Análise de desempenho de um coletor de calha parabólica para diferentes tubos absorvedores

Abstract

Climate change and rising oil prices are making renewable energy sources the center of public interest. Among renewable energy sources, solar energy is the most abundant. There are two main techniques for the direct use of solar energy: photovoltaic conversion and thermal conversion. The latter uses solar radiation collectors. The use of light energy concentration techniques allows solar collectors to be able to work at higher temperatures. The Parabolic Trough Collector (PTC) consists of an absorber, a transparent concentric cover and a parabolic reflector plate, and is coupled to a tracking mechanism. In the present project, the hardware of the Arduino platform and Light Dependent Resistors (LDRs) were used for the control system and a step motor for the actuation system. An improvement of a small PTC with automatic intermittent east-west tracking and manual north-south tracking was performed through the geometric modification of the absorber tubes inside the reflecting parabola. An analysis of the influence of different transversal areas, with diameters of 12.7 mm (1/2 in), 19.05 mm (3/4 in), 22.225 mm (7/8 in) and 28 mm, on the solar collector’s thermal efficiency was developed. The results show an increase in PTC thermal efficiency values as the transversal area increases. The increases were 11.49% for the 19.05 mm tube, 24.50% for the 22.225 mm tube and 28.92% for the 28 mm tube compared to the 12.7 mm tube in a flow rate of 64 mL/min, considering an average direct solar radiation of 800 W/m². The manufactured tubes were submitted to experimental tests at flow rates of 50 mL/min, 75 mL/min and 100 mL/min, and their characteristic curves were plotted. The performance increases were 10.98% for the 19.05 mm tube, 18.26% for the 22.225 mm tube and 8.37% for the 28 mm tube with a 100% increase in the volumetric flow rate from 50 mL/min to 100 mL/min, considering an average direct solar radiation of 800 W/m². Comparing the characteristic curves of this radiation from the manufactured tubes to obtain an outlet temperature of 60 °C for water disinfestation in agricultural areas and seed thermotherapy, the increases in thermal efficiency were 0.87% for the 22.225 mm tube and 9.57% for the 28 mm tube compared to the 19.05 mm tube.