Converter Array Research Articles

A 10-bit 33.3-kS/s 3.2-fJ/conversion-step single-ended counter-type SAR ADC with dual 5-bit CDAC arrays and counters in 65-nm CMOS

In this paper, an area-and-energy-efficient 10-bit single-ended (SE) counter-type (CT) analog-to-digital converter (ADC) with dual 5-bit capacitive digital-to-analog converter (DAC) arrays and dual 5-bit counters is presented. The 10-bit capacitive-DAC (CDAC) array and counter of the conventional counter-type structure are divided into two 5-bit CDAC arrays and counters, respectively. Significant improvements are achieved in comparison to the conventional counter-type ADC as follows: the capacitance of the most-significant-bit (MSB) capacitor is reduced by 96.88 % from 512C to 16C; the total capacitors are reduced by 93.75 % from 1024C to 64C; the total clock cycles required in the conversion phase are reduced by 93.94 % from 1023 to 62; the average switching energy is reduced by 98.55 % from 2390CV2 to 35CV2. The proposed ADC occupies a smaller area, performs much faster, and its power consumption is greatly reduced. This structure has a simple design with small sampling capacitor (32C) and does not require a rail-to-rail comparator compared to SAR ADCs. It is suitable for biomedical and multi-channel applications. The proposed ADC is post-layout simulated in 65-nm CMOS technology which occupies 0.016 mm2 area. This ADC has 85.43 nW power consumption, 9.64 ENOB on 33.33 kS/s sampling rate, and 3.23 fJ/conversion-step FOM.

Optimal array arrangement of oscillating wave surge converters: An analysis based on three devices

Wave farms consist of arrays of wave energy converters. However, ocean waves perturbed by one device will interact with its neighbouring devices, which can lead to more or less power yielded than operating the devices in isolation. Therefore, it is of significant interest to study the optimal layout and spacing to deploy multiple devices. In this work, a validated high-fidelity computational approach is used to study the optimal arrays of three oscillating wave surge converters by systematically varying the array arrangement in different wave conditions. The simulations demonstrate the wave interaction with multiple dynamic bodies and how this affects the overall power output. Furthermore, novel empirical rules are derived to design the optimal constructive layout for three devices in a given wave condition, and this can be extended to cases of numerous devices.

Multi-objective decision tool for the assessment of co-located wave-wind offshore floating energy parks

This study explores the benefits of co-locating wave-wind offshore energy parks. It focuses on the leeward effects of hypothetical wave energy converter (WEC) arrays deployed offshore Viana do Castelo, Portugal, and their synergies with the WindFloat Atlantic wind farm. Wave propagation through arrays of CECO WECs and their near-field effects were simulated using SNL-SWAN model. Furthermore, a multi-objective function to evaluate and choose between WEC arrays was defined. The analysis considered a wave height reduction area index, the q-factor and a new wave energy park layout assessment index (WLA). The effectiveness of arrays was dependent on incident wave direction, layouts with larger spacing achieved higher q-factors, while lower spacing provided better shield protection. The WLA, which considers both power absorption and wave height reduction, was introduced as a decision-making tool. The results also showed that some arrangements could increase accessibility to the turbines by circa 22%, representing about 1957 h/year added to the weather window for maintenance and operation of the park. In summary, this study provides a comprehensive analysis and a decision-making strategy for determining the optimal positioning of WEC arrays, contributing to the advancement of renewable energy technologies, promoting sustainable and efficient utilization of wave and wind resources.

Hydrodynamic performance of a three-unit heave wave energy converter array under different arrangement

A pile-restrained floating wave energy converter (WEC) array is proposed as an alternative to a single floater of the size of the array for use as a floating breakwater. The hydrodynamics of the WEC are modelled based on the Navier-Stokes equations and the model is verified by comparing its results with existing experimental data. The model then is used to characterize the array composed by a line of three WECs in terms of floater heaving, wave energy conversion, wave reflection, transmission and dissipation, for different layouts.In the examined array configuration, the aligned arrays exhibit superior performance compared to the staggered arrays, comprehensively considering both wave energy conversion and wave transmission. Specifically, when khi > 1.73, the wave energy conversion efficiency of the aligned array with a spacing of 0.1 times the WEC width ranges from 0.141 to 0.330, while the wave transmission coefficient ranges from 0.187 to 0.472, indicating the effectiveness of the arrays in simultaneously reducing wave transmission and converting wave energy under shorter-wavelength conditions. Compared to a single WEC of the same dimensions, the array exhibits a remarkable increase in wave energy conversion efficiency and effectively reduce wave reflection.

