• Energy Research

Abstract The paper suggests a multi-criteria approach for the energy refurbishment of historical buildings, proposing methodologies for the performance analysis, coupling several experimental and numerical studies. The target consisted in a rigorous evaluation procedure, in order to guarantee the necessary reliability of a numerical model of the system “building envelope/technical plants”, on which testing the technical and economical convenience of energy retrofit solutions. The paper collects the long work carried out by several Institutions during the last years, on Building “Palazzo dell’Aquila Bosco-Lucarelli”, a historical building located in Benevento (Southern Italy), currently analyzed in order to define the technical adoption of possible improving actions. The carried out studies, beyond the proposition of an operational methodology, are aimed to evidence a best-practice specified for the Italian territorial context, which has several historical buildings needing restoration. Carefully applied investigations, based on various methodologies and through several instrumentations, allowed the definition of a numerical model correspondent to the real building, defined also comparing the results with the historical requests of gas and electricity. Moreover, dynamic energy simulations tested the effectiveness, singularly and coupled, of several solutions for the building energy optimization. A significant potentiality of energy and economic optimization has been demonstrated.

Buildings account for an important share of global energy consumption and CO2 emissions, so increasing energy efficiency in buildings is essential to ensure an energy transition and sustainable development. In this study, we evaluate the energy benefits obtained through the application of a double-skin façade (passive façade). Subsequently, this is combined first with an airflow network and a control logic for opening windows (active façade) and secondly with transparent photovoltaic modules. The proposed measures are applied to an office building at the University of Naples Federico II. Building’s thermal and energy performance are evaluated using EnergyPlus software, starting from validated energy demands of the base buildings. Both passive and active façade provided a reduction in primary energy consumption for space conditioning, of about 17% and 28%, respectively, and in the total primary energy consumption, of about 4% and 9%, respectively. The best solution, with the maximum energy saving in total primary energy consumption, approximately 20%, is achieved with the active façade and with 80% of the outer layer of the double-skin façade covered by PV modules. The results show that transparent double-skin facades are promising and effective for energy retrofit.

This study presents new flow boiling heat transfer and pressure gradient results of R-134a flowing inside a 6.00 mm internal diameter, smooth and horizontal stainless steel tube. The measurements were made over a wide range of test conditions, where there is a lack of data in literature for the investigated geometry: evaporating pressure from 2.1 to 5.7 bar (saturation temperature between �8.8 C and 19.9 C), refrigerant mass flux from 197 to 472 kg m � 2 s � 1 and heat flux from 8.5 to 20.1 kW m � 2 . The experimental results allow to evaluate the dependence of the heat transfer coefficients and pressure gradients on the vapour quality, the saturation temperature, the refrigerant mass flux and heat flux. The new database of 254 points it is used determine the best predictive methods for heat transfer coefficients and pressure gradients; indeed, it is presented an analysis of the influence of the saturation pressure and of the mass flux on the error of these predictive methods.

The paper presents an exergetic analysis of a vapour compressor refrigeration plant when the refrigeration capacity is controlled by varying the compressor speed. The aim is performance evaluation of both the whole plant and its individual components. The analysis of the exergy flow destroyed in each device of the plant varying the compressor speed has been carried out in order to determine the relative irreversibility of the plant components. The vapour compression plant is subjected to a commercially available cold store. The compressor working with R22, R407C and R507 and designed for a revolution speed corresponding to 50 Hz supply current frequency, has been used varying the frequency in the range 30–50 Hz. In this range, the most suitable working fluids proposed as substitutes of R22, as R407C (R32/R125/R134a 23/25/52% in mass), R507 (R125/R143A 50/50% in mass) and R417A (R125/R134a/R600 46.6/50/3.4% in mass), have been tested. The variable-speed compressor is fitted with a pulse-width modulated source inverter (PWM) predominantly used in medium power applications due to its relatively low cost and high efficiency. The basic difference between variable speed refrigeration and conventional refrigeration systems is in the control of the system capacity at part-load conditions. The conventional refrigeration systems are characterized by compressor on/off cycles arising from by the thermostatic control. On the contrary when the inverter is used the capacity of the refrigeration system is matched to the load regulating the compressor motor speed. When the control of the compressor capacity is obtained by varying its speed there is an energy saving with respect to the thermostatic control. The best results of the exergetic analysis have been obtained using R22 followed by the non-azeotropic mixture designed as R407C that confirms, among the fluid candidates R22 substitution a better performance, shown also at the compressor nominal speed. Copyright © 2003 John Wiley & Sons, Ltd.

