In the context of reducing CO2 emissions and balancing energy supply and demand across the electricity grid, energy storage has become an important topic. Therefore, new energy policies are looking for more efficient and environmental friendly technologies. The aim of this research is to assist in the implementation of the renewable energies technologies and to improve the energy efficiency in well-known and established processes by recovering and storing heat. Moreover, the use of a recycled material as a storage media for thermal energy storage applications shows a more sustainable use of resources reducing at the same time the overall cost. In this research a novel composite, a recycled high density polyethylene (HDPE)/graphite(Cg) mixture, for medium temperature thermal energy storage application has been formulated and characterized. One common characteristic of polymers is their low thermal conductivities. This causes a slow thermal response when the PCM is used in high power applications. In this study the thermal conductivity properties of the HDPE/Cg were enhanced by the optimization of its manufacturing process and composition. Graphite content was added in different mass fractions into the PCM, and thermal properties were measured by means of Thermogravimetric analysis (TGA), Differential scanning calorimetry (DSC) and Laser Flash Analysis (LFA). The experimental results showed that the thermal conductivities are improved by the higher mass fraction of graphite. When the graphite content was in the ratio of 20 wt%, the thermal conductivity of the PCM increased from 0.51 W m(-1) K-1 up to 1.31 W m(-1) K-1. The secondary electron microscopy confirms a good homogeneity of the manufacturing process. Further chemical stability analysis was performed by means of charging and discharging processes. The cycled samples present good thermal property reliability at temperatures up to 250 degrees C. (C) 2016 Published by Elsevier B.V.