Converter Array Research Articles

Numerical study of the effect of central platform motion on the wave energy converter array

Placing multiple wave energy converters (WECs) on a central floating platform has become a popular approach of ocean energy conversion in recent years. The strong interaction between the WECs and the platform brings a number of technical challenges for the design of such devices. The present study investigates the effect of the platform motion on the WEC array of a typical ocean energy harvesting device, consisting of a central support platform and eight point-absorber WECs arranged in a circular array. The numerical studies are performed in four stages based on the potential flow solver ANSYS-AQWA. The power absorption of each WEC in isolation and of eight WECs operating simultaneously is evaluated and compared in the first and second stages respectively, revealing the interaction among the WECs. The effect of a fixed and a floating platform on the power absorption of the WEC array is investigated in the next two stages. The systematic studies demonstrates that the heave motion of the platform improves the power absorption of the WEC array for most wave frequencies tested, due to the increase in the wave-surface elevations around the WECs and the phase differences of the heave motion between the WECs and the platform. The opposite is true for the pitch motion of the platform. Furthermore, the WEC dimension, the platform dimension, and the distance between the WECs and the platform should be optimized based on the actual operating sea conditions to increase the wave-surface elevations and the phase differences. Otherwise, the motion of the platform should be restricted as much as possible.

Axisymmetric reflectors in wave energy converter arrays: Harnessing scattering to increase energy extraction

We elucidate the effect of non-extracting reflectors on the performance of wave energy converter (WEC) arrays. We consider an infinitely periodic row of converters parallel to an infinitely periodic row of discrete axisymmetric reflectors (C-R arrays), and we study how the spatial configuration affects energy extraction. Using a multiple-scattering algorithm for linear wave-array interactions, we conduct a series of simulations of C-R arrays for a range of spatial configuration parameters, wavenumbers, and wave incident angles. We find that C-R arrays can significantly increase energy extraction compared to a WEC array by itself. We offer a simplified theoretical model, based on the far-field response of periodic rows in isolation, which shows that the large increases in energy extraction result from the constructive Bragg and Laue interferences caused by wave interactions with the reflector row. For the pertinent case of incident waves of the WEC-resonant frequency, we find that optimized C-R arrays can achieve energy extraction gains of O(500%). Remarkably, the optimal C-R array extracts more energy than two rows of converters of optimal configuration even though the C-R array consists of only half as many WECs.

Wave power extraction by Multi-Salter's duck WECs arrayed on the floating offshore wind turbine platform

Integrating a floating offshore wind turbine (FOWT) and an array of wave energy converters (WECs) can be suggested as a proper new hybrid concept with the aim of more energy harvesting and lower total energy cost. This paper proposes the new composition of a floating braceless semi-submersible platform and an array of Salter's duck (SD) WECs. First, a comparison of the numerical results and experimental data of the hydrodynamics response of FOWT and the single SD WEC response are presented and discussed. Then, the performance of three-hybrid FOWT-Multi-Salter's Duck (FOWT-MSD) systems with 12SD, 15SD, and 18SD WECs has been investigated under different wave periods and wave heights. In the wave period of 6–9 s, 12SD absorbs more power than 15SD and 18SD. From T = 9–11 s, all three systems have the same absorbed power. For wave periods above 12 s, 15SD and 18SD have higher absorbed power than 12SD. No clear trend was observed for wave periods below 6 s. The effect of wave direction shows that at lower wave heights, maximum absorber power belongs to FOWT-12SD under zero wave direction. In contrast, increasing wave heights leads to a smaller difference between the three-hybrid systems at different wave directions.

SWELL: An open-access experimental dataset for arrays of wave energy conversion systems

