Scandinavian countries have a high demand of heating and cooling in indoor thermal comfort, representing 45% of the total energy use in Swedish residential and service sector . Nowadays the utilization of this district cooling or heating networks is increasing in the modern societies and some researchers have been studied its possibilities for future enhancements of the systems , .
The improvement of the storage efficiency results in a higher performance of the whole system, and thermal stratification is commonly used for this purpose . Stratification of the water in storage tanks is created by the difference in density between the cold and the hot water. Due to this density difference, cold water remains at the bottom of the tank while the hot water is placed at the top. The larger the temperature difference between these hot and cold water the better the efficiency of the storage. The intermediate region is called the thermocline which has been deeply studied over the years , . Some researchers have been focusing on the inlet distribution manifold which has holes drilled around the circumference to remove the momentum of the incoming fluid and inhibits mixing while allowing buoyancy forces. Yee and Lai  developed a numerical model for a rigid manifold to investigate the effect of various design parameters on the formation of thermal stratification in a water storage tank. Hence, Brown and Lai  investigated the effectiveness of a vertical porous manifold in the formation and maintenance of thermal stratification in a liquid storage tank.
It is well known that thermal energy storage (TES) plays an important role in both industrial and domestic applications, storing heat and cold when available and using it when needed. Therefore, it is of great interest for the researchers to study this type of TES system, enhancing the stratification and the energy storage density of them. One of the most attractive latent cold TES systems is the spherically capsulated phase change material (PCM) filled packed bed. And it also seems to be one of the most effective and convenient methods of encapsulation . Mehling et al.  studied numerically and experimentally the addition of PCM modules at the top of a hot water storage tank with stratification, adding higher storage density in the top layer. Nallusamy et al.  studied the thermal performance of a packed bed latent heat TES unit integrated with solar water heating system, concluding that the utilization of packed bed PCM reduces the size of the storage tank appreciably compared to conventional storage tank system. Some other researchers have been focusing in the numerical studies of heat transfer in packed bed latent heat storage , , .
Hence, in this study a packed bed storage tank with encapsulated PCM is analysed and compared with the same storage system without PCM during a charging process of the low phase change temperature PCM. As the temperature profile does not give the information in the best way to characterize clearly the stratification in a water storage tank dimensionless numbers which condense this information in a single parameter are studied . Therefore the aim of this paper is to analyse the stratification and the thermal characteristics of a water storage tank filled with PCM packed bed and compare it with the same tank without PCM.
2012. 284-285 p.