Performance evaluation of various nano heat transfer fluids in charging/discharging processes of an indirect solar air heating system

Abstract

In this study, an indirect charging/discharging solar-driven air heating system is assessed. In the charging process, a nano heat transfer fluid (NHTF) is thermally charged in a solar cylindrical cavity receiver (CCR) and then releases its thermal energy in a discharging process into the air in cross-contact with finned-tube heat exchangers (FTHEXs). In the experiments, a heating box with two consecutive FTHEXs is designed, and Syltherm 800 with Cu nanoparticles operates as the NHTF. The performance of the entire system in both single and dual FTHEXs modes is evaluated using various NHTFs, such as Therminol® 62, Therminol® 66, and Therminol® VP-1, and various nanoparticles, such as Ag, Cu, and Al2O3. It is found that the dual HEXs mode has a better performance than a single HEX mode. Therminol® 66 has a better performance in the charging process in the CCR, and Therminol® 62 exhibits a better thermal performance in both the charging and discharging processes. The average daily thermal efficiency (ADTE) of the CCR and the entire system for considered operational ranges can be improved up to 65.79 and 39.25%, respectively, for a single HEX mode and up to 69.93 and 42.83%, respectively, for a dual HEXs mode.