Energy Extraction Performance Research Articles

Propulsion and energy extraction performance of wave-powered mechanism considering wave and current

Wave-powered mechanism can extract wave energy as propulsion, enabling long-term ocean observation. The kinematic properties of wave propulsion mechanisms are susceptible to ocean current loads in addition to wave effects, presenting complex dynamic behaviors. In this paper, to understand the effects of ocean currents on the kinematic performances of wave propulsion mechanism, the dynamic performance of a wave propulsion mechanism in the combined condition of wave and current is analyzed based on numerical simulation method for fluid-rigid body coupling and experimental methods. The performance of the propulsion mechanism against the current under counter-current conditions and the contribution of flow to the propulsion performance under co-current conditions are discussed. The intrinsic relationship between wave height, current velocity and current direction is investigated, and the energy extraction performance of wave propulsion mechanism is analyzed. According to the results, the ability of propulsion mechanism to resist counter-current is obtained, and it is found that as the counter-current velocity increases, the hydrofoil will stall and the propulsion force and efficiency decreases significantly. Whilst, the co-current has a promoting effect on the forward of the mechanism, but within a certain range of co-current speed, the mechanism will be negatively affected by the shedding vortex. And the discrimination mechanisms for wave-dominated propulsion and current-dominated propulsion are obtained. This work reveals the combined effect of waves and currents on the motion performance of wave propulsion mechanism, providing a reference for the development of surface vehicles that utilize multiple ocean energies for synergistic propulsion.

An experimental study of vortex evolution around an offshore stationary dual-chamber oscillating water column converter

The Oscillating Water Column (OWC) is a promising Wave Energy Converter (WEC) due to its practicality and efficiency. Conventional single-chamber OWCs exhibit constraints in extracting energy over diverse wave periods, while dual-chamber OWCs demonstrate enhanced adaptability, leading to improved energy capture rates. This study investigates an offshore stationary dual-chamber OWC with an underlying transverse channel designed to facilitate wave ingress and augment energy extraction. Experiments across various wave periods reveal that dual-chamber OWCs with transverse channels achieve superior wave energy extraction compared to single-chamber counterparts. Time-resolved particle image velocimetry (TR-PIV) is employed for detailed flow field measurement to elucidate the complex hydrodynamic processes. Analysis of the water column oscillation and flow dynamics reveals an intensified interaction between hydrodynamic processes in the dual chambers, especially with elongated transverse channels. The Ω method is used to examine vortex evolution, indicating heightened complexity in vortex formation within dual-chamber OWCs compared to single-chamber OWCs. Notably, large-scale vortices emerging in the forward chamber impede oscillation and diminish energy extraction efficiency. Optimizing the design of transverse channel entries, frontal walls, and internal chamber corners is crucial to mitigate flow separation and vortex generation, thereby enhancing overall energy extraction performance.

Theoretical investigation on a heaving wave energy converter-dual-arc breakwater integration system array

The hydrodynamic performance of an array of heaving wave energy converter (WEC)-dual-arc breakwater integration systems was investigated using a theoretical model in the present paper based on the potential flow theory, with each dual-arc breakwater consisting of a bottom-mounted inner solid and outer perforated arc-shaped cylinder. The multiple scattering problem was solved using a hybrid iterative method. The mean transmission coefficient was introduced to quantify the wave attenuation performance. After successfully validating the theoretical model, the array effect and the hydrodynamic characteristics of the proposed integration system array are investigated. Focusing on the layout of a linear equidistant column, the calculation results indicate that the WEC and dual-arc breakwater array have mutual benefits in enhancing the wave energy extraction and wave attenuation performance. Compared to an array of bottom-mounted solid and porous cylinders, the proposed integration system array provides much better protection to the leeward region except at a very low wave frequency. In the considered frequency region, a closely arranged integration system array under the perpendicular incident waves has a very constructive array effect on the overall performance. For two staggered columns of integration systems, the wave energy absorption of the leeward column is improved and the sheltering effect to the leeward region is better at a high frequency as compared to the one-column layout; in contrast, the overall performance for two aligned columns is better at a low frequency.

