Water currents in e.g. unregulated rivers and tides are a considerable renewable energy resource. The technology needed to harness the energy in these streams has not yet matured, but research and development are ongoing. In the present paper the power coefficient of marine current turbines using vertical axis turbines is considered. When the width and height of the turbine are kept constant, the C_p - λ curve will depend mainly on the solidity. However, since all natural channels have vertical velocity variations due to friction against the sea floor, this will also affect the efficiency. The double multiple streamtube model was used to study the power coefficient as a function of solidity and vertical shear profile. Three turbine cases were studied: one turbine vertically aligned, and two horizontally aligned turbines rotating with opposite direction. The results show that the difference depends both on how much the velocity varies over the velocity profile, and on the shape of the velocity profile, where a linear profile causes a lower C_p compared to a logarithmic profile. The difference was especially prominent at low tip speed ratios for the horizontally aligned turbines with different rotational direction.
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