1,720,957 research outputs found
Theoretical and experimental study of solar thermal collector systems and components
Full text linkFlat-plate and evacuated tube collectors are the most widely used device to convert solar radiation into heat. In conventional applications they can provide energy for domestic hot water or space heating in combination with low water temperature systems.
Testing of thermal efficiency and optimisation of these solar thermal collectors are addressed and discussed in the present work.
A new set of experimental data has been used to evaluate the performance of flat-plate and evacuated collectors at various conditions. Efficiency is measured following the standard EN 12975: standard efficiency curves, input/output energy curves and daily efficiency curves have been obtained from tests.
This approach allows a more comprehensive comparison of the effective performance of the solar thermal collectors considered here.
A simulation tool of the flat-plate thermal collector is also proposed. The numerical model permits to obtain a “virtual collector” and provides information on the predicted performance without constructing prototypes. The three-dimensional steady-state model for flat-plate solar collector is validated comparing model predictions with experimental results obtained from experimental tests on two flat-plate collectors. Measures of the temperature profile in the copper sheet, in the glass cover and in the tubes, and an infrared analysis, are used to investigate the heat losses and optical characteristics.
The numerical model has been also used to analyse comparative effects of varying design choices, with regard to geometry and materials.
The evacuated tube collectors are in principle more expensive than flat-plate collectors and they are suitable for more higher temperature systems (space heating, absorption cooling machines..). Their geometry makes more difficult to fully characterize the collector performance. A new procedure to characterize these type of collectors has been developed and described in this thesis. Efficiency tests have been performed on a U-tube evacuated tubular collector, using a cylindrical absorber, both with and without external CPC (compound parabolic concentrator) reflectors.
Experimental results have been used to validate the new procedure and to provide information on the improvement of collectors’ performance obtained through the use of external CPC reflectors.I collettori solari termici maggiormente impiegati per convertire la radiazione solare in calore sono i collettori a tipologia piana e a tubi evacuati. Nelle applicazioni convenzionali essi vengono utilizzati per la produzione di acqua calda sanitaria o riscaldamento domestico in combinazione con sistemi a bassa temperatura.
Prove d’efficienza termica ed ottimizzazione di questi dispositivi solari sono affrontati e discussi in questa tesi.
Nuove prove sperimentali in varie condizioni sono state effettuate su collettori piani vetrati e a tubi evacuati per effettuare una nuova analisi comparativa. L’efficienza è stata misurata secondo le indicazioni della normativa europea EN 12975 ed inoltre si sono ricavate curve input-output e di efficienza media giornaliera.
Questo approccio permette un confronto più ampio delle prestazioni effettive dei collettori solari considerati.
Uno strumento di simulazione delle prestazioni dei collettori a geometria piana è anche proposto. Il modello numerico permette di ottenere un “collettore virtuale” e fornisce informazioni sulle prestazioni di un collettore senza costruirne un prototipo funzionale. Il modello tridimensionale stima le prestazioni di un collettore a geometria piana in condizioni stazionarie ed è stato validato confrontando le prestazioni stimate con i risultati sperimentali ottenuti su due collettori diversi. Misure di temperatura della piastra assorbente, della copertura vetrata, dei tubi dell’arpa ed analisi all’infrarosso sono stati utilizzati per investigare le dispersioni termiche e le caratteristiche ottiche della geometria considerata.
Il modello è inoltre stato usato per analizzare gli effetti di alcune scelte progettuali riguardanti la geometria e i materiali dei componenti del collettore.
I collettori solari a tubi evacuati sono in linea di principio più costosi dei collettori piani e sono idonei ad applicazioni a più alta temperatura (sistemi di riscaldamento, macchine ad assorbimento..). La loro geometria rende più difficile la caratterizzazione completa delle prestazioni del collettore. Una nuova procedura per caratterizzare questo tipo di collettori è stata sviluppata ed è qui descritta. Sono state eseguite prove sperimentali su un collettore a tubi evacuati, con tubi ad U, con assorbitore cilindrico, con e senza riflettori CPC (compound parabolic concentrator) esterni.
