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The object of research is a static transformer frequency converter, which can be used to combine parallel-operating power systems having different operating frequencies. The aim of the work is to study operational characteristics and evaluate strategies and laws of the proposed device from the point of view of developing methods and means of power transmission improving quality via electrical communication, containing a frequency converter based on phase-shifting transformer made according to zigzag-triangle scheme and controlled by power keys. In order to achieve the above object, it is proposed to sectionalize control windings providing a 48-position with a discreteness of 2.5 °, and a 24-position with a discreteness of 5 ° switching in each 120 ° sector of "coarse" regulation. Structural simulation models were built and computational experiments were carried out for electrical communication combining two power systems with frequencies of 60 and 50 Hz and 50 and 49.6 Hz, respectively. The most significant results are: a new circuit version of the frequency converter, various strategies for controlling the conversion process. The two-channel frequency converter allows to reduce by 2 times the number of "coarse" control keys, which switch not after 60°, but after 120°. Also succeeded to reduce the switching step from 5 degrees to 2.5 degrees, which made it possible to improve the conversion quality. The significance of the obtained results is that during regulation the number of keys connected in series in operation remains equal to 4, which can significantly improve the reliability indicators of the control system.

The article presents the results on the adsorption of the dye methylene blue (MB) from an aqueous solution on a nanocomposite DDT (nanosized titanium dioxide in the anatase phase precipitated on diatomite) and its components – diatomite D and TiO2. The effect on adsorption of the pH of the solution, the dose of the adsorbent, the initial concentration, and temperature of the dye solution was studied. The equilibrium data were fitted to the Freundlich, Langmuir, and Dubinin-Radushkevich isotherms equations. The obtained parameters and correlation coefficients showed that the adsorption of the dye on D and TiO2 was best described by the Langmuir isotherm model, while the adsorption isotherm of MB on DDT – by the Dubinin-Radushkevich model. The MB adsorption data on DDT and D, obtained at different solution temperatures (20, 30, and 40°C) revealed irreversible adsorption, which increased with an increment of T. Thermodynamic parameters of the process were calculated. The negative values of the Gibbs free standard energy ΔGo show that the adsorption of MB on these adsorbents is spontaneous. Positive values of ΔHo, characteristic of the endothermic process, were relatively high, which indicates that the interaction between the sorbent molecules and the adsorbate was not only physical, but also chemical. The dye adsorption on TiO2 decreases with increasing the solution temperature; it is physical and exothermic in nature, ΔGo being positive and ΔHo negative. The values of the activation energy of adsorption Ea of MB on DDT, D, and TiO2, calculated according to the Dubinin-Radushkevich equation, was equal to 10.74, 11.78, and -2.68 kJ/mol, respectively, thus indicating the ion-exchange mechanism of the adsorption of MB by DDT and initial diatomite and physical adsorption by TiO2. A negative value of the activation energy for a sample of TiO2 means the absence of a potential barrier during adsorption of MB on this adsorbent.

Sustainable economic development is also determined by the availability of primary energy supply. Geographical incoherence of energy production and consumption areas leads to new challenges in providing consumers with energy. The purpose of this work is to analyze and assess trends and problems related to the provision of energy resources, mainly of the natural gas for Republic of Moldova in the context of new challenges in the natural gas market and rhe changes in their transport infrastructure. The investigative methodology is based on the analysis of time series of production, consumption and import of energy resources at global and regional level in 1990-2017. It is proposed to using the linear approximation functions. The coefficients and the its signs of these functions ("plus" or "minus") and the value of and the determination coefficient are used for the quantitative analysis of the production, consumption and import trends of primary energy and natural gas. Europe and Asia ensure their energy balance by massive energy imports. The Southeast European region, including the Republic of Moldova, imports large sums of primary energy resources. The need to import natural gas in the Romania-Moldova-Ukraine sub-zone is currently estimated at about 12 million tons of oil equivalent. The new energy corridors for the transportation of natural gas will lead to new challenges regarding the natural gas supply of the Republic of Moldova. Information on possible scenarios of natural gas supply to the Republic of Molodova is presented and the estimated costs for natural gas transport for these scenarios are presented.

The purpose of the work was to study the basic constructional, energetic and technological features of electrochemical reactors (activators) used in processing of whey – a dairy by-product, but also a biologically very valuable liquid. The choice of optimal regimes for processing of various types of whey is specified, aimed at raising the efficiency of isolation of whey protein fractions in the protein-mineral concentrates while reducing energy intensity. It is also demonstrated that it is necessary to reasonably select such constructional features of electro activators as the distance between the electrodes and between the electrodes and the membrane as well as their geometric shape so as to avoid appearance of “dead” areas and to assure a high efficiency of the process. It was also important to find the most appropriate option of electric parameters and processing regimes (periodic or continuous). Besides, the account was taken of the ratio of the whey volume to the main electrode area; of the membrane type; of the initial content of dry matter in each whey type, i.e., its protein compositions; the concentration and composition of the anodic (secondary) liquid. The results of the investigation are of importance in the development of a wasteless technology of by-products processing.

The work deals with district heating systems based on cogeneration plants and the use of carbon dioxide heat pumps in them. Heat pumps are used in heating systems of buildings and use the heat of outdoor air as a source of low potential heat and, at the same time, the heat of return network water. The aim of the study is to develop the structure of a heat four-pole (HFP) for fitting of heat pumps with district heating system, determine the parameters of a heat exchanger with a variable surface area of the heat exchange installed in the return water line, and analyze one of the schemes of the HFP. The goal to be sought is achieved by solving the following tasks: development of requirements and justification of the HFP scheme, justification of it’s parameters. The most significant results are developed models of statics and dynamics of heat exchanger in HPF, means of the integration of carbon dioxide heat pumps into heat networks with high temperature schedules. The significance of the results obtained consists in deriving dependencies between the temperature schedule of heating system and HFP parameters, which can be used in the practice of designing a district heating systems with heat pumps. As a result of calculations, equations were obtained for the describing of the relationship between the area of the heat exchanger installed in the return water line, temperature drops at the ends of the heat exchanger, heat power of the heat exchanger.