Achieving large-scale commercial exploitation of ocean wave energy inherently encompasses the design and deployment of arrays of wave energy converters (WECs), in an effort to reduce the associated levelised cost of energy. In this context, understanding the interactions between devices in a controlled WEC array is hence essential to achieve optimal layout configurations, as well as to provide guidance on the area required for array installation, reliability, life-time, and overall cost of the farm. Successful achievement of these vital objectives for the wave energy industry has been constantly aided by the use of appropriate numerical models. Regardless of the specific modelling approach adopted, model reliability is always a major concern: Numerical models need to able be to represent reality to be useful in supporting the different stages of development, hence providing significant results for decision making. To test reliability of a model, experimental results are an invaluable asset for validation.Recognising the striking absence of real-world data concerning arrays of WEC systems, and its inherent value for model validation and data-based modelling purposes, we present, in this paper, an experimental campaign fully conducted with the sole objective of generating and providing an open-access dataset on WEC farms: SWELL (Standardised Wave Energy converter array Learning Library). The generated dataset, included alongside this manuscript, comprises an approximate total of ∼3000 variables and more than ∼108 datapoints, for up to 5 devices in 9 diverse WEC array layouts with different levels of interaction, and 19 carefully selected operating conditions. Four different categories of tests are considered, providing measures of key variables required for model validation and data-based modelling tasks. As such, SWELL provides a crucial resource to achieve confidence in numerical modelling, helping towards creating reliable tools for decision making in the WEC field, hence effectively supporting the pathway towards effective commercialisation of ocean wave energy.

On the mooring methodology of heaving point absorber arrays

This paper aims at developing a mooring methodology for floating point absorber arrays. To date, the literature gives no clear indications of the mooring influence on the potential energy conversion of a floating wave energy converter. Hence, this study targets to provide general insights into the dynamics of a floating wave energy converter under the influence of moorings. These are gained by conducting an experimental study on the motion response of a wave energy converter array in regular wave conditions using three different mooring systems: a taut line mooring, a vertical tension leg mooring, and a conventional slack catenary mooring. Results show that the potential energy conversion is strongly influenced by the choice of the mooring system and the incident wavelength. In intermediate wavelengths, a taut mooring system leads to a 8% increase in energy conversion potential. In contrast, the tension-leg moored array exceeds the potentially converted energy of the catenary moored array in wavelengths significantly longer than the wave energy converter’s length by over 80%. The results of this study provide an insight into the site-specific design of point absorber arrays and the best choice of the mooring system to increase the potential power output.

Analysing the influence of power take-off adaptability on the power extraction of dense wave energy converter arrays

The aim of this work is to assess the influence of different degrees of adaptability of the power take-off (PTO) system on the power absorption of dense wave energy converter (WEC) arrays. The adaptability is included in simulations through a transmission ratio that scales the force actuating the PTO relative to the force generated by the motion of a floater. A numerical model is used in which hydrodynamic interactions between floaters and nonlinearities in the PTO are considered. The lower computational cost of this numerical model makes it possible to study the power extraction of a dense WEC array in irregular waves to easily create power matrices and other performance metrics. The methodology is applied to the case study of the Ocean Grazer WEC to showcase the potential performance improvements achieved through the inclusion of a transmission ratio. The analysis shows that including a high degree of adaptability and choosing WEC array configurations and PTO designs specific to potential deployment locations early in the design process can lead to an increase in extracted power.

Numerical study of a novel hybrid system with the Wavestar wave energy converter array and a SPIC semi-submersible floating platform

To improve power generation efficiency and reduce costs in the industry of ocean renewable energy, one of the optimal strategies is the addition of wave energy converters (WECs) into the floating wind turbine. This paper proposes a novel hybrid wind-wave energy concept that comprises multiple Wavestar point absorption wave energy converters (WECs) and a SPIC semi-submersible floating wind turbine (FWT). ANSYS/AQWA software is used to perform numerical simulations based on three-dimensional potential flow theory. Three hybrid systems are designed with different numbers of Wavestar WECs, which are symmetrically arranged on the platform. Before the study, the hydrodynamic responses of the Wavestar WEC and the SPIC concept platform are simulated separately and compared with existing experimental data. After the validation of numerical models, the platform motions and the absorbed power of the WECs are investigated, which are the main factors in evaluating the performance of a hybrid system. After a series of numerical simulations, the results show that the absorbed power increase significantly with the increase in the number of the Wavestar WECs, while the platform motions also increase. It can be concluded that the wind-wave energy hybrid design needs to find a balance between the platform motions and the absorbed power of the WECs. Overall, this study can be used as a reference for the design of other novel hybrid ocean energy concepts.

