Power Capture Performance Research Articles

Interactive effects of deformable wave energy converters operating in close proximity

Flexible wave energy converters (FlexWECs) have been gaining increasing research and industrial interest as their deformable nature can potentially remedy the structural issues that limit the development of rigid WECs. To maximise the usage of space and infrastructure and improve energy efficiency, FlexWECs are normally deployed in close proximity, where the wave interaction with one device can influence others, signifying the opportunity to obtain energy efficiency enhancement from the interactions. To investigate the power capture performance of a FlexWEC array, this study employed a validated three-dimensional high-fidelity computational method to simulate the wave interaction with three FlexWECs in various array arrangements including power-take off. Based on systematic simulation cases, the present work analysed the relation between the geometrical characteristics of an isolated FlexWEC's perturbed wave field and the array's overall energy capture efficiency. The constructive interaction of the array was found the strongest when the longitudinal and lateral spacings of the array were 0.6 and 1 times of incident wavelength respectively, with a 15 % enhancement of overall captured energy compared to three devices operating in isolation. Overall, this study provides insights into the fluid-structure interaction of waves with multiple deformable structures, facilitating the modelling and planning of FlexWECs.

Optimizing wave energy harvesting through oscillating water columns in semi-submersible energy harvesting platforms: A comprehensive study on enhancing efficiency and competitiveness

In coastal regions with substantial wave energy potential, significant efforts have been dedicated to advancing both the Technology Readiness Level (TRL) and Technology Performance Level (TPL) of Wave Energy Converters (WECs). Despite their promising energy generation capabilities, the widespread adoption of WEC devices faces a notable challenge in the form of their relatively high Levelized Cost of Energy (LCOE). This economic hurdle hampers their competitiveness when compared to conventional fossil fuels and other renewable energy sources. Oscillating Water Columns (OWCs) stand out due to their noteworthy potential, primarily attributed to their low Operations and Maintenance (O&M) costs. To enhance the energy performance of OWC arrays and pave the way for hybrid offshore energy solutions, the integration of a semi-submersible Energy Harvesting Platform (EHP) is proposed. This integration involves configuring the array with specific OWC End-Stop limitations and geometry to establish resonant conditions during the predominant wave, thereby optimizing constructive interactions and desirable OWC motion with the EHP's heave, pitch, and roll. A Genetic Algorithm (GA) is employed for this optimization process. Additionally, a novel parameter, the In Situ Interaction Factor (ISIF) is introduced, to evaluate the EHP's power capture performance. This metric compares the actual energy harvested from the array of WECs on the platform to the cumulative energy harvested by each device in isolation. Notably, ISIF achieved a remarkable value of 1.99, highlighting the highly efficient constructive interaction achieved through the precise positioning of OWCs. This resulted in nearly doubling the power capture compared to 23 OWCs operating in isolation, underscoring the potential of the EHP in harnessing wave energy. Validation of the analytical analysis was conducted using the open-source Boundary Element Method (BEM) tool NEMOH, alongside reference to the experimental studies.

Design, modeling and performance analysis of a deformable double-float wave energy converter for AUVs

Autonomous underwater vehicles (AUVs) are one of the most important means of ocean exploration. However, the restricted energy supply poses a significant challenge to the advancement and practical application of AUVs. Based on the structural characteristics, this paper proposes a novel AUV, which integrates a deformable double-float wave energy converter (WEC), called DFWEC-AUV. The DFWEC empowers the AUV to capture wave energy to supply itself by oscillating double-float form in power generation mode. The structural design, focusing on the deformable float (DeF) and the onboard damping plate (DP), and the working principle, including mode switching and energy capture, are firstly introduced. Then, a double-float dynamic model of DFWEC-AUV oscillating in regular waves is established to reveal the energy capture mechanism in both frequency-domain and time-domain. Finally, the influence of the presence of DP, structural parameters (diameter of DP and deployment angle of DeF) and power take-off (PTO) system damping coefficient on the dynamic response characteristics and energy capture performance is analyzed. In addition, to verify the effectiveness of the performance analysis, a comparative simulation of parameter optimization is carried out under actual wave excitation. The results show that the addition of onboard DP is able to effectively enhance the energy capture performance of DFWEC-AUV, as well as increase the energy capture bandwidth. Moreover, the larger the diameter of the DP is, the greater the performance improvement of DFWEC-AUV is. Conversely, increasing the deployment angle of the DeF does not necessarily lead to better power capture performance. Additionally, it is observed that waves of a particular period are consistently associated with an optimal PTO damping coefficient, which leads to superior power capture performance. In comparative simulation, the energy capture performance of DFWEC-AUV after parameter optimization is increased by 4.7 times.

