Membrane distillation provides a feasible and optimal solution to potable water issues. The literature contains a number of studies and research studies that aim to understand the behavior of membrane distillation systems and to provide the best possible solutions under different conditions. The purpose of this article is to discuss the air gap membrane distillation (AGMD) specifically and its development to date. The areas for future research in the field of AGMD are suggested. Membranes used in AGMD were discussed, including nanocomposite membranes and graphene membranes. In addition, the long-term performance issues regarding membrane fouling and scaling and the ways to prevent and to reduce them were discussed. Performance parameters that have not been explored sufficiently, such as energy efficiency and performance ratio, are discussed. Evolution of new membrane distillation processes from AGMD, such as the material gap and permeate gap, and conductive gap membrane distillation, is discussed. A generalized theoretical model for heat and mass transfer is presented for air gap membrane distillation systems. Coupling AGMD to form a hybrid combination with renewable energy sources is considered as a good answer to energy specific issues. Hybrid renewable energy systems with AGMD are discussed in detail. Novel designs for coupling AGMD systems with different forms of renewable energies are suggested, which presents an excellent area to be considered for developing advanced hybrid AGMD systems. It is suggested that future research should include economic studies, long-run system performance, operational problems and maintainance requirements, and related issues for better understanding and better acceptance of AGMD systems for industrialization.
