Search

In transportation applications, filling high pressurised hydrogen tanks within a few minutes implies a fast and large temperature increase that can compromise the structural integrity of the storage system and the safety of the process. Common approaches to control this temperature increase mainly involve hydrogen pre-cooling using complex and energy-demanding refrigeration systems. This study proposes and assesses alternative and energy efficient thermal management strategies for fast filling of hydrogen tanks including the modification of the liner's thermal properties and the use of existing on-board refrigeration systems. The analysis is performed by means of a Computational Fluid Dynamics model validated against experimental observations. Results show that proposed solutions provide a robust and efficient alternative to hydrogen pre-cooling, potentially simplifying the hydrogen supply system, reducing its associated energy demand, maintaining the hydrogen temperature below admissible limits and increasing the state of charge of the tank. • Strategies are based on liner pre-cooling and its thermal properties modification. • Similar H 2 temperature is achieved when pre-cooling it or changing liner properties. • Energy consumption of liner pre-cooling is up to 42–68% lower than H 2 pre-cooling. • Liner heat capacity increase is more effective than thermal conductivity increase. • Strategies increase the SOC up to 3.5% points compared to H 2 pre-cooling. [ABSTRACT FROM AUTHOR]

Wettability has a key impact on the performance of LiBr-H 2 O horizontal-tube falling film absorbers working under typical operating conditions. Therefore, it is necessary to study the influence of the wettability properties on the film hydrodynamics and wetted area. This work analyses the influence of the contact angle (0–120°) on the transient film hydrodynamics and the wetted area in the droplet and jet flow modes (7 ≤ Re ≤ 53). The comparison between two-dimensional and three-dimensional models evidences that three-dimensional models are needed for incomplete wetting conditions. The influence of the contact angle on both the film hydrodynamics and wetted area is demonstrated using 3D unsteady CFD model. Furthermore, the results indicate that each Reynolds number has a maximum contact angle to wet the whole tube steadily. When the value of the contact angle is lower than this maximum value, the contact angle has not influence on the wetting ratio. In contrast, at higher values, the wetting ratio decreases as the contact angle increases. [ABSTRACT FROM AUTHOR]

Abstract: The goal of the present work is to characterise the influence of static prestrain on the elastic and dissipative properties of constrained viscoelastic films. Dynamic mechanical analysis (DMA) technique is considered for measuring the dynamic properties under several static prestrain levels, and a new material constitutive model is presented for characterising the frequency–prestrain dependence of the viscoelastic material. The results indicate that the static prestrain strongly affects the elastic and dissipative properties of the analysed viscoelastic material. The proposed material constitutive model properly describes the frequency–prestrain dependence of the tested viscoelastic material. [Copyright &y& Elsevier]

Abstract: A passive damping technique with viscoelastic materials is analysed in the present work. Concretely, the constrained layer damping (CLD) configuration, also known as viscoelastically damped sandwich structure, is considered. For an efficient dynamical analysis of viscoelastically damped structures, an accurate material characterisation and suitable mathematical models are necessary for reproducing the real material behaviour. With this aim, a CLD structure has been characterised in the frequency domain by means of experimental tests, considering both homogenised and multilayered approaches. The material-parameter extraction procedure is presented, and mathematical models are reviewed. For the homogenised characterisation, a mathematical model based on the Ross et al. model has been proposed, whereas for the multilayered characterisation, the constraining layers have been considered as linear elastic, and the viscoelastic core has been represented by a four-parameter fractional derivative model. Both material models have been considered for defining both homogenised and layered multimaterial numerical models by the finite element method. The numerical models have been validated by correlating the numerical and the experimentally measured dynamic responses. Both numerical models have reproduced the experimental dynamic response accurately for the bandwidth considered herein. [Copyright &y& Elsevier]

