3.2.2. Thermal Capacity Testing

Due to the results of the Micronal tests obtained by the DSC method, a very wide range of temperatures in the DHFMA tests of the plate containing this material was adopted. Since this type of research is extremely time-consuming, a temperature change step of 2K was assumed for the preliminary testing.

The heating stage started at 10 ◦C and finished at 32 ◦C. The cooling of the material was carried out immediately after the heating stage, so the first measurement value obtained after reversing the process was the temperature of 30 ◦C, and the last one was 8 ◦C. Table 2 shows the temperature values that were actually obtained in the apparatus during the measurements and mass heat capacity. Temperature values given in Table 2 differ insignificantly from the measurement conditions set in the FOX 314 apparatus. Figure 2 and Table 2 summarize the results of the measurement obtained during the heating stage, while Figure 3 and Table 3 summarize the results during the cooling stage.


**Table 2.** Mass thermal capacity of Micronal® PCM SmartBoard™ 23, melting stage: 10–32 ◦C.

**Figure 3.** Mass thermal capacity of Micronal® PCM SmartBoard™ 23, cooling stage: 30–8 ◦C. **Table 3.** Mass thermal capacity of Micronal® PCM SmartBoard™ 23, cooling stage: 30–8 ◦C.


Based on the measurement in the FOX apparatus, the volumetric heat capacity [J/m3·K] was obtained. On this basis, after conversion, the value of the mass heat capacity [J/kg·K] was calculated

The apparatus used for the tests described here does not have specialized or very expensive software for dynamic tests. Therefore, obtaining the results was an extremely time- and labor-intensive process. It was necessary to manually control the course of the research and observe the process of stabilization after changing the boundary conditions. The measurement step of the apparatus is approximately 0.7 s. The tested sample is a relatively thin gypsum board, for which the average stabilization time was close to 4 h. For this period, almost 21,000 measurement records were obtained, which had to be further copied and processed. With 12 measurement steps used in the research, the calculation sheet counted over 220,000 records for only one series of tests.

Figure 4 shows the combined results obtained during the heating and cooling of the Micronal® PCM SmartBoard™ 23 in one graph.

**Figure 4.** Thermal capacity of Micronal® PCM SmartBoard™ 23 for melting and cooling stages.

The temperature range in which the phase activity of the material can be observed starts at 16 ◦C and extends up to 26 ◦C. Due to the long duration and preliminary nature of the tests, a large (2K) step of temperature changes over time was adopted, and unfortunately, the stages of heating and cooling were carried out for the same temperature values, without any shift. Thus, the results and graphs obtained in this way are approximates only; they do not allow a reproduction of the real shape of the enthalpy curve or to clearly specify the phase transition temperature. Even with a 2K step, offsetting the heating and cooling cycles by 1K would yield additional information. According to the ASTM guidelines [9], the step of temperature changes can be as small as 0.5K. However, in order to maintain the accuracy of the measurement, several separate measurement series should be carried out for this purpose with a larger step, but mutually shifted by 1K or 0.5K.

The preliminary results of the PCM tests based on a new procedure and the nonstandard use of the FOX 314 apparatus allowed estimations that the phase change in the tested material takes place close to the temperature declared by the manufacturer. The maximum heat capacity of a gypsum board with PCM was 8772 J/kgK, and in terms of volumetric capacity, it was 7.15 MJ/m3K. The authors of the article [8] obtained, in the case of a gypsum board containing 25% paraffin-based PCM, ca. 6 MJ/m3K. The heat capacity values of the tested material during melting and solidification were not identical, showing the hysteresis effect. Solidification took place at a lower temperature than melting, while the maximum heat capacity values obtained were almost identical. Both observations differ from the characteristics of Micronal itself, obtained from the DSC study. The material containing PCM already in the liquid phase has a lower heat capacity than in the solid phase. A similar result was obtained by the authors of [8].

#### **4. Conclusions**

The conducted pilot tests of the thermal characteristics of the phase change material allow the following conclusions:



**Author Contributions:** Conceptualization, K.N., T.K., A.Z.-R. and U.B.; methodology, K.N. and T.K.; software, K.N.; validation, K.N. and T.K.; formal analysis, K.N., U.B., T.K. and A.Z.-R.; investigation, K.N.; data curation, K.N.; writing—original draft preparation, K.N., T.K and A.Z.-R.; writing—review and editing, U.B. and A.Z.-R.; visualization, K.N. and T.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy issues.

**Acknowledgments:** Special thanks to the member companies of Netzsch that provided data for our research. Publication cost of this paper was covered with founds of the Polish National Agency for Academic Exchange (NAWA): "MATBUD'2023—Developing international scientific cooperation in the field of building materials engineering" BPI/WTP/2021/1/00002, MATBUD'2023.

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