1,721,016 research outputs found
PCM MELTING IN A BOTTOM-HEATED ENCLOSURE
No full textThe melting process of a Phase Change Material (PCM) within a bottom-heated enclosure featuring vertical metallic fins is studied on a fundamental level, combining theoretical and numerical approaches. A scale analysis is performed in the absence of fins in order to identify the order of magnitude of the melt layer height and to predict the transition time from the conduction-driven melting to the convection-driven one. The influence of the presence of vertical fins on the evolution of the melting interface is studied by means of numerical simulations. The obtained results highlight the significance of the aspect ratio, which relates the enclosure's height to the fin spacing, on the melting phase. In particular, three different thermal behaviors are detected: Rayleigh–Bénard cells are observed during the melting in the cases with small aspect ratios, whereas conduction dominates in the cases with large aspect ratios. Interestingly, with intermediate spacing configurations, solid drops are generated when the horizontal flow channels originating from the fins come into contact. These solid drops are observed to descend and ultimately complete their melting on the heated bottom surface of the enclosure. The results shed light on the fundamental processes involved in the PCM melting and the dependence on the aspect ratio within the enclosure, helping identify the best configurations of heat exchangers filled with phase change materials
A new numerical method to determine isothermal g-functions of borehole heat exchanger fields
No full textAn iterative numerical method to determine g-functions of borehole fields with the boundary condition of uniform temperature and time-constant surface-averaged heat flux is proposed. The method employs boundary conditions of uniform time-dependent temperature that converge to time-constant surface-averaged heat flux. Two simulations are sufficient for technical purposes, while additional simulations yield very precise g-functions with the desired boundary condition. The method is applied to analyze the overestimation of the g-function yielded by the condition of uniform and constant heat flux and the effects of the buried depth and of the spacing between boreholes on the g-function, for a 3 × 2 borehole field
Energy saving obtainable by applying a commercially available M-cycle evaporative cooling system to the air conditioning of an office building in North Italy
No full textWe analyze the energy saving obtainable by coupling a commercially available M-cycle evaporative cooling system to a conventional refrigeration cycle, for the air conditioning of office buildings in North Italy. The analysis is performed through the dynamic simulation of a case study. The hourly energy needs for cooling, dehumidifying and air change, during July and August, of a real office building located in Milan are considered. The climate of Milan is humid in summer, so that the example refers to critical conditions for the application of this technology in Italy.
Two systems with M-cycle evaporative cooling are analyzed and compared to a traditional system: M-cycle pre-cooling of the external air, coupled to a refrigeration cycle; M-cycle cooling of the recirculated air, coupled to a refrigeration cycle with heat recovery. The results show that the second application of the M-cycle offers the best performance: it yields a 37.6% reduction in energy extracted by the refrigeration cycle, a 76% reduction in heating energy, and a 38% reduction in the total use of electric energy, under the assumption of electric heating. The reduction in heating energy is due to avoiding reheating after dehumidification, without decrease in thermal comfort
Dynamic simulation during summer of a reversible multi-function heat pump with condensation-heat recovery
No full textA numerical code for the hourly simulation of reversible electric air-to-water heat pumps has been implemented. It applies to reversible multi-function heat pumps, which, during summer, are used for both space cooling and domestic hot water (DHW) production, with recovery of the condensation heat to produce DHW while cooling. The method can be used both for on-off heat pumps and for inverter-driven ones, coupled with storage tanks for air-conditioning and for DHW production, and integrated by a gas boiler for DHW. The numerical code has been used to evaluate the summer performance of the multi-function inverter-driven heat pump employed in the retrofit of a residential building in Bologna (North-Center Italy). The results show a 30% seasonal saving in non-renewable primary energy with respect to a traditional solution, where the heat pump provides only air-cooling and the gas boiler provides DHW. A validation of the code by comparison with TRNSYS, in the case of cooling-only operation, is provided
Sizing effects on the energy performance of reversible air-source heat pumps for office buildings
No full textThis paper deals with the numerical analysis of the energy performance of HVAC systems for heating and cooling, based on a reversible electrical air-source heat pump. The aim of this study is to highlight in which way the energy consumption of these systems is influenced by the heat pump modulating capability, the heat pump sizing and the climate. A numerical model based on the bin method and the energy signature procedure, able to take into account the performance of the heat pump at partial load, has been followed in order to calculate the seasonal and annual performance of the HVAC system coupled to a typical office building, located in three different European locations (Frankfurt, Istanbul, Lisbon). Different kinds of heat pumps (mono-compressor on-off, multi-compressor and inverter-driven heat pumps) have been considered. The numerical results presented in this paper point out that the heat pump sizing strongly affects seasonal and annual HVAC energy consumption: the ratio between the full thermal capacity of the heat pump and the building peak load strongly influences the energy performance of the system. It has been demonstrated that, depending on the season, the seasonal performance of the above-mentioned kinds of heat pumps are influenced in a different way by their sizing
