Study On Preparation And Thermal Properties Of Phase Change Nanocomposites For Cool Storage

Cool storage technology is an important measure to shift electrical energy demand from peak period to off-peak period. At present, cool storage technology is applied mainly in air-conditioning systems. It is expected to extend it's application to industrial area by new technology. The research and development of low-temperature phase change materials for cool storage is of great importance to industrial application.In this dissertation, one kind of low temperature phase change material for cool storage was developed, which is suitable in beer industry application. Because this kind of material combines TiO₂ nanoparticles(20nm) and eutectic salt solution, it is called "Phase Change Nanocomposites for Cool Storage ". The TiO₂ nanoparticles can be cheaply obtained from the ordinary titanium dioxide powder.The research efforts for preparation and characterization the thermal properties of nanocomposites are indicated from the following aspects:(1)Preparation and suspension stability of the nanocompositesThe dissertation presents a preparation method of nanocomposites by directly mixing nanoparticles and eutectic salt solution. Some auxiliary dispersants are necessary to obtain evenly distributed and stabilized suspensions. Using this method, several nanocomposites with different nanoparticles volume fraction were prepared.The experimental result shows that dispersant and solution pH are two important factors of affecting the stability and evenness of nanocomposites.(2)Thermal conductivity measurement of nanocompositesA transient two hot-wire instrument for measuring the thermal conductivity of nanocomposites is developed. Experimental result shows that nanoparticales enhance the thermal conductivity of eutectic salt solution. Theoretical analysis of the thermal conductivity of nanocomposites is introduced to explain possible mechanisms contributing to enhancement of the thermal conductivity of nanocomposites.(3)Supercooling research of nanocompositesIn order to research the crystal effect of nanoparticals on eutectic salt solution, the freezing/melting experiment with the sample fluids is conducted. The result shows that nanocomposites can reduce the degree of supercooling, and with the increase ofnanoparticle volume fraction, the degree of supercooling decreases strongly. With 1.13 vol % nanoparticles suspended in the solution , the degree of supercooling was nearly eliminated.?Latent heat and specific heat of nanocompositesLatent heat and specific heat of nanocomposites are measured with Differential Scanning Calorimetry (DSC ).The value of latent heat is 254.2²79.5kJ/kg, and phase change temperature is about -8.5°C .After fifty times freezing/melting cycles, The latent heat and phase change temperature of nanocomposites still remain constant. Additionally, the specific heat of nanocomposites was measured under different temperature.?The viscosity of nonocomposites was also measured with the NXE-1B viscosimeter. Experimental results demonstrate that the T1O2 nanoparticles increase the viscosity of the salt solution. Furthermore, temperature and the volume fraction of nanoparticles are the main factors that affect the viscosity of nanocomposites.?Peformance of cool charge and discharge of nanoompositesExperiment made in a small cool storage apparatus verified the excellent thermal properties of nanoomposites. The cool charge/discharge rate and cool charge/discharge capacity increase greatly than that of eutectic salt solution without suspended nanoparticles, and the heat exchange coefficient outside the heat-transfer tube is also apparently enhanced.