Performance evaluation of tetrabutylphosphonium trifluoroacetate ionic liquid-water solution as refrigerant for adsorption refrigeration in chilled storage applications

This study evaluates tetrabutylphosphonium trifluoroacetate ([P4444][CF3COO]) ionic liquid (IL) as a solute for enhancing water-based adsorption refrigeration systems (ARS) under chilled-storage conditions. The work addresses the long-standing trade-off in aqueous refrigerants, where achieving strong freezing-point depression typically reduces vapor pressure, by examining whether [P4444][CF3COO] can sustain both thermodynamic advantages and system-level performance relative to NaCl. Experimental measurements revealed contrasting freezing behaviors between [P4444][CF3COO]– and NaCl–water solutions: NaCl followed classical colligative behavior with monotonic freezing point depression, while [P4444][CF3COO] exhibited nonlinear responses and double-freezing transitions indicative of IL-rich and water-rich domains. These ion-specific interactions and nanoscale organizational effects enabled freezing point depression without substantial vapor pressure reduction. System simulations of a single-effect silica-gel ARS confirmed the resulting performance benefits. At 1 °C evaporation, −3 °C freezing, and a 75 °C heat source, the [P4444][CF3COO]-based system delivered an SCE of 55.5 kJ/kg compared to 47.5 kJ/kg for NaCl, with the advantage increasing under deeper sub-zero freezing conditions (−4.5 to −7 °C) where the IL averaged 72.7 kJ/kg, equivalent to 55.1% higher than NaCl. Under −1 °C evaporation, the IL maintained stable SCE across the assumed freezing range, whereas NaCl became inoperable at −4.5 to −7 °C. Although higher heat-source temperatures improved both systems, [P4444][CF3COO] consistently produced higher SCE due to its ability to maintain higher vapor pressure at higher concentrations. Overall, the strong and stable cooling performance of [P4444][CF3COO] at deep sub-zero and low heat-source temperatures highlights its promise as a sustainable refrigerant for low-grade thermal energy-driven cooling.