1,721,676 research outputs found
Entropy generation analysisfor the design optimizationof solid oxide fuel cells
Full text linkPurpose - The aim of this paper is to investigate performance improvements of a monolithic solid oxide fuel cell geometry through an entropy generation analysis. Design/methodology/approach - The analysis of entropy generation rates makes it possible to identify the phenomena that cause the main irreversibilities in the fuel cell, to understand their causes and to propose changes in the design and operation of the system. The various contributions to entropy generation are analyzed separately in order to identify which geometrical parameters should be considered as the independent variables in the optimization procedure. The local entropy generation rates are obtained through 3D numerical calculations, which account for the heat, mass, momentum, species and current transport. The system is then optimized in order to minimize the overall entropy generation and increase efficiency. Findings - In the optimized geometry, the power density is increased by about 10 per cent compared to typical designs. In addition, a 20 per cent reduction in the fuel cell volume can be achieved with less than a 1 per cent reduction in the power density with respect to the optimal design. Research limitations/implications - The physical model is based on a simple composition of the reactants, which also implies that no chemical reactions (water gas shift, methane steam reforming, etc.) take place in the fuel cell. Nevertheless, the entire procedure could be applied in the case of different gas compositions. Practical implications - Entropy generation analysis allows one to identify the geometrical parameters that are expected to play important roles in the optimization process and thus to reduce the free independent variables that have to be considered. This information may also be used for design improvement purposes. Originality/value - In this paper, entropy generation analysis is used for a multi-physics problem that involves various irreversible terms, with the double use of this physical quantity: as a guide to select the most relevant design geometrical quantities to be modified and as objective function to be minimized in the optimization proces
Sensitivity analysis applied to the multi-objective optimization of a MCFC hybrid plant
Full text linkIn this paper, the multi-objective optimization of a molten carbonate fuel cell (MCFC) based hybrid plant fueled with landfill gas is performed. System operation is significantly affected by off-design conditions. These are due to variations methane concentration occurring as the landfill depletes, performance degradations of the components, particularly the fuel cell, and ambient conditions. For these reasons, the objective functions are defined considering the plant lifetime. Some of the parameters affecting the results, as the voltage degradation, the cost of fuel cell, the methane concentration and the unit cost of landfill gas can be only estimated or forecasted and their actual values are uncertain. Therefore, the optimization is performed considering a sensitivity analysis in order to estimate the effects of possible variations on the Pareto front. The following free design variables are considered: pressure and temperature operation of the MCFC, turbine inlet temperature, fuel mass flow rate. In addition, the optimal configuration of the heat exchanger network is selected for each set of the design variabl
Numerical Analysis of a medium scale latent energy storage unit for district heating systems
Full text linkThe present paper describes the application of computational fluid-dynamics (CFD) to the design and characterization of a medium scale energy storage unit for district heating systems. The shell-and-tube LHTES unit contains a technical grade paraffin (RT100) as phase change material (PCM) and uses water as heat transfer fluid (HTF). The system has been designed to transfer heat from the district to the building heating networks. After an initial description of the LHTES unit and a wide literature overview on the subject, the paper discusses the need for thermal enhancement to improve the thermal conductivity of the PCM. A solution based on a paraffin-graphite composite with a 15% graphite volume fraction has been found to be well performing in this particular application. Several operating scenarios characterized by heat requests ranging between 130 kWand 400 kWhave been explored and the main outputs presented as function of Re and St numbers. The timewise variations of other significant quantities such as liquid fraction, sensible and latent energy content, HFT outlet temperature and heat fluxes have been also presented and discussed. A final discussion on the possible system configurations shows that in comparison to traditional water storage systems for district heating, LHTES systems provide, depending on the chose alternative, higher energy storage densitie
Design improvement of circular molten carbonate fuel cell stack through CFD Analysis
Full text linkMolten carbonate fuel cell (MCFC) is a promising technology for distributed power generation. The core of an MCFC power generation unit is the stack, where various fuel cells are connected together in series and parallel in order to obtain the desired voltage and power. Stack geometry and configuration are major engineering topics, as inhomogeneous temperature or mass fractions cause inefficient performances of the fuel cells, as efficiency and power smaller than the expected and shorter lifetime. A detailed model is a useful tool to improve stack performances, through design improvements. In this paper, a 3D model of a stack composed of 15 circular MCFC, considering heat, mass and current transfer as well as chemical and electrochemical reactions is presented. The model validation is conducted using some preliminary experimental data obtained for an MCFC stack developed in the Fabbricazioni Nucleari laboratories. These results are examined in order to improve the stack configuration. It is shown that power density may be increased of about 20% through double side feeding. In addition, the average temperature gradients in the axial direction are reduced of more than 70%. Significant reductions in the temperature gradients, especially in transversal direction, can be achieved by adjusting the mass flow rate of cathodic gas supplied to the various cell
Malicious Cyber Operations Committed by Stateand Non-State Actors: The International Legal Landscape
No full textThe recent exponential increase in malicious cyberoperations
by both state and non-state actors is undermining national and
international peace and security, and delicate geo-strategic balances1.
This rise in threats in cyber space has become a critical global security
issue, as highlighted in the “Concept Note for the Security Council of
the United Nations”.
