Performance improvement of a PCM cold box by two bilayers configuration

A method to increase the storage time of frozen materials in a PCM cold box maintained at a sub-zero temperature has been investigated in this paper. The configuration of a single Phase Change Material (PCM) pack surrounding the payload and subsequently surrounded by an insulation layer is termed as a bilayer configuration. Our study shows that by changing the configuration into two bilayers, which is a series of PCM and insulation pairs, a significant advantage is gained in terms of the total time for which the payload stays at the desired frozen temperature. A parametric thermodynamic model is developed to calculate melting time of the PCM in a single and a two bilayer box configuration having same volume of the PCM and insulation materials. Rather counterintuitive, the model revealed that the optimum design is when the PCM mass is asymmetrically distributed in a two bilayer configuration. Using the properties of a eutectic PCM (melting point of −49.8 °C) and vacuum insulation panels, a storage time increase of more than 30% is achieved when the inner PCM mass is 10% of the total PCM mass. The ambient temperature considered in this example is 20 °C. Furthermore, it is shown that the storage time increases when the temperature difference between the ambient and the PCM melting increases for a two bilayer configuration.