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The aim of this research is to study the drying kinetics of vacuum-dried and freeze-dried bee honey produced from two different varieties: Sunflower honey (Helianthus Annuus L.) and Acacia honey (Robinia pseudo acacia L.). Vacuum drying treatments were carried out with the honey samples? initial temperatures of +25?C, -20?C and -40?C. Water content, total soluble solids, as well as the water activity of fresh and dried honey samples were determined. Freeze-drying of bee honey with initial sample temperature of -40?C has resulted in shorter drying time (7-9 hours), moisture content (10%-12%), water activity (0.405-0.427aW) and effective moisture diffusivity coefficient (8.26?10- -9.51?10-7[m2s-1]). The HPLC method was used when analyzing the impact that drying pre-treatments had on honey quality. The application of pre-treatments has led to an increase in hydroxy-methyl-furfural by 39% -71%, and a decrease in diastase activity by 17% -36%, all compared to fresh honey samples. The solutions of Verma model proved to be the best fit with the experimental results. [Projects of the Serbian Ministry of Education, Science and Technological Development, Grant no. 451-03-68/2020-14/200116 and Grant no. 451-03-68/2020-14/200105]

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The paper presents a mathematical model of the solar water heating system. The analyzed solar system consists of flat collectors, two water storage tanks, outer heat-exchanger and a source of auxiliary energy. The implemented model enables simulation of system operation and determination of appropriate energy characteristics of the system and its elements for different locations (climatic data), orientations of collectors, thermal characteristics of system components, as well as different dynamics of hot water consumption. Results of the simulation are presented for the days of a typical meteorological year for Belgrade and the daily profile of hot water consumption. The analysis of the energy performances of the system is presented. The results obtained by simulation were compared with the results obtained with the f-chart method of calculation.

In this article are presented experimental and numerical determinations of thermal transmittance performed on three different types of window frames (vinyl, aluminium and wooden) within the same insulated glass unit. Good agreement between experimental and numerical results was attained. Using the numerical models, thermal improvement techniques of the frames and their influence on thermal transmittance of frames were studied. The first thermal improvement technique was using the insulation materials inserted inside large air cavities. By filling the cavity of vinyl frame with the polyurethane foam, thermal transmittance of vinyl frame was lowered by 10%. The second technique was based on repeating the procedure with materials installed inside frames with the materials that have lower thermal conductivity. This technique can be applied on thermal breaks and on steel profiles inside cavities. The result of this thermal improvement (attained by replacing thermal break material with material that has lower thermal conductivity) was certain reduction of the thermal transmittance of frames, by 9%. Using stainless steel instead of the oxidized steel was reduction of the thermal transmittance of vinyl frame by 3%. For the case of wooden frames was analysed the influence of shifting glazing unit deeper into profile upon the thermal transmittance of the frame. Installing the glass unit by 5 mm deeper into the wooden frame reduced glass thermal transmittance by 5%.

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На основу података Енергетског биланаса Републике Србије за 2017. годину, потрошња финалне енергије је процењена на 8429 Mtoe, од чега је 48% укупне потрошње у сектору домаћинства, пољопривреда и јавно-комуналне делатности. У укупној структури коришћења финалне енргије, обновљиви извори енергије (само геотермална енергија и биомаса заједно) учествују са 12%. Осим квантитативне (енергијске) анализе, за потпуно разумевање свих важних аспеката коришћења енергије и успостављања одговарајуће енергетске политике, потребно је извршити и квалитативну (ексергијску) анализу, као и анализу утицаја коришћења различитих енергената на животну средину. Цене енергената треба да одражавају вредност искористивог дела, односно квалитет, њихове енергије, тј. ексергије. У раду је приказана термодинамичка и економска анализа коришћења различитих енергената у стамбеној згради грејане површине око 160 m 2 , за климатско подручје Београда. Поређени су различити енергенти за грејање и припрему санитарне топле воде: електрична енергија, природни гас, даљинско грејање, угаљ и обновљиви извори енергије. Дат је упоредни приказ потребне примарне енергије, емисије угљен-диоксида, ексергије, као и цена енергије и ексергије за различите енергенте. Кључне речи: Енергија; ексергија; цене енергената; обновљиви извори енергије; емисија CO2 According to the energy balance for Republic of Serbia for 2017, the total final energy needs for 2017 for Republic of Serbia are estimated as 8429 Mtoe, with a share of 48% estimated for households, agricultural, public and commercial sectors. In the structure of final energy consumption by fuel for 2017 in Serbia, the renewable energy sources are participating with a share of 12% (only geothermal energy and biomass). In order to provide better understanding of all important aspects of energy usage, beside the quantitative (energetic) analysis, it is necessary to perform also qualitative (exergetic) analysis, together with the analysis of different fuels usage impact on environment. Fuel costs should take into a consideration the value of usable part, with other words the quality of their energy, namely exergy. The paper presents thermodynamic and economic analysis of different fuels usage, for residential building, with net heated area cca 160 m 2 , for Belgrade weather data. Results are presented as a comparison between different fuels used for heating and domestic hot water preparation, such as: electricity, natural gas, district heating, coal and renewable energy sources. The comparison is shown for primary energy consumption, CO2 emission, exergy consumption and energy and exergy costs for different fuels.

More than one third of the world’s primary energy demand refers to residential sector. Heating is considered as one of the main part of the energy consumption in buildings. In this study, a thermodynamic sustainability assessment analysis of different energy sources for heating of residential building, with net floor heated area of 162 m2, for Belgrade weather data, was presented. Five options of energy sources were studied, namely: coal, natural gas, electricity, district heating and air-water heat pump. Energy and exergy analyses were conducted and appropriate efficiencies were determined. Energy and exergy flows in boundaries of the building and in the whole chain from primary to final values were analyzed. The environmental impact factor and exergetic sustainability index were determined for all considered energy sources. The exergy efficiency is very low in all analyzed cases, which further implies poor thermodynamic compatibility of energy quality from the supplied side and the energy used for building heating. It was shown that the highest exergy efficiency is for the case of heat pump utilization (about 6%), due to the energy used from environment. The minimum environmental impact factor (15.37) and maximum exergetic sustainability index (0.065) were found for the case of heat pump utilization.