Wave Energy Converters and Hydraulic Power Take-off: analysis of a possible control strategy

Wave energy conversion is one of the most promising but untapped worldwide technologies. In fact, wave energy is clean and renewable as the solar and wind ones, but also less mature and utilized. Moreover, several Wave Energy Converters (WECs) patents have been developed in the last decades, as evidence of the great potential beneath these systems. Among the different issues related to this technology, the most challenging is controlling such systems. Maximizing the captured energy is indeed the main goal of a WEC’s control mechanism. As for single-mode oscillating absorbers, it has been proven that a resonance condition between body velocity and excitation force must be achieved. This control solution is generally called phase control. Different phase control strategies have been realized since the 1970s and latching control is one of them. However, how to implement sub-optimal phase control in real seas remains an open problem. A numerical model of the wave energy converter and its power take-off system is proposed in this paper. More precisely, the model simulates the hydrodynamics of a WEC point absorber coupled with a hydraulic Power Take Off in regular waves. Additionally, a parametric latching control strategy is suggested to enhance the converter’s power capture performance.

Improving the power capture performance of deep-sea two-body counter-rotating wave energy using frequency domain optimization in regular and irregular waves

This paper presents a novel two-body counter-rotating wave energy converter (CR-WEC) designed to power deep-sea exploration equipment. In contrast to conventional two-body structures, which employ relative motion, the CR-WEC powers the Power Take-Off (PTO) via absolute motion to circumvent the issue of a limited working stroke. We present the power equation of CR-WEC in the frequency domain, based on the actuator disk model from the wind turbine field, in order to elucidate the energy absorption characteristics of this type of two-body absorber and investigate the influence of parameters on power acquisition. Experiments conducted as part of this study have confirmed this theory's applicability to the CR-WEC. Through an approximation, we derive the frequency point corresponding to the maximum value of the equation, which we refer to as the Power Maximum Frequency Point (PM-FP) in this paper. Under the external excitation of both regular and irregular waves, we investigate how parameter modification can enhance the absorber's capture characteristics in specific cases. After modifying the parameters according to the principles derived from the aforementioned research, the system's theoretical power output can more than double. In addition, this study's conclusions emphasize the benefits of CR-WEC with cascade expansion. By connecting multiple PTOs in series, power generation can be enhanced.

Performance characteristics and parameter analysis of a multi-DOF wave energy converter with hybrid power take-off systems

A multi-degree-of-freedom (multi-DOF) wave energy converter (WEC) with hybrid power take-off (PTO) systems may capture more wave energy than a traditional single-DOF WEC, which usually obtains energy from pitch or heave. Compared with the existing WECs with a single PTO, this paper presents the concept design of a multi-DOF WEC with hybrid PTOs named PUWEC, which has prismatic pairs in the hydraulic cylinder and universal pairs in the gear transmission system. Two corresponding PTOs are matched to the motion of the buoy in heave and pitch to enhance wave energy conversion. This work focuses on establishing a time-domain numerical model to simulate the coupled dynamics and power capture performance of the proposed PUWEC. The model tests are carried out in a wave tank to verify the validity of the numerical model. The results show that the numerical model is efficient in predicting the dynamic performance of the PUWEC. Regular and irregular wave conditions are considered in the numerical simulations to explore the performance characteristics of the devices. It is found that the maximum improvement of the captured power of PUWEC is 49.8 % compared with single-DOF PWEC under the regular wave frequency 0.42 Hz, and the capture width ratio of PUWEC is close to 30 % under three wave conditions. In addition, the motion trajectory of the center of gravity of the buoy is analyzed. With the increase in U-PTO damping, the trajectory of the float changes from horizontal bowl-shaped irregular path to inclined regular hollow elliptical orbit. The trajectory coefficient is introduced to provide insights into how the PUWEC improves the energy conversion. The actual working space of the PUWEC is then calculated, which is less than the design working space, indicating that the actual performance of the PUWEC is fully developed. The proposed PUWEC and constructed numerical model which considers the coupled dynamic performance of dual PTOs, can provide valid reference for the design of other multi-DOF WECs with hybrid PTOs.

