Preparation and Characteristics of Na2HPO4·12H2O-K2HPO4·3H2O/SiO2 Composite Phase Change Materials for Thermal Energy Storage

In this paper, a series of eutectic hydrated salts was obtained by mixing Na2HPO4·12H2O (DHPD) with K2HPO4·3H2O (DHPT) in different proportions. With the increase in the content of DHPT, the phase transition temperature and melting enthalpy of eutectic hydrated salts decreased gradually. Moreover, the addition of appropriate deionized water improved the thermal properties of eutectic hydrated salts. Colloidal silicon dioxide (SiO2) was selected as the support carrier to adsorb eutectic hydrated salts, and the maximum content of eutectic hydrated salts in composite PCMs was 70%. When the content of the nucleating agent (Na2SiO3·9H2O) was 5%, the supercooling degree of composite PCMs was reduced to the minimum of 1.2 °C. The SEM and FT-IR test results showed that SiO2 and eutectic hydrated salts were successfully combined, and no new substances were formed. When the content of DHPT was 3%, the phase transition temperature and melting enthalpy of composite PCMs were 26.5 °C and 145.3 J/g, respectively. The results of thermogravimetric analysis and heating–cooling cycling test proved that composite PCMs had good thermal reliability and stability. The application performance of composite PCMs in prefabricated temporary houses was investigated numerically. The results indicated that PCM panels greatly increased the Grade I thermal comfort hours and reduced energy consumption. Overall, the composite PCM has great development potential building energy conservation.


Introduction
Global greenhouse gas emissions will bring serious problems such as environmental pollution [1]. According to research reports, greenhouse gas emissions are mainly caused by the large consumption of energy [2,3]. Therefore, it is necessary to vigorously develop advanced energy storage technology. The thermal energy storage system is generally divided into thermochemical energy storage, sensible heat storage, and latent heat storage. The energy storage density of the thermochemical energy storage system is the largest, but it has the disadvantages of high cost, uncontrollable process, and strict requirements for equipment [4]. The sensible heat storage system is simple, but it has the defects of large device volume and low energy storage density [5]. The latent heat storage system releases and stores heat through the phase transition process of the phase change materials (PCMs) [6]. When the temperature exceeds the phase transition temperature, the surrounding heat will be absorbed and stored by PCMs. When the temperature is lower than the phase transition temperature, the heat stored by PCMs will be released. Due to its advantages of easy control and high energy storage density, it has been widely concerned by researchers [7,8].
PCMs, so its composite PCMs have better thermal properties [32,33]. At present, there are few reports on the research of eutectic hydrated salts/ silicon dioxide composite PCMs.
In this paper, a series of eutectic hydrated salts were obtained by mixing dipotassium hydrogen phosphate trihydrate (DHPT) and disodium hydrogen phosphate dodecahydrate (DHPD) with different contents. Colloidal silicon dioxide (SiO 2 ) was selected to adsorb eutectic hydrated salts. Sodium metasilicate nonahydrate was used as the nucleating agent to reduce the supercooling degree of composite PCMs. The properties of composite PCMs were tested and characterized, including the supercooling degree, phase change characteristics, micromorphology, chemical composition, thermal stability, and thermal reliability. The application of composite PCMs in prefabricated temporary houses was investigated through numerical simulation. In this work, composite PCMs based on eutectic hydrated salts with good performance were obtained, and their application potential in the construction field was demonstrated.

Preparation Process of Eutectic Hydrated Salts
Firstly, a certain amount of DHPD and DHPT were weighed and put into a sealable glass bottle. Then, a certain proportion of deionized water was dropped into the glass bottle. In addition, Na 2 SiO 3 ·9H 2 O was used as the nucleating agent to reduce the supercooling degree of PCMs. Finally, the eutectic hydrated salt solution was obtained by stirring them uniformly at 50 • C. The eutectic hydrated salt with different phase transition temperatures could be prepared by adding DHPT with different mass fractions.

Preparation Process of Composite PCMs
To improve the liquid leakage phenomenon of solid-liquid PCMs, colloidal silicon dioxide was employed as the porous material to absorb the eutectic hydrated salt. First, SiO 2 with different mass fractions was added to the eutectic hydrated salt solution prepared above. After stirring for 3 min, it was placed in an environment of 50 • C to melt the PCM, and then it was taken out and stirred again. This process was repeated several times to ensure that the PCM was fully adsorbed by SiO 2 . Finally, the composite PCM was obtained by cooling it at room temperature, and then it was put into the refrigerator for future use.

