Molten salts with high thermal conductivity play a key role to achieve a high thermal energy transfer efficiency during heat charging and discharging processes in a high-temperature concentrating solar energy system. In this work, a novel highly thermal conductive composite phase change material (CPCM) was designed by blending magnesium (Mg) particles with eutectic ternary carbonate salt (Li2CO3-Na2CO3-K2CO3) and used as heat transfer fluid (HTF) and/or thermal energy storage medium in advanced high-temperature concentrating solar power (CSP) plants. The thermo-physical properties of the composite systems were measured. Experimental results indicated that Mg particles dispersed in the molten salts with dendritic structure. The melting temperature of the CPCM had negligible change compared to pure carbonate salt, and the phase change enthalpy reached up to 160 J/g. The effective thermal conductivity was 1.93 W/(m K) for 2 wt% of Mg, which was enhanced by 45.11% compared to pure ternary carbonate salts. The upper limit working temperature of the CPCM was measured to be 725 degrees C in argon atmosphere. The wide working temperature range (Delta T = 325 degrees C) indicated its great thermal stability and high capacity of energy storage. (C) 2017 Elsevier Ltd. All rights reserved.