Preparation and heat storage characteristics of high-temperature phase change macrocapsules of chloride eutectic salt

Chloride salts, as high-temperature phase change materials (PCMs), have advantages in terms of high application temperature and high latent heat, but the intrinsic disadvantages such as high corrosiveness and easy leakage have seriously limited their development. Employing the NaCl–KCl binary salt as PCM, a double-shell cladding strategy with an inner layer of expanded graphite and an outer layer of ceramic was proposed, thereby, the chloride phase change macrocapsule with good thermal storage performance was successfully prepared. The double-shell capsules exhibited better leakage resistance and cycling stability. On this basis, the preparation process of double-shell eutectic salt macrocapsules was simplified by using an in-situ melting method, employing the mixed NaCl and KCl as raw materials directly that were transferred to eutectic binary salt inside the capsules during sintering. It is also proposed to improve the heat storage density of the capsules by increasing the capsule size and isostatically pressing the capsule. The NaK-20-200MPa sample indicated the largest heat storage density in the temperature range from 600 °C to 700 °C, with a mass heat storage density of 190.38 J/g, which was 2.85 % higher than that of the NaK-20-0MPa capsules and 36.99 % higher than that of the NaK-5-0MPa capsules. Additionally, the volume heat storage density of the NaK-20-200MPa capsules was 339.19 J/cm³, which was 14.69 % higher than that of the NaK-20-0MPa capsules and 21.52 % higher than that of the NaK-5-0MPa capsules. The good thermal performance and low cost of chloride PCM could ensure the encapsulated salt for use in high-temperature heat storage.