Cooling Capacity Test for MIL-101(Cr)/CaCl2 for Adsorption Refrigeration System

An MIL-101(Cr) powder material was successfully prepared using the hydrothermal synthesis method, and then the original MIL-101(Cr) was combined with different mass fractions of CaCl2 using the immersion method to obtain a MIL-101(Cr)/CaCl2 composite material. The physical properties of the adsorbent were determined by X-ray powder diffraction (XRD), an N2 adsorption desorption isotherm test, and thermogravimetric analysis (TG). The water vapor adsorption performance of the metal-organic frameworks MOFs was tested with a gravimetric water vapor adsorption instrument to analyze its water vapor adsorption mechanism. Based on the SIMULINK platform in the MATLAB software, a simulation model of the coefficient of performance (COP) and cooling capacity of the adsorption refrigeration system was established, and the variation trends of the COP and cooling capacity of the adsorption refrigeration system under different evaporation/condensation/adsorption/desorption temperatures was theoretically studied. MIL101-(Cr)/CaCl2-20% was selected as the adsorption material in the adsorption refrigeration system through the physical characterization of composite materials with different CaCl2 concentrations by means of adsorption water vapor test experiments. A closed adsorption system performance test device was built based on the liquid level method. The cooling power per unit and adsorbent mass (COP and SCP) of the system were tested at different evaporation temperatures (288 K/293 K/298 K); the adsorption temperature was 298 K, the condensation temperature was 308 K, and the desorption temperature was 353 K. The experimental results showed that COP and SCP increased with the increase in the evaporation temperature. When the evaporation temperature was 298 K, the level of COP was 0.172, and the level of SCP was 136.9 W/kg. The COP and SCP of the system were tested at different adsorption temperatures (293 K/298 K/303 K); the evaporation temperature was 288 K, the condensation temperature was 308 K, and the desorption temperature was 353 K. The experimental results showed that the levels of COP and SCP decreased with the increase in the adsorption temperature. When the adsorption temperature was 293 K, the level of COP was 0.18, and the level of SCP was 142.4 W/kg.

Molecules 2020, 25, 3975 3 of 20 40 • C. Wang [15] combined activated carbon fiber felts (ACF FELT) with 30% calcium chloride, and the water adsorption capacity reached 1.7 g H2O /g ads , higher than activated carbon, silica gel and even the compound of silica gel and calcium chloride. Chan [16] soaked zeolite 13X in 46% CaCl 2 solution, and the adsorption capacity increased by 0.4 g H2O /g ads at 25 and 75 • C and 870 Pa.
In this paper, MIL-101(Cr)/CaCl 2 composite materials with different CaCl 2 concentrations (10%, 20%, 30%) were prepared by the dipping method. Through physical characterization and water vapor adsorption performance experiments, MIL-101(Cr)/CaCl 2 -20%/water was selected as the pair of working media for the adsorption refrigeration system to test the system performance at different evaporation/adsorption temperatures.  10.30 • and 16.54 • , and the position of the diffraction peak is very consistent with the report [15]. The MIL-101(Cr)/CaCl 2 composite has the same diffraction peak as the original MIL-101(Cr), indicating that MIL-101(Cr) is still the main component of the MIL-101(Cr)/CaCl 2 composite. In addition, we can also find that the peak of XRD diffraction gradually decreases with the increase in CaCl 2 content, because the disordered hydrated salt molecules are filled into the mesopores of MIL-101(Cr) with the increase in CaCl 2 content [17].

XRD Analysis
Molecules 2020, 25, x FOR PEER REVIEW 3 of 20 30% calcium chloride, and the water adsorption capacity reached 1.7 g H2O /g ads , higher than activated carbon, silica gel and even the compound of silica gel and calcium chloride. Chan [16] soaked zeolite 13X in 46% CaCl2 solution, and the adsorption capacity increased by 0.4 g H2O /g ads at 25 and 75 °C and 870 Pa. In this paper, MIL-101(Cr)/CaCl2 composite materials with different CaCl2 concentrations (10%, 20%, 30%) were prepared by the dipping method. Through physical characterization and water vapor adsorption performance experiments, MIL-101(Cr)/CaCl2-20%/water was selected as the pair of working media for the adsorption refrigeration system to test the system performance at different evaporation/adsorption temperatures. Figure 1 shows the XRD spectra of MIL-101(Cr)/CaCl2 composites. It can be seen from the figure that the main characteristic diffraction peaks of MIL-101(Cr) samples appear at 2θ = 3.42°, 5.98°, 8.56°, 9.18°, 10.30° and 16.54°, and the position of the diffraction peak is very consistent with the report [15]. The MIL-101(Cr)/CaCl2 composite has the same diffraction peak as the original MIL-101(Cr), indicating that MIL-101(Cr) is still the main component of the MIL-101(Cr)/CaCl2 composite. In addition, we can also find that the peak of XRD diffraction gradually decreases with the increase in CaCl2 content, because the disordered hydrated salt molecules are filled into the mesopores of MIL-101(Cr) with the increase in CaCl2 content [17].

TG Analysis
The thermogravimetric results for MIL-101(Cr)/CaCl2 composites are shown in Figure 2. The weightlessness of the material mainly goes through two stages. As the temperature rises to 100 °C, the water in MIL-101(Cr) dissipates, and the weight loss in the first stage is caused by the loss of calcium chloride in the composite. At 300 °C, MIL-101(Cr) began to lose the crystal water in the structure, and by 400 °C, the framework was decomposed. The structure of the composite material was destroyed at 450 °C, which was delayed by 50 °C compared with MIL-101(Cr).

