An experimental study has been conducted in order to investigate the thermophysical properties of water-glycerol mixture-based nanofluids containing CuO nanoparticles, as phase change material (PCM) for cool thermal energy storage (TES) systems. The proportion of the water-glycerol in the mixture has been estimated as 80:20 (by weight) and the different mass fractions of CuO nanoparticles dispersed in the mixture are 0.1, 0.3, 0.5, 0.8, and 1.0 wt%, individually. The dispersion stability of the nanofluids employed as PCMs has been clearly ascertained by means of the sedimentation photograph and particle size analyzer. The thermophysical properties of the nanofluids, like phase change temperature and latent heat have been measured by differential scanning calorimetry. The thermal stability and reliability analyses were studied to evaluate the performance of the nanofluids for long-term operation. The effects of the concentration of nanoparticles and operating temperature of the PCMs on the viscosity of the nanofluids have been investigated. The laser flash analyzer has been used to ascertain the thermal conductivity enhancement of the nanofluids for all mass fractions of nanoparticles. The obtained results have shown that the prepared composite PCMs have achieved the desired properties, and they can be recommended as an efficient potential candidate for cool TES systems.