Computational fluid dynamics and experimental study of a model-scale two-body hinged raft wave energy converter

This paper proposes a novel three-dimensional oscillating pendulum wave energy converter (WEC) that integrates an oscillating float dock station. The device captures wave energy by utilizing both the pitch and roll motions of its primary float and the pendular motion of a buoy. A time-domain analysis method is used to numerically evaluate the hydrodynamic behavior and energy conversion efficiency of the WEC. In ANSYS AQWA, a multi-cantilever WEC model is employed to address the fluid-solid coupling, calculating the device’s motion response and capturing the width ratio under various environmental conditions. Additionally, by modifying key geometric parameters including float radius, length, and cantilever angle, the study examines the rotation at the articulation point and the capture width ratio variation for different device configurations. Results indicate that the device achieves a maximum capture width ratio at a float radius of approximately 120 mm under T = 1.4 s, and a 130 mm for wave periods of 1.5 s and 1.6 s. The highest average capture width ratio is reached at a power take-off (PTO) damping coefficient of 400 N·s/m. The study further investigates the effect of cantilever angle and float length, aiding in the optimization of these geometric parameters.

Extracting energy from ocean waves has become a heated topic since the energy crisis of the 2000s. Among all the different concepts and designs of Wave Energy Converter (WEC), point absorber is a widely adopted type with great potential, and various configurations and constraints are applicable to it. Here, the point absorber WECs with four different set‐up configurations are explored: single body heaving WEC, two‐body heaving WEC, two‐body WEC with a flat plate (Reference Model 3), and a two‐body WEC with a cylinder‐shaped second body. Dynamic models are established for each case and wave tank tests are conducted for verification. The results show that the power capture of a point absorber can benefit from several aspects: the two‐body WEC with a streamlined shape can double the wave capture width ratio (up to 66.5%) over the single‐body WEC or Reference Model 3, while coupling other motion or mooring dynamics can further improve the capture width ratio by 12% by increasing the relative motion stroke.

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

The performance of wave energy converters (WECs) in terms of energy capture presents considerable challenges in enhancing conversion efficiency. This research proposes a structural design and operational principle for an omnidirectional oscillating buoy WEC (OOBWEC), featuring six absorbers arranged in a circular configuration. To validate the proposed design and operational principle, experimental investigations were conducted within a wave flume. The experimental findings indicate that the capture width ratio (CWR) peaked at approximately 68.15% when the incident wave period was 1.8 s and the wave height was 80 mm. It was observed that as the wave period increased, the CWR initially rose before gradually declining. Conversely, an increase in wave height corresponded with a gradual decrease in the CWR. Notably, due to the angle of the incoming waves, the power captured by the forward absorber significantly exceeded that of the other absorbers. These results provide a basis for future numerical simulations, and further experimental studies will be conducted to optimize the WEC’s structure and improve its energy conversion efficiency.

Experimental investigation of breakwater-type WEC composed of both stationary and floating pontoons

While the field of wave energy has been the subject of numerical simulation, scale model testing, and precommercial project testing for decades, wave energy technologies remain in the early stages of development and must continue to prove themselves as a promising modern renewable energy field. A wave energy converter (WEC) concept currently being explored is the variable-geometry WEC (VGWEC), which aims to add an extra control option to WEC design. VGWECs attempt to incorporate controllable geometric features to adjust the floating body hydrodynamics to favor either power absorption, load shedding, or other operational goals. These variable geometry components have been proposed to be controlled on a sea-state-to-sea-state or wave-to-wave time scale depending on the force (or torque) and bandwidth limitations of the actuators required to manipulate just the controllable geometric hull features. Having control over both the WEC geometry components and the power takeoff (PTO) offers the potential to improve overall system performance and reliability if a cost-effective solution can be found for a given WEC architecture. This paper will present the recent developments and results of a VGWEC concept that incorporates variable-geometry modules into a two-body WEC. In the proposed VGWEC concept, the variable-geometry modules consist of air-inflatable bags in the surface float and a water inflatable ring in the subsurface body. The surface float is tethered directly to the subsurface body through tether lines, each connected to a separate PTO. Adjusting the geometry of both the surface and subsurface bodies along with the PTO coefficients can maximize power in design sea states while reducing motion response and PTO forces when transitioning to sea states where rated power is reached and load shedding is prioritized. The ability to transition between operating condition is expected to increase the sea state operational map and power capacity.

