Numerical Investigation into the Performance of an OWC Device under Regular and Irregular Waves

Abstract

A numerical investigation into the hydrodynamic efficiency of an oscillating water column (OWC) device for the production of energy from sea waves under the conditions of regular and irregular waves is proposed. The numerical simulations were carried out using a two-dimensional version of a recently published three-dimensional free-surface nonhydrostatic numerical model, which is based on a conservative form of the contravariant Navier–Stokes equations written for a moving co-ordinate system. The governing equations are spatially discretized by a finite volume shock-capturing scheme based on high-order wave-targeted essentially nonoscillatory reconstructions and an exact Riemann solver. Time discretization was performed by a predictor-corrector method that took into account the nonhydrostatic pressure component. The proposed numerical model allowed us to highlight the significant differences between the hydrodynamic efficiency obtained under irregular waves and those obtained under regular monochromatic waves and provides more realistic evaluations of the OWC device performances. The results of the above comparison showed a reduction in the hydrodynamic efficiency of the OWC from 0.78 to about 0.54 when passing from regular waves to the corresponding irregular ones. The model was applied to assess the potential energy production obtainable by a set of OWCs at the Cetraro harbor (southern Italy). The numerical results show that, by adopting the optimal dimensions of the OWC, the estimated mean annual energy production obtainable at the Cetraro harbor is equal to 1540.52 MWh, which corresponds to the energy production of about 10 wind turbines with a nominal power of 60 KW.

Numerical Study on the Performance of an OWC under Breaking and Non-Breaking Waves

“…The experimental measurements show that in the laboratoryscale OWC, the interactions with regular waves whose non-dimensional wave numbers were close to K h = 1.5 (corresponding to a wavelength λ = 3.56 m) produced a peak of hydrodynamic efficiency. As demonstrated in a previous work [13], such an efficiency peak is due to resonance phenomena, in which the pressure oscillations within the chamber favor free-surface oscillations and induce their amplification.…”

“…The results of the above-mentioned study demonstrated that the hydrodynamic efficiency of an OWC calculated by using irregular waves (which represent marine wave motion more realistically) is significantly lower than that calculated by using regular waves. As demonstrated in the present work, the evaluation of the OWC efficiency obtained in [13] (although more realistic than that obtained by using regular waves) may still be overestimated because it was calculated by a pneumatic model that is usually adopted in the literature [7][8][9]13], in which the simplifying hypothesis of air incompressibility was assumed. This hypothesis is fully justified only in the case of low air speeds, i.e., a low rate of change in the air volume in the OWC chamber.…”

“…. T(t) by Equation (13) and RT(t) by Equation (12), substituting them into Equation (11), and dividing by p(t)V(t), we obtain .…”

“…In the case of numerical studies of OWCs located inside the breaking zone or in shallow water, more complex hydrodynamic models are needed, such as those based on the numerical integration of the Navier-Stokes equations by shockcapturing numerical schemes [10][11][12]. This approach has been used in a recent study [13], in which some of the present authors made a comparison between the performances of an OWC subjected to regular waves and those obtained by using irregular waves characterized by the same wave energy. The results of the above-mentioned study demonstrated that the hydrodynamic efficiency of an OWC calculated by using irregular waves (which represent marine wave motion more realistically) is significantly lower than that calculated by using regular waves.…”

“…In the present paper, we present a development of the work of Cannata et al [13]. We propose a pneumatic model that overcomes the simplifying hypothesis of air incompressibility, and we evaluate the performances of an OWC in two different hydrodynamic conditions, one characterized by non-breaking waves and one by breaking waves.…”