Characterization
The supercooling degree of the material was tested by the T-history method, and the simple diagram of the testing device is displayed in Figure 1. The samples were poured into a sealed glass tube and compacted, and then placed in a test chamber (BPHJS-060B, Shanghai Yiheng Technology Instrument Co., Ltd., Shanghai, China) that could adjust the temperature and humidity. A K-type thermocouple was inserted and fixed at the center of the sample. Agilent 34970A data acquisition instrument was used to record the change in sample temperature with time.
The liquid leakage test was used to determine the appropriate content of eutectic hydrated salts in composite PCMs. The composite PCMs containing eutectic hydrated salts of different mass fractions were pressed into blocks and placed on clean filter papers. They were heated in an oven at 50 • C for 30 min and then taken out to carefully observe whether there were water stains. The liquid leakage test was used to determine the appropriate content of eutectic hydrated salts in composite PCMs. The composite PCMs containing eutectic hydrated salts of different mass fractions were pressed into blocks and placed on clean filter papers. They were heated in an oven at 50 °C for 30 min and then taken out to carefully observe whether there were water stains.
The scanning electron microscope (SEM, Zeiss Gemini 500, Jena, Germany) was selected to observe the micromorphology of the composite PCM and SiO2. Fourier transform infrared spectroscopy (FT-IR, Nicolet iN10, Waltham, MA, USA) was employed to test the chemical composition of the sample. The thermal characteristics of the sample, such as phase transition temperature and melting enthalpy, were characterized by the differential scanning calorimeter (DSC, NETZSCH 214 Polyma, Selb, Germany). The thermogravimetric analyzer (TGA, Mettler Toledo TGA2, Columbus, OH, USA) was selected to test the thermal decomposition temperature and thermal stability of the materials. For DSC and TG tests, the sample mass was about 10 mg. The test was conducted at a heating rate of 5 °C/min under a nitrogen atmosphere.
A heating-cooling cycling test was employed to evaluate the thermal reliability of the composite PCM. A small amount of the composite PCM was poured into a sealable bottle, and then it was put into a test chamber with adjustable temperature and humidity (BPHJS-060B, Shanghai Yiheng Technology Instrument Co., Ltd., Shanghai, China). For the temperature procedure of the test chamber, it took 10 min to lower the temperature to 0 °C, and then the sample was kept constant at 0 °C for 20 min. Then, it took 10 min to raise the temperature to 50 °C, and then the sample was kept constant at 50 °C for 20 min. The whole process was repeated 200 times.

Supercooling Degree
The cooling curves of the eutectic hydrated salt with different mass fractions of deionized water are displayed in Figure 2a, and the corresponding supercooling degree is shown in Table 1. It could be seen that the melting process of eutectic hydrated salts was divided into two stages when no deionized water was added. This may be related to the inherent defect that DHPD is easy to lose crystal water, which will lead to the decline of the thermal storage performance of eutectic hydrated salts. In this work, it was found that this phenomenon could be improved by adding deionized water to the material preparation process. When the mass fraction of deionized water was 15%, the supercooling degree of the sample was the minimum, which was 2.8 °C. Subsequently, the deionized water content was fixed to 15% of the mass of eutectic hydrated salts.
The cooling curve of the composite PCM is displayed in Figure 2b. When the content of DHPT was 0, 3, 6, 9, and 12% of the mass of eutectic hydrated salts, the supercooling degree was 4.5, 5.4, 6.4, 6.9, and 6.2 °C, respectively. The results showed that the The scanning electron microscope (SEM, Zeiss Gemini 500, Jena, Germany) was selected to observe the micromorphology of the composite PCM and SiO 2 . Fourier transform infrared spectroscopy (FT-IR, Nicolet iN10, Waltham, MA, USA) was employed to test the chemical composition of the sample. The thermal characteristics of the sample, such as phase transition temperature and melting enthalpy, were characterized by the differential scanning calorimeter (DSC, NETZSCH 214 Polyma, Selb, Germany). The thermogravimetric analyzer (TGA, Mettler Toledo TGA2, Columbus, OH, USA) was selected to test the thermal decomposition temperature and thermal stability of the materials. For DSC and TG tests, the sample mass was about 10 mg. The test was conducted at a heating rate of 5 • C/min under a nitrogen atmosphere.
A heating-cooling cycling test was employed to evaluate the thermal reliability of the composite PCM. A small amount of the composite PCM was poured into a sealable bottle, and then it was put into a test chamber with adjustable temperature and humidity (BPHJS-060B, Shanghai Yiheng Technology Instrument Co., Ltd., Shanghai, China). For the temperature procedure of the test chamber, it took 10 min to lower the temperature to 0 • C, and then the sample was kept constant at 0 • C for 20 min. Then, it took 10 min to raise the temperature to 50 • C, and then the sample was kept constant at 50 • C for 20 min. The whole process was repeated 200 times.

