Experimental and Numerical Study on the Thermal Response of the Lightweight Aggregate Concrete Panels Integrated with MPCM

Abstract

This paper determines the best design parameters and uses conditions of lightweight aggregate concrete panels containing microencapsulated phase change materials (MPCM-LWAC panels). The main work of this paper includes the followings: (1) The fundamental properties (dry density, thermal conductivity, and specific heat capacity) of MPCM-LWAC were researched to reveal the effect of MPCM dosage on these properties. (2) A model test was carried out to quantify the effect of MPCM dosage on the thermal response of the MPCM-LWAC panel exposed to realistic climate conditions. (3) The numerical simulation was conducted to investigate the effect of MPCM dosage, panel thickness, and outdoor temperature conditions on the thermal response of the MPCM-LWAC panel, which helps to determine its optimum design parameters and use condition. The results showed that the incorporation of MPCM results in lower dry density and thermal conductivity of MPCM-LWAC but higher specific heat capacity. The more MPCM dosage in the MPCM-LWAC panel with a thickness of 35 mm, the lower the energy demand to keep a comfortable interior temperature. Most notably, when the panel thickness exceeds 105 mm, the MPCM-LWAC panel with 5% MPCM only delays the peak temperature. Moreover, the optimal use condition for MPCM-LWAC panels is an average outdoor temperature of 25 °C, which makes the energy demand attain a minimum.

1. Introduction

A 2022 Research Report on China’s Building Energy Consumption and Carbon Emissions shows that our country’s total energy consumption nationwide has reached 45% of the total energy, and building energy consumption (energy consumed by heating, cooling, ventilation, hot water, lighting, household appliances, and cooking and other supplies) accounts for 21.3% of the total energy [1]. Among them, the energy consumption used for heating and cooling is upwards of 70% of building energy consumption [2,3,4,5]. The thermal performance of the building envelope plays a critical role in energy consumption for heating and cooling [6,7]. Hence, the measures to improve the thermal performance of building envelopes are imperative.

Abundant work has shown that adding the phase change materials (PCM) into concrete can improve the thermal inertia of concrete materials without increasing the energy consumption [8,9], which is due to the advantages of PCM with high energy storage density, absorbing or releasing energy in small temperature intervals, and the phase transition without accompanying volume change [10,11,12]. Much recent research has been conducted to investigate the properties of PCM concrete [13,14,15,16].

Zhu et al. [17] used the phase change ceramsite as coarse aggregate to prepare thermal storage concrete for investigating its thermal performance. This study found that thermal storage concrete can reduce indoor temperature fluctuations. Memon et al. [18] prepared the concrete with thermoregulatory properties by adding the porous lightweight aggregate adsorbed with PCM. They found that the energy storage concrete can flatten the indoor temperature fluctuations and keep the load away from peak hours. In the case of large day and night temperature differences, the thermoregulatory properties were further improved. Li et al. [19] carried out an experiment to investigate the thermal response of concrete with PCM, and the study showed that PCM concrete can effectively reduce the peak temperature and delay the appearance time of peak temperature. The temperature regulation ability of PCM concrete was directly proportional to the admixture of PCM. Urgessa et al. [20] first fabricated MPCM by using melamine formaldehyde resin to encapsulate PCM with low transition temperature. Then, the thermal storage concrete was prepared by adding the fabricated MPCM into the concrete mixture to investigate the thermal response of MPCM concrete slabs exposed to real climatic environments. Their findings showed that temperature fluctuations indoors were reduced when the ambient temperature was near the phase change temperature. Yin et al. [21] prepared the thermal storage foam concrete with PCM and investigated its thermal performance. The results showed that when the PCM additive is 15%, the time for the internal surface temperature of PCM foam concrete wall to reach the highest and lowest values were extended by 1.83 h and 1.16 h, respectively. Compared with the pure foam concrete wall, the best effect was achieved when the PCM foam concrete was placed on the inside of the wall.

