Adding nanoparticles almost enhance the thermal transport properties in thermal storage system. Nanoparticles could be used in two manners, the first one with Phase Change Material (PCM) as a nucleation agent to help the fast formation of crystals in solidification process. The second manner with Heat Transfer Fluid (HTF) to enhance the rate of heat transfer through the heat transfer process. In the present study a comparison between adding Al2O3 nanoparticles to either distilled water as PCM, and to aqua ethylene glycol solution of 50% wt as a HTF on the performance of cool storage system. A spherical capsule with 85% filling of its internal volume with PCM is used as a tested platform. A set of experiments to study the effect of adding Al2O3 nanoparticles to HTF at different volume fraction concentrations up to 1% for different HTF inlet temperature and volume flow rates on the solidified mass fraction, surface heat flux and complete solidification time, was performed. Comparing the present results with those for nanoparticles with PCM, it could be concluded that adding the nanoparticles to PCM alone is much beneficial to cool storage system than adding it to HTF alone.