Experimental assessment of combined sliding mode & moment-based control (SM[formula omitted]C) for arrays of wave energy conversion systems

Motivated by the lack of comprehensive experimental implementation and assessment of the potential benefit that can be achieved with energy-maximising optimal control solutions for arrays of wave energy converters (WECs), we present, in this paper, the development, design, experimental implementation, and performance appraisal, of optimal moment-based control for arrays of WEC systems. Both centralised and decentralised controllers are evaluated. Four different WEC array layout configurations are considered, with up to three 1:20 scale prototypes of the Wavestar WEC system operating simultaneously within the basin, subject to a variety of sea state conditions. In particular, the proposed controller, termed sliding-mode-moment-based controller SM2C, is composed of a receding-horizon moment-based reference generation process, and a subsequent proportional–integral–derivative-like continuous sliding mode tracking controller. This composite control structure is implemented and assessed experimentally, providing a detailed analysis of key performance metrics. We show that the proposed SM2C strategy is able to maximise energy absorption for all the considered WEC array layouts, with up to 2.8 times energy improvement when compared to the benchmark controller case. The findings of this experimental study show tangible proof of the performance enhancement that can be achieved in real arrays of WEC systems with the use of appropriate control technology, demonstrating not only the feasibility of the proposed SM2C strategy in itself, but the key role that control systems have to play in the pathway towards effective exploitation of the yet largely untapped wave energy resource.

Performance of a Raft-Type Wave Energy Converter with Diverse Mooring Configurations

The development and utilization of wave energy, heralded as a potential leading source of clean energy worldwide, have garnered considerable attention from the global research community. Among the diverse array of wave energy converters (WECs), the raft-type WEC stands out for its potential to efficiently harness and utilize wave energy, offering high energy conversion rates and a broad frequency response range. This paper delves into the evaluation of a raft-type WEC’s performance in various mooring configurations under different wave conditions. Our analysis primarily focuses on the dynamics of the two-body WEC using a weakly nonlinear three-dimensional potential flow solver. The considered device comprises two interconnected floating barges, incorporating a power take-off system at the hinged connection point. This investigation involves the use of equivalent linear damping to model the power take-off (PTO) system. To validate the numerical simulations, we conduct physical model experiments with WECs. Additionally, the coupling of the raft-type WEC’s dynamics and its mooring dynamics was examined, highlighting the performance differences between various mooring systems through a comparative analysis.

Study on the power performance of wave energy converters mounted around an offshore wind turbine jacket platform

With the deepening of renewable energy research, the economic-friendly multi-energy coupling development method is gradually favored. This paper proposes a jacket-mounted hybrid wind-wave energy converter, which includes a bottom-fixed offshore wind turbine (OWT) and a microarray composed of three point-absorbing wave energy converters (WECs). Numerical simulations were conducted to study the power performance of the wave energy converter array, and a model test with a scale of 1:8 was carried out to calibrate and validate the numerical model. The array converts wave energy through a hydraulic system, and a dynamic-link library was created to simulate the hydraulic power take-off (PTO) system that can take into account the real-time pressure changes of the accumulator. The time domain simulation results show that the maximum capture width ratio (CWR) of the WEC array exceeds 0.5 when the wave period is nearly to the natural period, and the optimal CWR of a single WEC exceeds 0.8. When the wave period is less than 2 times the natural period, the energy capture behavior of WEC is sensitive to the change in wave direction, and this sensitivity is weakened when the device forms an array. Specifically, a single WEC prefers waves to be incident from the back side, while the WEC array prefers waves to be incident obliquely. This work will provide design guidance for the engineering application of stationary hybrid wind-wave energy concepts.

