• 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.

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 A new analytical methodology aimed at characterizing the energy performance of new and existing buildings is proposed for both winter and summer and with reference to entire urban context, and the methodology is transferred into Geographic Information Systems (GISs). The main target is the evaluation of criticalities of the building stock, promoting effective design for new buildings and proper refurbishment of existing buildings. In this regard, the application of standardized procedures is too onerous if the goal is the qualitative distribution of the energy performances with reference to entire urban areas. The energy demands for space heating are evaluated by simplifying the methodology of the European Standard EN ISO 13790, while with reference to summer cooling, a new hybrid method has been specifically developed. The whole method was applied to the entire historical center of the Italian city of Benevento and, thus, to more than 500 buildings. Moreover, comparisons with measured/evaluated energy requests have been performed to identify the reliability of the methodology. Information on energy performances in the whole city center, moreover, allowed for the setting of “Urban Energy Maps,” a tool dedicated to local government, usable for advanced energy planning and territorial control in energy supply.

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.

In the vapour compression plants possible leaks can vary the composition of a zeotropic refrigerant mixture. The main aim of this paper is to verify experimentally if the plant performances are restored with a proper recharge, when leaks are imposed. As for the R407C mixture the percentage of the less volatile component (R134a) increases, while the percentage of the more volatile components (R32 and R125) decreases when leaks occur. The experimental tests have been realized determining refrigerant leaks from the liquid receiver placed at the condenser outlet and measuring the properties in steady state condition. The recharge is realized in different ways: with R407C, with R134a and R410A or with R32 R125 and R134a. The different solutions are then compared from the economical point of view, in terms of COP, refrigeration power and air temperature at the evaporator outlet, adopting an iteration method. This method determines the real composition of a zeotropic mixture working in a compression plant, when only the temperature and pressure values at the expansion valve inlet and outlet are known.