Experimental study of a WEC array-floating breakwater hybrid system in multiple-degree-of-freedom motion

Installing an array of wave energy converters (WECs) on an existing breakwater is viable to reduce the high cost and improve the reliability of wave energy applications. The interaction between WECs and floating breakwater under multi-degree-of-freedom motion is crucial but not well investigated, especially the impact of WECs on the mooring system of the breakwater. This article fills this gap by experimentally studying a hybrid system consisting of a rectangle floating breakwater and an array of oscillating body WECs. The interactions between the WEC and the breakwater, the effect of the WEC bottom shape on the hybrid system, and the performance of the hybrid system in waves with different nonlinearities are investigated. Results indicate that the wave amplification effect of the breakwater improves the wave energy conversion performance of the WEC by up to 324.6%. As the WEC absorbs wave energy, the mooring forces of the breakwater in the pitch and surge directions decrease by up to 19.8% and 47.4%, respectively. When the incident wave height increases, the energy extraction performance of the hybrid system improves while the wave attenuation performance decreases. This study provides a reference for the practical application of WEC-breakwater hybrid systems and test data for the validation of numerical methods.

Numerical investigation on the energy extraction characteristics of parallel multi foils arranged in columns

Abstract The oscillating foil is a focus device in the clean energy utilization field. The presence of an oscillating foil structure within the flow field significantly impacts the energy extraction capabilities of the system. In this paper, three types of parallel multi-foil structures arranged in columns are proposed. The energy extraction characteristics of diverse multi-foil structures under various oscillation conditions are analyzed numerically. The flow field structure and the impact of parallel multi foils on the aerodynamic characteristics are investigated. Results demonstrate that the multi-foil structure with a phase difference of 180° achieves the highest level of energy extraction performance, and the maximal mean power coefficient achieves 0.894, which is 11.03% higher than that of the structure with a smaller phase difference. The size of the trailing edge vortex under the structure with a phase difference of 180° is significantly larger, leading to an obvious fluctuation in the aerodynamic lift.

Hydrodynamic performance of vertical cylindrical wave energy absorbers in front of a vertical wall

This paper studies the hydrodynamic performance of vertical cylindrical absorbers in front of a vertical wall. All the absorbers are independent of each other and restricted to only the heave motion. Based on a linear potential flow theory, an analytical solution is developed for the problems of wave diffraction and radiation by absorbers. In the solving procedure, the hydrodynamic problem is first transformed into an equivalent problem in an open water domain using the image principle. The number of absorbers in the equivalent problem is twice that in the real problem, and the plane layout is symmetric about the original vertical wall. The velocity potential of the fluid domain is obtained using the method of variable separation, and the unknown expansion coefficients in the velocity potential are determined by the matched boundary conditions. The heave excitation force, added mass, radiation damping, motion response, and energy capture width of the absorbers are calculated. Case studies are presented to show the effects of the wall reflection and hydrodynamic interaction on the energy extraction performance of the wave energy converter (WEC) system. Subsequently, the WEC performance under the action of irregular waves is analyzed by considering an incident wave spectrum, and the mean annual absorbed power of the device is estimated by considering the wave data statistics at the actual sites. The results indicate that when the wave motion resonates with the absorber motion, the energy extraction performance of the absorbers is significantly improved. The performance of the absorbers can be effectively improved when the structures are close to the antinodes of a standing wave field. By designing a reasonable plane layout, the hydrodynamic interaction can play a constructive role in the performance of the WEC system.

Numerical study on the energy extraction characteristics of a flapping foil with movable lateral flaps

This study focuses on how to improve the energy extraction performance of flapping foils by using movable lateral flaps. Three flaps are designed firstly, including fixed lateral flaps (FLFs), movable lateral flaps on the pressure side (PMLFs) and movable lateral flaps on the suction side (SMLFs). The energy extraction characteristics of foils with different flaps at different reduced frequency f* are investigated and compared based on numerical simulations. The results show that PMLFs have a more positive effect on the energy extraction of the foil than other flaps. Then the effect of the geometrical parameters of PMLFs including opening amplitude β0, lateral flap length lf and position df on the energy extraction efficiency η of flapping foils is studied, respectively. When using the suitable PMLFs, a larger lift is obtained by improving the pressure distribution and velocity circulation around the foil, which increases η. It is found that β0 has an optimal value, and a failure point exists when the sum of lf and df is 0.9c. The best design scheme of PMLFs is at β0 = 30°, lf = 0.2c and df = 0.8c, which help the flapping foil obtain the largest η = 48.0% at f* = 0.18, with an improvement of 21.7% than that of the conventional foil.

