Enhancing the performance of hybrid wave-wind energy systems through a fast and adaptive chaotic multi-objective swarm optimisation method

This paper presents a practical design of a wave energy converter (WEC). The WEC is based on an innovative mechanical device power take-off (PTO) which can convert wave energy into electric energy by using a floating buoy. By using cable and pulley mechanism, the WEC can work in high efficiency by capturing wave energy in both surge and heave mode and converting the bidirectional motion of ocean waves into one-way rotation of an electric generator. First, the configuration is presented to describe working principle of the system. Then, the hydrodynamic forces are analyzed in the time domain. Moreover, the coupled mechanical and hydrodynamic model are simulated in Matlab/Simulink to investigate the performance of the proposed design. Finally, simulation results indicate that the proposed WEC can absorb wave energy in high efficiency and can be applied in the realistic condition.

The proper design of wave energy converters (WECs) is crucial for ensuring robustness in harsh wave climates without incurring the additional expense of unnecessary overdesign. The power take-off (PTO) mechanism, serving as a vital link between the moving body and the electric generator, is a key component in the design load analysis of WECs. However, the setting of PTO system parameters significantly impacts the dynamic behavior of the entire WEC system, leading to alterations in estimated loads. This work is dedicated to studying the influence of PTO control strategies on the identification of extreme loads of a heaving point absorber WEC. A nonlinear time-domain model is established to estimate the dynamic responses and loads of the WEC. Both PTO loads and end-stop loads under extreme conditions are examined, considering the wave climate of a realistic sea site. The results suggest that the PTO setting strategies significantly impact the extreme load exerted on both the PTO system and the end-stop system. Varying the PTO damping within a certain range could lead to a difference of 57% and 63% in short-term extreme loads for the PTO system and the end-stop system, respectively. Furthermore, the impacts of the PTO control strategy appear to be specific to each WEC component. The PTO parameters selected for reducing the extreme PTO loads might increase the extreme end-stop loads. A holistic examination is therefore recommended for estimating the extreme loads of WECs.

The aim of this study is to conduct a hydrodynamic analysis of a point absorber-type floating hemisphere with Power take-off(PTO) system for an optimum design of wave energy converter(WEC). In general, to predict the behavior of WEC, the equation of floating body motion can be solved in the frequency-domain analysis. A numerical model of PTO system is usually expressed as a linear damper-spring type force. In this study, however, a hydraulic-pump-type PTO system was modeled to reflect the behavior of WEC accurately. To describe the hydraulic PTO system, which is strongly nonlinear, the coulomb damping force was applied. In the time-domain analysis, Cummins equation was employed to consider the coulomb damping force in the equation of motion. Using the developed numerical model for a floating hemisphere WEC, regular wave analysis of hydrodynamic performance was conducted firstly. Heave RAOs were compared for various hydraulic pressures of the PTO system. The generated wave powers, expressed as the combination of buoy velocity and coulomb damping force, were also compared for various WEC conditions. Including PTO system with all possible damping, the modified equation of motion in irregular waves was finally solved to obtain the response of the hemispheric buoy. Significant displacement of the buoy and generated mean power of WEC were then obtained. Based on the parametric study on various wave conditions of selected areas, an optimum design and condition can be proposed to operate the present WEC.

Recent studies have focused on optimising wave energy converter (WEC) designs, maximising their power performance and techno‐economic feasibility. Reliability has yet to be fully considered in these formulations, despite its impact on cost and performance. In this study, this gap is addressed by developing a reliability‐based design optimisation framework for WEC hull geometries to explore the trade‐off between power performance and power take‐off (PTO) system damage equivalent loading (DEL). Optimised hull geometries for two sites are considered (from the centre of the North Sea and off the west coast of Norway), and two directions of motions (heave and surge). Results indicate that site characteristics affect the potential power production and DEL for an optimal WEC design. These are also affected by the direction of motion for power extraction, which also significantly changes optimal hull shape characteristics. Optimal surging WEC designs have edges facing oncoming wave directions, while heaving WECs have pointed bottoms, both to streamline movement. Larger, more convex WECs result in greater power production and DEL, while smaller, more concave WECs result in lesser power production and DEL. These findings underline the importance of considering WEC hull geometry in early design processes to optimise cost, power production, and reliability.

Direct-drive wave energy converter (WEC) and buoy control algorithms have shown great potential for renewable wave energy extraction in ideal conditions. However the actual power take-off (PTO) impacts are barely considered in the WEC design. This paper highlights the demands of designing the WEC wave-to-wire control from a global point of view by studying the actual PTO impacts. A permanent magnet linear electrical machine (LEM) PTO unit is simulated and controlled to fulfill the WEC buoy control requirements. Several state of the art control algorithms, which include singular-arc (SA) control, shape-based (SB) control, model predictive control (MPC), and proportional-derivative (PD) control, are applied to maximize the wave energy production (mechanical energy). Multiple types of electrical PTOs, including ideal PTO, unlimited PTO and limited PTO, are all implemented to evaluate WEC wave-to-wire performances. Further, the PTO copper loss model and the PTO actual efficiency maps are introduced and studied to improve the electrical PTO operation efficiency. To further assess the control schemes in various wave conditions, one-year PacWave ground-truth data is applied as well. Numerical simulations are conducted using MATLAB/Simulink and the Simscape toolbox. The electrical PTO unit is composed of a LEM, an ideal inverter, and an ideal energy storage system. The results show that the actual PTO will impact the constrained controls (MPC and SB) less comapring to unconstrained controls (SA and PD). Although SB can produce the maximum energy with the limited PTOs, it is not robust for all wave conditions. At the end of the paper, the possible solutions for improving the WEC wave-to-wire performances are also provided.

