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Unmanned surface vehicles (USVs) have been widely used in various fields credited for their high flexibility and excellent manoeuvrability. However, the limited onboard electricity of USVs restricts their applications in long-term working tasks. A promising option to address that problem is employing a wave-powered USV, where a wave energy converter is integrated into the USV. For a wave-powered USV, it is challenging to achieve a precise estimation of its power generation under complex marine environments. Although there are physics-based models of WECs used for the estimation and prediction of power generation of USVs, they cannot achieve satisfactory accuracy in practice because of the linearization and simplification of those mathematical modelling. To fill up this gap, this paper proposes a data-driven modelling method utilizing feature engineering and ensemble learning to estimate the mean power generation of a wave-powered USV, relying on time-serial measurements obtained from real physical wave-flume experiments under regular waves. Results show that the estimation achieves promising performance, where the minimum RMSEs are 3.359 mW and 6.148 mW for a harvester-to-wire model and a wave-to-wire model, respectively. Additionally, the proposed method shows great potential for real-sea applications, such as online power prediction and control. • A data-driven method for power estimation of a wave-powered USV is proposed. • The robustness of the proposed method is verified with the presence of noise. • The proposed method possesses practical significance, such as data-driven control. [ABSTRACT FROM AUTHOR]

Considering the practical application of wave energy generation, a comprehensive wave-to-wire (W2W) model is proposed in this paper. A novel wave energy conversion (WEC) system with a center of gravity adjustment mechanism is proposed. The proposed W2W model involves the coupling modeling between the absorber and the center of gravity adjustment mechanism as well as the coupling modeling between the absorber and the power take-off (PTO) system. Based on the coupling relationship between the absorber and the center of gravity adjustment mechanism, a resonance control strategy is proposed. In addition, based on the coupling relationship between the absorber and the PTO system, an impedance-matching control strategy is proposed. Combined with the above two control strategies, the two conditions of optimal power capture can be met at the same time, so the WEC system can realize the maximum power point tracking (MPPT). In addition, in order to ensure the safe operation of the WEC system under extreme wave conditions, a power constraint control strategy is proposed to constrain the absorbed power. Finally, the feasibility of the W2W model and the accuracy of the proposed control strategies are verified under the regular and irregular waves. • A comprehensive wave-to-wire model is proposed for the practical application. • A novel WEC with an adjustable natural period characteristic is proposed. • Coupling modeling of the wave-to-grid system is given. • The maximum power point tracking under regular and irregular waves is realized. • A power constraint control strategy is proposed to limit the power of the system. [ABSTRACT FROM AUTHOR]

Wave-to-Wire models play an important role in the development of wave energy converters. They could provide insight into the complete operating process of wave energy converters, from the power absorption stage to the power conversion stage. In order to cover a set of relevant nonlinear effects, wave-to-wire models are predominately established in the time domain. However, the low computational efficiency of time-domain modeling is hindering the extensive application of wave-to-wire models, especially in early-stage design and optimization where a large number of iterations are required. To address this issue, a spectral-domain wave-to-wire model is proposed, and the nonlinear effects are incorporated by stochastic linearization. This model can significantly reduce the computational load and maintain good accuracy. The reference concept studied in this paper is defined as a heaving point absorber coupled with a linear permanent-magnet generator. Four representative nonlinear effects involved in both the hydrodynamic stage and the electrical stage of the concept are considered. The proposed model is verified against a corresponding nonlinear time-domain wave-to-wire model, and a good agreement is observed. The relative error of the proposed spectral-domain wave-to-wire model is around 2 % in typical operational regions and is still within 7 % for wave states with large significant wave heights, regarding the estimate of the power conversion efficiency. Meanwhile, the computational load of the spectral-domain wave-to-wire model is reduced by 2 to 3 orders of magnitudes compared with the conventional time-domain approach. Finally, a case study of tuning the PTO damping to maximize power production is conducted to demonstrate the performance of the proposed spectral-domain wave-to-wire model. [ABSTRACT FROM AUTHOR]