TG Analysis
The thermogravimetric results for MIL-101(Cr)/CaCl 2 composites are shown in Figure 2. The weightlessness of the material mainly goes through two stages. As the temperature rises to 100 • C, the water in MIL-101(Cr) dissipates, and the weight loss in the first stage is caused by the loss of calcium chloride in the composite. At 300 • C, MIL-101(Cr) began to lose the crystal water in the structure, and by 400 • C, the framework was decomposed. The structure of the composite material was destroyed at 450 • C, which was delayed by 50 • C compared with MIL-101(Cr).  Figure 3 shows the N2 adsorption and desorption isotherms for the MIL-101(Cr)/CaCl2 composites. According to the classification of the BET adsorption isotherm curve and type-II adsorption-desorption isotherms, in the low pressure stage (p = 0-0.2/p0), the nitrogen adsorption quantity displays sharp growth, proving that it contains a certain amount of microporous materials. At a relative pressure of about 0.2, the material adsorbed a second time, indicating that the experimentally synthesized material contained two different cage structures [18]. The adsorption capacity in the middle section of the curve (p/p0 = 0.2-0.9) increased slowly with the increase in pressure. In the second half of the curve (p/p0 = 0.9-1.0), the adsorption line rose sharply and did not show adsorption saturation until the vapor pressure was close to saturation, indicating that the sample contained a certain amount of mesoporous and macroporous bulk filling due to capillary condensation.   Table 1 lists the pore structure parameters of the MIL-  Figure 3 shows the N 2 adsorption and desorption isotherms for the MIL-101(Cr)/CaCl 2 composites. According to the classification of the BET adsorption isotherm curve and type-II adsorption-desorption isotherms, in the low pressure stage (p = 0-0.2/p 0 ), the nitrogen adsorption quantity displays sharp growth, proving that it contains a certain amount of microporous materials. At a relative pressure of about 0.2, the material adsorbed a second time, indicating that the experimentally synthesized material contained two different cage structures [18]. The adsorption capacity in the middle section of the curve (p/p 0 = 0.2-0.9) increased slowly with the increase in pressure. In the second half of the curve (p/p 0 = 0.9-1.0), the adsorption line rose sharply and did not show adsorption saturation until the vapor pressure was close to saturation, indicating that the sample contained a certain amount of mesoporous and macroporous bulk filling due to capillary condensation.   Table 1 lists the pore structure parameters of the MIL-101(Cr) and MIL-101(Cr)/CaCl 2 composites. With the increase in CaCl 2 addition, the specific surface area, total pore volume and pore size distribution of microporous mesopores of MIL-101(Cr)/CaCl 2 composites decreased gradually. This is because the added CaCl 2 molecules occupy part of the pores of the MIL-101(Cr) material. For MIL-101(Cr)/CaCl 2 -30%, the BET specific surface area and total pore volume were only 193 m 2 /g and 0.071 cm 3 /g, which indicates that the CaCl 2 molecule almost completely blocked the pores of MIL-101(Cr).

N 2 Adsorption-Desorption Isotherms and Pore Size Analysis
Molecules 2020, 25, x FOR PEER REVIEW 5 of 20 101(Cr) and MIL-101(Cr)/CaCl2 composites. With the increase in CaCl2 addition, the specific surface area, total pore volume and pore size distribution of microporous mesopores of MIL-101(Cr)/CaCl2 composites decreased gradually. This is because the added CaCl2 molecules occupy part of the pores of the MIL-101(Cr) material. For MIL-101(Cr)/CaCl2-30%, the BET specific surface area and total pore volume were only 193 m 2 /g and 0.071 cm 3 /g, which indicates that the CaCl2 molecule almost completely blocked the pores of MIL-101(Cr).

Water Vapor Adsorption Isotherms
It can be seen from Figure 5 that the MIL-101(Cr)/CaCl2-10% adsorption isotherm line type is almost the same as the MIL-101(Cr) line type. The adsorption isotherms of MIL-101(Cr)/CaCl2-20% and MIL-101(Cr)/CaCl2-30% showed obvious changes, and the equilibrium adsorption capacity was higher than that of the original MIL-101(Cr). As can be seen from the adsorption isotherm, water absorbing capacity of calcium chloride itself leads to the low pressure area which in the water vapor adsorption experiment showed higher adsorption performance, the relative pressure adsorption amount compared with the original start going down after 0.4. This is due to the good water absorption characteristics of calcium chloride. The specific surface area of the material gradually became dominant; the specific surface area of the composites compared with MIL-101 decreased, so the relative pressure was greater than 0.4 after the adsorption performance was reduced. In particular, the MIL-101(Cr)/CaCl2-20% and MIL-101(Cr)/CaCl2-30% composite materials had a higher water adsorption capacity and equilibrium adsorption capacity than the original MIL-101(Cr) at low pressure. Therefore, the MIL-101(Cr)/CaCl2-20% and MIL-101(Cr)/CaCl2-30% materials are very attractive adsorbent materials for use in adsorption refrigeration applications. Although the equilibrium water adsorption capacity of the MIL-101(Cr)/CaCl2-30% composite adsorbent is high, it becomes agglomerated after water absorption continues to agglomerate after repeated drying, which significantly affects the material's adsorption capacity and limits its use in adsorption refrigeration applications. In summary, the MIL-101(Cr)/CaCl2-20% composite material was selected for its adsorption refrigeration performance.

