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The kinetic energy present in tidal currents and other water courses has long been appreciated as a vast resource of renewable energy. The work presented in this doctoral thesis is devoted to both the characteristics of the hydro-kinetic resource and the technology for energy conversion.

An assessment of the tidal energy resource in Norwegian waters has been carried out based on available data in pilot books. More than 100 sites have been identified as interesting with a total estimated theoretical resource—i.e. the kinetic energy in the undisturbed flow—in the range of 17 TWh. A second study was performed to analyse the velocity distributions presented by tidal currents, regulated rivers and unregulated rivers. The focus is on the possible degree of utilization (or capacity factor), the fraction of converted energy and the ratio of maximum to rated velocity, all of which are believed to be important characteristics of the resource affecting the economic viability of a hydro-kinetic energy converter.

The concept for hydro-kinetic energy conversion studied in this thesis comprises a vertical axis turbine coupled to a directly driven permanent magnet generator. One such cable wound laboratory generator has been constructed and an experimental setup for deployment in the river Dalälven has been finalized as part of this thesis work. It has been shown, through simulations and experiments, that the generator design at hand can meet the system requirements in the expected range of operation. Experience from winding the prototype generators suggests that improvements of the stator slot geometry can be implemented and, according to simulations, decrease the stator weight by 11% and decrease the load angle by 17%. The decrease in load angle opens the possibility to reduce the amount of permanent magnetic material in the design.

The conversion of kinetic energy in water currents into electricity has gained great attention the past years. The conversion systems are stand-alone units that typically consist of a turbine, driven by the water stream, with a generator connected to it. At the Division of Electricity at Uppsala University, research on a hydrokinetic energy conversion system is ongoing. In March 2013, a full-scale prototype was deployed in the river Dalälven at Söderfors.

This thesis is based on seven papers where the aim has been to assess the resource for hydrokinetic energy conversion. The existing hydrokinetic energy resource assessments in Scandinavia have been limited to the tidal energy found along the coast of Norway. The results from these assessments were unreliable, due to the lack of velocity data and the simple methodology used. One objective of this thesis was thus to measure the velocity in both tidal current and rivers, and evaluate models for predicting these values. Another objective was to study implications of the conversion of hydrokinetic energy, such as the degree of utilisation and the conversion efficiency, and effects on the surrounding flow and water level.

River discharge data was shown to give a good approximation of the velocity. However, non-linear behaviour of the velocity upon changing discharge cannot be approximated with discharge data. A model using tidal level data to estimate the velocity in fjord entrances was evaluated, and the model was shown to adequately estimate the cross-sectional average velocity. However, the maximum velocity in the horizontal cross-sectional profile was significantly higher than the cross-sectional average, and the model, in its current form, was not recommended to be used for resource estimations.

A high degree of utilisation, around 50%, was shown to be possible to achieve in both tidal and river currents, provided that the rated velocity is chosen properly. It was concluded that the rated velocity should be higher than the mean value, but lower than the value giving optimal conversion efficiency. Converting the kinetic energy of the flow to electricity in a river was shown to alter the water level upstream of the turbine. However, the increase in water level, caused by a hydrokinetic energy converter, was shown to be negligible compared to background friction.

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.