Thermal properties of phase change cement board with capric acid/expanded perlite form-stable phase change material

Abstract

Capric acid/expanded perlite form-stable phase change materials were prepared by vacuum adsorption. The phase change cement boards were prepared by adding capric acid/expanded perlite into cement mortar, and their thermal properties were tested. The results show that capric acid/expanded perlite improves the thermal storage capacity of the cement board; the latent heats of melting and freezing were 14.25 and 14.1 J g⁻¹, respectively, at 20 wt% capric acid/expanded perlite. Capric acid/expanded perlite enhanced the specific heat capacity of the cement board, and the higher the content of the capric acid/expanded perlite in the cement board is, the higher the equivalent specific heat capacity is. Capric acid/expanded perlite also significantly reduced the thermal conductivity and the heat storage coefficient, while at the same time, increased the thermal inertia coefficient of the cement board. Compared with the ordinary cement board, the thermal conductivity of phase change cement board with mass ratios of 10, 15, and 20 wt% of capric acid/expanded perlite decreased by 39.4%, 47.83%, and 52.49% at 20°C and 37.94%, 46.84%, and 50.63% at 50°C, respectively; the heat storage coefficients decreased by 34.07%, 40.62%, and 44.87% at 20°C and 30.25%, 35.59%, and 37.65% at 50°C, respectively; the thermal inertia coefficients increased by 8.75%, 13.78%, and 15.96% at 20°C, and 8.75%, 21.53%, and 26.21% at 50°C, respectively.

Keywords

Phase change materials phase change cement board building materials fatty acids thermal properties

Introduction

As a form of modern architecture, lightweight building materials are being widely used in recent years, especially in high-rise buildings, due to their lightweight, good earthquake resistance, and high degree of industrialization. However, these materials cause the indoor temperature to be higher during the day and lower during the night, which reduces thermal comfort because of low thermal inertia and heat capacity of lightweight materials. Phase change materials (PCMs), as a kind of latent heat energy storage materials with the merits of high-energy storage density, reusability, and approximately isothermal and easily controllable phase change process,^1,2 could be added to lightweight building materials, such as gypsum board, mortar, and others to effectively increase the thermal inertia and the heat capacity of lightweight buildings.³

Fatty acids are one of the commonly used PCMs in the application of building energy conservation because of their high heat capacity, good chemical stability, low supercooling, non-toxic nature, high corrosion resistance, small volume change, and low price.⁴ The direct mixing of PCMs with building materials can result in the leak of molten PCMs after repeated use, which may pollute the wall and decrease the effect of temperature control. Methods like, microencapsulation^5,6 and physical adsorption,^7,8 can effectively prevent the leakage of fatty acid PCMs. Compared to microencapsulation, physical adsorption, especially the adsorption of porous matrix which is cost-effective and operates more easily, is conducive to the utilization of PCMs in the construction field. Many researchers have prepared composite PCMs by combining PCMs with porous materials and studied their thermal properties. Zhang et al.⁹ added palmitic acid–stearic acid (PA-SA) eutectic mixture to expanded graphite (EG) by vacuum adsorption to prepare PA-SA/EG form-stable composite PCMs, and they observed that the content of PA-SA in the PA-SA/EG form-stable composite PCMs was as high as 92.8 wt% and the thermal conductivity increased 8.6 times. Xu and Li¹⁰ tested the adsorption capacity of diatomite to paraffin by preparing paraffin/diatomite/multi-wall carbon nanotubes (MWCNTs) cement. The phase change temperature and the latent heat of these composite PCMs were 27.12°C and 89.4 J g⁻¹, respectively, and the thermal conductivity increased by 42.45% at 0.26 wt% MWCNTs. Sun and Wang¹¹ prepared paraffin/expanded perlite (EP) cement mortar by direct mixing and tested its thermal and mechanical properties. The results showed that the optimal mixing ratio of paraffin mortar was 20 wt%, at which it exhibited good thermal properties and thermal reliability. Furthermore, the thermal conductivity of the composites could be increased by adding EG, and the best proportion was 5 wt%. All the above mentioned studies mainly focus on the enhancement in the thermal conductivity of stabilized composite PCMs with porous matrix which improve the heat transfer efficiency and shorten the energy charging/discharging time. There are few systematic studies on the parameters of thermal insulation performance of building components prepared using stabilized composite PCMs with porous matrix, including thermal conductivity, thermal storage coefficient, thermal inertia coefficient, and so on.