Water Vapor Adsorption Isotherms
It can be seen from Figure 5 that the MIL-101(Cr)/CaCl 2 -10% adsorption isotherm line type is almost the same as the MIL-101(Cr) line type. The adsorption isotherms of MIL-101(Cr)/CaCl 2 -20% and MIL-101(Cr)/CaCl 2 -30% showed obvious changes, and the equilibrium adsorption capacity was higher than that of the original MIL-101(Cr). As can be seen from the adsorption isotherm, water absorbing capacity of calcium chloride itself leads to the low pressure area which in the water vapor adsorption experiment showed higher adsorption performance, the relative pressure adsorption amount compared with the original start going down after 0.4. This is due to the good water absorption characteristics of calcium chloride. The specific surface area of the material gradually became dominant; the specific surface area of the composites compared with MIL-101 decreased, so the relative pressure was greater than 0.4 after the adsorption performance was reduced. In particular, the MIL-101(Cr)/CaCl 2 -20% and MIL-101(Cr)/CaCl 2 -30% composite materials had a higher water adsorption capacity and equilibrium adsorption capacity than the original MIL-101(Cr) at low pressure. Therefore, the MIL-101(Cr)/CaCl 2 -20% and MIL-101(Cr)/CaCl 2 -30% materials are very attractive adsorbent materials for use in adsorption refrigeration applications. Although the equilibrium water adsorption capacity of the MIL-101(Cr)/CaCl 2 -30% composite adsorbent is high, it becomes agglomerated after water absorption continues to agglomerate after repeated drying, which significantly affects the material's adsorption capacity and limits its use in adsorption refrigeration applications. In summary, the MIL-101(Cr)/CaCl 2 -20% composite material was selected for its adsorption refrigeration performance.

Effect of Different Desorption Temperatures on System Cooling Capacity and COP
The effect of the desorption temperature Tg2 on cooling capacity and COP is shown in Figure 6. It can be seen from the figure that the cooling capacity of the system increased with the increase in the desorption temperature. The total cycle time was 9000 s. From the COP, there was an optimal desorption temperature. When Te = 288 K, Tc = 308 K, Ta2 = 298 K, the optimal desorption temperature was 351 K. At this time, the system cooling capacity was 118 kJ and the COP was 0.24. Before the optimum desorption temperature, the COP value increased with the desorption temperature in a certain temperature range. The higher the desorption temperature, the more water vapor is desorbed from the adsorption bed, so the more water vapor is adsorbed by the material. The more the corresponding cooling capacity, the greater the amount of cooling. However, when the desorption temperature exceeds the optimum desorption temperature, the COP decreases as the desorption temperature increases because the desorption temperature increases and the heat consumed by the system cycle also increases, causing the COP to decrease.

Effect of Different Desorption Temperatures on System Cooling Capacity and COP
The effect of the desorption temperature T g2 on cooling capacity and COP is shown in Figure 6. It can be seen from the figure that the cooling capacity of the system increased with the increase in the desorption temperature. The total cycle time was 9000 s. From the COP, there was an optimal desorption temperature. When T e = 288 K, T c = 308 K, Ta 2 = 298 K, the optimal desorption temperature was 351 K. At this time, the system cooling capacity was 118 kJ and the COP was 0.24. Before the optimum desorption temperature, the COP value increased with the desorption temperature in a certain temperature range. The higher the desorption temperature, the more water vapor is desorbed from the adsorption bed, so the more water vapor is adsorbed by the material. The more the corresponding cooling capacity, the greater the amount of cooling. However, when the desorption temperature exceeds the optimum desorption temperature, the COP decreases as the desorption temperature increases because the desorption temperature increases and the heat consumed by the system cycle also increases, causing the COP to decrease.

Effect of Different Desorption Temperatures on System Cooling Capacity and COP
The effect of the desorption temperature Tg2 on cooling capacity and COP is shown in Figure 6. It can be seen from the figure that the cooling capacity of the system increased with the increase in the desorption temperature. The total cycle time was 9000 s. From the COP, there was an optimal desorption temperature. When Te = 288 K, Tc = 308 K, Ta2 = 298 K, the optimal desorption temperature was 351 K. At this time, the system cooling capacity was 118 kJ and the COP was 0.24. Before the optimum desorption temperature, the COP value increased with the desorption temperature in a certain temperature range. The higher the desorption temperature, the more water vapor is desorbed from the adsorption bed, so the more water vapor is adsorbed by the material. The more the corresponding cooling capacity, the greater the amount of cooling. However, when the desorption temperature exceeds the optimum desorption temperature, the COP decreases as the desorption temperature increases because the desorption temperature increases and the heat consumed by the system cycle also increases, causing the COP to decrease.  Effect of desorption temperature on refrigeration capacity and coefficient of performance (COP).

Effect of Different Condensation Temperatures on System Cooling Capacity and COP
Calculation parameters: T e = 288 K, T a2 = 298 K, T g2 = 353 K. The effect of condensation temperature Tc on cooling capacity and COP is shown in Figure 7. It can be seen from the figure that as the condensing temperature increases, the cooling capacity and COP of the system decrease. When T c = 313 K, the cooling capacity was 10.04 W and the COP was 0.17. This is because the condensation temperature is not conducive to the condensation of the refrigerant vapor, reducing the desorption amount of the refrigerant, thereby reducing the COP of the system. At the same time, the adsorption delay occurs when the condensation temperature rises, so that the adsorption amount decreases, causing the cooling capacity to decrease accordingly. It can be seen from the above law that reducing the condensation temperature of the adsorption refrigeration system can improve the performance of the system.

Effect of Different Condensation Temperatures on System Cooling Capacity and COP
Calculation parameters: Te = 288 K, Ta2 = 298 K, Tg2 = 353 K. The effect of condensation temperature Tc on cooling capacity and COP is shown in Figure 7. It can be seen from the figure that as the condensing temperature increases, the cooling capacity and COP of the system decrease. When Tc = 313 K, the cooling capacity was 10.04 W and the COP was 0.17. This is because the condensation temperature is not conducive to the condensation of the refrigerant vapor, reducing the desorption amount of the refrigerant, thereby reducing the COP of the system. At the same time, the adsorption delay occurs when the condensation temperature rises, so that the adsorption amount decreases, causing the cooling capacity to decrease accordingly. It can be seen from the above law that reducing the condensation temperature of the adsorption refrigeration system can improve the performance of the system.

