Demonstration of long-term cyclic sorption of ammonia in modified expanded graphite-calcium chloride composites for practical applications

The long-term viability of the ammonia-calcium chloride (NH3-CaCl2) pair in commercial adsorption refrigeration systems faces challenges due to the disintegration of calcium chloride (CaCl2) during multiple ammonia sorption cycles. To address this issue, we conducted a comprehensive study, starting from lab-scale investigations to experiments with full-scale commercial refrigeration systems. We prepared three calcium chloride-expanded graphite (CaCl2:EG/4:1) composite samples (labelled B-1, B-2, and B-3) with varying water content (20-45% by weight), and also used a conventional CaCl2-activated carbon-white cement composite (CaCl2: AC : WC/16 : 4 : 1, sample A). The composites were coated over fin tubes wrapped around with wire mesh for this purpose. These composites were tested for structural integrity over 300 continuous cycles in a customized constant-volume single fin-tube adsorption/desorption bed. While all samples initially exhibited similar sorption capacity triangle chi of approximate to 0.6 - 0.7kgNH(3)/kg CaCl2, sample A experienced a significant deterioration of approximately 60% in capacity within 300 cycles due to agglomeration and volumetric expansion. In contrast, the EG-based samples demonstrated remarkable stability, with B-3 maintaining triangle chi approximate to 0.57kgNH(3)/kg CaCl2 even after 800 cycles. We applied B-3 in a 2 TR adsorption refrigerator that uses biomass as fuel to demonstrate <15% cooling power attenuation even after prolonged operation over 800 cycles in practical scenarios. We believe these engineering interventions are necessary to encourage the broader adoption of ammonia-based adsorption refrigeration systems, which are a sustainable choice due to their zero global warming and ozone depletion potentials.