• The impact of absorption determination methods for LiBr − H 2 O is analysed via CFD. • The employed methodology considerably changes falling film absorption performance. • Fick method is more sensitive than Higbie to interface's mass fraction variations. • Fick 1D without imposition approach shows best agreement with experimental results. [Display omitted] Different methodologies have been used in the literature to determine the absorbed vapour mass flux in falling film absorbers. So far, however, there has been little discussion about the impact of different approaches in the numerical models' performance. This study compared different methods for considering absorption in LiBr − H 2 O vertical tube-type falling film absorbers operating at both inlet subcooling and equilibrium conditions. This comparison was made based on three different criteria: the way of considering the absorption, the method to determine the absorption rate, and the assumption of the existing type of diffusion at the interface. The influence of the methodology was demonstrated using 2D multiphase CFD model. The effect of different operating conditions on the methods was also examined. Finally, the results of the numerical models were compared with three experimental works, and the most suitable model was chosen, which properly agrees with them. [ABSTRACT FROM AUTHOR]

Fatigue life estimation methods based on time-domain and rainflow counting techniques are widely recognised and accepted for their reliability. However, applying them in scenarios involving random loads with multiple large-scale time series becomes impractical due to high computational costs. This challenge is particularly significant for offshore renewable energy systems. Therefore, it is essential to employ more efficient frequency domain fatigue life calculation models. This study critically evaluates various spectral fatigue assessment methods for estimating damage caused by stress loads in the offshore environment. In this research, 25 spectral methods are analysed and compared using synthetic tension signals. The evaluation criteria include the results' reliability and ease of implementation, leading to the development of a scoring scheme as a systematic and straightforward ranking method. The results reveal that the Tovo–Benasciutti method excels in accurately estimating rainflow damage in bimodal processes, closely followed by the empirical α 0.75 , Han–Ma, Low, Modified Tovo–Benasciutti, Dirlik, and Jun–Park models. Other popular methods, such as the Narrow-band and Wirsching–Light methods, yielded poor results, and their use is discouraged. Furthermore, the spectral methods analysed in the benchmarking study have been implemented in Python, and the code has been released as open source. • 25 publicly available spectral methods are evaluated for ORE applications. • Synthetic signals that effectively reproduce realistic offshore loads are designed. • Spectral methods can reduce computational time by more than 80%. • Tovo–Benasciutti method is shown to be the most suitable for ORE applications. • RFC PDF Approximation methods show consistently reliable performance. [ABSTRACT FROM AUTHOR]

Given the current maturity of hydrogen technologies, accurate and computationally efficient numerical models are crucial for improving their understanding and development. Traditional models are either computationally prohibitive or lack the capacity to assess the spatial distribution of critical variables. The present paper suggests a hybrid numerical model for Fuel Cells (FCs) and electrolysers, articulating the coupling between a 1D analytical model and a Computational Fluid Dynamics (CFD) model via a nonlinear regression. Results present an initial validation of the 1D analytical model and the CFD model, and, once the validation is proven successful, the hybrid model is evaluated with a relatively long and highly varying loading profile that covers a wide range of the potential operational points of a FC. The hybrid model shows promising results, showing fidelity levels similar to CFD models, including the capacity to assess the spatial distribution, and a low computational cost. Hence, this hybrid model is demonstrated to be an attractive tool for the design of FCs and electrolysers, optimisation of thermal management and control strategies, degradation analysis, and techno-economic analysis. [ABSTRACT FROM AUTHOR]

An application-agnostic procedure is outlined for checking the validity of momentum-deficit-based drag measurements performed under different turbulent conditions in a wind tunnel. The approach defines a two-step methodology: the first stage characterizes the turbulent flowfield generated downstream a passive grid through a set of statistical parameters. Acceptable values for such parameters are determined by means of two criteria: compliance with the threshold value set by an analysis of the experimental uncertainties, and fulfilment of the isotropic condition for ensuring a well-established turbulent flowfield. Those two prerequisites define a set of turbulent configurations for which the momentum-deficit-based technique applies feasibly. The second stage of the procedure is configuration-specific, and undertakes drag measurements upon a NACA0021 airfoil subjected to a set of different turbulent configurations. It is shown that performing measurements under invalid turbulent conditions leads to inconsistent drag curves, which serves for defining a validity map based on the testable cases. [ABSTRACT FROM AUTHOR]

