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Sufficiently accurate and low-cost estimation of tidal velocities is of importance when evaluating a potential site for a tidal energy farm. Here we suggest and evaluate a model to calculate the tidal velocity in fjord entrances. The model is compared with tidal velocities from Acoustic Doppler Current Profiler (ADCP) measurements in the tidal channel Skarpsundet in Norway. The calculated velocity value from the model corresponded well with the measured cross-sectional average velocity, but was shown to underestimate the velocity in the centre of the channel. The effect of this was quantified by calculating the kinetic energy of the flow for a 14-day period. A numerical simulation using TELEMAC-2D was performed and validated with ADCP measurements. Velocity data from the simulation was used as input for calculating the kinetic energy at various locations in the channel. It was concluded that the model presented here is not accurate enough for assessing the tidal energy resource. However, the simplicity of the model was considered promising in the use of finding sites where further analyses can be made.

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.

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. 

From the commercial point of view it may be beneficial to installa set of marine current turbines forming a park, by analogy with windparks. Consequently, this motivates research on park configurations.An array of ten vertical axis marine current turbines is simulatedto study how the distance between the turbines affects the performanceof the park for different flow directions. The simulations are performedusing a two-dimensional vortex method. An array of identical turbinesis created, where all turbines are on a single line. The turbinesare operated at the tip speed ratio, which corresponds to the highestpower coefficient for a single turbine. The distance between the turbinesis varied and the total power from the array is compared to the turbinespacing for different flow directions.Additionally, flow data from a real site is used to find an optimalorientation of the line of turbines. The performance of the arrayis estimated for the site as a function of turbine spacing.

The use of in-stream energy converters in rivers is an area of research that is still in its preliminary stages. The driving force of river flows is the potential energy the water gains when it precipitates on mountainsides, and this energy is traditionally converted by hydropower stations, where dams are used to create a larger head. Using an in-stream energy converter would be advantageous in areas restricted by regulation. In this paper the effects of using these converters on the upstream water level in a river are studied. This has been done both with an analytical model and with a numerical model. The analytical model described the water level increase due to energy capture to depend on how large fraction of the channel that is blocked by the turbine. It was also shown that as the converter induces drag on the flow, and as energy is lost in wake mixing, the total head loss will be a sum of energy capture and energy losses. The losses correspond to a considerable fraction of the total head drop. The numerical model was used to evaluate these results. The model used was the 3D numerical model MIKE from the DHI Group in Sweden. Turbines were modelled with an inbuilt function in the program. The results from the model did not correspond to the analytical results, as the energy capture was equal to the head drop in the program. (c) 2010 Elsevier Ltd. All rights reserved.

The Division of Electricity at Uppsala Universityis developing an experimental hydrokinetic power station for instreamexperiments at a site in a river. The purpose of this paperis to present some of the design choices made in the constructionof the experimental station. For background purposes, an outlineof the research project as a whole is also given.

The experimental station will be deployed in the Dal¨alvenRiver at S¨oderfors, whence the project derives its name. Thesite was selected based on several technical and non-technicalreasons. The system comprises a vertically oriented cross-streamaxis turbine and a directly driven permanent magnet generator tobe situated on the riverbed. The necessary power electronics forcontrol and power conversion will be housed in a small measuringstation on shore.

The paper discusses several aspects of the project, thatmight be of interest to other researchers in the field. Variousdesign choices, where different properties become the limiting ordeciding factor in different cases, are discussed along with theirrespective advantages and disadvantages. A brief outlook as tothe future of the project is also given.

As interest in renewable energy sources is steadily on the rise, tidal current energy is receiving more and more attention from politicans, industrialists, and academics. In this article, the conditions for and potential of tidal currents as an energy resource in Norway are reviewed. There having been a relatively small amount of academic work published in this particular field, closely related topics such as the energy situation in Norway in general, the oceanography of the Norwegian coastline, and numerical models of tidal currents in Norwegian waters are also examined. Two published tidal energy resource assessments are reviewed and compared to a desktop study made specifically for this review based on available data in pilot books. The argument is made that tidal current energy ought to be an important option for Norway in terms of renewable energy.

A hydro-kinetic energy project has been underway in Sweden since 2000, and an in-stream prototype setup for experiments at a site in a Swedish river is now in progress. The system comprises a vertical axis turbine and a directly driven permanent magnet generator. Methods and choices used in designing the system are described here. The turbine and generator are evaluated based on measurements and CFD simulations of conditions at the site for the experimental setup.