Thermal Performance Analysis of Aluminum Alloy Phase Change Panels for Regions with Hot Summers and Warm Winters

: Utilizing phase change materials (PCMs) in passive energy-saving wall panels to regulate indoor temperatures during hot seasons can improve people’s thermal comfort and reduce the energy consumption of air conditioning systems. This study is based on the hot summer and warm winter climatic characteristics of Hainan. According to local meteorological data and residents’ living habits, a suitable phase change temperature of approximately 28 ◦ C was determined. A composite PCM of paraffin and stearic acid n-butyl ester was prepared and tested for thermal performance. Encased in an aluminum box with non-penetrating aluminum rods to enhance heat transfer, the phase change panel was applied to the inner side of exterior walls. Thermal tests demonstrated that increasing the mass ratio of stearic acid n-butyl ester to paraffin lowers the melting point and latent heat. At a 3:7 mass ratio, the melting point of the composite PCM was 28.30 ◦ C, and the latent heat was 128.26 J/g. The 20 mm thick panel maintained a stable phase change process, with unheated surface temperatures between 28 ◦ C and 29 ◦ C for up to 180 min. Compared to panels without aluminum rods, those with rods exhibited a 20% longer phase change time, extended heat transfer paths, and reduced liquid-phase convective heat transfer rates, demonstrating improved PCM utilization. Therefore, the phase change panel with non-penetrating aluminum rods exhibits excellent insulation and temperature control properties.


Introduction
Phase change materials (PCMs) are substances that can change their state while maintaining a constant temperature, thereby providing latent heat during the phase transition.Latent heat storage technology [1] leverages the absorption or release of heat during the phase change process, addressing the limitations of sensible heat storage, such as the inability to retain heat over extended periods.Additionally, PCMs offer a high energy storage density and do not involve chemical reactions.By utilizing the unique latent heat storage properties of PCMs, they can be integrated into building materials to create phase change wall panels.These panels can enhance the thermal regulation of building envelopes, effectively moderating indoor temperatures during high-temperature seasons and reducing the energy consumption of air conditioning systems in order to achieve significant energy savings in buildings.
The low thermal conductivity and slow heat storage and release rates of most PCMs are significant factors limiting their widespread application [2].Enhancing heat transfer has always been a key focus in the field of thermal energy storage and is crucial for the advancement of phase change thermal storage technology.Extensive research and experiments have been conducted to enhance heat transfer in PCMs.Nourani [3] dispersed Al 2 O 3 nanoparticles into paraffin at different mass fractions.When the mass fraction was 10%, the thermal conductivity of the PCM increased by 31% in the solid state and by 13% in the liquid state.Aqib [4] incorporated 2%, 4%, and 6% multi-walled carbon nanotubes (MWCNTs) into paraffin and compared the internal temperatures.The results indicated that as the mass fraction of MWCNTs increased, the temperature rise rate of the composite PCM improved.The composite PCM containing 6% MWCNTs exhibited the best temperature rise effect and thermal performance.Zheng [5] found that the melting time of copper foam/paraffin composite PCM was reduced by 20.5% compared to pure paraffin.Shang [6] prepared a composite PCM with a mass ratio of 4:1 between the PCM and expanded graphite using the melt blending method.Its thermal conductivity reached a maximum value of 3.084 W/(m•K) at 40 • C. Oliveski [7] investigated the effect of fin length-to-diameter ratio on the phase change process in a rectangular cavity.They found that fins with a high length-to-diameter ratio exhibited melting and solidification rates that were 16% and 15% faster, respectively, compared to fins with a low length-to-diameter ratio.Currently, heat transfer enhancement methods are mainly divided into three categories: the first method involves adding high thermal conductivity metal nanoparticles or carbon nanotubes to the PCMs; the second method involves combining PCMs with high thermal conductivity porous materials, such as metal foams or expanded graphite; the third method involves incorporating high thermal conductivity metal fins, such as plate-like or needlelike fins, into the PCMs.Adding metal fins can increase the heat transfer area, thereby accelerating convective heat transfer within the PCMs and enhancing the heat storage and release rates.Due to their robust structure and the increasingly mature manufacturing techniques, metal fins have been widely used in various heat exchange devices, such as heat sinks and heat exchangers.
To address the issue of liquid phase leakage during the phase change process of solidliquid PCMs, encapsulation methods have emerged as a key breakthrough for applying this technology in the construction field [8].Abbasi [9] mixed the PCM with cement mortar to produce a hybrid mortar.Pomianowski [10] added microencapsulated paraffin to concrete to produce phase change concrete.Zhang [11] embedded the PCM into gypsum boards to create "sandwich"-structured phase change energy storage gypsum boards.Fuentes [12] and Dhaidan [13] investigated the melting process of PCMs in plate energy storage units, taking into account multiple factors including material properties and encapsulation structures.The encapsulation methods for PCMs primarily include direct mixing, macroscopic encapsulation, microscopic encapsulation, and shape-stabilized encapsulation.Among these methods, macroscopic encapsulation [14] stands out, as it maximizes the preservation of the inherent thermophysical properties of PCMs, thereby avoiding performance degradation.Additionally, macroscopic encapsulation offers larger encapsulation volumes and higher energy storage densities, leading to more effective energy storage.Moreover, it is relatively cost-effective compared to other encapsulation methods.
Currently, research on the utilization of PCMs in building walls is increasingly abundant.Zhang [15] proposed a phase change wall that can switch positions with the internal insulation layer.This design can save 89% of the energy required for heating during a severely cold winter.Ling [16] calculated the heat storage and release capacity of phase change walls on sunny and cloudy days.The results confirmed the positive role of PCMs in long-term improvements of the thermal environment inside greenhouses.Androniki [17] studied the impact of phase change layers at different positions within walls under various climate conditions.It can be concluded that in non-air-conditioned indoor spaces, the phase change layer is most effective when placed near the inner surface of the wall.Diaconu [18] proposed a phase change wall composed of an insulation layer, a phase change layer, and insulating materials, and determined the phase change temperature of the PCM to maximize energy savings.The results showed that this wall can effectively reduce the peak heating and cooling loads throughout the year.Luo [19] proposed a solar phase change wall with dual channels and intermediate insulation that enables passive solar heating and cooling.It was found that the phase change wall delayed the occurrence of peak temperature and reduced the peak temperature.Zhang [20] developed a heat transfer model for the melting and solidification process of PCM-filled brick walls and conducted a numerical analysis of the thermal response of the wall.The results showed that under fluctuating outdoor temperature conditions, the PCM effectively enhanced the heat storage capacity of the brick wall and improved indoor thermal comfort.
This study, based on the climatic characteristics of the Hainan region, prepared paraffin/butyl stearate composite PCMs with suitable phase change temperatures using a melt blending method.Aluminum boxes were used to encapsulate the PCMs, and nonpenetrating pin-shaped aluminum rods were incorporated.These structures form aluminum alloy phase change panels, which were applied to the inner side of exterior walls.Thermal experiments under different working conditions were conducted to explore the heat transfer process and mechanism of the phase change panels, the role of the pin-shaped aluminum rods, and the practical application effects of the phase change panels.Unlike previous studies on phase change wall panels, this study developed composite PCMs with adjustable phase change temperatures, making them applicable in various scenarios.Additionally, using aluminum boxes to encapsulate the PCMs simultaneously addresses the issues of liquid phase leakage and low thermal conductivity of solid-liquid PCMs.The study results provide new experimental insights for the performance regulation of PCMs and the optimization design of phase change wall panels.