Supercooling Degree
The cooling curves of the eutectic hydrated salt with different mass fractions of deionized water are displayed in Figure 2a, and the corresponding supercooling degree is shown in Table 1. It could be seen that the melting process of eutectic hydrated salts was divided into two stages when no deionized water was added. This may be related to the inherent defect that DHPD is easy to lose crystal water, which will lead to the decline of the thermal storage performance of eutectic hydrated salts. In this work, it was found that this phenomenon could be improved by adding deionized water to the material preparation process. When the mass fraction of deionized water was 15%, the supercooling degree of the sample was the minimum, which was 2.8 • C. Subsequently, the deionized water content was fixed to 15% of the mass of eutectic hydrated salts.
The cooling curve of the composite PCM is displayed in Figure 2b. When the content of DHPT was 0, 3, 6, 9, and 12% of the mass of eutectic hydrated salts, the supercooling degree was 4.5, 5.4, 6.4, 6.9, and 6.2 • C, respectively. The results showed that the combination of eutectic hydrated salts and SiO 2 could not play a positive role in decreasing the supercooling degree. Moreover, with the gradual increase in the content of DHPT, the melting platform of composite PCMs became smaller, which meant that its heat storage capacity decreased. It could also be found that the initial melting temperature of the composite PCM decreased with the increase in the content of DHPT. For building application, the mass fraction of DHPT was determined to be 3% by comprehensively considering the heat storage capacity and phase transition temperature. in Figure 2c. The results showed that the supercooling degree of the composite PCM decreased when Na2SiO3·9H2O was added as the nucleating agent. When the content of Na2SiO3·9H2O was 2, 3, 4, 5, 6, and 7% of the mass of eutectic hydrated salts, the supercooling degree was 3.7, 4.0, 3.4, 1.2, 3.6, and 4.2 °C, respectively. Therefore, when the mass fraction of Na2SiO3·9H2O was 5%, it was most helpful in reducing the supercooling degree of composite PCMs.    The cooling curve of the composite PCM containing the nucleating agent is displayed in Figure 2c. The results showed that the supercooling degree of the composite PCM decreased when Na 2 SiO 3 ·9H 2 O was added as the nucleating agent. When the content of Na 2 SiO 3 ·9H 2 O was 2, 3, 4, 5, 6, and 7% of the mass of eutectic hydrated salts, the supercooling degree was 3.7, 4.0, 3.4, 1.2, 3.6, and 4.2 • C, respectively. Therefore, when the mass fraction of Na 2 SiO 3 ·9H 2 O was 5%, it was most helpful in reducing the supercooling degree of composite PCMs.

Optimum Content of Eutectic Hydrated Salts
The liquid leakage test results of the composite PCM are displayed in Figure 3. It could be seen from Figure 3a that composite PCMs could be pressed into blocks and shaped. After the composite PCMs were heated, the eutectic hydrated salts melted. It could be found from Figure 3b that when the content of eutectic hydrated salts was 65% or 70%, no trace of liquid leakage was observed on the filter paper. With the gradual increase in the content of eutectic hydrated salts, such as 75% or more, more and more water stains appeared. This phenomenon is mainly determined by the pore structure inside the supporting material. When the content of eutectic hydrated salts was too large, the pore volume of SiO 2 could not adsorb all PCMs, which led to the leakage of eutectic hydrated salts during melting. Therefore, based on the experimental results, the optimal content of eutectic hydrated salts in the composite PCMs was about 70%.

