1. Introduction
The choice of appropriate building materials is among the most crucial research actions discussed in the context of the bioclimatic architecture concept to reduce energy consumption in buildings. The implementation of phase change materials (PCMs) into the building envelope is among the most investigated solutions to make the building sector more sustainable [
1]. Applying PCMs to assist heating and cooling allows a reduction in the energy consumption of buildings with a positive impact on climate change [
2,
3,
4].
PCMs are energy-dense materials that absorb or release energy as latent heat during a phase transition process in a narrow temperature range, thereby enabling the efficient use of thermal energy [
5]. Depending on the materials’ nature, it is possible to classify them as solid–liquid or solid–solid phase change materials [
1]. Solid–liquid phase change materials (SL-PCMs) absorb heat during the solid–liquid phase transition [
6]. The solid–liquid PCM type is currently adopted in the building sector in the form of micro-encapsulated paraffins, salt hydrates, eutectic mixtures or water-based ice systems [
7].
Solid–solid phase change materials (SS-PCMs) absorb thermal energy in the reversible transition from a solid in the crystalline or semi-crystalline phase to another solid in the crystalline, semi-crystalline or amorphous phase [
8]. SS-PCMs exhibit inherent advantages over solid–liquid types, i.e., smaller volume variation, no leakage issues, no need for encapsulation, and no nucleating agents needed to prevent supercooling, which makes them suitable for applications requiring long-term performance [
9,
10,
11,
12].
Direct incorporation, macro-encapsulation, micro-encapsulation, immersion, and shape stabilization are examples of techniques used to implement PCMs in the building envelope [
13,
14]. The integration in cement mortars can be realized, i.e., by embedding microcapsules of PCM inside the mixture or by impregnating the pure cement [
15].
The investigation of the influence of PCM integration on the thermal characteristics of building materials as well as the introduction of methods to integrate them in cementitious mortars has gained interest in recent years. Frazzica et al. [
16] investigated the thermal performance of two commercial micro-encapsulated hybrid cement mortar PCMs, Micronal DS 5038X and Micronal DS 5040X, for warm climate applications. The composites with nominal melting points of 23 °C and 26 °C in different percentages were produced and experimentally characterized. A numerical model, validated with the experimental data, was used to perform a parametric analysis aimed at defining an optimized melting temperature in order to reduce the overall energy consumption inside buildings both in winter and summer. The results showed that a melting temperature of 27 °C and a PCM mass fraction of 15% allowed to improve the comfort conditions by about 15% compared to pure cement mortars.
Wi et al. [
17] prepared a bio-based mortar/microencapsulated PCM (B28) composite specimen for external insulation plastering. They showed that the use of the proposed material with a latent heat capacity of 67.92 and 71.48 J/g during the endothermic and exothermic processes achieved significant thermal improvement. In detail, the time-lag effect was improved by 59.4% with 10 wt.% of PCM, indicating excellent thermal storage performance. Hattan et al. [
18] evaluated the thermal behavior of a shape-stabilized PCM incorporated in building plastered walls. The proposed PCM, PEG 600, showed a high potential to provide thermal energy storage capacity in external masonry wall systems by mitigating indoor temperature fluctuation as well as reducing and shifting the peak temperature. A homemade experimental setup was employed to test wall specimens with different PCM contents and water-to-cement ratios. Nemeth et al. [
19] evaluated the heat storage capability and the effects on the thermal characteristics of paraffin PCM (Micronal
® from BASF) microcapsule-loaded gypsum panel plaster. Through an experimental setup, two model houses, the reference one without PCM, were subjected to the same winter climatic conditions. They showed that the peak temperatures were delayed by ~2 h owing to the higher thermal inertia of the PCM walls. Moreover, both the minimum and maximum surface temperature values of the external walls in the PCM-plaster-lined house were ~2 °C higher than those of the reference house with better benefits for human comfort. Mi’ziane et al. [
20] performed a numerical study to simulate the transient thermal behavior of a passive solar wall incorporating microencapsulated PCM. They showed a reduction in the peak temperature of ~1 °C and a shift of about 6.67 h in the peak-hour load due to the increase in the thermal inertia of the wall. Moreover, they showed that the oscillations in the amplitude of the heat flux and the temperature were affected by the wall thickness. Increasing the thickness from 1 cm to 3 cm, the maximum heat flux on the internal wall decreased from 43.98 W/m
2 to 25.41 W/m
2 while the maximum temperature from 27.94 °C to 25.88 °C. Dobri et al. [
21] developed a semi-analytical model to describe the transient heat transfer in micro-encapsulated paraffin in gypsum plaster walls for building applications under constant flux conditions. They showed that the energy demands of an HVAC system were reduced by 15 to 20% with PCM volume loadings as low as 5%. Ramirez et al. [
22] performed steady-state and transient simulations to investigate the thermal performance of solid–solid PCM-based acrylic plaster for thermal energy storage in building applications. They showed that the indoor temperature decreased by 67.26%, whereas the thermal lag increased by 9%. They assessed the viability of the proposed SS-PCM materials to control temperature fluctuations in relation to direct contact with outdoor/indoor applications. Baccega and Bottarelli [
23] carried out experimental tests at the lab scale and numerical simulations with COMSOL Multiphysics to characterize a lime-based plaster with the addition of commercial A28 granular paraffin PCM. One reference lime-based plaster and one with incorporated 10% wt of granular PCM were applied to the external side of a wall. They showed a reduction in the incoming energy between 9% and 18%.