EP when used as the porous matrix effectively prevents the leakage of molten PCMs, and it is also a commonly used building material with the advantages of low thermal conductivity, good fire resistance, high heat resistance, and low density. The combination of fatty acids and EP not only increases the thermal inertia of the materials, but also helps in the storage of energy to adjust the indoor temperature and improve the thermal comfort, effectively solving the problem of low heat capacity and thermal inertia of lightweight buildings.

Considering the average outdoor air temperature during hot summer in south China, which is about 30°C, and the economic efficiency, we chose capric acid (CA) as the PCM and EP as the matrix material in the present study. The CA/EP form-stable PCMs were prepared by the vacuum adsorption method, and cement boards with 10, 15, and 20 wt% CA/EP were obtained by mixing the CA/EP form-stable PCMs and cement mortar. Subsequently, a comprehensive study on the thermal characteristics of the PCMs-based cement boards was conducted, which is expected to provide certain reference values for the application of these PCMs in the field of thermal insulation and temperature control.

Experimental

Materials and equipment

CA (99%) used as the PCM was purchased from Aladdin Industrial Corporation, Shanghai, China. EP (60–100 meshes) was supplied by Dalian Zhongde Perlite Factory and was used as the supporting material to prepare form-stable PCM. Portland cement and river sand were used to prepare the cement board.

The schematic diagram of the equipment used for the preparation of the CA/EP composite is shown in Figure 1. A magnetic stirrer was used to heat and mix the composite uniformly. A one-necked flask on the left side was used to hold the mixture, a separating funnel was used to drop the CA solution, and a condenser pipe and a one-necked flask on the right side were used to condense and collect the alcohol vapor, respectively. All the glass containers and the vacuum pump formed a system to take the vacuum condition.

Figure 1.

Equipment used for the preparation of CA/EP composite PCM.

Preparation of CA/EP form-stable PCM

The CA/EP form-stable PCM was prepared by vacuum impregnation. The EP was dried at 70°C for 2 h in an oven prior to its use in the preparation of CA/EP composite samples. The CA/EP composite samples were prepared according to the processes reported in the literature.¹² A series of CA/EP composite samples with different mass ratios of CA namely, 45, 50, 55, 60, and 65 wt% was prepared. It was found that the CA/EP composite sample with CA mass ratio of 55 wt% exhibited no leakage of melted CA after heat treatment at 50°C for 2 h. This sample is hereafter referred as form-stable PCM.

Preparation of phase change cement board

Two molds with dimensions of 300 mm × 300 mm × 30 mm and 300 mm × 300 mm × 100 mm were used to prepare the cement board. Portland cement and river sand with mass ratio of 1:2 and the CA/EP form-stable PCM with a specific mass ratio were homogeneously blended. Tap water was added to the mixture powder and stirred until the mortar was sticky. Subsequently, the mortar was poured into the mold and vibrated by an internal vibrator until some mud appeared on the surface. The surface of the mortar was smoothed and coated with a plastic wrap to prevent desorption of moisture. The mold was removed after 24 h. Then, the phase change cement board was maintained at room temperature for 3–5 days and dried at 110°C in an oven for 24 h. The mass ratios of the CA/EP form-stable PCM added to prepare the cement board were 10, 15, and 20 wt%, and accordingly, three kinds of phase change cement boards were prepared.

