Thermal Performance and Energy Conservation Effect of Grain Bin Walls Incorporating PCM in Different Ecological Areas of China

Abstract

China, as one of the largest grain production countries, is faced with a storage loss of at least 20 billion kilograms each year. The energy consumption from grain bin buildings has been rising due to the preferred environmental demand for the long-term storage of grain in China. A prefabricated phase change material (PCM) plate was incorporated into the bin walls to reduce energy consumption. The physical model of PCM bin walls was numerically simulated to optimize the latent heat and phase change temperature of PCMs for ecological grain storage area. The thermal regulating performance of the prefabricated PCM plate on the grain bin wall was optimized. It was indicated that a higher value of latent heat of the PCM is more suitable for the hotter region for storing grain in bins in this paper. The energy saving did not increase in the same proportion as the increase in latent heat, suggesting a diminishing return. In this study, the optimal latent heat ranged from 180 to 250 kJ/kg. The values of phase change temperature were selected as 31 °C, 28 °C, and 28 °C for Guangzhou, Zhengzhou, and Harbin cities, respectively, corresponding to hot, warm, and cold climates. The percentages of energy saving were 12.5%, 14.8%, and 17.5% with the corresponding phase change temperatures, which showed an advantage of the PCM used in grain bin walls.

1. Introduction

The energy consumption of buildings accounts for 40% of the total world’s primary energy consumption [1]. The major portion of the energy consumption in buildings is to maintain a comfortable interior environment, which is mainly related to the requirement of heating or cooling [2]. The thermal performance of building walls greatly affects the energy consumption to provide a required indoor temperature. Commonly, lightweight heat insulation materials, such as expanded polystyrene (EPS) and extruded polystyrene (XPS), have been widely used to isolate the effect of outdoor conditions on the interior environment of buildings [3,4]. However, the disadvantages of these insulation materials are insufficient durability, inflammability, and complicated construction, etc.

Nowadays, PCMs as a renewable energy technology have been increasingly applied in many fields, such as buildings [5,6], foundations [7,8], and highways [9,10]. Thus, PCM walls are suitably used to construct low-energy-consumption buildings [11,12]. The desired properties of PCMs are high thermal storage capacity, suitable phase change temperature (T_m), chemical stability, low cost, small volume change during solidification, and availability without flammability, toxic distortion, and crystallization problems [13]. During the phase change process, the PCMs can absorb (or release) a great amount of energy at almost isothermal temperature or in a very narrow temperature range [14].

In order to select a suitable PCM for the specified requirement, the thermal storage capacity and phase change temperature of the PCM should be first considered. The thermal storage capacity of PCMs is mostly affected by their latent heat [15]. Paraffin has a high thermal storage capacity, which is extensively used as a PCM [16]. It can be directly incorporated into the building materials, e.g., concrete or plaster, although it might cause the leakage of the PCM [17]. To solve the leakage problem, the composite PCM, for example, high-density polyethylene complexed with paraffin, can effectively prevent the leakage of the PCM during the phase melting process [18]. Thus, different composite PCMs are developed and used in different fields.

The selection of the phase change temperature of PCMs plays a role in greatly utilizing the PCMs, which depends on both the targeted demand of the indoor temperature and the effect of solar radiation [19]. The indoor temperature can be decreased and smoothed in a narrow temperature amplitude range due to the use of PCM walls because the exterior heat transition is successfully delayed by the PCMs [20]. Therefore, the usage of the PCM walls can enhance the energy saving as well as decrease the carbon emissions during the grain storage for the grain bins. To evaluate the energy saving, different analysis methods were applied in terms of the reduction in cooling energy consumption or the total heat gain [21]. In most studies from the literature, the energy saving is represented by comparing the total heat gain reduction of building walls with and without PCMs [22].

The previous studies reported that the PCMs have been capable of regulating the indoor temperature by application in civil buildings [23,24]. The PCMs were usually mixed into cement mortar and concrete to construct the phase change boards or walls [25,26]. Thus, the incorporation of the PCM may enhance the heat storage capacity of these walls. Meanwhile, the PCMs were numerically studied for the PCM-based walls or roofs [27,28]. The high heat storage mass of the PCM was proven to enhance the thermal performance of building walls and roofs, maintaining a comfortable indoor environment.

As for grain bins in China, the technologies of air-conditioning and ventilation cooling have been widely applied to maintain the grain storage temperature in bins, especially in summer, which should be below 25 °C for indoor temperature [29,30]. However, the previous cooling technologies generate high energy consumption and even induce environmental pollution. In this paper, the PCMs were proven to be useful in the grain bin wall to enhance the energy savings during the long-term grain storage.

