Search Results (19)

This study investigates the hydrodynamic responses and energy harvesting performance of a hemispherical point-absorber wave energy converter (WEC) in uniform current. A frequency-domain Rankine source method (RSM) is developed to rigorously account for current-modified free-surface conditions, and an approximate free-surface Green-function method (AFSGM) is implemented to assess practical applicability under weak-current assumptions. The numerical settings for body, free-surface, and radiation-boundary discretizations are determined through convergence tests. Model validation is performed by comparing motion responses against published benchmark results under both zero-current and current conditions. The effects of current and motion constraints are examined for surge–heave free and heave-only cases. Results show that current can amplify the heave response and that surge freedom enhances heave motion through coupling effects, leading to increasing discrepancies between RSM and AFSGM as current strengthens. For heave-only motion, AFSGM provides practically acceptable predictions within |Fr| ≤ 0.045, while noticeable differences appear near resonance beyond this range, for which RSM is recommended. Energy harvesting is evaluated using a linear PTO damping model, revealing that current alters the capture width ratio (CWR) and shifts the optimal PTO damping and frequency, indicating the necessity of considering current in performance assessment and PTO design. Full article

Ocean waves constitute a vast renewable resource, yet most linear generator-based wave energy converters (WECs) rely on single-degree-of-freedom (SDOF) linear oscillators that exhibit narrow resonance bandwidths and utilise sliding components prone to wear. To address these limitations, this paper presents a nonlinear three-degree-of-freedom (3DOF) magnetic spring power-take-off (PTO) system for broadband wave energy harvesting. The device comprises three axially levitated NdFeB permanent magnets, each coupled to an independent copper coil, forming a compact, friction-free generator column. A coupled electromechanical state-space model was developed and experimentally validated on a laboratory-scale test rig. The 3DOF PTO exhibited three distinct resonance modes at approximately 35, 48, and 69 rad s⁻¹, enabling multi-mode energy capture across a broad frequency range. Under identical excitation (6.5 N amplitude and 3.13 Hz excitation force), the 3DOF configuration achieved a 114.5% increase in RMS voltage compared with the SDOF design and a 44.10% improvement over the 2DOF benchmark, confirming the effectiveness of the coupled resonance mechanism. The levitated magnetic architecture eliminates mechanical contact and lubrication, reducing wear and maintenance while improving long-term reliability in marine environments. A preliminary life-cycle assessment estimated a cradle-to-gate carbon intensity of 40–80 g CO₂-eq kWh⁻¹, significantly lower than that of conventional hydraulic PTOs, owing to reduced steel use and recyclable magnet assemblies. The proposed 3DOF magnetic spring PTO thus offers a sustainable, low-maintenance, and high-efficiency solution for next-generation ocean-energy converters. Full article

This study introduces a novel double dumbbell-shaped flux-switching linear tube generator (DDFSLG) for ocean wave energy conversion. The innovative architecture features a uniquely shaped stator and translator, distinguishing it from conventional linear generators. Unlike traditional systems, the DDFSLG is housed in a cylindrical buoy. The translator oscillates axially within the stator. This eliminates the need for motion rectification and reduces mechanical friction losses in the power take-off (PTO) system. These design advancements result in high power output and improved performance. The DDFSLG’s three-phase coil circuit is another key innovation, improving electrical performance and stability in irregular wave conditions. We conducted comprehensive experimental validation using an MTS-250 kN testing system, which demonstrated strong agreement between theoretical predictions and measured results. We compared star and delta coil connections to assess how circuit configuration affects power output and efficiency. Furthermore, hydrodynamic simulations using the JONSWAP spectrum and ANSYS AQWA software (Ansys 13.0) provide detailed insight into the system’s dynamic response under realistic oceanic conditions. Full article

Due to the increasing share of battery electric buses (BEBs) in cities, depots need to be adapted to the increasing load demand. The integration of renewable energy sources (RESs) into a depot can increase the self-consumption, but optimal sizing is required for a cost-efficient and reliable operation. Accordingly, this paper introduces a co-design optimization framework for a depot microgrid, equipped with photovoltaics (PVs) and an energy storage system (ESS). Three European cities are considered to evaluate the effect of different environmental conditions and electricity prices on the optimal microgrid design. Accurate models of the different subsystems are created to estimate the load demand and the power generation. Different energy management strategies (EMSs), developed to properly control the power flow within the microgrid, are compared in terms of operational costs reduction, one of which was also experimentally validated using a hardware-in-the-loop (HiL) test setup. In addition, the total cost of ownership (TCO) of the depot microgrid is analyzed, showing that an optimally designed depot microgrid can reduce the charging-related expenses for the public transport operator (PTO) by 30% compared to a scenario in which only the distribution grid supplies the BEB depot. Full article

