Geometrical Analysis of an Oscillating Water Column Converter Device Considering Realistic Irregular Wave Generation with Bathymetry

Abstract

Given the increasing global energy demand, the present study aimed to analyze the influence of bathymetry on the generation and propagation of realistic irregular waves and to geometrically optimize a wave energy converter (WEC) device of the oscillating water column (OWC) type. In essence, the OWC WEC can be defined as a partially submerged structure that is open to the sea below the free water surface (hydropneumatic chamber) and connected to a duct that is open to the atmosphere (in which the turbine is installed); its operational principle is based on the compression and decompression of air inside the hydropneumatic chamber due to incident waves, which causes an alternating air flow that drives the turbine and enables electricity generation. The computational fluid dynamics software package Fluent was used to numerically reproduce the OWC WEC according to its operational principles, with a simplification that allowed its available power to be determined, i.e., without considering the turbine. The volume of fluid (VOF) multiphase model was employed to treat the interface between the phases. The WaveMIMO methodology was used to generate realistic irregular waves mimicking those that occur on the coast of Tramandaí, Rio Grande do Sul, Brazil. The constructal design method, along with an exhaustive search technique, was employed. The degree of freedom H1/L (the ratio between the height and length of the hydropneumatic chamber of the OWC) was varied to maximize the available power in the device. The results showed that realistic irregular waves were adequately generated within both wave channels, with and without bathymetry, and that wave propagation in both computational domains was not significantly influenced by the wave channel bathymetry. Regarding the geometric evaluation, the optimal geometry found, H1/Lo = 0.1985, which maximized the available hydropneumatic power, i.e., the one that yielded a power of 25.44 W, was 2.28 times more efficient than the worst case found, which had H1/L = 2.2789.

Constructal Design Applied to an Oscillating Water Column Wave Energy Converter Device under Realistic Sea State Conditions

“…This can be justified by the fact that, for this ratio, L is larger, causing greater compression and decompression of the air inside the hydropneumatic chamber. It is important to highlight that this fluid dynamic behavior was also observed in previous works, such as Mocellin et al [23] and Gomes et al [20]. As shown in Figure 23, the highest static pressure peak occurred during the decompression of the hydropneumatic chamber, Figure 24 illustrates the mass flow rate within the turbine duct during simulation periods of 15 min for different geometries corresponding to various values of H 1 /L.…”

“…In addition, it should be noted that energy losses due to the power take-off (PTO) system were not considered; hence, the investigation concerning the OWC performance was developed considering its available power. This simplification has been adopted in several studies regarding the OWC WEC device [21][22][23]58,59]. Therefore, considering the turbine was not the original goal of this study.…”

“…Gomes et al [59] also performed a similar application of the constructal design methodology by varying H 1 /L and employing it to perform a geometric evaluation of an OWC device, for which the authors found an optimal ratio of H 1 /L = 0.2152, a value similar to that obtained in the present study. Mocellin et al [23] investigated the same H 1 /L values and device geometries as the ones used in the present study. The authors, however, focused on employing the WaveMIMO methodology to evaluate the hydrodynamic performance of the OWC WEC device in another possible installation site with a different wave climate.…”

“…This methodology was employed in the present study to assess the wave climate of a coastal region, aiming to reproduce an OWC WEC device subjected to a realistic sea state. In a similar approach, Mocellin et al [23] used this methodology to analyze the influence of bathymetry on the generation and propagation of realistic irregular waves and to geometrically optimize an OWC WEC device. The authors, however, focused on evaluating the performance of the device under realistic waves in the coastal region of the municipality of Tramandaí, Brazil.…”

“…Due to the random and chaotic nature of sea waves, few studies have made use of irregular waves [14,[24][25][26], and even fewer have employed realistic sea data [23,27]. In this lies the objective of the present study, which was to perform numerical simulations using irregular waves, originating from realistic sea data, as a means to analyze the geometry and performance of an OWC WEC device located in the coastal region of Rio Grande, Rio Grande do Sul, Brazil by using the constructal design method.…”