Researches On Preparation And Properties Of Expanded Graphite-based Composite Phase Change Thermal Storage Materials For Solar Thermal Medium-temperature Applications

Solar energy is the most abundant and renewable clean energy on the earth. Phasechange material (PCM) that stores and releases a lot of heat during its phase change processcan be used to improve the utilization efficiency of solar energy. Expanded graphite (EG)with strong adsorption ability and good thermal conductivity has been widely used as thematrix to absorb organic PCMs, obtaining a large number of shape-stable composite PCMswith enhanced thermal conductivity and good heat storage capacity. However, the phasetransition temperature of all the EG-based composite PCMs reported previously are below80℃, which are not suitable for medium-and high-temperature solar heat storage systems.Therefore, it is necessary to develop novel EG-based composite PCMs with the phasetransition temperature above80℃for solar thermal applications.In this thesis, three kinds of organic PCMs with the phase transition temperatures of noless than80℃, sebacic acid (SA), RT100and mannitol were selected to prepare EG-basedcomposite PCMs. Their microstructure and thermal properties were characterized by SEM,XRD, FT-IR, DSC and TG. It is shown from the XRD patterns of the three kinds of EG-basedcomposites that, the SA/EG and RT100/EG composites are just the integration of SA orRT100with EG, wheareas the mannitol/EG composite exhibits obvious changes in diffractionpeaks, resulting in a disappearance in heat storage capacity of the composite. It has beendetermined on the leakage testing and SEM analysis that, the optimal mass percentage of SAin the SA/EG composite is found to be around85wt%, and that of RT100in the RT100/EGcomposite PCM is around80wt%. The solid-liquid phase change temperature of thecomposite PCMs are the same as those of the corresponding organic PCMs, and their latentheat are equivalent to the calculated values based on the mass fractions of the correspondingorganic PCMs in the composites. It is revealed by the3000heating-cooling cycling test thatthe two composites have good structure stability and thermal reliability.The SA/EG and RT100/EG composite PCMs were formed into several round blocks bydry pressing with a home-made cylindrical mould under the pressure of1.0×106kg/m2,followed by the investigations on thermal conductivity, thermal properties and thermalexpansivity. It is found that the two EG-based composite PCMs have good formability. Thethermal conductivity of the formed EG-based composite PCMs is higher than that of the onesbefore dry pressing, and increases with the packing density. The formed composite PCMs exhibit a little loss in latent heat after dry pressing, a slight volume expansion but no PCMleakage is observed of the formed composite PCMs during their phase change process. Theseresults indicate that the SA/EG and RT100/EG composite PCMs are promising in medium-temperature solar heat storage systems.