Effect of energy recovery on the performance of a spiral wound vacuum assisted air gap membrane distillation system

Advancing membrane distillation (MD) requires innovative module configurations that enhance permeate flux and energy efficiency. Multi-envelope MD operated in vacuum-assisted air-gap mode (V-AGMD) has emerged as an effective alternative to the established conventional thermal desalination, offering higher energy efficiency and modular scalability. In this study, a physics-based model of a pilot-scale spiral-wound V-AGMD module is used to design and evaluate single- and two-stage energy-recovery configurations (parallel/series), quantifying gains in gained output ratio (GOR), reductions in specific thermal energy consumption (STEC), and productivity trade-offs. The model's predictions demonstrate close agreement with experimental findings, based on approximately 72 measurements originating from independent sources cited in the literature. A comprehensive physical analysis examines variations in air gap width, membrane length, both low and high operating conditions, such as temperature as well as the feed rate. GOR values initially increase with membrane channel length; they reach a peak before declining due to the balance between vapor transport irreversibility and the diminishing temperature gradient through the membrane. The operating and design conditions envelope exerts a strong, quantifiable influence on performance. Fresh water productivity shows a remarkable increase of 788.1 % with a rising feed flow rate from 100 to1000 l/h, while at 40 °C. Narrowing the air gap from 8.0 to 0.75 mm decreases the STEC from 1295 to 365.9 kWh/m³. The considerable result achieved in this work is observed in the detailed analysis of energy recovery methods that leads to a substantially high GOR. In a two-stage series configuration, the AS24 modules achieve the lowest STEC of 34.8 kWh/m³, corresponding to GOR of 18.96.