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A building energy performance gap can be illustrated as the difference between the theoretical (methodologically defined) and the actual energy consumption. In EU countries, Energy Performance Certificates are issued when buildings are constructed, sold, or leased. This information is the first step in order to evaluate the energy performance of the building stock. In Serbia, when issuing an energy certificate, the adopted national methodology recognizes only energy consumption for heating. The main purpose of this paper is to evaluate the energy gap and estimate the relevance of an Energy Performance Certificate to meet the national energy efficiency or carbon target. An Energy Performance Certificate determines the theoretical residential and commercial building energy efficiency or its “design intent”. This research stresses the necessity of measuring and achieving reductions in actual energy consumption through system regulation and consumers’ self-awareness in buildings. The research compares the performance of the building stock (135) that is connected to the District Heating System (DHS), with its own integrated heat meter, to Individual Gas Boiler (IGB) systems (18), in the city of Novi Sad, Serbia, built after 2014. For the purpose of comparing energy consumption, 16 buildings were selected that are very similar in terms of design, operation, and location. The data used are derived from metered consumption data, official evidence of city service companies, and Energy Performance Certificates of the considered buildings. We have determined that IGB systems have a much wider specific annual performance gap (11.19–101 kWh/m2a) than the buildings in the DHS (3.16–18.58 kWh/m2a).

This paper deals with an analysis of a two-dimensional viscous fluid flow between the two parallel plates inclined with respect to the horizontal plane, where the lower plate is heated and the upper one is cooled. The temperature difference between the plates is gradually increased during a certain time period after which it is temporarily constant. The temperature distribution on the lower plate is not constant in x-direction, there is a longitudinal sinusoidal temperature variation imposed on the mean temperature. We have investigated the wave number and amplitude influence of this variation on the subcritical stability and the onset of the Rayleigh-Bénard convective cells, by direct numerical simulation of 2D Navier-Stokes and energy equation.

In this paper a new method for burned mass fraction - pressure relation, x-p relation, for two-zone model combustion calculation is developed. The main application of the two-zone model is obtaining laminar burning velocity, SL, by using a pressure history from a closed vessel combustion experiment. The linear x-p relation by Lewis and Von Elbe is still widely used. For linear x-p relation, the end pressure is necessary as input data for the description of the combustion process. In this paper a new x-p relation is presented on the basis of mass and energy conservation during the combustion. In order to correctly represent pressure evolution, the model proposed in this paper needs several input parameters. They were obtained from different sources, like the PREMIX software (with GRIMECH 3.0 mechanism) and GASEQ software, as well as thermodynamic tables. The error analysis is presented in regard to the input parameters. The proposed model is validated against the experiment by Dahoe and Goey, and compared with linear x-p relation from Lewis and Von Elbe. The proposed two zone model shows sufficient accuracy when describing the combustion process in a closed vessel without knowing the end pressure in advance, i.e. both peak pressure and combustion rates can be sufficiently correctly captured

Abstract. Obtaining all acceptable locations is one of the main tasks

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The goal of this paper is to establish the optimal operating regime of the observed perforated plate air/water heat exchanger in a wide range of parameters. The experimental investigation was carried out in a package of three perforated plates which were placed in the experimental chamber and heated by hot water. A fan with the variable air volume flow was connected to the experimental chamber, so the air flow rates varied from 100 to 300 m 3 /h, while the water flow varied from 0.03 to 0.06 m 3 /h. The thermocouples were attached to the surface of the middle perforated plate in the package along its upwind and downwind sides, as well as at the inlet and outlet of the chamber and between the perforated plates. During each experiment, the readings of thermocouples were recorded alongside with the air and water volume flow and temperatures of water at the inlet and outlet of the chamber. In order to predict the performance of the observed perforated plate heat exchanger, NTU-Effectiveness analysis was performed on the basis of the experiment results and analytical relations. Experimental results showed that the effectiveness of the perforated plate heat exchanger can be calculated the same as for the concentric tube counter flow. At the end of the paper, the optimal operating point in the range of varied parameters was determined.

Wind power assessment in complex terrain is a very demanding task. Modeling wind conditions with standard linear models does not sufficiently reproduce wind conditions in complex terrain, especially on leeward sides of terrain slopes, primarily due to the vorticity. A more complex nolinear model, based on Reynolds averaged Navier-Stokes equations have been used. Turbulence was modeled by modified two-equations k- model for neutral atmospheric boundary layer conditios, written in general curvelinear nonorthogonal coordinate system. The full set of mass and momentum conservation equations as well as turbulence model equations are numerically solved, knowing as computational fluid dynamics (CFD) technique. A comparison of the application of linear model and nonlinear model is presented. Considerable discrepancies of estimated wind speed have been obtained using linear and nonlinear models. Statistics of annual electricity production vary up to 30% of the model site. Even anemometer measurements directly at a wind turbine's site do not necessarily deliver the results needed for prediction calculations, as extrapolations of wind speed to hub height is tricky. The results of the simulation are compared by means of the turbine type, quality and quantity of the wind data and load factor. Finally, the comparison to the results with the measured data at 10, 30 and 50m.