Characterization of new selective coatings, made of granite and chrome, for solar collectors

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 low-cost 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.