Comparative study on power capture performance of oscillating-body wave energy converters with three novel power take-off systems

Wave energy is a significant source of renewable energy harnessed by wave energy converters (WECs). However, due to the relatively high levelised cost of energy (LCoE), wave energy has not attained a commercial stage yet. To minimise the LCoE, since the optimum (uncontrolled) WEC design typically differs from the optimum controlled WEC design, control co-design (CCD) techniques are essential. With CCD, the WEC control-related aspects are taken into account from the start of the WEC design phase and, ideally, the best control-informed WEC design is then achieved. This paper specifically focuses on CCD for an oscillating-water-column (OWC) WEC, equipped with a Wells turbine and a bypass valve. In essence, a parametric CCD approach is devised to find the optimum (control-informed) turbine rotor diameter, and bypass valve diameter, for the considered OWC WEC. In particular, the optimum design parameters minimise a ‘simplified’ LCoE, which is chosen as a suitable performance function. Despite the LCoE is primarily sensitive to the power take-off size, rather than to the bypass valve size, peak-shaving control with a bypass valve potentially increases the capacity factor and, consequently, can minimise the LCoE for small-to-medium sized turbines.

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.

Ocean energy technologies are in their developmental stages, like other renewable energy sources. To be useable in the energy market, most components of wave energy devices require further improvement. Additionally, wave resource characteristics must be evaluated and estimated correctly to assess the wave energy potential in various coastal areas. Multiple algorithms integrated with numerical models have recently been developed and utilized to estimate, predict, and forecast wave characteristics and wave energy resources. Each algorithm is vital in designing wave energy converters (WECs) to harvest more energy. Although several algorithms based on optimization approaches have been developed for efficiently designing WECs, they are unreliable and suffer from high computational costs. To this end, novel algorithms incorporating machine learning and deep learning have been presented to forecast wave energy resources and optimize WEC design. This review aims to classify and discuss the key characteristics of machine learning and deep learning algorithms that apply to wave energy forecast and optimal configuration of WECs. Consequently, in terms of convergence rate, combining optimization methods, machine learning, and deep learning algorithms can improve the WECs configuration and wave characteristic forecasting and optimization. In addition, the high capability of learning algorithms for forecasting wave resource and energy characteristics was emphasized. Moreover, a review of power take-off (PTO) coefficients and the control of WECs demonstrated the indispensable ability of learning algorithms to optimize PTO parameters and the design of WECs.

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

An Oscillating Wave Surge Converter (OWSC) is a Wave Energy Converter (WEC) that consists of a bottom-hinged flap which oscillates due to wave action. Extensive research has been performed on this type of WEC through small scale experimental wave tank tests. One of the key challenges of experimental testing is replicating the characteristics of the Power Take-Off (PTO) system of the equivalent full scale WEC. Many scale models rely on simplified mechanical designs to simulate a PTO system. This can often restrict the experimental research into the influence of PTO design and control strategies of WECs. In order to model PTO systems and control strategies more accurately other tools are needed. This paper describes the design and build of a PLC controlled Force Feedback Dynamometer (FFD) system that enables the testing of more sophisticated control strategies applicable to an OWSC through fast application of a variable PTO damping torque. A PLC system is shown to be a viable control for PTO strategy investigations through velocity triggered damping levels. Examples of both PTO and position control strategies are presented.

Multi-scale concurrent design of a 100 kW wave energy converter

The technology of ocean Wave Energy Converters (WEC) has dramatically developed during recent years. These WECs apply various power take-off (PTO) system which are continuing to improve. Different types of PTOs such as hydroelectric, electromagnetic and air turbine systems have been introduced in different relevant studies. The most significant challenge in ocean WECs is related to different power take-off systems. This work introduces a new development of a WEC from a concept of taking energy from ocean waves to the prototype stage and testing along the southern coast of the Caspian Sea. The point absorber Single-body WEC, which is a popular design, was selected to model the proposed system under different conditions, including irregular and regular waves. The bi-directional motion of a wave is converted to the unidirectional motion which rotates the generator by the proposed PTO system that named “Mechanical Gear Rectifier” (MGR), in which a new pinion system is applied. A prototype of our small-scale WEC was designed, built, and then a close analysis of it is performed. Finally, the performance of proposed WEC carefully tested in different wave conditions (regular and irregular waves). The simulation results demonstrate the high efficiency of the proposed MGR-WEC.

Flexible membrane structures for wave energy harvesting: A review of the developments, materials and computational modelling approaches

Presented herein is the performance of a pontoon-type wave energy converter (WEC) similar to that of the ‘Floater Blanket’ WEC, which consists of a grid of interconnected floating elements. The current WEC design takes into account the elastic deformation of the structure under wave action and investigates its efficiency in generating wave energy as compared to its counterparts that achieve this via rigid body motions. The power take off (PTO) systems are installed at the seabed and attached to the WEC to generate energy via the vertical motions of the WEC’s structural modules. Three types of pontoon-type WEC designs are investigated, i.e. with hinges, with a reduced number of hinges and without hinges. In addition, the influence of the flexibility of the structural modules towards the energy generation is also investigated in order to assess its performance when behaving like an elastic material. The pontoon-type WEC’s structural modules with fewer or without hinge connections are preferable due to their ease in installation, while allowing for flexible deformation in the structural design would yield savings in material cost.