Comparative assessment of AGMD and VMD systems configured with flat sheet and hollow fibre membranes: an energy and exergy, thermo-economic, and environmental analysis

Membrane Distillation (MD) systems, particularly Air Gap Membrane Distillation (AGMD) and Vacuum Membrane Distillation (VMD) in Flat Sheet (FS) and Hollow Fibre (HF) configurations, demonstrate strong potential for freshwater production, Yet, there remains a lack of comparative studies evaluating their energy, exergy, economic and environmental performance under similar conditions. This study presents a comparative analysis of four MD systems under similar operating conditions: FS-AGMD, HF-AGMD, FS-VMD and HF-VMD, and considers their respective costs and operation in Western Australia. The analysis was performed for single-stage and multi-stage configurations of up to ten modules, both with and without Heat Recovery (HR), to enhance efficiency and reduce the Cost of Water (COW). These systems were assessed in terms of energy, exergy, cost and environmental impacts, with results showing that for an optimal number of modules, HF-VMD achieved the highest water production (0.84 m 3 .d −1 ), while FS-AGMD had the lowest (0.16 m 3 .d −1 ). Further, COW depended on the number of stages, and decreased with additional stages, reaching the lowest values with HR: 13.39 USD.m −3 (Eight-Stage HF-VMD), 18.26 $.m −3 (Seven-Stage FS-VMD), 34.15 $.m −3 (Five-Stage HF-AGMD), and 52.67 $.m −3 (Seven-Stage FS-AGMD). Our analysis shows that the COW of HF-VMD with HR can be reduced to 12.55 $.m −3 when the water production increases to 1 m 3 .d −1 . In addition, results indicate that HF-AGMD exhibits the least exergy destruction, whereas HF-VMD produces the highest CO 2 and NO x emissions. This multi-stage evaluation emphasises the influence of heat recovery on performance and cost-efficiency.