Numerical investigation of a solar/waste energy driven sorption/desorption cycle employing a novel adsorbent bed

This paper presented a numerical investigation into a solar/waste energy driven sorption/desorption cycle employing a novel LiCl-Sillicon-Gels adsorbent bed. Several adsorbent materials available for air drying, e.g., silica gel, zeolites, silica gel haloid compound, consolidated composite desiccant, etc., were compared each other, leading to the selection of a most suitable desiccant material (i.e., LiCl-Sillicon-Gels), which has an adequate regeneration temperature of 80 °C and relatively higher moisture absorption capacity of 0.5 g/g. A dedicated adsorbent bed structure was devised to allow both solar radiation and warm air (generated from the waste heat) to pass through to vaporize the water reserved in the voids of the bed. The mass and energy conservation principles were applied to both air and adsorbent within the bed, leading to the development of a specialist mathematical model able to characterize and evaluate the performance of the sorption/desorption cycle. On this basis, the desorption process driven by both solar radiation and waste heat and associated sorption process were simulated side by side. The performance of the sorption/desorption cycle, represented by moisture extraction volume (Dme), moisture extraction/removal efficiencies (ƞme/ηmr), and dehumidification coefficient of performance (DCOP), and their correlations with the major operational factors, e.g. swapping time of working mode, parametrical data of the process/regeneration air and solar radiation, were investigated and characterized. The results of the research indicated that the system can achieve a good performance (i.e., moisture extraction volume of 7-7.2 g/kg, moisture extraction/removal efficiencies of 0.4-0.5 and 0.5 to 0.57, and DCOP of 0.35-0.37) under a typical wet climatic condition (i.e., 30-35 °C and 70-80% RH). Increasing solar radiation intensity to 1,800 W/m2 could lead to a significant rise in DCOP (from 1 to 5). Furthermore, the geometrical set-up of the adsorbent chambers/beds and cycle was optimized, giving the recommended air flow channel length of 0.7-0.9 m and air flow turning number of 5-7. The swapping time of the beds in terms of the function is suggested to 2.5-3 h. Compared to the conventional adsorption system, the new system can achieve around 90% saving in fossil fuel energy use. In summary, the paper made first of its kind effort in designing, characterizing and optimizing a novel solar/waste energy driven sorption/desorption cycle with LiCl-Sillicon-Gels as the bed filling material, which would help realization of the sustainable air treatment process in both building and industrial sectors, thus contributing to the energy saving, carbon emission reduction, as well as sustainable development on the global scale.