Lightweight aggregate concrete (LWAC) with a low self-weight and low thermal conductivity was used to prepare the thermal storage concrete by adding MPCM, which helps to develop the building structures by optimizing thermal insulation, energy storage and energy conservation [22]. However, there has been relatively little literature to study how to fully utilize the latent heat stored in concrete in practical applications. Considering the fact that the content of MPCM determines the energy storage density of MPCM-LWAC panels, the thickness affects the heat transfer process, and the outdoor temperature conditions directly determine that of the phase transition behavior, this research will study the effect of three influencing factors mentioned above on the thermal response of MPCM-LWAC panels, which contributes to yield the best design parameters and usage conditions for MPCM-LWAC panels. In this research, non-load-bearing MPCM-LWAC panels with different MPCM dosages were prepared. The effects of MPCM dosage on the dry density, thermal conductivity, and specific heat capacity of MPCM-LWAC were first investigated by performing a series of experiments. Then, the influence of MPCM dosage, thickness, and outdoor temperature conditions on the thermal response of MPCM-LWAC panels was researched by using MATLAB R2022b. Based on this, the energy demand was calculated to assess the energy-saving potential of MPCM-LWAC panels.

2. Test Materials and Methods

2.1. Raw Materials

The MPCM used in this study was purchased from Hebei Ruosen Technology Co., Ltd. It consists of an octadecane core and methyl methacrylate shell. The core/shell ratio, density, and size of MPCM are 6:4, 0.88 g·cm⁻³, and 1–3 μm. The experimental results obtained from the differential scanning calorimetry (DSC) test are shown in Figure 1a. From Figure 1a, it can be found that the latent heat of MPCM is 199.86 J/g for the endothermic cycle, and 169.33 J/g for the exothermic cycle. Based on the DSC curve (Figure 1a), we calculated the specific heat capacity of MPCM, as shown in Figure 1b.

The ceramsite with different particle sizes was selected as the lightweight coarse aggregate (LWCA) and lightweight fine aggregate (LWFA). Their basic properties were tested and shown in Figure 2.

In addition, P·O42.5 ordinary Portland cement was purchased.

2.2. Mixing Proportion

The proportion of MPCM-LWAC is given in

Table 1. The proportion of MPCM-LWAC (kg/m³).

Sample	Water	Cement	LWCA	LWAF	MPCM
MPCM-LWAC-0.0%	187	311.7	455.5	845.80	0
MPCM-LWAC-2.5%	187	311.7	455.5	824.66	21.15
MPCM-LWAC-5.0%	187	311.7	455.5	803.51	42.29
MPCM-LWAC-7.5%	187	311.7	455.5	782.37	63.44
MPCM-LWAC-10.0%	187	311.7	455.5	761.22	84.58

. In this research, the method of equal mass replacement LWFA with MPCM was used to prepare the MPCM-LWAC specimens. In addition, the content of MPCM (not exceeding 10%) has been intensely studied in the preliminary study. For details, see ref. [23]. The specimen named MPCM-LWAC-0.0% in Table 1 was the LWAC without MPCM, which served as the reference group. The specimen named MPCM-LWAC-2.5% indicated that the MPCM dosage is 2.5%. The meanings of the other specimen names are followed by analogy.

2.3. Specimen Preparation

Figure 3a,b provide the preparation process and preparation overview of the MPCM-LWAC specimen, respectively. For this research, three groups of specimens were prepared. The first group was cubic specimens with a side of length 100 mm, the second group was cylindrical specimens with dimensions of Φ200 × 400 mm², and the third group was the panel with a dimension of 300 mm × 300 mm × 35 mm. These three groups of specimens were used to determine the dry density, thermal conductivity, and thermal response, respectively.

2.4. Test Methods

Dry density: The dry density of the selected specimen was calculated by Equation (1).

$$\rho =\frac{m_d}{V}$$

(1)

where “ρ”, “m_d”, and “V” are the dry density, drying mass, and volume of MPCM-LWAC, respectively.

