High Temperature Phase Change Materials for Thermal Energy Storage Applications: Preprint

To store thermal energy, sensible and latent heat storage materials are widely used. Latent heat thermal energy storage (TES) systems using phase change materials (PCM) are useful because of their ability to charge and discharge a large amount of heat from a small mass at constant temperature during a phase transformation. Because high-melting-point PCMs have large energy densities, their use can reduce energy storage equipment and containment costs by decreasing the size of the storage unit. Using cascaded PCMs, with equally spaced melting points and with high thermal properties, the TES is significantly enhanced. However, currently there is not enough information on the thermal properties of molten salt systems at high temperatures. Molten salt PCM candidates for cascaded PCMs were evaluated for the temperatures near 320°C, 350°C, and 380°C. These temperatures were selected to fill the 300°C to 400°C operating range typical for parabolic trough systems, that is, as one might employ in three-PCM cascaded thermal storage. Such systems require a series of PCMs with melting points spanning the range of TES. The molten salt systems considered in this study were KNO3-KCl-KBr, NaCl-KCl-LiCl and MgCl2-KCl-NaCl. Because the majority of the salts studied were hygroscopic, a handling and mixing protocol under controlled atmosphere was employed. The mixing procedure was carefully controlled because the sample weight required for the thermal property tests is on the order of a few milligrams, so a perfectly homogenized sample is essential for accurate results. Heat capacity, latent heat, transformation temperatures, thermal stability, and viscosity were measured. Before performing these evaluations, chemical stability (corrosion) of the container materials with the molten salts was evaluated in a controlled atmosphere furnace under nitrogen gas. The materials tested were 316 stainless steel (SS316), high purity aluminum (Al1100), aluminum-manganese alloys (Al3003) and aluminum oxide (Al2O3). Based on the results, the best candidate for temperatures near 320°C was the molten salt KNO34.5wt%KCl. For the 350°C and 380°C temperatures, the evaluated molten salts are not good candidates because of the corrosiveness and the high vapor pressure of the chlorides.