ADVANTAGES AND DISADVANTAGES OF SOLAR ENERGY PRODUCTION AND USE

The energy released through nuclear fusion on the Sun expands into space in the form of electromagnetic radiation. A total of 1.5x109 TWh reaches the surface of the Earth. About 30% of the energy returns into space by reflexion, whereas 70% of the energy is absorbed on the Earth’s surface (1,05x109 TWh). This amount is greater than the total coal and oil reserves taken together. Using solar energy has its specificities. First, the energy of the Sun records great fluctuations during the day and year. Second, there is a great daily, seasonal, yearly and meteorological variability of the Sun’s radiation accompanied by a very low congruence of the production and consumption of the energy of the Sun. Third, there is a low degree of the conversion of the Sun’s energy to electrical energy. The growth of production and an ever-increasing mass use of solar systems, especially so in China, will for sure exert an influence on a reduction in the prices of these systems in the future. Due to its specificity, the surplus of produced solar energy has to immediately be used or stored in the batteries that are still a major cost of the construction of solar systems. It is more frequently the case that the current surplus of produced solar energy is delivered to electrical power enterprises “to be stored”, whereas the same is taken from the electrical grid during the time of a deficit

Regionalna obeležja tržišne proizvodnje voća i grožđa u Srbiji

In the paper analyzes the trends in the development of market production of fruit (on the example of the apple and the plum) and grapes in Serbia from 1976 to 2015. The grouping of the Serbian districts according to the degree of the market production of fruit and grapes in 2015 was performed by a cluster analysis, on the basis of the six features of production, five features of the capacities, and five features of development. According to the data for 2015, the degree of the marketability of apples in Serbia was 47.7%, plums 15.9%, and grapes 18.3%. The Serbia-North Region shows a surplus in the production of apples, and a deficit in the production of plums (-181.7%) and grapes (-99.1%). The Serbia-South Region has a surplus in the production of the analyzed kinds of fruit (the apple accounting for 43.0%, and the plum 50.9%) and grapes (45.2%).U radu je analizriana regionalna obeležja tržišne proizvodnje voća (na primeru jabuke i šljive) i grožđa po oblastima Srbije u periodu 1976-2015. godine. Grupisanje oblasti u Srbiji prema stepenu tržišne proizvodnje voća i grožđa u 2015. godini izvršeno je klaster analizom na osnovu 6 obeležja proizvodnje, 5 obeležja kapaciteta i 5 obeležja razvijenosti. Prema podacima za 2015. godinu, stepen tržišnosti jabuka u Srbiji iznosio je 47.7%, šljiva 15.9% i grožđa 18.3%. Region Srbija-Sever suficitaran je u proizvodnji jabuka, a deficitaran u proizvodnji šljiva (-181.7%) i grožđa (-99.1%). Region Srbija-Jug suficitaran je u proizvodnji analiziranih vrsta voća (jabuka 43.0% i šljiva 50.9%) i grožđa (45.2%)

A microbearing gas flow with different walls' temperatures

An analytical solution for the non-isothermal 2-D compressible gas flow in a slider microbearing with different temperatures of walls is presented in this paper. The slip flow is defined by the continuity, Navier-Stokes and energy continuum equations, along with the velocity slip and the temperature jump first order boundary conditions. Knudsen number is in the range of 10-3-10-1, which corresponds to the slip flow. The ratio between the exit microbearing height and the microbearing length is taken to be a small parameter. Moreover, it is assumed that the microbearing cross-section varies slowly, which implies that all physical quantities vary slowly in x-direction. The model solution is treated by developing a perturbation scheme. The first approximation corresponds to the continuum flow conditions, while the second one involves the influence of rarefaction effect. The analytical solutions of the pressure, velocity, and temperature for moderately high Reynolds numbers are presented here. For these flow conditions the inertia, convection, dissipation, and rate at which work is done in compressing the element of fluid are presented in the second approximation, also

Navier-stokes-fourier analytic solutions for non-isothermal couette slip gas flow

