Bemrosetta: An open-source hydrodynamic coefficients converter and viewer integrated with Nemoh and Foamm

Boundary Element Method (BEM) solvers are extensively used to obtain the hydrodynamic coefficients required to model hydrodynamic forces in oating marine structures. BEM solvers require the discretization of the submerged device surface as a mesh to compute the hydro-dynamic coefficients as radiation damping and added mass, response amplitude operators and linear and second-order exciting forces. Each of these solvers need particular input files and mesh formats, and save the results in specific file formats. Typically, the input and output files are incompatible between different solvers. Researchers handle this problem by converting model results through homemade spreadsheets or macros made in scripting languages. BEMRosetta was created to allow loading and saving the input files, mesh geometries and the hydrodynamic coefficients, in different formats. Furthermore, it also includes a mesh viewer. Additionally, BEM-Rosetta can calculate di erent parameters from the mesh and the hydrodynamic coefficients. Through its integration with the Finite-Order hydrodynamic Approximation by Moment-Matching (FOAMM) toolbox, BEMRossetta allows the state- space model of the radiation convolution term for the desired degrees of freedom be obtained

A post-processing technique for removing ‘irregular frequencies’ and other issues in the results from BEM solvers

Within the wave energy community, hydrodynamic coefficients obtained from boundary element methods (BEMs) are commonly used to predict the behaviour of wave energy converters (WECs) in response to incident waves. A number of commercially-available BEM solvers exist, with a number of open-source alternatives also available. While open-source solvers have an obvious cost advantage compared to their commercial counterparts, the results from such solvers are often susceptible to so-called ‘irregular frequencies’, which arise from ill-conditioning in boundary integral problems, and result in large under- or over-estimation of hydrodynamic parameters at certain excitation frequencies. Furthermore, while commercial solvers may employ techniques to suppress the effects of irregular frequencies, such solvers may, under certain circumstances, exhibit other problems in the hydrodynamic results produced. For example, the results obtained for the added mass at high frequencies, and the infinite frequency added mass for a water column, may be incorrect. The current work first focusses on an approach to remove the effects of irregular frequencies from the results obtained for the radiation damping of a particular WEC geometry. The use of radiation damping results to obtain values for the added mass, through the use of the Ogilvie relations, is then considered. The technique described herein has been implemented in BEMRosetta, an open-source tool which allows a user to view the results from various BEM solvers, as well as converting input files between solvers. The results presented in this paper have been obtained using the BEMRosetta implementation

Design, validation and application of wave-to-wire models for heaving point absorber wave energy converters

Ocean waves represent an untapped source of renewable energy which can significantly contribute to the energy transition towards a sustainable energy mix. Despite the significant potential of this energy source and the multiple solutions suggested for the extraction of energy from ocean waves, some of which have demonstrated to be technically viable, no commercial wave energy farm has yet been connected to the electricity grid. This means that none of the technologies suggested in the literature has achieved economic viability. In order to make wave energy converters economically viable, it is essential to accurately understand and evaluate the holistic behaviour and performance of wave energy converters, including all the different conversion stages from ocean waves to the electricity grid. This can be achieved through wave tank or open ocean testing campaigns, which are extremely expensive and, thus, can critically determine the financial sustainability of the developing organisation, due to the risk of such large investments. Therefore, precise mathematical models that consider all the important dynamics, losses and constraints of the different conversion stages (including wave-structure hydrodynamic interaction and power take-off system), known as wave-to-wire models, are crucial in the development of successful wave energy converters. Hence, a comprehensive literature review of the different mathematical approaches suggested for modelling the different conversion stages and existing wave-to-wire models is presented, defining the foundations of parsimonious wave-to-wire models and their potential applications. As opposed to other offshore applications, wave energy converters need to exaggerate their motion to maximise energy absorption from ocean waves, which breaks the assumption of small body motion upon which linear models are based. An extensive investigation on the suitability of linear models and the relevance of different nonlinear effects is carried out, where control conditions are shown to play an important role. Hence, a computationally efficient mathematical model that incorporates nonlinear Froude-Krylov forces and viscous effects is presented. In the case of the power take-off system, mathematical models for different hydraulic transmission system configurations and electric generator topologies are presented, where the main losses are included using specific loss models with parameters identified via manufacturers’ data. In order to gain confidence in the mathematical models, the models corresponding to the different conversion stages are validated separately against either high-fidelity well-established software or experimental results, showing very good agreement. The main objective of this thesis is the development of a comprehensive wave-to-wire model. This comprehensive wave-to-wire model is created by adequately combining the subsystems corresponding to the different components or conversion stages. However, time-step requirements vary significantly depending on the dynamics included in each subsystem. Hence, if the time-step required for capturing the fastest dynamics is used in all the subsystems, unnecessary computation is performed in the subsystems with slower dynamics. Therefore, a multi-rate time-integration scheme is implemented, meaning that each subsystem uses the sample period required to adequately capture the dynamics of the components included in that conversion stage, which significantly reduces the overall computational requirements. In addition, the relevance of using a high-fidelity comprehensive wave-to-wire model in accurately designing wave energy converters and assessing their capabilities is demonstrated. For example, energy maximising controllers based on excessively simplified mathematical models result in dramatic consequences, such as negative average generated power or situations where the device remains stuck at one of the end-stops of the power take-off system. Despite the reasonably high-fidelity of the results provided by this comprehensive wave-towire model, some applications require the highest possible fidelity level and have no limitation with respect to computational cost. Hence, the simulation platform HiFiWEC, which couples a numerical wave tank based on computational fluid dynamics to the high-fidelity power take-off model, is created. In contrast, low computational cost is the main requirement for other applications and, thus, a systematic complexity reduction approach is suggested in this thesis, significantly reducing the computational cost of the HiFiWEC platform, while retaining the adequate fidelity level for each application. Due to the relevance of the nonlinearity degree when evaluating the complexity of a mathematical model, two nonlinearity measures to quantify this nonlinearity degree are defined. Hence, wave-to-wire models specifically created for each application are generated via the systematic complexity reduction approach, which provide the adequate trade-off between computational cost and fidelity level required for each application

