Thermal energy storage (TES) system is an essential component of any concentrating solar thermal power (CSP) plant to ensure a reliable plant operation even at night or cloudy weather. To enhance the TES capacity, a one-step method was proposed to synthesize nano-salts by in-situ production of CuO nanoparticles, via a high temperature decomposition of copper oxalate, in a binary salt used as a phase change material (PCM). The specific heat of the nano-salt both for solid and liquid phases were measured by differential scanning calorimetry (DSC) with the weight fraction of CuO nanoparticles varied from 0.1 to 3.0 wt%. The maximum specific heat increment of 7.96% in solid phase and 11.48% in liquid phase, were achieved at a CuO nanoparticle concentration of 0.5 wt%. A forming of intermediate layers composing of needle-like structures between nanoparticles and the salt was observed. The mixing model considering such an intermediate layer can be used to explain the observed specific heat enhancement at low particle concentrations. Both latent heat and onset temperature were decreased with increasing concentrations of CuO nanoparticles, while the melting temperature range was increased. When considering both latent heat and sensible heat contributions, the maximum increment of TES was achieved as 4.71% at 0.5 wt% CuO concentration in the temperature range from 160 degrees C to 300 degrees C. (C) 2016 Elsevier Ltd. All rights reserved.