The explicit and reliable analytical solutions for steady plane compressible non-isothermal Couette gas flow are presented These solutions for velocity and temperature are developed by macroscopic approach from Navier-Stokes-Fourier system of continuum equations and the velocity slip and the temperature jump first order boundary conditions. Variability of the viscosity and thermal conductivity with temperature is involved in the model. The known result for the gas flow with constant and equal temperatures of the walls (isothermal walls) is verified and a new solution for the case of different temperature of the walls is obtained Evan though the solution for isothermal walls correspond to the gas flow of the Knudsen number (Kn lt = 0.1), i. e. to the slip and continuum flow, it is shown that the gas velocity and related shear stress are also valid for the whole range of the Knudsen number. The deviation from numerical results for the same system is less than 1%. The reliability of the solution is confirmed by comparing with results of other authors which are obtained numerically by microscopic approach. The advantage of the presented solution compared to previous is in a very simple applicability along with high accuracy

Influence of transport coefficients' dependence on temperature for gas flow in microbearing

The paper presents an analytical solution for the non-isothermal compressible gas flow in a slide microbearing with different temperatures of walls. The gas flow is defined by the Navier-Stokes-Fourier system of the continuum equations and first order boundary conditions. Knudsen number corresponds to the slip and continuum flow (Kn lt = 10(-1)) and Reynolds number is moderately high, so inertia needs to be included. The solution is obtained by perturbations with the first approximation that relates to the continuum flow and the second one that involves second-order effects: the rarefaction, inertia, convection, dissipation, and rate at which work is done in compressing the element of fluid. The presented model analyzes the influence of the dependence of transport coefficients on temperature. The obtained analytical solution for the pressure, velocity, and temperature is approved by a comparison with the results of other authors. The microbearings can often be a part of MEMS, so the presented method and the obtained analytical solution can serve for solving similar non-isothermal shear-driven or pressure-driven problems. The paper gives an estimation about the error in values for microbearing mass flow and load capacity if the dependence of transport coefficients on temperature are neglected

A constant wall temperature microbearing gas flow

U radu je analizirano neizotermsko dvodimenzijsko stišljivo strujanje gasa u mikroležaju konstantnih i jednakih temperatura zida. Vrednost Knudsenovog broja je između 10-3 i 10-1, što odgovara režimu strujanja sa klizanjem. Ovaj režim strujanja definiše se jednačinama kontinuuma: Navije-Stoksovom i jednačinom energije i graničnim uslovom klizanja i temperaturskog skoka na zidu. Strujanje gasa je dozvučno, pa se mali parametar definiše kao ε = κM2/Re. Osim toga pretpostavljeno je se poprečni presek mikrokanala menja sporo, što dovodi do spore promene svih veličina u pravcu strujanja. Rešenje je dobijeno perturbacionom metodom. Prva aproksimacija predstavlja rešenje za slučaj strujanja gasa bez klizanja, dok se u drugoj aproksimaciji javlja uticaj klizanja i inercije. Dobijena su analitička rešenja za raspodelu pritiska, brzine i temperature u mikroležaju pri umereno velikim vrednostima Rejnoldsovog broja.A non-isothermal two-dimensional compressible gas flow in a slider microbearing with constant and equal wall temperature is investigated in this paper analytically. The slip flow is defined by the Navier-Stokes and energy continuum equations along with the velocity slip and the temperature jump first order boundary conditions. Knudsen number is in the range of 10-3 to 10-1, which corresponds to the slip flow. The gas flow is subsonic and the ratio κM2/Re is taken to be a small parameter. Moreover, it is assumed that the microbearing cross-section varies slowly, which implies that all physical quantities vary slowly in x-direction. The model solution is treated by developing a perturbation scheme. The first approximation corresponds to the continuum flow conditions, while the second one involves the influence of inertia as well as rarefaction effect. The analytical solutions of the pressure, velocity and temperature for moderately high Reynolds numbers are obtained