Code-to-code nonlinear hydrodynamic modelling verification for wave energy converters: Wec-sim vs. nlfk4all

In the wave energy conversion field, simulation tools are crucial for effective converter and controller design, but are often prone to become very casespecific, in both structure and parameter selection. This is due to majorly different working principles and diverse importance of nonlinear effects, at times requiring adhoc modelling approaches. To tackle this challenge, WECSim (Wave Energy Converter SIMulator) was born from the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories, providing a unique simulation platform for all WECs. Nonlinearities related to timevarying wetted surface, especially important in floating WECs, are included in WEC-Sim through a mesh-based computation of nonlinear Froude-Krylov forces. Virtually arbitrary geometries can be considered, thanks to the discretized representation of wetted surfaces, at the price of a significant increase in computational burden. This paper considers a time-effective alternative, implemented in the open-source toolbox called NLFK4ALL, applicable to the popular and wide family of axisymmetric floaters. The Spar-buoy floating oscillating water column device is considered, particularly challenging due to a submerged volume composed of several different sections. The accuracy of WEC-Sim and NLFK4ALL is verified by a preliminary cross-comparison, using independent methods to compute virtually same effects. Fixed-body numerical experiments are used to quantify nonlinearities and compare not only he accuracy, but also the computation burden. Results show that both methods provide almost identical results, although WEC-Sim doubles computational requirements

Nonlinear Hydrodynamic Models for Heaving Buoy Wave Energy Converters

Numerical models for heaving buoy wave energy converters are a fundamental tool for device design and optimization, power production assessment and model-based controller design. Ideally, models are required to be easy to implement, simple, accurate and computationally efficient. Unfortunately, such features are often conflicting and a compromise has to be reached to define an appropriate model structure. A very common choice is to assume a small amplitude of motion and linearize the model. Despite the attractiveness of computational convenience, linear models quickly become inaccurate when large motion occurs. In particular, the implementation of a control strategy, which aims to increase power absorption, enlarges the operational space of the device and significantly enhances the impact of nonlinearities on the model. There are different possibilities to approach the representation of nonlinearities in heaving point absorbers, each of them characterized by a different level of complexity, computational time requirements and accuracy. This paper compares six different methods: one of them fully-nonlinear (implemented in a computational fluid dynamics environment) and the others based on a linear model with the progressive inclusion of nonlinear restoring force, nonlinear Froude-Krylov force and viscous drag

Nonlinear Hydrodynamic Models for Heaving Buoy Wave Energy Converters

Numerical models for heaving buoy wave energy converters are a fundamental tool for device design and optimization, power production assessment and model-based controller design. Ideally, models are required to be easy to implement, simple, accurate and computationally efficient. Unfortunately, such features are often conflicting and a compromise has to be reached to define an appropriate model structure. A very common choice is to assume a small amplitude of motion and linearize the model. Despite the attractiveness of computational convenience, linear models quickly become inaccurate when large motion occurs. In particular, the implementation of a control strategy, which aims to increase power absorption, enlarges the operational space of the device and significantly enhances the impact of nonlinearities on the model. There are different possibilities to approach the representation of nonlinearities in heaving point absorbers, each of them characterized by a different level of complexity, computational time requirements and accuracy. This paper compares six different methods: one of them fully-nonlinear (implemented in a computational fluid dynamics environment) and the others based on a linear model with the progressive inclusion of nonlinear restoring force, nonlinear Froude-Krylov force and viscous drag

A reduced wave-to-wire model for controller design and power assessment of wave energy converters

