Vol 55, No 5 (2019)

A promising direction in photovoltaic and heat power engineering is the development of highly efficient photothermovoltaic (РTV) systems to convert the thermal energy of heated bodies into electrical energy. They have important advantages over other thermal power devices. It is known that heating negatively affects the operation of many semiconductor devices, but it increases the efficiency of photothermovoltaic element. The process of the appearance of voltage and electric current in the p-Si–n-(Si₂)_{1 – x – y} (Ge₂)_x(ZnSe)_y structure during uniform heating both in the dark and in light was studied in this work. The uniform heating of the p‑Si–n-(Si₂)_{1 – x – y}(Ge₂)_x(ZnSe)_y heterostructure both in the dark by thermal heating and photoheating and in light by solar radiation generated an electric current and a potential difference. The dark current generated by photoheating in the studied temperature range has a greater value of three orders of magnitude than in the case of thermal heating. However, the potential difference generated by photoheating slightly decreases with increasing temperature, but its value is almost two orders of magnitude greater than in the case of thermal heating. There is also a slight decrease in the photocurrent and potential difference at increasing temperature. Since the composition of the substrate-film intermediate region changes continuously from Si to the p-Si–n-(Si₂)_{1 – x – y}(Ge₂)_x(ZnSe)_y epitaxial film, a graded-gap layer with a smoothly varying composition prevents breaks in the energy zones of the p–n structure. Due to the variability in the intermediate region, an energy barrier arises, mainly for holes, which contributes to the appearance of an additional separating field, which is determined by the gradient of the band gap of this layer. Therefore, the hole current in this structure that is caused by photothermally generated electron-hole pairs can be significant up to higher temperatures. The low efficiency of the studied structure is clearly associated with the recombination of the main parts of the photothermally generated charge carriers.

This paper presents an intelligent maximum power point tracker which combines the conventional perturbation and observation method and an artificial neural network. The proposed controller overcomes the drawback of the conventional perturbation and observation under partial shading conditions. A MATLAB model for a photovoltaic system is developed so as to simulate system’s performance and characteristic under partial shading condition. This model includes models for PV array, DC–DC boost converter as well as the proposed algorithm. Results show that the proposed algorithm is able to track the global maxima accurately regardless the number of surrounding local maxima points with an efficiency of 95%.

A review of experimental and theoretical studies on technology for producing the single-crystal substitutional solid solution of p-(GaAs)_{1 – x – y}(Ge₂)_x(ZnSe)_y on (GaAs) with the given crystallographic orientation using the liquid-phase epitaxy method is presented in this paper. The methods for determining the optimal parameters of the grown epitaxial films are given. Based on the analysis of literature data and the experimental results of the authors, the optimal parameters of epitaxial films are established. The solid solutions (GaAs)_0.69(Ge₂)_0.17(ZnSe)_0.14 have a sphalerite structure and possible configurations of the arrangement of atoms and molecules are determined. It has been established that paired Ge atoms partially replace GaAs molecules in the defective regions of the matrix lattice, and zinc selenide molecules form nanoislands on the surface layer of the GaAs_1 – xGe_x solid solution, which have the geometric shape dome, with lateral dimensions of 55–65 nm. Research investigating current transfer mechanisms in n-GaAs–p-(GaAs)_{1 – x – y}(Ge₂)_x(ZnSe)_y hetero-structures at room temperature show that in the voltage range from 0.7 to 3 V degree depends J = АV^α (α₁ = 2, α₂ = 2.7 and α₃ = 3) are observed, which arise due to the formation of complex recombination complexes. The noticeable photosensitivity of n-GaAs–p-(GaAs)_0.69(Ge₂)_0.17(ZnSe)_0.14 hetero-structure begins at the photon energy of 1.16 eV, and the maximum is observed at the photon energy of 1.38 eV. In the short-wave region of the emission spectrum, peaks are observed with maxima at photon energies of 1.37, 1.47, 1.65, 1.88, 2.3, and 2.62 eV, as well as the gentle portion at 2.26–2.46 eV. Analysis using the Gaussian approximation showed that these features of the photo-spectrum are related to the state of the atoms of the constituent components and their interaction when illuminated by sunlight. It has been established that heterostructures have the property of controlled selective photosensitivity.