Effects of Different Adsorption Temperatures on Cooling Capacity and COP of the System
Calculation parameters: Te = 288 K, Tc = 308 K, Tg2 = 353 K. The effect of condensation temperature Ta2 on cooling capacity and COP is shown in Figure 8. It can be seen from the figure that as the adsorption temperature increases, the cooling capacity and COP of the system decrease. The adsorption temperature increases, so that the difference between the adsorption temperature and the initial adsorption temperature decreases, and then the cyclic adsorption amount also decreases, eventually leading to a decrease in system COP. When Te = 288 K, Tc = 308 K, Tg2 = 353 K, and Ta2 = 308 K, the minimum cooling capacity and COP were 8.81 W and 0.15, respectively.

Effects of Different Adsorption Temperatures on Cooling Capacity and COP of the System
Calculation parameters: T e = 288 K, T c = 308 K, T g2 = 353 K. The effect of condensation temperature T a2 on cooling capacity and COP is shown in Figure 8. It can be seen from the figure that as the adsorption temperature increases, the cooling capacity and COP of the system decrease. The adsorption temperature increases, so that the difference between the adsorption temperature and the initial adsorption temperature decreases, and then the cyclic adsorption amount also decreases, eventually leading to a decrease in system COP. When T e = 288 K, T c = 308 K, T g2 = 353 K, and T a2 = 308 K, the minimum cooling capacity and COP were 8.81 W and 0.15, respectively.  Figure 7. It can be seen from the figure that as the condensing temperature increases, the cooling capacity and COP of the system decrease. When Tc = 313 K, the cooling capacity was 10.04 W and the COP was 0.17. This is because the condensation temperature is not conducive to the condensation of the refrigerant vapor, reducing the desorption amount of the refrigerant, thereby reducing the COP of the system. At the same time, the adsorption delay occurs when the condensation temperature rises, so that the adsorption amount decreases, causing the cooling capacity to decrease accordingly. It can be seen from the above law that reducing the condensation temperature of the adsorption refrigeration system can improve the performance of the system.

Effects of Different Adsorption Temperatures on Cooling Capacity and COP of the System
Calculation parameters: Te = 288 K, Tc = 308 K, Tg2 = 353 K. The effect of condensation temperature Ta2 on cooling capacity and COP is shown in Figure 8. It can be seen from the figure that as the adsorption temperature increases, the cooling capacity and COP of the system decrease. The adsorption temperature increases, so that the difference between the adsorption temperature and the initial adsorption temperature decreases, and then the cyclic adsorption amount also decreases, eventually leading to a decrease in system COP. When Te = 288 K, Tc = 308 K, Tg2 = 353 K, and Ta2 = 308 K, the minimum cooling capacity and COP were 8.81 W and 0.15, respectively.

Effects of Different Evaporation Temperatures on Cooling Capacity and COP of the System
Calculation parameters: T a2 = 298 K, T c = 308 K, T g2 = 353 K. The effect of evaporation temperature (T e ) on the cooling capacity and COP is shown in Figure 9. It can be seen from the figure that the system's cooling capacity and coefficient of performance COP both increase with the increase in the evaporation temperature. When the evaporation temperature increased from 288 K to 303 K, the cooling capacity and COP-initially 8.81 W and 0.15-increased to 16 W and 0.28. This occurs because (1) the higher the evaporation temperature, the higher the evaporation pressure; (2) the larger the pressure difference in the adsorption bed, the smaller the mass transfer resistance; (3) the larger the adsorption amount of the adsorbent, the larger the sensible heat of the adsorption bed. Ultimately, the cooling capacity and coefficient of COP performance both increase as the evaporation temperature increases. Calculation parameters: Ta2 = 298 K, Tc = 308 K, Tg2 = 353 K. The effect of evaporation temperature (Te) on the cooling capacity and COP is shown in Figure 9. It can be seen from the figure that the system's cooling capacity and coefficient of performance COP both increase with the increase in the evaporation temperature. When the evaporation temperature increased from 288 K to 303 K, the cooling capacity and COP-initially 8.81 W and 0.15-increased to 16 W and 0.28. This occurs because (1) the higher the evaporation temperature, the higher the evaporation pressure; (2) the larger the pressure difference in the adsorption bed, the smaller the mass transfer resistance; (3) the larger the adsorption amount of the adsorbent, the larger the sensible heat of the adsorption bed. Ultimately, the cooling capacity and coefficient of COP performance both increase as the evaporation temperature increases.  Figure 10 shows the water vapor adsorption capacity of the adsorption refrigeration system over time at different evaporation temperatures. It can be seen from the figure that when Ta2 = 298 K, Tc = 308 K, Tg2 = 353 K, the evaporation temperature was 288 K, the adsorption capacity was 0.625 g/g; when the evaporation temperature was 293 K, the adsorption capacity was 0.715 g/g; when the evaporation temperature was 298 K, the adsorption capacity was 0.845 g/g. We can find that as the evaporation temperature increases, the equilibrium adsorption amount of water vapor temperature also increases. This is because the evaporation pressure increases and the pressure difference in the refrigeration system also increases, which is beneficial to MIL-101(Cr)/CaCl2-20% composite adsorption.

