## 5 Theoretical Effectiveness of the PCM Storage Tank  
The analysis of the stored heat gain was conducted for three rooms of the building described in Section 3: the kitchen, the dining and living room. The calculations presented below were performed using real data from measurements conducted between February 2017 and February 2018. For the entire period, excess heat gain (SEHG) was defined as the sum of all temporary heat gains. According to Equation (1), TEHG is positive if the temperature increases during a single time step with respect to the previous time step and the temperature is higher than the base temperature (20 \({}^{\circ}\)C). The assumption is made that it is possible to accumulate excess heat gain and utilise it later. The device filled with RT24 described in Section 4 was used in the storage tank. The SEHG values achieved in the entire heating season can be seen in Figure 6, except for the intervals when they were equal to 0. The values were defined in 10-min time steps. The average TEHG value for the entire heating season was 19.3 kJ. It is important to note that these values only refer to the three rooms that belong in the day zone of the building. Figure 7 shows the temporary level of charge of the PCM storage tank. Maximal values are the result of limiting the energy capacity of the storage tank, set as 848 kJ and achieved independently during the time period. As seen in Figure 7, the storage tank is effectively used throughout the entire duration of the analysis period, particularly in the winter months from December to February. The changes show that the storage tank is regularly charged and discharged, with hardly any periods when it is not in use. The period of zero values in March is due to no measurement data as a result of faulty measuring equipment. The total amount of heat stored and released into the room is dependent on the assumed airflow. The value of SEHG achieved during the heating season is 88 MJ assuming an airflow of 140 m\({}^{3}\)/h, which corresponds to air exchange only in the kitchen and the living room. Increasing the airflow to the maximal throughput of exchanger (420 m\({}^{3}\)/h), improves the stored energy potential to 208 MJ, which corresponds to an electricity reduction of 33%. Figure 5: The scheme of the phase change material storage tank installation with heat exchanger: (**a**) Horizontal cross section; (**b**) Vertical cross section. The temperature history of the PCM in the storage tank is presented in Figure 8. It can be noticed that material is working above 21 \({}^{\circ}\)C which is the set point that is determined by the indoor temperature. The container is charged or the energy can be regained when the indoor temperature is above the set point. The maximum value of the PCM temperature is below 26 \({}^{\circ}\)C. This means that the material is not overloaded during the whole period of time that was analysed. This is confirmed by the results presented in Figure 9. The melting fraction of PC is below, but almost 1.0. The lower limit is around 0.45. This is an effect of the PCM temperature in the tank, which is above 21 \({}^{\circ}\)C. In that case, some of the paraffin is melted. Figure 6: Excess of heat gains during the analysed heating season for different airflow rates. Figure 7: Temporary charging/discharge of the storage tank for different airflow rates. An additional study was devoted to a numerical analysis of the analysed building and other scenarios. This analysis enabled us to determine the heat gain storage potential for other building construction solutions, including timber frame constructions (low thermal mass). It is expected that the efficiency of heat gain storage in lightweight construction buildings will be much higher than in the case of heavily constructed buildings. For the comparative analysis, the results from the building energy performance simulation was used.