Viscous Effects and Energy Recovery Optimization for Freely Floating and Bottom Fixed Wave Energy Converters

Abstract

This work deals with the study of two types of wave energy converters (WEC’s) taken as representative models of the bottom fixed systems and the freely floating ones. The considered freely floating WEC (FFWEC), a multi-body articulated system, consists of two horizontal cylinders connected with a flat plate and with axis parallel to the waves. The bottom fixed WEC, a point absorber (PAWEC), consists of a unique horizontal cylinder oscillating under the action of sea waves and connected to the seabed through an extensible Power Take Off device. For each system, a mathematical model is developed and is based on a balance of forces involving, in addition to the gravity and the Archimedean thrust, the Morison force, representing the added mass and damping effects as well as additional viscous effects. An evolutionary algorithm based method is used for the identification of the appropriate values of added mass and drag coefficients. The importance of the optimization of the sizing parameters for each of the systems with respect to the energy recoverable by the WEC’s is shown, and an evolutionary algorithm method is also used for their optimization. For the PAWEC, the results shows that the recovered energy is a decreasing function of the drag coefficient and that the decay is more marked when the wave pulsation increase. This decrease remains however limited and is about 5% when varies from 0 to 6. However, for the FFWEC the results show a less predictable situation as the drag coefficient increase due to the fact that the FFWEC model is more sensitive to a parameter’s variation. For the FFWEC, it is found that the viscous effects can act in both directions, in favor or against a recovery of energy recovery according to the range of values of the other coefficients. It is also important to note that taking account of viscous effects can lead to corrections of more than 50% in the calculation of the energy recovered by the FFWEC.