Summer Performances of Reversible Air-to-water Heat Pumps with Heat Recovery for Domestic Hot Water Production
No full textAbstractA numerical model for the seasonal performance evaluation of electric air-to-water reversible heat pumps during summer is presented. The model employs the bin-method, as indicated by the standards EN 14825 and UNI/TS 11300-4, but also considers domestic hot water (DHW) production through condensation heat recovery. The model evaluates the heat pump Seasonal Energy Efficiency Ratio (SEER) as function of the heat pump typology (multi-compressor, inverter-driven). The energy saving potential of DHW production integrated with the heat pump cooling function with respect to traditional separate cooling and DHW devices is analyzed as function of the building demand and of the heat pump typology
Climate Influence on Seasonal Performances of Air-to-water Heat Pumps for Heating
No full textAbstractA mathematical model for the evaluation of the seasonal performances of electric air-to-water heat pumps for heating is used to analyze the efficiency of on-off heat pumps, multi-compressor heat pumps and heat pumps with inverter compressor, integrated by electric heaters as back-up system, in the service of several buildings located in different Italian climates.The paper points out the importance of a good dimensioning of the heat pump as a function of the building energy signature and of the climate of the city where the building is placed, in order to enhance the system seasonal efficiency
Annual Performances of Reversible Air Source Heat Pumps for Space Conditioning
No full textAbstractThe paper presents the results obtained by a numerical simulation of a heating and cooling system based on a reversible air-to-water electric heat pump and electric resistances as back-up. According to the procedure suggested by the current standards EN 14825 and UNI/TS 11300-4, by using the bin method the influence of outdoor conditions and of the typology of heat pump installed has been investigated by determining the value assumed by the seasonal coefficient of performance (SCOPon), the seasonal efficiency ratio (SEER) and the annual performance factor (APF). The numerical results allow discussing the rules for an optimal heat pump sizing in a fixed site
Influence of sizing strategy and control rules on the energy saving potential of heat pump hybrid systems in a residential building
Full text linkHybrid heat pump systems are a suitable solution to mitigate the well-known disadvantages of air-source heat pumps, such as energy losses linked to defrost cycles and a significant reduction of their heating capacity during the most severe part of the season. In hybrid systems the heat pump is sized to satisfy only a fraction of the building peak load and is coupled to a second heater (back-up device), which helps the heat pump during the coldest part of the winter. In this paper, a series of dynamic simulations has been performed to calculate the seasonal performance of hybrid systems based on an air-to-water heat pump and to assess the optimal configuration of the system. Results point out that the energy performance of these systems strongly depends on the heat pump sizing, the back-up device typology and the control algorithm used for the activation of the heat generators. It is demonstrated that the adoption of hybrid systems in which the heat pump is coupled to a gas boiler allows to obtain relevant primary energy savings. The overall seasonal efficiency can be increased up to 6% and 22%, if compared to monovalent systems respectively based on a heat pump or a gas boiler, only if the heaters are activated following an alternative operating mode, with a cut-off temperature selected between the design and the bivalent temperature. On the contrary, if the back-up device of the hybrid system is an electric resistance, the heaters have to work in parallel during the whole heating season and the only achievable advantage is that the heat pump can be slightly under-sized with respect to the nominal building load
Effetto del clima sull’efficienza stagionale di sistemi di riscaldamento basati su pompe di calore aria-acqua
No full textIn questo lavoro è utilizzato un modello matematico per analizzare l’effetto di diverse condizioni climatiche italiane sulle prestazioni stagionali di sistemi di riscaldamento a pompa di calore elettrica aria-acqua. Attraverso il modello impiegato, viene evidenziata l’importanza del dimensionamento della pompa di calore in funzione della tipologia di macchina (on-off, multi-compressore o con inverter), della firma energetica dell’edificio e del clima della località in cui si trova.This work employs a mathematical model to evaluate the effect of different typical Italian climates on the seasonal performances of heating plants based on electric air-to-water heat pumps. The importance of the right heat pump sizing is highlighted as a function of the machine typology (on-off, multi-compressor, inverter-driven), of the building energy signature and of the climate
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