Arno Krenzinger

Cooling of cylindrical vertical tanks submitted to natural internal convection

A numerical and an experimental analysis of velocity and temperature fields inside a storage tank submitted to natural convection is presented. The analysis was performed in two stages. In the first stage, the temperature profile along the vertical axis of the storage tank was obtained experimentally and numerically, for cooling time ranging from 45 to 60 h. The numerical analysis was carried out using a transient bi-dimensional model in cylindrical co-ordinates. In the second stage, after the numerical code validation, 40 cases of cooling with four aspect ratios, five insulation thicknesses, and two different volumes were simulated. In all simulations, thermal losses for the environment in all tank walls (side, top and bottom) were considered. Two correlations for the Nusselt number, encompassing all the forty cases, were obtained with these results.

PV hydro hybrid systems

The energy systems based on renewable resources face several acceptance difficulties mainly due to high initial costs and to low final incomes. A way to improve the effectiveness of these systems is the use of more than a source of energy, mainly when the employed sources present some type of complementarity. An interesting combination is obtained with the use of hydro and solar energy resources, that it allows to take advantage of the complementarity among two sources and it allows to investigate the storage of energy in water reservoirs and in batteries. The configurations with a bus in cc would assist demands of until 1 kW to 2 kW, while configurations involving a bus in ca evidently would reach larger powers. This article discusses PV hydro hybrid systems, characterizing some configurations, discussing operation strategies, commenting on the effects of the complementarity in time among the energy sources and evaluating that complementarity in the State of Rio Grande do Sul, in the south of Brazil, besides presenting preliminary estimates of costs.

Estimation of radiation incident on bifacial albedo-collecting panels

In this paper we present a method to predict the radiation incident on both front and back faces of a bifacial photovoltaic panel surrounded by several diffusely reflecting surfaces. The incident irradiance on the panel is obtained from a mathematical model that considers both the effect of the interreflections between the reflector surfaces and the effect of the collector shadow. The model was validated experimentally for several albedo reflector configurations. The results of applying this model to several configurations of reflector planes using data for Madrid are presented. It is shown that in some cases an increasement in annual radiation incident on bifacial photovoltaic panels of nearly 70% can be achieved when compared to the radiation incident on conventional monofacial panels.

Development of paints with infrared radiation reflective properties

Large buildings situated in hot regions of the Globe need to be agreeable to their residents. Air conditioning is extensively used to make these buildings comfortable, with consequent energy consumption. Absorption of solar visible and infrared radiations are responsible for heating objects on the surface of the Earth, including houses and buildings. To avoid excessive energy consumption, it is possible to use coatings formulated with special pigments that are able to reflect the radiation in the near- infrared, NIR, spectrum. To evaluate this phenomenon an experimental study about the reflectivity of paints containing infrared-reflective pigments has been made. By irradiating with an IR source and by measuring the surface temperatures of the samples we evaluated: color according to ASTM D 2244-14, UV/VIS/NIR reflectance according to ASTM E 903-12 and thermal performance. Additionally, the spectral reflectance and the IR emittance were measured and the solar reflectance of the samples were calculated. The results showed that plates coated with paints containing IR-reflecting pigments displayed lower air temperature on the opposite side as compared to conventional coatings, indicating that they can be effective to reflect NIR and decrease the temperature of buildings when used in roofs and walls.

Artificial Intelligence Technics Applied to Analisys of Photovoltaic Energy Systems

This paper presents a procedure to determine the energetic behavior of photovoltaic systems through genetic algorithms and artificial neural networks. The method divides the I-V Curve of the photovoltaic generator in three regions. The best results are achieved for the region around the maximum power point, which can be advantageous for grid-connected systems. The method allows determining the energy delivered by the system and the inverter performance from a few experimental data. The output is a vector comprising the DC power, the maximum power point tracker efficiency, the inverter efficiency and the AC power.

Evacuated Tube Collectors - Influence of thermophysical properties in numerical simulations by CFD

Solar liquid heating systems using evacuated tube collectors currently have a better performance than flat plate collectors for high temperature operation. This is due to reduced heat losses by convection, as a result of the vacuum layer over the inner tube. The solar collectors with vacuum tubes are a novelty in Brazil, in addition, its use on a commercial scale is less than 10 years and there are many questions to be answered about their use and efficiency. To get a better understanding of the operation of this equipment, it has started a program of study through a collaboration between the NETE - UNISINOS (Study Center Thermal and Energy of University of Vale do Rio dos Sinos) and LABSOL-UFRGS (Solar Energy Laboratory of Federal University of Rio Grande do Sul). The methodology consists in the numerical simulation by CFD (Computer Fluid Dynamics) and validate the methodology through numerical and experimental results. The results of velocity fields and mean temperature indicate that the methodology is quite adequate.

Analysis of Maximum Power Point Tracking Efficiency of Inverters in Photovoltaic Systems

Grid-connected photovoltaic systems directly convert solar energy into electrical energy delivering to the distribution grid a clean and renewable energy. These systems are basically formed by an array of photovoltaic modules and inverters. The inverters are responsible for converting electrical energy of direct current to electrical energy of alternating current. In addition to performing this conversion, the inverter is also responsible for the management of the energy delivered to the grid and for performing the maximum power point tracking. The maximum power point tracking is a control process in which the inverter tries to keep the photovoltaic generator operating in a region of its characteristic curve where the product current versus voltage reaches its maximum value in order to optimize the extraction of power from the photovoltaic generator. This paper presents a theoretical review of topologies and algorithms and also present results of tests using ten inverter models from different manufacturers. The inverter electrical characteristic measured and analyzed was the maximum power point tracking efficiency.

Description and Analysis of Inverters Used in Photovoltaic Systems

Grid-connected photovoltaic systems directly convert solar energy into electrical energy delivering to the distribution grid a clean and renewable energy. These systems are basically formed by an array of photovoltaic modules and inverters. The inverters are responsible for converting direct current to alternating current. This paper presents a study of electrical characteristics of inverters used in grid-connected photovoltaic systems from a theoretical and experimental analysis. The inverters tests were performed at Solar Energy Laboratory at Federal University of Rio Grande do Sul where it was used a 4,8 KWp grid-connected photovoltaic system and ten inverters of different manufacturers. The inverters electrical characteristics measured and analyzed were direct current to alternating current conversion efficiency. The paper shows the efficiency curves of the inverters and average performance index are used for rating. The analysis of the behavior of the inverters improves the understanding of the operating of such equipment and their dynamic interaction with other grid-connected photovoltaic system components.