Ocean waves are a promising source of renewable energy. In this paper, we briefly introduce the characteristics of ocean wave energy and summarize the principles of harvesting ocean energy by wave energy converters. We also review the prototypes or commercial devices deployed in real sea between 2005 and July 2016. In addition, we present the concept of a wave-to-wire model as a framework to systematically review and compare control strategies for wave energy conversion systems, with a focus on the numerical and experimental validation. [ABSTRACT FROM AUTHOR]

The active control of wave energy converters with oil-hydraulic power take-off systems presents important demands on the electrical drives attached to their pumps, in particular on the required drive accelerations and rotational speeds. This work analyzes these demands on the drives and designs reliable control approaches for such drives by simulating a wave-to-wire model in a hardware in-the-loop simulation test rig. The model is based on a point absorber wave energy converter, being the wave, hydrodynamic and oil-hydraulic part simulated in a computer that sends and receives signals from the real embedded components, such as the drive generator, controller and back-to-back converter. Three different control strategies are developed and tested in this test rig and the results revealed that despite the drive limitations to acceleration levels, well above 1 × 10 4 rpm/s, these do not significantly affect the power take-off efficiency, because the required acceleration peaks rarely achieve these values. Moreover this drive is much more economical than an oil-hydraulic and equivalent one that is able to operate at those peaks of acceleration. [ABSTRACT FROM AUTHOR]

Control of wave energy converters (WECs) has been very often limited to hydrodynamic control to absorb the maximum energy possible from ocean waves. This generally ignores or significantly simplifies the performance of real power take-off (PTO) systems. However, including all the required dynamics and constraints in the control problem may considerably vary the control strategy and the power output. Therefore, this paper considers the incorporation into the model of all the conversion stages from ocean waves to the electricity network, referred to as wave-to-wire (W2W) models, and identifies the necessary components and their dynamics and constraints, including grid constraints. In addition, the paper identifies different control inputs for the different components of the PTO system and how these inputs are articulated to the dynamics of the system. Examples of pneumatic, hydraulic, mechanical or magnetic transmission systems driving a rotary electrical generator, and linear electric generators are provided. [ABSTRACT FROM AUTHOR]

The development of wave energy has attracted wide attention. This paper proposes a conceptual design of a station featuring direct drive 3-degree-of-freedom (3-DoF) wave energy converter (SJTU-WEC), of which the numerical and experimental performances are studied. This article first introduces the power generation principle of the SJTU-WEC, of which the wave-to-wire (W2W) model is simplified and established as a 2-DoF model, including frequency domain and time domain. Then a 1:12.5 prototype is manufactured and tested in wave tank to study its performance. By comparing numerical and experimental results, the accuracy of the W2W model is verified. The experimental results show that the energy conversion efficiency reaches up to 28% under the specific wave condition. Finally, based on the W2W model, the power generation performance of the full-scale SJTU-WEC is obtained. In Lianyungang Sea areas, the average annual power generation of a single SJTU-WEC is calculated to be 108W. •The wave-to-wire (W2W) model of 2-DoF simplified wave energy converter (SJTU-WEC) is established. •A 1:12.5 scaled prototype is manufactured and tested in wave tank to assess its performance. •The W2W model is verified and used to predict the full-size SJTU-WEC's performance in practical sea areas. [ABSTRACT FROM AUTHOR]

The construction of the prototype OWC plant proposed development requirements of the wave-to-wire model for the overall performance predictions, which should balance the computational cost and accuracy. In this study, a CFD-based wave-to-wire model was developed to couple the air chamber and air turbine. The steady numerical model was employed to derive the relationship between the air pressure drop and the incident airflow velocity, which was induced to the air chamber model in the numerical wave tank using the porous zone module. The numerical predictions on the free surface elevation and air pressure variation in the chamber, the torque output of the turbine in regular and irregular wave scenarios, and the efficiencies were validated by the experimental data. By considering the air compressibility, the wave-to-wire model was employed to predict the performance of the 500 kW Yongsoo prototype plant. The interaction mechanism among energy converting processes and the overall performance of the plant under the real-sea condition were carefully analyzed and reported. • A CFD-based wave-to-wire model was developed for the OWC wave energy converter. • The air pressure drop of an impulse turbine was induced to the air chamber model. • The computationally economical model showed good agreement with experimental tests. • The interaction mechanism among energy converting processes were analyzed. • The annual electricity production of a prototype OWC plant was predicted. [ABSTRACT FROM AUTHOR]