Investigations on the use of a KFm-4 profile to improve the energy extraction performance of a fully-activated flapping foil

Numerical simulations are performed to evaluate the effects of KFm-4 profile geometry, reduced frequency, and Reynolds number on the energy extraction performance of a fully-activated flapping foil. First, assuming that the flow is laminar, the governing equations are solved at a Reynolds number of Re=1100 to investigate the effects of step position, step thickness, step angle, trailing edge shape, and reduced frequency on the performances of the flapping foil. Then, the performances of the flapping foil are computed for Reynolds numbers up to 5.0105, assuming that the flow is fully turbulent for Re higher than 104. The obtained results show that the performance of the KFm-4 flapping foil is higher than that of the baseline profiles and that the maximum energy extraction efficiency is obtained when the step is located at the position Xm/c=0.5 and the step thickness is equal to em=0.06c, while the step angle has a negligible effect. Moreover, higher efficiency is obtained when the KFm-4 profile is built with a flat trailing edge. The results also show that the optimum reduced frequency is f∗=0.14 and that the efficiency of the KFm-4 flapping foil increases slightly with increasing Reynolds number.

Hydrodynamic responses and layout optimization of wave energy converter arrays consisting of five-degree-of-freedom truncated cylinders in front of a vertical wall

The hydrodynamic responses and layout optimization of a group of cylindrical wave energy conversion devices (WEC) in front of a fully reflecting vertical wall are investigated. Each truncated floating cylinder can oscillate with five degrees of freedom, i.e., surge, sway, heave, roll, and pitch. Based on the linear water wave theory, an analytical solution is developed for the hydrodynamic problem. The results of specific parameter studies suggest that the wall reflection effect significantly improves the energy extraction performance of the WEC array with the appropriate parameter conditions. A multi-level optimization method based on a genetic algorithm is developed. This paper investigates the optimal layout of the six WEC arrays, composed of 2–7 buoys, respectively. Additionally, the impact of other degrees of freedom (DOFs), besides the heave mode, on the hydrodynamic performance of the array is investigated. For β ≤ π/12, there is no need to consider the impact of other DOFs on the energy extraction in heave mode. The dimensionless amplitudes of other DOFs gradually decrease as the equivalent constraint stiffness increases. For k0a > 1.0, the heave amplitude and energy capture performance of the WEC array are significantly smaller. However, the amplitudes of other DOFs still have considerable magnitudes for k0a > 1.0. Therefore, for the sea area with high-frequency incident waves (k0a > 1.0), setting up a power takeoff system on other DOFs of each buoy to extract energy is a feasible solution to improve the performance of the WEC array.

Effect of boundary conditions on energy harvesting of a flow-induced snapping sheet at low Reynolds number

Recent studies on the snap-through motion of elastic sheets have attracted intense interest in energy-harvesting applications. However, the effect of boundary conditions (BCs) on energy extraction performance still remains an open question. In this study, we explored the snapping dynamics and energy-harvesting characteristics of the buckled sheet at various conditions using fluid–structure interaction simulations at a Reynolds number Re = 100. It was found that the front boundary condition (BC) dramatically affects the sheet's snapping dynamics, e.g., the pinned or relatively soft front BC triggers the sheet's instability easily and thus boasts the collection of potential energy. In the snap-through oscillation state, a stiffer rear BC results in a larger improvement in the sheet's energy collection compared with a minor effect of front BC. Meanwhile, the enhancement can also be achieved by adjusting the rear rotational spring stiffness up to 1.125 × 10−4, after which it remains nearly constant, as observed in the case of EI* = 0.004. This introduction of an elastic BC with krs* = 1.125 × 10−4 not only efficiently enhances energy extraction but significantly reduces stress concentration and, as a result, greatly prolongs the sheet's fatigue durability, especially for the stiffer sheet with EI* = 0.004. The effect of three other governing parameters, including the length ratio ΔL*, sheet's bending stiffness EI*, and mass ratio m*, on the sheet's energy-harvesting performance were also explored. The result shows that increasing ΔL* and EI* could improve the total energy harvested, primarily by enhancing the elastic potential energy, particularly in the aft half of the sheet. In contrast, increasing m* mainly enhances the kinetic energy collected by the sheet's central portion, thus improving the total energy-extracting performance. This study provides an in-depth insight into the dynamics of a buckled sheet under various BCs, which may offer some guidance on the optimization of relevant energy harvesters.