China is one of the largest grain production countries, with an annual yield of more than 0.65 trillion kg. However, improper grain storage can bring in a loss of more than 20 billion kg each year, equivalent to the food of 100 million people for one year. One of the most efficient ways to reduce grain storage loss is to maintain a low temperature within grain bins. In this paper, the thermal performance of bin walls with PCMs was studied in order to provide a suitable temperature for the long-term stored grain in different climate regions around China. As known, the required temperature of stored grain should be below 25 °C according to the Chinese code [31], at which grain insects will not be produced to decrease the quality of grains or even damage grains. The PCM bin wall can effectively regulate the temperature of the stored grain; thus, it can greatly decrease the energy consumption by air-conditioner cooling, especially in summer.

In this paper, the prefabricated PCM plate was integrated into the concrete wall to construct the PCM bin wall, which was composed of a 40 mm thick PCM layer and a 330 mm thick concrete layer. The PCM layer was located on the exterior side of the concrete wall, which aimed to absorb the exterior heat flux and regulate the temperature of storage grain stored in bins. The physical model of PCM bin walls was numerically simulated to optimize the latent heat and phase change temperature of PCMs for different ecological grain storage areas in China. Due to the different climate effects in different grain storage regions, three typical cities were taken into consideration, representing the hot climate region in Guangzhou city, the warm climate region in Zhengzhou city, and the cold climate in Harbin city around China, respectively. The thermal regulating performance of the PCM bin wall was optimized. Finally, the corresponding energy savings of PCM walls were presented by comparing with the common concrete bin wall.

2. Methodology

2.1. Physical Model

Each typical week in Guangzhou, Zhengzhou, and Harbin city was selected to analyze the thermal performance of grain bin walls, which represent the hot, warm, and cold climates for grain storage in China, respectively.

The typical week was determined by the hottest summer week in July in these three cities. Climatic conditions of each typical week were represented by the hourly outdoor solar air temperature T_eq, as shown in Figure 1 and Equation (1). It combines the outdoor dry bulb temperature T_ex, solar radiation equivalent temperature ρ·I/h_ex, and the external surface temperature of longwave radiation t_γ [32].

$$T_{\mathrm{eq}}{=T}_{\mathrm{ex}}+\frac{\rho ·I}{h_{\mathrm{ex}}}-t_{\mathsf{\gamma }}$$

(1)

where ρ is the absorption coefficient of the external surface, I is the solar radiation intensity (W/m²), and h_ex is the convective heat transfer coefficient of the exterior surface (W/m·K). Here, the value of t_γ was 1.8 W/m² and ρ was 0.48 W/m·K.

Two types of walls were modeled in the simulations. One was named the common bin wall, which was purely a concrete layer with a total width of 370 mm. The other was called a PCM bin wall consisting of a 330 mm thick concrete layer and a 40 mm thick PCM layer presented in Figure 2. The height of the two walls was 2000 mm. The thermal and physical properties of the PCM and concrete are detailed in

Table 1. Thermal properties of materials used in the model [33,34,35].

Materials	Density (kg/m3)	Specific Heat (kJ/kg·K)	Thermal Conductivity (W/m2·K)	Latent Heat (kJ/kg)
Concrete	2400	1.030	1.23	0
PCM	890	1.500	0.8	60–250

. The initial temperature of the wall was 25 °C. Here, T_eq was used as the exterior surface temperature of the wall.

2.2. Governing Equations and Boundary Conditions

The assumptions were as follows:

(1)

A one-dimensional heat transfer was assumed.

(2)

The PCM layer was a pure, homogeneous, and isotropic material.

(3)

The thermal contract resistance between the PCM layer and concrete layer was negligible.

(4)

The surrounding radiations except solar were ignored.

(5)

Heat generation, radiation heat transfer, and natural convection in materials were not considered.

(6)

The top and bottom boundary of the wall were assumed in adiabatic conditions.

According to the assumptions given above, the concrete heat conduction equation can be expressed by Equation (2) [12]:

$${\rho }_1{c}_1\frac{\partial T}{\partial t}=\frac{\partial }{\partial x}({\lambda }_1\frac{\partial T}{\partial x})$$

(2)

where ρ₁ is the density of concrete (kg/m³), c₁ is the specific heat of concrete (J/kg·K), λ₁ is the thermal conductivity of concrete (W/m²·K), T denotes temperature (°C), and t is time. The apparent heat capacity method was used to simulate the phase change process of the PCM. The heat equation of the PCM layer, considering the variation in the latent heat with temperature, is described by Equation (3) [22]:

$${\rho }_2{c}_2\frac{\partial T}{\partial t}=\frac{\partial }{\partial x}{(\lambda }_2\frac{\partial T}{\partial x})$$

(3)

where ρ₂ is the density of the PCM, c₂ is the specific heat of the PCM, and λ₂ is the thermal conductivity of the PCM (W/m²·K).

