*2.2. PCM Use E*ffi*ciency*

One of the basic items of technical information of PCM is the transition temperature, usually given as one specified value. It is common knowledge that the real physical course of the PCM occurs within a temperature range of a few degrees. Additionally, the transition temperature range is usually di fferent for the freezing and melting curves because of hysteresis. The total transition from solid to liquid and vice versa does not occur when the real temperature fluctuations around the PCM and do not cover the whole range of both phase change courses. In these conditions, the large amounts of heat are neither stored nor released and the operating results of the system do not meet expectations. Berardi and Soudian discussed this aspect extensively, suggesting the use of night cooling to obtain full solidification of the melted material [33] and reported the frequency of PCM activation and the percentage of time with full solidification.

The relatively wide phase-change temperature ranges and the thermal conditions in a room not covering this range can be the cause of only a partial solidification or melting. Such situations occur in practice when the entire transformation range goes beyond the minimum or maximum air temperature in the interior or when the time of maintaining a su fficiently low temperature is too short, and the material only partially solidifies.

In the situation of partial phase change, it is necessary for the simulation algorithm to break the initial process and switch to the second enthalpy curve without reaching the end of phase change. Moreles et al. stated that even all the numerical models tested were not fully able to replicate the behavior of PCM layers if the PCM did not melt or re-solidify completely [31].

The transition process between the heating and cooling enthalpy–temperature curves in the case of incomplete phase change was investigated by Delcroix et al. [34]. In their experiment, the PCM-equipped wall sample was quickly transferred from a cold to a hot environment and conversely. For the interrupted cooling process, the PCM followed an enthalpy curve that was very close to the heating curve. In the case of the interrupted heating process, PCM followed a new curve that was located between the heating and cooling curve. It was revealed that the transition process was sensitive to the actual values of the boundary conditions. In another paper, Delcroix et al. [25] showed that varying heat transfer rates have a significant impact on the phase change temperature range and the hysteresis between heating and cooling curves. Higher rates increase the hysteresis and shift the phase change temperature towards colder temperatures. The authors suggested adjusting a PCM testing method to the perspective conditions of its application. All these results of testing and observations mean that a precise description of the material properties is di fficult.

The problems highlighted above are known to people involved in PCM research, but they are poorly understood by designers or potential investors. The e ffective design of a building in which PCM is applied requires extensive and precise information about the material used, and second, a simulation tool that would allow e ffective modelling of such a phenomenon.

The introduction of the hysteresis modelling and temperature-dependent enthalpy in the EnergyPlus ™ program improved the simulation capabilities of this tool. However, the above-highlighted aspect of incomplete PCM transition should be taken into account when modelling phase change phenomena.