Thermal conductivity: Thermal conductivity of prepared cylindrical specimens was tested by a concrete thermophysical parameter tester (Figure 4a). Significantly, the hole with a diameter of 40 mm passing through the specimen must be set in the specimen’s center. Before the experiment, sealing rings were placed on the upper and lower ends of the specimen to ensure that heat only flowed radially. Furthermore, the water and thermometer were added and arranged in the set hole and outer cavity of the specimen, as shown in Figure 4b. The specimen was first heated up to 60 °C at the beginning of the test. Then, the temperature was kept constant until the specimen reached thermal equilibrium, after which the inner and outer temperatures of the specimen and the heat flow passing through the specimen were recorded every 10 min. Finally, the thermal conductivity for the specimen was calculated according to Equation (2).

$$\lambda =\frac{QIn\frac{r_2}{r_1}}{2\pi L(T_2-T_1)}$$

(2)

where “Q” is the heat transfer from the center of the specimen to the peripheral area in the unit time, KJ/h; “r₁”, “r₂”, and “L” are inner aperture, diameter, and specimen height respectively, m; “T₁” and “T₂” are the temperature at the center of the specimen and the temperature of the cooling water respectively, °C.

Specific heat capacity: According to Equation (3), we calculated the specific heat capacity. Parameters in Equation (3) can be acquired from the DSC curves of the empty crucible (as baseline), sapphire, and MPCM-LWAC.

$$C_p=\overline{C_p}\cdot \frac{\overline{m}}{m}\cdot \frac{y}{\overline{y}}$$

(3)

where “$C_P$” and “$\overline{C_P}$” are the specific heat capacity of MPCM-LWAC sample and sapphire, J/g·°C; “y” and “$\overline{y}$” are the distance difference between MPCM-LWAC sample and sapphire in the vertical coordinate; “m” and “$\overline{m}$” are the masses of MPCM-LWAC sample and sapphire, mg.

Thermal response: A model box with inner dimensions of 300 × 300 × 300 mm³ was made to investigate the thermal response. To prevent heat transfer around the model box, the five-sided model box was insulated with a 40 mm rubber and plastic plate. The panels with a size of 300 × 300 × 35 mm³ were inserted into the top of the box. The sensors of heat flow and temperature were arranged on the inside and outside surfaces of the tested panel for recording the heat flow and temperature. The model boxes were placed in a real climate environment (summer in Xi’an) for 14 days (15–29 June) for real-time testing (Figure 5).

3. Numerical Simulation

3.1. Heat Transfer Model

To investigate the effect of MPCM dosage, panel thickness, and outdoor temperature conditions on the thermal response of MPCM-LWAC panels, a numerical simulation was performed. In order to simplify the model, it is assumed that: (i) heat is transferred only along the panel’s thickness direction; (ii) the MPCM-LWAC panel is homogeneous and isotropic; (iii) natural convection when the PCM is melted into a liquid is ignored; and (iv) there is no heat source inside the panel. Based on these assumptions, the equation for one-dimensional heat transfer across the MPCM-LWAC panel is shown in Equation (4).

$$k\frac{{\partial }^{2}T}{\partial x^2}=\rho C_P(T)\frac{\partial T}{\partial t}$$

(4)

where “k”, “ρ”, and “x” are the thermal conductivity, dry density, and thickness of MPCM-LWAC, respectively, W/(m·°C), kg/m³, and mm; “C_p(T)” is the specific heat capacity as a function of the temperature of MPCM-LWAC, J/(g·°C).

3.2. Numerical Solution

The implicit difference method was applied to solve the mathematical model. As shown in Figure 6, the MPCM-LWAC panel was discretized into a number of nodes, where the distance (thickness) between two adjacent nodes is Δx.

Based on the energy balance equation and implicit difference method, the difference equation of each node inside the panel was obtained, as shown in Equation (5).

$$\left\{\begin{array}{ccc}k\frac{T_2^{t+\Delta t}-T_1^{t+\Delta t}}{\Delta x}+h_{in}(T_{in}^{t+\Delta t}-T_1^{t+\Delta t})=\rho C_p\frac{\Delta x}{2}\frac{T_1^{t+\Delta t}-T_1^t}{\Delta t}& (i=1)& \\ k\frac{T_{m-1}^{t+\Delta t}-T_m^{t+\Delta t}}{\Delta x}+k\frac{T_{m+1}^{t+\Delta t}-T_m^{t+\Delta t}}{\Delta x}=\rho C_p\Delta x\frac{T_m^{t+\Delta t}-T_m^t}{\Delta t}& (2\le i\le N-1)& \\ k\frac{T_{N-1}^{t+\Delta t}-T_N^{t+\Delta t}}{\Delta x}+h_{out}(T_{out}^{t+\Delta t}-T_N^{t+\Delta t})=\rho C_p\frac{\Delta x}{2}\frac{T_N^{t+\Delta t}-T_N^t}{\Delta t}& (i=N)& \end{array}\right.$$