Fred Olsen is currently testing their latest wave energy converter (WEC), Lifesaver, outside of Falmouth Bay in England, preparing it for commercial operation at the Wavehub test site. Previous studies, mostly focusing on hydrodynamics and peak to average power reduction, have shown that this device has potential for increased power extraction using reactive control. This article extends those analyses, adding a detailed model of the all-electric power take-off (PTO) system, consisting of a permanent magnet synchronous generator, inverter and DC-link. Time domain simulations are performed to evaluate the PTO capabilities of the modeled WEC. However, when tuned towards reactive control, the generator losses become large, giving a very low overall system efficiency. Optimal control with respect to electrical output power is found to occur with low added mass, and when compared to pure passive loading, a 1% increase in annual energy production is estimated. The main factor reducing the effect of reactive control is found to be the minimum load-force constraint of the device. These results suggest that the Lifesaver has limited potential for increased production by reactive control. This analysis is nevertheless valuable, as it demonstrates how a wave-to-wire model can be used for investigation of PTO potential, annual energy production estimations and evaluations of different control techniques for a given WEC device. [ABSTRACT FROM AUTHOR]

This paper aims to investigate electric dynamics and improve electric power generation of an isolated wave energy converter that uses a linear permanent magnet generator as the power take-off system, excited by regular or irregular waves. This is of significant concern when considering actual operating conditions of an offshore wave energy converter, where the device will encounter different sea states and its electric load needs to be tuned on a sea-state-to-sea-state basis. To that end, a fully coupled fluid-mechanical-electric-magnetic-electronic mathematical model and an optimization routine are developed. This proposed time-domain wave-to-wire model is used to simulate the hydrodynamic and electric response of a wave energy converter connected to specific electric loads and also used in an optimization routine that searches optimal resistive load value for a wave energy converter under specific sea states. Sample results are presented for a point-absorber type wave energy converter, showing that the electric power generation of a device under irregular waves can be significantly improved. • A wave-to-wire model is developed for a point-absorber type wave energy converter (PAWEC). • A new optimization procedure that identifies optimal electric load is proposed. • Hydrodynamic and electric performance of a wave energy converter under random sea states are investigated. [ABSTRACT FROM AUTHOR]

The Tupperwave device is a closed-circuit oscillating water column (OWC) wave energy converter that uses non-return valves and two large fixed-volume accumulator chambers to create a smooth unidirectional air flow, harnessed by a unidirectional turbine. In this paper, the relevance of the Tupperwave concept against the conventional OWC concept, that uses a self-rectifying turbine, is investigated. For this purpose, wave-to-wire numerical models of the Tupperwave device and a corresponding conventional OWC device are developed and validated against experimental tests. Both devices have the same floating spar buoy structure and a similar turbine technology. The models include wave-structure hydrodynamic interaction, air turbines and generators, along with their control laws in order to encompass all power conversion stages from wave to electrical power. Hardware-in-the-loop is used to physically emulate the last power conversion stage from mechanic to electrical power and hence validate the control law and the generator numerical model. The dimensioning methodology for turbines and generators for power optimisation is explained. Eventually, the validated wave-to-wire numerical models of the conventional OWC and the Tupperwave device are used to assess and compare the performances of these two OWC type wave energy device concepts in the same wave climate. The benefits of pneumatic power smoothing by the Tupperwave device are discussed and the required efficiency of the non-return valves is investigated. [ABSTRACT FROM AUTHOR]