| The rapid development of global economy leads to energy demand increase quickly. However, conventional fossil energy sources are limited and their uses are restricted due to the emission of harmful gases which are responsible for climate changes and environmental pollution. Composite phase change materials (PCM) can be used for energy storage and utilization due to it can absorb or release large latent heat during the melting or solidifying process when surrounding temperature increases and decreases, which is helpful to improve energy efficiency and develop new renewable energy and can prevent PCM from leaking and harmful interaction with the environment when phase change occurs. In this work, we have determined the ranks of important PCMs using Analytic Hierarchy Process (AHP) and VlseKriterijumska Optimisacija IKompromisno Resenje (VIKOR) technique. Stearic acid is found to be the best PCM for latent heat thermal energy storage systems within the range of our reseach and it also encapsulated with the polymer wall or inorganic carrier. The structure and properties of capsules and loaded phase change materials were studied, and the self-assembly mechanism of the composites were analyzed. Also, the effects of fabricated technology on particle size distribution, thermal properties and structures were stuied and the relationship between the phase transiton behavior with surface interreaction of PCM and matrix was explored.Selection of an optimal PCM according to engineering requirements, such as high sensible heat, latent heat, heat of conduction and light weight, is a crucial and tedious task. It depends upon various thermo-physical properties including latent heat of fusion, density, thermal conductivity, specific heat capacity. Therefore, Analytic Hierarchy Process and VlseKriterijumska Optimisacija IKompromisno Resenje, the approach of multi-attribute decision making, are employed for PCMs selection. Based on the characteristics of PCM, the valuation system of three level and six different attributes was constructed and the evaluation model based on AHP and VIKOR method were established. At last, the VIKOR and AHP method were used to sort the attributes of alternative PCMs comprehensively. The results show that the two methods are effective and practical for selecting PCMs and stearic acid is the optimal PCM within the candidants for thermal energy storage.In order to identify the validity of fabricating microencapsulated phase change material by ultraviolet irradiation-initiated method, the stearic acid/polymethyl methacrylate (PMMA) microcapsules were prepared. The structural characteristics and thermal properties of the microcapsules were also determined by various techniques. The results show that the form-stable microencapsulated PCM with core/shell structure is formed and the maximum encapsulated proportion of SA in the composite is51.8wt.%without melting PCM seepage from the composite. In the shape stabilized microencapsulated PCM, the polymer acts as supporting material to form the microcapsule cell preventing the leakage of PCM from the composite and the SA acts as a PCM encapsulated in the cell of PMMA resin. The oxygen atom of carbonyl group of skeleton is interacted with the hydrogen atom of hydroxyl group of SA. The melting and freezing temperatures and the latent heats of the composite PCM are measured as60.4℃,50.6℃and92.1J-g-1,95.9J-g-1, respectively. The results of hermal cycling test show that the thermal reliability of the composite PCM has an imperceptible change.In order to further optimize the preparation technology and reduce the particle size of capsules, the polymethyl methacrylate encapsulated eicosanoic-stearic acid (EA-SA) eutectic nanocapsules were prepared by ultraviolet photoinitiated emulsion polymerization and various characterization techniques were employed to investigate the influence of preparation methods on thermal properties and particle size distribution (PSD). The results show that the particle size decreases and PSD narrows with the increase of agitation speed, reduction of initiator and monomer concentration, existence of cross-linking agent and stabilized at agitated speed higher than5000rpm and monomer concentration lower than0.15mol-L-1. However, latent heats of the capsules decreases with the increase of monomer and initiator concentration. Type of emulsifier in emulsion has significant effects on PSD and phase change properties of EA-SA/PMMA and nonionic emulsifier is suitable for reducing particle size and enhancing heat storage ability. Morphology and chemical characteristic analysis indicate that spherical nanocapsules with average diameter of46nm were successfully fabricated and its maximum encapsulation ratio is68.8wt.%without leakage of core material. The melting and crystallizing temperatures and latent heats of capsules are determined as56.9℃and54.5℃,126.4J-g-1and128.3J-g-1, respectively. Accelerated thermal cycling test shows that the nanocapsules have good thermal and chemical reliability after repeated thermal cycling. Besides, the super-cooling problem of PCMs is reduced dramatically by forming nanocapsules.In order to entrust electrical conductivity for phase change materials, the composite phase change material using SA as core material and polyaniline (PANI) as wall materials were perapared though chemical oxidation method. The structure, thermal properties, thermal reliability, thermal conductivity, heat storage or release performance and electrical conductivity of the dual-functional compsosite were also determined. The results show that the polymerization temperature has significant effects both on the morphology and phase change properties of SA/PANI, but the type of the doping acid and the type of initiator are only effect the morphology or thermal performance respectively. In addition, the the type of doping acid has negligible effects on thermal performance and the type of initiator has ignore effects on morphology. Self-assembly mechanism analysis show that the assembly forces of flaky composite is hydrogen bond and the assembly force in spherical composite is electrostatic force. The optimal technology for preparation SA/PANI composite which has high thermal storage capacity and excellent stability is using sulfamic acid as doping agent, APS and BPO as mixed initiator and polymerized at room temperature. The maximum encapsulated proportion of SA in the flake SA/PANI composite is53.8wt.%and the melting and crystallization temperature is55.6℃and50.7℃respectively, corresponding phase transition enthalpy is98.0J-g-1and97.5J-g-1. The conductive ability of the composite is determined as0.4336S-cm-1. All of the conclusions indicate that the composite has a better thermal conductivity, good stability and conductive ability.In order to stuied the effect of preparation methods on the structure and thermal properties fully, two kinds of phase change materials using SA as PCM and activated montmorillonite as matrix were prepared. The composite PCM was characterized using scanning electron microscope, Fourier transformation infrared analysis technique and the thermal properties, thermal reliability and heat storage/release performance were determined by differential scanning calorimetry, FT-IR and thermal cycling test. Also, the pahse change foam cement containing composite PCM which synthesized by melting impregnation method was prepared and the temperature adjusting performance was also studied. The results show both composites have similar morphology, structure and comparable latent heats with SA intercalated into a-MMT gallery. Thermal reliability and thermal cycling test obviously indicate that preparation technology affects the thermal properties. Latent heat of the composite prepared by melting impregnation method changs by-0.59%for fusion and-1.01%for solidification, whereas, it decline by39.71%and40.89%for the composite prepared by solution immersed technology after600thermal cycling. In other words, the melting impregnation method is a potential candidate for preparing reliable composite PCMs. The result of temperature adjusting test show that the time of phase change cement increased172%when temperature increasing from20℃to70℃. |
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