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Wireless monitoring systems for the marine environment are important for rapidly growing subsea developments. The power supply of wireless sensor nodes within the monitoring systems, however, is a major challenge. This study proposes a novel piezoelectric wave energy converter (pWEC) device to power the wireless sensing nodes. Unlike previous studies, the proposed device utilizes contactless pWEC technology in which a spring pendulum provides a two-stage frequency amplification of 3.8 times for low-frequency wave environments. The pWEC device consists of a floating body, inner pendulum, spring pendulum, magnets and piezoelectric sheets. In order to harvest the energy from relatively low frequency ocean waves, the pWEC device is designed to have an enhanced energy-capturing frequency. The effects of internal pendulum mass, spring pendulum weight, pendulum length and spring stiffness on wave energy absorption are investigated using theoretical and numerical analysis combined with laboratory experiments. The slider that drives the motion of the piezoelectric sheet vibrates at up to 3.8 times the wave frequency. To test the piezoelectric generators in the laboratory environment, a mechanical structure is set up to simulate the motion of the external floating body and the internal wave energy converter under the action of waves. When the four piezoelectric plates are arranged horizontally, the average output power per plate is increased by 2.4 times, and a single piezoelectric plate can generate an average of 10 mW of power. The proposed piezoelectric wave energy converter device has the potential to provide long-term energy supply for small ocean monitoring platforms at remote locations with reasonable wave energy resources. [ABSTRACT FROM AUTHOR]

In this paper, a novel hybrid wave energy converter (WEC)‐floating breakwater system consisting of three floating pontoons with power take‐off (PTO) modules is developed to extract wave energy from heave motion and attenuate waves to protect the coast. Unlike previous studies of this subject, the present three pontoons are in close proximity so that they may share the same foundation to reduce the cost further and both pontoons with step change drafts and same drafts are investigated. A systematic experimental study was carried out in a wave flume to examine the hydrodynamic characteristics of the hybrid system under regular waves. The experimental results are used to assess the influences of the WEC‐breakwater integration and the configuration of pontoons on the hydrodynamic performance of the hybrid system as a WEC and a breakwater. It was found that the integration of multiple energy‐extracting pontoons is robust in smoothing out the power output fluctuations, and the floating pontoons with same drafts gave arise to higher energy conversion efficiency while the floating pontoons with step change drafts are more effective in attenuating wave energy. A parametric analysis is able to identify the balanced performance of the system as a WEC and a breakwater. [ABSTRACT FROM AUTHOR]

Abstract: In this study we investigate how the wave energy deficit in the lee of an array of overtopping type wave energy converting devices (WECs), redistributes with distance from the array due to the natural variability of the wave climate and wave structure interactions. Wave directional spreading has previously been identified as the dominant mechanism that disperses the wave energy deficit, reducing the maximum wave height reduction with increasing distance from the array. In addition to this when waves pass by objects such as an overtopping type WEC device, diffracted waves re-distribute the incident wave energy and create a complex interference pattern. The effect of wave energy redistribution from diffraction on the wave energy shadow in the near and far field is less obvious. In this study, we present an approximate analytical solution that describes the diffracted and transmitted wave field about a single row array of overtopping type WECs, under random wave conditions. This is achieved with multiple superpositions of the analytical solutions for monochromatic unidirectional waves about a semi-infinite breakwater, extended to account for partial reflection and transmission. The solution is used to investigate the sensitivity of the far field wave energy shadow to the array configuration, level of energy extraction, incident wave climate, and diffraction. Our results suggest that diffraction spreads part of the wave energy passing through the array, away from the direct shadow region of the array. This, in part, counteracts the dispersion of the wave energy deficit from directional spreading. [Copyright &y& Elsevier]

• Hydrodynamic behaviour of a hull-wall system is extensively investigated over the range of hull draft ratio d/h ∈[0.4, 0.98]. • Enlightened by the low-frequency acoustic resonance in cavities, Helmholtz resonance of water waves in the gap is introduced. • Significant flow blockage phenomenon is observed as the hull approaches the bottom boundary. • Influence of wave nonlinearity on the behaviour of the Helmholtz-type hull-wall system is illustrated. Viscous and nonlinear effects on the gap resonance between a deep-draft hull and a vertical wall are investigated numerically, with particular attention to the low-frequency response, i.e., Helmholtz resonance. Viscous-flow computations indicate that the free-surface response in the gap initially increases and then decreases as the hull draft-to-depth ratio varies from 0.4 to 0.98. Conditions in which there is near complete dissipation and minimal wave reflection (i.e., K d = 0.99, K r = 0.07) are identified and the effect of wave height is evaluated. Vorticity contours are used to study the flow field between the hull and boundaries. Significant flow blockage is shown to occur when the hull approaches the bottom boundary, attenuating the gap resonance. In this case, full development of these vortices and water transport into and out of the gap is restricted, leading to increased wave reflection. Numerical simulations for various incident wave heights demonstrate that the Helmholtz resonance still results in substantial wave dissipation (i.e., K d > 0.89), which is in contrast to the behaviour observed for conventional gap problems. The findings may inform innovative designs for long wave absorbing breakwaters and Oscillating Water Column devices. [Display omitted] [ABSTRACT FROM AUTHOR]

