1,720,959 research outputs found
Advanced sequential dual evaporator domestic refrigerator/freezer: System energy optimization
Full text linkStudy of innovative techniques aimed at reducing energy consumption in domestic refrigeration system
Full text linkI refrigeratori domestici maggiormente diffusi nell'UE sono refrigeratori combinati frigorifero-congelatore tipicamente con due porte di apertura. I due scomparti operano a temperature molto diverse, tipicamente 4°C e -18°C. Tuttavia, solo i modelli con due compressori sono attualmente in grado di far evaporare il fluido refrigerante a due temperature di evaporazione molto diverse tra loro in modo avvicinarsi alle temperature dell’aria dei compartimenti.
L’attività di dottorato si focalizza su un particolare circuito frigorifero chiamato “sequential dual evaporator” (SDE, ovverosia circuito a doppio evaporatore sequenziale) che attiva gli evaporatori del reparto congelatore (FC) e del reparto frigorifero (RC) in modo alternato utilizzando un singolo compressore. In questo modo, solamente un solo reparto alla volta è raffreddato consentendo di raggiungere più alte temperature di evaporazione e, conseguentemente, maggiori COP durante il funzionamento del reparto RC. Il problema principale del sistema SDE è una potenza di raffreddamento estremamente elevata prodotta dal compressore volumetrico ed uno scambio termico modeste alle elevate temperature di evaporazione proprie del funzionamento del reparto RC.
Due prototipi innovativi di circuito SDE con compressore a velocità variabile sono introdotti in questo lavoro. Il primo prototipo impiega un materiale a cambiamento di fase (PCM) a contatto diretto con l’evaporatore del reparto RC per accumulare un’elevata quantità di energia durante il raffreddamento. Esaurita la fase di carica del PCM, questo scambia continuamente calore con l'aria del reparto RC con un meccanismo di convezione naturale con un tasso di scambio molto modesto che ha un effetto positivo sulla conservazione degli alimenti freschi. Di conseguenza, la temperatura dell'aria nel reparto RC è molto più stabile ed il reparto RC resistenza può operare per periodi prolungati anche in caso di interruzione improvvisa dell’alimentazione della rete eletrica.
Il secondo prototipo realizza un circuito aggiuntivo collegato alla linea del liquido in ingresso all’evaporatore del reparto congelatore. Il refrigerante, passando attraverso il PCM del reparto RC, va incontro ad un forte sottoraffreddamento che risulta in uno spostamento del carico termico dal reparto FC al reparto RC. Teoricamente lo spostamento del carico termico al reparto RC migliora il COP complessivo del refrigeratore domestico.
Le prestazioni di entrambi i prototipi sono state analizzate sperimentalmente. Le prestazioni del prototipo del circuito SDE con PCM sono state confrontate con le prestazioni del circuito SDE senza PCM. Il prototipo SDE-PCM ha mostrato un miglioramento significativo della COP del reparto RC fino al 29% ed un miglioramento del COP complessivo del refrigeratore domestico pari al 9%. Inoltre, l’analisi ha permesso di comprendere che la migrazione del refrigerante ed il design del condensatore svolgono un ruolo essenziale nel raggiungimento di elevate efficienze. Infine, la temperatura dell’aria del reparto RC è stata stabilizzata e il periodo di spegnimento del compressore per il raffreddamento del reparto RC è stato prolungato da 1 ora a quasi 13 ore.
Il prototipo di circuito che prevede il sottoraffreddamento aggiuntivo del refrigerante non ha mostrato dei benefici significativi in termini di efficienza energetica, lasciando aperta la possibilità a miglioramenti futuri di cui è stata delineata una traccia.The most popular cold appliances in EU are combined two doors refrigerator-freezers. The two compartments function at very diverse air temperatures usually 4°C and -18°C. Nevertheless only the models with two compressors are currently able to evaporate refrigerant at two significantly different evaporation temperatures to match compartments air temperatures.
This PhD activity focused on the particular refrigeration circuit called sequential dual evaporator (SDE) which operates freezer (FC) and refrigerator (RC) compartments evaporators in an alternative mode with single compressor. Hence only one evaporator is cooled at the time and high evaporation temperature and COP can be achieved during RC operation. The principal problem of the SDE system is an extremely large cooling capacity delivered by volumetric compressor and low heat transfer rate at increased RC evaporation temperature.
Two innovative concepts of SDE circuit with variable speed compressor are presented in this work. The first concept employs phase change material in direct contact with visible RC evaporator to accumulate high cooling capacity. PCM then continuously absorbs heat from the RC air by natural convection at very small heat rate which has positive effect on the fresh food preservation. The RC air temperature is much more stable and resistance of the RC to grid power failure is improved.
The second concept implements supplementary loop attached to the freezer liquid line, which by passing through RC PCM can additionally subcool liquid refrigerant and shift load from the FC to RC. Theoretically shifting thermal load to RC improves overall appliance COP.
Both concepts were turned to the appliance prototypes and experimentally tested. Performance of SDE-PCM concept was compared to the performance of the baseline SDE circuit without PCM. The SDE-PCM concept showed significant improvement in the RC COP up to 29% and overall appliance COP was raised by 9%. It was also understood that refrigerant migration and design of condenser play essential role in reaching high efficiency. RC air temperature was stabilized and the RC compressor OFF period was extended from 1 hour to almost 13 hours.
The concept with additional subcooling loop didn't confirm expected energy efficiency benefit and suggestions for further studies were drawn.DIPARTIMENTO DI ENERGIA25JOPPOLO, CESARE MARIABOTTANI, CARLO ENRIC
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
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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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