Design an Optimum Air Duct for Oscillating Water Column Wave Energy Device

Oscillating Water Column (OWC) is a technology that harvests wave energy to generate electricity. It works by harnessing the wave-induced pressure oscillation in the opening mouth of OWC. The oscillation of the pressurised air is extracted using a turbine placed at the top of the OWC structure. Although OWC has been studied for many years, designing an optimum air duct for such devices is still an open research area. The air duct plays a crucial role in the energy conversion process, as it is responsible for directing the airflow towards the turbine, and its design can greatly affect the performance and efficiency of the system. The article aims to study how the air duct design impacts the power output form. Moreover, the result will be compared with the conventional air duct design. The OWC performance is analysed using Computational Fluid Dynamic (CFD) software to identify which shape of the air duct has high efficiency. Autodesk Inventor is used to design the OWC devices modified from the basis design. Then these designs are imported into CFD software to simulate and obtain the energy extraction result. The CFD software is chosen to simulate the hydrodynamics problem. In this study, the oval design shows significant improvement in the energy extraction performance compared to the basis design. The oval design managed to produce 9.81, 10.02, and 11.66 J/kg of mechanical energy compared to 9.81, 9.82, and 10.04J/kg as prompted by the basis air duct design at the wave height of 0.2505, 0.75, and 2.25m, respectively. The result shows that the non-edge produces higher air pressure than the sharp edge of the air duct shape. Thus, the oval air duct design is more efficient, and it may increase the performance of the OWC wave energy converter device at a low wave height of 0.75 m.

Analytical investigation on a wave energy converter-dual-arc breakwater integration system

A wave energy converter (WEC)-breakwater integration system consisting of a heaving WEC and a bottom-mounted dual-arc breakwater is proposed. The energy extraction performance and wave attenuation performance of the integration system were investigated using a theoretical model based on linear potential flow theory. The permeability of the dual-arc breakwater is considered in the theoretical model. The singularity at the edges of the perforated arc-shaped wall was solved using the multiterm Galerkin method. The unknowns in the expressions of velocity potential were solved using the eigenfunction expansion matching method. After successful validation, a comparative study between the models with dual-arc breakwater of different permeability was carried out first. Then, a parametric study on the configuration with a solid inner and a perforated outer arc-shaped wall is carried out. The performance of the proposed integration system is further investigated in multi-directional irregular waves, and it was found that the fluctuations of the average power capture width ratio are much lower compared with that under unidirectional regular wave conditions. The overall performance of the integration system is better with a larger directional spreading parameter.

Imitating Fishtail Motion to Increase Efficiency of Flapping Foil Energy Harvesters

After a 2D simulation on both single and tandem flapping foil with two types of flapping trajectories, there was a significant role in energy extraction performance.

Effects of wake interaction on energy extraction performance of tandem semi-active flapping foils

A numerical investigation was carried out to analyze the interactions between semi-active tandem flapping foils at various tandem distances with a chord-based Reynolds number of 1100. Results indicate that with a tandem distance of less than 1.5 chord lengths and released in-phase, both foils exhibited terminal periodic motions with a nonzero mean stagger distance. In contrast, under the other conditions, the two foils ended up with periodic flapping motions without stagger. Due to the high-pressure region near the leading edge of the aft foil, the heaving motion of the fore foil resulted in lower energy extraction performance than that of single foil, when the tandem distance was less than 5 chord lengths. However, as the tandem distance increased, the fore foil acted like a single foil. The aft foil demonstrated significant fluctuations in performance parameters when subjected to the wake of the fore foil. The favorable interaction between the wake and aft foil resulted in lower power consumption for pitching and enabled the aft foil to extract an additional 15.2% power compared to a single foil. Conversely, during the unfavorable wake–foil interaction, the pitching motion of the aft foil consumed more energy than energy extraction from the heaving motion, leading to net energy consumption. The initial inter-foil pitching phase difference also significantly influenced the performance of the aft foil. Two models, the global phase and the wake phase model, affect these tandem configurations, both proving effective in capturing these effects with the wake phase model displaying notable efficacy.