A low settling time switching scheme for SAR ADCs with reset‐free regenerative comparator

SummaryThis paper presents an energy‐efficient fully differential switching scheme for successive approximation register (SAR) analog‐to‐digital converters (ADCs). During the sampling phase, the top and bottom plates of all capacitors except most significant bit (MSB) capacitors are grounded in digital‐to‐analog converter (DAC) arrays. The input signals are bottom plate sampled on MSB capacitors. This technique can reduce the settling time by more than 87.5% in comparison with the conventional switching scheme. Furthermore, a novel reset‐free regenerative comparator is unveiled in this paper. The proposed comparator is armed to amplify its inputs both during the reset and evaluation phases. In comparison with a conventional single‐ended comparator, the proposed comparator can reduce power consumption above 90% for the almost same input‐referred offset voltage. The proposed scheme is designed with a resolution of 8‐bit and a sampling rate of 90 MS/s in a standard 65‐nm CMOS technology. The simulation results certify that the ADC dissipates 363‐μW power with a 1.2‐V supply voltage and achieves a 7.13 effective number of bits (ENOBs), yielding a 28.8 fJ/conversion step Nyquist rate Walden FOM.

A polarization converter array based on microcavity patterned hydrophobic layer induced twisted-radial liquid crystals

We report a facile method to prepare an array of liquid-crystal polarization converters (LCPCA) based on hydrophobic microcavity arrays. The reactive ion etching (RIE) technique was chosen to treat the homeotropic polyimide (PI⊥) layers through a metal mask with a through-hole array. Under the selective RIE treatment, a hydrophobic–hydrophilic pattern is obtained due to the increase in surface energy in the etched area from ∼34.6 to ∼73.2 mN/m. The etched area forms a microcavity with a diameter of 520 μm, depth of ∼23 nm, and center distance of 690 μm, which is consistent with the dimensions of the metal mask. Since the walls of the microcavity retain hydrophobicity, the liquid-crystal (LC) molecules show a radial arrangement in the microcavity. Combining a substrate covered by a parallel rubbed homogeneous polyimide (PI∥) layer with the substrate covered by a microcavity-patterned PI⊥ layer can form a “sandwich” structure. The LC molecules in the microcavities are arranged in a twisted-radial texture, which can convert linearly polarized light to radially or azimuthally polarized light depending on the polarization direction of the incident light. The proposed method has the merits of simple steps, controllable size, and easy to fabricate large-area arrays. Our LCPCA has potential applications in tight focus, material processing, biological tissue analysis, particle capture, and optical data storage.

Optimization of heterogeneous arrays of wave energy converters

Optimizing the layout of devices in an array of wave energy converters (WECs) is one of the core aspects of WEC array design. More often than not, WEC arrays are designed to contain identical devices. However, to further improve power generation, this paper investigates how devices of varying dimensions can affect the overall performance of the array; the array containing devices of different sizes is referred to as a heterogeneous array in this work. The heterogeneity is achieved by varying the radius and draught of cylindrical buoys. We measured the performance of the heterogeneous array against the homogeneous array containing the same number of devices in an identical layout, assuming the summation of volumes of all devices is the same for both the heterogeneous and the homogeneous arrays. The power from the array is computed using a time-domain array dynamic model and an optimal constrained control. The hydrodynamic coefficients used as input in the dynamic model are computed using a semi-analytical method to enable computationally efficient optimization. The optimization was performed using a genetic algorithm. Arrays of 3, 5, and 7 devices are presented in this paper. For the tested wave conditions, it was found that in some wave conditions a heterogeneous array can achieve a performance improvement of up to 40% over the corresponding homogeneous array.

Parametrical study on an array of point absorber wave energy converters

Based on the developed high-fidelity time-domain simulation framework, this paper presents a parametric study on arrays of point absorber wave energy converters under irregular waves. Its purpose is to assess the effects of coupling among array configuration parameters, array interactions, and array layouts, and to provide guidance for array design. Parameters considered include the incident wavelength, draft, slenderness ratio, bottom shape, Power Take-Off damping, and mooring stiffness. It is shown that the device parameter configuration has a negligible effect on the optimization of the array layout and the performance improvement obtained by tuning the configuration of each device in the array is not significant compared to using the optimal value of the isolated device for the whole array. Therefore, optimizing the parameter configuration of the isolated devices first, and then the layout of the array composed of these devices, is proved to be a more recommended optimization procedure. Finally, the obtained findings and recommended optimization procedures effectively guide the design of a 10-PA-WEC array in a realistic scenario. And the array optimization results support the findings of the parameter study.