Wave power extraction of flap-type wave energy converter array mounted at the stepped bottom topography

Wave power extraction of flap-type wave energy converter array mounted at the stepped bottom topography

Design and Control of Hydraulic Power Take-Off System for an Array of Point Absorber Wave Energy Converters

The development of wave energy converter (WEC) arrays is an effective way to reduce the cost of levelized energy and facilitate the commercialization of WECs. This study proposes a hydraulic power take-off (PTO) system for an array of point absorber wave energy converters (PA-WECs) and designs a control system using a novel algorithm called the improved simplified universal intelligent PID (ISUIPID) controller and the adaptive matching controller including an improved artificial gorilla troops optimizer (IGTO) to improve and stabilize the output power of PA-WEC arrays. Simulations under varying irregular wave states have been carried out to verify the validity of the mathematical model and the control system. The results show that the designed IGTO has faster convergence speed and better convergence accuracy in solving the optimal linear damping coefficient of the generator, and the proposed ISUIPID controller provides superior performance in tracking the speed of the hydraulic motor under the changing sea states. In addition, the capture power and output power of the array of PA-WECs are improved and the electrical energy can be output stably under the designed control system. The array of PA-WECs with the proposed control system will become an independent, stable, efficient, and sustainable power supply system.

Design of Nupix-A2, a Monolithic Active Pixel Sensor for heavy-ion physics

The High Intensity heavy-ion Accelerator Facility (HIAF) is being constructed to generate intense beams of primary and radioactive ion for a wide range of research fields. Hence, a Monolithic Active Pixel Sensor (MAPS) named Nupix-A2 has been developed in a 130-nm High Resistivity CMOS process. The Nupix-A2 can simultaneously measure the particle hit' energy, arrival time, and position. It consists of a 128 × 128 pixel array, a digital-to-analog converter array, and a digital control module. The Nupix-A2 can measure energy deposition from 300 e- to over 50 ke- and time duration from 13 μs to 140 μs. This sensor also offers full readout mode and fast readout mode. In full-readout mode, all pixels measure arrival time and energy, suitable for real-time beam monitoring and image reconstruction. In fast-readout mode, only particle hit's positions are detected and read out. This paper presents the design of the Nupix-A2.

Fast time-domain model for the preliminary design of a wave power farm

This study presents a novel, fast time-domain model developed for an array of interacting point-absorber wave energy converters. The model is validated using experimental wave tank data. The point-absorbers, based on Uppsala University’s design, are arranged in a symmetric grid and interact with scattered and radiated waves while constrained to the heave motion. The model employs linear potential flow theory to solve the hydrodynamic coefficients in the frequency domain and employs Cummins’ formulation to solve the equations of motion in the time domain. Modeling an array of wave energy converters in the time domain yields a system of integro-differential equations, featuring convolution terms in the excitation and radiation forces. This implies that past waves radiated by the body continue to impact future dynamics. Irregular long-crested waves, generated from the Bretschneider spectrum, serve as the incident waves for the study. The model’s accuracy in capturing the dynamics and power absorption of the farm is demonstrated through validation against experimental data from a 1:10 scaled prototype of a six-point-absorber array. Despite inherent differences between the experimental and numerical set-ups, the model accurately represents the farm’s behavior. Furthermore, an efficiency test reveals that the numerical scheme approximates the performance of wave power farms comprising 6, 12, 24, 48, and 96 interacting devices within a maximum computational time of 20 s. Overall, this research presents a novel and accurate time-domain model for analyzing an array of point-absorber wave energy converters. The model’s ability to capture the dynamics and power absorption, along with its efficiency in simulating larger wave power farms, make it a valuable tool for the preliminary design stage.

Power and dynamic performance of a floating multi-functional platform: An experimental study

Hybrid platforms which integrate a floating offshore wind turbine and a wave energy converter are a promising technology as the wave energy converter can lead to a more stable platform and potentially lower costs. In the present work, a semi-submersible floating offshore wind turbine platform integrating with an array of wave energy converters (WECs) in the form of an oscillating water column (OWC) is presented and tested experimentally. The hydrodynamic properties of the multi-functional platform were investigated under a series of regular and irregular wave tests considering both operational and survival sea states. The influence of various parameters (including incident wave directions and chamber opening ratios) on the dynamic responses and the wave energy capturing were systematically studied. The introduction of the oscillating water column can not only capture wave energy but also reduce the heave, roll and pitch responses, especially for long waves kh ≤ 2.0, kh ≤ 1.4 and kh ≤ 1.76 respectively. The maximum reduction in heave, roll and pitch are 35 %, 55.4 % and 13.8 %, respectively. The chamber air orifice size was analyzed and optimized. From the survival sea state tests, it is found that the introduction of the OWC WECs decreases the maximum amplitudes of the platform motions in all five freedoms and the nacelle acceleration.

