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Marine currents, i.e. water currents in oceans and rivers, constitute a large renewable energy resource. This thesis presents research done on the subject of marine current energy conversion in a broad sense. A review of the tidal energy resource in Norway is presented, with the conclusion that tidal currents ought to be an interesting option for Norway in terms of renewable energy. The design of marine current energy conversion devices is studied. It is argued that turbine and generator cannot be seen as separate entities but must be designed and optimised as a unit for a given conversion site. The influence of support structure for the turbine blades on the efficiency of the turbine is studied, leading to the conclusion that it may be better to optimise a turbine for a lower flow speed than the maximum speed at the site. The construction and development of a marine current energy experimental station in the River Dalälven at Söderfors is reported. Measurements of the turbine's power coefficient indicate that it is possible to build efficient turbines for low flow speeds. Experiments at the site are used for investigations into different load control methods and for validation of a numerical model of the energy conversion system and the model's ability to predict system behaviour in response to step changes in operational tip speed ratio. A method for wake measurements is evaluated and found to be useful within certain limits. Simple models for turbine runaway behaviour are derived, of which one is shown by comparison with experimental results to predict the behaviour well.

Nowadays, renewable energy is in full growth. In particular, offshore wind farms will be at the centre of UK energetic strategy in the coming years. However, other types of marine renewable are still at an early development stage. That is the case for tidal energy. Many projects have been undertaken but there is no candidate for competitive commercial applications yet. Deltastream is one of these numerous pioneering projects. It consists of a set of three marine current turbines mounted on a triangular base put down onto the seabed. The device is not moored and no harm is done to the environment. However, that makes the structure more sensitive to water flows. And it is important to ensure that it will remain at its location and not being carried along with the tidal streams. Using CFD, the present study aims to evaluate the drag on the nacelles of the structure and come up with solutions to reduce it as much as possible.

This thesis investigates the load control of a marine current energy converter using a vertical axis turbine mounted on a permanent magnet synchronous generator. The purpose of this thesis is to show the work done in the so far relatively uncharted territory of control systems for hydro kinetic energy conversion. The work is in its early stage and is meant to serve as a guide forfuture development of the control system. An experimental power station has been deployed and the first results are presented. A comparison between three load control methods has been made; a fixedAC load, a fixed pulse width modulated DC load and a DC bus voltage control of a DC load. Experimental results show that the DC bus voltage control reduces the variation of rotational speed with a factor of 3.5. For all three cases, the tip speed ratio of the turbine can be kept close to the expected optimal tip speed ratio. However, for all three cases the average extracted power was significantly lower than the average power available in the turbine times the estimated maximum power coefficient. A maximum power point tracking system, with or without water velocity measurement, should increase the average extracted power. A simulation model has been validated using experimental data. The simulated system consists of the electrical system and a hydrodynamic vortex model for the turbine. Experiments of no load operation were conducted to calibrate the drag losses of the turbine. Simulations were able to predict the behaviour in a step response for a change in tip speed ratio when the turbine was operated close to optimal tip speed ratio. The start position of the turbine was varied in the simulation to view the influence on the step response from a changed turbine position relative to the direction of the water flow. / <p>Funders: J Gust Richert, Bixia Miljöfond</p>

The marine current turbine (MCT) is an exciting proposition for the extraction of renewable tidal and marine current power. However, the numerical prediction of the performance of the MCT is difficult due to its complex geometry, the surrounding turbulent flow and the free surface. The main purpose of this research is to develop a computational tool for the simulation of a MCT in turbulent flow and in this thesis, the author has modified a 3D Large Eddy Simulation (LES) numerical code to simulate a three blade MCT under a variety of operating conditions based on the Immersed Boundary Method (IBM) and the Conservative Level Set Method (CLS). The interaction between the solid structure and surrounding fluid is modelled by the immersed boundary method, which the author modified to handle the complex geometrical conditions. The conservative free surface (CLS) scheme was implemented in the original Cgles code to capture the free surface effect. A series of simulations of turbulent flow in an open channel with different slope conditions were conducted using the modified free surface code. Supercritical flow with Froude number up to 1.94 was simulated and a decrease of the integral constant in the law of the wall has been noticed which matches well with the experimental data. Further simulations of the marine current turbine in turbulent flow have been carried out for different operating conditions and good match with experimental data was observed for all flow conditions. The effect of waves on the performance of the turbine was also investigated and it has been noticed that this existence will increase the power performance of the turbine due to the increase of free stream velocity.

<p>This thesis involves the development of a system for control of a marine current energy conversion system. The control system is developed on the principles of load control, i.e. it aims to control the rotational speed of the turbine by controlling the power extracted from the generator. The system operates by feedback of the generator DC-voltage and current as well as the speed of the water current passing through the turbine. An IGBT-transistor controlled by an AVR-microcontroller executes control of the generator and hence the turbine. A digitally implemented PID-controller serves as the fundamental automatic control regime. The control system can be operated from a PC-application connected to the microcontroller through a serial wire connection. From the graphical user interface ofthe PC-application the system operator can set the system control parameters and monitor the state of the generator and turbine. The control system can be set to keep the turbine operating at a desired tip speed ratio, rotational speed or generator voltage. Further, for purposes of indoor testing of the control system a separate system, a motor control system, was developed as a part of this thesis work. The purpose of the motor control system is to enable simulating the behavior of a turbine with a motor driving the generator instead of an actual turbine. The motor control system operates by control of an ACS800 variable frequency drive that is connected to the motor. The motor control system allows its operator to feed in data describing the variations in water speed over time as well as data describing how the simulated turbine's power coefficient depends on its tip speed ratio. From this data the motor control system continuously calculates the torque that should be put on the generator axis by the motor. Results from test runs of the system show that the performance of the system is good. The system responds quickly to changes in the control parameters. Also the system manages to keep the specified control parameter quite well even during rapid changes in the water speed.</p>