Effects of DHPT on Phase Transition Characteristics
The change in phase transition temperature and melting enthalpy of composite PCMs with the content of DHPT is shown in Figure 4 and Table 2. When no additional deionized water was added during the preparation of composite PCMs, two broad peaks could be seen in the DSC curve, which might be related to the loss of crystal water from hydrated salts. At this time, the thermal properties of the composite PCM were poor, and its melting enthalpy was only 129.9 J/g. After adding the proper amount of deionized water in the preparation process, it could be observed that the second peak on the DSC curve was significantly reduced, which was conducive to improving the thermal properties of the composite PCM. When adding DHPT to DHPD, the phase transition temperature and melting enthalpy of composite PCMs decreased, which was consistent with the results of the cooling curves. As the mass fraction of DHPT increased from 0% to 12%, the melting enthalpy decreased from 150.3 J/g to 122.4 J/g, and the phase transition temperature dropped from 29.6 • C to 22.9 • C. When the content of DHPT was 3%, the melting enthalpy (145.3 J/g) of composite PCMs decreased slightly, and the phase transition temperature (26.5 • C) was suitable for building applications, so it was selected for subsequent research.
ials 2021, 14, x FOR PEER REVIEW 7 of the pore volume of SiO2 could not adsorb all PCMs, which led to the leakage of eutect hydrated salts during melting. Therefore, based on the experimental results, the optim content of eutectic hydrated salts in the composite PCMs was about 70%.

Effects of DHPT on Phase Transition Characteristics
The change in phase transition temperature and melting enthalpy of composi PCMs with the content of DHPT is shown in Figure 4 and Table 2. When no addition deionized water was added during the preparation of composite PCMs, two broad peak could be seen in the DSC curve, which might be related to the loss of crystal water fro hydrated salts. At this time, the thermal properties of the composite PCM were poor, an its melting enthalpy was only 129.9 J/g. After adding the proper amount of deionize water in the preparation process, it could be observed that the second peak on the DS curve was significantly reduced, which was conducive to improving the therm properties of the composite PCM. When adding DHPT to DHPD, the phase transitio temperature and melting enthalpy of composite PCMs decreased, which was consiste with the results of the cooling curves. As the mass fraction of DHPT increased from 0% 12%, the melting enthalpy decreased from 150.3 J/g to 122.4 J/g, and the phase transitio temperature dropped from 29.6 °C to 22.9 °C. When the content of DHPT was 3%, th melting enthalpy (145.3 J/g) of composite PCMs decreased slightly, and the pha transition temperature (26.5 °C) was suitable for building applications, so it was selecte for subsequent research.

Morphology and Structure
The micro morphology of the composite PCM and SiO2 is presented in Fig  could be seen from Figure 5a that SiO2 molecules agglomerated into a three-dim cluster structure through intermolecular force. These abundant pore structures e to well adsorb eutectic hydrated salts. It was observed from Figure 5b that the

Morphology and Structure
The micro morphology of the composite PCM and SiO 2 is presented in Figure 5. It could be seen from Figure 5a that SiO 2 molecules agglomerated into a three-dimensional cluster structure through intermolecular force. These abundant pore structures enabled it to well adsorb eutectic hydrated salts. It was observed from Figure 5b that the particles became larger. Through the capillary force and hydrophilic groups (such as hydroxyl) of SiO 2 , the eutectic hydrated salts were adsorbed in the pore structure and on the particle surface. Therefore, SiO 2 could prevent the leakage of eutectic hydrated salts when phase transition occurred.

