Comparing Energy Yields From Fixed and Yawing Horizontal Axis Marine Current Turbines in the English Channel

Abstract

At some sites with high tidal stream velocities 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), Race of Alderney (Alderney, Channel Islands/Normandy, France) and St. Catherine’s Point (Isle of Wight, UK) — 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) at the location of a tidal diamond, allowing a direct tidal analysis. For the three sites, the available data were analyzed into harmonic constituents and then extrapolated into the future. For each year’s worth of predictions, the cubed speeds as a function of time were sorted into bins to form a histogram. The annual power output for each design of turbine was then calculated using the known performance at each value of cubed speed. This process was repeated for each year over an 18.6-year lunar nodal cycle in order to ascertain the inter-annual variation in power output.