Parametric analysis and multi-objective optimization of a membrane distillation liquid desiccant regenerator with heat/moisture recovery and potable water production

The regenerator plays a crucial role in determining the energy efficiency of green liquid desiccant air-conditioning systems. This paper presents a novel air gap membrane distillation regenerator featuring heat recovery and portable water production. Based on a validated numerical model, performance evaluation of the regenerator combined with an external heat exchanger is conducted in terms of regeneration capacity (RC) and energy consumption (Qin). The comparison of flow patterns demonstrates that the counter-flow has a slightly higher RC, while the parallel-flow has a significantly lower Qin and is therefore preferable from the perspective of COP. The effects of key parameters including feed and coolant desiccant inlet temperatures (Tf,in & Tc,in), mass flow rate (mdil), membrane length (L), air gap thickness (δag), desiccant concentration (Xdil) and solution heat exchanger effectiveness (εshe) on the performance are investigated. The results show that Tf,in is the dominant factor on performance compared to Tc,in and mdil, and increasing L leads to a marked growth in both RC and COP regardless of the raised Qin. Additionally, it is found that narrowing the air gap from 3 mm to 1 mm can improve RC and COP by 69.9 % and 50.1 % respectively. As for εshe, it has no influence on RC but remarkable impacts on Qin and COP. Moreover, the heat recovery by the coolant channel is proved to save nearly 8 % of energy input under the involved conditions. Finally, the NSGA-Ⅱ based multi-objective optimization is performed to obtain the Pareto front for maximizing RC and minimizing Qin. And the optimal solution through TOPSIS decision-making offers a RC = 975.8 g/h and a Qin = 1938 W. The findings provide a basis for design and application of the novel regenerator.