Precise mathematical models are essential for designing energy maximising control strategies and assessing power production capabilities of Wave Energy Converters (WECs). However, commonly used mathematical models excessively simplify Wave-Structure Hydrodynamic Interactions (WSHIs) and/or the Power Take-Off (PTO) system, due to the need for fast models, resulting in poor control strategies and misestimated power production estimates. The suitability of a reduced Wave-toWire (rW2W) model is studied here, which includes only the necessary dynamics and losses of the WSHI and PTO system, significantly reducing mathematical requirements of a comprehensive High-Fidelity W2W (HFW2W) model. Results demonstrate that the controller designed using the rW2W model is very similar to that designed via the HFW2W model, ensuring a satisfactory performance of the WEC. In addition, power production capabilities of a WEC assessed using the rW2W and HFW2W models show very similar results, with differences of up to 5% in the annual mean power production

A reduced wave-to-wire model for controller design and power assessment of wave energy converters

Precise mathematical models are essential for designing energy maximising control strategies and assessing power production capabilities of Wave Energy Converters (WECs). However, commonly used mathematical models excessively simplify Wave-Structure Hydrodynamic Interactions (WSHIs) and/or the Power Take-Off (PTO) system, due to the need for fast models, resulting in poor control strategies and misestimated power production estimates. The suitability of a reduced Wave-toWire (rW2W) model is studied here, which includes only the necessary dynamics and losses of the WSHI and PTO system, significantly reducing mathematical requirements of a comprehensive High-Fidelity W2W (HFW2W) model. Results demonstrate that the controller designed using the rW2W model is very similar to that designed via the HFW2W model, ensuring a satisfactory performance of the WEC. In addition, power production capabilities of a WEC assessed using the rW2W and HFW2W models show very similar results, with differences of up to 5% in the annual mean power production

Linearisation-based nonlinearity measures for wave-to-wire models in wave energy

It is important to consider nonlinear effects when designing controllers to maximise generated energy in wave energy converters (WECs). Due to the substantial extra computation and complexity added when considering nonlinearities in the controller calculations, quantifying the extent of nonlinearity in WECs’ behaviour is crucial to avoid designing overcomplicated control strategies. This paper suggests two nonlinearity measures to quantify the nonlinearity degree of wave-to-wire (W2W) models in steady-state, using the best linear approximation identified through a minimisation problem as a benchmark. The first measure, referred to as the original nonlinearity measure, evaluates the nonlinear effects of the wave-absorber hydrodynamic interaction. The second measure, referred to as the power nonlinearity measure, quantifies the nonlinear effects in power take-off (PTO) systems, considering the quadratic response of the power signal. The degree of nonlinearity of two WEC models, a partially-nonlinear hydrodynamic model with an ideal PTO model and a complete nonlinear W2W model, is evaluated using monochromatic and polychromatic waves over a wide range of wave periods and heights, covering the whole operational space of a WEC

Egiazko (in)dependentziez: energiaren ikuspegia Hego Euskal Herrian

The present study intends to clarify the philosophical grounds of system energetics, ecosophy and sustainability, following the path introduced by Ulazia (2016) in a previous issue of Gogoa (Ulazia 2016), in order to establish the foundations of a pragmatic strategy for the energetic sovereignty of the Basque Country. First, avoiding excessive reductionism, we emphasize the importance of the problem of energy sovereignty for a real (in)dependence of a country, showing its relevance in the geopolitical sense. Using Lund’s foundational research as analogy, the energy strategy of an independent Basque Country is suggested, taking into account the energy on the Danish situation consumption and generation data of Basque Autonomous Community and Upper Navarre. Currently, an important lack of renewable energy sources is observed in the current energy mix, which is a fundamental point in the sustainable development of a state. Hence, these empirical facts show that any discussion about independence is meaningless if the subject about the local and distributed implementation of renewable energies is not considered.; Lan honetan Gogoaren aurreko ale batean Ulaziak (2016) hasitako bideari jarraitzen diogu, sistema-energetikaren, ekosofiaren eta jasangarritasunaren oinarri filosofikoak argitu eta gero, Euskal Herriaren burujabetza energetikorako estrategia pragmatiko baten fundamentuak ezartzeko. Erredukzionista izan nahi gabe, energiaren arazoaren garrantzia nabarmentzen dugu hasieran; izan ere, herri baten egiazko (in)dependentziari buruzko eztabaidan, oro har, aipatzen dena baino garrantzizkoagoa da energia, eta horren lekuko dira artikuluan aipatzen diren zenbait adibide geopolitiko. Danimarkako egoera eta Lunden lana erreferentzia gisara hartuta, Euskal Herrirako plan estrategiko baten oinarrien eta ereduen arteko konparaketa egingo dugu; horrela, EAEren eta Nafarroa Garaiaren egungo energiaren sorkuntza eta kontsumo-datuak aintzat hartuta. Energia berriztagarrien aparteko eskasia nabarmentzen da egin den ikerketan, eta horixe da, hain zuzen, herri baten garapena jasangarria dela esateko ezinbesteko baldintzetako bat. Beraz, emaitza enpirikook frogatzen dutenez, independentziari buruzko edozein debate hutsala da, debate horrek energia berriztagarrien inplementazio lokalaren eta bananduaren (deszentralizatuaren) gaia mahaigaineratzen ez badu