Energy and Economic Analysis of Multistage Vacuum-Assisted Air-Gap Membrane Distillation with Different Flow Configurations

This study investigates the recovery of hypersaline water using membrane distillation (MD) implemented through multistage vacuum-assisted air-gap membrane distillation (MSV-AGMD). The proposed approach represents an advanced desalination concept characterized by high thermal efficiency and practical applicability. Despite the growing research interest in MD, the design and optimization of multistage systems remain insufficiently addressed. In this study, parallel, series, and mixed MSV-AGMD flow configurations are developed and evaluated within a unified modeling framework. A comprehensive energetic and thermo-economic assessment is conducted to quantify configuration-dependent performance. An energy analysis is first conducted to examine system productivity and energy efficiency across a range of design and operating parameters, including feed and coolant temperatures, feed flow rate, air-gap vacuum pressure, and the number of stages. Performance indicators such as permeate flux and gained output ratio (GOR) are employed to quantify system efficiency. Results indicate that the parallel configurations (D-1 and D-2) exhibit the highest energy-recovery potential, with configuration D-2 achieving a maximum GOR of 12.33. Moreover, an economic evaluation highlights the cost-effectiveness of the proposed systems, identifying an optimal operating point with a minimum levelized cost of water of $3.184/m³ at a feed flow rate of 1000 L/h, representing the best trade-off between energy efficiency and economic feasibility.