A Design Study of Marine Current Turbine-Generator Combinations

Marine currents are an offshore source of renewable energy of increasing importance, with the development of technology for electricity generation from tidal currents or low-head river currents advancing at a quick pace. Two of the major components of a marine current power plant are the generator and the turbine. It is not sufficient to design these components separately, but a system approach, where the power plant is seen as one entity, must be taken to achieve best overall efficiency. In the present paper, the performance of three different combinations of direct-driven permanent magnet generator with cross-stream axis marine current turbine is examined numerically under the variation of water flow speed. The design case chosen is that of a shallow river or tidal channel, where the cross-sectional area limits the physical size of the power plant. The units are designed for a power output of 10 kW at a water current velocity of 1 m/s. Turbines for three different rotational speeds are considered, each in combination with a corresponding generator. The three turbine-generator systems are designed according to similar design criteria to allow for comparisons. The turbines are modelled using an in-house code, based on the double multiple streamtube model. Corrections are made due to the finite aspect ratio and tip losses of the blades. Experimental data for the lift and drag coefficients for different Reynolds numbers are used in the model. The generators are modelled using a FEM tool that has been validated with experimental results. The three generators are designed for the same nominal voltage and with a low load angle to allow for overload operation. The overall performance of each of the three systems is studied under varying flow velocity. The main conclusion is that all three machines exhibit essentially the same performance behaviour, which means that the choice of nominal operational speed for a power plant will not be a major design constraint. Turbines with higher rotational speed allow for a more compact generator design within the limits of the design parameters used in this study. However, this also entails certain mechanical constraints on the turbine. Due to the restricted cross-sectional area in the channel, it is clear that at least one of the three systems would have to be placed with the axis of rotation in a horizontal rather than vertical position.