Modification Of Phase Change Materials Based On Carboxylic Acids For Low-temperature Thermal Energy Storage

Phase change materials(PCMs)can not only solve the problem of matching energy in time and space,but also improve energy utilization efficiency,thus achieving the goal of saving energy and protecting environment,for which PCMs are widely concerned by researchers in the field of energy storage.Currently,there are fewer PCMs that are suitable for application in ice storage air-conditioning,cold chain transport and movable heating.Carboxylic acids as PCMs have potential application values for their high latent heat,numerous sources,environment protecting,etc.However,they also exist some problems,such as poor cycle stability,low thermal conductivity,and mismatch problem between phase change temperature and actual application temperature.To put these materials into practice,it is very important to make modification research on their properties.This thesis made characterization analysis on caprylic-nonanoic acid binary system,nonanoic-lauric acid binary system and oxalic acid dihydrate-boric acid binary system by using differential scanning calorimeter(DSC),thermal conductivity apparatus(TCA),synchronized thermal analyzer(STA),scanning electron microscope(SEM),fourier transform infrared spectroscopy(FTIR),etc.Moreover,the existing problems of above PCMs were also studied and perfected.Specific research contents and results are as follows:1.Caprylic-nonanoic acid binary system(1)Thermophysical properties,such as phase change temperature and latent heat,were measured for the binary system with different molar fractions of caprylic acid,and the system's binary phase diagram was also constructed.The phase diagram showed that the binary system existed a eutectic point and its eutectic temperature was 1.83? when the molar fraction of caprylic acid was 0.483,but the eutectic temperature was not well matched with practical application demand.On the other hand,a binary mixture in which the molar fraction of caprylic acid was 0.814 had aphase change temperature of 7.6?and latent heat of 123 J/g,which was very suitable for applying to ice storage air-conditioning and cold chain transport.More importantly,it presented a reversible phase-change behavior in similar with the eutectic point.Therefore,the binary mixture was selected as an investigated PCM for applications.(2)DSC was used to measure cycle stability of the binary mixture in which the molar fraction of caprylic acid was 0.814,and results showed that its melting curves had a high reproducibility while freezing curves existed a certain movement towards low temperature area with thermal cycle numbers increasing,which meant that the supercooling degree had a rising trend when thermal cycle numbers increased.To improve the crystallization property and cycle stability of the binary mixture,expanded graphite(EG)was coupled with the binary mixture to prepare composite PCMs.The results showed that the leakage of composite PCM was effectively controlled after addition of 10wt% EG,and the thermal conductivity increased 25%correspondingly.Besides,crystallization property and cycle stability were both improved,which can be attributed to the addition of EG that played a role of nucleating agent and promoted crystallization process for its porous structure.2.Nonanoic-lauric acid binary system(1)Phase change temperatures and latent heats of the binary system with different mass fractions of lauric acid were measured,and phase diagram of the binary system was established according to phase change temperature data,showing that the binary system existed a eutectic point and its eutectic temperature was 4.75? when the mass fraction of lauric acid was 16%.Moreover,the Schr?der-Van Laar equation was used to predict the eutectic point,which was highly identical with the experiment result.(2)The eutectic mixture had a lower latent heat,which limited its application.By analyzing DSC curves and corresponding thermophysical properties data,a binary mixture in which the mass fraction of lauric acid was 96% had a stable peak shape and its melting point and latent heat of melting were 41? and 163 J/g separately,which made it very suitable for thermal energy storage.In addition,thethermophysical properties and chemical composition of the binary mixture exhibited a good stability after running 1000 thermal cycles.3.Oxalic acid dihydrate-boric acid binary system(1)To reduce supercooling degree of eutectic mixture of the binary system,NaCl was selected from 15 feasible nucleating agents through a lot of experiments.When the addition rate of NaCl was 1wt%,the supercooling degree of the eutectic mixture was reduced from 12.80? to 3.64?,and kept around 3.37? on average during 100 thermal cycles.At the same time,its latent heat of melting only reduced 6.4% after thermal cycle test,while latent heat of freezing almost kept unchanged,which indicated that the cycle stability of composite PCM got improved obviously in comparison with that of the eutectic mixture.(2)To improve thermal conductivity of the eutectic mixture,ultrafine powders of SiC with different weights and 1wt% NaCl were coupled with the eutectic mixture,and results showed that the suitable addition rate of SiC was 9wt% for improved thermal conductivity about 16.5% and good cycle stability.