Numerical study on bottom-hinged plate wave energy converter geometry design

This study focuses on the fluence on the power harvest performance exerted by plate geometry, damping setting and number of plates. Linear potential fluid theory is applied to calculate the hydrodynamic coefficients of the plate in frequency domain, and motion of bottom-hinged plate with different damping coefficients in time domain. The power capture performance is carried out based on the character of the plate motion. The performance of the device in different wave periods is studied and the adaption of the device is evaluated. The parameterized study is conducted to research the width, thickness, and the cross section of the plate. It is found that a two-dimensional plate model cannot fully reveal the hydrodynamic characteristic of the bottom-hinged plate, the width of the plate is an important factor and should take into consideration. The thickness of the plate is not a sensitive factor. The cross section of the plate has a distinct influence on plate performance. The number of the plate segment with a fixed whole width is studied. It is found that dividing plate cannot improve the performance of the plate, and dividing the plate cannot improve the performance of the plate while facing non-front-inducing waves either.

Low-frequency energy capture and water wave attenuation of a hybrid WEC-breakwater with nonlinear stiffness

Wave energy is attractive for its generous, sustainable and clean, the combination of floating breakwater (FB) and wave energy converter (WEC) is an economical approach to capture wave energy and attenuate waves. The conventional hybrid WEC-FB system is ineffective for low-frequency waves. To overcome this shortcoming, this paper proposed a novel WEC-FB with nonlinear stiffness mechanism to improve its wave attenuation and power capture performance. A hybrid semi-analytical nonlinear frequency-domain approach including the eigenfunction expansion matching method (EEMM) and multi-harmonic balance method (MHBM) is proposed to solve the nonlinear waves-structures interaction problem. The performance of the nonlinear WEC-FB is demonstrated by comparing with conventional linear counterpart, and the underlying reason for the effect of nonlinear mechanism is explored. The phase control mechanism for improving low frequency performance of wave elimination and energy capture is revealed by using the analytical method. This study shows that the introduction of the nonlinear mechanism can both improve the wave attenuation and energy absorption performance, especially in the low-frequency wave region.

Power output estimation of WEC with HF radar measured complex representative sea states

Wave tank model testing has been widely used to assess the performance of Wave Energy Converters (WEC) in different technology readiness levels (TRL). At early stage the use of simple wave conditions such as regular waves and parametric wave spectrum JONSWAP or Pierson-Moskowitz spectrum is acceptable. However at later stages there is a need to use site specific complex wave conditions representative of potential prototype deployment sites. In previous research, 10 different regrouping methods on HF radar measured wave spectrum were tested to find out the most representative sea states for tank testing. It has been shown that by using the K-means clustering technique, the characteristics of wave conditions can be well preserved. In order to assess the power capture performance of a typical WEC in these representative sea states, the RM3 point absorber has been simulated. By analysing how well the average power output predicted from different representative sea-state selection methods compares with the total power output prediction, it is shown that the non-directional wave spectrum K-means clustering method provides the most representative sea states and, for a point absorber, with a very accurate estimation of the total power output, which is not the case by using a traditional binning method. The importance of using the complex site-specific sea states rather than simplified parametric JONSWAP sea states to obtain the accurate total power estimation has also been shown.

A novel discrete control for wave energy converters with a hydraulic power take-off system

In this paper, a novel discrete control strategy is proposed to improve power-capture performance of wave energy converters (WECs) with a hydraulic power take-off (PTO) system. In this novel discrete control strategy, separate switching valves are introduced such that each chamber of hydraulic cylinders can be actively and independently connected to a high-pressure gas accumulator or a low-pressure gas accumulator by controlling the switching action of the switching valves. Thus, it increases the flexibility of the PTO system and makes the PTO system more controllable. The effectiveness of the novel discrete control on improving power-capture performance of a raft-type WEC and the optimal switching command of the switching valves for maximizing power capture are studied. Benefit brought by the novel discrete control is investigated in terms of power improvement and the control effect of the novel discrete control is compared with that of load control and latching control. Results show that the novel discrete control can effectively improve power-capture performance of the raft-type WEC and theoretically the corresponding benefit reaches a peak value of 2 and 2.5 in regular and irregular waves, respectively. The novel discrete control has an advantage over load control and can be complementary to latching control.

Wave energy converter optimization based on differential evolution algorithm

A parameter optimization method for an oscillating buoy-type wave energy converter (WEC) is presented from the perspective of submerged buoy volume; which affects both the power production and cost investment. The effects of submerged buoy volume on the optimal power capture are studied using the differential evolution algorithm and linear potential flow theory. The cost indictor V/P (V and P representing submerged buoy volume and power capture, respectively) is introduced to measure the cost-effectiveness of the WECs. The power capture and cost performance of the optimized WECs in regular and irregular waves are discussed. The results show that the WEC with the large power take-off (PTO) damping has good adaptability to a range of wave frequencies; however, the optimal submerged buoy volume is not cost-efficient due to the consequent relatively large cost indicators. The best choice of WEC in terms of cost-effectiveness is one designed with a large PTO damping and a buoy of slightly small non-optimal submerged volume.

