Search

• A real-time nonlinear model predictive controller to maintain the optimal wave energy extraction. • Wave condition preview based on the recurrent neural network identifier. • A pragmatic method for obtaining the optimal reference buoy velocity. A real-time nonlinear model predictive controller is presented for point-absorbing wave energy converter (PA-WEC) to maintain the optimal wave energy extraction by tracking the reference PA-WEC velocity. In order to reduce computational cost, the predictive controller is designed using wave condition preview and the Taylor series expansion based nonlinear optimization to determine the q-axis current control signals while the constraints on both the device velocity and q-axis current are respected. The dynamics of the PA-WEC is modeled and details of the nonlinear model predictive controller are presented. A pragmatic method is proposed to obtain the optimal reference PA-WEC velocity by observing the time instants when the excitation force passes a threshold. A long short-term memory recurrent neural network (LSTM-RNN) identifier is designed to identify the wave condition term for implementing the controller. The overall stability of the closed control system including the predictive control and the LSTM-RNN identifier is proved. The proposed nonlinear predictive controller is validated under realistic wave conditions. The results indicate that the proposed control allows higher PA-WEC power production than the conventional control under representative irregular wave conditions. [ABSTRACT FROM AUTHOR]

A spherical point absorbing wave energy converter was simulated in floating and submerged conditions for regular and irregular waves. A nonlinear stiffness was included to augment a linear stiffness and damper control system. The linear control parameters were optimised for each regular and irregular wave condition. The optimal linear control stiffness for the floating system was negative to counteract the hydrostatic stiffness. The nonlinear stiffness was shown to increase the converted power if no linear control stiffness was used, and reduce the converted power if optimal linear control parameters were used for regular waves. Similarly, for irregular conditions, nonlinear stiffness degrades the performance of the system when compared to optimised parameters, but enhances the robustness of the system to changing sea states and stochastic phases. Therefore, a nonlinear stiffness mechanism may improve the power output for poorly tuned control systems arising from plant uncertainty for point absorbing wave energy converters. • Nonlinear stiffness can provide negative stiffness to counteract hydrostatic stiffness. • No improvement to the converted power due to nonlinear stiffness compared to perfect conditions. • Nonlinear stiffness enhances the robustness for changing sea states. [ABSTRACT FROM AUTHOR]

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. • A practical model was presented to accurately calculate the hydrodynamic coefficients. • A coupled motion equation was developed to model the rigidity and damping constraints in a hybrid system. • The study examined the synergistic effects between a wind platform and PTO damping. • Added WECs significantly reduce the pitch motion response of the wind platform. • Increasing the submergence depth of the base column has a negative impact on the hybrid system. [ABSTRACT FROM AUTHOR]

DPC(clustering by fast search and find of density peaks) is an efficient clustering algorithm. However, DPC and its variations usually cannot detect the appropriate cluster centers for a dataset containing sparse and dense clusters simultaneously, resulting in the unique clustering within a dataset cannot being found. To remedy these limitations, we propose an Adaptive Nearest Neighbor Density Peak Clustering algorithm, referred to as ANN-DPC. It introduces the adaptive nearest neighbors for a point, so as to define the accurate local density of the point. Moreover, it partitions points into super-score, core, linked and slave points, and proposes techniques to detect appropriate cluster centers through introducing super-core point with higher local density to absorb the other super-core points sharing adaptive nearest neighbors with it and the dependency vector for finding next cluster center. Furthermore, novel assignment strategies are proposed by leveraging the adaptive nearest neighbors combing with breadth first search and fuzzy weighted adaptive nearest neighbors, so as to assign non-center points to the most appropriate clusters. Extensive experiments on real-world and synthetic datasets demonstrate the superiority of the proposed ANN-DPC algorithm over the counterparts in precisely detecting the cluster centers and the unique clustering within a dataset. [Display omitted] • Introducing adaptive nearest neighbors for a point to define its accurate local density and partition it as a super-core, core, linked, or slave point. • Introducing super-core point absorbing technique and the dependency vector to detect the appropriate cluster centers within a dataset. • New assignment strategies are present by leveraging the adaptive nearest neighbors combining with breadth first search and fuzzy weighted adaptive nearest neighbors. • ANN-DPC algorithm is introduced by leveraging these aforementioned contributions. • Extensive experiments demonstrate the superiority of the ANN-DPC over existing counterparts. [ABSTRACT FROM AUTHOR]