Preparation of Composite PCMs
Hainan Province in China is located in a hot summer and warm winter region, with the summer climate predominantly hot and humid.Based on a thermal comfort survey of Hainan residents and the research presented in the literature, it has been found that long-term residents of Hainan have adapted to the local climate.With good ventilation, they can feel comfortable even if the outdoor temperature exceeds 36 • C, as long as the indoor temperature remains below 28 • C [21,22].Therefore, the suitable phase change temperature for the Hainan region is around 28 • C.
Paraffin and butyl stearate, commonly used organic solid-liquid PCMs, have become prominent in recent research due to their high phase change latent heat, chemical stability, non-corrosiveness, and cost-effectiveness.To meet the specific needs of buildings in Hainan, composite PCMs with suitable phase change temperatures can be prepared by adjusting the mass ratio of paraffin and butyl stearate [23,24].The paraffin selected for this study had a solid density of 760 kg/m 3 , a specific heat capacity of 2140 J/(kg•K), a thermal conductivity of 0.21 W/(m•K), and a melting point of 30 • C to 32 • C. The butyl stearate selected for the study had a liquid density of 865 kg/m 3 , a specific heat capacity of 2300 J/(kg•K), a thermal conductivity of 0.15 W/(m•K), and a solidification point of 19 Different mass ratios of butyl stearate/paraffin composite PCMs were prepared using a melt blending method.The temperature of a constant-temperature heating magnetic stirrer was set to 80 • C. The constant-temperature heating magnetic stirrer was manufactured by Yuexin Instrument Company of Changzhou City, China.Once the water bath reached 80 • C, a beaker containing paraffin was placed into the water bath.After the paraffin had completely melted, different mass ratios of butyl stearate were added.The stirrer was then turned on to mix the materials thoroughly.After mixing for approximately 1 h, the mixture was allowed to cool naturally, resulting in composite PCMs with different mass ratios.Table 1 shows the mass of n-butyl stearate (m 1 ), the mass of paraffin (m 2 ), and the mass ratio of the two (m 1 :m 2 ) in different experimental groups.

Thermal Performance Testing of PCMs
The melting point and latent heat of fusion of the composite PCMs were determined using Differential Scanning Calorimetry (DSC) [25].Figure 1 shows the key instruments for DSC testing.First, 3 to 10 mg of the sample was weighed using an electronic balance with an uncertainty of 1 mg, with a crucible and tweezers, and then sealed with a press mold machine.The DSC instrument was preheated with a nitrogen gas flow.After preheating, the sample was placed on the instrument's sample tray.Then, the temperature range was set between 0 • C and 60 • C, with a heating and cooling rate of 5

Thermal Performance Testing of PCMs
The melting point and latent heat of fusion of the composite PCMs were determined using Differential Scanning Calorimetry (DSC) [25].Figure 1 shows the key instruments for DSC testing.First, 3 to 10 mg of the sample was weighed using an electronic balance with an uncertainty of 1 mg, with a crucible and tweezers, and then sealed with a press mold machine.The DSC instrument was preheated with a nitrogen gas flow.After preheating, the sample was placed on the instrument's sample tray.Then, the temperature range was set between 0 °C and 60 °C, with a heating and cooling rate of 5 °C/min controlled by the computer software.During the heating and cooling process, the temperature and heat flow data of the sample were recorded.
By analyzing the DSC curves, the melting point and latent heat of fusion of the sample were determined.The melting point was identified as the intersection between the tangent at the point of maximum slope on the left side of the melting peak with the baseline.The latent heat of fusion was calculated as the area enclosed between the melting peak and the baseline.To minimize errors, samples were taken from different positions of the material and the experiment was repeated three times.The average value was used as the final test result.

Preparation of Phase Change Panels
The aluminum alloy phase change panels were prepared using a macroscopic encapsulation method.Aluminum alloy boxes were selected as the encapsulation material.Drawing inspiration from heat sink designs, non-penetrating pin-shaped aluminum rods were placed inside the aluminum box to ensure full contact with the PCM.This design increases the heat exchange surface area to enhance heat transfer.The cover panel and aluminum box were tightly sealed using a locking mechanism, forming the aluminum alloy phase change panel.
Aluminum alloy is not only lightweight and strong, but also has excellent thermal conductivity, which effectively transfers heat between the environment and the PCM, thereby improving the overall energy efficiency of the system.Figure 2 shows the specific structure of the aluminum alloy phase change panel.To emphasize the pin-shaped By analyzing the DSC curves, the melting point and latent heat of fusion of the sample were determined.The melting point was identified as the intersection between the tangent at the point of maximum slope on the left side of the melting peak with the baseline.The latent heat of fusion was calculated as the area enclosed between the melting peak and the baseline.To minimize errors, samples were taken from different positions of the material and the experiment was repeated three times.The average value was used as the final test result.

