1,720,978 research outputs found
Assessment and perspectives of heat transfer fluids for csp applications
Full text linkDifferent fluid compositions have been considered as heat transfer fluids (HTF) for concentrating solar power (CSP) applications. In linear focusing CSP systems synthetic oils are prevalently employed; more recently, the use of molten salt mixtures in linear focusing CSP systems has been proposed too. This paper presents a comparative assessment of thermal oils and five four nitrate/nitrite mixtures, among the ones mostly employed or proposed so far for CSP applications. The typical medium-size CSP plant (50 MWe) operating with synthetic oil as HTF and the “solar salt” as TES was considered as a benchmark. In the first part of the paper, physical properties and operation ranges of different HTFs are reviewed; corrosion and environmental issues are highlighted too. Besides an extensive review of HTFs based on data available from the open literature, the authors report their own obtained experimental data needed to thoroughly compare different solutions. In the second part of the paper, the impact of the different HTF options on the design and operation of CSP plants are analyzed from techno-economic perspectives
Thermophysical, environmental, and compatibility properties of nitrate and nitrite containing molten salts for medium temperature CSP applications: A critical review
No full textThe production of electric energy from solar radiation is nowadays one of the most investigated and developed “carbon free” technology. A throughout investigation of the ceramics most commonly used as heat transfer fluids and/or heat storage materials for concentrating solar power systems (i.e. alkaline and heart alkaline nitrate/nitrite mixtures) is here reported. The study stems by the need to base materials selection on an accurate and critical knowledge of all their characteristics, including their thermophysical, environmental compatibility, and economic features. At this purpose, a rating criterion have been established, to readily show the advantages and disadvantages of each material, and to highlight which characteristics of the examined materials need to be further investigated and improved. Nitrate/nitrite mixtures have also been compared with other ceramics used for thermal storage, such as solid fillers, liquid metals, other salt mixtures, or phase change materials
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Performance of an indirect packed bed reactor for chemical energy storage
No full textChemical systems for thermal energy storage are promising routes to overcome the issue of solar irradiation discontinuity, helping to improve the cost-effectiveness and dispatchability of this technology. The present work is concerned with the simulation of a configuration based on an indirect-packed bed heat exchanger, for which few experimental and modelling data are available about practical applications. Since air shows advantages both as a reactant and heat transfer fluid, the modelling was performed considering a redox oxide based system, and, for this purpose, it was considered a pelletized aluminum/manganese spinel. A symmetrical configuration was selected and the calculation was carried out considering a heat duty of 125 MWth and a storage period of 8 h. Firstly, the heat exchanger was sized considering the mass and energy balances for the discharging step, and, subsequently, air inlet temperature and mass flow were determined for the charging step. The system performances were then modelled as a function of the heat exchanger length and the charging and discharging time, by solving the relative 1D Navier-Stokes equations. Despite limitations in the global heat exchange efficiency, resulting in an oversize of the storage system, the results showed a good storage efficiency of about 0.7
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Chemical CSP storage system based on a manganese aluminium spinel
No full textChemical storage systems are a promising innovative route to overcome the issue of the solar irradiation storage, resulting as cost effective and with high energy density. A main problem with these kinds of materials is to design a synthesis method for preparing stable reactive structures, presenting at the same time a high volumetric charging/discharging enthalpy. At this purpose, a size controlled spinel was produced, characterized and investigated regarding its thermophysical and kinetics properties. The obtained powder presents an average diameter between 100 and 200 μm and an energy density of 133 J/g and an experimental test was carried out to verify the spinel morphology stability under thermal cycles. The specific heat is similar to other structured chemical storage system and makes the spinel feasible to be used also as sensible accumulation medium. Despite the relatively high particles size, and the expected small exposed reactive area, the charging, and especially discharging reaction rates resulted particularly favourable and comparable with the reported behaviour of micrometric powders. The particularly simple preparation method plus the cost effectiveness of the precursors leads to a quite convenient expected cost for the storage material, absolutely similar to commercially available accumulation systems
Predictive model for the phase diagrams of ternary mixtures composed of calcium, lithium and sodium/potassium nitrates
Full text linkThe study and development of multi-component alkaline nitrates systems is of utmost interest to investigate low melting and low cost HTFs and HSMs, to be used in CSP plants. At this aim, several binary and ternary mixtures have been proposed as feasible and promising materials in the scientific literature. In order to evaluate the most feasible HTF and HSM, the simulation campaign proposed in this work was focused on economical and readily available compounds, ruling out rare and costly species. A semi-predictive model, based on the regular solution theory, was employed to draw the ternary phase diagrams of LiNO3/NaNO3/Ca(NO3)2 and LiNO3/KNO3/Ca(NO3)2, from which it was possible to select the lower melting zones and the compositions. The binary sub-systems were the starting point for the employed approach. The results showed a good agreement with literature experimental data, and new low melting compositions were simulated and experimentally validated. It is noteworthy that it was possible to identify in great detail the zones of the two ternaries presenting the lowest freezing points and this can allow the design of interesting and cost-effective low melting mixtures
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Performance of an indirect packed bed reactor for chemical energy storage
No full textChemical storage systems (CS TES) are promising innovative routes to overcome the issue of the solar irradiation discontinuity, in order to make the CSP technology cost effective and with high energy density.
The aim of the present work concerns the simulation of a TES system based on an indirect-packed bed heat exchanger (HX). The purpose is to investigate these types of configurations as few experimental and modelling data are available about practical applications.
Given the possibility to use air both as reactant and HTF, the simulation was performed taking into account a spinel-based system characterized in a previous work, presenting low cost and toxicity and prepared with a size feasible for packed bed reactors.
A symmetrical configuration was selected for the modelling and the calculation was carried out considering a 125MWth and a storage period of 8 hours.
Firstly, considering the mass and energy balances for the discharging step, the minimum size of the heat exchanger was calculated and then, for the charging step, the HTF inlet temperature and mass flow were determined. By solving the relative 1D Navier-Stokes equations, the performances in function of the HX length over the charging and discharging time were modelled. Despite the necessity to use relatively high temperatures in the charging phase, the results show a good storage efficiency of the system considered
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