Cooperative model predictive control for Wave Energy Converter arrays

It is well known that commercial Wave Energy Converters (WECs) are likely to be deployed in arrays, which gives the possibility to enhance the energy harvesting properties of the wave farm as a whole. Advanced control strategies are necessary to exploit the potential and capture as much energy as possible in a limited sea space. Centralized control is a natural benchmark, as it sees the array as a large-scale system, making energy maximization a large plantwide problem. However, centralized control has a high requirement for computational efficiency. In this paper, a cooperative model predictive control (MPC) is proposed to achieve energy maximization while reducing computational cost. A modified array model is constructed to capture the dynamics of interactions in arrays, explicitly showing the linear couplings in the system state, control input, and excitation force of each subsystem. Sensitivity analyses are performed to evaluate the performance for different array layouts, separation distances, and sea states. The results show that cooperative MPC can achieve an energy capture performance that approximates centralized MPC, outperforming decentralized MPC significantly, and is well adapted to sea states. Furthermore, the proposed method has the potential to improve computational efficiency by 14.86%–51.14% for simulations in the study.

A time domain approach for the optimal control of wave energy converter arrays

Wave energy converters typically use various control methods to extract energy from ocean waves. The objective of the control system is to optimize the energy extraction process, taking into account the dynamics of the system and the wave conditions. The task of deriving the optimal control laws of wave energy converter arrays for regular and irregular waves using the Pontryagin minimum principle was previously investigated in the literature. The result is a combination between the singular arc and bang-bang control laws. For irregular waves, some complexity arises due to the radiation state-space representation, which requires ignoring the hydrodynamic coupling terms related to the added mass and radiation-damping coefficients; this reduces the computational complexity of the control force but adversely affects the solution's accuracy. Also, the derived control laws are specific to a particular wave condition. Recently, the optimal control of a flexible buoy wave energy converter was derived using the convolution representation for the radiation force. In this work, the optimal control laws of flexible buoy wave energy converters are modified to simulate wave energy converter arrays; then, the results are compared to those obtained by dropping the hydrodynamic radiation coupling terms. Although using a convolution representation adds computational complexity to the optimal control problem, it generates an equation that is generic to any wave condition, can be used with any wave spectrum, and provides an expression for the switching condition.

Fast time-domain model for an array of interactive point-absorbers


 
 
 A fast time-domain model is developed for an array of six interacting point-absorber wave energy converters, based on the design originated at Uppsala University. The point-absorbers are placed in a symmetric grid, were each row contains one pair. The devices interact by scattered and radiated waves, while they are restricted to move in one degree of freedom, heave. Under the assumption of linear potential flow theory, the hydrodynamic coefficients for the excitation and radiation forces are obtained using an analytical multiple-scattering method. The equations of motion are solved directly in the time domain following the Cummins’ formulation. Modelling an array of wave energy converters in the time domain comes down to solving a system of integro-differential equations, were convolution terms appear in the computation of the excitation and radiation forces. In the majority of wave farm models, frequency-domain approaches are used to solve the equations of motion, since time-domain models are more computationally demanding and significantly more challenging to develop. This is not only because of the numerical integration involved, but especially due to the computation of the convolution term accounting for the radiated water waves on the free-surface, implying that waves radiated by the body in the past continue to affect the dynamics in the future. Regardless the computational effort associated with time-domain approaches, their use is required for realistic control applications and complex device dynamics, like non-linearities due to the power take-off configuration. In particular the non-linear effects that arise during the wave energy conversion chain are treated as time-varying coefficients within the system of differential equations describing the motion. Input to the numerical scheme are irregular, long-crested waves, obtained from the Bretschneider spectrum, corresponding to four, different sea states. It is of high interest to study whether the linear numerical model simulates accurately the performance of each interactive surface buoy in response to the irregular waves. Therefore, the numerical results for the full array configuration are compared with experimental data. At this point we emphasize that there are not a lot of experimental works considering arrays of point- absorbers, due to the complexity and costs associated with the task. Therefore, finding a set of data to validate the array model is not trivial. The experimental results we use were carried out in the COAST Lab at Plymouth University, UK, corresponding to a 1:10 scaled prototype of an array of point-absorbers. The set-up consists of six ellipsoid floats free to move in six degrees of freedom and connected via ropes and pulleys to individual power take-off systems. Despite the highly non-linear effects in the physical experiment, the free motion of the buoys in all directions, and the power take-off configuration, the numerical scheme is able to accurately capture the heaving motion of the buoys and their power absorption.
 
