Liquid-state dipolarcaloric refrigeration cycle with nitrate-based salts.

The environmental burden of vapor compression refrigeration has driven interest in alternatives. Caloric refrigeration cycles offer a path forward, but most rely on solid-state materials with limited temperature lift, low performance, and poor fluidity, which hinder scalability. We introduce a liquid-phase dipolarcaloric refrigeration cycle utilizing endothermic dissolution of nitrate-based salts regenerated through electrodialysis. This cycle achieves large adiabatic temperature changes and high coefficients of performance. We identified effective saltwater pairs and validated the cycle experimentally, supported by thermodynamic modeling. Among these pairs, ammonium nitrate is suited for refrigeration, and potassium nitrate is appropriate for air conditioning. The system uses abundant, low-cost materials, and its fluidic nature ensures efficient heat transfer and scalability. This work establishes dipolarcaloric cooling as a viable alternative for environmentally responsible refrigeration. Editor's summary: The large adiabatic temperature changes that occur when salts dissolve in water can be harnessed for cooling, offering an alternative to vapor compression cycles. Kim et al. demonstrate such a system that operates by dissolution of nitrate salts; the concentrated salts are then regenerated using electrodialysis. Ammonium nitrate, with an adiabatic temperature change of 37.3°C, is suited for refrigeration, and potassium nitrate, with an adiabatic temperature change of 18.6°C, could be used for air-conditioning. —Phil Szuromi INTRODUCTION: Cooling systems are fundamental to industrial and domestic applications, providing thermal management for comfort, safety, and efficient operation. Although the electrically driven vapor compression cycle remains a solid approach, its reliance on refrigerants with high global warming potential and on mechanical compressors presents critical environmental and efficiency challenges. Although solid-state caloric systems such as elastocaloric, electrocaloric, and magnetocaloric technologies have been explored as alternatives, their progress has been limited by small entropy changes, low temperature lift, inefficient heat transfer, and poor scalability. A different strategy is required to achieve sustainable, high-performance cooling. RATIONALE: A liquid-state refrigeration cycle has been developed based on the strong endothermic dissolution of nitrate salts in water, which is defined as the dipolarcaloric effect—a phenomenon that arises from interactions between ionic compounds and polar solvent molecules. Regeneration of the dissolved salts is achieved through electrodialysis, enabling closed-loop operation. This liquid-phase configuration combines large isothermal entropy changes with efficient convective heat transfer, and it further offers durability and practical scalability. The inherent flowability of aqueous solutions ensures facile circulation and system integration, overcoming the barriers in solid-state caloric systems. The dipolarcaloric cycle establishes a practical path toward compressor-free and climate-friendly cooling, using abundant materials consisting of water and nitrate-based salts. RESULTS: We identified effective dipolarcaloric materials through thermodynamic modeling. Their working behavior is accurately predicted by using an enthalpy chart as a function of temperature and weight concentration. Ammonium nitrate achieves adiabatic temperature changes of 37.3°C and isothermal entropy changes up to 531 J/(kg·K), and potassium nitrate reaches 18.6°C and 390 J/(kg·K). When integrated into a refrigeration cycle, the potassium nitrate–water pair delivers coefficients of performance of 9.37 to 4.32 across a 5° to 10°C temperature span, with the cold-side reservoir maintained at 20°C, suitable for air conditioning. The ammonium nitrate–water pair obtains coefficients of performance of 3.07 to 1.65 over a 5° to 20.7°C span with the hot-side reservoir set at 20°C, enabling refrigeration and subzero operation. Salt regeneration by electrodialysis sustains 70 to 85% current efficiency with stable cycling. Specific cooling powers of 22 to 41 W/liter are achieved in a single stack, and scaling to a 15-pair device yields 287 to 1965 W/liter, demonstrating both feasibility and practical scalability. The ideal dipolarcaloric cycle is partially consistent with the Carnot framework, with deviations including a temperature glide during heat exchange and isenthalpic mixing, while practical operation is constrained by irreversibility. CONCLUSION: This study establishes the liquid-state dipolarcaloric refrigeration cycle, utilizing endothermic dissolution of nitrate-based salts in water, coupled with electrodialysis regeneration. Potassium nitrate is identified as a candidate for air conditioning, and ammonium nitrate enables refrigeration. The use of aqueous-liquid working fluids ensures high feasibility and integration potential, while delivering large adiabatic temperature change and isothermal entropy change, high energy efficiency, and scalable cooling power. This positions dipolarcaloric refrigeration as a practical and sustainable alternative to vapor compression technologies. Liquid-state dipolarcaloric refrigeration cycle.: The system uses endothermic dissolution of nitrate salts in water with electrodialysis-based regeneration. Mixing concentrated and dilute solutions produces an adiabatic temperature change, while convective liquid flow enables efficient heat absorption from the cold reservoir. Electrodialysis restores concentration differences, and heat release to the hot reservoir ensures continuous closed-loop operation. [ABSTRACT FROM AUTHOR]