During the preparation process of the phase change cement broad, two common cement boards with thicknesses of 30 and 100 mm were also prepared. The physical properties of the prepared cement boards are listed in Table 1.

Table 1.

Physical properties of the prepared cement boards.

Mass ratio of CA/EP	0 wt%		10 wt%		15 wt%		20 wt%
Thickness (mm)	31	104	31	104	32	103	32	102
Mass (kg)	5.6	18.7	3.8	12.8	3.6	11.5	3.4	11.03
Density (kg m⁻³)	2008	1998	1362	1368	1250	1241	1181	1201
Average density (kg m⁻³)	2003		1365		1246		1191

CA: capric acid; EP: expanded perlite.

Characterization

The specific heat capacity, the phase change temperature, and the latent heat of the phase change cement boards were tested using a differential scanning calorimeter (DSC, TA Q20, USA) at 5°C/min under a constant stream of argon at a flow rate of 50 mL/min. The DSC test temperature was in the range of 10°C–50°C, and the accuracies of the temperature and the enthalpy measurements were 0.1°C and 0.4%, respectively.

The thermal conductivity and the heat storage coefficient of the phase change cement board at 20°C and 50°C were measured based on the one-dimensional heat conduction principle of a semi-infinite object using the constant power plane heat source method. The schematic diagram of the test system is shown in Figure 2. The test system consists of a voltage regulator (range: 0–430 V, TSGC2J-3; Changcheng Electrical Apparatus Group Co., Ltd.), heating films (polyimide, 300 mm × 300 mm, power rating: 360 W, heat-resisting temperature: 180°C), samples, thermocouples (T-type, accuracy: ±0.1°C), a data logger (Agilent 34970A), and a computer. The cement boards with thicknesses of 30 and 100 mm are represented as samples I and II, respectively, in the schematic diagram of the test system. The cement boards were placed symmetrically at the two sides of the heating film as shown in Figure 2. This makes the thermal transfer to the cement boards at the two sides equivalent. The temperature distribution was measured at three test points. Test point 1 was set at the center of the heating film, test point 2 was set at the center between samples I and II, and test point 3 was set at the center of the top of sample II. The profile diagram of the arrangement of the thermocouples is shown in Figure 3. In the experiment, the test was stopped when the temperature of test point 3 changed by more than 0.1°C. This is because it dissatisfies the condition of the semi-infinite heat transfer model.

Figure 2.

Device diagram of the test system using the constant power plane heat source method.

Figure 3.

Profile diagram showing the arrangement of the thermocouples.

Results and discussion

Thermal properties of the phase change cement boards

Figure 4 shows the DSC curves of the CA/EP form-stable PCM and the phase change cement boards. The thermal properties obtained from the DSC curves are listed in Table 2. The melting and freezing temperatures and the latent heat of the CA/EP form-stable PCM were found to be 30.91°C, 28.07°C, and 72.64 J g⁻¹, 71.13 J g⁻¹, respectively. The CA/EP form-stable PCM exhibits suitable thermal properties for building energy conservation. The melting temperatures of the phase change cement boards with 10, 15, and 20 wt% CA/EP form-stable PCM were 30.11°C, 30.57°C, and 30.76°C, respectively. These values indicate that the melting temperature of the phase change cement board slightly increased with increase in the content of the CA/EP form-stable PCM. The freezing temperatures of the phase change cement boards were 29.54°C, 29.15°C, and 28.82°C at 10, 15, and 20 wt% of CA/EP form-stable PCM, respectively. This indicates that the trend of the freezing temperature of the phase change cement board is opposite to that of the melting temperature. Moreover, the temperature range of the phase change process of the phase change cement board shown in Figure 4 is suitable for temperature regulation in regions with hot summer.

Figure 4.

DSC curves of the CA/EP form-stable PCM and the phase change cement boards: (a) melting and (b) freezing.

Table 2.

Thermal properties of the CA/EP form-stable PCM and the phase change cement boards.

