The availability of Thermal Energy Storage systems in Concentrated Solar Power plants makes them suitable to handle the gap between energy supply and power demand. Increasing the total thermal energy storage capacity of the Thermal Energy Storage materials used is of interest to improve their efficiency. In this work the thermal energy storage of the so called solar salt (60% NaNO3 - 40% KNO3) was improved by adding a phase change material composed of Al-Cu alloy nanoencapsulated with an aluminium oxide layer naturally formed when exposed to oxygen. The resistance of the oxide shell to thermal cycling up to 570 degrees C and its compatibility with the molten salt were proved. The specific heat and the total thermal energy storage were evaluated at different solid mass loads. Although the specific heat and thus the sensible heat storage decreases with solid content, the contribution of the phase change enthalpy and the latent heat storage can increase the total thermal energy storage up to a 17.8% at constant volume basis comparison. Besides, the thermal conductivity of the nanofluid was increased when adding the nanoparticles improving its heat transfer performance under some particular conditions. (C) 2018 Elsevier Ltd. All rights reserved.