System Performance Analysis at Different Evaporation Temperatures
It can be seen from Figure 11 that as the evaporation temperature increases, the cooling capacity increases. When the evaporation temperature was 298 K, the cooling capacity of the system increased by 5.47 W compared with that of the system when the evaporation temperature was 288 K.
During the experiment, as the evaporation temperature increases, the system cooling capacity increases, resulting in an upward trend in the system's COP and SCP. It can be found from Figure 12 that the COP of the system was 0.103 when the evaporation temperature was 288 K. When the evaporation temperature rose to 298 K, the COP of the system was 0.172, which was increased by 67%. For SCP, when the evaporation temperature was 288 K, the value was 82.2 W/kg, and when the evaporation temperature was 298 K, the SCP value was 136.9 W/kg, which is an increase of nearly 66.5%. Therefore, in the refrigeration process, in the case of meeting the cooling demand, the evaporation temperature of the system should be increased as much as possible.  Figure 10 shows the water vapor adsorption capacity of the adsorption refrigeration system over time at different evaporation temperatures. It can be seen from the figure that when T a2 = 298 K, T c = 308 K, T g2 = 353 K, the evaporation temperature was 288 K, the adsorption capacity was 0.625 g/g; when the evaporation temperature was 293 K, the adsorption capacity was 0.715 g/g; when the evaporation temperature was 298 K, the adsorption capacity was 0.845 g/g. We can find that as the evaporation temperature increases, the equilibrium adsorption amount of water vapor temperature also increases. This is because the evaporation pressure increases and the pressure difference in the refrigeration system also increases, which is beneficial to MIL-101(Cr)/CaCl 2 -20% composite adsorption.

System Performance Analysis at Different Evaporation Temperatures
It can be seen from Figure 11 that as the evaporation temperature increases, the cooling capacity increases. When the evaporation temperature was 298 K, the cooling capacity of the system increased by 5.47 W compared with that of the system when the evaporation temperature was 288 K.
During the experiment, as the evaporation temperature increases, the system cooling capacity increases, resulting in an upward trend in the system's COP and SCP. It can be found from Figure 12 that the COP of the system was 0.103 when the evaporation temperature was 288 K. When the evaporation temperature rose to 298 K, the COP of the system was 0.172, which was increased by 67%. For SCP, when the evaporation temperature was 288 K, the value was 82.2 W/kg, and when the evaporation temperature was 298 K, the SCP value was 136.9 W/kg, which is an increase of nearly 66.5%. Therefore, in the refrigeration process, in the case of meeting the cooling demand, the evaporation temperature of the system should be increased as much as possible. Molecules 2020, 25, x FOR PEER REVIEW 9 of 20      Figure 13 shows the variation of system adsorption over time at different adsorption temperatures. It can be seen from the figure that when T e = 288 K, T c = 308 K, T g2 = 353 K, the adsorption temperature was 293 K, the adsorption capacity was 0.9 g/g; when the adsorption temperature was 298 K, the adsorption capacity was 0.66 g/g; when the adsorption temperature was 303 K, the adsorption capacity was 0.565 g/g. From this we can see that as the adsorption temperature decreases, the amount of water vapor adsorption in the system gradually increases. The increase in the adsorption temperature causes the relative pressure to decrease. It can be seen from the adsorption isotherm of Figure 5 that as the relative pressure decreases, the equilibrium adsorption capacity decreases.

System Performance Analysis at Different Adsorption Temperatures
Molecules 2020, 25, x FOR PEER REVIEW 10 of 20 2.6.2. System Performance Analysis at Different Adsorption Temperatures Figure 13 shows the variation of system adsorption over time at different adsorption temperatures. It can be seen from the figure that when Te = 288 K, Tc = 308 K, Tg2 = 353 K, the adsorption temperature was 293 K, the adsorption capacity was 0.9 g/g; when the adsorption temperature was 298 K, the adsorption capacity was 0.66 g/g; when the adsorption temperature was 303 K, the adsorption capacity was 0.565 g/g. From this we can see that as the adsorption temperature decreases, the amount of water vapor adsorption in the system gradually increases. The increase in the adsorption temperature causes the relative pressure to decrease. It can be seen from the adsorption isotherm of Figure 5 that as the relative pressure decreases, the equilibrium adsorption capacity decreases. It can be seen from Figure 14 that the cooling capacity decreases with increasing adsorption temperature. When the adsorption temperature was 293 K, the system cooling capacity increased by 4.94W compared with that of when the adsorption temperature was 303 K. It can be seen from Figure 15 that the COP and SCP of the system decrease with the increase in adsorption temperature, and the downward trend is very obvious. When the adsorption temperature It can be seen from Figure 14 that the cooling capacity decreases with increasing adsorption temperature. When the adsorption temperature was 293 K, the system cooling capacity increased by 4.94W compared with that of when the adsorption temperature was 303 K.
Molecules 2020, 25, x FOR PEER REVIEW 10 of 20 2.6.2. System Performance Analysis at Different Adsorption Temperatures Figure 13 shows the variation of system adsorption over time at different adsorption temperatures. It can be seen from the figure that when Te = 288 K, Tc = 308 K, Tg2 = 353 K, the adsorption temperature was 293 K, the adsorption capacity was 0.9 g/g; when the adsorption temperature was 298 K, the adsorption capacity was 0.66 g/g; when the adsorption temperature was 303 K, the adsorption capacity was 0.565 g/g. From this we can see that as the adsorption temperature decreases, the amount of water vapor adsorption in the system gradually increases. The increase in the adsorption temperature causes the relative pressure to decrease. It can be seen from the adsorption isotherm of Figure 5 that as the relative pressure decreases, the equilibrium adsorption capacity decreases. It can be seen from Figure 14 that the cooling capacity decreases with increasing adsorption temperature. When the adsorption temperature was 293 K, the system cooling capacity increased by 4.94W compared with that of when the adsorption temperature was 303 K. It can be seen from Figure 15 that the COP and SCP of the system decrease with the increase in adsorption temperature, and the downward trend is very obvious. When the adsorption temperature It can be seen from Figure 15 that the COP and SCP of the system decrease with the increase in adsorption temperature, and the downward trend is very obvious. When the adsorption temperature was 293 K, the COP and SCP of the system were 0.18 and 142.4 W/kg, respectively. When the adsorption temperature was raised to 303 K, the COP and SCP of the system were reduced to 0.12 and 93 W/kg, respectively, and the performance of the system was nearly doubled compared with the adsorption temperature of 293 K. Therefore, in the adsorption refrigeration system, the adsorption temperature has a significant influence on the performance of the system. was 293 K, the COP and SCP of the system were 0.18 and 142.4 W/kg, respectively. When the adsorption temperature was raised to 303 K, the COP and SCP of the system were reduced to 0.12 and 93 W/kg, respectively, and the performance of the system was nearly doubled compared with the adsorption temperature of 293 K. Therefore, in the adsorption refrigeration system, the adsorption temperature has a significant influence on the performance of the system.