	Melting temperature (°C)	Melting latent heat (J/g)	Freezing temperature (°C)	Freezing latent heat (J/g)
CA/EP	30.98	72.64	28.07	71.13
10 wt% PCM cement	30.11	7.03	29.54	7.08
15 wt% PCM cement	30.57	10.85	29.15	10.61
20 wt% PCM cement	30.76	14.25	28.82	14.1

CA: capric acid; EP: expanded perlite; PCM: phase change material.

It can be found from Table 2 that the latent heats of phase change of the phase change cement boards are approximately equal to the products of the latent heats and the mass ratios of the CA/EP form-stable PCM in the cement boards. This implies that there is little loss in the energy storage capacity of the prepared phase change cement board. Although the latent heat of phase change of the phase change cement board is low, it can store sufficient latent heat energy required for a whole building.

The equivalent specific heat capacity of the phase change cement board was measured by DSC, and the results are shown in Figure 5. It can be observed that the equivalent specific heat capacity of the ordinary cement board is constant in the test temperature range, which is about 0.94 J g⁻¹ K⁻¹. The equivalent specific heat capacity of the phase change cement board is also constant during the process without any phase change and is higher than that of the ordinary cement board. The equivalent specific heat capacity of the phase change cement board increased with increase in the content of the CA/EP form-stable PCM. This is attributed to the high specific heat capacity of the CA/EP form-stable PCM. The equivalent specific heat capacity of the phase change cement board presents a peak in the phase change temperature range, and the peak value of the equivalent specific heat capacity increases with the increase of the content of the CA/EP form-stable PCM. A lot of heat is absorbed by the PCM during the phase change process, which is reflected as the equivalent specific heat capacity.^6,13

Figure 5.

Equivalent specific heats of the ordinary and the phase change cement boards.

Thermal conductivity of the phase change cement board

The thermal conductivity of the ordinary cement board was measured to be 0.9160 and 0.9321 W m⁻¹ K⁻¹ at 20°C and 50°C, respectively. The test results of the thermal conductivities of the phase change cement boards are shown in Figure 6. It can be found that the thermal conductivity of the phase change cement board is reduced after the addition of the CA/EP form-stable PCM, and the thermal conductivity increases with the increase of the content of the CA/EP form-stable PCM. In general, heat transfer is mainly caused by elastic waves. The thermal conductivity of a dense material increases when elastic waves can propagate easily through it. A large number of holes were formed in the phase change cement board when the CA/EP form-stable PCM with a low density was added to prepare the cement board. In addition, the compactness of the cement board was reduced, which slowed down the heat transfer. Therefore, the phase change cement board showed a lower thermal conductivity. Moreover, CA used as the PCM can store latent heat, and the phase change process occurs at a nearly constant temperature which is equivalent to extending the temperature difference of heat transfer. This also resulted in lower thermal conductivity. The test results of the present study are consistent with those of Meshgin and Xi¹⁴ and Zhang et al.¹⁵ Compared with the ordinary cement board, the thermal conductivities of the phase change cement boards decreased by 39.4%, 47.83%, and 52.49% at 20°C and 37.94%, 46.84%, and 50.63% at 50°C when the contents of CA/EP form-stable PCM were 10, 15, and 20 wt%, respectively.

Figure 6.

Variations in the thermal conductivities of the ordinary and the phase change cement boards before and after phase transition.

The thermal conductivity of the phase change cement board at 50°C is higher than that at 20°C because the heat conduction and heat radiation between the holes are enhanced by the high temperature of 50°C, and the convection of the melted PCM in the phase change cement board also enhances the heat transfer.

