Control strategies for improving power performance of two-body heaving wave energy devices

This paper addresses the hydrodynamic analysis of a generalized twobody system (2B), a wave energy convertor containing a power take off (PTO) system for the purpose of energy conversion. The aim is to study the behaviour of the system and the effect of different control strategies on the power performance. For such a device, resonance can be obtained at two different modes of motion. A different version of the 2B system could be designed to modify their occurrence, for example, a system 1 for the first mode and a system 2 for the second mode. The differences in the dynamics and in energy absorption between the two systems are assessed and compared. The following methodology has been used. First we started with hydrodynamic modelling of the 2B system point absorber. The hydrodynamic models provide insight to the amplitude responses and power absorption behaviour in the frequency domain. Next these models are extended to time domain models for these devices. This allows studying and optimizing the non linear effects such as forces and damping on the PTO system. Finally the time domain model allows studying the control of the devices. Three types of control have been assessed, including passive loading, equivalent saturation control and maximum stroke control. In this study the control strategies are applied only on one sea state. Conclusions based on that analysis have been drawn in this work. However time domain models, including control strategies, allow us to estimate annual power production of 2B system by consideration of all sea states found in a wave climate.