(5)

where “$T_m^t$”, “$T_m^{t+\Delta t}$” are the temperatures of node m at moments t and (t + ∆t), respectively, °C; “T_in”, “T_out” are the internal and external ambient temperatures of the panel, respectively, °C; “h_in, “h_out” are the heat transfer coefficients of the inner and outer surfaces of the panel, respectively, W/(m²·K).

3.3. Simulation Verification

Indoor and outer surface temperatures monitored in the model test (Section 2.4) were used as the temperature boundary conditions in the simulation study. The change of inner surface temperature (T₁) with time was obtained. The T₁ from the experiment and numerical simulation are presented in Figure 7. Figure 7 shows that the change trends of simulated data were basic coincidences with that of experimental data; however, a slight difference exists in the temperature. To evaluate the reliability of numerical solutions, the relative error (RAM) between the test results and simulation results was calculated using Equation (6). The calculation results showed that for panels with different MPCM dosages (0.0%, 2.5%, and 5.0%), the RAM is 2.0%, 2.1%, and 2.5% (≤5%), respectively, indicating the reliability of the numerical results.

$$RAM=A\mathrm{verage}\left(\left|\frac{T_{1sim}-T_{1ref}}{T_{1ref}}\right|\times 100\%\right)$$

(6)

where “T_1sim” and “T_1ref” are the inner surface temperature of the MPCM-LWAC panel obtained by simulation and testing, respectively.

3.4. Simulated Working Conditions

According to the Thermal Design Code of Civil Building (GB50176-2016) [24], the convective heat transfer coefficients of the inner and outer surface of the panel can be set to 8.7 W/(m²·K) and 19.0 W/(m²·K), respectively. To research the effects of MPCM dosage, thickness, and outdoor temperature conditions on the thermal response of the MPCM-LWAC panel, the following three working conditions were designed to carry out numerical simulation studies. In addition, Equation (7) was used to simulate the outdoor temperature.

Case 1: Effect of MPCM dosage on thermal response.

Keeping the thickness of the panel at 35 mm, the dosages of MPCM in the MPCM-LWAC panel were 0.0%, 2.5%, 5.0%, 7.5%, and 10%, respectively.

Case 2: Effect of panel thickness on thermal response.

The MPCM content was taken as 0.0% and 5.0% (0.0% as a reference group), and the panel thicknesses were taken as 35 mm, 70 mm, 105 mm, and 140 mm.

For Case 1 and Case 2, A and B in Equation (7) were identified as 26 °C and 10 °C, respectively.

Case 3: Effect of outdoor temperature conditions on thermal response.

Keeping the MPCM dosage (5.0%) and panel thickness (70 mm) unchanged, the different outdoor temperature conditions were simulated by changing the A and B in Equation (7). For Case 3, In order to simulate the thermal response of MPCM-LWAC panels exposed to various seasonal temperature conditions, A was set to vary from 0 to 40 °C in steps of 5 °C. On the other hand, in order to investigate the effect of one-day temperature difference on the thermal response of MPCM-LWAC panels, B was set to different values, and this study took 4 °C, 6 °C, 8 °C, and 10 °C.

$$T_{out}=\mathrm{A}+\mathrm{B}sin\left(\frac{\pi }{12}t\right)$$

(7)

where “A” represents the average outdoor temperature, °C; “B” represents the outdoor temperature amplitude, °C; “t” represents the time, h.

4. Results and Discussion

4.1. Analysis of Fundamental Properties

The fundamental properties (dry density, thermal conductivity, and specific heat capacity) of MPCM-LWAC with different MPCM dosages were tested and given in Figure 8 and Figure 9.

