Design of compact microscale geometries for ammonia-water desorption

This article presents the development of two novel compact heat and mass exchange concepts for use as the desorber in small-capacity ammonia-water absorption systems. Both concepts feature a counterflow orientation of the vapor and solution streams, in which the generated vapor rises through small channels within the component, flowing past the heated ammonia-water solution. Heat input is supplied by a hot coupling fluid flowing through a microchannel array adjacent to the vapor/solution channels in a counterflow orientation with the solution stream. This geometry provides high heat transfer coefficients, enabling a significant reduction in component size. This arrangement is also found to produce high-purity ammonia vapor, which leads to reduced rectification loads and a higher coefficient of performance of the system compared to more conventional designs. Additionally, both designs can be configured to include an analyzer and rectification section, leading to more compact integrated system packages and a reduction of external fluid connections and cost. A model of the coupled heat and mass exchange process associated with ammonia-water desorption in these concepts is developed. The proposed design is compact, scalable, versatile, and easily mass-produced. This concept can also be extended for use in other multi-component heat and mass transfer applications.