Development of a novel isothermal heat and mass exchanger for humidity control

Efficient energy technologies are becoming increasingly important for mitigating climate change. In this study, a desiccant-based sorption system was developed to reduce energy consumption and subsequent greenhouse gas emissions, specifically in dehumidification systems. A novel isothermal heat and mass exchanger was designed, experimentally tested, and its performance verified through a numerical study. The novel concept is based on transferring the heat released during adsorption from the high-humidity stream to the desorption section of the system, where the drier air stream is processed. This heat transfer creates ideal isothermal conditions during the adsorption/desorption processes, enabling cooler adsorption and warmer desorption. Using a substrate with high thermal conductivity helps maintain the desiccant coating at a constant temperature (isothermal) during both adsorption and desorption, significantly improving the system's kinetics compared to non-uniform substrates, where temperature swings in the airflow degrade overall performance. The desiccant material, calcium chloride (CaCl2) confined within mesoporous silica gel and using polyvinyl alcohol as a binder, was coated on both sides of the substrate. One side underwent desorption to a cold air stream, while the other adsorbed water from a hot, humid air stream. The heat exchanger was tested under varying operating conditions, including the temperature and relative humidity of the hot and cold air streams, the system's flow rates, and the cycle times for adsorption and desorption. This study demonstrates that the optimized design is largely independent of inlet temperature and relative humidity; however, its performance is influenced by other operating conditions. The optimal values achieved for the dehumidification coefficient of performance and normalized moisture removal capacity were 2.05 and 42 kg/h & sdot;m2, respectively. The developed heat exchanger outperforms the commercial desiccant wheel in terms of dehumidification coefficient performance by a factor of four, while the desiccant wheel has double the moisture removal capacity per frontal surface area. However, at the same moisture removal capacity, the developed system consumes 75 % less energy compared to the desiccant wheel.