Study of lift enhancing mechanisms via comparison of the flapping trajectory of tandem flapping foil

Designing an aero/hydrovehicle that mimics natural swimmers such as fish is a challenging task. In the present study, an attempt has been made to mimic the fish tail motion that generates the lift energy. Two fishtail trajectories are replaced with two National Advisory Committee for Aeronautics 0012 foils in a tandem configuration for energy extraction. The energy induced due to lift force of the hind wing while flapping along fishtailed trajectory in single and tandem configurations is determined and compared with that of the simple flapping foil having heave and pitch motion only. The impact of various factors such as foil gap (G), phase angle (φ), and influence of Strouhal number (St) on the energy extraction performance is also investigated at a Reynolds (Re) number of 1173. The results show that a fishtail motion trajectory and foil gap ratio of 3c–4c (c being chord length) would enhance the energy extraction performance of the hindwing. The present research provides a new vision for the improvement of biomimetic power generators, as it aims to improve energy efficiency through the use of wake vortices.

Numerical investigation on the hydrodynamics of a hybrid OWC wave energy converter combining a floating buoy

The present paper investigates a hybrid breakwater-oscillating water column (OWC) system by integrating a heaving floater wave energy converter (WEC). Both the water oscillating of the OWC and the wave-induced relative motion of the floater hinged in front of the OWC device are employed to extract wave energy. A three-dimensional wave tank is numerically modelled by Star-CCM + Computational Fluid Dynamics (CFD) software to investigate the hydrodynamic performance of the hybrid system. The hydrodynamic characteristics of symmetric and asymmetric floaters in terms of wave attenuation and energy extraction performance are examined. Four floaters with different bottom shapes are considered. The results indicate that the asymmetric floaters lead to a higher capture width ratio (CWR) and better wave attenuation performance, such that CWR of the hybrid system integrated with the Berkley Wedge bottom shape can reach up to 81.2%. The effects of the radius ratio, the submerged opening height, and the distance between the oscillating buoy (OB) and OWC device are further analyzed. The findings of this paper reveal that the geometrical parameters of the hybrid system have significant effects on wave energy absorption efficiency and hydrodynamic performance. The results provide advice on the design of wave energy devices.

Performance Optimization of SSHC Rectifiers for Piezoelectric Energy Harvesting

In the past decades, inductor-based synchronized switch harvesting on inductor (SSHI) rectifiers have been widely employed in many active rectification systems for piezoelectric energy harvesting. Although SSHI rectifiers achieve high energy extraction performance compared to passive full-bridge rectifier (FBR), the performance greatly depends on the inductor employed. While a larger inductor can achieve higher performance, the system form factor is also increased, which is counter to system miniaturization in many applications. To solve this issue, an efficient synchronized switch harvesting on capacitors (SSHC) rectifier was proposed recently. Instead of using large inductors, the SSHC rectifier employs on-chip or off-chip flying capacitors to achieve comparable or higher performance. In previous studies, the flying capacitors are chosen equal to the inherent capacitance of the piezoelectric transducer (PT) to achieve 1/3 voltage flipping efficiency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(\eta _{F})$ </tex-math></inline-formula> for a 1-stage SSHC rectifier and 4/5 flipping efficiency for a 8-stage SSHC rectifier. This brief presents that the flipping efficiency can be further increased to 1/2 for a 1-stage SSHC rectifier if the flying capacitor is chosen to be much larger than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C_{P}$ </tex-math></inline-formula> and the 4/5 flipping efficiency can be achieved by employing only 4 flying capacitors.