Manure distribution in soil creates a ground environment that is conducive to crop cultivation. However, the lumping and concentration of manure in the field can occur, hindering the fertilization of the soil for plant growth, and the randomization of nutrients under different soil depths accelerates it. To overcome the challenges associated with agricultural testing, such as high cost, inclement weather, and other constraints, computational analysis is often used. In this study, rotary operations are performed using the discrete element method (DEM) to ensure the uniform distribution of manure and four soil layers. DEM analysis was conducted with three experimental factors, and simulation sets were designed using the Box-Behnken central combination method. The DEM results were evaluated using the uniformity index (UI), and the field test of the rotary operation was performed with the set showing the most uniform distribution among the results. Due to undistinguishable particles in reality, the uniformity was validated by a comparison of the chemical characteristics of the L₁ and L₅ in terms of before and after the rotary operation. The DEM parameter of the soil was determined by performing field measurements at different soil depths (0–20 cm), and this parameter was calibrated by conducting a penetration test. The Box–Behnken central combination method was implemented using the following factors: tillage depth (X₁), PTO revolution speed (X₂), and forward machine velocity (X₃). These factors were obtained using the UI regression model and the response surface method. In the results, it was indicated that the UI was affected by the factors in the following order: X₁ > X₂ > X₃. The optimized factor values were X₁ = 25 cm, X₂ = 800 RPM, and X₃ = 1.8 km/h, leading to a UI of 6.07, which was consistent with the analysis results. The operating parameters were maintained throughout the field test, and the acquired data were input into the measurement system. The lowest UI value of 6.07 had the strongest effect on decreasing the disparity between L₁ and L₅, especially in terms of pH, organic matter, P, Ca, and Mg. In summary, the results indicated that soil distribution can be controlled by adjusting mechanical parameters to ensure uniform chemical characteristics across various soil depths. Full article

Optimizing the design and operational parameters for tillage tools is crucial for improved performance. Recently, artificial intelligence approaches, like ANN with learning capabilities, have gained attention for cost-effective and timely problem solving. Soil-bin experiments were conducted and data were used to develop ANN and regression models using gang angle, velocity ratio, soil CI, and depth as input parameters, while tractor equivalent PTO (PTO_eq) power was used as an output. Both models were trained with a randomly selected 90% of the data, reserving 10% for testing purposes. In regression, models were iteratively fitted using nonlinear least-squares optimization. The ANN model utilized a multilayer feed-forward network with a backpropagation algorithm. The comparative performance of both models was evaluated in terms of R² and mean square error (MSE). The ANN model outperformed the regression model in the training, testing, and validation phases. A well-trained ANN model was integrated with the particle-swarm optimization (PSO) technique for optimization of the operational parameters. The optimized configuration featured a 36.6° gang angle, 0.50 MPa CI, 100 mm depth, and 3.90 velocity ratio for a predicted tractor PTO_eq power of 3.36 kW against an actual value of 3.45 kW. ANN–PSO predicted the optimal parameters with a variation between the predicted and the actual tractor PTO_eq power within ±6.85%. Full article

The power take-off (PTO) of a wave energy converter (WEC) converts mechanical power extracted from the waves into electrical power. Increasing PTO performance under several operational conditions is therefore essential to reduce the levelized cost of energy of a given wave energy concept and to achieve higher levels of technology readiness. A key task in the WEC design will then be the holistic assessment of the PTO performance in combination with other subsystems. It is hence important that WEC designers are aware of the different modeling options. This paper addresses this need and presents two alternative wave-to-wire modeling approaches based on a 250 kW modular electromechanical PTO coupled to an oscillating wave surge converter (OWSC) device. The first is a detailed and accurate offline model. The second model is a simplified and faster version of the first, being adequate for rapid analyses and real-time (RT) simulation. The paper presents the benchmarking of the offline model against the RT model and the hardware-in-the-loop (HIL) tests of the PTO. The normalized root-mean-square error (NRMSE) is considered as a quantitative indicator for the measurement of real-time and HIL test results against the offline simulation. Results show that the dynamics of the offline model are well represented by the RT model with execution times up to 10 times faster. The offline model also depicts well the behavior observed in the HIL tests with the NRMSE values for the PTO position, velocity, and force above 0.90, which shows the HIL test results replicates with fidelity the dynamic behavior of the complete model. Meaningful differences are however present and highlighted in this paper. An understanding of the advantages and drawbacks of these three approaches is fundamental to properly design a WEC during its project cycle and validate PTO concepts with a certain level of simplification. Full article

