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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%.