Design, Control and Optimization of a water sloshing tank system coupled with ISWEC

The work performed in this thesis concerns the implementation of a water sloshing tank system (U-Tank) in the Inertial Sea Wave Energy Converter (ISWEC) in order to maximize the energy harvested from ocean waves. The conventional use of the water sloshing tank is like damping and stabilizing system, it creates a stabilizing force directly opposing the forces created by the passing wave in order to reduce the roll motion of ships. The purpose for which U-tank system was conceived is overturned and its simple working principle is exploited to broaden the performance bandwidth of the ISWEC device (designed for low-medium energetic sea-states) to oceanic applications. Firstly, the influence of the design parameters of a passive U-Tank on its dynamic properties was analyzed and then an optimal design study based on the U-shaped tank geometry variation was performed to define the envelope curve of the Hydrodynamic pitch Response Amplitude Operator curves which guarantee the achievement of a maximum resonance condition for the coupled system at a given peak period. Due to technological limitations which make the resonance matching of the device with the incoming wave impractical through the only variation of geometrical and inertial properties of the device, the dynamic properties of the coupled system can be modified with an active control system leaving the device geometry unchanged. The integration of the water sloshing tank (U-Tank) technology in a pitching-resonant WEC is based on the idea of implementing a slow-tuning control logic of the U-Tank to tune the resonance of the device to a specific excitation frequency. The shift of the resonance period in the Hydrodynamic pitch Response Amplitude Operator, through control of the sloshing dynamics of the water inside the U-Tank, allows the ’slow-tuning’ of the WEC with incoming sea-state. In order to achieve this result, we have developed different control laws which affect the coupled dynamic system through the U-Tank dynamics. A multi-objective optimization algorithm is implemented in order to optimize the weights of the different control laws and for each of them the achievable performances were assessed.