Thermal Performance And Application Of Sodium Acetate Trihydrate As Phase Change Material

Latent heat storage using phase change materials (PCMs) is one of the most efficient ways of storing thermal energy. Unlike the sensible heat storage method, the latent heat storage method provides much higher storage density, with a smaller temperature difference between storing and releasing heat. The salt hydrate PCMs having different melting points (7-120℃) are attractive materials for use in thermal energy storage due to their high volumetric storage density, relative-ly high thermal conductivity and moderate costs. However, most of these materials have severe supercooling problems and undesirable phase separation which have limited their application.Sodium acetate trihydrate (SAT) is a typical salt hydrate PCM. It is a promising heat storage material because of its relatively large latent heat of fusion and its melting temperature which makes it suitable for applications such as solar energy storage for domestic hot water. Many re-searches were carried out in this study to prevent supercooling and phase separation of this ma-terial and improve its thermal performances. The main work and achievements are as follows:1. The thermal behavior of SAT was studied by using thermogravimetry-differential scan-ning calorimetry (TG-DSC). The crystal water of SAT evaporates all the time during the heating process due to water vapor pressure difference. Thus the hydrate decomposes into anhydrous salt and loses the ability of storing heat energy. The water evaporation can be inhibited effectively through the silicone oil coverage method.2. The solubility in water of salt increases with the aid of ultrasonic energy and phase sepa-ration can be effectively inhibited. After ultrasound was applied to the melt for a while, nuclea-tion occurred, which was then followed by the growth of the fine crystal and supercooling was reduced. However, the effect of ultrasound on suppressing supercooling cannot be repeated.3. In CH3COONa·3H2O-Na2HPO4·12H2O phase change system, Na2HPO4 formed during the heating and cooling cycles and it promoted the nucleation of SAT. This system was found very stable in the test over 9 times. 4. Adding xylitol accelerated the dehydration of SAT and its phase change performance was also affected. The temperature of SAT-xylitol mixture increased steadily during charging process, besides, the solidification was suppressed. Xylitol was not a suitable thickening agents for the mixture of CH3COONa·3H2O with Na2HPO4·12H2O because it weakened the effect of the nuc-leator and made the supercooling degree increased and unstable.5. The thermal performance of the latent heat storage system base on SAT and the sensible heat storage system based on water was studied. The former system has many advantages in ca-pacity and quality of the heat energy stored.