To offer point absorber wave energy converters (WECs) as a bankable product on the marine renewable energy market, multiple WECs will be installed together in an array configuration. The wave energy community (research and industrial) has identified the urgent need for available realistic and reliable data on WEC array tests in order to perform a better WEC array optimization approach and in order to validate recently developed (non-linear) numerical models. The ‘WECfarm’ project is initiated to cover this scientific gap on necessary experimental data. The ‘WECfarm’ experimental setup consists 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 realized with a Speedgoat Performance real-time target machine, offering the possibility to implement advanced WEC array control strategies in the MATLAB-Simulink environment. The presented article describes the experimental setup, the performed tests and the results of the test campaign using a single, isolated ‘WECfarm’ WEC in April 2021 at the wave basin of Aalborg University (AAU), Denmark. A Coulomb and viscous friction model is determined to partly compensate for the drivetrain (motor, gearbox, rack and pinion) friction. A system identification (SID) approach is adopted considering the WEC system to be composed of two single input single output (SISO) models, the radiation and the excitation model. Radiation tests yield the intrinsic impedance. Excitation tests yield the excitation frequency response function. Adopting an impedance matching approach, the control parameters for the resistive and reactive controller are determined from the complex conjugate of the intrinsic impedance. Both controllers are tested for a selection of regular wave conditions. The performed experimental test campaign using an isolated ‘WECfarm’ WEC allows a full evaluation of the WEC design prior to extending the setup to five WECs. Within the ‘WECfarm’ project, an experimental campaign with a five-WEC array in the Coastal and Ocean Basin (COB) in Ostend, Belgium, is under preparation. Full article

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. Full article

In this work, the hydrodynamic performance of a novel wave energy converter (WEC) configuration which combines a moonpool platform and a javelin floating buoy, called the moonpool–javelin wave energy converter (MJWEC), was studied by semianalytical, computational fluid dynamics (CFD), and experimental methods. The viscous term is added to the potential flow solver to obtain the hydrodynamic coefficients. The wave force, the added mass, the radiation damping, the wave capture, and the energy efficiency of the configuration were assessed, in the frequency and time domains, by a semianalytical method. The CFD method results and the semianalytical results were compared for the time domain by introducing nonlinear power take-off (PTO) damping; additionally, the viscous dissipation coefficients under potential flow could be confirmed. Finally, a 1:10 scale model was physically tested to validate the numerical model and further prove the feasibility of the proposed system. Full article

The aim of this work is to show that a significant increase of the efficiency of a Wave Energy Converter (WEC) can be achieved already at an early design stage, through the choice of a turbine and control regulation, by means of an accurate Wave-to-Wire (W2W) modeling that couples the hydrodynamic response calibrated in a wave flume to a Hardware-In-the-Loop (HIL) test bench with sizes and rates not matching those of the system under development. Information on this procedure is relevant to save time, because the acquisition, the installation, and the setup of a test rig are not quick and easy. Moreover, power electronics and electric machines to emulate turbines and electric generators matching the real systems are not low-cost equipment. The use of HIL is important in the development of WECs also because it allows the carrying out of tests in a controlled environment, and this is again time- and money-saving if compared to tests done on a real system installed at the sea. Furthermore, W2W modeling can be applied to several Power Take-Off (PTO) configurations to experiment different control strategies. The method here proposed, concerning a specific HIL for testing power electronics and control laws for a specific WECs, may have a more general validity. Full article

This paper introduces a new point-absorber wave energy converter (WEC) with a moonpool buoy—the moonpool platform wave energy converter (MPWEC). The MPWEC structure includes a cylinder buoy and a moonpool buoy and a Power Take-off (PTO) system, where the relative movement between the cylindrical buoy and the moonpool buoy is exploited by the PTO system to generate energy. A 1:10 scale model was physically tested to validate the numerical model and further prove the feasibility of the proposed system. The motion responses of and the power absorbed by the MPWEC studied in the wave tank experiments were also numerically analyzed, with a potential approach in the frequency domain, and a computational fluid dynamics (CFD) code in the time domain. The good agreement between the experimental and the numerical results showed that the present numerical model is accurate enough, and therefore considering only the heave degree of freedom is acceptable to estimate the motion responses and power absorption. The study shows that the MPWEC optimum power extractions is realized over a range of wave frequencies between 1.7 and 2.5 rad/s. Full article