The exterior surface of the PCM layer was simulated by the combined effects of convective and solar radiation. Meanwhile, the interior surface of the concrete layer was exposed by convective heat transfer. Thus, the mathematical boundary conditions of the exterior surface can be expressed by Equation (4) [32]:

$$-{\lambda }_2\frac{\partial T}{\partial x}{=h}_{\mathrm{ex}}\left(T_{\mathrm{eq}}-T_{\mathrm{ex}}\right){+\rho I+\epsilon \sigma (T}_{\mathrm{eq}}^4{-T}_{\mathrm{ex}}^4)$$

(4)

where h_ex is the heat transfer coefficient of the exterior surface with a value of 18.3 W/m²·K [32], ρ is the absorption coefficient of the external surface, I is the solar radiation intensity (W/m²), ρI represents the absorption of solar radiation, and ε is the surface emissivity. σ is the Stefan–Boltzmann constant with a value of 5.67 × 10⁻⁸ W/m²·K⁴ [32]. The heat transfer in the interior surface of the wall is represented as Equation (5):

$$-{\lambda }_1\frac{\partial T}{\partial x}=h_{\mathrm{in}}\left(T_1-T_{\mathrm{in}}\right)$$

(5)

where h_in is the convective heat transfer coefficient of the interior surface (8.7 W/m²·K) [32], and T₁ and T_in are the indoor temperature and indoor air temperature, respectively.

The heat flux was investigated to analyze the energy saving. The instantaneous heat flux q_(t) was defined as the heat flux going through the wall, which can be expressed by Equation (6):

$$q_{\left(t\right)}=\{\begin{array}{c}{h}_{\mathrm{in}}\left(T_{\mathrm{out}}-T_{\mathrm{in}}\right) \mathrm{for} \mathrm{heat} \mathrm{storage} \mathrm{cycle}\\ h_{\mathrm{in}}\left(T_{\mathrm{in}}-T_{\mathrm{out}}\right) \mathrm{for} \mathrm{heat} \mathrm{release} \mathrm{cycle}\end{array}$$

(6)

where T_out is the outdoor air temperature. Heat storage cycles occurred during solar radiation after sunrise in the daytime, corresponding to heat flux increasing through the wall. In addition, the heat release cycle was identified after sunset at night, corresponding with heat flux decreasing through the wall.

The weekly heat gain Q_gain can be calculated by Equation (7), which was integrated by q_(t) in a week including the heat storage cycle and the heat release cycle. The weekly heat gain difference represented as ∆Q_gain in Equation (8) was expressed by the difference between the common bin wall $Q_{\mathrm{gain}-0}$ and PCM bin wall $Q_{\mathrm{gain}-\mathrm{PCM}}$, as shown in Figure 3. Then, the energy saving can be obtained by Equation (9).

$$Q_{\mathrm{gain}}={{\displaystyle \int }}_0^{168}q\left(\mathrm{t}\right)dt$$

(7)

$$\mathrm{\Delta }{Q}_{\mathrm{gain}}{=Q}_{\mathrm{gain}-0}-Q_{\mathrm{gain}-\mathrm{PCM}}$$

(8)

$$E_{\mathrm{saving}}=\frac{△Q_{\mathrm{gain}}}{Q_{\mathrm{gain}-0}}$$

(9)

The economy analysis of the PCM bin wall was analyzed based on the energy saving analysis. Q_season was the electricity saving of the PCM bin wall in whole cooling season, as shown in Equation (10) [36].

$$Q_{\mathrm{season}}=\mathrm{\Delta }Q·A_1·\frac{\tau }{168}$$

(10)

where A₁ is the total area of exterior walls, m²; τ is the total time in the cooling season, 2160 h [37]; 168 is the time of one week, h. P is the cost saving of electricity in the whole cooling season, which can be expressed as Equation (11) [36].

$$P=\frac{Q_{\mathrm{season}}}{\mathit{EER}}·a$$

(11)

where EER is the air-conditioning cooling energy efficient ratio, 3.2; a is the unit price of electricity, 0.56 CNY/kW·h [37].

The thermal resistance, heat storage coefficient, and thermal inertia index were used to evaluate the thermal properties of grain bin walls. Thermal resistance (R_wall) was calculated by Equation (12), consisting in the sum of the thermal resistance of every grain bin wall. The heat storage coefficient (S₁₆₈) represented the heat storage capacity of the concrete layer and PCM layer, which was calculated by Equation (13). The thermal inertia index D calculated by Equation (14) indicates the periodic attenuated temperature waves within the wall [38].

$$R_{\mathrm{wall}}=\sum R_{\mathrm{i}}=\sum \frac{{\delta }_{\mathrm{i}}}{{\lambda }_{\mathrm{i}}}$$

(12)

$$S_{168}=\sqrt{\frac{2\mathsf{\pi }}{168}{c}_1{\lambda }_1{\rho }_1}=\sqrt{\frac{2\mathsf{\pi }}{168}{·\lambda }_2{\rho }_2\frac{△h}{△t}}$$

(13)

$$D=\sum D_{\mathrm{i}}=\sum R_{\mathrm{i}}{·S}_{168}$$

(14)

where δ_i (i = 1,2) is the thickness of the concrete layer or PCM layer, Δh is the enthalpy of the PCM, and Δt is the phase change diameter of the PCM. D_i (i = 1, 2) is the thermal inertia index of the concrete layer or PCM layer.

