This paper presents the experimental investigation on the thermal properties and viscosity of the new organic ester phase change material embedded with the silver-titania hybrid nanocomposite (HyNPCM) with the mass proportions ranging from 0.1% to 1.5%. The HyNPCM embedded with the surface functionalized hybrid nanocomposite exhibited improved thermal conductivity from 0.286 W/m K to 0.538 W/m K, congruent phase change temperature (6.8 degrees C), high latent heat capacity (90.81 kJ/kg), substantial reduction in the supercooling degree (1.82 degrees C), thermal stability (191 degrees C) and chemical stability, while compared to the pure PCM. Experimental results reveal that, the freezing and the melting times of the HyNPCM were reduced by 23.9% and 8.5% respectively, when compared to the pure PCM. The increased mass proportion of HyNC resulted in the increased viscosity up to 3.89%, which suggests the existence of relative dependencies between the thermal properties and the viscosity of the HyNPCM. In total, the improved thermal properties and the heat storage potential of the HyNPCM has facilitated them to be considered as a viable candidate for the cool thermal energy storage applications in buildings without sacrificing energy efficiency. (C) 2013 Elsevier Ltd. All rights reserved.