Morphology and Structure
The micro morphology of the composite PCM and SiO2 is presented in Figure 5 could be seen from Figure 5a that SiO2 molecules agglomerated into a three-dimension cluster structure through intermolecular force. These abundant pore structures enabled to well adsorb eutectic hydrated salts. It was observed from Figure 5b that the partic became larger. Through the capillary force and hydrophilic groups (such as hydroxyl) SiO2, the eutectic hydrated salts were adsorbed in the pore structure and on the parti surface. Therefore, SiO2 could prevent the leakage of eutectic hydrated salts when pha transition occurred. The FT-IR spectrum of SiO2, DHPT, DHPD, and composite PCMs is presented Figure 6. For the spectra of composite PCMs (CPCM), the characteristic absorption pea at 1096 cm −1 and 4667 cm −1 correspond to the antisymmetric stretching vibration a The FT-IR spectrum of SiO 2 , DHPT, DHPD, and composite PCMs is presented in Figure 6. For the spectra of composite PCMs (CPCM), the characteristic absorption peaks at 1096 cm −1 and 4667 cm −1 correspond to the antisymmetric stretching vibration and bending vibration of Si-O-Si, respectively. The absorption peak at 1096 cm −1 was also the stretching vibration peak of HPO 4 2− . In addition, the stretching vibration peak of HPO 4 2− also appeared at 859 cm −1 and 544 cm −1 . Only the characteristic absorption peaks of HPO 4 2− and SiO 2 appeared in the composite PCMs, and no other peaks appeared. The results indicated no chemical reaction between eutectic hydrated salt and SiO 2 , but only physical adsorption.

Thermal Stability
The TGA curves of SiO 2 , eutectic hydrated salts, and composite PCMs are shown in Figure 7. Because SiO 2 has hygroscopicity, it could be seen from the figure that it had weight loss during the process of heating to 600 • C. The weight loss of SiO 2 was the water content in SiO 2 , which was 6.57%. For the eutectic hydrated salt, its weight loss came from the evaporation of crystal water. When the sample temperature was higher than 330 • C, the weight of eutectic hydrated salts basically did not decrease, and the total weight loss was 63.35%. For composite PCMs, the weight loss process was similar to that of the eutectic hydrated salt, and its total weight loss was 46.22%. According to the weight loss of SiO 2 , eutectic hydrated salts, and composite PCMs, it could be calculated that the content of eutectic hydrated salts in composite PCMs was about 69.8%, which was consistent with the theoretical value of 70%.
Materials 2021, 14, x FOR PEER REVIEW 9 bending vibration of Si-O-Si, respectively. The absorption peak at 1096 cm −1 was also stretching vibration peak of HPO4 2-. In addition, the stretching vibration peak of HP also appeared at 859 cm −1 and 544 cm −1 . Only the characteristic absorption peaks of HP and SiO2 appeared in the composite PCMs, and no other peaks appeared. The res indicated no chemical reaction between eutectic hydrated salt and SiO2, but only phys adsorption.

Thermal Stability
The TGA curves of SiO2, eutectic hydrated salts, and composite PCMs are show Figure 7. Because SiO2 has hygroscopicity, it could be seen from the figure that it weight loss during the process of heating to 600 °C. The weight loss of SiO2 was the w content in SiO2, which was 6.57%. For the eutectic hydrated salt, its weight loss came f the evaporation of crystal water. When the sample temperature was higher than 330 the weight of eutectic hydrated salts basically did not decrease, and the total weight was 63.35%. For composite PCMs, the weight loss process was similar to that of eutectic hydrated salt, and its total weight loss was 46.22%. According to the weight of SiO2, eutectic hydrated salts, and composite PCMs, it could be calculated that content of eutectic hydrated salts in composite PCMs was about 69.8%, which consistent with the theoretical value of 70%.

Thermal Reliability
The DSC curve and corresponding thermal characteristics of composite PCMs after 200 heating and cooling cycles are displayed in Figure 8 and Table 3. It could be seen that after 200 heating and cooling cycles, the thermal properties of composite PCMs had only

Thermal Reliability
The DSC curve and corresponding thermal characteristics of composite PCMs after 200 heating and cooling cycles are displayed in Figure 8 and Table 3. It could be seen that after 200 heating and cooling cycles, the thermal properties of composite PCMs had only changed a little. The phase transition temperature and the peak temperature of composite PCMs increased by about 0.6 • C. The melting enthalpy of the composite PCMs decreased from 145.3 J/g to 139.3 J/g. The results showed that the defects, such as phase separation, were improved, and it had good thermal stability. It can be imagined that the thermal properties of composite PCMs will not be seriously damaged during long-term use. Other similar studies were summarized in Table 4. After comprehensive comparison, the composite PCMs prepared in this study had high latent heat, so they have good potential in thermal energy storage applications.

