Small horizontal axis free-flow turbines for tidal currents

This thesis deals with small horizontal axis free-flow turbines for harnessing the kinetic energy of marine currents driven by the tidal cycle. The overall objective of this work is the development of innovative optimal rotor geometries for these small horizontal axis turbines which feature a) the maximum annual energy production at a given site of deployment, b) a minimum axial thrust in order to reduce cost for foundation at the seabed or for mooring of a floating platform, and c) a minimum sensitivity to cavitation for a low immersion depth in case of floating platform systems. Within this thesis, a novel turbine design method, tailor-made for small horizontal axis turbines, is developed. Key feature is the integration of an enhanced semi-analytic blade-element-momentum method and a cavitation inception model into a multi-objective optimization scheme. Control strategies for fixed- und variable-pitch turbines are included. For four generic classes of tidal current velocity distributions, representing potential tidal current resources of different energy levels, fixed- und variable-pitch turbines are designed. The predicted annual energy production of the optimized fixed-pitch turbine is between 22% and 27% above a classical design. The optimized variable-pitch versions promise even larger improvements. The new design method has been validated successfully within comprehensive model scale turbine tests in a towing tank and cavitation tunnel. Moreover, in several months' full-scale tests in a real tidal current, the predicted power and thrust characteristics of a selected close-to-production turbine was confirmed.