Figure 8 shows that the dry density and thermal conductivity of MPCM-LWAC decreases as the MPCM dosage increases. When the dosage of MPCM increases from 0.0% to 10%, the dry density and thermal conductivity decrease by 9.0% and 19.0%, respectively. The reasons for this phenomenon may be that the density and thermal conductivity of MPCM are lower than those of the substituted shale ceramic sand. Moreover, the addition of MPCM leads to an increase in porosity, and air has a low density and low thermal conductivity (0.0257 W/(m·°C) [25]. Figure 9 shows that in the tested temperature range, the temperature has almost no influence on the specific heat capacity of LWAC without MPCM. Within the range of phase transition temperature, the specific heat capacity of LWAC with MPCM varies considerably.

4.2. Analysis of Thermal Response

4.2.1. Effect of MPCM Dosage on Thermal Response

The effect of MPCM dosage on T₁ is shown in Figure 10a. According to Figure 10a, the temperature difference (∆T_dosage) between the MPCM-LWAC panels with the MPCM dosage of 2.5%, 5.0%, 7.5%, and 10% and the MPCM-LWAC-0.0% panel were calculated, as shown in Figure 10b. Combining Figure 10a,b, it can be seen that with the increase in MPCM dosage, the peak temperature was delayed on one hand, and the fluctuation of T₁ was decreased on the other. This phenomenon is particularly noticeable in the melting (from 24.9 °C to 28.9 °C) and freezing (from 20 °C to 25.1 °C) temperature range of MPCM. Compared to the MPCM-LWAC-0.0% panel, the peak of T₁ for the MPCM-LWAC-10% panel is reduced by 0.4 °C and its occurrence is delayed by 0.51 h.

Figure 11a shows the variation of heat flux of MPCM-LWAC panels when the MPCM dosage is 0.0%, 2.5%, 5.0%, 7.5%, and 10%. From Figure 11a, it can be seen that within the range of phase transition temperature, the more MPCM dosage, the smaller the change in the flux flow on the inner surface of the tested panel. Furthermore, the energy demand [26], an indicator assessing the energy-saving potential, can be obtained by calculating the cumulative heat flux through the inner surface of the tested panel. The results are given in Figure 11b. Figure 11b shows that when the MPCM dosage is 0.0%, 2.5%, 5.0%, 7.5%, and 10%, the energy demand of the MPCM-LWAC panel is 0.781 KWh/m², 0.759 KWh/m², 0.742 KWh/m², 0.717 KWh/m², and 0.687 KWh/m², respectively. That is to say, compared to the panel without MPCM, the energy demand of other panels with different MPCM dosages was reduced by 2.8%, 5.0%, 8.2%, and 12.0%, in turn.

4.2.2. Effect of Panel Thickness on the Thermal Response

The panels with different thicknesses (Case 2 in Section 3.4) were simulated to investigate the effect of panel thickness on T₁. The results are shown in Figure 12a. From Figure 12a, it can be seen that the fluctuation of T₁ in both the tested panels with and without MPCM decreases as the panel thickness increases. This is mainly due to the increase in thickness caused by the reduction of heat transfer in the unit area of the panel. For the MPCM-LWAC-0.0% panel and MPCM-LWAC-5.0% panel, when their thickness increased from 35 mm to 140 mm, the peak temperature (T₁) declined by 2.9 °C and 3.8 °C and was delayed by 3.21 h and 4.64 h, respectively. For the tested panel with the same thickness and different MPCM dosage, the temperature difference (∆T_thickness) was calculated according to Figure 12a, as shown in Figure 12b. A significant finding from Figure 12b was that when the panel thickness exceeded 105 mm, the increase in MPCM dosage did not cause the increase in ∆T_thickness, but only delayed the time when the maximum value of ∆T_thickness appears.

For the MPCM-LWAC-0.0% panel and the MPCM-LWAC-5.0% panel with different thicknesses, the heat flux and energy demand were given in Figure 13a,b. Figure 13b shows that when the panel thickness increases from 35 mm to 140 mm, the energy demand decreases from 0.781 KWh/m² to 0.394 KWh/m² for the MPCM-LWAC-0.0% panel, and decreases from 0.742 KWh/m² to 0.255 KWh/m² for the MPCM-LWAC-5.0% panel. For an increase in MPCM-LWAC-5.0% panel thickness from 35mm to 70mm, 105mm, and 140mm, the energy demand is reduced by 25.7%, 48.7%, and 65.6%, respectively.