Coupling performance of two tandem and side-by-side inverted piezoelectric flags in an oscillating flow

The flow-induced vibration characteristics and energy extraction performance of two flexible inverted piezoelectric flags arranged in tandem and side-by-side configurations in an oscillating flow are studied. An immersed boundary-lattice Boltzmann method is employed for Reynolds number of 100, mass ratio of 2.9 and non-dimensional bending stiffness of 0.26 which correspond to the maximum flapping amplitude for a single inverted flag in a uniform flow. 2D simulations are conducted by varying the ratio R of the frequency of the oscillating flow to the fundamental natural frequency of the flag and the horizontal velocity amplitude (Au) of the flow. Three coupling regimes at Au=0.5 are identified for both tandem and side-by-side flags: chaotic oscillations regime I (0.1≤R≤1), large periodic and symmetric oscillation regime IIa1.1≤R≤1.5andIIb(2.1≤R≤3.0), and small periodic and asymmetric oscillation regime III(1.6≤R≤2). The maximum mean electrical power coefficient C¯P occurs in regime IIa at R=1.5 with, α (piezo-mechanical coupling parameter) = 0.5, and β (piezo-electric tuning parameter) = 1.5. C¯P is 0.1 for a tandem upstream flag, 0.068 for the tandem downstream flag and 0.1 for both side-by-side flags, and is respectively 120%, 300% and 213%, higher than that of the corresponding flag in the uniform flow. This improvement is attributed to the higher flapping angular amplitude (180°), the higher ratio of the flapping frequency of the flags to the oscillating frequency of the flow (virtually constant at 0.5), and constructive vortex interaction in regime IIa.

Effect of the Dynamic Froude–Krylov Force on Energy Extraction from a Point Absorber Wave Energy Converter with an Hourglass-Shaped Buoy

Point absorber wave energy converter (WEC) control strategies often require accurate models for maximum energy extraction. While linear models are suitable for small motions, the focus is on the nonlinear model of an hour-glass shaped buoy undergoing large vertical displacements. Closed-form expressions for the static and dynamic Froude–Krylov forces are developed. It is shown that, in general, the dynamic and static forces are of similar magnitude, which is not the case for a spherical buoy. While the dynamic force reduces the amplitude of the net buoy force, its shape predicts a larger buoy response than if neglected, causing the nonlinear terms to have an even more significant effect. An input-state feedback linearizing controller is developed to show how the nonlinear model can be used in a control law. A 2.5 m buoy example is simulated to illustrate the approach of tracking an arbitrary displacement reference. For the case considered, the extracted power is 30% larger when the nonlinear dynamic FK force is used in the control law. The hourglass buoy is also compared to a spherical buoy to illustrate differences in their response to regular waves and energy extraction when using the same control laws. A spherical buoy diameter of 7.5 m was required to obtain the same power output as a 5 m tall hourglass buoy. A power-force-amplitude (PFA) metric is introduced to compare energy extraction performance and power take-off requirements. The hourglass buoy’s PFA was 13% larger than the spherical buoy implying that it can produce similar power but with less control effort.

Numerical study on the energy extraction performance by flapping foils in a density stratified flow

An energy extractor based on fully-activated flapping foils operating in a density stratified flow is studied numerically. The effect of stratification strength characterizing as the internal Froude number on the energy extraction performance is first investigated by fixing a typical set of parameters. The hydrodynamic characteristics of flapping foils with different kinematics in the stratified regime are then examined by varying the pitching amplitude. Our results suggest that the stratification effect decreases the energy harvesting efficiency owing to resulting poor synchronization between forces and flapping motion. Within the considered parametric space, the minimum energy extraction performance is identified at the Froude number around Fr=2, giving rise to an attached boundary layer without the favorable flow separation and leading edge vortex shedding. Nevertheless, a sudden increase of extracted power is observed when the stratification level is enhanced to Fr=1, which is attributed to the distinct flow structures generated by the induced internal waves. For the varying kinematics, the Froude number corresponding to the minimum performance is found to increase as the pitching amplitude becomes higher. It is indicated that the induced internal waves tend to dominate the flow patterns at a highly strong stratification, whereas the kinematics of flapping foils still play a crucial role in determining the vortex dynamics for the moderate and weak stratification strength.