## 2 Materials and Methods
Air exchange during the day is 64.8 m\({}^{3}\)/h, while night ventilation is 1768 m\({}^{3}\)/h. The 650 FF standard specifications also assumed internal heat gains of 200 W-60% through long-wave radiation and 40% through convection, infiltration: 0.5 air change/h, 100% sensible Mechanical System: 100% convective air system, 100% efficient with no duct losses and no capacity limitation, no latent heat extraction, soil Temperature: 10 \({}^{\circ}\)C continuous. Most PCM materials are characterized by hysteresis resulting from the interaction between the processes taking place inside the material. The complexity of the processes that influence the hysteresis effect means that this phenomenon is not fully explained yet and, thus, difficult to model. There are often significant differences, up to 15% [37; 38], between the amount of energy required to melt the material and consolidate it. This fact is difficult to explain from the perspective of the law of conservation of energy. The report [38] contains a hypothesis that energy is stored mechanically through deformation of the matrix, which is a consequence of the formation of dilatations in the PCM structure. The need for more theoretical and experimental research in this area is commonly emphasized. The Energy Plus program, in which the model was made, has open access to the source code (written in C++). This allows user intervention and adjusting the program to your own needs. The source code of the program has been modified to enable the extension of the application functionality in terms of PCM hysteresis. Modification of the algorithm introduces only one change in the interface with data to the program--in the place of entering the coordinates of one curve (temperature and the corresponding enthalpy value); the second, alternative enthalpy value is also entered. The modified algorithm takes into account the hysteresis of materials by predicting the behavior of PCM during material heating and cooling. In the basic variant, it is assumed that the phase changes took place in the entire volume of the material (Figure 4). The actual course of the phase changes is more complicated and is based on the transition from \"Cooling curve state\" to \"heating curve state\", defined as an occurrence of the third type a condition called \"transition curve state\". The states are represented by the following colors: blue--cooling curve; red--heating curve; violet--transition curve. When the cooling phase of the completely melted material has begun (curve 0-1-2, Figure 5a) and before the phase change starts (point 2), the heat is released in a sensible way. After the phase change process has started, the energy is released in both latent and sensible ways (curve 2-3). At this point (point 3), the heating of the material begins again. Thus far, the enthalpy value follows the cooling curve state. The re-start of heating results  
Figure 4: Phase change hysteresis process represented as a function of temperature [26]. in the termination of the cooling curve state and the start of the transition curve state (curve 3-4). For the transition curve, the enthalpy value is determined from either the heating curve or the cooling curve. At point 4, the process of melting of the partially solidified material restarts. At this moment, the transition curve state ends and the heating curve state will restart (curve 4-5 in Figure 5b). After the completion of the phase change, the material again accumulates heat only in a sensible way (curve 5-1) while remaining in the heating curve state. The equivalent specific heat of a PCM in case of the heating curve is calculated in the following way:  
\[c_{p}(T)=\frac{H_{i,h}^{j}-H_{i,h}^{j-1}}{T_{i}^{j}-T_{i}^{j-1}} \tag{1}\]  
where \(H_{i,h}^{j}\)-heating enthalpy of a discrete node '\(i\)' at a time-step '\(j\)' (and respectively '\(j-1\)') [I/kg].