Preparation of Phase Change Panels
The aluminum alloy phase change panels were prepared using a macroscopic encapsulation method.Aluminum alloy boxes were selected as the encapsulation material.Drawing inspiration from heat sink designs, non-penetrating pin-shaped aluminum rods were placed inside the aluminum box to ensure full contact with the PCM.This design increases the heat exchange surface area to enhance heat transfer.The cover panel and aluminum box were tightly sealed using a locking mechanism, forming the aluminum alloy phase change panel.
Aluminum alloy is not only lightweight and strong, but also has excellent thermal conductivity, which effectively transfers heat between the environment and the PCM, thereby improving the overall energy efficiency of the system.Figure 2 shows the specific structure of the aluminum alloy phase change panel.To emphasize the pin-shaped aluminum rods and the PCM inside the aluminum box, the aluminum alloy cover panel and locking mechanism are not shown in the figure.Figure 3 shows the schematic of the aluminum box with the aluminum rods.As illustrated, the dimensions of the aluminum box were 400 mm × 400 mm × 20 mm with a wall thickness of 1 mm.Pin-shaped aluminum rods were uniformly welded to the inner surface of the box with a spacing of 50 mm between adjacent rods.Each rod had a length of 12 mm and a radius of 1 mm.The aluminum box was filled with composite PCMs with the specified phase change temperature in Section 2.    By effectively combining the phase change panel with the building envelope, the latent heat utilization of the PCM is increased, and the temperature control effect of the phase change wall panel will be enhanced.This can achieve the goal of regulating indoor temperature and reducing air conditioning energy consumption.It is necessary to show a brief explanation of its actual application scenario, for the structure of the phase change panel is closely related to its function within the building envelope.As shown in Figure 4, the phase change panel was installed on the inner side of the exterior wall.The pin-shaped aluminum rods were connected to the side of the panel that had direct contact with the heat source, without penetrating the PCM.This design ensures that the PCM can quickly transfer heat with the indoor air, while the heat transferred through the main structure of the wall can be stored by the PCM, thereby delaying and reducing the indoor temperature peak.By effectively combining the phase change panel with the building envelope, the latent heat utilization of the PCM is increased, and the temperature control effect of the phase change wall panel will be enhanced.This can achieve the goal of regulating indoor temperature and reducing air conditioning energy consumption.It is necessary to show a brief explanation of its actual application scenario, for the structure of the phase change panel is closely related to its function within the building envelope.As shown in Figure 4, the phase change panel was installed on the inner side of the exterior wall.The pin-shaped aluminum rods were connected to the side of the panel that had direct contact with the heat source, without penetrating the PCM.This design ensures that the PCM can quickly transfer heat with the indoor air, while the heat transferred through the main structure of the wall can be stored by the PCM, thereby delaying and reducing the indoor temperature peak.By effectively combining the phase change panel with the building envelope, the latent heat utilization of the PCM is increased, and the temperature control effect of the phase change wall panel will be enhanced.This can achieve the goal of regulating indoor temperature and reducing air conditioning energy consumption.It is necessary to show a brief explanation of its actual application scenario, for the structure of the phase change panel is closely related to its function within the building envelope.As shown in Figure 4, the phase change panel was installed on the inner side of the exterior wall.The pin-shaped aluminum rods were connected to the side of the panel that had direct contact with the heat source, without penetrating the PCM.This design ensures that the PCM can quickly transfer heat with the indoor air, while the heat transferred through the main structure of the wall can be stored by the PCM, thereby delaying and reducing the indoor temperature peak.In the heating process, the surface of the phase change panel in contact with the heating plate was designated as the heating surface and the opposite surface as the non-heating surface.The temperatures at the measurement points were recorded every minute using a multi-channel temperature tester.The average temperature of the measurement points on each side of the phase change panel was taken as the corresponding surface temperature.The indoor air temperature was recorded in real time using a Stevenson screen.To prevent varying environmental temperatures from affecting the test results, the non-heating surface and all peripheral surfaces of the phase change panel were covered with insulation cotton at the same area to reduce convective heat transfer between the phase change panel and the air.The multi-channel temperature tester and thermocouple wires were manufactured by Yongpeng Instrument Company of Shenzhen City, China, and the Stevenson screen was manufactured by Renke Electronic Technology Company of Jinan City, China.Figure 6 shows the layout of the thermocouple measurement points, which are symmetrically distributed around the center of the phase change panel.The distance between adjacent measurement points was 80 mm, with the positions of the measurement points on both surfaces of the phase change panel being identical.In the structure of the phase change panel, the thermal conductivity of the material surrounding the aluminum box was significantly higher than that of the PCM, exhibiting the characteristics of a thermal bridge [26].Due to the higher temperatures in these areas, the thermocouple measurement points were positioned as close to the center of the phase change panel as possible to more accurately analyze the heat transfer mechanism of the panel.In the heating process, the surface of the phase change panel in contact with the heating plate was designated as the heating surface and the opposite surface as the non-heating surface.The temperatures at the measurement points were recorded every minute using a multi-channel temperature tester.The average temperature of the measurement points on each side of the phase change panel was taken as the corresponding surface temperature.The indoor air temperature was recorded in real time using a Stevenson screen.To prevent varying environmental temperatures from affecting the test results, the non-heating surface and all peripheral surfaces of the phase change panel were covered with insulation cotton at the same area to reduce convective heat transfer between the phase change panel and the air.The multi-channel temperature tester and thermocouple wires were manufactured by Yongpeng Instrument Company of Shenzhen City, China, and the Stevenson screen was manufactured by Renke Electronic Technology Company of Jinan City, China.