Comparison of Experimental Results with Simulation Results
The experimental results and simulation results are summarized in Tables 2 and 3. The comparison shows that the simulation results basically reflect the variation of system performance under different working conditions. However, as far as the specific values are concerned, the experimental results are slightly lower than the simulation results, mainly because of the following three reasons: (1) The power of the electric heating furnace of the adsorption bed was too large, and the heat generated during the desorption process was excessive, resulting in the system sensible heat increasing and the system performance decreasing. (2) The specific heat capacity selected during the simulation process was mostly fixed, and the specific heat capacity changed with the experimental temperature during the experiment; (3) The leakage heat of the whole experimental system was better. There was more, and the simulation process ignored the heat that the system loses into the environment.

Comparison of Experimental Results with Simulation Results
The experimental results and simulation results are summarized in Tables 2 and 3. The comparison shows that the simulation results basically reflect the variation of system performance under different working conditions. However, as far as the specific values are concerned, the experimental results are slightly lower than the simulation results, mainly because of the following three reasons: (1) The power of the electric heating furnace of the adsorption bed was too large, and the heat generated during the desorption process was excessive, resulting in the system sensible heat increasing and the system performance decreasing. (2) The specific heat capacity selected during the simulation process was mostly fixed, and the specific heat capacity changed with the experimental temperature during the experiment; (3) The leakage heat of the whole experimental system was better. There was more, and the simulation process ignored the heat that the system loses into the environment.

2.
With the increase in CaCl 2 content, the total pore volume and pore diameter distribution of MIL-101(Cr)/CaCl 2 composite material gradually decreased. When CaCl 2 concentration reached 30%, the pore volume of MIL-101(Cr) was almost completely blocked by molecules.

3.
The COP simulation model showed that the optimal desorption temperature and COP were 351 K and 0.24, 0.1 higher than MIL-101(Cr).

Synthesis of MIL-101(Cr)/CaCl 2 Composites
MIL-101(Cr)/CaCl 2 composites were prepared by the immersion method [17]. About 0.4 g of MIL-101(Cr) was weighed and dried in an oven at 100 • C for 8 h. The MIL-101(Cr) sample was cooled to room temperature and then immersed in 10%, 20%, and 30% CaCl 2 solutions (soaked at room temperature for 2 h). Finally, the soaked material was subjected to centrifugation, washed with deionized water, and centrifuged again to remove excess calcium chloride. The sample was collected and completely dried overnight in a vacuum oven at 373 K.

Materials Characterization
X-ray diffraction (XRD) was used to analyze the image of the composite. The diffraction peak was used to determine whether the composite was successfully synthesized. The thermogravimetric analysis (TG) was used to test the relationship between material quality and temperature, and its composition and thermal stability were studied. The specific surface area and pore structure of the composite were analyzed by N 2 adsorption-desorption isotherm. The water vapor adsorption isotherm, adsorption kinetics and differential adsorption heat curves of the composite adsorbent were obtained by the gravimetric method, and the huge space and potential in the adsorption refrigeration application were preliminarily determined.

Measurement of Water Vapor Adsorption Isotherms
The water vapor adsorption isotherm, adsorption kinetics curve and adsorption heat curve of the material were tested by Conta's DVS Advantage water vapor adsorption instrument. A weight about 40 mg of the sample was vacuum dried at 373 K to remove the solvent molecules from the material and adsorb the water vapor into the pores. The material was then cooled to room temperature and placed in the crucible of the instrument, then the interface was clicked on to run the program. During the test, the temperature of the adsorption bed and the crucible were set to 298 K and 308 K, respectively.

Experimental Rig of Single-Bed Adsorption System with New Type Adsorbent Filling Method
The experimental platform of adsorption refrigeration system adopts the same structure as the previous study [21], as shown in Figure 16. We changed the adsorption bed filling method. Figure 17 shows the adsorption bed fabrication process. We took a 30 cm long stainless steel tube with a diameter of 22 mm, opened a certain number of 5 mm holes, this was the mass transfer hole. The empty stainless steel tube was wrapped with two 300-mesh screens, and clamped to the stainless steel tube with a clamp to prevent the adsorbent powder from being drawn into the pipeline of the system during vacuuming, which affects the normal operation of the experiment. We welded the stainless steel tube with the mass transfer holes into the center of a thick stainless steel tube (adsorption bed) with a length of 30 cm and a diameter of 52 mm. At this time, the inner diameter of the small stainless steel tube was the mass transfer channel of the whole adsorption bed. The composite adsorbent material was then filled into the gap between the thick stainless steel tubes, and the adsorbent bed was welded and sealed and attached to the system via a vacuum ball valve. The adsorption bed had sufficient mass transfer space to enhance the heat and mass transfer capacity of the adsorption bed. The structure of the adsorption bed is shown in Figure 18.

