**6. Conclusions**

Numerous experimental tests confirmed that the hysteresis of the phase change process has a noticeable effect on heat accumulation in PCM. It is, therefore, necessary to include this phenomenon in commonly used simulation programs. This article presented the principles of a novel EnergyPlus™ simulation module, which enables the hysteresis effect of phase change in building materials to be taken into account.

In the developed algorithm, apart from the basic difference associated with a different course of the heating and the cooling phenomenon of the material, it is possible to model an incomplete phase change too. This feature of the algorithm seems to be a unique and important aspect of the developed algorithm because incomplete transition quite often appears in the practical applications of PCM and adversely affects the achieved energy effects. The verification of the new calculation module confirmed its usefulness in simulation analyses. The relative calculation error of surface temperature was less than 1%, while in the case of the simplified model without hysteresis, the simulation error would be even a few times higher. We conducted experimental research and created a simulation model that confirmed the practical effects of PCM application as a passive measure of overheating protection. The addition of 30% phase changing material to the gypsum board increased its effective heat capacity several times and doubled the maximum cooling power of PCM wall cladding.

However, the practical effects of PCM application are often much lower than the expectations associated with them. This may be related to several factors that are poorly understood by building designers. One of these is the phase change temperature, declared by the material manufacturer as a specified numerical value, while in reality, the transformation process covers a temperature range of a few degrees. The incomplete phase transition is largely related to the fact that the phase change does not take place at one temperature but in a fairly wide temperature range. Effective phase change temperature ranges of the PCM 23 applied in this research, measured in the DSC apparatus, was 17.8 °C–31.5 °C. As a result, the heat capacity of PCM can be only partially used. Another reason for poor performance is an insufficient temperature fluctuation or too short a heating or cooling stage, during which the material is not fully charged or discharged.

Both experimental tests in the climate chamber, as well as simulation analysis, confirmed the effectiveness of PCM enriched gypsum board as an energy accumulator.

**Author Contributions:** Conceptualization, A.Z.-R., T.K., U.B.; methodology, A.Z.-R., T.K.; software, A.Z.-R., validation, T.K.; writing—review and editing, A.Z.-R., T.K., U.B. All authors have agreed to the published version of the manuscript.

**Funding:** This research was funded by NSERC.

**Conflicts of Interest:** The authors declare no conflict of interest.