Thermal Testing of Phase Change Panels
Figure 6 shows the layout of the thermocouple measurement points, which are symmetrically distributed around the center of the phase change panel.The distance between adjacent measurement points was 80 mm, with the positions of the measurement points on both surfaces of the phase change panel being identical.In the structure of the phase change panel, the thermal conductivity of the material surrounding the aluminum box was significantly higher than that of the PCM, exhibiting the characteristics of a thermal bridge [26].Due to the higher temperatures in these areas, the thermocouple measurement points were positioned as close to the center of the phase change panel as possible to more accurately analyze the heat transfer mechanism of the panel.Figure 6 shows the layout of the thermocouple measurement points, which are symmetrically distributed around the center of the phase change panel.The distance between adjacent measurement points was 80 mm, with the positions of the measurement points on both surfaces of the phase change panel being identical.In the structure of the phase change panel, the thermal conductivity of the material surrounding the aluminum box was significantly higher than that of the PCM, exhibiting the characteristics of a thermal bridge [26].Due to the higher temperatures in these areas, the thermocouple measurement points were positioned as close to the center of the phase change panel as possible to more accurately analyze the heat transfer mechanism of the panel.The determination of the heating temperature and duration is directly related to the analysis results of the heat transfer mechanism of the phase change panel.The heating temperatures and durations for different experimental groups are shown in Table 2.The determination of the heating temperature and duration is directly related to the analysis results of the heat transfer mechanism of the phase change panel.The heating temperatures and durations for different experimental groups are shown in Table 2. Experimental group 1 tested the effect of aluminum rods on enhanced heat transfer.The side of the phase change panel with the welded aluminum rods was used as the heating side, allowing heat to be directly transferred through the aluminum rods.This setup was used to find the impact of the aluminum rods on the melting process of the internal PCM.The heating temperature was set to 45 • C and the heating duration was 6 h.
Experimental group 2 served as the control group for experimental group 1.An aluminum alloy phase change panel without aluminum rods was used.The heating temperature was set to 45 • C and the heating duration was 6 h.Since this phase change panel did not have welded aluminum rods, it is unnecessary to specify the heating side of the panel.
Experimental group 3 simulated the effect under actual operating conditions, where the phase change panel was positioned on the inner side of the exterior wall, with the side welded with aluminum rods facing the interior.In Hainan, building exterior walls experience high temperatures from 9:00 to 15:00 during the summer, with surface temperatures exceeding 40 • C. Due to the presence of an insulation layer, the outer surface temperature of the phase change panel can reach up to 35 • C. Therefore, the side of the phase change panel without welded aluminum rods was used as the heating side, with the heating temperature set to 35 • C and the heating duration set to 6 h.
Experimental group 4 served as the control group for experimental group 3, testing the effect of the heating side position on the experimental results.The side of the phase change panel with welded aluminum rods was used as the heating side, with the heating temperature set to 35 • C and the heating duration set to 6 h.
In all four experimental groups, the dimensions of the phase change panels and the amount of the PCM used were the same.

Variation in Melting Point and Latent Heat
Table 3 presents the melting point (T) and latent heat of fusion (∆H) of composite PCMs with different mass ratios.Figure 5 presents the DSC curve of the sample prepared in experimental group A 2 , which illustrates the variation in the sample's heat flow (H) with temperature.As seen in Figure 7, the composite PCM exhibits a single thermal peak during both the melting and solidification processes.This indicates that the composite PCM stores or releases all the heat, demonstrating excellent thermal response performance.As shown in Table 3, with the increase in the mass fraction of butyl stearate, both the melting point and latent heat of fusion of the composite PCMs decrease.This is because the melting point and latent heat of fusion of butyl stearate are lower than those of paraffin.The sample prepared in experimental group A 2 has a melting point of 28.30 • C and a latent heat of fusion of 128.26 J/g.The PCM begins to melt at 28.30 • C, fully absorbing and storing heat, thereby maintaining the indoor temperature to around 28 • C, which meets the requirements for an appropriate phase change temperature in Hainan.Therefore, when the mass ratio of butyl stearate to paraffin is 3:7, a composite PCM with a suitable phase change temperature can be prepared by mixing these two organic PCMs.

Heat Transfer Process of Phase Change Panels
Figure 8 illustrates the temperature variation curves on both surfaces of phase change panels in experimental groups 1 and 2, with heat transfer process of experimental group 1 annotated.Due to the rapid temperature changes on the heating surface of the phase change panel and the varying temperature distribution of the PCM at different locations, the temperature change on the non-heating surface is used as the standard for dividing the overall heat transfer process of the phase change panel.From Figure 6, it can be seen that the heat transfer process of the phase change panel is divided into three main stages [27].
Appl.Sci.2024, 14, x FOR PEER REVIEW 10 of 16 After 300 min, the temperature increase of both the heating and non-heating surfaces of the phase change panels slows down and approaches a critical value.Due to the thermal resistance of the PCM, there is a fixed temperature difference between the heating and non-heating surfaces, and the entire phase change panel eventually reaches thermal equilibrium.In terms of the heating rate, the sensible heat storage phases in both the solid and liquid states are relatively fast.The heating rates during these two phases are primarily influenced by the specific heat capacity, density, and thermal conductivity of the PCM.The driving force for heat transfer is the temperature difference, which is greatest at the initial stage when the temperature difference between the PCM and the heating surface is the largest.As the heating process progresses, the temperature difference gradually decreases, leading to a slowdown in heat transfer.Additionally, the specific heat capacity of liquid paraffin is higher than that of solid paraffin, resulting in a slightly higher heating rate during the solid-phase sensible heat stage compared to the liquid phase.The heating rate during the latent heat storage phase is relatively slower, mainly due to the heat transfer mechanism in this stage and the melting point and latent heat of fusion of the PCM.

