Enhanced performances of macro-encapsulated phase change materials (PCMs) by intensification of the internal effective thermal conductivity

Performances of spherical macrocapsules (nodules) currently used in latent heat-based thermal energy storage (TES) industrial units have been enhanced by the addition of graphite particles to the phase change material (PCM). Two different graphite types, namely graphite flakes (GF) and expanded natural graphite (ENG), have been tested at constant PCM content in the nodule. Using water as PCM, both graphite types have been proven to lead to significant reduction in storage/discharge durations (up to 35% and 58% for a graphite load of only 13%wt) without reduction in storage capacity. Therefore, enhancement using ENG greatly enhances efficiency, but it is also more expensive. GF maybe preferred, considering both its ease of use and economical issues. At the highest experimented graphite load (13%wt) the overall thermal behavior of the nodule is advantageously improved, with simultaneously no apparent supercooling,a very stable phase change plateau, and very sharp and straight sensible heat exchange periods. The graphites induce both extensive thermal power enhancement and improvement in thermal behaviors. These experimental results have been simulated using numerical Comsol®-based models with success. The simulated charge/discharge steps have shown that the air gap present in the nodules induces modifications in the phase change front profile only at the beginning of the periods.