1,721,029 research outputs found
Investigation of compressors and dual-source heat pumps working with natural refrigerants
No full textThis thesis focuses on the investigation of the performance different types of compressors and the study of dual-source heat pumps using natural refrigerants.
Compressor efficiency is a crucial factor in improving system performance and is influenced by the operating conditions. Therefore, experimental studies were conducted on three different compressors. The first was a rotary compressor using CO2 as refrigerant, where the effects of compressor speed, high pressure, and evaporation temperature were analyzed, leading to the development of new performance correlations. The second was a large swept volume scroll compressor working with low-GWP refrigerants, with experimental data compared to existing numerical correlations. Lastly, a reciprocating compressor operating with propane was tested to investigate the impact of suction superheating. Additionally, a preliminary 3D CFD model of a reciprocating compressor was developed to investigate the heat transfer across the wall separating the suction and discharge chambers.
The first type of compressor was tested in a dual-source heat pump. Utilizing dual-source systems presents an effective solution for air-source heat pumps, which are typically limited by low outdoor air temperatures. An experimental study was conducted on a solar-air dual-source heat pump using CO2 as the refrigerant, designed to exploit air and solar radiation as thermal sources. This system features two evaporators: a finned coil heat exchanger and three PV-T collectors, which can operate individually or simultaneously. The configuration allowing the simultaneous use of both evaporators is entirely novel and enhances system performance by eliminating the need to manage a switch between the two sources. Performance comparisons were made between the three different evaporation modes: air-mode, solar-mode, and an innovative simultaneous-mode.
A numerical model of the solar-air dual-source heat pump was developed and validated against experimental data. The model can evaluate performances in different operating conditions. Each component was modelled using a physical approach, which enabled the creation of a new model for heat pumps using CO2 and propane for hot water production in the building sector. This comprehensive model facilitates the design of both the heat pump and the water circuit, while also enabling the simulation of the system's seasonal performance under various thermal demands and in different locations
Government Bond Yield Spreads
No full textIl presente lavoro raccoglie 4 contributi sul tema dei differenziali sui tassi di interesse esistenti tra i membri dell'unione monetaria europea.I investigate the determinants of sovereign yield spreads in EMU
Experimental study of low-GWP refrigerants in a chiller: system performance and compressor efficiencies
No full textComparative analysis of CO2 and propane heat pumps for water heating: seasonal performance of air and hybrid solar-air systems
No full textThis study numerically compares the seasonal heating performance (SCOP) of three 15 kW heat pumps using natural refrigerants: two with carbon dioxide and one with propane. The propane system is an air-source heat pump (R290-AHP). The carbon dioxide systems include an air-source heat pump (R744-AHP) and a dual-source solar-air heat pump (R744-SAHP) equipped with finned-coil and photovoltaic-thermal evaporators that can work simultaneously. In the case of a transcritical carbon dioxide cycle, the use of several low-temperature sources is a promising solution to improve the performance of the system by enhancing the exploitation of renewable energy sources. Since the efficiency of air and solar-based systems is related to weather conditions and location, there is the need for accurate models to evaluate the SCOP. In this work, a numerical model has been developed to design the three 15 kW heat pumps and assess the SCOPs under variable space heating and domestic hot water demand profiles, using climatic data for Rome and Strasbourg. The results indicate that the R290-AHP consistently achieves the highest SCOP, while the R744-AHP performs the lowest. The R744-SAHP overperforms the R744-AHP by approximately 4 % regardless of heating demand characteristics. In particular, the results show that the performance of the three heat pumps is significantly influenced by the distribution of the thermal load throughout the day. Specifically, when the thermal load is concentrated during daylight hours, the heat pumps can operate at a higher SCOP, especially for the R744-SAHP, and also increase, up to 44 %, the self-consumed photovoltaic energy produced
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
Anharmonic Assignment of the Water Octamer Spectrum in the OH Stretch Region
Full text linkWe interface the quasi-classical trajectory approach with an ab initio potential energy surface for water to assign the vibrational spectroscopical features of the OH stretch region of the water octamer cluster, which is considered to be a precursor of ice. An attempt by Li et al. to assign their recent reference experiment involved lower-level calculations based on an ad hoc scaled harmonic approach. Differently from the conclusions of this previous assignment, which invoked the contribution of 5 conformers and a solvated form of the water heptamer in the spectrum, we find out that the spectroscopic features can be related to the 4 conformers of the octamer lying lower in energy
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