Magnesium chloride saltwater solutions as phase change material for sub-zero thermal energy storage

Abstract

Introduction: Efficient sub-zero latent heat thermal energy storage (LHTES) is critical for decarbonizing refrigerated transport. While aqueous magnesium chloride (MgCl₂) is a highly promising inorganic phase change material (PCM), its industrial deployment is severely limited by phase transition anomalies, severe supercooling, and aggressive corrosivity.

Materials and methods: This study investigates the thermodynamic and electrochemical behaviour of MgCl₂ solutions (14–25 wt.%) to resolve existing literature discrepancies regarding hypereutectic phase boundaries and to establish optimal containment parameters.

Results: Differential scanning calorimetry (DSC) and bulk-scale cooling protocols revealed that rapid thermal gradients kinetically suppress primary hydrate precipitation in 25 wt.% solutions, inducing a metastable state that solidifies en masse near −34 °C rather than the predicted equilibrium liquidus. Furthermore, macro-scale evaluations (500 g) demonstrated significant thermal lag compared to micro-scale baselines, driven by the low intrinsic thermal conductivity of the crystalline matrix. To resolve the containment bottleneck, potentiodynamic polarisation and 480 h ASTM G31 immersion tests evaluated aluminium, copper, and 316L stainless steel. Aluminium exhibited rapid passive layer breakdown and severe chloride-induced pitting, evidenced by Al(OH)₃ precipitation. Conversely, 316L stainless steel demonstrated superior localised corrosion resistance and a stable passivation layer (i_corr ≈ 1.5 µA cm⁻²).

Conclusions: Ultimately, a 21 wt.% eutectic MgCl₂ solution paired with 316L stainless steel provides the optimum thermodynamic and structural configuration for next-generation sub-zero heat exchangers.