Performance Optimization of Wave Energy Converters Through Bistable Mechanisms: A Theoretical-Experimental Comparative Study

Chile, a country located in South America, boasts exceptional wave energy potential, with some of the most promising conditions globally. For this reason, a few years ago, academics from the Universidad del B´ıo-B´ıo (UBB), located in the city of Concepci´on, joined forces to form the GROW-E UBB (Group of Renewable Ocean Energy). One of the most important milestones of this group is the design, manufacturing,and installation of a real-scale WEC at sea this year. This WEC is a point absorber type, as shown in Figure 1. This WEC, and generally all point absorber type WECs, are characterized by high hydrostatic stiffness. Consequently, they have a high natural frequency in comparison to the wave excitation frequency, which typically results in a low response.

Therefore, and as a result of a collaborative project between GROW-E UBB, the University of Edinburgh, and RWTH Aachen University, a solution was sought to increase the device’s dynamic response. A simple and promising solution is the use of a nonlinear stiffness mechanism, with a negative stiffness region, which reduces the system’s natural frequency and brings it closer to the excitation frequency.

Increasing the system’s response implies improving its performance and making the WEC more competitive. For this reason, a study of the WEC was initiated at a 1:13 scale. The nonlinear stiffness mechanism, or bistable system, which consists of a compression spring, was integrated, as shown in Figure 2. The dynamic response of the system was calculated using potential flow theory, obtaining the hydrodynamic coefficients in ANSYS AQWA. These results were contrasted with experimental measurements in a wave flume, Figure 3. The results show that the use of the nonlinear stiffness mechanism effectively increases the system’s response Figure 4, as indicated by previous studies. However, there is a poor correlation between the numerical and experimental results, where the response is overestimated by the theoretical calculations. By increasing the value of the radiation damping, a good agreement between the experimental and theoretical results is achieved.

Due to this, it is concluded that potential flow theory does not adequately represent the dynamic system when a mechanism that increases the device’s displacement is added, moving it out of the linear regime. From this, the need for high-fidelity numerical models arises to fully capture the complex dynamics of these systems. Therefore, as future work, it is proposed to perform the analysis with CFD, using the RANS (Reynolds-Averaged Navier-Stokes) equations. This is expected to provide a much more realistic representation of the interaction between the water and the buoy. Finally, the motivation to continue conducting research collaborations between GROW-E UBB and other research groups worldwide is made clear.