2.3. The Validation and Mesh Independence of Physical Model

The experiment conducted by Pasupathy et al. [39] was used to verify the reasonability of the numerical method. The experimental room (1.22 m × 1.22 m × 2.44 m) was constructed to assess the effect of the PCM layer incorporated in the roof on regulating the room temperature. The indoor temperature was set-up at a constant temperature of 27 °C; in addition, the outdoor and indoor heat transfer coefficients were 5 W/m²·K and 1 W/m²·K, respectively.

Figure 1. The temperature distribution in the three cities in July [40].

Figure 1. The temperature distribution in the three cities in July [40].

Figure 2. Physical model of grain bin wall (L1: 40 mm; L2: 330 mm).

Figure 2. Physical model of grain bin wall (L1: 40 mm; L2: 330 mm).

Figure 3. Weekly heat gain description of bin wall.

Figure 3. Weekly heat gain description of bin wall.

Figure 4a represents the ambient temperature, the experimental and the numerical results of the indoor surface temperatures are presented, as well as a maximum difference of 0.85 °C and a relative error of 3.26% between the experimental and numerical values. The difference was mainly caused by the variation in the outdoor environment, which may influence the heat transfer in the roof. Thus, the numerical results showed a good agreement with the experimental results, which provided greater reasonableness for the numerical method.

In Figure 4b, the physical model of the grain bin wall was divided into four kinds of element numbers, corresponding to 450, 1900, 7400, and 29,600. The outdoor temperature was selected by the Zhengzhou region. It can be seen that the four temperature curves presented a great uniformity, which proved the mesh independence of the physical model.

3. Results

In the current work, the thermal performance of different PCM bin walls was evaluated in every typical week of the three cities mentioned above. Two parameters of the PCM were mainly taken into consideration, including the latent heat (60, 90, 120, 180, and 250 kJ/kg) and phase change temperature (26 °C to 33 °C). The thermal performance of the bin wall with or without the PCM was evaluated by its heat flux, heat gain, and temperature amplitude.

3.1. Effects of Latent Heat of PCM

The latent heat of the PCM had a significant effect on the thermal performance of the wall, which primarily affected the regulation capacity of the indoor temperature of the grain bin. In this paper, the latent heat of the PCM ranged from 60 to 250 kJ/kg, considering the common value used in the literature and also the potential maximum value obtained from the tests in the future.

3.1.1. Effects of Latent Heat on T_in and Heat Flux of the Bin Wall

Obviously, the maximum latent heat can provide better regulation capacity of the building. However, the maximum utilization rate of latent heat may differ. Simultaneously, different climates would not affect the use of latent heat. Therefore, Guangzhou was typically selected to evaluate the effect of the latent heat of the PCM on the thermal performance of the bin wall because the difference is very little for different climates.

In Guangzhou, there is a great demand to control the temperature condition of the bin for safely storing grain because of its hot climate. The variations in the indoor temperature T_in with different latent heats of the PCM in the PCM bin wall are described in Figure 5. The peak indoor temperature of the PCM bin wall reduced compared to that of the common bin wall. As shown in Figure 5 during the fifth day, the highest reduction of 0.62 °C was shown by the PCM layer with a latent heat of 250 kJ/kg, while the smallest reduction of 0.3 °C was presented by the PCM layer with a latent heat of 60 kJ/kg. As shown in Figure 6, similar periodic changes of the interior heat flux in the PCM bin wall were also presented during the typical week. The peak instantaneous heat flux of the PCM bin wall gradually reduced by 3 to 5.5 W·h·m⁻² compared to the common bin wall. The reason is that the heat energy was effectively absorbed by the PCM layer because of its state from solid to liquid. The heat transition can be profitably decreased by the PCM layer of the bin wall. It is worth noticing that the indoor heat flux was also reduced with increasing latent heat of the PCM.

3.1.2. Effects of Latent Heat on Heat Gain of the Bin Wall

To evaluate the energy saving associated with the effect of latent heat of the PCM, the weekly heat gain $Q_{gain}$ was investigated in Guangzhou city. As described in Figure 7, $Q_{gain}$ gradually decreased with increasing latent heat of the PCM. The weekly heat gains $Q_{gain}$ were 2621, 2390.5, 2317.7, 2293.3, 2196.4, and 2125.1 W·h·m⁻², corresponding to the common bin wall and PCM bin walls with latent heats of 60, 90, 120, 180, and 250 kJ/kg. Thus, the energy saving $E_{saving}$ was 18.9% for a latent heat of 250 kJ/kg, while it was 16.2%, 12.5%, 11.6%, and 8.8% for latent heats of 180, 120, 90, and 60 kJ/kg, respectively. It can be concluded that the energy saving was promoted with increasing latent heat of the PCM.

