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"Wave power devices"
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Wave power extraction from multiple oscillating water columns along a straight coast
The integration of oscillating water column (OWC) wave energy converters into a coastal structure (breakwater, jetty, pier, etc.) or, more generally, their installation along the coast is an effective way to increase the accessibility of wave power exploitation. In this paper, a theoretical model is developed based on the linear potential flow theory and eigenfunction matching method to evaluate the hydrodynamic performance of an array of OWCs installed along a vertical straight coast. The chamber of each OWC consists of a hollow vertical circular cylinder, which is half embedded in the wall. The OWC chambers in the theoretical model may have different sizes, i.e. different values of the radius, wall thickness and submergence. At the top of each chamber, a Wells turbine is installed to extract power. The effects of the Wells turbine together with the air compressibility are taken into account as a linear power take-off system. The hydrodynamic and wave power extraction performance of the multiple coast-integrated OWCs is compared with that of a single offshore/coast-integrated OWC and of multiple offshore OWCs. More specifically, we analyse the role of the incident wave direction, chamber size (i.e. radius, wall thickness and submergence), spacing between OWCs and number of OWCs by means of the present theoretical model. It is shown that wave power extraction from the coast-integrated OWCs for a certain range of wave conditions can be significantly enhanced due to both the constructive array effect and the constructive coast effect.
Journal Article
A Review of Point Absorber Wave Energy Converters
There are more than thousands of concepts for harvesting wave energy, and wave energy converters (WECs) are diverse in operating principles, design geometries and deployment manners, leading to misconvergence in WEC technologies. Among numerous WEC devices, the point absorber wave energy converter (PAWEC) concept is one of the simplest, most broad-based and most promising concepts that has been investigated intensively all over the world. However, there are only a few reviews focusing on PAWECs, and the dynamical advancement of PAWECs merits an up-to-date review. This review aims to provide a critical overview of the state of the art in PAWEC development, comparing and contrasting various PAWEC devices and discussing recent research and development efforts and perspectives of PAWECs in terms of prototyping, hydrodynamic modelling, power take-off mechanism and control.
Journal Article
Broadband near-perfect capture of water wave energy by an array of heaving buoy wave energy converters
Arrays of heaving buoy type wave energy converters (WECs) are a promising contender to harness the renewable power of ocean waves on a commercial scale but require strategies to achieve efficient capture of wave energy over broad frequency bands for economic viability. A WEC-array design is proposed for absorption over a target frequency range in the two-dimensional water wave context by spatially grading the resonant properties of WECs via linear spring–damper power take-off mechanisms. The design is based on theories for rainbow reflection and rainbow absorption, which incorporate analyses based on Bloch wave modes and pole–zero pairs in complex frequency space. In contrast to previous applications of these theories, the influence of a higher-order passband and associated pole–zero pairs are shown to influence absorption at the high-frequency end of the target interval. The theories are used to inform initialisations for optimisation algorithms, and an optimised array of only five WECs is shown to give near-perfect absorption ($\\geq$99 %) over the target interval. Broadband absorption is demonstrated when surge and pitch motions are released, for irregular sea states, and for incident wave packets in the time domain, where the time-domain responses are decomposed into Bloch modes to connect with the underlying theory.
Journal Article
Wave scattering by plate array metacylinders of arbitrary cross-section
Metastructures composed of a closely spaced plate array have been widely used in bespoke manipulation of waves in contexts of acoustics, electromagnetics, elasticity and water waves. This paper focuses on wave scattering by discrete plate array metastructures of arbitrary cross-sections, including isolated vertical metacylinders, periodic arrays and horizontal surface-piercing metacylinders. A suitable transform-based method has been applied to each problem to reduce the influence of barriers in a two-dimensional problem to a set of points in a one-dimensional wave equation wherein the solution is constructed using a corresponding Green's function. A key difference from the existing work is the use of an exact description of the plate array rather than an effective medium approximation, enabling the exploration of wave frequencies above resonance where homogenisation models fail but where the most intriguing physical findings are unravelled. The new findings are particularly notable for graded plate array metastructures that produce a dense spectrum of resonant frequencies, leading to broadband ‘rainbow reflection’ effects. This study provides new ideas for the design of structures for the bespoke control of waves with the potential for innovative solutions to coastal protection schemes or wave energy converters.
Journal Article
Extending limits for wave power absorption by axisymmetric devices
The theoretical limit for absorption of energy in monochromatic water waves of wavelength $\\lambda$ by axisymmetric wave energy converters operating in rigid-body motion was established in the 1970s. The maximum mean power generated by a device absorbing due to heave motion is equivalent to that contained in $\\lambda /2{\\rm \\pi}$ length of an incident wave crest. For devices absorbing through surge and/or pitch motions the so-called capture width doubles to $\\lambda /{\\rm \\pi}$. For devices absorbing in both heave and surge/pitch the capture width increases further to $3\\lambda /2{\\rm \\pi}$. In this paper it is demonstrated that it is theoretically possible to extend the capture width for axisymmetric wave energy converters without bound through the use of generalised (non-rigid-body) modes of motion. This concept is applied to vertical cylinders whose surface is surrounded by an array of narrow vertical absorbing paddles. A continuum approximation is made to the paddle motion which simplifies the problem and allows strategies to be developed for setting the springs and dampers that control the power absorption. Results demonstrate that a cylinder of fixed size can absorb as much power as demanded from a plane incident wave although the practical limitations of linear theory are rapidly breached as that demand increases unless the size of the cylinder increases in proportion. In this paper we do not explore these limits in detail or further practical design considerations, such as imposing motion constraints. The continuum approximation is tested against a discrete paddle simulation for accuracy.
