The present work consisted in obtaining and studying selective surfaces for applications in low-cost flat plate solar collectors, using residues from the granite industry. Five different surfaces were studied, varying the percentage by weight: 100% granite powder, 75% granite powder + 25% titanium oxide, 50% granite powder + 50% titanium oxide, 25% granite powder + 75% titanium oxide and 100% titanium oxide. For the tests, an experimental wooden bench was built, and it was possible to simulate the conditions of a flat plate solar collector. For characterization of the surfaces, SEM techniques, infrared analysis and UV-VIS absorbance determination were used, as well as graphs with surface temperatures and with radiation in the sun tests. The efficiency of the surfaces was determined by the ratio of the absorptivity through the emissivity, as well as the trademark MRTiNOX. An efficiency of 23.58 was obtained for this, while for the 50% granite - 50% titanium surface the value of 23.30 (closest to the trade mark) was calculated. Therefore, replacing the traditional components of selective surfaces with granite proved to be a satisfactory solution, contributing to the reduction of costs with solar energy.

The depletion of fossil fuel reserves and climate change caused by atmospheric pollution has led the human being to seek alternatives that are less damaging to the environment. The concern and the awareness of the population open space for the study of renewable energies to be deepened, among them the best use of solar energy.The use of alternative materials to replace selective surfaces is a natural trend, since improvements in surface efficiency are usually sought while attempting to reduce costs. Composite substances are already used to obtain some selective surfaces, and, as a result, the search for better processes awakens research on more appropriate and lower cost materials, which represents a great scientific potential in the evolution of these technologies.Thus, the present work consisted in obtaining and studying selective surfaces for applications in lowcost flat plate solar collectors, using residues from the granite industry. Five different surfaces were studied, varying the weight percentage: 100% granite powder, 75% granite powder + 25% chromium oxide, 50% granite powder + 50% chromium oxide, 25% granite powder + 75% chromium oxide and 100% chromium oxide.For the tests, an experimental wooden stand was built, and it was possible to simulate the conditions of a flat plate solar collector. To characterize the surfaces, scanning electron microscope (SEM) techniques, infrared analysis, X-ray diffraction and UV-VIS absorbance determination, as well as graphs with surface temperatures and with radiation, during the tests in the sun, were used.The surfaces` efficiency was determined by the ratio of the absorptivity by the emissivity, as well as the trademark MRTiNOX, a commercial selective surface applied on copper substrate. For this, an efficiency of 23.56 was obtained, while for the 50% granite and 50% chromium surfaces the value of 23.27 (closest to the trade mark) was calculated. Therefore, replacing the traditional components of selective surfaces with granite proved to be a satisfactory solution, contributing to the reduction of costs with work involving solar energy.

The depletion of conventional energy sources and the concern with the environmental impacts of the use of fossil fuels sparked interest in researching new energy sources and improving existing processes. In this context, the solar energy presents itself as one of the most promising energy sources on the planet, given its wide availability and applicability in thermal processes. However, its use still represents a great technological and economic challenge, because many systems that use this energy still have low efficiency and high cost, which makes them uncompetitive in competition with systems using other energies. With that, the search for the improvement of the processes awakens the researches in more adequate and cheaper materials, which represents a great scientific potential in the evolution of these technologies. Thus, the present work proposes to obtain and analyze a selective surface for applications in solar-thermal collectors, using CRFO (compound formed by chromium and iron oxides), varying the percentage by weight, and then mixed with aluminum oxide. For the tests, an experimental bench was built to simulate a flat plate solar-thermal collector in real operating conditions. Some characterizations were carried out, such as: techniques scanning electron microscopy (SEM), infrared analysis and UV-VIS absorptivity determination, as well as the graphs with surface temperatures and with radiation during the tests in the Sun for the composite (CRFO and aluminum oxide) and for the commercial surface (MRTiNOX). With the results obtained, it was verified in the field test that one of the produced selective surfaces obtained a result of absorptivity of 0.94 and 0.0349 of emissivity, which is close to that of the commercial surface. The results were confirmed in the characterization by UV-VIS, which showed high absorptivity values in the visible light range.

Solar energy is the most promising energy source, due to its great availability and applicability in thermal energy applications. However, researchers still experience technological and economical challenge, since many systems that use this energy still have low efficiency and high cost. In this way, the development of new materials and technologies to increase the efficiency of solar thermal collectors is both a challenge and a necessity. In this context, the objective of this work is to obtain and analyze selective surfaces for solar thermal collectors, using cashew nut shell liquid. The cashew nut shell liquid can be classified as technical or natural, depending on the mode of extraction of cashew liquid. An experimental bench was built to simulate a flat plate solar collector under real operating conditions. For comparative purposes, the tests were performed between the cashew nut shell liquid and the commercial surface (MRTiNOX). In order to verify the structure morphology and the chemical composition of the surface, analyzes were performed by scanning electron microscopy. In order to identify the presence of components after the sintering process, the infrared analysis technique was used. To analyze the surface absorbance, the ultraviolet–visible spectroscopy absorbance technique was used. With the tests in real conditions, it was possible to perform the temperature measurements, and later, with the energy balance, the absorptivity, emissivity, and efficiency were calculated. The technical cashew nut shell liquid presented efficiency of 42.86%, while the MRTiNOX, 41.8%. In contrast, natural cashew nut shell liquid obtained efficiency of 31.28%. Thus, the use of technical cashew nut shell liquid, a low-cost regional product, was presented as a viable and satisfactory solution for cost reduction in solar thermal collectors.