Parametric study on power capture performance of an adaptive bistable point absorber wave energy converter in irregular waves

The power capture performance of an adaptive bistable point absorber wave energy converter (WEC) in irregular waves is investigated in this paper. Based on the linear potential flow theory and Cummins equations, the equation of motion for the WEC is established in time domain. Then a parametric study is performed on the power capture performance of the WEC by considering the influences of different wave and system parameters such as the spring combination parameter, the stiffness of auxiliary springs and main springs, the original length value of the main spring, damping coefficient of the PTO systems and the significant wave height. The results show that in an irregular wave, each parameter will have a certain impact on the performance of the device. Appropriate device parameters need to be selected according to actual environmental parameters to ensure that the adaptive bistable WEC has better power capture performance than its linear and conventional bistable counterparts.

Evaluation of long-term power capture performance of a bistable point absorber wave energy converter in South China Sea

Nowadays, extensive research has been conducted to generate electricity from ocean waves, but there are not too many studies on the capture of wave energy in a specific ocean area. As the largest sea area in China's offshore waters, the South China Sea contains a large amount of wave energy and is regarded as a natural wave energy power plant. A comparative study was conducted to investigate the effects of wave energy converter (WEC) parameters on the long-term power capture performance of a bistable point absorber WEC installed in the South China Sea in this paper. Combined with the joint probability distribution of significant wave height Hs and peak wave period Tp in South China Sea, a mathematical model was developed for the evaluation of long-term power performance of WECs. The governing equations of the point absorber WEC were established using the hybrid frequency-time domain method. The dynamic response and captured power can be obtained after numerically solving the equations of motion with the 4th order Runge-Kutta Method. The results show that the bistable mechanism has great advantages in the long-term captured power of WEC. In general, a circular truncated cone-shaped buoy WEC with a larger buoy size and smaller nonlinear parameter tend to result in better long-term power capture performance. The insights given in this study provide a motivation and foundation for developing a complete set of performance optimization methods of point absorber WECs installed in South China Sea.

Numerical and experimental study on hydrodynamic performance of multi-level OWEC

The performance of a multi-level overtopping wave energy converter (OWEC) has been numerically and experimentally investigated in a two-dimensional wave tank in order to study the effects of opening width of additional reservoirs. The device is a fixed OWEC consisting of an inclined ramp together with several reservoirs at different levels. A particle-based numerical simulation utilizing the Lattice Boltzmann Method (LBM) is used to simulate the flow behavior around the OWEC. Additionally, an experimental model is also built and tested in a small wave flume in order to validate the numerical results. A comparison in energy captured performance between single-level and multi-level devices has been proposed using the hydraulic efficiency. The enhancement of power capture performance is accomplished by increasing an overtopping flow rate captured by the extra reservoirs. However, a noticeably large opening of the extra reservoirs can result in a reduction in the power efficiency. The overtopping flow behavior into the reservoirs is also presented and discussed. Moreover, the results of hydrodynamic performance are compared with a similar study, of which a similar tendency is achieved. Nevertheless, the LBM simulations consume less computational time in both pre-processing and calculating phases.

Performance Analysis of an Adaptive Bistable Point Absorber Wave Energy Converter Under White Noise Wave Excitation

Performance analysis is conducted for an adaptive bistable point absorber wave energy converter under white noise wave excitation. The adaptive bistable wave power capture mechanism is composed of two oblique springs responsible for bistable characteristics, and two auxiliary springs realizing the ‘adaptive’ feature. White noise wave excitation is adopted to explore the effects of multi-frequency input in absence of different energy distribution characteristics. Time domain simulations are performed based on Cummins equations derived from linear potential flow theory. Results indicate that the adaptive bistable mechanism still bears a better power performance than its linear counterpart which may be well explained by its inherent ‘phase control’ feature in white noise wave excitation. A parametric study is also carried out to investigate how the power capture performance of the adaptive bistable point absorber wave energy converter is affected by main and auxiliary spring stiffness, random phase of wave excitation and power-take-off damping.

Power capture performance of a heaving wave energy converter for varying brad/bpto ratio

In this study, power capture performance of a heaving wave energy converter has been optimized by investigating the effect of variation of power take-off damping. The wave energy converter model used in this study utilizes the sliding discrete Fourier transform technique to estimate the local wave frequency. The power take-off parameters are adjusted according to the estimated wave frequency. As a part of the study, two different cases are evaluated. In the first case, the conventional approach based on using power take-off damping equal to radiation damping which is a function of the estimated wave frequency is considered. In the second part of the study, as a proposed approach, the power take-off damping is calculated with a gain factor. This gain factor varies the hydrodynamic analysis-based frequency-dependent radiation damping by multiplying with it. The results show that the captured power results can be increased from 1.64 to 10.38%, while, in average, the power capture increase covering all datasets is 5.95%.