Abstract Thermal bridges are weak areas of the building envelope, determining heat flows higher than those characterizing the common dispersing surfaces (i.e., walls without discontinuities). This phenomenon induces uncontrolled thermal losses and hygiene problems, connected to the possible vapor condensation and mold growth. Presently, the numerical codes for the energy audits, as, for instance, EnergyPlus, carry out zero-dimensional (indoor air) and one-dimensional (conduction heat transfer) analyses, approximately estimating the thermal bridge effects on the seasonal heating demand of buildings. In this paper, results obtained by means of the simplified 1-D models are compared to those obtained by more sophisticated models 2-D or 3-D, in order to point out differences in terms of equivalent conductivity and thermal transmittance. Then, dynamic simulations compare the outcomes in terms of seasonal energy consumption. A typical office building has been considered, analyzing – according to three different approaches – the thermal bridge represented by the roof structure. The outcomes, with reference to several Italian climates, show that a proper modelling is necessary. In particular, an over estimation of the heat losses, determined by an approximate evaluation, induces higher cost of refurbishing, higher cooling energy requests in summer, minor thermal comfort in naturally ventilated buildings.

Abstract Building energy simulation programs – in most cases – solve heat conduction through walls by considering one-dimensional heat flows, neglecting thermal bridges. The paper shows a new method for implementing bi-dimensional and three-dimensional heat transfer in dynamic energy simulation software, allowing a great improvement of their capabilities. The new procedure starts from the theory of state space representation of transfer functions, and then introduces simplifications for reducing computational time and the required CPU sources. Starting from a first case study, aimed to verify the achievable correspondence of the simplified new method compared to the original one, two common thermal bridges have been deeply analysed, comparing the proposed methodologies and numerical solution based on using finite volume methods. The investigated building structures determine bi-dimensional heat flows because of discontinuities in both materials and geometry. By comparing the achieved outcomes to those derived by much more onerous CFD studies, several cases changing grid refinement, timestep, ambient condition and so on, the proposed method shows its suitability, with maximum errors never higher than 4.5%, also under hourly-variable outdoor temperature and solar radiation.

Abstract Long-period food storage vertical freezers are small cooling capacity appliances usually working with HC. The particular system architecture and the small thermal load allow the use of natural draft wires and tube heat exchangers with large heat transfer areas. Despite of the good performance achieved, high refrigerant charges with respect to the cooling capacity of the system are required, due to the high volumes of the heat exchangers. In this work an experimental analysis of an apparatus with a typical arrangement of the heat exchangers and capillary tube is presented. The system was first designed according to the current standards to reach high performances. Then the working conditions of the system were completely characterized experimentally in the whole range of refrigerant charges compatible with steady operation. Some general conclusions about the systems with this kind of architecture are furnished about the determination of the thermodynamic optimum operating condition and, hence, the determination of the optimum charge.

Abstract Several studies show the potential benefits achievable by recurring to roof vegetation. Really, little literature investigates the economic feasibility of such solution. The paper verifies utility of green roofs, under environmental and energy point of views, by considering all the aspects that influence their performances. With reference to several climates, intensity of rainfalls, needs of irrigation and kind of building use, a large parametric analysis evaluates the technical and economical feasibility of green roofs applied to a modern office building, considering various vegetations and different external coatings. The scarce amount of rainfall – and thus the irrigation cost – can nullify the savings in energy demand for air-conditioning. Moreover, even if green roofs show satisfactory performance if monthly rainfalls do not imply significant additional watering – the economic investigation shows scarce convenience for well-insulated buildings, above all if the higher initial cost of a green roof, compared to traditional roofing coating, is computed. Finally, cool roofs, by means of high-reflective and high-emissive coatings, are suitable solutions in warm climates, strongly improving the summer performances, with low extra costs for installation and maintenance.