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.

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In this article, a thermal energy storage system is studied from the irreversibility point of view. The system consists of some phase change materials arranged in series. The phase change materials are located in the annular space between two concentric cylinders while the working fluid is flowing in the inner cylinder. Charging and discharging processes occur periodically. The conservation equations of energy for the working fluid and the phase change materials are solved numerically. The numerical results are validated when compared with available experimental results. The effect of different physical and geometrical factors on the irreversibility of the system is studied. The results show that the number of phase change materials and their arrangement have great influence on the irreversibility of the system. The temperature distribution inside the phase change materials affects the availability of the system. The proper arrangement of the phase change materials can reduce the irreversibility or increase the availability of the system.