Experimental Rig of Single-Bed Adsorption System with New Type Adsorbent Filling Method
The experimental platform of adsorption refrigeration system adopts the same structure as the previous study [21], as shown in Figure 16. We changed the adsorption bed filling method. Figure 17 shows the adsorption bed fabrication process. We took a 30 cm long stainless steel tube with a diameter of 22 mm, opened a certain number of 5 mm holes, this was the mass transfer hole. The empty stainless steel tube was wrapped with two 300-mesh screens, and clamped to the stainless steel tube with a clamp to prevent the adsorbent powder from being drawn into the pipeline of the system during vacuuming, which affects the normal operation of the experiment. We welded the stainless steel tube with the mass transfer holes into the center of a thick stainless steel tube (adsorption bed) with a length of 30 cm and a diameter of 52 mm. At this time, the inner diameter of the small stainless steel tube was the mass transfer channel of the whole adsorption bed. The composite adsorbent material was then filled into the gap between the thick stainless steel tubes, and the adsorbent bed was welded and sealed and attached to the system via a vacuum ball valve. The adsorption bed had sufficient mass transfer space to enhance the heat and mass transfer capacity of the adsorption bed. The structure of the adsorption bed is shown in Figure 18.

Experimental Rig of Single-Bed Adsorption System with New Type Adsorbent Filling Method
The experimental platform of adsorption refrigeration system adopts the same structure as the previous study [21], as shown in Figure 16. We changed the adsorption bed filling method. Figure 17 shows the adsorption bed fabrication process. We took a 30 cm long stainless steel tube with a diameter of 22 mm, opened a certain number of 5 mm holes, this was the mass transfer hole. The empty stainless steel tube was wrapped with two 300-mesh screens, and clamped to the stainless steel tube with a clamp to prevent the adsorbent powder from being drawn into the pipeline of the system during vacuuming, which affects the normal operation of the experiment. We welded the stainless steel tube with the mass transfer holes into the center of a thick stainless steel tube (adsorption bed) with a length of 30 cm and a diameter of 52 mm. At this time, the inner diameter of the small stainless steel tube was the mass transfer channel of the whole adsorption bed. The composite adsorbent material was then filled into the gap between the thick stainless steel tubes, and the adsorbent bed was welded and sealed and attached to the system via a vacuum ball valve. The adsorption bed had sufficient mass transfer space to enhance the heat and mass transfer capacity of the adsorption bed. The structure of the adsorption bed is shown in Figure 18.

Experimental Procedure for Performance Test of Adsorption Refrigeration System
The schematic diagram of the experimental system is shown in Figure 19. The experimental steps are as follows: Adsorption bed heating vacuum process: because the system requires high vacuum, it is necessary to heat the adsorption bed before the cycle starts, set the heating temperature to 100 °C, then turn on the vacuum pump for 30 s. Then the vacuum ball valve 1 and the vacuum ball valve 3 are opened, and the adsorption bed is heated and evacuated until the internal pressure of the adsorption bed is maintained at about −101kPa. The vacuum ball valve 1 is closed, the constant temperature water bath is turned on, and the adsorption bed is cooled to the adsorption temperature.
To remove excess air from the evaporator/condenser: open the vacuum ball valve 2 on the basis of the previous step and remove excess air from the evaporator/condenser until the pressure in the evaporator/condenser reaches the evaporation temperature. Saturate the pressure, then turn off the vacuum ball valve 1 and then turn off the vacuum pump.
Adsorption-evaporation process: the vacuum ball valve 2 and the vacuum ball valve 3 are opened, and the adsorption bed is connected with the evaporator. The refrigerant in the evaporator changes from a liquid state to a gaseous state and enters the adsorption bed to be adsorbed by the adsorbent, and then the temperature and pressure in the adsorption bed will increase. When the valve is not opened, the pressure in the evaporator remains at the condensation pressure, and the pressure difference between the adsorber and the evaporator is the difference between the condensation pressure and the evaporation pressure. At the beginning of the adsorption process, the pressure inside the evaporator will gradually decrease to the evaporation pressure, and the pressure difference between the adsorber and the evaporator will gradually decrease to zero. At the same time, the water bath that opens the adsorbent bed cools the adsorbent bed to ensure that the adsorbent bed temperature is maintained at the adsorption temperature. The liquid water in the evaporator evaporates into a heat exchange between the water vapor and the external environment, so that the temperature of the evaporator water bath is continuously reduced as the adsorption process progresses, thereby generating a refrigeration effect. The adsorption process ends when the adsorbent bed pressure is the same as the evaporator pressure. The initial adsorption time, changes in evaporator/condenser level, and adsorption end time are recorded.
Heating process: the adsorption bed is heated before the desorption process. The adsorption bed is heated to 100 °C by an electric heating tube, during which the internal temperature and pressure of the adsorption bed are both increased. When the pressure in the adsorption bed is greater than the pressure in the condenser, the temperature rise is completed.

