Análise experimental do desempenho térmico de um coletor solar de tubos evacuados assistido por termossifões com nanofluido de pentóxido de nióbio

Abstract

This work presents an experimental investigation of the thermal performance of a thermosyphon-assisted evacuated tube solar collector operating with niobium pentoxide nanofluid as the working fluid under real meteorological conditions in the city of Ponta Grossa, PR, Brazil. Initially, niobium pentoxide nanoparticles were synthesized using the Sol-Gel method, followed by calcination to obtain the crystalline phase and subsequently characterized by scanning electron microscopy and X-ray diffraction. Nanofluids were then prepared at mass concentrations of 1,0 g/L and 0,1 g/L using a dispersion protocol based on ultrasonication and mechanical stirring to ensure suspension homogeneity. In the first experimental stage, thermosyphons were evaluated under laboratory conditions with different working fluids, and thermal resistance was determined for different heat inputs and cooling flow rates. Both nanofluids reduced the thermal resistance of the thermosyphons compared to distilled water, with maximum reductions of 21% for the 1,0 g/L concentration and 36% for the 0,1 g/L concentration. Based on these results, the 0,1 g/L nanofluid was selected to fill the thermosyphons integrated into the evacuated tube solar collector. In the second stage, the solar collectors were evaluated under real cloudy and clear-sky conditions. On the cloudy day, the collector operating with niobium pentoxide nanofluid achieved an average thermal efficiency of 58.4%, compared to 43.0% for the conventional collector, corresponding to an increase of 15.4%. Under clear-sky conditions, the average efficiency of the collector with nanofluid reached 54.9%, whereas the collector equipped with water-filled thermosyphons achieved 40.4%, resulting in an increase of 14.5%. The results demonstrate that the use of niobium pentoxide nanofluid reduces the thermal resistance of thermosyphons and enhances the thermal efficiency of thermosyphon-assisted evacuated tube solar collectors, highlighting its potential for application in solar thermal systems.