Exhaustive closed loop behavior of an one degree of freedom first-generation device for harnessing energy from marine currents

Resumen

The use of ballast systems to carry out automatic emersion/immersion maneuvers for first generation tidal energy converters (TECs) has aroused the interest of researchers and technicians as new technique by which lower installation and operation and maintenance (O&M) costs (a reduction of the installation costs by 10% and O&M by 15%). Very simple dynamic models have been obtained and subsequently employed in order to propose various control schemes with which to carry out this sort of maneuvers in devices with different degrees of freedom. This paper provides a detailed study of the closed loop behavior of a gravity-based first generation TEC, which performs only vertical movements with a single degree of freedom. The dynamic behavior of the set (system + controller) is analyzed when the parameters, obtained from its nominal dynamic model and used to design the controller, cannot correspond faithfully with the real parameters of the system. The effects of large uncertainties on the rigid body and the influence on the viscous terms of the added masses of the device are analyzed. The effect of strong additive disturbances owing to external forces or non-perfect null buoyancy during the performance of emersion/immersion maneuvers is also studied. The effectiveness of the proposed control system and its excellent behavior, even under non-nominal conditions and in the presence of strong external disturbances, together with the performance of the proposed emersion/immersion strategy has been demonstrated by means of numerical simulations and experimental trials on a laboratory prototype. A very simple criterion for the design of controllers is also proposed.