An analytical model is applied to investigate the dynamic performance of a type of hybrid system (PAWT) with a semi-submersible floating offshore wind turbine (FOWT) coupled to an array of point-absorbing wave energy converters (PA). In this process, the linear potential flow theory and eigenfunction matching method are employed to study the wave–structure interactions, and a coupled set of equations is proposed to evaluate the motion response in frequency-domain. To account for the pronounced impact of nonlinear factors inherent in the hybrid system, a constellation of mathematical models has been integrated into Cummins equation framework, including aerodynamic, mooring lines, power take-off (PTO), and viscosity modeling. After running the convergence analysis and model validation, the present model is employed to clarify the effects of PTO damping, WEC radius, layout, and real sea states on the motion response and wave energy capture efficiency. Results demonstrate that integrating Wavestar-type WECs is more beneficial for improving the pitch stability of FOWT than heaving-type. Although augmenting the number of integrated WECs cannot enhance FOWT stability, it does contribute to broadening the bandwidth of the capture width ratio (CWR). Notably, the power take-off (PTO) is simplified as a linear damping to model a linear permanent magnet generator (PMG) in this study. Furthermore, the interest of this work lies in the full-process analytical model itself, which is found to be efficient in modeling the interaction of the wind-wave field and can be used in future hybrid system projects. • A full-process analytical model has been developed for hybrid systems. • Multiple types of engineering-practical hybrid systems are introduced. • The effect of PTO damping, WEC radius, layout, and sea states are discussed. • Revealing the effect factors and interconnections on the dynamic characteristic. [ABSTRACT FROM AUTHOR]

A mathematical model is developed to investigate the performance of a hybrid system comprising semi-submersible floating offshore wind turbines (FOWT) coupled to an array of point-absorbing wave energy converters (WECs) under irregular waves and dynamic winds. In this study, the hydrodynamic interaction of the hybrid system is conducted by applying the matching-method of eigenfunctions to solve the diffraction and radiation velocity potentials. Non-linearities of the hybrid system are taken into account in the Cummins framework, covering aerodynamic, catenary mooring, PTO system, fluid viscosity modeling, and frequency–time domain transformation of hydrodynamic coefficients. 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 (including radius, draft, PTO damping, and layout) on the performance of a hybrid system. Nevertheless, a significant aspect of this work revolves around the development of a precise and efficient mathematical model, capable of accurately computing hydrodynamic coefficients for specific marine structures and evaluating the motion response of the interconnected floaters. Additionally, the study offers valuable insights into the preliminary design of hybrid systems and lays a mathematical foundation along with corresponding code for hybrid system projects. • An accurate and efficient model is presented to calculate the hydrodynamic coefficients of hybrid systems. • A practical mathematical model has been developed to evaluate the dynamic performance of coupled marine structures. • Integrating WECs significantly reduce the power spectrum density in pitch mode of the wind platform. • Linear PTO contributes more to platform stability than quadratic and constant PTO systems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

With recent advances in offshore floating wind and wave energy technology, questions have emerged as to whether the two technologies can be combined to reduce their overall levelised cost of energy. In this paper, the potential for combining a floating offshore wind turbine to a point absorbing wave energy converter is investigated. The focus of the investigation is how much power might be produced by a combined floating wind and wave energy converter system, and the resultant changes in motion of the floating wind platform. A model for the combined wave and wind system is developed which uses the standardised NREL OC3 5 MW spar type wind turbine and a cylindrical buoyant actuator (BA), which is attached to the spar via a generic wave power take-off system (modelled as a spring-damper system). Modelling is conducted in the frequency domain and the tests span a wide range of parameters, such as wave conditions, BA sizes, and power take-off coupling arrangements. It is found that the optimal (with respect to power production) BA size is a draft and radius of approximately 14 m. It is found that this BA can theoretically produce power in the range of 0.3 to 0.5 MW for waves with a significant wave height of 2 m, and has the potential to produce power greater or near to 1 MW for waves with a significant wave height of at least 3 m. However, it is also found that, in terms of the relative capture width, significantly smaller BAs are optimal, and that these smaller BA sizes less significantly alter the motion of the floating wind platform. [ABSTRACT FROM AUTHOR]