In this study, the influence of Zirconia (ZrO 2 ) and Titania (TiO 2 ) nanopaticles on liquid–solid phase tran- sition of aqueous nanofluids with/without Poly vinyl pyrrolidone as surfactant are experimentally com- pared. A cooling generation apparatus based on the compression refrigeration cycle has been used to ex- plore the solidification behavior of nanofluids as phase change materials. The experimental results show that ZrO 2 and TiO 2 nanoparticles considerably reduce the solidification supercooling degree of deionized water (as basefluid). Only adding 0.04 wt% ZrO 2 and TiO 2 nanoparticles to base fluid, the percentage of reduction in supercooling degree attained 81% and 65%, respectively. The results reveal that although the presence of surfactant in nanofluids reduces the supercooling degree and slightly solidification time of both ZrO 2 and TiO 2 nanofluids; but it has no influence on onset nucleation time. Comparison of ZrO 2 and TiO 2 nanofluids with/without surfactant presents that ZrO 2 provides faster solid layers formation and has more energy saving potential in storage systems due to its lower supercooling degree.

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