Experimental investigation on the thermal behavior of nanofluid direct absorption in a trough collector

In the present work, a solar direct absorption parabolic trough collector is prepared for studying the efficiency enhancement methods. The trough collector was manufactured with a width of 0.7 m and the height of 2 m steel mirror reflector. The main issue in this collector is its absorber tube which is made of borosilicate glass. The glass-glass tube has high transmissivity for longwave radiation and increasing the performance of the collector. The outlet temperature and the thermal efficiency were compared by using of various nanofluids. ASHRAE Standard 93-2010 was used for the operating conditions. The volume fractions of 0.1%, 0.2%, and 0.3% MWCNT/ethylene glycol (EG) and nanosilica/EG were used as various working fluids. The results showed that the temperature difference for 0.1% nanosilica/EG was 2.2 K, for 0.2% nanosilica/EG was 5.3 K, and 7.7 K was obtained for 0.3% nanosilica/EG. while this parameter for 0.1% and 0.2% MWCNT/EG were 5.8 K and 9.3 K. Also, the outlet temperature for 0.3% MWCNT/EG was 15.7 K more than the base fluid. The thermal efficiency of 0.3% carbon nanotube/EG has the highest value, and it is averagely 17% higher than the base fluid. In addition, the optical efficiency of this collector is 71.4%, and the time constant is 8.31 min.