На английском языке. Магистерская диссертация. Работа выполнена в
National University of Singapore, 2003, 129 +XII pp.
Summary
Microencapsulated PCMs are micron size phase change materials enclosed in a protective wrapping. The microcapsule prevents the leakage of the material during its phase change. It also provides larger heat transfer area per unit volume of heat storage vessel. It could be used in solar energy storage, waste heat utilization, and space heating and cooling. This study investigated the use of complex coacervation and spray drying methods for microencapsulating paraffin wax by polymeric materials (gelatin and acacia) in an aqueous system. Experiments on operational variables to select suitable conditions were carried out for complex coacervation. Encapsulation efficiency was found to be higher when the products had lower core to coating ratios. The optimum condition for various core to coating ratios was found to be 10 minutes homogenizing time and the amount of cross-linking agent 6~8 ml. Non-linear regression was used to correlate the encapsulation efficiency and the parameters studied. In spray-drying method, decrease in encapsulation efficiency with increase in the ratio of core to coating was observed. The optimum core to coating ratio was found to be 1:2.
In the studies on thermal performance analysis by Differential Scanning Calorimetery (DSC), the effect of core to coating ratio on energy storage/release capacities was investigated. The higher paraffin wax content in the sample gave the higher energy storage/release capacities. The energy storage/release capacities of the coacervated microcapsules were higher than those of the spray-dried samples. Energy storage/release capacities were found to be in the range of 91-239 J/g for microencapsulated PCMs prepared under different conditions.
Further characterization for both coacervated and spray-dried samples focused on surface morphology and inner structure by using microtone and Scanning Electron Microscopy (SEM). SEM analysis showed that spray-dried samples were more regular and spherical in shape compared to coacervated samples. Both samples contained a few small globules. Size of coacervated particles ranged from 3.3 to10.5 μm. Spray-dried microcapsules had a diameter ranging from 1.3-10.1 μm. The inner structure characterization showed that both coacervated and spray-dried samples consisted of a polymeric matrix surrounding numerous globules.
The thermal stability of both coacervated and spray-dried samples was estimated by using Thermogravimetry (TG) analysis. Thermal decomposition temperatures of core and coating materials were determined from TG output curves. The decomposition temperature of paraffin wax existed between 200 and 300°C, and the decomposition temperature of polymer network (gelatin and acacia) was observed between 300 and 400°C. The TG output curve for spray-dried samples.had two peaks between 300 and 400°C. The second extra peak showed the decomposition temperature of unreacted coating materials. Thermal stability of microencapsulated PCMs was also checked through accelerated thermal (melt/freeze) cyclic tests. Both samples were subjected to thermal cycle tests up to 2000 cycles. DSC analysis was carried out to measure energy storage/release capacities, melting temperature and specific heat capacity after specific number of cycles. Both the coacervated and spray-dried samples showed good thermal stability throughout cycling process. Fourier Transform Infrared Spectophotometry (FTIR) analysis also confirmed distinct chemical stability of both samples throughout thermal cycling. Finally, the thermal performance of the PCM was carried out in as fluidized bed heat exchanger. Heat transfer between the spray-dried encapsulated PCM and air was studied during heating and cooling process. It was found that the time taken for charging and discharging the capsules was about 760 and 600 seconds, respectively. Total energy and release amount were found to be 2953 and 2431 J. Therefore, it was observed that it was efficient heat exchange system.
Microencapsulated PCMs are micron size phase change materials enclosed in a protective wrapping. The microcapsule prevents the leakage of the material during its phase change. It also provides larger heat transfer area per unit volume of heat storage vessel. It could be used in solar energy storage, waste heat utilization, and space heating and cooling. This study investigated the use of complex coacervation and spray drying methods for microencapsulating paraffin wax by polymeric materials (gelatin and acacia) in an aqueous system. Experiments on operational variables to select suitable conditions were carried out for complex coacervation. Encapsulation efficiency was found to be higher when the products had lower core to coating ratios. The optimum condition for various core to coating ratios was found to be 10 minutes homogenizing time and the amount of cross-linking agent 6~8 ml. Non-linear regression was used to correlate the encapsulation efficiency and the parameters studied. In spray-drying method, decrease in encapsulation efficiency with increase in the ratio of core to coating was observed. The optimum core to coating ratio was found to be 1:2.
In the studies on thermal performance analysis by Differential Scanning Calorimetery (DSC), the effect of core to coating ratio on energy storage/release capacities was investigated. The higher paraffin wax content in the sample gave the higher energy storage/release capacities. The energy storage/release capacities of the coacervated microcapsules were higher than those of the spray-dried samples. Energy storage/release capacities were found to be in the range of 91-239 J/g for microencapsulated PCMs prepared under different conditions.
Further characterization for both coacervated and spray-dried samples focused on surface morphology and inner structure by using microtone and Scanning Electron Microscopy (SEM). SEM analysis showed that spray-dried samples were more regular and spherical in shape compared to coacervated samples. Both samples contained a few small globules. Size of coacervated particles ranged from 3.3 to10.5 μm. Spray-dried microcapsules had a diameter ranging from 1.3-10.1 μm. The inner structure characterization showed that both coacervated and spray-dried samples consisted of a polymeric matrix surrounding numerous globules.
The thermal stability of both coacervated and spray-dried samples was estimated by using Thermogravimetry (TG) analysis. Thermal decomposition temperatures of core and coating materials were determined from TG output curves. The decomposition temperature of paraffin wax existed between 200 and 300°C, and the decomposition temperature of polymer network (gelatin and acacia) was observed between 300 and 400°C. The TG output curve for spray-dried samples.had two peaks between 300 and 400°C. The second extra peak showed the decomposition temperature of unreacted coating materials. Thermal stability of microencapsulated PCMs was also checked through accelerated thermal (melt/freeze) cyclic tests. Both samples were subjected to thermal cycle tests up to 2000 cycles. DSC analysis was carried out to measure energy storage/release capacities, melting temperature and specific heat capacity after specific number of cycles. Both the coacervated and spray-dried samples showed good thermal stability throughout cycling process. Fourier Transform Infrared Spectophotometry (FTIR) analysis also confirmed distinct chemical stability of both samples throughout thermal cycling. Finally, the thermal performance of the PCM was carried out in as fluidized bed heat exchanger. Heat transfer between the spray-dried encapsulated PCM and air was studied during heating and cooling process. It was found that the time taken for charging and discharging the capsules was about 760 and 600 seconds, respectively. Total energy and release amount were found to be 2953 and 2431 J. Therefore, it was observed that it was efficient heat exchange system.