Experimental Procedure for Performance Test of Adsorption Refrigeration System
The schematic diagram of the experimental system is shown in Figure 19. The experimental steps are as follows: Molecules 2020, 25, x FOR PEER REVIEW 15 of 20 Desorption-condensation process: set the temperature of the water bath where the condenser is set to the condensation temperature and open the vacuum ball valve 2. Due to the pressure difference between the adsorption bed and the condenser, the gaseous refrigerant will enter the condensation through the pipeline under the action of pressure and with the desorption, the pressure difference between the adsorber and the condenser decreases. The condenser condenses into a liquid refrigerant, and the temperature of the condenser water bath increases. When the internal pressure of the condenser is equal to the internal pressure of the adsorbent bed, the condensation process ends.
Cooling process: when the desorption process is finished, the adsorption bed is still in a high temperature and high pressure state, which is not conducive to the next cycle, so the adsorption bed needs to be cooled before the start of the next cycle. The circulating water is opened to the adsorption bed. When the pressure in the adsorption bed drops to the saturation pressure corresponding to the evaporation temperature, the cooling process ends and the next adsorption cycle begins. The parameters of electric heating tube, pressure transmitter and constant temperature water bath used in the above experiment are shown in Tables 4-6.  Figure 19. Schematic diagram of adsorption performance test.
Adsorption bed heating vacuum process: because the system requires high vacuum, it is necessary to heat the adsorption bed before the cycle starts, set the heating temperature to 100 • C, then turn on the vacuum pump for 30 s. Then the vacuum ball valve 1 and the vacuum ball valve 3 are opened, and the adsorption bed is heated and evacuated until the internal pressure of the adsorption bed is maintained at about −101kPa. The vacuum ball valve 1 is closed, the constant temperature water bath is turned on, and the adsorption bed is cooled to the adsorption temperature.
To remove excess air from the evaporator/condenser: open the vacuum ball valve 2 on the basis of the previous step and remove excess air from the evaporator/condenser until the pressure in the evaporator/condenser reaches the evaporation temperature. Saturate the pressure, then turn off the vacuum ball valve 1 and then turn off the vacuum pump.
Adsorption-evaporation process: the vacuum ball valve 2 and the vacuum ball valve 3 are opened, and the adsorption bed is connected with the evaporator. The refrigerant in the evaporator changes from a liquid state to a gaseous state and enters the adsorption bed to be adsorbed by the adsorbent, and then the temperature and pressure in the adsorption bed will increase. When the valve is not opened, the pressure in the evaporator remains at the condensation pressure, and the pressure difference between the adsorber and the evaporator is the difference between the condensation pressure and the evaporation pressure. At the beginning of the adsorption process, the pressure inside the evaporator will gradually decrease to the evaporation pressure, and the pressure difference between the adsorber and the evaporator will gradually decrease to zero. At the same time, the water bath that opens the adsorbent bed cools the adsorbent bed to ensure that the adsorbent bed temperature is maintained at the adsorption temperature. The liquid water in the evaporator evaporates into a heat exchange between the water vapor and the external environment, so that the temperature of the evaporator water bath is continuously reduced as the adsorption process progresses, thereby generating a refrigeration effect. The adsorption process ends when the adsorbent bed pressure is the same as the evaporator pressure. The initial adsorption time, changes in evaporator/condenser level, and adsorption end time are recorded.
Heating process: the adsorption bed is heated before the desorption process. The adsorption bed is heated to 100 • C by an electric heating tube, during which the internal temperature and pressure of the adsorption bed are both increased. When the pressure in the adsorption bed is greater than the pressure in the condenser, the temperature rise is completed.
Desorption-condensation process: set the temperature of the water bath where the condenser is set to the condensation temperature and open the vacuum ball valve 2. Due to the pressure difference between the adsorption bed and the condenser, the gaseous refrigerant will enter the condensation through the pipeline under the action of pressure and with the desorption, the pressure difference between the adsorber and the condenser decreases. The condenser condenses into a liquid refrigerant, and the temperature of the condenser water bath increases. When the internal pressure of the condenser is equal to the internal pressure of the adsorbent bed, the condensation process ends.
Cooling process: when the desorption process is finished, the adsorption bed is still in a high temperature and high pressure state, which is not conducive to the next cycle, so the adsorption bed needs to be cooled before the start of the next cycle. The circulating water is opened to the adsorption bed. When the pressure in the adsorption bed drops to the saturation pressure corresponding to the evaporation temperature, the cooling process ends and the next adsorption cycle begins.
The parameters of electric heating tube, pressure transmitter and constant temperature water bath used in the above experiment are shown in Tables 4-6.

Basic Assumptions of the Theoretical Model
For an adsorption refrigeration system, its internal energy flow is a complex process, and it is impossible to fully express such a complicated energy flow process by mathematical expression. There is no need to consider such a complicated problem in the engineering application, so the complex problem can be simplified in mathematical modeling, regardless of the heat transfer process of internal load during energy flow, and the lumped parameter method is used to describe the energy flow process. The system parameters and constants involved in the simulation process are shown in Table 7. In order to simplify the calculation, the following assumptions were made: (1) The temperature and vapor pressure inside the entire adsorption bed are uniform.
(2) The water is uniformly adsorbed by the MIL-101(Cr)/CaCl 2 composite material, and the water is liquid in the MIL-101(Cr)/CaCl 2 composite material. (3) The pressure difference between the adsorption bed and the condenser are ignored, the pressure difference between the adsorption bed and the evaporator are ignored. (4) The heat conduction of the casing connected between the adsorption bed and the condenser or the evaporator is ignored, assuming complete insulation between the adsorption bed, the condenser and the evaporator. (5) In addition to accidental heat exchange between hot water, cooling water and chilled water, the system ignores the cold/heat in the environment. (6) The specific heat capacity of the MIL-101(Cr)/CaCl 2 composite, the specific heat capacity of the adsorbent bed and the specific heat capacity of the refrigerant are constant.

Mathematical Model of the Basic Cycle
The simulation model of this cycle is based on COP Equation (1).
The seven heat calculation formulas involved are as follows: