The influence of flow damping on the maritime natural cave performance

Maritime Natural Caves (MNCs) are real and natural representations of shoreline Oscillating Water Column (OWC) devices. Recently, one particular MNC located in Cidade Velha on Santiago Island, Cabo Verde, has been the focus of several studies aimed at analyzing its behavior and energy performance under different wave climate conditions. This study investigates the operation of this MNC, focusing on the impact of airflow damping caused by its power take-off mechanism, represented here by orifices (ORs) with different cross-section areas and Wells turbines with various rotor blade stagger angles (B) on its energy extraction and production capacity. Our study showed that the power available from the MNC is linked to the flow damping characterized by the area contraction coefficient and the linear damping coefficient for turbines. The optimum flow contraction coefficient was found to be 𝐶􀯖 = 0.269, which maximizes both the average and peak power available from the cave, reaching 439.5 𝑊 and 4237.9 𝑊, respectively. When using turbines, the average power available ranged from 38.3 W to 80.5 W, which was considerably lower than the range observed with orifices (132.2 W – 454.1 W). This suggests that the damping capacity of the turbines could be improved. The turbine with the highest damping coefficient (B=15) produced the highest values of both available and converted power. However, it showed limitations in effectively converting the energy made availabThe turbines with moderatele by the MNC. The turbines with moderate damping, between 2.52 𝑐𝑚𝐻 m3/s (turbine with B=0) and 2.84 𝑐𝑚𝐻 m3/s (B=10), demonstrated better energy conversion performance with average efficiency of 23.6% and 20.7%, respectively. The remaining turbines exhibited lower average efficiency: 16.6% (B=15) and 13.3% (B=5). However, all turbines showed losses of efficiency when the MNC operated with high airflow rates. The turbine with B=0 had a contraction coefficient of 0.545, significantly higher than the optimal found for the orifices. The ideal contraction coefficient could be achieved by increasing both the turbine blades and hub by 60%.