At the present time, 92% of the PV market is made using single crystal or polycrystalline wafer silicon. However, producing power with these cells remains expensive compared to conventional power generation. Thin film solar cells have been developed to reduce production costs, especially those based on cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS). Despite a number of successes in the development of thin film solar cells, some problems associated with the toxicity of cadmium and with the high cost of indium and gallium remain, which has prompted researchers to search for alternative materials for solar cells. Novel low cost and high efficiency zinc tin selenide (Zn_xSn_1 – xSe) thin film solar cells are without these drawbacks. However, there is no information in the literature about this new material. The samples of Zn_xSn_{1 – x}Se films were fabricated using the chemical molecular beam deposition (CMBD) method at atmospheric pressure in hydrogen flow. ZnSe and SnSe powders with 99.999% purity were used as precursors. The temperature of precursors varied in the range of 850–950°C. Films were deposited at substrate temperature of 500–600°C. Borosilicate glass was used as a substrate. The results showed that the composition of the samples changed toward ZnSe with the temperature of the substrate. The grain size of the samples increased from 2–5 μm to 15–17 μm at substrate temperatures of 500 and 550°C, respectively. At a substrate temperature of 600°C, the grain size decreased to 3–5 μm, which is possibly due to an increase in the ZnSe content. The X-ray diffraction pattern has shown that the samples have ZnSe, SnSe, Se, and Sn phases.

Low-potential solar plants, including flat-plate water-heating collectors (FSWHC), are complex systems in terms of heat engineering. A problem in the development of both stationary and non-stationary heat engineering FSWHC models is to specify convective heat exchange in SWHC channels. This paper considers the features and conditions of convective heat transfer in SWHC channels to water. It is shown that the water flow hydrodynamic and thermal stabilization sections in FSWHCs and at potential speeds of their heat carriers reach only 10 to 13 cm in length and the convective heat exchange in SWHCs can be calculated at a constant value of Nu. It is found that the differences in temperature between the walls of a SWHC channel and between the wall and the liquid will not exceed 10 and 5°C, respectively. These conditions make mixed free and forced convection in SWHC channels possible. Since the heat mainly comes from the upper walls of the channel, this determines the need for pilot studies to determine the distribution of temperature on the walls and in the liquid along the channel length and the difference in temperature between them. The heat exchange in SWHC channels can be heavily affected by free convection. An important issue when elaborating design models of heat exchange in SWHCs is the possibility of applying the convective heat exchange formulas used for plates to the walls of SWHC channels.

This paper presents the results of the first stage of verification of the generalized computational algorithm for determining the geometrical, dynamical, and energy parameters of the “parabolic trough concentrator – tube heat receiver” solar receiver system. This algorithm is based on the dimensionless coupled mathematical model of heat and mass exchange in the “Sun – parabolic trough concentrator – tube receiver” system. The Monte Carlo ray tracing method was used to calculate irregular heat flux from the concentrator to the tube receiver surface. The 3D mathematical model of heat exchange in the tube receiver was solved using the finite volume method. The mathematical model took local climatic and geographical features into account, as well as micro- and macroscopic imperfections of the concentrator surface. The dimensional problem of heat exchange in the tube receiver of the solar parabolic trough system was solved at the first stage of verification. A simplified physical model was described. It was assumed that the glass envelope around the tube receiver was removed. The dimensional mathematical model of heat exchange in the tube receiver is based on the system of Navier–Stokes equations for viscous incompressible liquid with constant physical properties. The mathematical model with real boundary conditions was solved numerically. An analytical solution for the test problem with trivial boundary conditions was obtained. The results of the test numerical problem were compared with analytical solution results and were found to be in close agreement. Numerical experiments with real boundary conditions were conducted using the computational algorithm. The numerical data were compared with the results of field experimental studies and showed nearly total agreement, which proved the adequacy of the basic mathematical model and the resulting numerical algorithm.

This work considers the current status of meteorological stations of actinometric observations in Uzbekistan. The data obtained from six modern ground-based meteorological stations in Uzbekistan are compared with the satellite observations and reanalysis data from NASA POWER, ERA5, and SARAH-E. It is shown that the comparative deviations of the data of NASA POWER from the ground monthly amounts of solar radiation on the horizontal surface for most regions of Uzbekistan from April to September do not exceed 8%, which is quite acceptable for the design of solar power plants. The comparative deviations of the data of ERA5 and SARAH1E for the same period of the year do not exceed 8 to 12%. According to the results of the calculation studies on verifying global horizontal solar radiation using the three aforementioned databases, these can be used in engineering calculations. When performing calculation studies, one can select any base depending on the required information, i.e. daily or hourly solar radiation values.