Heat Transfer Mechanism of Phase Change Panels
Figure 9 presents the temperature difference variation curves on both surfaces of phase change panels in experimental groups 1 and 2, with the heat transfer process of experimental group 1 annotated.A smaller temperature difference between the heated and unheated sides of the phase change panels signifies a faster heat transfer process and greater heat storage efficiency.Stage one is the sensible heat storage stage in the solid phase before the phase change.During this stage, the PCM on the non-heating side has not yet melted and remains in a solid state.The initial temperature is relatively low and the rate of temperature increase is rapid.For experimental group 1, this stage is located in the OA section of the curve and lasts from 0 to 40 min, while for experimental group 2, it is located in the OB section and lasts from 0 to 60 min.This indicates that experimental group 1 starts the next stage 20 min earlier than experimental group 2.
Stage two is the latent heat storage stage during the phase change.In this stage, the PCM partially melts and exists in a solid-liquid coexistence state, resulting in a distinct temperature plateau.The temperature of the PCM rises slowly within the phase change range of 28 • C to 29 • C. As the temperature on the heating surface continues to rise, the PCM accelerates its melting.After 160 min, the PCMs inside the panels of both experimental groups 1 and 2 are completely melted, and both groups start the next stage simultaneously.For experimental group 1, this stage is located in the AC section of the curve and lasts from 40 to 160 min, while for experimental group 2, it is located in the BC section and lasts from 60 to 160 min.
Stage three is the sensible heat storage stage in the liquid phase after the phase change.In this stage, the PCM is completely melted and remains in a liquid state.The temperature continues to rise, but the rate of temperature increase gradually decreases.For experimental group 1, this stage is located in the CD section of the curve, while for experimental group 2, it is located in the CE section.During this stage, both experimental groups 1 and 2 occur between 160 and 300 min, but the temperature of experimental group 1 is higher than that of experimental group 2.
After 300 min, the temperature increase of both the heating and non-heating surfaces of the phase change panels slows down and approaches a critical value.Due to the thermal resistance of the PCM, there is a fixed temperature difference between the heating and non-heating surfaces, and the entire phase change panel eventually reaches thermal equilibrium.
In terms of the heating rate, the sensible heat storage phases in both the solid and liquid states are relatively fast.The heating rates during these two phases are primarily influenced by the specific heat capacity, density, and thermal conductivity of the PCM.The driving force for heat transfer is the temperature difference, which is greatest at the initial stage when the temperature difference between the PCM and the heating surface is the largest.As the heating process progresses, the temperature difference gradually decreases, leading to a slowdown in heat transfer.Additionally, the specific heat capacity of liquid paraffin is higher than that of solid paraffin, resulting in a slightly higher heating rate during the solid-phase sensible heat stage compared to the liquid phase.The heating rate during the latent heat storage phase is relatively slower, mainly due to the heat transfer mechanism in this stage and the melting point and latent heat of fusion of the PCM.

Heat Transfer Mechanism of Phase Change Panels
Figure 9 presents the temperature difference variation curves on both surfaces of phase change panels in experimental groups 1 and 2, with the heat transfer process of experimental group 1 annotated.A smaller temperature difference between the heated and unheated sides of the phase change panels signifies a faster heat transfer process and greater heat storage efficiency.During the solid-phase sensible heat storage stage, heat transfer occurs primarily through conduction.At this stage, the temperature has not yet reached the melting point of the PCM.The thermal conductivity of the aluminum alloy and the PCM is the key factor influencing the temperature difference variation.Initially, the aluminum alloy on the heating surface absorbs heat, causing its temperature to rise.Subsequently, it transfers heat to the PCM.Due to the PCM's relatively low thermal conductivity, heat transfer to the unheated side occurs slowly, leading to a rapid increase in the temperature difference be- During the solid-phase sensible heat storage stage, heat transfer occurs primarily through conduction.At this stage, the temperature has not yet reached the melting point of the PCM.The thermal conductivity of the aluminum alloy and the PCM is the key factor influencing the temperature difference variation.Initially, the aluminum alloy on the heating surface absorbs heat, causing its temperature to rise.Subsequently, it transfers heat to the PCM.Due to the PCM's relatively low thermal conductivity, heat transfer to the unheated side occurs slowly, leading to a rapid increase in the temperature difference between the two sides of the phase change panel.
During the latent heat storage stage, the heat transfer mechanism involves both the latent heat storage properties of the PCM and liquid-phase convective heat transfer.As the internal temperature of the phase change panel rises, the liquid phase volume of the PCM gradually expands.The buoyancy resulting from density changes causes the liquid PCM to move along the heating surface to the top, continuously cutting through the unmelted solid PCM until it is fully melted.This liquid-phase convective heat transfer accelerates the PCM's attainment of its melting point.Most of the heat transferred through the heating surface is absorbed by the PCM, causing the temperature on the unheated side to rise slowly.As the heating surface temperature continues to increase, the temperature difference between the two sides also increases.
During the liquid-phase sensible heat storage stage, heat transfer occurs primarily through the convective heat transfer of the liquid PCM.At this stage, the PCM is completely melted.When the temperature of the heating surface reaches its upper limit, the convective heat transfer of the liquid PCM causes the temperature of the unheated side to rise rapidly, gradually reducing the temperature difference between the two sides.Once the phase change panel reaches thermal equilibrium, the temperature difference remains relatively constant.

The Role of Aluminum Rods
As shown in Figure 8, during the solid-phase sensible heat storage stage, the phase change panel in experimental group 1 transitioned to the next stage 20 min earlier than that in experimental group 2. This is because the aluminum rods accelerate the melting of the surrounding solid PCM, and the absorption of heat during melting suppresses the temperature rise of the heating surface, thereby reducing the heat transferred from the heating surface.
During the latent heat storage stage, part of the PCM near the aluminum rods had already melted.At this stage, the heat transfer mechanism is primarily characterized by the latent heat storage properties of the unmelted PCM and the convective heat transfer of the melted liquid PCM.The dense arrangement of aluminum rods decreases the rate of liquid-phase convective heat transfer and increases the phase change time of the unmelted PCM, thereby extending the overall phase change process of the panel.Due to these factors and the almost identical mass and total heat absorption of the PCM in both groups, experimental groups 1 and 2 simultaneously reached the liquid-phase sensible heat storage stage at the same temperature.As shown in Figure 9, during the solid-phase sensible heat storage stage, the temperature difference curve of experimental group 1 shows a downward trend compared to that of experimental group 2. This is because part of the heat from the heating surface is transferred through the aluminum rods to the PCM near the unheated side and then to the unheated surface, reducing the temperature difference between the two sides of the panel.
The final results indicate that the overall phase change time of the phase change panel in experimental group 1 increased by 20%, and the temperature differences in all three stages were smaller than those in experimental group 2. This suggests that the aluminum rods effectively enhance heat transfer, increase the thermal conduction path within the phase change panel, and reduce the thermal resistance of heat transfer.Consequently, the phase change panel achieves better temperature regulation between the heated and unheated surfaces, improves internal heat transfer, optimizes heat distribution, and enhances the utilization of the PCM.

The Simulation Application Effect of Phase Change Panels
Figure 10 shows the temperature variation curves on both surfaces of phase change panels in experimental groups 3 and 4, with the heat transfer process of experimental group 4 annotated.As illustrated, for experimental group 3, the latent heat storage stage is located in the BD section of the curve and lasts from 120 to 300 min, while for experimental group 4, it is located in the AC section and lasts from 80 to 240 min.This indicates that experimental group 4 completes the solid-phase sensible heat storage stage 40 min earlier than experimental group 3 and reduces the phase change time by 20 min.
stages were smaller than those in experimental group 2. This suggests that the aluminum rods effectively enhance heat transfer, increase the thermal conduction path within the phase change panel, and reduce the thermal resistance of heat transfer.Consequently, the phase change panel achieves better temperature regulation between the heated and unheated surfaces, improves internal heat transfer, optimizes heat distribution, and enhances the utilization of the PCM.

The Simulation Application Effect of Phase Change Panels
Figure 10 shows the temperature variation curves on both surfaces of phase change panels in experimental groups 3 and 4, with the heat transfer process of experimental group 4 annotated.As illustrated, for experimental group 3, the latent heat storage stage is located in the BD section of the curve and lasts from 120 to 300 min, while for experimental group 4, it is located in the AC section and lasts from 80 to 240 min.This indicates that experimental group 4 completes the solid-phase sensible heat storage stage 40 min earlier than experimental group 3 and reduces the phase change time by 20 min.This is because both groups 3 and 4 have aluminum rods, which similarly weaken liquid-phase convective heat transfer.When the heating side is on the welded aluminum rod side, the aluminum rods act as efficient heat transfer paths, accelerating the melting of the PCM in the vicinity of the rods.This reduces the thermal resistance of the solid PCM near the heating surface and increases the overall melting rate of the PCM, thereby shortening the phase change time.Conversely, when the heating side is on the non-welded aluminum rod side, during the entire solid-phase sensible heat storage stage and the initial phase of the latent heat storage stage, the temperature of the PCM near the unheated surface remains relatively low.At these stages, the heat transfer enhancement effect of the aluminum rods is not fully realized, which better utilizes the thermal resistance of the PCM and prolongs the phase change time.
Therefore, in practical applications of phase change panels, it is advisable to position the phase change panel on the inner side of an exterior wall, with the aluminum rods connected to the side of the panel facing the indoor heat source and not penetrating the PCM.This configuration ensures that the heat transferred through the main structure of the wall is absorbed by the PCM, thereby delaying and reducing the indoor peak temperature.
As shown in Figure 8, the phase change time for experimental group 1 is 120 min.Under identical conditions, with heating applied to the welded aluminum rod side, the phase change time for experimental group 4 extends to 160 min.This discrepancy arises because the heating temperature for experimental group 1 is higher than that for experimental group 4. In practical applications of phase change panels, it is crucial to incorporate an adequate amount of insulation material to optimize the heat storage performance of the PCM within the wall panels.
Field measurements of building exterior walls in Hainan during the summer indicate that from 9:00 a.m. to 3:00 p.m., with adequate sunlight, the exterior surface temperature of the walls can rise rapidly, exceeding 40 • C. Due to the insulation layer within the wall, the temperature on the side of the phase change panel near the exterior can reach up to 35 • C. As shown in Figure 8, the temperature on the interior side of the phase change panel increases slowly, with the rate of increase gradually diminishing.After 120 min, the temperature reaches approximately 28 • C, at which point the PCM begins to melt.The heat transferred through the main structure of the wall is absorbed by the phase change panel, maintaining the temperature on the interior side between 28 • C and 29 • C for up to 180 min.This demonstrates the excellent insulation and temperature control effects of the phase change panel, delaying and reducing the peak indoor temperature, enhancing thermal comfort [28], reducing the usage time of air conditioning [29], and lowering the building's energy consumption.

