• Energy Research

Since 2016, Ecole Centrale de Nantes has been investigating a radically new technology for conversion of the far-offshore wind energy into sustainable fuel, called the FARWIND energy system. It relies on mobile wind energy converters (energy ships) that are not grid-connected. Thus, the converters include on-board power-to-X plants for storage of the produced energy. In [16], we investigated the feasibility of hydrogen as the energy vector. It was found to be challenging because of high transportation and distribution cost due to the low volumetric energy density of hydrogen for standard conditions of temperature and pressure. In this paper, we first investigate other options which include synthetic natural gas, methanol, Fischer-Tropsch fuel and ammonia. Comparison of those options indicates that methanol is the most promising option. Then, net energy efficiency and cost of FARWIND-produced methanol is estimated. Despite the fact that the net energy efficiency is found to be smaller than for the hydrogen solution, it is shown that the methanol cost could be competitive for the transportation fuel market on the medium to long term.

The International Energy Agency Technology Collaboration Programme for Ocean Energy Systems (OES) initiated the OES Wave Energy Conversion Modelling Task, which focused on the verification and validation of numerical models for simulating wave energy converters (WECs). The long-term goal is to assess the accuracy of and establish confidence in the use of numerical models used in design as well as power performance assessment of WECs. To establish this confidence, the authors used different existing computational modelling tools to simulate given tasks to identify uncertainties related to simulation methodologies: (i) linear potential flow methods; (ii) weakly nonlinear Froude–Krylov methods; and (iii) fully nonlinear methods (fully nonlinear potential flow and Navier–Stokes models). This article summarizes the code-to-code task and code-to-experiment task that have been performed so far in this project, with a focus on investigating the impact of different levels of nonlinearities in the numerical models. Two different WECs were studied and simulated. The first was a heaving semi-submerged sphere, where free-decay tests and both regular and irregular wave cases were investigated in a code-to-code comparison. The second case was a heaving float corresponding to a physical model tested in a wave tank. We considered radiation, diffraction, and regular wave cases and compared quantities, such as the WEC motion, power output and hydrodynamic loading.

This paper synthesizes the technical feasibility study carried out for a hybrid ocean energy converter, with balanced wind and wave contributions. The solution envisaged involves a 100m diameter circular barge equipped with floating oscillating wave surge converters (OWSCs). This floating structure is mounted with a 5MW wind turbine. The present study covers power performance estimations, structural analysis and mooring design calculations. The first section describes the “Wave to Wire” model programmed in both frequency and time domain. The mathematical and hydrodynamic assumptions are highlighted together with the numerical model. The second part starts with the assessment of the performances of this device, carried out on in-house simulation codes. Based on combined wave and wind resources, the annual average absorbed power figures are compared with published results for existing ocean energy converters. The total rated power of the combined system reaches 10MW. Eventually, the last section approaches practical topics, directly related to the capital and operational costs inherent to an industrial development phase. The total steel mass is estimated first, from structural calculations carried out for a selection of 3D static loads cases. Then, a technical solution for the mooring system is presented together with the envisaged installation procedure.

This article presents the novel methodology used in the software InWave to address the problem of wave energy converters (WEC) modelling. The originality compared to other recently developed tools lies in a fast semi-recursive multibody dynamic solver which integrates a flexible hydrodynamic solver. The multibody solver works in time domain and is fully nonlinear. It solves the dynamic of systems formed of a fixed or free base articulated with any number of bodies that can be floating or not, with branchy structure ([1]). The integrated hydrodynamic solver is a linear potential flow solver based on boundary elements method. It uses the generalized degrees of freedom approach ([11]). Combined with a relative coordinate parameterization, it allows for a minimization of the number of hydrodynamic boundary value problems that have to be solved, thus allowing a reduction of computational time both for BEM computations and time domain simulations. Time domain reconstruction is performed to link hydrodynamic loads with the multibody dynamic solver. Interaction between bodies through radiation is thus taken into account. InWave is a complete WEC modelling tool including incident wave generation, multibody dynamic solver, hydrodynamic solver, power take-off and mooring models, post-processing and visualization. A successful comparison with the linear potential flow solver Aquaplus ([5]) on a basic cylinder test case is carried out. Finally, a complex test case on a Langlee-like device is presented, comparing InWave results with those from the NumWec project ([2]). A good agreement between both models is found, which increases the confidence in InWave algorithms and implementation.

Aiming at amplifying the energy productive motion of wave energy converters (WECs) in response to irregular sea waves, the strategies of discrete control presented here feature some major advantages over continuous control, which is known to require, for optimal operation, a bidirectional power take-off able to re-inject energy into the WEC system during parts of the oscillation cycles. Three different discrete control strategies are described: latching control, declutching control and the combination of both, which we term latched–operating–declutched control. It is shown that any of these methods can be applied with great benefit, not only to mono-resonant WEC oscillators, but also to bi-resonant and multi-resonant systems. For some of these applications, it is shown how these three discrete control strategies can be optimally defined, either by analytical solution for regular waves, or numerically, by applying the optimal command theory in irregular waves. Applied to a model of a seven degree-of-freedom system (the SEAREV WEC) to estimate its annual production on several production sites, the most efficient of these discrete control strategies was shown to double the energy production, regardless of the resource level of the site, which may be considered as a real breakthrough, rather than a marginal improvement.

Knowledge of the wave perturbation caused by an array of Wave Energy Converters (WEC) is of great concern, in particular for estimating the interaction effects between the various WECs and determining the modification of the wave field at the scale of the array, as well as possible influence on the hydrodynamic conditions in the surroundings. A better knowledge of these interactions will also allow a more efficient layout for future WEC farms. The present work focuses on the interactions of waves with several WECs in an array. Within linear wave theory and in frequency domain, we propose a methodology based on the use of a BEM (Boundary Element Method) model (namely Aquaplus) to solve the radiation-diffraction problem locally around each WEC, and to combine it with a model based on the mild slope equation at the scale of the array. The latter model (ARTEMIS software) solves the Berkhoff’s equation in 2DH domains (2 dimensional code with a z-dependence), considering irregular bathymetries. In fact, the Kochin function (a far field approximation) is used to propagate the perturbations computed by Aquaplus into Artemis, which is well adapted for a circular wave representing the perturbation of an oscillating body. This approximation implies that the method is only suitable for well separated devices. A main advantage of this coupling technique is that Artemis can deal with variable bathymetry. It is important when the wave farm is in shallow water or in nearshore areas. The methodology used for coupling the two models, with the underlying assumptions is detailed first. Validations test-cases are then carried out with simple bodies (namely heaving vertical cylinders) to assess the accuracy and efficiency of the coupling scheme. These tests also allow to analyze and to quantify the magnitude of the interactions between the WECs inside the array.