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Marine currents, e.g. tidal currents, ocean currents and unregulated water courses are characterized by a fairly steady or regular flow and pose a significant potential for electricity generation. An ongoing research project at Uppsala University is looking into an energy conversion system with a vertical axis turbine and a permanent magnet direct drive generator placed directly in the water flow, i.e. without using a dam. This choice of technology is intended to provide a simple and robust system with low maintenance needs and minimal environmental impact. During 2006 an experimental setup has been built. It consists of a frequency converter, a motor and a gearbox to drive the generator, the generator itself, and a resistive load to consume the generated power. The generator is two meters in diameter and is built on an elevated structure over the motor and gearbox. A stainless steel structure supports the cable wound stator. The permanent NdFeB magnets are fastened in milled grooves in the rotor. The experimental setup will be used for verification of a simulation and optimization program.

Marine currents can be an effective source for renewable electric energy conversion. In this paper a direct drive permanent magnet synchronous generator capable of being connected to a vertical axis turbine for marine current energy conversion is presented. This 5 kVA, 120 pole, 10 rpm prototype generator was initially designed for ocean current speeds of 1.5 m/s using a finite element based electrical machine design software. Unlike most conventional generators this machine has a cable wound stator. To evaluate the electrical and mechanical performance characteristics of this machine, laboratory experiments are being carried out using a speedcontrolled motor drive system and resistive three phase loads. First measurements of voltage and current show good agreement between the modelled and built generator.

A low-speed permanent magnet (PM) cable wound generator for electrical energy conversion from marine or tidal currents has been designed and constructed. A key feature of this variable speed direct drive generator is its capability to efficiently generate electricity from tidal currents with very low velocities, in the order of 1 m/s. In arriving at an appropriate design for the generator typical characteristics of tidal currents were considered. Using these characteristics as input, and accounting for the electromagnetic losses, detailed computer simulations using a finite-element method software were carried out to come up with the final design. Various parameters that can influence the generator design are presented. An experimental set-up has been constructed based on the above-mentioned design in order to study the electrical and mechanical performance of the generator through a variety of experiments. The power input for this set-up is a variable speed motor, capable of operating the generator at rotational speeds of 0–16 r/min, representing tidal currents with very low velocities. The generator presented in this paper may be beneficial for a better understanding of an appropriate design and layout of tidal energy conversion systems.

Extracting energy from a free-flow marine current using a vertical axis fixed pitch turbine requires a generator that can handle varying speeds and loads, since such a turbine gives maximum power capture for a fixed tip speed ratio. A prototype of such a generator has been designed and constructed. In this paper, its variable speed and load operation is evaluated, both in terms of how the constructed generator performs in relation to simulations, and in terms of how the generator could perform with three different fixed pitch turbines. Measurements of root mean square (RMS) voltage and current differ 10% from simulations. Performance analysis with example turbines shows that the generator can match fixed tip speed ratio operation of several turbines for current speeds between 0.5 and 2.5 m/s.

This paper presents experiments and measurements on a low speed permanent magnet cable wound generator for marine currentenergy conversion.Measurements were made for no load and nominal load (4.44Ω/phase) conditions at nominal speed (10 rpm).For either load condition, the magnetic fields in the air gap were also measured. The measurements on the generator werecompared with the corresponding finite element method simulations used to design the machine. It is shown in the paper thatmeasurements and corresponding case simulations show good agreement. At nominal speed, the measured and simulated loadvoltages (nominal load) differ less than 1% for the rms values and less than 5% for peak values. At no load, measured and simulatedvoltages had larger differences, that is, <9% for rms values and <5% for peak values. Harmonic analyses of measured and simulatedphase voltages and currents show only the presence of third harmonics. The percentage of harmonics in the measured data wascomparable with the corresponding predictions of the simulations. The discussions and results presented in the paper could bebeneficial for future design of efficient and reliable marine current energy converter systems.

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.

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.

Renewable energy must be stored in order to make itreliable. Flywheels are capable of storing high amounts ofenergy and can also be used as power buffers, due to theirhigh power densities. This paper investigates a way tosmooth the power output from renewable energy converters(wave, wind and marine current) by adding a doublewoundflywheel energy storage to the system. Simulationsshow that a ramp-controlled flywheel energy storage woulddrastically smooth the short time power from a wave energyconverter but not be that appropriate for longer termenergy storage. The power quality enhancement producedby the addition of the flywheel to the system is alsosimulated and discussed.