Physical Model
To study the building energy-saving potential of composite PCMs, the impact of composite PCMs on the thermal performance of prefabricated temporary houses was evaluated. The model diagram of the room is shown in Figure 9. The length, width, and height of the building model were 5.62, 3.80, and 2.85 m, respectively. There was a window with a width of 1.2 m and a height of 1.5 m on the north and south walls. There was also a door with a width of 0.9 m and a height of 2.1 m on the south wall. The roof of prefabricated temporary houses was a double-pitch roof. The geometry of the room model

Physical Model
To study the building energy-saving potential of composite PCMs, the impact of composite PCMs on the thermal performance of prefabricated temporary houses was evaluated. The model diagram of the room is shown in Figure 9. The length, width, and height of the building model were 5.62, 3.80, and 2.85 m, respectively. There was a window with a width of 1.2 m and a height of 1.5 m on the north and south walls. There was also a door with a width of 0.9 m and a height of 2.1 m on the south wall. The roof of prefabricated temporary houses was a double-pitch roof. The geometry of the room model in this paper represented the typical traditional prefabricated temporary house in the current Chinese market. The structural details of prefabricated temporary houses and the thermophysical properties of materials involved in the building model are presented in Tables 5 and 6, respectively. In this section, the prefabricated temporary house with and without PCM panels was named PCM rooms and reference rooms, respectively.

Envelope Roof and Wall Door Floor Window With PCM
Without PCM Outside layer 0.5 mm steel sheet 0.5 mm steel sheet 0.5 mm steel sheet 0.5 mm steel sheet 6 mm low-E glass Layer 2 90 mm rock wool 100 mm rock wool 100 mm rock wool 100 mm rock wool 12 mm air layer Layer 3 10 mm PCM panel 0.5 mm steel sheet 0.5 mm steel sheet 10 mm plywood 6 mm glass Layer 4 0.5 mm steel sheet Energyplus v9.2 was selected as a tool for numerical simulation. The heat balance algorithm of conduction finite difference was adopted to simulate the PCM. The Chinese Standard Weather Data of Beijing, which is located in the cold region, was selected as the external environment. Because the thickness of the steel sheet was too thin, it needed to be ignored in the simulation. The infiltration ventilation rate and time step were set to 0.5 air change per hour (ACH) and 3 min, respectively. From the perspective of energy saving,   Energyplus v9.2 was selected as a tool for numerical simulation. The heat balance algorithm of conduction finite difference was adopted to simulate the PCM. The Chinese Standard Weather Data of Beijing, which is located in the cold region, was selected as the external environment. Because the thickness of the steel sheet was too thin, it needed to be ignored in the simulation. The infiltration ventilation rate and time step were set to 0.5 air change per hour (ACH) and 3 min, respectively. From the perspective of energy saving, the air conditioning temperature in the cooling season  and heating season  were set to 18 and 26 • C, respectively. The "HVAC Template: Zone: Ideal Loads Air System" object was selected to simulate the energy consumption of prefabricated temporary houses.
The predicted mean vote and predicted percentage of dissatisfied models (PMV-PPD) were employed to evaluate the residential thermal comfort of prefabricated temporary houses. The metabolic rate and air velocity were considered to be 1.0 met and 0.15 m/s, respectively. The clothing level was set at 0.5 clo from May to September, and 1.0 clo for other months. The number of people in prefabricated temporary houses was considered as 2, and the working efficiency value was set as 0. Based on the evaluation standard for the indoor thermal environment in civil buildings (GB/T 50785-2012), indoor thermal conditions are classified into three categories. As the PPD and absolute value of PMV become smaller, the thermal comfort becomes better, as shown in Table 7. Table 7. Evaluation grade of indoor thermal comfort.

