  A number of wave energy controllers tune the power take-off (PTO) system to
the frequency of incident waves. Since real ocean waves are non-stationary by
nature and not defined by a single frequency component, the PTO can be either
tuned at a constant frequency characterized by the local spectrum, or
continuously tuned to a representative wave frequency. In either case, a
time-frequency representation of the waves is expected since the wave profile
changes over time. This paper discusses about the PTO tuning problem for
passive and reactive controllers, in real waves, by comparing different methods
for time-varying frequency estimation: the extended Kalman filter (EKF),
frequency-locked loop (FLL), and Hilbert-Huang transform (HHT). The aim is to
verify the impact of such methods on the absorbed and reactive powers, and the
PTO rating. It is shown that the mean estimated frequency of the EKF, and FLL,
converges respectively to the mean centroid frequency, and energy frequency, of
the excitation force spectrum. Moreover, the HHT mean frequency has no
correlation with the spectral statistical properties. A comparison of the
energy absorbed shows that up to 37% more energy is obtained with the HHT over
the other estimation methods. Numerical simulations are performed with sea
elevation data from the Irish coast.
