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

Abstract

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 B1, B2, and B3) 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 ∆χof≈0.6−0.7kgNH3/kgCaCl2, 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 B3 maintaining ∆χ≈0.57kgNH3/kgCaCl2 even after 800 cycles. We applied B3 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.