| Summary: | At many locations with high tidal stream velocities – and potential for tidal stream energy generation – the flow is approximately rectilinear, that is to say the flow direction is always 0 degrees or 180 degrees with respect to a particular orientation. At some sites, however, there is an appreciable change in flow direction (‘swing’) away from 180 degrees between the two maxima of flow speed. In order to assess the performance of horizontal axis marine current turbines in non rectilinear currents, measurements of a model rotor have been made in a towing tank. Curve fits have been calculated as a function of the cosine of the yaw angle squared and the thrust as cosine of the yaw angle. The curve fits have been used in a case study to investigate the impact of fixed-orientation or yawing rotor designs on average annual energy output, at three locations in the English Channel. All three sites are of the type where flow is accelerated around a headland or cape, but their tidal streams vary in deviation from rectilinearity. For two of the sites - Portland Bill (Dorset, UK) and Race of Alderney (Alderney, Channel Islands/Normandy, France) - available data consisted of tidal stream diamonds printed on Admiralty navigational charts. These rely on local tidal elevations for interpolation of tidal streams. At the other site – St. Catherine’s Point, Isle of Wight, Hampshire – current meter measurements of duration one month were available from the British Oceanographic Data Centre (BODC), allowing a direct tidal analysis. …
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