Discussion
During the solid-phase sensible heat storage stage, a small portion of the PCMs undergo melting, but the primary heat transfer mechanism of the phase change panel remains as thermal conduction.In this stage, the heating rate and heating duration of the unheated side of the phase change panel reflect its insulation performance.
As shown in Figure 10, with the start of the heating process, the temperature of the heated side of the phase change panels in experimental groups 3 and 4 rapidly rises until it exceeds 30 • C. The heat simultaneously transfers from the heated side to the unheated side.The temperature of the unheated side rises slowly, with the rate of increase diminishing over time.Therefore, during the solid-phase sensible heat storage stage, the insulation performance of the phase change panel improves as the heating time increases.
The unheated side in experimental group 4 reaches 28 • C at 80 min, while that in experimental group 5 reaches 28 • C at 120 min.The heating duration for both groups exceeds one hour, with a temperature difference of only 7 • C before and after heating.This is because both paraffin and n-butyl stearate have very low thermal conductivities, resulting in a low thermal conductivity for the composite PCMs.Even though the exterior of the phase change panel has high thermal conductivity due to the aluminum alloy material, the overall thermal conductivity is determined by the internal PCMs, causing the panel to exhibit high thermal resistance.
The difference between experimental groups 1 and 4 lies in the heating temperature.The heating temperature for experimental group 1 is 45 • C, while for experimental group 4, it is 35 • C. As shown in Figure 8, the temperature on the unheated side of experimental group 1 reaches 28 • C at 40 min, with a temperature difference of 5 • C before and after heating.Meanwhile, the temperature on the unheated side of experimental group 4 reaches 28 • C at 80 min, with a temperature difference of 7 • C before and after heating.Therefore, during the solid-phase sensible heat storage stage, the heating rate of experimental group 4 is lower than that of experimental group 1.As the heating temperature decreases, the insulation performance of the phase change panel improves.
These phenomena demonstrate that in addition to excellent temperature control effects, the phase change panel also has outstanding insulation performance, which can play an important role during hot seasons.
In a recent similar study, Li et al. [30] used COMSOL Multiphysics to optimize the position of the phase change layer, the melting point of PCMs, and different loading conditions.The authors defined the intensity of thermal discomfort as the period during which the temperature is outside the chosen thermal comfort range of 22 • C to 28 • C. It was found that the optimal position is obtained for PCMs placed close to the inner layer of the wall, and the selection of the melting point of PCMs is highly dependent on the loading condition.The authors recommend PCMs with a melting point between 26 • C and 30 • C for cooling.These conclusions further confirm the rationality of determining the appropriate phase change temperature and the placement of the phase change layer in the wall in the study.
Kulkarni [31] analyzed and compared the thermal performance of the direct incorporation of PCMs into mortar.The results showed that phase change mortar also has good thermal performance, capable of reducing peak temperatures by 3 • C to 5 • C.However, the compression test showed a reduction in compressive and mechanical strength with the addition of PCMs.This phenomenon demonstrates the advantages of macro-encapsulation in practical applications.When PCMs are encapsulated in aluminum boxes to form phase change panels, and these panels are installed as decorative layers on the inner side of exterior walls, they do not affect the mechanical properties of the main wall structure.Therefore, the amount of PCMs can be flexibly adjusted to better achieve temperature control, thereby improving the overall performance of the wall.

Conclusions
To address the issues of low thermal conductivity and liquid phase leakage in the application of solid-liquid PCMs in building walls, an aluminum alloy phase change panel was proposed.The phase change panel was applied to the inner side of exterior walls.Thermal experiments were conducted to investigate the heat transfer process and mechanisms of the phase change panel, the role of pin-shaped aluminum rods, and the practical application effects of the phase change panel.The main conclusions are as follows: Firstly, as the mass fraction of stearic acid n-butyl ester increases, both the melting point and latent heat of fusion of the composite PCMs decrease.When the mass ratio of stearic acid n-butyl ester to paraffin is 3:7, the composite PCM exhibits a melting point of 28.30 • C, meeting the suitable phase change temperature requirements for the Hainan region.
Secondly, during the three stages of the heat transfer process in the phase change panel, the solid-phase sensible heat storage stage is dominated by thermal conduction, the latent heat storage stage involves both the latent heat storage properties of the PCM and liquid-phase convective heat transfer, and the liquid-phase sensible heat storage stage is primarily driven by the liquid-phase convective heat transfer of the PCM.
Thirdly, the aluminum rods extend the heat transfer path and reduce the liquid-phase convective heat transfer rate of the PCM.Compared to phase change panels without aluminum rods, those with aluminum rods exhibit a 20% longer phase change time and a smaller temperature difference between the two surfaces.
Finally, in practical application, when the exterior side of the panel reaches 35 • C, the interior side reaches 28 • C after 120 min, initiating the PCM melting.The heat transferred through the wall structure is absorbed by the PCM, maintaining the interior temperature between 28 • C and 29 • C for up to 180 min.This demonstrates the excellent thermal insulation and temperature regulation effects of the phase change panel.

Figure 1 .
Figure 1.The key instruments for DSC testing: (a) electronic balance; (b) press mold machine; (c) DSC instrument.

Figure 1 .
Figure 1.The key instruments for DSC testing: (a) electronic balance; (b) press mold machine; (c) DSC instrument.
1, and the volume of the PCM approximately matched the volume of the aluminum box.All aluminum boxes were manufactured by Chengxin Metal Materials Company of Changzhou City, China.Appl.Sci.2024, 14, x FOR PEER REVIEW 5 of 16 aluminum rods and the PCM inside the aluminum box, the aluminum alloy cover panel and locking mechanism are not shown in the figure.

Figure 3
shows the schematic of the aluminum box with the aluminum rods.As illustrated, the dimensions of the aluminum box were 400 mm × 400 mm × 20 mm with a wall thickness of 1 mm.Pin-shaped aluminum rods were uniformly welded to the inner surface of the box with a spacing of 50 mm between adjacent rods.Each rod had a length of 12 mm and a radius of 1 mm.The aluminum box was filled with composite PCMs with the specified phase change temperature in Section 2.1, and the volume of the PCM approximately matched the volume of the aluminum box.All aluminum boxes were manufactured by Chengxin Metal Materials Company of Changzhou City, China.

Figure 2 .
Figure 2. The specific structure of the aluminum alloy phase change panel: (a) untreated aluminum box; (b) aluminum box with welded aluminum rods; (c) aluminum box filled with the PCM.

Figure 3 .
Figure 3.The schematic of the aluminum box with aluminum rods.

