Modelling the hydrokinetic energy resource for in-stream energy converters

Abstract: Hydrokinetic energy, referring to the energy contained in moving water, is a renewable energy source that has gained much attention the past years. The energy is found in all moving water masses, but is only economical to convert for water masses moving with high velocity, i.e. likely around 1 m/s and above. This energy can for example be found in tidal, ocean and river currents which flow through narrow straits and channels. Along the west coast of Norway, there are many sites where kinetic energy conversion would be possible due to the strong current present. The driving force behind the currents is the tidal wave that progresses northward along the coast and increases in strength. The models that so far have been used for estimating the resource in Norway have been shown to be uncertain since they do not account for the fact that the velocities and the water levels are altered when energy is extracted. These effects can be simulated with numerical models. A channel in the Dal river, the Söderfors channel, is situated downstream a hydropower plant and was simulated with the numerical model MIKE. The water level alteration due to turbines was simulated. It was shown to be a lot less than the water level alteration caused by the level change in the downstream lake. Velocity profiles measured at several different locations were used to estimate how the power coefficient was changed. Four turbine configurations were studied and it was shown that changes in the power coefficient were prominent only for a vertical shear profile with a strong gradient. At the Division of Electricity, studies have been conducted on how to convert hydrokinetic energy to electricity since 2003. The main idea has been to use a system that limits the need for maintenance. The concept studied is a vertical axis turbine directly coupled to a permanent magnet generator. The Söderfors channel has, due to aspects such as the flow properties and velocity, been chosen as a site for an experimental station.