To increase the amount of energy captured from a vibrating buoy in the ocean with a simple mechanism, this paper proposes a two-body point absorber wave energy converter (WEC) with a tuned inerter. The tuned inerter mechanism consists of a spring, a linear damping element, and a component called inerter. This mechanism was originally proposed in the field of civil engineering as a structural control device which can absorb energy from vibrating structures effectively by taking advantage of the resonance effect of the inerter part. In addition to this mechanism where a generator is used as the linear damping element, the current of the generator for the power take-off system is controlled based on the algorithms proposed in literature to achieve further improvement of the power generation capability. In this research, a detailed analytical model of the proposed WEC is introduced and developed. Then the power generation performances of full-scale WEC models are assessed through numerical simulation studies using WAMIT software and it is shown that the current-controlled WEC with the proposed mechanism achieves an 88% increase compared to the conventional one for the JONSWAP spectrum with 6 s peak period and 1 m significant wave height.

This dissertation focuses on a Computational Fluid Dynamics (CFD) study of offshore floating structures, particularly floating Wave Energy Converter (WEC) devices. The open-source CFD software, OpenFOAM®, is evaluated for its effectiveness in simulating relevant phenomena. The study presents the hydrodynamic analysis and optimization of a novel axisymmetric single cylindrical WEC equipped with a moonpool, called MoonWEC. The primary aim is to optimize the design of a small-scale model of the MoonWEC device by minimizing pitch rotations, enhancing heave motion, and facilitating its moonpool oscillations using CFD. A set of new laboratory experiments was conducted to characterize the hydrodynamic response of the device. These experiments were numerically reproduced in OpenFOAM. A detailed explanation of the numerical setup in OpenFOAM is provided in the thesis for the readers to reproduce the CFD model for any moored floating structure. The heave and pitch responses were analyzed by comparing experimental and numerical data, showing good agreement. The results showed that the MoonWEC device can achieve the preferred excitation in heave, but it can also undergo the unpreferred excitation in pitch. By shifting the centre of mass, MoonWEC was numerically optimized to reduce the pitch rotations without affecting the heave oscillations. The moonpool motion was also analyzed for the optimized small-scale model. The study concludes that the CFD model in OpenFOAM, coupled with the MooDy library, can accurately simulate the hydrodynamics of the MoonWEC. The optimization study shows that the centre of mass of the device is a key parameter in controlling the natural period of the device in pitch. The analysis of the moonpool shows that the MoonWEC has the potential to generate significant power. This research contributes to the ongoing efforts to develop wave energy as a sustainable and renewable energy source.

