**1. Introduction**

Modern buildings are characterized by significantly improved thermal insulation and the other solutions aimed at minimizing energy demand. A frequently used techno- logy for erecting buildings is a technology based on durable structural elements forming the skeleton filled with thermal insulation. Heat transfer through such partitions is minimized, but the disadvantage of this technology is the low thermal mass. In addition, oversized glazing often causes a disproportion between the solar gains and the limited thermal capacity of the building. The inability to counteract significant temperature fluctuations in the rooms has a very negative effect on the microclimate and the feelings of people staying in them, as well as on the efficiency of solar energy use.

The ability to store excess heat energy and release it when the room cools down results in the better use of solar radiation in the winter and reduces the risk of space overheating in the winter and summer. The occurrence of a too-high temperature is usually connected with the use of expensive and energy-intensive air-conditioning equipment. In low energy buildings, the ability to store energy becomes particularly important.

The lack of a time coincidence of the demand and possible supply of energy gains contribute to the development of research on heat and cold storage. One of the effective ways of passive accumulation of thermal energy in buildings is the use of materials that change the state of aggregation, usually marked with the abbreviation PCM (phase changing material).

**Citation:** Nowak, K.; Kisilewicz, T.; Berardi, U.; Zastawna-Rumin, A. Thermal Performance Evaluation of a PCM-Integrated Gypsum Plaster Board. *Mater. Proc.* **2023**, *13*, 39. https://doi.org/10.3390/ materproc2023013039

Academic Editors: Katarzyna Mróz, Tomasz Tracz, Tomasz Zdeb and Izabela Hager

Published: 20 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

During the change of aggregation phase, a large amount of heat is accumulated or released. Particularly in buildings erected using light construction technologies, the application of additional energy storage in the form of PCM proves to be very effective [1]. PCM can contribute to reducing the peak demand on cooling and allow for more even operation of the air conditioning system, also reducing the need for heating at night. Thanks to the use of PCM, it is possible to avoid or at least reduce the currently frequent effects of the overheating of buildings with excessive glazing [1,2].

Phase change materials offered for building use can take the form of:


This article presents the results of preliminary tests of the thermal capacity of a plasterboard integrated with PCM. The purpose of these studies was to test a new research technique, the necessary conditions for conducting research and a completely different than designed application of the FOX 314 apparatus for measuring the thermal conductivity of building materials. The apparatus and its original software were designed to conduct tests in stationary conditions, while thermal capacity tests require dynamic conditions.

#### **2. Measurement Methods of PCM Thermal Characteristics**

The proper selection of the PCM type and its quantity requires the precise experimental determination of the thermal characteristics of heterogeneous products.

Differential scanning calorimetry (DSC) is one of the most widely used PCM measurement methods because of the ease with which different thermodynamic data can be obtained. DSC measures the amount of heat needed to raise the temperature of the test sample in a given temperature range [4]. On this basis, the phase change temperature, enthalpy, heat capacity and specific heat can be determined. However, the DSC method is applicable to millimeter-scale samples with weight in the order of a few milligrams. The DSC method also requires relatively homogeneous samples. Due to the large heterogeneity of composites on a small-scale sample, determining the average PCM content in a mass-produced product requires many tests, which is troublesome and expensive, and may be subject to a large error of sample randomness [3]. In the case of composite materials (PCM—enhanced building products), with unevenly distributed PCM in the structure, testing according to the DSC method does not make sense.

In search of a possibly fast and at the same time precise method that can be used in determining the thermodynamic parameters of large samples of PCM-reinforced building components, a dynamic method using a plate apparatus was developed.

In the Oak Ridge National Laboratory, a dynamic method using a plate apparatus was developed to measure the content of phase-change material in composite samples [6,7]. The plate apparatus is basically used to measure the thermal conductivity of materials in a steady state of heat conduction. However, it can also be used to measure the dynamic thermal properties of samples of the tested materials, in which the heat flux stabilization is relatively slow due to the ongoing phase change. The developed method is referred to as dynamic HFMA or DHFMA. Since the beginning of its development, the DHFMA method has been subject to constant modifications. The theory of the method and test procedure has been described by the creators of the method [8].

In an isothermal process such as a phase transition, the enthalpy change (ΔH) is equal to the amount of heat absorbed (heat input) or released (heat output) during the process (ΔQ). The heat capacity (cp) is the temperature derivative of the enthalpy (H), i.e., cp = dH/dT.

Assuming there is a linear relationship between enthalpy and temperature for small temperature increments (ΔT):

$$\mathbf{c}\_{\text{P}} = \mathbf{d} \mathbf{H} / \mathbf{d} \mathbf{T} \approx \Delta \mathbf{H} / \Delta \mathbf{T} = \Delta \mathbf{Q} / \Delta \mathbf{T},\tag{1}$$

where:

ΔH—enthalpy change [J/m2];

ΔQ—energy amount absorbed or released during a given process [J/m2];

ΔT—temperature increase [K].

In the DHFMA method, the temperature of the sample is changed in small steps and the resulting heat flow into or out of the sample is measured during this process. The heat capacity at an average temperature is then determined using Equation (1). The DHFMA method uses a conventional HFMA instrument containing at least one heat flow sensor on each of the isothermal plates. The sample is placed parallel to the plates while the sides are thermally insulated to achieve near-adiabatic conditions. Both isothermal plates are kept at the same temperature. A temperature step is then applied to both plates and the heat flows through the plates are measured until a thermal equilibrium is reached where the heat flow values become negligible. In the case of a step change in temperature, the change in enthalpy per unit area of the sample is calculated by integrating the heat flow rates over time (2):

$$
\Delta \mathbf{H} = \Delta \mathbf{Q} = \sum [\{(\mathbf{q} \mathbf{U}\_i - \mathbf{q} \mathbf{U}\_{\text{final}}) + (\mathbf{q} \mathbf{L}\_i - \mathbf{q} \mathbf{L}\_{\text{final}})\} \pi] \tag{2}
$$

where:

qUi—heat flux flowing through the upper plate, recorded at time intervals τ [W/m2];

qLi—heat flux flowing through the bottom plate, recorded at time intervals τ [W/m2]; qUfinal—the value of the remaining heat flux of the upper plate, caused by side heat losses in the equilibrium state [W/m2];

qLfinal—the value of the remaining heat flux of the lower plate, caused by side heat losses in the equilibrium state [W/m2];

τ—time step between readings [s].

The volumetric heat capacity of the PCM-enhanced component is determined as follows (3):

$$\mathbf{c}\_{\text{V}} = (1/\text{l}) \cdot (\Delta \mathbf{Q} / \Delta \mathbf{T}),\tag{3}$$

where l is the thickness of the sample. It should be noted that the DHFMA method can be used to measure the heat capacity of any solid or liquid material. Recently, ASTM introduced the test standard C1784 for measuring the thermal storage properties of phase change materials and products, based on the DHFMA method [9].
