  Wind turbines are often controlled to harvest the maximum power from the
wind, which corresponds to the operation at the top of the bell-shaped power
coefficient graph. Such a mode of operation may be achieved implementing an
extremum seeking data-based strategy, which is an invasive technique that
requires the injection of harmonic disturbances. Another approach is based on
the knowledge of the analytic expression of the power coefficient function, an
information usually unreliably provided by the turbine manufacturer. In this
paper we propose a globally, exponentially convergent on-line estimator of the
parameters entering into the windmill power coefficient function. This
corresponds to the solution of an identification problem for a nonlinear,
nonlinearly parameterized, underexcited system. To the best of our knowledge we
have provided the first solution to this challenging, practically important,
problem.