Non-isothermal rarefied gas flow in microtube with constant wall temperature

In this paper, pressure-driven gas flow through a microtube with constant wall temperature is considered. The ratio of the molecular mean free path and the diameter of the microtube cannot be negligible. Therefore, the gas rarefaction is taken into account. A solution is obtained for subsonic as well as slip and continuum gas flow. Velocity, pressure, and temperature fields are analytically attained by macroscopic approach, using continuity, Navier-Stokes, and energy equations, with the first order boundary conditions for velocity and temperature. Characteristic variables are expressed in the form of perturbation series. The first approximation stands for solution to the continuum flow. The second one reveals the effects of gas rarefaction, inertia, and dissipation. Solutions for compressible and incompressible gas flow are presented and compared with the available results from the literature. A good matching has been achieved. This enables using proposed method for solving other microtube gas flows, which are common in various fields of engineering, biomedicine, pharmacy, etc. The main contribution of this paper is the integral treatment of several important effects such as rarefaction, compressibility, temperature field variability, inertia, and viscous dissipation in the presented solutions. Since the solutions are analytical, they are useful and easily applicable

Rarefied gas flow in microtubes at low Reynolds numbers

U radu se proučava osnosimetrično, izotermsko, stišljivo strujanje gasa sa klizanjem, pri malim vrednostima Rejnoldsovog broja. Problem je rešen primenom jednačine kontinuiteta i Navije-Stoksovih jednačina, zajedno sa Maksvelovim graničnim uslovom prvog reda. Analitički rezultati su dobijeni primenom perturbacione metode. Dobijena rešenja se dobro slažu sa poznatim eksperimentalnim rezultatima drugih autora.Low Reynolds number, axisymmetric, isothermal, compressible, slip gas flow in microtubes is investigated in this paper. The problem is solved by the continuum equations, continuity and Navier-Stokes, along with Maxwell first order boundary condition. The analytical results are obtained by perturbation method. The solutions show a good agreement with experimental results

Non-isothermal rarefied gas flow in microtube with constant wall temperature

In this paper, pressure-driven gas flow through a microtube with constant wall temperature is considered. The ratio of the molecular mean free path and the diameter of the microtube cannot be negligible. Therefore, the gas rarefaction is taken into account. A solution is obtained for subsonic as well as slip and continuum gas flow. Velocity, pressure, and temperature fields are analytically attained by macroscopic approach, using continuity, Navier-Stokes, and energy equations, with the first order boundary conditions for velocity and temperature. Characteristic variables are expressed in the form of perturbation series. The first approximation stands for solution to the continuum flow. The second one reveals the effects of gas rarefaction, inertia, and dissipation. Solutions for compressible and incompressible gas flow are presented and compared with the available results from the literature. A good matching has been achieved. This enables using proposed method for solving other microtube gas flows, which are common in various fields of engineering, biomedicine, pharmacy, etc. The main contribution of this paper is the integral treatment of several important effects such as rarefaction, compressibility, temperature field variability, inertia, and viscous dissipation in the presented solutions. Since the solutions are analytical, they are useful and easily applicable

Rarefied gas flow in microtubes at low Reynolds numbers

U radu se proučava osnosimetrično, izotermsko, stišljivo strujanje gasa sa klizanjem, pri malim vrednostima Rejnoldsovog broja. Problem je rešen primenom jednačine kontinuiteta i Navije-Stoksovih jednačina, zajedno sa Maksvelovim graničnim uslovom prvog reda. Analitički rezultati su dobijeni primenom perturbacione metode. Dobijena rešenja se dobro slažu sa poznatim eksperimentalnim rezultatima drugih autora.Low Reynolds number, axisymmetric, isothermal, compressible, slip gas flow in microtubes is investigated in this paper. The problem is solved by the continuum equations, continuity and Navier-Stokes, along with Maxwell first order boundary condition. The analytical results are obtained by perturbation method. The solutions show a good agreement with experimental results