Heat transfer enhancement of a thermal storage unit consisting of a phase change material and nano-particles

A thermal energy storage unit consisting of two concentric cylinders is studied in this article. The thermal storage unit is filled with a phase change material (PCM). In order to improve the performance of the unit two different methods were used and the methods were analyzed from irreversibility point of view. In the first method the unit is charged periodically, meaning that the temperature gradient inside the PCM drops between two consecutive charging periods and builds up during the next charging period. In the second method nano-particles are added to the PCM, where their high thermal conductivity improves the performance of the unit. The governing equations describing the heat exchange between the working fluid and the PCM were solved numerically. In order to validate the numerical predictions, sample results were compared with the reliable experimental results available in the literature. The agreement between the two sets of results proves the accuracy of numerical predictions. The numerical results indicate that the techniques explained in this paper significantly reduce the entropy generation of the system. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4747824]