Energy efficiency of direct contact membrane distillation

Membrane distillation (MD) is a promising technology due to its ability to function using low temperature differences and low-quality heat sources, thus allowing it to operate on solar or waste heat. The flux and energy efficiency of MD are influenced by temperature and concentration polarization, process conditions, and membrane-related parameters like thickness, tortuosity, thermal conductivity, pore size, and porosity. To date, a comprehensive review of membrane and distillation parameters on energy consumption has not yet been conducted. Accordingly, this review introduces the central energy parameters for MD (e.g., energy efficiency, gained output ratio, etc.) and discusses the reported impacts of membrane properties, mass and heat transfer, feed water properties, and system parameters on the energy parameters. The application of solar energy to direct contact MD (DCMD) is also discussed. A critical analysis of the energy efficiency of DCMD processes will help to establish its strengths and limitations and provide a road map for the development of this technology for both large-scale and portable applications.