 

Effects of control strategies on the performance of floating WEC point absorbers operating attached to a breakwater by time-domain simulator

The estimation of the performance of Wave Energy Converter (WEC) arrays of the type of simple floaters operating in nearshore and coastal areas, characterized by variable bottom topography, is important for the estimation of the wave power absorption and determination of the operational characteristics of the system and could significantly contribute to the efficient design and layout of WEC farms. For this purpose, full 3D models based on Boundary Element Method have been developed, supporting the systematic use for optimization studies. Apart from WEC arrays in nearshore and coastal regions a promising possibility is the installation at the exposed side of port breakwaters with significant energy potential. In this work, the analysis of the above system is extended to the prediction of WEC performance in multichromatic incident waves under latching control strategy using a time-domain simulator, showing improvement of performance in particular conditions.

Experimental Investigation into the Air Compressibility Scaling Effect on OWC Performance and Wave Height

Wave energy converter arrays have the potential to provide coastal protection in addition to generating power from incoming waves. As part of a wider experimental study to investigate dual-use applications, this paper presents the results of wave flume testing conducted to analyse the performance characteristics of a single, generic Oscillating Water Column (OWC) device, in preparation for the next phase of study that will focus on multi-device arrays. The specific focus of this flume testing was to characterise parameters such as Response Amplitude Operator (RAO), Capture Width Ratio (CWR) and Phase Response, as well as the device’s effect on the local wave field. A potential limitation when using scaled experimental results for OWCs are the differing scaling factors that should be applied to the device’s submerged volume (λ³) and air volume (λ²) which, together with the Power Take Off (PTO) damping, can greatly affect the air-spring stiffness experienced within the OWC. A subset of 34 tests (out of a total 134) were conducted under monochromatic wave conditions with wave heights of 20 mm or 40 mm and wave periods ranging from 1.2 s to 2.2 s. In these tests the PTO damping was varied by adjusting the orifice diameter, while the air volume was varied via an adjustable auxiliary air chamber. Results show that for the smaller orifice diameters (i.e., higher damping) tested, air-spring stiffness played a significant role and counterintuitively increased with increased air volume. Effects of the single OWC device on the wave field within the flume were also investigated. Results revealed that while there is a marked difference when comparing the OWC to an identically-shaped blockage, there was no significant measurable difference in the wave height change observed for all the damping and air volume parameter settings that were modelled, despite a general trend when comparing to the empty flume.