• A breakdown of the uncertainty analysis of measurands providing individual contributions. • Operative condition variations take aerodynamic coefficients outside uncertainty intervals. • Temperature variations due to wind tunnel operation constitute the critical factor. • The effect of operative condition variations on aerodynamic coefficients is non-linear. • An analysis of the operative conditions is to be performed regardless of the experimental set-up. A twofold analysis is performed on an experimental set-up comprising a low-Reynolds wind tunnel, with the aim of quantifying the effect of measurand uncertainties and operative condition variations in the lift and drag curves of a NACA0021 airfoil. The first part of the study focuses on an uncertainty analysis of basic measurands used for calculating standard aerodynamic magnitudes, showing the individual contributions of each of the factors affecting the overall uncertainty ranges. The second part intends to show how those uncertainties are propagated to lift and drag coefficients, and to calculate the effect of varying operative conditions on those curves. It is found that, when changes in operative conditions are left unattended, their influence on derived magnitudes may grow as large as to cause the resultant data to lie outside the estimated uncertainty intervals. Additionally, those variations are shown to depend non-linearly on the operative condition changes that produce them. [ABSTRACT FROM AUTHOR]

Green hydrogen (H 2) generation in far-offshore locations is being proposed as an alternative for the decarbonisation of the current fossil fuel-based energy system. However, due to the limited experience and low level of maturity of the technology, an accurate and comprehensive assessment of their technical and economic feasibility is crucial. To this aim, the present paper suggests a holistic numerical model for the techno-economic assessment of diverse H 2 -carrying energy vectors, such as gaseous (GH 2) and liquid H 2 (LH 2), and ammonia (NH 3), in far-offshore wind farms. The model includes all the relevant stages, from energy generation, to conversion, transportation, and storage. Hence, the model enables the assessment of broad offshore areas for site selection, sensitivity analyses to identify the most relevant aspects, and optimisation procedures to identify the most suitable H 2 -carrying energy vector. The consistency of the model is first verified against the literature and then applied to regions off the Iberian Peninsula and Balearic Islands. Results show that, compared to bathymetry or distance to port, wind resource availability is the most crucial factor to ensure the economic feasibility of the projects. Additionally, for any given site, the full electric alternative turns out to be lower than any H 2 -based energy vector. Among the studied H 2 -carrying energy vectors, GH 2 is the least expensive one and nearly competitive with electricity, as LH 2 and NH 3 show between two- to three- times greater expenses. • A techno-economic model is developed for assessing offshore H 2 -carrying vectors. • The model includes H 2 generation, conversion, transportation, and storage stages. • To ensure economic feasibility, wind resource becomes the most crucial factor. • The cost of the full electric vector turns to be lower than any H 2 -carrying vector. • Gas H 2 shows two to three times lower expenses compared to liquid H 2 and ammonia. [ABSTRACT FROM AUTHOR]

Highlights • Contact angle of the aqueous LiBr solution on different materials are presented. • Contact angle – Solution surface linear relation is observed in all materials. • Three Metals show a hygrophilic performance, while PTFE showed hygrophobic one. • An estimation of the minimum wetting rate has been performed by obtained results. • The contact angle should be considered in LiBr–H 2 O components analysis and design. Abstract Wetting surface is a very important issue for the design of absorption applications and heat exchangers. The contact angle is deemed essential in wettability studies; However, LiBr aqueous solution contact angle studies are limited. This work analyses the contact angle of LiBr aqueous solution in the range of 0–55% mass fraction on different material surfaces: copper, aluminum, stainless-steel and polytetrafluoroethylene (PTFE) under atmospheric conditions. A sessile drop technique was used for measuring the contact angles, and a linear relation between solution surface tension and contact angle is observed in the tested materials. The study of three metals show hydrophilic performance (θ < 90°), whereas the PTFE shows hydrophobic performance. Additionally, the effect of the selection of the material, and consequent effect on the contact angle, on the minimum wetting rate and film thicknesses is presented under the working conditions of the absorption technologies. From the wettability point of view, the results show that using stainless-steel and aluminum leads to a slightly better performance than a copper made heat exchanger. [ABSTRACT FROM AUTHOR]

