Ca(NO3)2-NaNO3/expanded graphite composite as a novel shape-stable phase change material for mid- to high-temperature thermal energy storage

A novel shape-stable Ca(NO3)2-NaNO3/expanded graphite composite phase change material (PCM) for mid- to high-temperature TES was successfully synthesized by the developed impregnation and sintering two-step method. The low-cost binary salt Ca(NO3)2-NaNO3 was chosen as the PCM and expanded graphite (EG) was as the structural supporting material and thermal conductivity enhancer. The results showed that the binary eutectic nitrate and EG have good chemical compatibility, and the composite with 7 wt% EG can overcome the problems of cracks or liquid leakage during the phase change. Benefiting from the established better conductive passageway within the composites, the thermal conductivity of the composite with 7 wt% EG can be significantly increased by about 7.3 times in comparison with the binary eutectic nitrate. However, the measured phase change latent heats of the shape-stable phase change materials (SSPCMs) are markedly lower than the theoretical ones, meaning some losses of the latent heat during the material fabrication process. The developed SSPCM also showed little change of the phase change properties after 500 thermal cycles, indicating it has good thermal reliability for a long term period. Compared with other PCMs for mid- to high-temperature applications, this novel cost-effective composite SSPCM is very promising due to the wide operating temperature range and high thermal conductivity.