Assessment and validation of wave excitation force estimators in operative conditions

For a reliable development of energy-maximising optimal control strategies in wave energy, wave excitation force estimators are fundamental components in the control loop, since the optimality of the solution depends on their precision. In the control deployment phase, it is necessary to test the precision of wave estimates to assess the eventual margin of improvement for optimal energy extraction. Nonetheless, current strategies for wave estimator validation are based on the possibility of repeating with high precision the wave signal, and blocking the device in a predefined position, assumptions which are possible only in certain test facilities. This study proposes a novel estimator validation strategy, termed closed-loop validation, based on the analysis of the system motion only, which is suitable for testing excitation force estimators under realistic (open-sea) conditions. For this purpose, two excitation force observers, based on a random walk and a harmonic oscillator wave description, are developed, on the basis of the energy conversion device identified model. In particular, the device is a WaveStar prototype at the Aalborg University facility tank, in Denmark. The observers (and the identified model) are validated under irregular sea states, with both the classical and the proposed scheme, proving the effectiveness of the latter in demonstrating the excitation force estimator performance without requiring a direct force measurement.