Considerable testing and modeling are required in order to fully realise, efficiently develop, and successfully industrialise the wave energy converters (WECs). Numerical modeling, full-scale measurements, and scaled prototype testing are the various methodologies that can be applied to model WECs and predict the dynamic response. Mathematical WEC models form the basis of model-based energy maximising control and directly affect the ability of model-based controllers to maximise energy capture. Linear WEC models are attractive in leading to simpler control designs, but may not cover the complete operational space. One solution is to identify a range of linear models at different operating points, which give a measure of the underlying nonlinear behaviour [1], [2]. This model set can then be used to extract a nominal model, and an associated uncertainty region, which can be used as a basis for a robust WEC controller synthesis process, such as articulated in [3]. Recently, such an approach has been adopted using data generated from a high-fidelity numerical computational fluid dynamics (CFD) model [4]. However, numerical wave tanks (NWTs) and physical wave tanks differ significantly in terms of the range of tests which can be performed, and the contamination which can affect the measurements used to determine the data-based models e.g. measurement noise, numerical effects, wave reflections, etc [5]. As a result, the determination of nominal models and uncertainty regions in a physical wave tank may provide some advantages (and disadvantages) which need to be examined carefully. In addition, the range of post-processing techniques which could, or should, be applied to the different experimental/numerical domains, to improve the fidelity of the identified models, may differ between domains. In this paper, experimental testing of a WEC, by recreating a wave field similar to real-life conditions and a small-scale version of the device, is used to understand the hydrodynamic behaviour and to obtain an accurate dynamic model for WECs, which are considered to be essential towards optimal WEC design. Physical wave tank experiments, even though having their own disadvantages, overcome some difficulties of CFD-based NWT experiments, most notably huge computation time, problems in accurate representation of viscous fluids, uncertainty in the specification of an appropriate turbulence model, and propagation of incident waves [6]. In this study, representative linear models of a point-absorber type WEC from a physical wave tank in the wave basin at Aalborg University are determined which give insight into the system dynamics and provide a basis for robust control of WECs. Among different stimulation techniques to excite the system dynamics in physical wave tank tests, the particular types of excitation signals covering the complete range of frequencies and amplitudes of the system dynamics, while considering limitations on the range of excitation signals or the wave tank reflections, are adopted for the determination of representative linear models. Moreover, a full investigation is carried out to ascertain the appropriate post-processing needed to optimise the signals as the basis for model identification. The model identification includes the non-parametric frequency response by means of empirical transfer function estimate (ETFE).

The development of wave energy converters (WECs) faces several technical challenges, particularly enhancing the capturing efficiency, improving the conversion of mechanical to electric energy, and reducing energy losses in the transmission of electricity to land-based facilities. The present study is an assessment of the interaction between an oscillating buoy-type wave energy converter (WEC) and waves using experimental and numerical methods. A small-scale model was tested in a wave tank to evaluate its energy capturing efficiency, taking wave heights and periods as independent variables. The recorded data were used to validate OpenFOAM (version 9.0) simulations, which provided insights into system response characteristics. The findings highlight the critical role of resonance in optimizing energy capture, with maximum efficiency observed for medium wave periods, and with specific buoy configurations. The study also identified an inverse relationship between the capture width ratio and wave height, suggesting the need for customized buoy designs, tailored to specific sea states. The integrated approach used in this research provides a comprehensive understanding of WEC behaviour and offers valuable insights for advancing wave energy technologies and improving their sustainability and efficiency in diverse marine environments.

Incorporating wave energy converters (WECs) into existing oceanographic instrument systems and offshore floating platforms can not only enhance the performance of these applications but reduce operational expenses. This article studies a system integrating a floater WEC with a floating spar platform via the inerter pendulum vibration absorber with a power take-off (IPVA-PTO) mechanism, with a focus on random wave excitation. Experiments and simulations performed on a simplified system in which the WEC is held fixed and radiation damping is absent reveal that the power spectral density (PSD) of the system consists of odd-order superharmonics when the peak frequency of wave excitation is equal to the natural frequency of the system. It is found that the odd-order superharmonics are created by the IPVA and have a strong correlation with an enhancement in power output. Simulations without the aforementioned simplifications confirm the odd-order superharmonics and the correlation, and demonstrate an improvement in the capture width ratio (CWR) of 161.4% at resonance without compromising the response amplitude operator (RAO) of the spar, in comparison with a linear benchmark with optimal electrical damping.

