Stratification analysis in packed bed thermal energy storage systems

District cooling and heating networks are increasing in many countries, especially in the Scandinavian countries. Some of the systems have become small for the needs of the population and they have to be enhanced in order to reach the cooling or heating necessities. Here, thermal energy storage system of a district cooling network is studied. Phase change material (PCM) is used in order to enhance the energy density and the stratification of the water tank. The experimental set up consists mainly of a cylindrical storage tank with a capacity of 3.73 L filled with spherically encapsulated PCM. The PCM used is PK6 from Rubitherm GmbH with a storage capacity of 175 kJ/kg between −2 °C and 13 °C. Many methods to characterize water tank stratification, such as graphical (dimensional and non-dimensional) and numerical figures based on temperature distribution (degree of stratification, first law efficiencies, second law efficiencies, other efficiencies as MIX number), are used to analysed and characterized two storage tanks, one of them with the inclusion of PCM packed bed during both charging and discharging processes.

► Graphical and numerical figures are used to study the thermal stratification in both PCM and non-PCM tank.
► Energy and exergy efficiencies were found to have no relationship with thermal stratification.
► Thermocline analysis was found to describe thermal stratification in charging and discharging.
► MIX, stratification and Richardson number describes thermal stratification during charging.