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Effect of the borehole thermal resistance on the performance of a ground-coupled heat pump system
No full textThe effects of the borehole thermal resistance on the annual performance of a ground coupled heat pump system
are analyzed by means of a dynamic simulation code developed by the authors, based on the g-functions
determined by Zanchini and Lazzari (Energy 59, 2013, 570–580; Energy 70, 2014, 444–455). The code simulates
the borehole heat exchangers, the heat pump and the storage tanks, and applies to multifunction heat pumps
with or without a variable-frequency inverter.
The hourly heat loads for heating, cooling-dehumidifying and domestic hot water production of a residential
building with 6 apartments located in Bologna, North-Center Italy, are considered. The ground heat exchanger
examined is composed of two BHEs with diameter 152 mm, each 150 m long, at a distance of 6.08 m from each
other, placed in a soil with thermal conductivity 1.8 W/(mK). Four different BHE kinds are considered: single U-tube
with shrank spacing 94 mm and grout thermal conductivity either 1 W/(mK) or 1.6 W/(mK), double U-tube
with shrank spacing 85 mm and grout thermal conductivity either 1 W/(mK) or 1.6 W/(mK). The working fluid is a
mixture of water and 25% ethylene glycol, with flow rate 16 liters per minute per BHE.
The results show that, due to higher shrank spacing and higher Reynolds number, the annual performance of
single U-tube BHEs is almost equal to that of double U-tube ones, while the grout thermal conductivity has a
slightly more relevant effect. Replacing single U-tube BHEs with grout thermal conductivity 1 W/(mK) by double
U-tube BHEs with grout thermal conductivity 1.6 W/(mK) yields about 1% reduction of the annual use of electric
energy by the heat pump
Valutazione delle prestazioni energetiche ed ambientali di impianti basati su pompe di calore operanti con diversi fluidi refrigeranti
No full textIn questo lavoro sono state valutate attraverso il software TRNSYS le prestazioni energetiche ed ambientali ottenibili da un impianto di tipo idronico, basato su una pompa di calore elettrica ed accoppiato ad un edificio mono-familiare di recente costruzione situato a Padova. In particolare, sono state considerate due tipologie di pompa di calore, una macchina aerotermica ed una geotermica, entrambe operanti con fluidi refrigeranti alternativi: R-410A e R-454B.
Le simulazioni numeriche effettuate hanno confermato come l’utilizzo di una pompa di calore geotermica permetta di incrementare le prestazioni energetiche complessive dell’impianto di circa il 10%, a parità di refrigerante. L’impatto ambientale delle configurazioni impiantistiche studiate è stato valutato per mezzo del TEWI (Total Warming Equivalent Impact). L’utilizzo del refrigerante R-454B comporta da un lato un lieve calo delle prestazioni energetiche complessive dell’impianto, pari al 2-3% su base annuale, mentre consente di ottenere una marcata diminuzione del TEWI. Considerando infatti l’attuale fattore di emissione italiano dell’energia elettrica prelevata da rete, le emissioni totali lungo l’intero ciclo di vita della pompa di calore calano di circa il 25% grazie all’utilizzo del fluido a basso GWP. Tuttavia, seguendo l’attuale trend di riduzione del fattore di emissione ed assumendolo nullo al 2050, tale riduzione può arrivare in prospettiva fino all’80%, azzerando così le emissioni indirette di CO2 dell’impianto
Comparison between metallic-foam and finned-tube water-to-air heat exchangers
No full textThe thermal performance of two water-to-air heat exchanger prototypes, with the air-side conventional finned surface replaced by metal foams, is investigated. Two different metal-foam packaging techniques (S-type, K-type) are tested. The heat exchangers are inserted in the same fan-coil case, equipped with a 3-speed fan, to compare their performance under the same operative conditions. A 10 PPI aluminum foam with 96% porosity shows similar pressure drops and air flow rates with respect to the traditional finned fan-coil. Nonetheless, a strong penalty in terms of heat transfer rate is observed with respect to the conventional finned surface, due to the worse tubes-foam thermal contact and to the 60% lower surface-to-volume ratio of the metal-foam. Due to the presence of by-pass corridors for air, the performance of the K-type heat exchanger is worse than that of the S-type. Also the fin efficiency of the metal foam prototypes is lower than that of the traditional heat exchanger. The experimental data highlight that metal foams cannot compete with conventional fins if the tubes-foam thermal contact is not as good as that of finned surfaces
Influence of the hydronic loop configuration on the energy performance of a CO2 heat pump for domestic hot water production in a multi-family building
