Análise experimental da conversão fototérmica em nanofluidos de ouro e nanotubos de carbono em água

Abstract

The present work experimentally investigated the behavior of Au/Water and MWCNT/Water nanofluids under realistic conditions of photothermal conversion using direct absorption solar collector. Nanofluids (gold and carbon nanotubes) at low volumetric concentrations (1 to 40 ppm and 1 to 300 ppm, respectively) were obtained by sonication process. An apparatus was built for simultaneous exposition of the different concentrations of nanofluids as well as base fluid to the solar radiation without any solar concentration device. The results showed that the nanoparticles have excellent photothermal conversion capability, even at very low concentrations. It also showed a remarkable change in temperature of nanofluids at the first hour of exposition and a linear augmentation in the total storage energy as function of nanoparticle mass for concentrations from 1 to 10 ppm. The heating behavior of carbon nanotube nanofluid with higher concentration (>10ppm) showed to be very similar (both temperature variation and maximum temperature). For 10ppm concentration of carbon nanotubes it was observed an increase of 90% in the temperature in comparison to the pure water, presenting a better result obtained by the 40ppm gold nanofluid (~75% higher). The analysis of specific absorption rate (SAR) showed an exponential decay with the concentration increase. The analysis of the stored energy ratio (SER) showed that the 40ppm volumetric concentration presented better results. After five days of tests, gold nanofluids has shown clear evidences of sedimentation, differently from the behavior presented by the carbon nanotubes nanofluids. The combined analysis of the studied parameters indicates that exists an optimal volumetric concentration, near 40ppm and 10ppm, for solar applications of gold and carbon nanotubes nanofluids. In comparison of nanoparticles nature, the higher stability, lower cost and good results presented by carbon nanotubes indicates that it is a better choice for solar applications under the same conditions presented on this work.