High precision and real-time acquisition system for interface stress measurement in bridge bearing

Since the damage of the bridge structure may cause great disasters, it is necessary to monitor its health status, especially the bridge bearing, the important connecting component of the bridge's upper and lower structures. Nowadays, manual inspection is the main method to get the information of the bridge bearings’ work status. However, occasional damage of bridge bearing may not be detected in time, and sometime the installation position of the bearing makes the manual inspection on bridge bearing difficult and even impossible. Therefore, in order to know the work status of the bridge bearings timely, an intelligent remote monitoring system for the bridge bearing is developed. A 32-channel real-time acquisition system is designed by using an AD7768-1 analog-to-digital converter and Xilinx Spartan-6 Field Programmable Gate Array for interface stress continuously acquired in the bridge bearing. To assure the good linearity and low noise performance of the monitoring system, the data acquisition card is meticulously designed to reduce noise from both hardware and software and realize high-precision acquisition. Through the establishment of the monitoring server, the compressive stress data can be displayed synchronously and the overpressure situation can be alarmed in real-time. The experimental results show that the accuracy of the calibrated sensor is within 1.6%, and the detection error of the acquisition board is less than 200 μV. The acquisition system is deemed to have considerable advantages in accuracy and applicability.

Experimental research on three parallel backward bent duct buoys

Backward bent duct buoy (BBDB) is a high-efficiency wave energy converter. It can be paralleled to form a wave energy farm. In this paper, a wave energy converter (WEC) array composed of three parallel BBDBs was tested. The array's performance and properties were reported. And, an effective method to improve the arrays' energy-capturing ability by setting the spacing above zero was proposed and verified. This method is helpful for the construction of BBDB wave energy farm.

Wave interaction and energy absorption from arrays of complex-shaped point absorbers

Water wave interactions with arrays of wave energy converters are numerically investigated based on the interaction theory. The converter is a heaving point absorber that can harness the ocean wave energy through up-and-down movements. A semi-analytical hybrid method is developed that combines the boundary element method and the interaction theory. The developed numerical method is verified against theoretical solutions for arrays of truncated vertical circular cylinders. Three different array layouts are studied in detail. It is found that trapped waves exist at critical wave numbers just below the cutoff values, and the peak load on the middle device increases with the number of devices in head waves. With the increase in the complexity of the array layout, significant wave force enhancement is observed, leading to a broader range of magnitude and stronger variations over the frequency band in beam waves. Moreover, variations of the q-factor show that there are some remarkable “bright spot” regions, indicating that the wave energy absorption there is locally optimized against wave conditions. By arranging the layout in a more randomized way, the optimal conditions for maximized power output can be hard to achieve, but the maximum power output can increase to a higher level.

Modelling and Installation of Grid Connected Rooftop Solar PV System

The 100 kW Grid Connected Roof-Top (GCRT) Solar PV System erected at the SSGMCE, Shegaon Campus is modelled and simulated in this research. One of the renewable energies that is widely accessible is solar energy. In order to get the most out of solar energy, it is vital to use it properly. To meet the rising need for energy, more renewable sources must be used. A PV array's conversion efficiency decreases as a result of environmental parameter changes. Therefore, maximum power tracking, or MPPT, is employed. In order to boost energy production, MPPT detects peak power. Developing a PV array model, synchronising it with the grid, and implementing it in MATLAB Simulink are all covered in this work. This system comprises of a solar PV array, an IGBT inverter, and a grid connection. The power plant's performance is also described.

Layout study of wave energy converter arrays by an artificial neural network and adaptive genetic algorithm

In this paper, the influence of the spatial configuration of a wave energy converter (WEC) array upon total power output is investigated. A power prediction model has been proposed to improve the efficiency of calculating the total power from the array with guaranteed accuracy. The prediction model consists of two main components. One is the automatic generation model for training samples by establishing an interface between MATLAB and AQWA. The other is the artificial neural network (ANN) adaptive training model improved by the adaptive genetic algorithm (AGA), which helps to improve the prediction accuracy by 80% according to experimental validation and simulation comparison. The traditional genetic algorithm (GA) has been widely used in WEC arrays optimization. The method proposed in this paper has improved the GA optimization efficiency by about 50%. Arrays with different numbers of devices are studied in irregular waves measured in the previous sea trials, and similar optimal layouts appear in all cases, which allows extrapolation of the results to even larger arrays. The solution space calculated by the optimization algorithm is further analyzed based on separate considerations of different spacing between devices and wave directions. The resulting behaviour is expected to guide engineering applications.

