Thermal Conductivity Enhancement of High Temperature Phase Change Materials for Concentrating Solar Power Plant Applications
Phase change materials (PCMs) can be used in latent heat storage systems to store or release heat by going through phase transformation. In the present work, an in-depth survey of PCMs thermal properties are investigated. It was concluded that Alkali nitrate salts are suitable PCM candidates for the medium temperature heat storages because of their moderate latent heat of fusion (100-300 KJ/Kg), low cost, and good phase-change properties such as no-phase segregation and supercooling. However similar to most other PCM candidates, Alkali nitrate salts exhibit very low thermal conductivity (<1 W/K.m), which limits their use, unless their effective thermal conductivities are enhanced.
In this research, first thermal conductivities of potassium nitrate and sodium nitrate were measured in the temperature range of 25-400°C, using transient hot wire method. It was found the thermal conductivities of potassium and sodium nitrate to be 0.62 W/K.m, and 0.57 W/K.m at 25°C, respectively. In addition, it was observed that thermal conductivity of sodium nitrate is approximately constant in the temperature range of 25-400°C while that of potassium nitrate decreases with an increase of temperature beyond 140°C. A DSC analysis indicated that this decrease is because of the orthorhombic to rhombohedral crystal transformation of potassium nitrate at 140°C.
To improve the thermal conductivities of potassium and sodium nitrate, PCM composites consisting of aluminum and copper foams and the salts were prepared and investigated. The effective thermal conductivities of aluminum-PCM composites were found to be in the range of 1.7-7.2 W/K.m while the effective thermal conductivities of copper-PCM composites were in the range of 6.3- 19.5 W/K.m. The measurements of effective thermal conductivity of the composite at molten state exhibited no changes for Al- and Cu- composites. However those of Al- and Cu- decreased by about 25%.
The effect of graphite flakes on aluminum-PCM and copper-PCM composites was studied to further increase their conductivities as well as improving their latent heat storage capacities. The effective thermal conductivities of Al-PCM-C composites were found to be in the range of 3.1-13.3 W/K.m while the effective thermal conductivities of Cu-PCM-C were measured to be in the range of 11.9-78.1W/K.m.