Journal Article
Weakly nonlinear theory for a gate-type curved array in waves
We analyse the effect of gate surface curvature on the nonlinear behaviour of an array of gates in a semi-infinite channel. Using a perturbation-harmonic expansion, we show the occurrence of new detuning and damping terms in the Ginzburg–Landau evolution equation, which are not present in the case of flat gates. Unlike the case of linearised theories, synchronous excitation of trapped modes is now possible because of interactions between the wave field and the curved boundaries at higher orders. Finally, we apply the theory to the case of surging wave energy converters (WECs) with curved geometry and show that the effects of nonlinear synchronous resonance are substantial for design purposes. Conversely, in the case of subharmonic resonance we show that the effects of surface curvature are not always beneficial as previously thought.
Journal Article
Optimisation of the geometry of axisymmetric point-absorber wave energy converters
We propose a scientifically rigorous framework to find realistic optimal geometries of wave energy converters (WECs). For specificity, we assume WECs to be axisymmetric point absorbers in a monochromatic unidirectional incident wave, all within the context of linearised potential theory. We consider separately the problem of a WEC moving and extracting wave energy in heave only and then the more general case of motion and extraction in combined heave, surge and pitch. We describe the axisymmetric geometries using polynomial basis functions, allowing for discontinuities in slope. Our framework involves ensuring maximum power, specifying practical motion constraints and then minimising surface area (as a proxy for cost). The framework is robust and well-posed, and the optimisation produces feasible WEC geometries. Using the proposed framework, we develop a systematic computational and theoretical approach, and we obtain results and insights for the optimal WEC geometries. The optimisation process is sped up significantly by a new theoretical result to obtain roots of the heave resonance equation. For both the heave-only, and the heave-surge-pitch combined problems, we find that geometries which protrude outward below the waterline are generally optimal. These optimal geometries have up to 73 % less surface area and 90 % less volume than the optimal cylinders which extract the same power.
Journal Article
Performance Analysis of a Floating Wind–Wave Power Generation Platform Based on the Frequency Domain Model
Integrating wave energy converters (WECs) onto floating offshore wind turbine platforms has emerged as a recent focal point of research aiming to achieve synergistic marine energy utilization and enhance the spatial efficiency of renewable energy. The power performance of WECs relies on hydrodynamic interactions with the floating platform. However, the coupled dynamic response and power generation mechanism remain unclear. This study establishes a multi-body model for the constrained motion of floating-platform and point-absorber WECs in the frequency domain. The power performance of WECs under different arrangements is compared and optimized. The performance of different hydrodynamic models in the South China Sea is analyzed further. The results indicate that exceptional peak performance is achieved when a single point absorber is placed on the floating platform. However, its performance under the full spectrum of wave frequencies in real sea conditions is suboptimal. Conversely, as the number of point absorbers on the floating platform increases, the performance of the hybrid system becomes more stable in real sea conditions. Furthermore, the array arrangement of point absorbers on the floating platform leads to multiple peaks in their power performance, and in selected array arrangements, the average power generation at specific frequencies is significantly superior to that of a single point absorber on the floating platform.
Journal Article
Assessment of Combined Wave and Wind Energy Resources of the Marine Pastures in Shandong Peninsula
The rapid development of the Blue Economic Zone on the Shandong Peninsula has generated a huge demand for renewable energy. Sensibly developing and utilizing these resources can provide a stable energy supply for marine pastures. However, assessing energy development potential must be done before the resources are developed. Using the fifth‐generation atmosphere reanalysis dataset (ERA5) from the European Centre for Medium‐Range Weather Forecasts (ECMWF) over nearly 15 years, this study assessed the combined wave and wind energy potential around the Shandong Peninsula’s marine pastures. A method was proposed to determine sites for combined wave and wind energy, analyzing the abundance, availability, stability, and correlation (CO) of these two resources in the sea surrounding the marine pastures. This novel approach addresses the limitations of traditional, disparate evaluation systems for wave and wind resources, enhancing assessment efficiency and accuracy. Utilizing the Joint Zoning Coefficients (JZCs) derived from these four indexes and considering the constraints related to the distance from the marine pastures, the VIOKR methodology was used to identify the four key sites most suitable for resource development. The JZC innovatively incorporates indicator weights and optimal station construction distance, a feature often overlooked in previous multi‐indicator assessments, to provide a more scientific basis for site selection. At these sites, the propagation direction as well as the cross‐CO with different time lags were analyzed. Finally, the most suitable wave energy converters (WECs) and wind turbines (WTs) have been identified, and the distribution of power generation times for different device combinations has been analyzed. This comprehensive device selection and power generation analysis aims to maximize energy conversion efficiency and ensure stable power output for marine pastures. The study findings offer decision‐making support for the combined wave and wind energy resources in the Shandong Peninsula.
Journal Article