Characterization and application of a selective coating for solar collectors from of the cashew nut shell liquid

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.