Evaluation of Formate Salt PCM's for Latent Heat Thermal Energy Storage

This work examines formate salts as potential phase change materials (PCMs) for middle-high temperature (<= 250 degrees C) latent heat thermal energy storage applications. The thermophysical properties of three formate salts were characterized: pure sodium formate and binary blends of sodium/potassium formate and sodium/calcium formate. The stability of formate PCM's was evaluated by thermal cycling using differential scanning calorimetry where sodium formate and sodium/potassium formate appeared stable over 600 cycles, while sodium/calcium formate exhibited a monotonic decrease in heat of fusion over the test period. A longer test with sodium formate led to gas release and decomposition of the salt. FTIR analysis of the PCM showed degradation of formate to oxalate. T-history experiments with 50-g PCM quantities demonstrated a bulk supercooling of only 2-3 degrees C for these salts. Thermal conductivity enhancement of over 700% was achieved by embedding aluminum in the solid PCM. Finally, mild carbon steel was immersed in molten sodium formate for up to 2000 h. Sodium formate was found to be non-corrosive, as calculated by mass loss and confirmed by cross-sectional high-resolution microscopy. FTIR analysis of the PCM after 2000 h shows oxidation at the free surface, while the bulk PCM remained unchanged, further indicating a need to protect the formate from atmospheric exposure when used as a PCM.

Automated summary

The study explores the potential use of formate salts as PCMs for thermal energy storage, highlighting the importance of stability and protection against oxidation. Experimental results showed that sodium formate and sodium/potassium formate exhibited good stability, while sodium/calcium formate showed a decrease in heat of fusion. It is crucial to protect formate from atmospheric exposure to maintain its integrity when used as a PCM.