Spectral control co-design of wave energy converter array layout

Wave energy systems are designed to maximise energy absorption from the oscillatory motion of waves, which can make a crucial contribution in a new carbon free generation matrix [1]. Thus, the global ocean energy market (including wave and tidal) is expected to grow by more than 700\% by 2028 [2]. However, the harsh conditions faced in the ocean, the strong variability of the resource, and the high force/velocity ratios pose significant challenges to the development of the different ocean energy technologies. Therefore, to date, wave energy converter (WEC) technology is still under development and still needs a significant progress to enhance the energy conversion efficiency and, consequently, to become commercial viable. Crucially, to meet these existing challenges, there are a number of key considerations to progress the effectiveness and efficiency of WEC technologies. In particular, energy maximising control systems, to maximise energy absorption, and the optimisation of WEC array schemes, to take advantage of constructive interaction effects, are currently considered as some of the key drivers for the development of efficient WECs [1]. Recent studies have shown that total design of WECs, analysing the impact of each individual component on the rest of the system, is a key methodology to achieve effective designs. For example, the levelised cost of energy (LCoE) is considered in the objective function in [3], analysing the interplay between the control system based on a spectral controller and the specifications of the power take-off (PTO) system defined as the maximum stroke and force. This methodology has been generally labelled as `co-design', or, when the development is carried out in a control-aware manner, `control co-design'. Similarly, in [4], the interaction between control and optimal WEC geometry is studied. In addition, as discussed before, another key driver in achieving effective wave energy system is the operation of WEC arrays. In particular, in [5] and [6] different array layouts are analysed in terms of computational efficiency and control, respectively. Considering the results in [5], where methods based on harmonic balance techniques are used to analyse the computational demand related to different array layouts, an assessment of the impact of different array layouts on the LCoE, is performed in this study. Figure 1 illustrates different array layouts (a), and the impact of different separation and number of WECs on the resulting LCoE (b). (Figure in PDF abstract)For the implementation of this study, a spectral control methodology is considered to virtually achieve optimal control solutions, even in constrained scenarios. In tandem, different layout templates, composed of multiple point absorber WECs, are considered. To achieve a clear global indicator based on the LCoE, each layout is analysed, in terms of the separation distance and the number of WECs, as well as capital and operation expenditure (CapEx and OpEx, respectively). Following the methodological guidelines considered in [3], a general co-design scheme is designed, essentially based on an exhaustive search method, indicating the interplay between different array layouts and LCoE.

A 3D BEM Model for the Hydrodynamic Analysis and Design of Heaving WEC Arrays Attached to a Breakwater

Assessment of the performance of arrays of wave energy converters (WECs) of the type of simple heaving floaters, operating in nearshore and coastal areas, is essential to estimate their power production capacity. The effective design and layout of such WEC farms can be supported by studying the system’s operational characteristics based on the wave climatology of the deployment region. Installation along the exposed side of harbor breakwaters in sea areas with substantial incoming energy potential is a promising option. In this study, a 3D hydrodynamic model based on the boundary element method (BEM) is presented and discussed, aiming to evaluate the performance of WEC arrays consisting of multiple heaving bodies attached to the exposed side of a harbor breakwater, modelled as a vertical wall, considering its reflective effects, as well as hydrodynamic interactions between the multiple floating devices. Numerical results of the predicted power performance for various configurations, including interactions of multiple WECs with the nearshore topography and the breakwater wall, as well as the effects of power take off (PTO) parameters, are presented and discussed. Finally, a case study is presented for a selected coastal site at the port of Heraklion, located in the north-central part of Crete Island in the South Aegean Sea, characterized by relatively increased wave energy potential, using long-term climatological data, illustrating the method and its applicability as a supporting tool for optimal design of WEC arrangements.

Hydrodynamic analysis of hybrid system with wind turbine and wave energy converter

A practical model is developed to investigate the performance of a hybrid system with semi-submersible floating offshore wind turbines (FOWT) coupled to an array of point-absorbing wave energy converters (WECs). In this study, the boundary value problem is solved by applying the matching-method of eigenfunctions to solve a complex-shaped hybrid system and the velocity potential can be decomposed into radiation and diffraction problems. For each component in the structure, we consider it consists of three coaxial cylinders of different dimensions, making our mathematical model applicable to many marine structures. Within the framework of a linearized theory, We develop the coupled equations of motion to model the stiffness and damping constraints and to evaluate the effect of coupled motion between the floating platform and vertical truncated cylinders, taking into account wind forces, mooring lines, power take-off (PTO) systems and viscous effects on the hybrid system. For such a system, the combination of the OC4-DeepCwind platform with an array of point-absorbing WECs is investigated in this study. After running the convergence analysis and model validation, the present model is employed to perform a multiparameter effect analysis. Case studies are presented to clarify the effects of WEC parameters (i.e. radius, draft, PTO damping and layout), base column submergence, wave heading and frequency on the motion response of wind platform and mean capture width ratio of the WECs array. Our results provide insights into the relationship between the variables analyzed and the performance of the hybrid system. Moreover, the theoretical model developed in this study accurately calculates the hydrodynamic coefficients and motion performance of some marine structures.