Preparation and thermal performance enhancement of sodium thiosulfate pentahydrate- sodium acetate trihydrate /expanded graphite phase change energy storage composites

Heat energy storage using phase change materials (PCMs) in electric radiant floor heating system (ERFHS) is a favorable solution to the improvement of energy efficiency. In this paper, the sodium thiosulfate pentahydrate (Na2S2O3•5H2O,STP)- sodium acetate trihydrate (CH3COONa•3H2O, SAT) eutectic mixture was prepared by adding 30 wt.% STP and 70 wt.% SAT. Sodium carboxymethyl cellulose (CMC) and sodium pyrophosphate (Na4P2O7,TSPP) were added to fabricate the modified STP- SAT phase change material. The results showed that almost no phase separation occurred in STP- SAT eutectics with 1 wt.% CMC and 0.4 wt.% TSPP, and its supercooling degree was reduced to 0.3 °C. The thermal properties of the composite phase change material (CPCM) with different contents of expanded graphite (EG) were studied. And the experimental results showed that the composite containing 8% EG displayed a high phase change enthalpy (186.0 kJ/kg) and an ignorable supercooling degree. The addition of EG increased the thermal conductivity of STP-SAT from 0.748 to 2.92 W/m•K. The thermal reliability of CPCM was confirmed through the thermal cycle test and thermogravimetric analysis (TGA). After 200 thermal cycles, CPCM exhibited fantastic thermal stability and reliability.