As shown in Figure 6, when latent heat changed from 60 to 180 kJ/kg, the energy saving of the PCM bin wall enhanced by 3.7% with every latent heat increment of 60 kJ/kg. For the case from 180 to 250 kJ/kg, the energy saving enhanced by 2.7%. Thus, the energy saving of the PCM bin wall optimized by increasing latent heat, but the energy saving did not increase in the same proportion as an increase in latent heat, suggesting a diminishing return.

3.2. Effects of Phase Change Temperature of PCM

The outdoor temperature will greatly affect the phase change temperature in different climate regions. Thus, all cases in the three cities were studied. The hourly indoor temperatures of the grain bin wall at different phase change temperatures are shown in Figure 8 and Figure 9 for these three cities. For all the cases in one typical week, there were slight fluctuations in the indoor temperature T_in during the initial time. After that, the T_in rapidly increased to its peak value, and then it decreased gradually to the valley value for each case. A heat storage cycle occurred during solar radiation after sunrise, which showed an increasing T_in, while the heat release cycle was identified after sunset when the T_in began to decrease. As shown in Figure 8, the phase change temperature of the PCM had a significant effect on the thermal performance of the grain bin wall. The distribution of T_in of the PCM wall was compared to that of the common bin wall without the PCM layer. It was observed that T_in obviously varied for the common bin wall and the PCM bin wall. The fluctuation in temperature was effectively smoothed with the application of the PCM in the bin wall.

3.2.1. Effect of Phase Change Temperature on T_in

The phase change temperature T_m was investigated during one typical week in summer to evaluate its effect on the bin wall in the three cities mentioned above.

As shown in Figure 8a, the distributions of T_in of the PCM bin wall and the common bin wall in Guangzhou are presented. It can be demonstrated that the T_in curve linearly increased from 0 to 27 h. Then, the amplitude of T_in attained different peak and valley values with the outdoor temperature oscillating.

With the application of the PCM layer in the bin wall, the peak T_in of the PCM bin wall was reduced by 0.2 to 0.4 °C on the sixth day. The highest T_in reduction of 0.4 °C was shown in the PCM bin wall with T_m changing from 31 °C to 33 °C, whereas the PCM bin wall with T_m of 29 °C presented a smaller T_in reduction of 0.2 °C. The phenomenon was due to a suitable T_m profitably decreasing the T_in of the PCM bin wall.

The amplitude of T_in of the PCM bin wall was smoothed in comparison to that of the common bin wall. When T_m was 31 °C, the amplitude of T_in of the PCM bin wall was lower than 0.2 °C. It was indicated that the PCM with suitable T_m can effectively absorb and release the heat energy during the heat storage and release cycle, which can produce more energy saving. A T_m of 31 °C was identified as the optimal phase change temperature of the PCM in Guangzhou city.

Figure 8b displays the indoor temperature profiles of the bin wall with and without the PCM in Zhengzhou, corresponding to the warm climate region. It can be seen that the PCM with T_m of 27 °C can maintain a lower T_in than the PCM bin wall with T_m of 28 °C during the initial 4 days. Then, a higher fluctuation in T_in was presented by the PCM bin wall with T_m of 27 °C. This was because the PCM had completely melted, which could not prevent the heat interaction with the outdoor temperature. It is obvious that a stabilized amplitude was presented by the PCM bin wall with T_m of 28 °C, and the maximum reduction in T_in was 0.4 °C. Thus, the PCM bin wall with T_m of 28 °C could maintain a stabler temperature for the grain storage. The optimal phase change temperature was 28 °C in Zhengzhou city.

For the case of a cold climate region as Harbin, its variation in T_in is represented in Figure 8c for the common bin wall and different PCM bin walls. In this investigation, the PCM bin wall with T_m of 28 °C was optimal to regulate the T_in of the grain bin. Taking the common bin wall as a reference, the largest T_in reduction of 0.4 °C was presented by the PCM bin wall with T_m of 28 °C and it also tends to maintain a T_in below 26 °C. Obviously, a higher T_m, for example, 29 or 30 °C, used in the cold regions could not be effective. Meanwhile, a PCM with a lower T_m, for example, 26 or 27 °C, may release less of the stored heat energy to the outdoor environment during the heat release cycle, which is low efficiency for the heat cycle. Thus, the thermal performance of the PCM bin wall with T_m of 28 °C was especially optimal in this region.