Tidal currents and rivers are promising sources of renewable energy given that suitable turbines for kinetic energy conversion are developed. To be economically and technically feasible, a velocity distribution that can give a high degree of utilization (or capacity factor), while the ratio of maximum to rated velocity is low would be preferable. The rated velocity is defined as the velocity at which rated power is achieved. Despite many attempts to estimate the resource, however, reports on the possible degree of utilisation from tidal currents and rivers are scarce. In this paper the velocity distribution from a number of regulated rivers, unregulated rivers and tidal currents have been analysed regarding the degree of utilisation, the fraction of converted energy and the ratio of maximum to rated velocity. Two methods have been used for choosing the rated velocity; one aiming at a high fraction of converted energy and one aiming at a high degree of utilisation. Using the first method, with a rated velocity close to the maximum velocity, it is unlikely that the turbine will reach the cut-out velocity. This results in, on average, a degree of utilisation of 23% for regulated rivers, 19% for unregulated rivers and 17% for tidal currents while converting roughly 30-40% of the kinetic energy. Choosing a rated velocity closer to the mean velocity resulted in, on average, a degree of utilisation of 57% for regulated rivers, 52% for unregulated rivers and 45% for tidal currents. The ratio of maximum to rated velocity would still be no higher than 2.0 for regulated rivers, 1.2 for unregulated rivers and 1.6 for tidal currents. This implies that the velocity distribution of both rivers and tidal currents is promising for kinetic energy conversion. These results, however, do not include weather related effects or extreme velocities such as the 50-year velocity. A velocity factor is introduced to describe what degree of utilisation can be expected at a site. The velocity factor is defined as the ratio U-max/U-rate at the desired degree of utilisation, and serves as an early indicator of the suitability of a site.

This thesis involves the development of a system for control of a marine current energy conversion system. The control system is developed on the principles of load control, i.e. it aims to control the rotational speed of the turbine by controlling the power extracted from the generator. The system operates by feedback of the generator DC-voltage and current as well as the speed of the water current passing through the turbine. An IGBT-transistor controlled by an AVR-microcontroller executes control of the generator and hence the turbine. A digitally implemented PID-controller serves as the fundamental automatic control regime. The control system can be operated from a PC-application connected to the microcontroller through a serial wire connection. From the graphical user interface ofthe PC-application the system operator can set the system control parameters and monitor the state of the generator and turbine. The control system can be set to keep the turbine operating at a desired tip speed ratio, rotational speed or generator voltage. Further, for purposes of indoor testing of the control system a separate system, a motor control system, was developed as a part of this thesis work. The purpose of the motor control system is to enable simulating the behavior of a turbine with a motor driving the generator instead of an actual turbine. The motor control system operates by control of an ACS800 variable frequency drive that is connected to the motor. The motor control system allows its operator to feed in data describing the variations in water speed over time as well as data describing how the simulated turbine's power coefficient depends on its tip speed ratio. From this data the motor control system continuously calculates the torque that should be put on the generator axis by the motor. Results from test runs of the system show that the performance of the system is good. The system responds quickly to changes in the control parameters. Also the system manages to keep the specified control parameter quite well even during rapid changes in the water speed.

The performance of a horizontal axis marine current turbine is predicted by three numerical methods, vortex lattice method MPUF-3A, boundary element method PROPCAV and a commercial RANS solver FLUENT. The predictions are compared with the experimental measurements for the same turbine model. A fully unsteady wake alignment is utilized in order to model the realistic wake geometry of the turbine. A lifting line theory based method is developed to produce the optimum circulation distribution for turbines and propellers and a lifting line theory based database searching method is used to achieve the optimum circulation distribution for tidal turbines. A nonlinear optimization method (CAVOPT-3D) and another database-searching design method (CAVOPT-BASE) are utilized to design the blades of marine current turbines and marine propellers. A design procedure for the tidal turbine is proposed by using the developed methods successively. Finally, an interactive viscous/potential flow method is utilized to analyze the effect of nonuniform inflow on the performance of tidal turbines. / text

Renewable energy is harnessed from continuously replenishing natural processes. Some commonly known are sunlight, water, wind, tides, geothermal heat and various forms of biomass. The focus on renewable energy has over the past few decades intensified greatly. This thesis contributes to the research on developing renewable energy technologies, within the wind power, wave power and marine current power projects at the division of Electricity, Uppsala University. In this thesis grid connection of permanent magnet generator based renewable energy sources is evaluated. A tap transformer based grid connection system has been constructed and experimentally evaluated for a vertical axis wind turbine. Full range variable speed operation of the turbine is enabled by using the different step-up ratios of a tap transformer. This removes the need for a DC/DC step or an active rectifier on the generator side of the full frequency converter and thereby reduces system complexity. Experiments and simulations of the system for variable speed operation are done and efficiency and harmonic content are evaluated.  The work presented in the thesis has also contributed to the design, construction and evaluation of a full-scale offshore marine substation for wave power intended to grid connect a farm of wave energy converters. The function of the marine substation has been experimentally tested and the substation is ready for deployment. Results from the system verification are presented. Special focus is on the transformer losses and transformer in-rush currents. A control and grid connection system for a vertical axis marine current energy converter has been designed and constructed. The grid connection is done with a back-to-back 2L-3L system with a three level cascaded H-bridge converter grid side. The system has been tested in the laboratory and is ready to be installed at the experimental site. Results from the laboratory testing of the system are presented. / Wind Power / Wave Power / Marine Currnet Power