The brief literature review above shows that the vast majority of studies on PCMs has been focused on solid–liquid type.
Some recent studies have shown promising properties of the PCMs referred to as PlusIce [
24,
25,
26]. This class allows to work in a wide range of operating temperatures between +4 °C and +89 °C covering the majority of the chilled water, heat recovery and heating applications in the building sector. Specifically, within the PlusIce class, the solid–solid type X (i) offers the chance to overcome the intrinsic drawbacks of SL-PCMs mentioned above and (ii) presents interesting physical properties that are promising for thermal storage applications. However, a thorough characterization of its properties and thermal performance is still lacking. For the first time, the present paper proposes a comprehensive characterization of the commercial PlusIce X25 integrated into pure cement mortar IN200 for thermal storage in the building envelope via experimental tests and numerical simulations.
A detailed finite element model implemented in COMSOL Multiphysics® 5.6 was validated and calibrated based on the experimental results. The model was used to evaluate, through parametric simulations, the benefits deriving from the application of the proposed solid–solid phase change material as a building construction material.
Most of the studies available in the literature—when carrying out parametric investigations—focus on one or two aspects only (typically, PCM fraction and melting temperatures). In contrast, in the present work, the numerical model represents the basis for giving a wide perspective on the application of the investigated composites towards their real application in buildings. In particular, the optimal mass fraction, PCM-loaded plaster thickness, phase transition temperature and wall stratigraphy are analyzed and discussed, thus making a step forward in the experimental application in realistic conditions. The numerical simulations showed a high potential for follow-ups and future studies. The PCM-loaded plaster’s benefits were discussed in terms of the inner surface temperature and inbound heat flux reduction, the heat latent stored and the attenuation of the temperature peaks.
4. Conclusions
In the present work, the thermal performance of a hybrid cement mortar–PCM composite has been tested experimentally and studied numerically. The experimental results confirmed the advantages of the PCM-loaded plaster; the validation with experimental data showed a reliable prediction of the external surface temperature of the specimen.
Experimental results were used to validate and calibrate a finite element model implemented in COMSOL Multiphysics® 5.6. The model was used to perform a parametric analysis aimed at investigating (i) the effect of the PCM mass fraction, transition temperature and PCM thickness on the thermal performance of the specimen; (II) the thermal behavior of the specimen under real Mediterranean climatic summer conditions; (III) the benefits of the PCM integration for different placements in the external wall of a building.
The results showed that better thermal comfort can be obtained by increasing the PCM mass fraction due to the higher absorption of latent heat. It was found that the energy saving increased almost linearly from 17.7% to 7.46% as the PCM mass fraction increased from 5% to 25%. The PCM thermal regulation ability as well as its storage performance are affected by the phase-transition temperature whose optimal value is dictated by time- and site-specific climatic parameters. The results showed that under summer conditions, the integration of a PCM with a higher transition temperature reduced the daytime wall temperature and increased the nighttime wall temperature. In detail, for the reference day, 17 July, during the sunlight hours, the inner surface temperature and the inbound heat flux increased by 4.71 °C and 4.4 °C and by 39.05 W/m2 and 36.53 W/m2 at 25 °C and 30 °C, respectively.
Increasing the PCM-loaded plaster thickness, a better performance in terms of the heat storage and thermal regulation of the wall can be obtained. It was found that the maximum inner surface temperature reduced from 1.47% to 9.72% as the thickness increased from 15 to 30 mm. The study carried out showed that the choice of a reasonable thickness is a crucial design step to improve (i) the storage efficiency of buildings’ walls, and (ii) the economic and technical potential of the PCM application.
The effect of the PCM-loaded plaster location on the thermal performance of the wall structure was investigated in order to ascertain the ideal position. It was found that when placing the composite on the interior side of the building structure, the transition phase is slower with higher benefits in using the PCM as a storage material. Due to the highest heat flux reduction, this PCM-loaded plaster location represents a reasonable approach to creating better thermal comfort conditions.
Note that the study carried out in this paper was carried out considering the thermal behavior of PCM mortar only for typical Mediterranean climatic conditions. Weather conditions are crucial to determining the degree of interaction between the PCM and the indoor and outdoor environments. The study of the proposed PCM mortar in a wide range of climates will be the focus of future research.