3.2.2. Effect of T_m on the Heat Flux

Figure 9 displays the indoor heat flux of PCM bin walls with different T_m in the three cities. As depicted in Figure 9a, a uniform downtrend of peak heat flux was presented by the instantaneous heat flux profiles in Guangzhou. The peak heat flux of the common bin wall was 21.2 W·h·m⁻², which appeared on the sixth day. Taking the common bin wall as a reference, the indoor heat flux of the PCM bin wall with T_m of 32 °C was reduced by 3.7 W·h·m⁻². It was followed by the PCM bin wall with T_m of 31 °C, which was reduced by 3.3 W·h·m⁻². However, the maximum heat flux reduction of the PCM bin wall was presented by the PCM with T_m of 31 °C during the other days. The reason was that the outdoor temperature on the sixth day was higher than those on the other days, which resulted in the different reduction in peak heat flux.

As shown in Figure 9b,c, the indoor heat flux clearly decreased in Zhengzhou and Harbin. As mentioned above, the thermal performance of the PCM with T_m of 28 °C was optimal in both areas. In comparison to the common bin wall, the indoor heat flux of the PCM bin wall with T_m of 28 °C was reduced by 3.5 W·m⁻² and 3.7 W·m⁻² in Zhengzhou and Harbin, respectively. The PCM with an optimal phase change temperature can easily absorb the outdoor heat flux during the heat storage cycle. Furthermore, the heat release capacity can be enhanced during the heat release cycle. Therefore, the PCM wall with an optimal phase change temperature had the greatest capacity for heat storage and release.

3.2.3. Effect of T_m on Energy Saving

To obtain a preferred energy saving result, the effect of T_m on the energy saving of PCM bin walls was analyzed in the three cities.

As shown in

Table 2. Weekly heat gain and energy saving results with different Tm for hot grain storage region.

City	Parameters	Common Bin Wall	PCM Bin Wall
			Phase Change Temperature of PCM (°C)
			29	30	31	32	33
Guangzhou	Q (W·h·m−2)	2621.0	2244.6	2242.2	2293.1	2372.8	2403.7
	ΔQ (W·h·m−2)	/	376.5	378.8	327.4	248.3	217.3
	$E_{saving}$ (%)	/	14.4	14.5	12.5	9.5	8.3

,

Table 3. Weekly heat gain and energy saving results with different T_m for warm grain storage region.

City	Parameters	Common Bin Wall	PCM Bin Wall
			Phase Change Temperature of PCM (°C)
			27	28	29	30	31
Zhengzhou	Q (W·h·m−2)	1324.3	1095.4	1128.0	1166.0	1209.2	1236.8
	ΔQ (W·h·m−2)	/	228.9	196.2	158.2	115.1	87.5
	$E_{saving}$ (%)	/	17.3	14.8	11.9	8.8	6.6

,

Table 4. Weekly heat gain and energy saving results with different T_m for cold grain storage region.

City	Parameters	Common Bin Wall	PCM Bin Wall
			Phase Change Temperature of PCM (°C)
			26	27	28	29	30
Harbin	Q (W·h·m−2)	1189.6	945.0	1056.8	981.5	1036.7	1099.7
	ΔQ (W·h·m−2)	/	244.6	132.8	208.1	152.9	89.9
	$E_{saving}$ (%)	/	20.6	11.6	17.5	12.9	7.6

, the maximum heat gain $Q_{gain}$ was 2621.03 W·h·m⁻², 1324.23 W·h·m⁻², and 1189.6 W·h·m⁻² for the common bin wall in Guangzhou, Zhengzhou, and Harbin, respectively. Taking the corresponding common bin wall as a reference, the maximum heat gain differences $\mathrm{\Delta }{Q}_{gain}$ were 378.82 W·h·m⁻², 228.88 W·h·m⁻², and 244.59 W·h·m⁻², with energy savings $E_{saving}$ of 14.5%, 17.3%, and 20.6%, respectively. It was concluded that the higher energy saving can be provided by the PCM layer with a lower T_m during the typical week for all the cases.

In the hot climate regions, for example, in Guangzhou, the exterior climate maintained a continuous high temperature, and the maximum exterior temperature was 51 °C in the summer week. The higher exterior temperature may consume more energy to regulate the grain storage temperature in Guangzhou. As shown in Table 2, it was observed that the energy saving $E_{saving}$ was decreased by 14.5% to 8.3% in the typical week. The energy savings $E_{saving}$ of the PCM bin wall with T_m of 29 °C and 30 °C were practically identical at 14.4% and 14.5%, respectively. Additionally, the energy saving $E_{saving}$ of the PCM bin wall with T_m of 31 °C was decreased by 12.5%, which was followed by the PCM bin wall with T_m of 32 °C and 33 °C.

