A theoretical and experimental investigation of a solid desiccant air dehumidification system using shell-tube heat and mass exchanger configuration

The building sectors represent about 40% of total primary energy consumption and more than 30% of CO2 emissions globally. In hot and humid climates, the need for air conditioning for thermal comfort contributes to around 60% of domestic buildings energy consumption. This is aggravated by the widespread use of inefficient and energy intensive mechanical vapour compression air conditioning systems and the way air humidity is controlled. An extensive literature review of air dehumidification systems was conducted to map out the state of existing technology. This re-enforced the status of existing passive cooling technologies operating according to the principle of adsorption, absorption and evaporation as offering promising solutions but of limited performance. This research aims to investigate potential design enhancement of solid desiccant based air dehumidification systems that can extend the applicability of evaporative cooling systems to hot and humid regions. However, this requires overcoming the common problems associated with solid desiccant systems in regards to removing large amount of heat released during the adsorption process and residual heat from the regeneration process. Therefore, this thesis proposes an innovative solid desiccant air dehumidification system arranged in the form of a shell-tube heat and mass exchanger configuration. The proposed air dehumidification arrangement is designed to enhance both heat and mass transfer between the solid desiccant layer and its surrounding during the adsorption and regenerations process. The design incorporates a solid desiccant annular layer inside the heat exchanger tubes. A bundle of these desiccant filled tubes was enclosed in cylinder with two openings to form a tube-shell heat and mass exchanger. A rolled thin copper mesh was used to support the solid desiccant particles against the tube wall while allowing air to flow through the centre of the tube. A comprehensive mathematical model expressing heat and mass transfer balance of the system was developed and the governing equations were discretised into a MATLAB codes program. The results of the mathematical model were presented in details investigating the effects of different operating parameters such as tube length, diameter, air temperature, humidity ratio and flow velocity. To validate the computer results, a proof-of-concept laboratory prototype and test rig were built and tested under controlled air temperature and humidity. It was particularly noted that while optimum tube length was found for better overall performance, it was also observed that moisture removal was more efficient under smaller air velocity. For higher dehumidification capacity and efficiency, shorter cycle time was however more favourable. Furthermore, the experimental test results emphasized the importance of the removal of heat of adsorption from the desiccant layer. By using secondary air to help remove heat during moisture adsorption and to evacuate residual heat from preceding regeneration process, the prototype was able to provide air with constant humidity ratio of around 8 g/kg lower than the ambient air for a dehumidification cycle of 5 minutes. It was also shown that dehumidification performance would be severely reduced if secondary air / post-regeneration cooling was not employed. Furthermore, the investigations under different testing conditions revealed that dehumidification capacity increased 50% with higher airflow while moisture removal would be more efficient under lower air velocity, and higher regeneration temperature improved dehumidification rate by up to 28% at the expense of higher heating energy. It was also noted that while drier outlet air could be provided with lower inlet air humidity, dehumidification effect was more significant with higher inlet air humidity. Last but not least, the experimental test results were also used to refine the mathematical model and introduce corrections to the governing equations mainly concerning heat storage of the system’s structure and its effect on moisture adsorption and desorption rates