Assembled PTO based on an array of double-acting hydraulic cylinders for WECs: From Conceptual Design to an Adjusted Detailed Model

Abstract

All Wave Energy Converters (WECs) based on wave activated bodies comprises a Power Take- Off (PTO) system among other subsystems like the reaction mechanism, the supervisor of the system and the electrical generator. One of the most applied PTO systems consists of oil high pressure hydraulic devices. These devices are able to apply high forces, to store large quantities of energy through accumulators and to provide smoother power output to the motor coupled to a generator. In these systems the poor efficiency and the oil leakages contaminating the environment are considered main drawbacks. Despite of this, they are widely used in several promising WECs with the aim of optimizing the harvested wave energy along the time. The initial challenge to absorb an oscillating movement of ±30º at 5rad/s as a maximum angular speed absorbing up to 16000Nm from a specific WEC, led to the development of a simplified hydraulic model before manufacturing a PTO prototype to be verified in a Test Bench (Figure 1). The experimental results of PTO under different conditions have been used to adjust a full detailed PTO Model using Mathworks® software platform. This work presents a patented oil high pressure hydraulic PTO prototype based on an array of four double-acting hydraulic cylinders. This prototype has been designed and completely modelled as a proof concept at 1:4 scale being able to apply a variable Coulomb type damping torque through the activation of each hydraulic cylinder independently and through the modification of geometrical parameters easily. The complete model of the PTO has been accurately tuned up through adjustment of model parameters using the results of the experimental tests. This will allow the study of control strategies to optimize the extracted wave energy from a specific WEC, like point-absorbers.