Figure 2 .
Figure 2. The specific structure of the aluminum alloy phase change panel: (a) untreated aluminum box; (b) aluminum box with welded aluminum rods; (c) aluminum box filled with the PCM.

Figure 3
shows the schematic of the aluminum box with the aluminum rods.As illustrated, the dimensions of the aluminum box were 400 mm × 400 mm × 20 mm with a wall thickness of 1 mm.Pin-shaped aluminum rods were uniformly welded to the inner surface of the box with a spacing of 50 mm between adjacent rods.Each rod had a length of 12 mm and a radius of 1 mm.The aluminum box was filled with composite PCMs with the specified phase change temperature in Section 2.1, and the volume of the PCM approximately matched the volume of the aluminum box.All aluminum boxes were manufactured by Chengxin Metal Materials Company of Changzhou City, China.

Figure 2 .
Figure 2. The specific structure of the aluminum alloy phase change panel: (a) untreated aluminum box; (b) aluminum box with welded aluminum rods; (c) aluminum box filled with the PCM.

Figure 3 .
Figure 3.The schematic of the aluminum box with aluminum rods.

Figure 3 .
Figure 3.The schematic of the aluminum box with aluminum rods.

Figure 4 .
Figure 4.The actual application scenarios of the phase change panel.

Figure 5
Figure 5 is a schematic diagram of the thermal testing apparatus of the phase change panel.As shown in the figure, the phase change panel was placed vertically, with five Ktype thermocouple measurement points arranged at the center on both sides.The uncertainty of the thermocouples was 0.5 °C.The heating temperature was set using an intelligent digital temperature control box with an uncertainty of 1 °C, which heated the cast aluminum heating plate with the thermal resistance.The dimensions of the heating plate were 400 mm × 400 mm × 20 mm, matching those of the phase change panel.Once the temperature of the cast aluminum heating plate was stable, it was placed on one side of the phase change panel, ensuring that the contact surfaces were fully aligned.The intelligent digital temperature control box and cast aluminum heating plate were manufactured by Dayu Electric Company of Xinghua City, China.In the heating process, the surface of the phase change panel in contact with the heating plate was designated as the heating surface and the opposite surface as the non-heating surface.The temperatures at the measurement points were recorded every minute using a multi-channel temperature tester.The average temperature of the measurement points on each side of the phase change panel was taken as the corresponding surface temperature.The indoor air temperature was recorded in real time using a Stevenson screen.To prevent varying environmental temperatures from affecting the test results, the non-heating surface and all peripheral surfaces of the phase change panel were covered with insulation cotton at the same area to reduce convective heat transfer between the phase change panel and the air.The multi-channel temperature tester and thermocouple wires were manufactured by Yongpeng Instrument Company of Shenzhen City, China, and the Stevenson screen was manufactured by Renke Electronic Technology Company of Jinan City, China.

Figure 4 .
Figure 4.The actual application scenarios of the phase change panel.

2. 4 . 16 Figure 5 .
Figure 5 is a schematic diagram of the thermal testing apparatus of the phase change panel.As shown in the figure, the phase change panel was placed vertically, with five K-type thermocouple measurement points arranged at the center on both sides.The uncertainty of the thermocouples was 0.5 • C. The heating temperature was set using an intelligent digital temperature control box with an uncertainty of 1 • C, which heated the cast aluminum heating plate with the thermal resistance.The dimensions of the heating plate were 400 mm × 400 mm × 20 mm, matching those of the phase change panel.Once the temperature of the cast aluminum heating plate was stable, it was placed on one side of the phase change panel, ensuring that the contact surfaces were fully aligned.The intelligent digital temperature control box and cast aluminum heating plate were manufactured by Dayu Electric Company of Xinghua City, China.Appl.Sci.2024, 14, x FOR PEER REVIEW 7 of 16

Figure 5 .
Figure 5.A schematic diagram for the thermal testing apparatus of the phase change panel.

Figure 5 .
Figure 5.A schematic diagram for the thermal testing apparatus of the phase change panel.

Figure 6 .
Figure 6.The layout of the thermocouple measurement points.

Figure 6 .
Figure 6.The layout of the thermocouple measurement points.

16 Figure 7 .
Figure 7.The DSC curve of the sample prepared in experimental group A2.

Figure 8 Figure 7 .
Figure 8 illustrates the temperature variation curves on both surfaces of phase change panels in experimental groups 1 and 2, with the heat transfer process of experimental group 1 annotated.Due to the rapid temperature changes on the heating surface of the phase change panel and the varying temperature distribution of the PCM at different lo-

Figure 8 .
Figure 8.The temperature variation curves on both surfaces of phase change panels in experimental groups 1 and 2 (the stages are set according to group 1).

Figure 8 .
Figure 8.The temperature variation curves on both surfaces of phase change panels in experimental groups 1 and 2 (the stages are set according to group 1).

16 Figure 9 .
Figure 9.The temperature difference variation curves on both surfaces of phase change panels in experimental groups 1 and 2 (the stages are set according to group 1).

Figure 9 .
Figure 9.The temperature difference variation curves on both surfaces of phase change panels in experimental groups 1 and 2 (the stages are set according to group 1).

Figure 10 .Figure 10 .
Figure 10.The temperature variation curves on both surfaces of phase change panels in experimental groups 3 and 4 (the stages are set according to group 4).
• C to 21 • C. Paraffin wax was produced by Shengbang Plastic Business Department of Dongguan City, China, and n-butyl stearate was produced by Xinfuhai Company of Zhangjiagang City, China.

Table 1 .
Mass ratio of paraffin to n-butyl stearate in different experimental groups.
• C/min controlled by the computer software.During the heating and cooling process, the temperature and heat flow data of the sample were recorded.

Table 2 .
Heating temperatures and durations for different experimental groups.

Table 2 .
Heating temperatures and durations for different experimental groups.

Table 3 .
Melting point and latent heat of composite PCMs with different mass ratios of stearic acid n-butyl ester (m 1 ) to paraffin (m 2 ).