The growing demand of energy coupled with the worldwide commitments in favor of renewable technologies turns marine renewable sources especially attractive, including wave energy. In particular, the oscillating water column technology is shown to be one of the most promising technologies, for which the design of efficient air turbines is crucial. The large number of geometrical parameters that such turbines own, together with the fact that the wave climate conditions affect the overall behavior, call for a tool that combines both aspects for optimizing the turbine design. A genetic-algorithm-based approach is proposed, which runs over a blade-element-method coupled with a stochastic modeling of the sea-state. The optimization method computes an overall fitness function given as a weighted average of the stochastic turbine efficiency over a set of most relevant sea-states. The suggested methodology is proven capable of finding the global fitness maximum upon a 6-dimensional parametric space. Additionally, it allows identifying and discarding the geometrical parameters that are shown to be negligible for the computation of the fitness function. Hence, a reduced parametric space results in a 2-dimensional region comprised by the hub-to-tip ratio and the tip solidity, which permits refining the search and analyzing their individual effects upon the turbine design. It is shown that the optimal configuration corresponds to trade-off solutions between the hub-to-tip ratio and the tip solidity, avoiding both an excessive rotational speed and an exceeding blockage of the turbine. • A holistic analytical tool is developed for optimizing Wells turbine design parameters. • A thorough mathematical derivation is provided for implementing the analytical code. • A novel, wave-climate-related cost function is proposed as an optimization metric. • A coarse optimization searching is shown to discard non-influential design parameters. • A refined optimization yields relationships among the effects of relevant parameters. [ABSTRACT FROM AUTHOR]

The decarbonisation of the energy system will imply the use of unexplored locations and technologies. In this sense, generation of green H 2 in far-offshore farms may be an alternative. However, the offshore green H 2 generation potential is commonly assessed by using constant conversion rates. This paper presents a holistic and dynamic mathematical model coupling the aero-hydrodynamic model with the polymer electrolyte membrane (PEM) electrolyser model. The model enables a more accurate and comprehensive analysis of the realistic H 2 generation capacity considering the power-to-gas system efficiency. First, the model of the PEM electrolyser is validated and a wide sensitivity analysis is computed in order to report the impact of the operational conditions. Then, H 2 production is assessed focused on (i) the stack performance and (ii) the system performance. The system performance results in a efficiency reduction of up to 10% in the case of FOWTs and up to 70% in WECs due to the fluctuations of the power signal. Hence, results highlight the need for a holistic system assessment instead of focusing on the stack. Finally, transient effects are considered, concluding that in highly fluctuating applications, such as renewables, external heat sources may improve the performance of the system. • A holistic & dynamic model coupling ORE systems with a PEM electrolyser is built. • Stack and System efficiencies are assessed under different operational conditions. • Stack efficiency is shown to be highest at low current densities, i.e. low partial loads. • System efficiency and economical viability are higher close to nominal power. • H 2 generation efficiency is poorer in WECs due to the more significant fluctuations. [ABSTRACT FROM AUTHOR]

Wind tunnel tests are carried out upon a NACA0021 airfoil subjected to transitional Reynolds numbers. Transitionally-operating airfoils show a high sensitivity to external conditions and pose relevant measurement issues for capturing the physical processes adequately. On the global side, the employed set of techniques measures lift forces directly and uses the momentum-deficit method for drag coefficients. Locally, the development of transitional structures is acknowledged via surface pressure measurements carried out by pressure taps together with oil-flow visualizations. The coupling of such techniques with a well-founded uncertainty analysis shows two relevant aspects of the measurement protocolization: on the one hand, the limitations of either the global or local methods for completely accounting for all transitional phenomena. On the other hand, the fact that combining the proposed set of different measurement techniques with a systematic protocol is a mandatory requirement for achieving a holistic characterization of transitionally-operating airfoils. • Load and pressure measurements, momentum-deficit method and oil-flow visualizations. • Uncertainty analysis addresses scale-effect on transitionally-operating airfoils. • Limitations of the piezoelectric balance for carrying aerodynamic drag measurements. • Surface pressure measurements and oil-flow visualizations limited to local analysis. • Different measurement techniques for performing a thorough aerodynamic characterization. [ABSTRACT FROM AUTHOR]