Obviously, the energy saving $E_{saving}$ of the PCM bin wall with T_m of 30 °C was higher than the other cases. However, referring to the above, the T_m of 31 °C was more optimal than in the other cases. The main reason is that the PCM bin wall with T_m of 30 °C had a higher indoor temperature fluctuation than that with T_m of 31 °C. When the outdoor temperature fell in between the PCM with T_m of 29 °C and 31 °C, more heat energy can be released by the PCM with T_m of 29 °C. Thus, it is reasonable that the maximum energy saving effect may not correspond to the optimal phase change temperature. The energy saving $E_{saving}$ of the PCM bin wall with an optimal T_m of 31 °C attained 12.5%.

In the warm climate region of Zhengzhou and cold region of Harbin, the exterior temperatures presented a lower temperature of 20 °C in the nighttime, while the exterior temperature represented a sharp rise in the daytime. Although the exterior temperatures in Zhengzhou and Harbin were lower than that of Guangzhou, the cooling energy demand was also extensively required to control the grain storage temperature in summer. Thus, it was necessary to use the PCMs in the bin walls to reduce the energy consumption.

Table 3 displays a gradual downtrend of energy saving, which was presented by PCM bin walls with T_m of 27 to 31 °C in Zhengzhou city. The energy savings $E_{saving}$ were 17.3%, 14.8%, 11.9%, 8.8%, and 6.6%, corresponding to PCMs with T_m values of 27 °C, 28 °C, 29 °C, 30 °C, and 31 °C. It was concluded that the energy saving $E_{saving}$ of the PCM bin wall was 14.8% with an optimal T_m of 28 °C.

As detailed in Table 4, the energy savings $E_{saving}$ of PCM bin walls were provided for the case in Harbin, which were 20.6%, 11.6%, 17.5%, 12.9%, and 7.6%, corresponding to PCMs with T_m values of 26 °C, 27 °C, 28 °C, 29 °C, and 30 °C. It can be seen that the energy saving $E_{saving}$ of the PCM bin wall was 17.5% with an optimal T_m of 28 °C. It is worth emphasizing that the highest energy saving with the PCM bin wall may not remain a stable thermal environment during the typical week. In the initial 3 days, the maximum energy saving was provided by the PCM with T_m of 27 °C and the PCM with T_m of 26 °C, corresponding to Zhengzhou and Harbin, respectively. Its phase change temperature was lower than those of other PCMs, which contributed to more heat energy absorbed into the PCM layer. When the PCM with T_m of 26 °C (or with T_m of 27 °C) completely melted, its interaction with the exterior environment was then restricted. Then, the heat flux was only reduced by the thermal resistance of the concrete wall and PCM layer. To sum up, the heat energy had been effectively decreased by the PCM with T_m of 28 °C.

Based on the above investigation, the energy saving of PCM bin walls could attain 12.5%, 14.8%, and 17.5%, corresponding to the optimal phase change temperature in Guangzhou, Zhengzhou, and Harbin.

4. Analysis and Discussion

In general, the thickness of the common concrete grain bin wall is 370 mm. The heat transition is resisted by the thermal resistance of the concrete wall, which cannot remain a stable interior environment for the stored grain. Thus, it is necessary to optimize the structure of the grain bin wall. In this study, a 40 mm thick PCM layer followed by a 330 mm thick concrete layer are modeled as the PCM bin wall. The PCM layer is set on the exterior surface of the wall, which mainly aims to reduce the heat transiting into the indoor environment [39].

As shown in

Table 5. The thermophysical properties of bin wall with and without PCM [38].

	PCM Thickness (mm)	Thermal Resistance (m2·K/W)	Regenerative Indicator (W·m−2·K)	Thermal Inertia Index
Concrete bin wall	-	0.3	10.66	3.21
PCM bin wall	40	0.32	39.97	15.63

, the thermal inertia index and heat storage coefficient of the PCM bin wall are approximately four times higher than those of the concrete bin wall. It is indicated that the inward heat transition can be effectively reduced by the PCM incorporated in the bin wall.

With the outdoor heat going through the PCM layer, the PCM absorbs and releases heat energy with a solid–liquid phase change process. In the heat storage cycle, the inward heat energy continuously charges the PCM bin wall. Much heat energy is resisted by the thermal inertia of the bin wall and absorbed by the PCM as latent heat. It also can maintain a relevant constant PCM temperature as long as both solid and liquid phases coexist. After the PCM completely melts, its interaction with the exterior environment is restricted. Then, the heat energy only reduces by the thermal resistance of concrete. On the other hand, a portion of heat energy will transfer to the interior of the grain bin, resulting in an increasing indoor temperature. During the heat release cycle, the stored energy in the PCM primarily releases to the outdoor environment. Then, the indoor temperature reduces with the heat energy transferring to the outdoor environment. In total, the PCM changes its state between solid and liquid with the outdoor temperature changing; therefore, it can automatically regulate the indoor temperature by the PCM itself.