Based on the necessity for enclosure protection of temperature and relative humidity sensors installed in a hostile environment, a wind tunnel was used to quantify the time that the sensors take to reach equilibrium in the environmental conditions to which they are exposed. Two treatments were used: (1) sensors with polyvinyl chloride (PVC) enclosure protection, and (2) sensors with no enclosure protection. The primary objective of this study was to develop and validate a 3-D computational fluid dynamics (CFD) model for analyzing the temperature and relative humidity distribution in a wind tunnel using sensors with PVC enclosure protection and sensors with no enclosure protection. A CFD simulation model was developed to describe the temperature distribution and the physics of mass transfer related to the airflow relative humidity. The first results demonstrate the applicability of the simulation. For verification, a sensor device was successfully assembled and tested in an environment that was optimized to ensure fast change conditions. The quantification setup presented in this paper is thus considered to be adequate for testing different materials and morphologies for enclosure protection. The results show that the boundary layer flow regime has a significant impact on the heat flux distribution. The results indicate that the CFD technique is a powerful tool which provides a detailed description of the flow and temperature fields as well as the time that the relative humidity takes to reach equilibrium with the environment in which the sensors are inserted.

The hybrid system of wave energy converters (WECs) using coastal structures is an attractive issue in terms of a decrease in construction costs and an improvement of the ability to capture wave energy. Most studies on the utilization of reflected waves from structures, which is one of the hybrid systems, are limited to mathematical analysis based on linear theories. Therefore, this paper presents fundamental experimental results in the presence of a reflection wall simplified as a coastal structure behind a bottom-hinged flap-type WEC under unidirectional regular waves. The behavioral characteristics and the power generation efficiency ke of the flap were investigated, focusing on wave steepness, initial water depth, and distance from the reflection wall. The results show that the condition of the initial water depth being smaller than the flap height is more effective in terms of avoiding unstable rotating of the flap. The maximum ke appeared slightly far from the node position of the standing waves because the flap shape and the power take-off (PTO) damping induce the phase difference between the reciprocating behavior of the flap and the period of the standing wave. The results imply that the optimum position of a WEC is dependent on WEC shape, PTO damping, and installation water depth.

The motion of some geometries of a buoy in a two-body wave energy converter (WEC) system is studied under regular wave excitations. The buoy is coupled to a heave damping plate where the coupling system models a simplified power takeoff (PTO) system. A frequency-domain analysis is conducted on the two-body model to predict the wave-induced motion behavior of the bodies considering the mutual effects of two bodies and the PTO system. The designed buoys are axisymmetric and have an identical water-plane area and submerged volume. The model was analyzed in identical conditions for different buoys so that the effect of the buoy shape on its dynamic properties and the hydrodynamic coefficients, as well as the absorption efficiency of the two-body WEC model, could be evaluated. The impact of the buoy shape and its hydrodynamic coefficients on the optimal and suboptimal design of the PTO system is studied. The results indicate that a suitable geometry for the buoy can have beneficial effects on the improved efficiency of the two-body WEC model, increasing the relative displacement between the two bodies and, also, allowing an optimal design of the PTO system. Furthermore, the buoy shape can reduce the difference between the damping coefficients of the PTO system and the buoy in order to create the resonance condition in the WEC system.

Model predictive control (MPC) has been considered as one important feed-forward optimal control strategy for ocean wave energy converter (WEC) targeted on power maximization. The capability of MPC to handle system constraints (ex. stroke, velocity, actuator limitations), and the availability to provide optimal solution for linear system provide potential for the implementation of such algorithm in the WEC control. However, currently, only active MPC control has been introduced for single and two-body WECs. Such control strategy may introduce negative power during the optimization process, since the power take-off (PTO) damping has no constraint. In this paper, we proposed a hybrid MPC strategy in limiting both the PTO damping force and PTO damping to avoid negative power generation during cost function minimization (negative power minimization) for the two-body WEC. The problem is formulated into a quadratic programming (QP) problem targeted at power maximization. However, the standard QP problem formulation cannot be directly applied to the semi-active control problem due to the PTO damping constraints. Therefore, the problem is reformulated as a Mixed-integer Quadratic Programming (MIQP) problem, which contains logical switch to select constraint matrices based on the sign of the relative velocity between the buoy and submerged body. The optimal solution is compared with those of the active MPC control strategy and the passive model with the same irregular wave input.