A novel triple oscillating floater wave energy converter (WEC)-breakwater hybrid system was proposed to harvest wave energy and protect coast. A series of comprehensive laboratory experiments were conducted to investigate the effects of varying power take-off (PTO) and configuration of floaters on wave energy conversion and attenuation efficiency under regular and irregular waves. Wave energy reflection, transmission, loss and conversion are derived from experimental data to identify the optimum PTO damping force and draft of the floaters. It was found that as the initial draft gradient of floaters or PTO damping increases, the floater displacement and the wave transmission decreases, and the system makes the transition from a dissipative to reflective state. When the PTO damping of three floaters increases towards the wave maker, the wave energy absorption is largely determined by the draft gradient of three floaters, as the draft of each floater dictates the energy losses. In addition, the hybrid system with different PTO damping for each of the three floaters, exhibits an optimum frequency bandwidth for both wave power conversion efficiency (K e exceeding 20%) and wave attenuation. Remarkably, the present WEC-breakwater hybrid system is capable to achieve high efficiency in wave power conversion and wave attenuation at the same time, particularly for irregular waves. [ABSTRACT FROM AUTHOR]

A theoretical model based on the linear potential flow theory and eigenfunction expansion-matching method is developed to analyze the interaction between water waves and multi-chamber oscillating water columns (OWCs) embedded in a caisson array. The semi-analytical solution consists of the diffraction and radiation components of periodic OWCs. The velocity singularity at the tip of the OWC chamber walls is resolved by implementing a Chebyshev polynomial expansion. The unknown expansion coefficients are determined by the continuity equations of velocity and pressure at the interface of subdomains. The semi-analytical solution is verified using the Haskind relation and the conservation law of wave energy flux. It is found that multi-chamber OWCs have higher hydrodynamic efficiency over a broader frequency bandwidth than traditional single- and dual-chamber OWCs. The maximum hydrodynamic efficiency η increases with increasing of chamber number (J), from 0.5 (J = 1) to nearly 1.0 (J = 8 and 12). It is also found that a larger incident wave angle leads to a narrower frequency bandwidth for energy capturing. Increasing the incident wave angle from π/3 to 5π/12 results in a decrease of the first cutoff frequency k c from 2.41 to 2.02, and therefore results in the energy capture bandwidth narrowing accordingly. Both constructive and destructive hydrodynamic interactions between caissons array and OWCs are observed over the tested frequency range. Interestingly, when configuration of the OWC chambers is reversed, the transmission coefficient remains unchanged. • A theoretical model for wave interaction with multi-chamber OWCs embedded in a caisson array is developed. • Multi-chamber OWCs achieve a higher hydrodynamic efficiency than the single- and dual-chamber OWCs. • Both constructive and destructive hydrodynamic interactions between OWCs and the caisson array are observed. [ABSTRACT FROM AUTHOR]

In this paper, we theoretically investigate a multi-resonant oscillating water column (OWC) breakwater array. Based on linear potential flow theory, an extension of semi-analytical solution for wave interaction with periodic OWCs was developed for periodic OWCs with arbitrary chamber number. Both the diffraction and radiation problem for periodic multi-chamber OWCs are solved and verified using Haskind relation and wave energy flux conservation law. Parametrical analysis was conducted to examine the hydrodynamic characteristics of the present OWC breakwater system. Results indicate that multiple resonance of OWCs in piston mode led to multiple peaks of hydrodynamic efficiency, therefore broaden the effective frequency bandwidth with significant wave power absorption and coastal protection capability. However, the resonance of OWCs in sloshing mode in along-shore direction significantly weakens the wave power extraction. The triggering of the sloshing mode resonance mainly depends on the along-shore length of OWC unit and the incident wave angle. For larger along-shore OWC unit length (2 l > h, within the present calculations), the effective frequency bandwidth is significantly influenced by the incident wave angle. Notably, when the incident wave angle exceeds π/4, the effective bandwidth decreases by half approximately. However, for narrower lengths (2 l < h) , the hydrodynamic performance of the OWC array demonstrates reduced sensitivity to the variations of the incident wave angle. [ABSTRACT FROM AUTHOR]