Full text linkIn this work, a numerical analysis of the influence of the hydronic loop on the energy performance of a CO2 heat pump dedicated to DHW production for an apartment
block located in Bologna (Italy) is presented. The energy model of the whole heating system, implemented in TRNSYS17, has been validated by means of a monitoring
campaign performed during the winter season of 2017- 2018. The experimental results highlighted a poor and unexpected energy performance of the heat pump. The comparison between experimental and numerical results showed a significant penalty of the heat pump performance due to an erroneous use of the hot stratified
thermal storage system. Outcomes of this paper confirm that CO2 heat pumps are very sensible to the temperature of the fresh water at the inlet of the gas cooler. This value can be strongly reduced thanks to the presence of the stratified tank in the hydronic loop
Experimental Study of A LTES Made By a Finned Heat Exchanger Immersed In a Paraffinic PCM
No full text–In this paper, a new Latent Thermal Energy Storage (LTES) system, made by a finned tube heat exchanger immersed in the commercial paraffinic Phase Change Material (PCM) RT44HC is studied experimentally. The storage capacity and the thermal performance of the LTES during both melting and solidification processes of the PCM was analysed for different test conditions using water as heat transfer fluid. In particular, experimental tests were performed by considering several values of hot and cold water temperature at the heat exchanger inlet, ranging from 55 to 70 °C and from 14 to 29 °C during charging and discharging processes, respectively. For each test, three different values of the water flow rate through the heat exchanger (25, 50, 100 kg/h) were considered. Results show how the thermal power increases considerably by charging (discharging) the LTES with higher (lower) inlet water
temperature and with a higher water mass flow rate. For a fixed inlet water temperature and for growing flow rates, an increase in thermal power up to 1.16 kW occurs. For fixed flow rates, a thermal power increase of 49.5% is registered going from 50 to 70°C of the inlet water temperature. The thermal performance of the LTES are also compared with those obtained using pure water as heat storage material, outlining that the storage capacity of the system using PCM instead of water is increased of about 300-400%
Numerical modelling and seasonal performance analysis of air-to-water heat pumps using low-GWP refrigerant R-454B as an alternative to R-410A
No full textIn this paper, a numerical model for air-to-water heat pumps is presented and validated with experimental data. The model is used with the aim to test the differences, in terms of seasonal energy performances, obtained by using the low-GWP refrigerant R-454B as alternative to R-410A. Each component of the heat pump (i.e. compressor, heat exchangers, fans, expansion valve) is modelled and its thermodynamic performance is evaluated following nested iterative cycles. Different configurations of the heat pump have been simulated. Results confirm that R-454B is a valid alternative to R-410A. An increase of the heat pump seasonal energy
efficiency ratio (SEER) up to 2.5% can be obtained with the low-GWP refrigerant
Dynamic modelling of a dual-source heat pump system through a Simulink tool
Full text linkIn this paper, the performance of a reversible Dual-Source Heat Pump (DSHP) system, able to exploit renewable energy from, alternatively, air and ground sources, is evaluated by using Matlab-Simulink. The actual source exploited depends on a simple control strategy on the basis of the current external air temperature. Yearly dynamic simulations have been carried out by coupling the DSHP to a detached residential building located in Bologna, in which heating and cooling loads are strongly unbalanced, and coupled to a borehole heat exchangers (BHEs) field. Different case studies have been analysed in which the length of the borefield has been modified. The obtained results show that an optimal switching temperature can be determined to maximise the Annual Performance Factor (APF) for a fixed BHE field length. Additionally, it has been demonstrated that DSHPs can be very useful in order to reduce the total required length of the borehole heat exchangers and, consequently, the associated costs, and to solve the problems linked to the ground temperature drift, which can be originated by the presence of an undersized borefield and/or by unbalanced building loads. As a consequence, DSHPs can be suggested for the retrofitting of traditional ground-coupled heat pump systems in presence of undersized BHEs
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
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