Small-scale, portable generation of electricity from ocean waves provides a versatile solution to power the ocean sensors network, in addition to the traditional large-scale wave energy conversion facilities. However, one issue of small-scale wave energy convertor (WEC) is the low capturable power density, challenging the design of the efficient power take-off (PTO) system. To tackle this challenge, in this paper, an electrohydraulic PTO system with compressed energy storage was proposed to boost output power of a portable WEC. Lumped-parameter kinematics and dynamics of the four-bar mechanism, the fluid dynamics of the digital fluid power circuit, and the mechanical and volumetric power losses were modeled and experimentally validated. Initial test results of the 0.64 m² footprint prototype showed that the inclusion of storage improved the averaged electric power output over 40 times compared to the traditional architecture, and the proposed device can deliver up to 122 W at peaks. Full article

This paper describes experimental research on a floating moored Oscillating Water Column (OWC)-type Wave-Energy Converter (WEC) carried out in the wave flume of the Coastal Engineering Research Group of Ghent University. This research has been introduced to cover the existing data scarcity and knowledge gaps regarding response of moored floating OWC WECs. The obtained data will be available in the future for the validation of nonlinear numerical models. The experiment focuses on the assessment of the nonlinear motion and mooring-line response of a 1:25 floating moored OWC WEC model to regular waves. The OWC WEC model motion has 6 degrees of freedom and is limited by a symmetrical 4-point mooring system. The model is composed of a chamber with an orifice on top of it to simulate the power-take-off (PTO) system and the associated damping of the motion of the OWC WEC model. In the first place, the motion response in waves of the moored floating OWC WEC model is investigated and the water surface elevation in the OWC WEC chamber is measured. Secondly, two different mooring-line materials (iron chains and nylon ropes) are tested and the corresponding OWC WEC model motions and mooring-line tensions are measured. The performance of these two materials is similar in small-amplitude waves but different in large wave-amplitude conditions. Thirdly, the influence of different PTO conditions is investigated by varying the diameter of the top orifice of the OWC WEC model. The results show that the PTO damping does not affect the OWC WEC motion but has an impact on the water surface elevation inside the OWC chamber. In addition, an unbalanced mooring configuration is discussed. Finally, the obtained data for a moored cubic model in waves are presented, which is a benchmarking case for future validation purposes. Full article

The Power Take-Off (PTO) system is the key component of a Wave Energy Converter (WEC) that distinguishes it from a simple floating body because the uptake of the energy by the PTO system modifies the wave field surrounding the WEC. Consequently, the choice of a proper PTO model of a WEC is a key factor in the accuracy of a numerical model that serves to validate the economic impact of a wave energy project. Simultaneously, the given numerical model needs to simulate many WEC units operating in close proximity in a WEC farm, as such conglomerations are seen by the wave energy industry as the path to economic viability. A balance must therefore be struck between an accurate PTO model and the numerical cost of running it for various WEC farm configurations to test the viability of any given WEC farm project. Because hydrodynamic interaction between the WECs in a farm modifies the incoming wave field, both the power output of a WEC farm and the surface elevations in the ‘near field’ area will be affected. For certain types of WECs, namely heaving cylindrical WECs, the PTO system strongly modifies the motion of the WECs. Consequently, the choice of a PTO system affects both the power production and the surface elevations in the ‘near field’ of a WEC farm. In this paper, we investigate the effect of a PTO system for a small wave farm that we term ‘WEC array’ of 5 WECs of two types: a heaving cylindrical WEC and an Oscillating Surge Wave Energy Converter (OSWEC). These WECs are positioned in a staggered array configuration designed to extract the maximum power from the incident waves. The PTO system is modelled in WEC-Sim, a purpose-built WEC dynamics simulator. The PTO system is coupled to the open-source wave structure interaction solver NEMOH to calculate the average wave field $\eta$ in the ‘near-field’. Using a WEC-specific novel PTO system model, the effect of a hydraulic PTO system on the WEC array power production and the near-field is compared to that of a linear PTO system. Results are given for a series of regular wave conditions for a single WEC and subsequently extended to a 5-WEC array. We demonstrate the quantitative and qualitative differences in the power and the ‘near-field’ effects between a 5-heaving cylindrical WEC array and a 5-OSWEC array. Furthermore, we show that modeling a hydraulic PTO system as a linear PTO system in the case of a heaving cylindrical WEC leads to considerable inaccuracies in the calculation of average absorbed power, but not in the near-field surface elevations. Yet, in the case of an OSWEC, a hydraulic PTO system cannot be reduced to a linear PTO coefficient without introducing substantial inaccuracies into both the array power output and the near-field effects. We discuss the implications of our results compared to previous research on WEC arrays which used simplified linear coefficients as a proxy for PTO systems. Full article