4.2.3. Effect of Outdoor Temperature Conditions on the Thermal Response

The effect of outdoor temperature conditions on T₁ of the tested panel was studied. Three groups of outdoor temperature conditions were selected and given in Figure 14a. Figure 14b gives the temperature difference (∆T_outdoor) of T₁ of tested panels under different outdoor temperature conditions. Figure 14a,b shows clearly that when the average outdoor temperature is 0 °C, 25 °C, and 40 °C, the ∆T_outdoor (the tested panel without MPCM and with 5.0% MPCM) are −0.6~0 °C, −1.4~1.3 °C, and −0.3~0.6 °C, respectively. This finding helps to determine the optimal outdoor temperature conditions for the application of the MPCM-LWAC panel.

Similarly, the energy demand of the tested panels under various outdoor temperature conditions was calculated, and the results were given in Figure 15a. Figure 15a shows that for the MPCM-LWAC-0.0% panel and the MPCM-LWAC-5.0% panel, with the increasing of outdoor average temperature, the energy demand first decreases, reaches its minimum at an average outdoor temperature of 25 °C, and then increases. That is to say, the energy demand is much lower when the average outdoor temperature is close to the melting temperature of MPCM and the outdoor temperature fluctuation Is low throughout the day. According to Figure 15a, the reduction in energy demand of the tested panels was calculated and shown in Figure 15b. As can be seen from Figure 15b, the energy demand reduction of the MPCM-LWAC-5.0% panel achieves the maximum (about 15~17%) at the average outdoor temperature of 25 °C. For the higher (>30 °C) and lower (<20 °C) outdoor average temperatures, the addition of MPCM has a smaller impact on reducing energy demand. The reason for this phenomenon was closely related to the phase-transition temperature range of MPCM. In addition, Figure 15b also shows that energy demand was reduced by about 2% at the higher and lower outdoor average temperatures, which may be caused by the decrease in thermal conductivity of LWAC incorporated with MPCM.

5. Conclusions

With the objective of surveying the optimum design parameters and the best working conditions of the MPCM-LWAC panel, experimental and numerical research was carried out to determine the effect of MPCM dosage, panel thickness, and outdoor temperature conditions on the thermal response of the tested panels. The main findings of this research are as follows.

(1) With the increasing of MPCM dosage, dry density and thermal conductivity of MPCM-LWAC panels decrease but specific heat capacity increases. Comparing MPCM-LWAC-10.0% with MPCM-LWAC-0.0%, its dry density and thermal conductivity decrease by 9.0% and 19.0%, respectively.

(2) In terms of the effect of MPCM dosage on thermal response, the addition of MPCM contributes to the reduction and delay of the inner surface temperature of the MPCM-LWAC panel, and such reduction and delay are proportional to the MPCM dosage. For the MPCM-LWAC panel with a thickness of 35 mm, when the dosage of MPCM is increased from 0% to 10%, the lower energy demand maintaining the comfortable interior temperature is reduced by 12.0%.

(3) In terms of the effect of panel thickness on thermal response, the optimal value of the thickness of the MPCM-LWAC panel is 105 mm. For the MPCM-LWAC-5.0% panel, when panel thickness is greater than 105 mm, the addition of MPCM only delays the peak temperature. In addition, when the thickness increases from 35 mm to 140 mm, the energy demand is reduced by 65.6%.

(4) The best use condition of the MPCM-LWAC panel is obtained by quantitative analysis of the effect of outdoor temperature conditions on its thermal response. With the increasing average outdoor temperature, energy demand decreases at first and then increases. When the average outdoor temperature is 25 °C, energy demand reaches the minimum (0.224 KWh/m²).

(5) This study determines the best design parameters and use conditions of the MPCM-LWAC panel, providing a basis for the application of MPCM-LWAC in building envelope structures.