Analysis of Thermal Performance
The thermal comfort hours and energy consumption of prefabricated temporary houses are shown in Table 8. For the reference room, the thermal comfort hours of Grade I, Grade II, and Grade III were 2187, 945, and 5498 h, respectively. When the 10 mm rock wool was replaced by the 10 mm PCM panel, the thermal comfort hours of Grade I and Grade II increased by 804 and 190 h, respectively. This meant that the PCM panel could play a positive role, which was better than the rock wool. The indoor temperature of prefabricated temporary houses in May is displayed in Figure 10. For the reference room, the room temperature was between 15 and 40 • C. Such a large temperature fluctuation made the thermal environment of prefabricated temporary houses unable to meet people's requirements. For the PCM room, PCMs could absorb the heat transferred from the outside in the daytime, which was conducive to decreasing the peak temperature. In addition, when the temperature at night dropped, the heat stored during the day was released, thus increasing the night temperature. It could be seen from Figure 10 that the temperature fluctuation of the PCM room had decreased significantly. Therefore, the thermal comfort of the PCM room was greatly improved compared with the reference room. Moreover, the integration of PCMs into prefabricated temporary houses also contributed to the reduction in energy consumption. According to Table 8, the energy consumption of the reference room and PCM room was 2775 and 2656 kWh, respectively. The energy consumption of the PCM room was 119 kWh less than that of the reference room, and the energy saving rate was about 4.3%. In general, the PCM panel had the potential for energy conservation in the application of prefabricated temporary houses.

Conclusions
In this paper, composite PCMs with tunable phase transition temperature were prepared. DHPT was blended with DHPD to form eutectic hydrated salts. SiO2 and Na2SiO3·9H2O were used as the adsorption carrier and nucleating agent of eutectic hydrated salts, respectively. The properties of composite PCMs were tested and characterized, and their application performances were studied. The main conclusions were as follows: 1. Adding 15% deionized water in the preparation of eutectic hydrated salts was helpful in improving the thermal storage performance of PCMs. Na2SiO3·9H2O effectively reduced the supercooling degree of the composite PCM. When its content was 5%, the supercooling degree was 1.2 °C . 2. The maximum adsorption capacity of SiO2 on eutectic hydrated salt is about 70%.
The SEM and FTIR test results confirmed that eutectic hydrated salts and SiO2 were combined, and the combination did not generate new substances. The TGA test results showed that the PCMs had good thermal stability, and the content of eutectic hydrated salts calculated according to the TGA results was basically consistent with the theoretical value. 3. With the increase in the content of DHPT, the melting enthalpy and phase transition temperature of composite PCMs decreased gradually. When the content of DHPT was 3%, the melting enthalpy of the composite PCM was 145.3 J/g, and its phase transition temperature (26.5 °C ) was suitable for the field of building energy conservation. After 200 heating and cooling cycles, its melting enthalpy and phase transition temperature changed very little, which indicated that it had good thermal reliability. 4. The numerical simulation results showed that composite PCMs had good application performance in prefabricated temporary houses. By replacing the 10 mm insulation

Conclusions
In this paper, composite PCMs with tunable phase transition temperature were prepared. DHPT was blended with DHPD to form eutectic hydrated salts. SiO 2 and Na 2 SiO 3 ·9H 2 O were used as the adsorption carrier and nucleating agent of eutectic hydrated salts, respectively. The properties of composite PCMs were tested and characterized, and their application performances were studied. The main conclusions were as follows:

1.
Adding 15% deionized water in the preparation of eutectic hydrated salts was helpful in improving the thermal storage performance of PCMs. Na 2 SiO 3 ·9H 2 O effectively reduced the supercooling degree of the composite PCM. When its content was 5%, the supercooling degree was 1.2 • C.

2.
The maximum adsorption capacity of SiO 2 on eutectic hydrated salt is about 70%. The SEM and FTIR test results confirmed that eutectic hydrated salts and SiO 2 were combined, and the combination did not generate new substances. The TGA test results showed that the PCMs had good thermal stability, and the content of eutectic hydrated salts calculated according to the TGA results was basically consistent with the theoretical value.

3.
With the increase in the content of DHPT, the melting enthalpy and phase transition temperature of composite PCMs decreased gradually. When the content of DHPT was 3%, the melting enthalpy of the composite PCM was 145.3 J/g, and its phase transition temperature (26.5 • C) was suitable for the field of building energy conservation. After 200 heating and cooling cycles, its melting enthalpy and phase transition temperature changed very little, which indicated that it had good thermal reliability.

4.
The numerical simulation results showed that composite PCMs had good application performance in prefabricated temporary houses. By replacing the 10 mm insulation board with the 10 mm PCM panel, the thermal comfort hours of Grade I increased from 2187 h to 2991 h, and the energy consumption decreased from 2775 kWh to 2656 kWh.