Thermal storage coefficient of the phase change cement board

The thermal storage coefficient of the ordinary cement board was found to be 11.30 and 11.38 W m⁻² K⁻¹ at 20°C and 50°C, respectively. The variations in the thermal storage coefficients of the phase change cement boards at different contents of the CA/EP form-stable PCM at 20°C and 50°C are shown in Figure 7. It can be observed that the thermal storage coefficient of the phase change cement board decreases with increase in the content of the CA/EP form-stable PCM. This is because the density of the CA/EP form-stable PCM is low and the phase change cement board becomes a light structural component after the addition of the CA/EP form-stable PCM. In addition to the density, the thermal conductivity of the phase change cement board decreased significantly as mentioned before. In other words, although the addition of the PCM increased the specific heat capacity of the cement board, the rate of increase is very small even with the addition of 20 wt% CA/EP form-stable PCM. Therefore, the thermal storage coefficient of the phase change cement board is decreased by the addition of the CA/EP form-stable PCM. The percentage decrements in the thermal storage coefficients are about 34.07%, 40.62%, and 44.87% at 20°C, and 30.25%, 35.59%, and 37.65% at 50°C for 10, 15, and 20 wt% of CA/EP form-stable PCM, respectively.

Figure 7.

Variations in the coefficients of thermal storage of the ordinary and the phase change cement boards before and after phase transition.

Thermal inertia coefficient of the phase change cement board

The thermal inertia coefficient of a cement board can be obtained using equation (1)

ξ = \frac{S}{λ}

(1)

where S is the thermal storage coefficient and λ is the equivalent thermal conductivity. The thermal inertia coefficients of all the prepared cement boards obtained using equation (1) are shown in Table 3.

Table 3.

Thermal inertia coefficients of the ordinary and the phase change cement boards.

Samples		0 wt%	10 wt%	15 wt%	20 wt%
Thermal conductivity (W m⁻¹ K⁻¹)	20°C	0.916	0.5551	0.4779	0.4352
Thermal conductivity (W m⁻¹ K⁻¹)	50°C	0.9321	0.5785	0.4955	0.4602
Heat storage coefficient (W m⁻² K⁻¹)	20°C	11.3	7.45	6.71	6.23
Heat storage coefficient (W m⁻² K⁻¹)	50°C	11.34	7.91	7.33	7.07
Thermal inertia coefficient (m⁻¹)	20°C	12.34	13.42	14.04	14.31
Thermal inertia coefficient (m⁻¹)	50°C	12.17	13.67	14.79	15.36

Figure 8 shows the variation of the thermal inertia coefficient of the phase change cement board at different contents of CA/EP form-stable PCM. It can be observed that the addition of the CA/EP form-stable PCM increases the thermal inertia coefficient effectively regardless of the test temperature (20°C or 50°C). In addition, the thermal inertia coefficients of the phase change cement boards increase with increase in the content of the CA/EP form-stable PCM. The thermal inertia coefficients of the phase change cement boards increase by about 8.75%, 13.78%, and 15.96% at 20°C and 8.75%, 21.53%, and 26.21% at 50°C when the mass ratios of the CA/EP form-stable PCM are 10, 15, and 20 wt%, respectively.

Figure 8.

Variations in the thermal inertia coefficients of the ordinary and the phase change cement boards before and after phase transition.

Conclusion

In this study, CA/EP form-stable PCM was prepared by vacuum adsorption and then added to cement mortar to prepare phase change cement board. The effects of CA/EP form-stable PCM on the thermal properties of the phase change cement board were studied. The results show that the addition of the CA/EP form-stable PCM increases the heat storage capacity of the cement board, and the latent heats of melting and freezing of the phase change cement board with 20 wt% CA/EP form-stable PCM were measured to be 14.25 and 14.1 J g⁻¹, respectively. The equivalent specific heat capacity and the thermal inertia coefficient of the phase change cement board increased with increase in the content of the CA/EP form-stable PCM. Furthermore, the thermal conductivity and the thermal storage coefficient of the phase change cement board decreased with increase in the content of the CA/EP form-stable PCM.

Footnotes

Academic Editor: Oronzio Manca

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The work is supported by Sichuan Province Youth Science and Technology Innovation Team of Building Environment and Energy Efficiency (no. 2015TD0015).

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