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Background: Wave energy represents one of the most promising renewable energies due to its great theoretical potential. Nevertheless, the electrical compliance of grid-connected systems is a great issue nowadays, due to the highly stochastic nature of wave energy. Methods: In this paper, a Hybrid Energy Storage System (HESS) consisting of a Li-ion battery and a flywheel is coupled to a Wave Energy Converter (WEC) that operates in grid connected mode. The study is performed using real yearly wave power profiles relating to three different sites located along the European coasts. The Simultaneous Perturbation Stochastic Approximation (SPSA) principle is implemented as real-time power management strategy for HESS in wave energy conversion systems. Results: Obtained results demonstrate how the proposed HESS and the implementation of the SPSA power management coupled to a WEC allow a reduction of more than 80% of power oscillations at the Point of Common Coupling (PCC), while proving the robustness of the developed management strategy over the investigated sites. Moreover, the average energy penalty due to the HESS integration results slightly higher than 5% and battery solicitation is reduced by more than 64% with respect to the flywheel solicitation, contributing to extend its lifetime. Conclusions: HESS integration in renewable generation systems maximizes the WEC production while smoothing the power at the PCC. Specifically, flywheel-battery HESS together with the implemented power management strategy could provide a great flexibility in the view of increasing power production from waves, strongly mitigating the variability of this source while enhancing grid safety and stability.

A Power Take-Off (PTO) is responsible to convert wave to electrical power and is one of the core sub-systems of any Wave Energy Converter (WEC). As result, assessing the PTO performance is one of the key aspects of the WEC design and, looking at the optimization of all WEC subsystems, it directly impacts the LCOE assessment and the commercialization of the technology. To address this need, it is crucial to approach WEC modeling with the awareness of the different techniques. The paper presents part of the results of the IMAGINE R&D program developed with the support of European Union's H2020 research and innovation programme. This project is aimed at the design and test of a novel PTO drive intended for a range of WEC configurations. In particular, the paper presents a complete simulation platform, which allows realistic simulations of the performance of a 250 kW modular electro-mechanical generator (EMG) when coupled to an oscillating wave surge converter (OWSC) device. This model is compared to a corresponding simplified model more suitable for real-time (RT) hardwarein-the-loop (HIL) testing. A comparison between the two is provided as fundamental check to guarantee that the simulated environment is as similar as possible to the realspace and hence to validate the product maturity even though with a certain level of simplification. Keywords-wave energy; wave energy conversion; systems modelling; power take-off; ballscrew; electric generator; test bench.Recently, a modular PTO system developed by UM-BRAGROUP based on an a novel electrical machine that converts slow-speed, reciprocating linear motion into electric power with high efficiency and reliability was introduced [7] and experimentally validated [8]. Also, the modular nature of this PTO system was explored during

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.

This paper describes the activities related to the design, manufacturing and commissioning of an innovative Hardware-In-the-Loop test rig for linear Power Take-Off testing. The rig is characterised by a fully coupled architecture in which three electro-mechanical units integrating a ballscrew and an electrical machine can actuate on a linear axis, either as motor or generator. A preliminary mechanical design of the test rig was carried out by identifying the most demanding conditions. The electrical and mechanical designs were assessed through a de-risking simulation of the overall test rig set-up, considering faults between the motors and respective power converters. The resulting rig setup includes a structure that embeds the three units, an electrical control panel and a control system. The use of electro-mechanical units increases the flexibility of the setup and simplifies the test of extreme conditions such as maximum output power or actuation force. Moreover, it allows reusing the power produced by the generating devices, thus reducing operational costs of the tests. The control system integrates a real-time hardware-in-the-loop simulation platform, a supervisory control and data acquisition systems. Those offer not only the possibility of easily tuning parameters but also testing new control strategies, operational situations, and failures of a power-take-off system in very realistic conditions.