Moreover, the application of the PCM in the walls is desired for maintaining the thermal stability in the thermal cycling process. In our previous work, the leakage test was examined to validate the thermal stability of the composite PCM. There was no leakage of the composite PCM when the heating source rose from 20 °C to 110 °C [35]. The previous studies had heated and cooled the composite PCM from 20 °C to 80 °C to prove its thermal stability by the accelerated thermal cycling analysis [41,42]. In this work, the PCM bin wall displays a great regulating temperature effect on the interior surface temperature in the whole week. The PCM layer can effectively absorb the heat source in the daytime, while releasing the stored energy to the outdoor environment in the nighttime. Thus, the PCMs may provide a great thermal stability in the thermal cycling process.

A suitable PCM latent heat is essential to effectively regulate indoor temperature. The effect of latent heat on the thermal performance is improved with the increase in latent heat [27], whereas the energy saving does not show a continuous increase with the same increment in latent heat. Zhou et al. [43] stated that high latent heat has an important effect on the daily energy storage when the PCM can complete the melting–solidification process during a day. Kishore et al. [21] reported a rise in the cumulative heat gain with the increase in latent heat, while the heat gain increase was reduced. Thus, it can be demonstrated that the maximum utilization rate of latent heat might appear as an increase in latent heat.

The phase change temperature is crucial for the thermal performance of the PCM bin wall for different climate regions. The optimal phase change temperatures are 31 °C, 28 °C, and 28 °C, corresponding to Guangzhou, Zhengzhou, and Harbin. Li et al. [44] reported that the optimal phase change temperature was 36 °C in Las Vegas, NV, USA. It is primarily affected by outdoor inward heat energy. However, some prior studies reported that the optimal phase change temperature is close to the indoor temperature, such as 22 to 24 °C [28]. The difference is mainly caused by the difference of building constructions and climates.

The optimal phase change temperature varies with climate change [44]. In a hot climate region such as in Guangzhou, the PCM phase change temperature is higher than that in the warm climate region of Zhengzhou and cold climate region of Harbin. Mechouet et al. [45] demonstrated that both the thickness and phase change temperature of the PCM used in different climate regions need to be selected, to achieve different energy saving optimization results. Yu et al. [46] reported that compared with the reference wall, the optimum phase change temperature of PCMs varied with different climatic regions. Obviously, climates play a decisive role in the selection of PCMs for different climatic regions.

In this study, the energy saving differs with different PCM latent heats and phase change temperatures in different climate regions. The energy saving is positively promoted by suitable latent heats and phase change temperatures of the PCM, and the energy saving effect can be profitably realized by PCMs in different climates.

According to the physical model of the grain bin (length 36 m × width 24 m × height 8.3 m) [47], the PCM bin walls with optimal phase change temperatures were used to analyze the economy analysis of the grain bin in

Table 6. The economy analysis of PCM bin walls in different grain storage regions [36].

Grain Storage Regions	ΔQgain/W·h·m−2	Qseason/kW·h	P/CNY
Guangzhou-Tm—31 °C	327.4	2095.7	366.7
Zhengzhou-Tm—28 °C	196.2	1256.1	219.8
Harbin-Tm—28 °C	208.8	1337.1	234.0

. The Q_season values of three grain storage regions were 2095.7 kW·h, 1256.1 kW·h, and 1337.1 kW·h, corresponding to Guangzhou, Zhengzhou, and Harbin. Furthermore, the cost of electricity can be saved by 366.7 CNY, 219.8 CNY, and 234.0 CNY for three grain storage regions in the cooling season. It can be seen that the application of the PCM layer in the grain bin is an effective method to decrease the electricity consumption in different grain storage regions.

5. Conclusions

In this study, the thermal performance of the PCM bin wall was simulated, considering the heat storage and release cycle process in three cities for safe grain storage. The main goal was to analyze the effect of latent heat and phase change temperature of the PCM on the thermal performance of the PCM grain bin wall. The obtained results are as follows.

(1)

The latent heat of the PCM is vital to its heat storage capacity to maximally regulate the indoor temperature of the grain bin. The cumulative heat gain presents an uptrend with the rise in latent heat. By contrast, a downtrend is observed for the heat gain increment. The selection of latent heat of the PCM should consider the effective utilization rate of the PCM, rather than the direct selection of high PCM latent heat. The maximum utilization rate of latent heat might appear when the latent heat falls within 180 to 250 kJ/kg.

(2)

The optimal phase change temperature is highly dependent on the climate conditions. The hot solar-air temperature directly requires a higher phase change temperature of the PCM. Proper phase change temperatures are selected by 31 °C, 28 °C, and 28 °C in Guangzhou, Zhengzhou, and Harbin city, respectively.

(3)

The energy saving of the PCM bin wall mainly depends on the phase change temperature and latent heat of the PCM. Based on the above investigation, the energy saving of the PCM bin wall can attain 12.5%, 14.8%, and 17.5%, corresponding with the optimal phase change temperature in Guangzhou, Zhengzhou, and Harbin.