Membrane distillation crystallization of ammonium nitrate solutions to enable sustainable cold storage: Electrical conductivity as an in-situ saturation indicator

Abstract

A novel refrigeration cycle is proposed using crystallized ammonium nitrate as a thermochemical energy storage material that is recharged with membrane distillation. Cooling is provided when needed by the dissolution of ammonium nitrate in water, an endothermic process that absorbs heat from the surroundings. The dilute solution is then concentrated to recrystallize the ammonium nitrate using membrane distillation, a thermally driven separation process capable of operating at low temperatures and high solution concentrations. Unlike conventional vapor compression refrigeration systems, this process does not rely on refrigerants with high global warming potential and could potentially utilize low-grade heat as the primary energy input rather than electricity. Membrane distillation was found to be capable of crystallizing ammonium nitrate from water at temperatures between 50 and 70 °C. The membranes tested were found to be very resistant to damage by ammonium nitrate crystallization, and performance (water flux, thermal efficiency, and rejection) was maintained even after repeated exposure to crystallization. Additionally, conductivity was found to be a reliable and consistent indicator of concentration and showed potential for use in monitoring and control systems.