I risultati sperimentali sono stati utilizzati per validare la nuova procedura e per fornire indicazioni sull’incremento dell’efficienza dovuto all’impiego dei riflettori CPC
An improved procedure for the experimental characterization of optical efficiency in evacuated tube solar collectors
No full textThe standard EN 12975-2 provides guidelines for testing solar collectors both in stationary and quasi-dynamic conditions. The second test method allows the optical efficiency of flat-plate collectors and even evacuated tube collectors to be determined by applying the extended multiple linear regression. However, in the case of tubular shape collectors, the available procedure requires a large number of data, above all for the determination of the transversal incidence angle modifier, which is the parameter describing the optical response of the absorber tube to the direct beam on the plane normal to the tube axis. Here, an improved procedure to determine the transversal incidence angle modifier is presented and validated against experimental data. For this purpose, efficiency tests in quasi-dynamic conditions have been performed following the standard EN 12975-2 on a U-tube evacuated tubular collector, using a cylindrical absorber, both with and without external CPC (compound parabolic concentrator) reflectors. The validation has been performed by comparing the efficiency curve and the curve of incidence angle modifier to the ones that are obtained by means of other available methods. The main advantages of the present new procedure are the followings: it provides a continuous curve of the incidence angle modifier and it does not require to subdivide the incidence angle range in many intervals. Therefore, it does not require a minimum number of data points for each data subset and thus it is less demanding in terms of required number of tests. © 2011 Elsevier Ltd
Development and experimental validation of a numerical model for flat-plate solar collectors
No full textExperimental analysis of thermal performance of flat plate and evacuated tube solar collectors in stationary standard and daily conditions
No full textNew comparative tests on two different types of solar collectors are presented in this paper. A standard glazed flat plate collector and an evacuated tube collector are installed in parallel and tested at the same working conditions; the evacuated collector is a direct flow through type with external compound parabolic concentrator (CPC) reflectors.
Efficiency in steady-state and quasi-dynamic conditions is measured following the standard EN 12975-2 and it is compared with the input/output curves measured for the whole day.
The first purpose of the present work is the comparison of results in steady-state and quasi-dynamic test methods both for flat plate and evacuated tube collectors. Beside this, the objective is to characterize and to compare the daily energy performance of these two types of collectors. An effective mean for describing and analyzing the daily performance is the so called input/output diagram, in which the collected solar energy is plotted against the daily incident solar radiation. Test runs have been performed in several conditions to reproduce different conventional uses (hot water, space heating, solar cooling).
Results are also presented in terms of daily efficiency versus daily average reduced temperature difference: this allows to represent the comparative characteristics of the two collectors when operating under variable conditions, especially with wide range of incidence angles
Experimental daily energy performance of flat plate and evacuated tube solar collectors
No full textNew comparative tests on different types of solar collectors are presented in this paper. Tests have been performed at the solar energy conversion laboratory of the University of Padova. Two standard glazed flat plate collectors and one evacuated tube collector are installed in parallel; the evacuated collector is a direct flow through type with external CPC (compound parabolic concentrator) reflectors. The present test rig allows to make measurements on the flat plate, on the evacuated collector or on both simultaneously, by simply acting on the valves to modify the circuit. In this paper measurements of the performance of the evacuated tube collector and flat plate collectors working at the same conditions are reported. Efficiency in stationary conditions is measured following the standard EN 12975-2 [1] and it is compared with the input/output curves measured for an entire day. The main purpose of the present work is to characterize and to compare the daily energy performance of the two types of collectors. An effective mean for describing and analyzing the daily performance is the so called input/output diagram, in which the collected solar energy is plotted against the daily incident solar radiation. Test runs have been performed in several conditions to reproduce different conventional uses (hot water, space heating, solar cooling).</jats:p
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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