Fig. 1. Sketch of the system simulated

This system with only a water store is used as the reference system for this study. More detailed information can be found in R. Heimrath and M. Haller [5].

1.1. Storage system

The storage system used in these simulations has different modules of PCM inside the storage tank. To supply the DHW demand, the PCM modules are placed at the top part of the store. The PCM selected is a granular PCM-graphite compound of about 90 vol.% of sodium acetate trihydrate and 10 vol.% graphite.

The properties of this PCM, provided by the manufacturer, are a density of 1.35 kg/L, a melting point of 58°C, a heat capacity of 2.5 kJ/kg-K, an enthalpy of 180-200 kJ/kg, and a thermal conductivity of 2- 5W/m-K.

To supply the heating demand, the PCM modules are placed in the middle part of the tank from a relative high of 1/4 to 3/4, occupying half part of the tank. The PCM selected is a paraffin with a melting point of 48 °C. No more technical data is available.

The PCM simulated is placed into the storage tank in cylindrical modules. The amount of PCM into the store depends on the value of the filling ratio (Parea) and the length of the modules. The filling ratio (Parea) is defined as the cross-sectional area of all modules divided by the cross-sectional area of the storage. Three different values of the Parea ratio were simulated: 0.25, 0.5 and 0.75. When the PCM is added only for DHW supply, those ratios mean a PCM volume ranging from, approximately, 2% to 10% (depending on the module length); when the PCM is used to supply DHW and heating demand, the PCM ratio is from 8% to 25%.