Aim of this paper is to focus on cyberoperations during
peacetime, and to serve as a preliminary foundation for the subsequent
chapters of this book, which will explore both the normative
frameworks and positions of non-Western countries in cyberspace,
and the roles of international organizations in promoting common
understandings, collaboration and international cooperation for the
sake of an open, secure, stable, accessible and peaceful digital
environmen
Cybersecurity governance and normative frameworks: non-western countries and international organizations perspectives
Full text linkInformation and communications technologies (ICTs) are not
anymore a new phenomenon, they are part of day-to-day life. Legal
discussion over ICTs, however, is still, ongoing, despite a wide
consensus on the applicability of international law to cyberspace. How
international law applies to ICTs is still a matter of discussion by
States, as disagreements and uncertainties have not been overcome
yet.
In the international discussion on how international law applies to
cyberspace, two fronts have emerged that are often described as 'the
West and the Rest'. The first includes North-American and European
countries which share common and rather coherent understanding of
the key issues of international law applicable to cyber operations. The
other category is broader and more diverse. Yet, in the legal discourse,
the non-Western countries are often addressed in an over-simplifying
manner that fails to consider the nuances between the opposing
interpretations and legal positions.
Thus, this book aims to bring together perspective of Non-
Western States and international organizations, in order to analyse and
bring forward the different approaches to international law and
cyberspace from a comparative perspectiv
Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancement
Full text linkThe present paper describes the analysis of the melting process in a single vertical shell-and-tube latent heat thermal energy storage (LHTES), unit and it is directed at understanding the thermal performance of the system. The study is realized using a computational fluid-dynamic (CFD) model that takes into account of the phase-change phenomenon by means of the enthalpy method. Fluid flow is fully resolved in the liquid phase-change material (PCM) in order to elucidate the role of natural convection. The unsteady evolution of the melting front and the velocity and temperature fields is detailed. Temperature profiles are analyzed and compared with experimental data available in the literature. Other relevant quantities are also monitored, including energy stored and heat flux exchanged between PCM and HTF. The results demonstrate that natural convection within PCM and inlet HTF temperature significantly affects the phase-change process. Thermal enhancement through the dispersion of highly conductive nanoparticles in the base PCM is considered in the second part of the paper. Thermal behavior of the LHTES unit charged with nano-enhanced PCM is numerically analyzed and compared with the original system configuration. Due to increase of thermal conductivity, augmented thermal performance is observed: melting time is reduced of 15% when nano-enhanced PCM with particle volume fraction of 4% is adopted. Similar improvements of the heat transfer rate are also detecte
Evaluation of a high-order central-difference solver for highly compressible flows out of thermochemical equilibrium
No full textA high-order shock-capturing central finite-difference scheme is evaluated for numerical simulations of hypersonic high-enthalpy flows out of thermochemical equilibrium. The scheme is an extension to thermochemical out-of-equilibrium flows of the technique presented in Sciacovelli et al. (2021) for high-speed flows in chemical nonequilibrium. It relies on a standard tenth-order accurate central-difference approximation of the inviscid fluxes, supplemented with a high-order accurate nonlinear artificial dissipation term of ninth-order accuracy in smooth flow regions. Close to flow discontinuities, a shock-capturing low-order term is activated based on a highly selective shock sensor. To enable robust and non oscillatory solutions in regions of strong discontinuities of the thermodynamic variables, including the vibrational temperature, a shock detector consisting of a combination of a pressure-based term and Ducros’ vorticity/dilatation sensor is applied to all conservation equations except that of vibrational energy. For the latter, a shock sensor based on the vibrational temperature itself is adopted instead, to account for the loose coupling between vibrational energy and pressure. The accuracy and robustness of the proposed approach is demonstrated for a variety of thermochemical non-equilibrium configurations, ranging from one-dimensional benchmarks to three-dimensional turbulent flows, for which the ILES capabilities of the selective high-order numerical dissipation are also showcased
Taking cyberattacks seriously: the (likely) Albanian cyber aggression and the Iranian responsibility.
No full textAim of this paper is the analysis of the likely Albanian cyber aggression launched on July and September 2022 by two groups of cyber criminals, presumably acting from the Islamic Republic of Iran. The Author starts from the examination of the forensic activity to attribute these attacks. The first part of the examination deals with the technical attribution and it looks for the digital evidence of the attacks to attribute them to the Iranian criminal group.
The second part deals with the use of the technical attribution to conduct the legal attribution to declare the Iranian State responsibility. The latter can be articulated in the responsibility i) for ordering a cyber-attack that has reached the threshold of the prohibition of force, under Article 2, par. 4, of the United Nations Charter, ii) for violating the principle of international law of due diligence, because eventually Iran did not avoid that the attacks have been conducted from the digital infrastructures of its territory. The latter solution seems to be preferable based on the digital evidence open source.
Lastly, the article shed light on the role of private security tech companies, like Mandiant and Microsoft, in attribution because they perform government-like roles, as they possess the technologies to investigate that are needed by States, but without check and balances and any public constraints, that is usually applied to national intelligence systems
Protagonista o spettatore del proprio destino? Intervista postuma a Imre Kertész
No full textL`articolo propone un`intervista immaginaria al premio Nobel ungherese Imre Kertész (1929-2016), sui seguenti temi: a) scelta della lingua autoriale; b) qualità della missione dello scrittore dopo Auschwitz; c) significato dell`Olocausto come cultura; d) importanza della riflessione intellettuale sui temi più generali della storia contemporanea.Le citazioni su cui si basa l`intervista provengono dall`opera postuma di Kertész Lo spettatore. Annotazioni 1991-2001, appena uscita da Bompiani nella traduzione di Antonio Sciacovelli </p
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