber. Since March 2006, multiple wave energy converters have been deployed on the Swedish west coast outside the town of Lysekil. The technology is verified by long-term operation during at sea and satisfactory reliability of the electricity generation.

Magnus Hedlund, "Electrified Integrated Kinetic Energy Storage", Digital  Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1504


The electric car is a technically efficient driveline, although it is demanding in terms of the primary energy source. Most trips are below 50 km and the mean power required for maintaining speed is quite low, but the system has to be able to both provide long range and high maximum power for acceleration. By separating power and energy handling in a hybrid driveline, the primary energy source, e.g. a battery can be optimised for specific energy (decreasing costs and material usage). Kinetic energy storage in the form of flywheels can handle the short, high power bursts of acceleration and decceleration with high efficiency.

This thesis focuses on the design and construction of flywheels in which an electric machine and a low-loss magnetic suspension are considered an integral part of the composite shell, in an effort to increase specific energy. A method of numerically optimising shrink-fitted composite shells was developed and implemented in software, based on a plane stress assumption, with a grid search optimiser. A composite shell was designed, analysed numerically and constructed, with an integrated permanent magnet synchronous machine. .[...] 

Nicole Carpman, "Resource characterization and variability studies for marine current power", Digital  Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1499

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. .[...] 

Weija Yang, "Hydropower plants and power systems: Dynamic processes and control for stable and efficient operation", Digital  Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1494,


As the largest global renewable source, hydropower shoulders a large portion of the regulation duty in many power systems. New challenges are emerging from variable renewable energy (VRE) sources, the increasing scale and complexity of hydropower plants (HPPs) and power grid. Stable and efficient operation of HPPs and their interaction with power systems is of great importance.

Theoretical analysis, numerical simulation and on-site measurement are adopted as main study methods in this thesis. Various numerical models of HPPs are established, with different degrees of complexity for different purposes. The majority of the analysis and results are based on eight HPPs in Sweden and China..[...] 

Jonas Nøland Kristiansen, "A new paradigm for large brushless hydrogenerators: Advantages beyond the static system", Digital  Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1491


The grid code, FIKS, from the Norwegian transmission system operator (TSO), Statnett, states that synchronous generators > 25MVA, must have a static excitation system. However, an improved brushless excitation system is in operation on some commercial power plants (36MVA, 93.75rpm & 52MVA, 166.67rpm) with grid-assisting performance beyond the conventional static system. The convenional diode bridge is replaced with a remote-controlled thyristor bridge on the shaft. If wireless communication is not allowed, a control signal through brushes should be employed instead. The thesis explores the expected new era for large brushless hydrogenerators. The proposed brushless system have benefits of reduced regular maintenance due to elimination of brushes and reduced unscheduled maintenance due to redundancy; causing a redused cost-of-energy. A six-phase exciter design with a hybrid-mode thyristor bridge interface leads to improved fault-tolerance, better controllability, minimized torque pulsations and reduced armature currents of the exciter. Excitation boosting (EB) capability is included in the brushless system without additional components or circuitry, contrary to the static excitation system. The brushless excitation system is made insensitive to voltage dips in the interconnected grid, causing improved fault ride-through (FRT) capability and power system stabilizer (PSS) actions.