Comparative study on solar-driven combined liquid desiccant regeneration and freshwater production systems integrated with membrane distillation: Energy, exergy, and economic analysis

As a water treatment technology combining thermal evaporation and membrane separation, membrane distillation (MD) holds promise for regenerating liquid desiccant and supplying drinking water simultaneously in hybrid air-conditioning systems. The present work aims to compare the potential of two commonly used compact MD configurations, direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD), for liquid desiccant regeneration under various working conditions. Based on a thermodynamic model, the performance of two solar-driven MD systems are systematically evaluated and compared from an energetic, exergetic and economic perspective. The results show that the AGMD system is preferable at a low operating temperature or solution flow rate compared to the DCMD system in terms of regeneration capacity, energy efficiency and cost-effectiveness. Furthermore, the AGMD system is found to be more competitive in handling highly saline desiccant than the DCMD system since the latter suffers from increasingly remarkable driving force reversal with the rise in concentration. As for the design parameter, the AGMD system exhibits an approximatively linear trend in regeneration capacity with extending membrane length, while for the DCMD system there exists an optimal membrane length at which it can yield the highest regeneration capacity. Additionally, the AGMD system outperforms the DCMD system regarding exergy destruction and exergy efficiency. The seasonal analysis indicates that the availability of DCMD system ranges between 60.7 % and 88.9 % during summer in Hong Kong, lower than that of the AGMD system owing to its less demand for thermal energy. These findings offer valuable insights into suitable selection of MD configurations employed in air-conditioning systems towards sustainable buildings.