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Abstract: Household water heating systems incorporating phase change materials (PCMs) can store solar thermal energy and reduce energy consumption. MgCl2·6H2O-Mg(NO3)2·6H2O eutectics have a melting point near 60 °C with a large energy storage density suitable for use in the thermal energy storage unit of a water heating system. However, the low thermal conductivity of the eutectic limits its heat extraction rate. In this study, we combine the eutectic and expanded graphite (EG) to prepare a high thermal conductivity composite PCM and study its cycling performance over 200 thermal cycles. The thermal conductivity of the composite PCM increased from 0.5 to 3.7 W m−1 K−1, ensuring faster heating and cooling. The images obtained from scanning electron microscopy and leakage testing showed that the maximum loading mass fraction of the eutectic in the EG reached 85 wt %. Infrared spectra and X-ray diffractometry patterns confirm that composites with compositions between those of EG and the eutectic were composed of the physical mixture. Thermogravimetric analysis indicated that the composite PCM lost less mass at 300 °C compared to the pure eutectic, and differential scanning calorimetry demonstrated that the composite PCM had a melting point of 52.3 °C and a small subcooling degree of 2.7 °C, compared to 56.6 and 18.4 °C for the pure eutectic, respectively. All these measurements demonstrated that the composite PCMs changed less after 200 thermal cycles than the pure eutectic, which had an observable stratification caused by phase separation. The composite PCM developed was thermally conductive, shape-stabilized, and more thermally stable during cycling than the pure eutectic.Graphical Abstract: [ABSTRACT FROM AUTHOR]