Numerical Study on Energy Harvesting Efficiency of Multi-stage Overtopping Wave Energy Converters

Abstract

A viscous solver based on RANS simulations is used to numerically investigate flow physics as well as performance of wave energy devices used to harvest ocean wave power based on overtopping principle. Several validations have been done for VOF model and overtopping rates in comparison with laboratory experimental data, analytical solutions and a comparable study. The regular linear wave is utilized in the present study as an incident wave using dynamic mesh in which the generated wave profile is in satisfactory agreement with the linear wave theory.

The baseline model is stationary buoyant overtopping wave energy converter in which several geometrical design strategies have been applied in order to study their effects on the wave energy harvesting mechanism and performance. The partial hydraulic efficiency based on captured crest energy is used as the main performance indicator in the present study. A design strategy in order to maximize the efficiency is applied by the mean of multi-stage reservoirs and their opening called slot at different crest heights. The hydraulic efficiency of different device layouts is compared with that of single-stage device to determine the effect of the geometrical design. Based on two-dimensional simulation, The results show optimal trends giving a significant increase in overtopping energy. The optimal efficiency is obtained at relative slot width of 0.15 and 0.2 for variable slope and fixed slope devices respectively. Moreover, the use of adaptive slot width is studied. It has been concluded that the effect of adaptive slot on performance is unnoticeable.

Results of two-dimensional simulations show that the design of overlap ramp has a slight effect on overall hydraulic efficiency whether the slot layout is adaptive or non-adaptive. However, the partial efficiency is noticeably effected. Particularly, the larger overlap distance gives the greater hydraulic efficiency of the main reservoir and subsequently the smaller hydraulic efficiency of the lower ones. This seemingly influence on control strategy of power take-off unit. Additionally, the influence of submerged depth of multi-stage device installed on breakwater has been studied. The land-based device has greater efficiency in energy capture since the transmission wave is eliminated and is partly converted into overtopping energy. The deeper submerged depth results in the greater efficiency and finally tends to converge to a certain value.

Three-dimensional simulations have also been performed and compared to the two-dimensional results. The wider span or aspect ratio gives the greater capture crest energy. The result implies that further increasing the span, the efficiency tends to closely converge to that of two-dimensional device.