Experimental investigation on stability and evaluation of nanorefrigerant applied on organic Rankine cycle system

The enhancement of thermal conductivity by nanorefrigerants improves the performance of the organic Rankine cycle system compared to traditional refrigerants, but the consequent poor stability and high viscosity prevent their application in thermodynamic systems. In this paper, the stability, viscosity and thermal conductivity of three mono nanorefrigerants and two hybrid nanorefrigerants are comprehensively analyzed. The results of transmittance and absorbance indicate that the hybrid nanorefrigerants have better stability compared to mono nanorefrigerants, which the absorbance of Al2O(3)/TiO2-R123 and ZnO/TiO2-R123 hybrid nanorefrigerants only decreased by 0.349 and 0.529 within 24 h. Furthermore, in order to fill the gap in pH studies of nanorefrigerants, the effect of pH on stability and viscosity is emphasized. It is revealed that the stability and viscosity are related to the pH of the isoelectric point, which shows better stability and lower viscosity of the nanorefrigerant while the pH is far away from the equipotential point, and the ZnO/TiO2-R123 hybrid nanorefrigerant reaches the optimal stability at pH 8. Finally, the thermal conductivity of ZnO-R123 is 1.4-2 times higher and 1.1-1.5 times higher than that of TiO2-R123 and ZnO/TiO2-R123 (4:6), respectively, which present the best overall thermophysical properties based on the PER results.