Power capture performance of hybrid wave farms combining different wave energy conversion technologies: The H-factor

In this paper we consider hybrid wave farms, in which different types of WEC are combined, through a case study involving oscillating water columns (OWCs) and point-absorbers (PAs). A new parameter, called “H-factor”, is introduced to compare hybrid (multi-type) and conventional (single-type) wave farms in terms of wave power capture. We develop an ad hoc semi-analytical model to calculate the H-factor in a computationally efficient manner, and apply it to investigate how the H-factor and, consequently, the power capture, depend on: (i) the spacing and layout of the WECs, (ii) the type of WEC technology, and (iii) the wave conditions. We discuss the influence of these factors and, in the process, show that the H-factor is a valuable decision-aid tool. For specified wave conditions and layout limitations, a conventional wave farm may not be the most efficient option as a result of a destructive array effect, whereas a hybrid farm can be more efficient if a constructive hybrid effect occurs (if the H-factor value is above unity). This constructive hybrid effect can even overcome the destructive array effect for specified cases, demonstrating the potential advantage of hybrid wave farms relative to conventional wave farms.

On establishing generalized analytical phase control conditions in two body self-reacting point absorber wave energy converters

It is widely suggested that step gains in wave energy converter power capture performance must be realized to achieve economic viability. One method of fulfilling power capture gains is to invoke resonant conditions between the device and the incoming ocean wave. However, a general method that can establish the prerequisites for achieving resonant conditions in an arbitrarily complex wave energy converter architecture is nonexistent.In this work, we present an analytical procedure, built on the mechanical circuit framework, for identifying the resonant conditions of an arbitrarily complex wave energy converter architecture. To demonstrate the procedure, we select three complex two body point absorber devices as a case study, each with a geometry controllable feature set. Through invoking resonant conditions in each architecture, we illustrate how the choice of topology has significant influence on the power capture characteristics of the WEC device. Selecting the highest performing architecture, we then reveal how the analytical equations can be applied to promote technology innovation by supplying design criterion prior to locking down the WEC design. Finally, we apply the analytics within a numerical case study and present a hierarchy describing the incremental performance improvements realized through implementing steps in control complexity for this device.

Dynamics and high-efficiency of a novel multi-stable energy harvesting system

Scavenging energy from external vibration with low-frequencies and low-level intensities has always been a huge challenge for the energy harvesting. To remedy this key issue, this study is to present a novel multi-stable electromagnetic transduction energy harvesting system by adjusting the geometric parameters of linear springs. Here, we prove that the multi-stable energy harvesting system composed of a nonlinear restoring mechanism can significantly improve the efficiency of the energy converter. The electromechanical model is set up, and the corresponding equations are derived by the Extended Averaging Method and Fokker–Planck equation analysis. The response under harmonic excitation exhibits that, compared to a mono-stable and a bi-stable system, with a tri-stable or a quad-stable system, we can achieve a higher-energy interwell motion with large-amplitude periodic oscillation, leading to significant increases of the harvested current and power. Moreover, improved the expected value of the mean square current and mean harvested power can also be attained under the Gaussian white noise with a small intensity. The results show that the proposed multi-stable system is a feasible design that can improve the power capture performance of energy harvesting from low-level excitations.

Numerical investigation of the dynamic response and power capture performance of a VLFS with a wave energy conversion unit

This paper numerically investigates the dynamic response and power capture performance of a Very Large Floating Structure (VLFS) with a Wave Energy Conversion (WEC) unit. The VLFS is composed of two hinged modules of which the relative pitch motion induced by waves is converted into electricity by the WEC unit. The discrete-module-beam-bending based hydroelasticity method is used to establish the equations of motion for the VLFS. A theoretical model is derived for optimizing the power take-off damping of the WEC unit. Based on the numerical results, it is found that (i) placing hinge near the foreside of VLFS is a good option for both the reduction of hydroelastic response and the enhancement of power absorption; (ii) for medium and long waves a larger wave incidence angle corresponds to a larger vertical displacement, a smaller bending moment and a decreased power capture width ratio of the VLFS; and (iii) the decrease of structural bending stiffness brings an increase of vertical displacement and a reduction of bending moment while its effect on power absorption is insignificant except for a very large bending stiffness.