Hydrokinetic Resource Assessment : Measurements and Models

Abstract: 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.