Experimental Modelling of Point-Absorber Wave Energy Converter Arrays: A Comprehensive Review, Identification of Research Gaps and Design of the WECfarm Setup

Commercial wave energy exploitation will be realised by placing multiple wave energy converters (WECs) in an array configuration. A point-absorber WEC consists of a floating or submerged body to capture wave energy from different wave directions. This point-absorber WEC acts as an efficient wave absorber that is also an efficient wave generator. Optimising the WEC array layout to obtain constructive interference within the WEC array is theoretically beneficial, whereas for wind farms, it is only important to avoid destructive interference within an array of wind turbines due to wake effects. Moreover, the WEC array layout should be optimised simultaneously with the applied control strategy. This article provides a literature review on the state of the art in physical modelling of point-absorber WEC arrays and the identification of research gaps. To cover the scientific gap of experimental data necessary for the validation of recently developed (nonlinear) numerical models for WEC arrays, Ghent University has introduced the “WECfarm” project. The identified research gaps are translated into design requirements for the “WECfarm” WEC array setup and test matrix. This article presents the design of the “WECfarm” experimental setup, consisting of an array of five generic heaving point-absorber WECs. The WECs are equipped with a permanent magnet synchronous motor (PMSM), addressing the need for WEC array tests with an accurate and actively controllable power take-off (PTO). The WEC array control and data acquisition are realised with a Speedgoat Performance real-time target machine, offering the possibility to implement advanced WEC array control strategies in the MATLAB-Simulink model. Wave basin testing includes long- and short-crested waves and extreme wave conditions, representing real sea conditions. Within the “WECfarm” project, two experimental campaigns were performed at the Aalborg University wave basin: (a) a testing of the first WEC in April 2021 and (b) a testing of a two-WEC array in February 2022. An experimental campaign with a five-WEC array is under preparation at the moment of writing.

Experimental and numerical investigation on an optimization method of heaving buoy wave energy converter arrays based on a given target wave spectrum

In this paper, an active optimization method of a wave energy converter (WEC) array that focuses on the characteristics of the random sea condition is proposed. Physical model tests were conducted in both regular and irregular wave conditions. Three representative regular wave cases were determined from the target wave spectrum using the method. A rigid cylindrical heaving buoy model with different constant power take-off (PTO) damping forces and drafts was applied to find the optimal isolated WEC parameter under each regular wave condition. Three equal buoys with respective optimized parameters were arranged rectilinearly into a WEC array for irregular wave tests. The power output of arrays with different settings are compared, indicating the advantages of the different optimized array and the significance of unit order. To amplify the optimization effect, a supplementary numerical investigation of a 5-body WEC array was carried out using the boundary element method (BEM). The results demonstrate the method and show the influence of the number of different units.

Design of multi neutron-to-gamma converter array for measuring time resolved ion temperature of inertial confinement fusion implosions.

The ion temperature varying during inertial confinement fusion implosions changes the amount of Doppler broadening of the fusion products, creating subtle changes in the fusion neutron pulse as it moves away from the implosion. A diagnostic design to try to measure these subtle effects is introduced-leveraging the fast time resolution of gas Cherenkov detectors along with a multi-puck array that converts a small amount of the neutron pulse into gamma-rays, one can measure multiple snapshots of the neutron pulse at intermediate distances. Precise measurements of the propagating neutron pulse, specifically the variation in the peak location and the skew, could be used to infer time-evolved ion temperature evolved during peak compression.

Predictive Control for a Wave-Energy Converter Array Based on an Interconnected Model

This paper proposes a model predictive control (MPC) method based on an interconnected model to maximize the ocean wave energy extracted by a wave-energy converter (WEC) array. In the proposed method, a formally uniform interconnected model is applied to represent the dynamics of an array consisting of an arbitrary quantity of WECs, simultaneously considering the hydrodynamic interaction among all the WEC devices. First, the WEC devices and their hydrodynamic interaction are represented in an interconnected model that describes the networked dynamics of a variety of WEC arrays with distinct spatial geometry layout of the WEC devices deployed in the sea field. Second, based on the presented model, an MPC method is applied to achieve the coordinated control of the WEC array to improve its energy conversion efficiency under the constraints of buoy position and control force. Third, a hardware-in-the-loop (HIL) platform is developed to simulate the WEC array’s physical operating conditions, and the proposed method is implemented on the platform to test its performance. The test results show that the proposed MPC method using the interconnected model has a higher energy harvesting efficiency than the classic MPC method.