Empirical modeling and thermophysical evaluation of potassium formate and glycerol aqueous matrices for sub-zero latent thermal energy storage

Abstract:

This study evaluates the formulation, phase behavior, and thermophysical properties of aqueous potassium formate (KF) and glycerol mixtures developed to optimize sub-zero latent thermal energy storage (LHTES) loops. The primary objective is to balance solid-phase phase change material (PCM) storage capacity with the fluid dynamics of liquid-phase secondary heat transfer fluids (HTFs), specifically minimizing viscosity to ensure low pumping power and reliable loop circulation. For solid-phase setups (KF mass fractions: 5-25%), empirical modeling and experimental validations indicated a linear drop in freezing point from -4.1 °C to -14.2 °C. While the theoretical energy storage capacity remained stable at approximately 369 kJ, experimental measurements showed that actual stored energy dropped from 498.8 kJ to 468.0 kJ as solute concentrations peaked. This performance gap is driven by a steep rise in fluid viscosity (from 1.05 to 2.81 cP), which suppresses convective heat transfer efficiency, alongside localized phase segregation. 
To achieve a reliable, pumpable secondary cooling loop that avoids complete solidification within deep-freeze zones, Response Surface Methodology (RSM) was deployed to engineer multi-component liquid HTFs. Formulations blending 20% KF with 20% and 30% glycerol successfully suppressed ice crystallization, remaining entirely fluid at the -18 °C deep-freeze baseline threshold. Thermophysical property testing established that the KF20G20 mixture yields an optimal trasde-off for low-temperature networks, maintaining an elevated specific heat capacity (3.87 Jg·K) and thermal conductivity (0.523 Wm·K) while minimizing viscosity to 7.32 ±0.12 cP. This formulation preserves excellent fluid mobility, preventing excessive flow resistance and maximizing pumpability in low-temperature industrial applications.