Energy efficiency comparison of single-stage membrane distillation (MD) desalination cycles in different configurations

Membrane distillation (MD) has shown potential as a means of desalination and water purification. As a thermally driven membrane technology which runs at relatively low pressure, which can withstand high salinity feed streams, and which is potentially more resistant to fouling, MD could be used for desalination where reverse osmosis is not a good option. The use of thermal energy, rather than electrical energy, and the fact that MD membranes can withstand dryout make this technology attractive for renewable power applications as well. However, most research on MD has focused on maximizing membrane flux as opposed to minimizing energy consumption and cost, and current MD systems suffer from poor energy efficiency compared to other desalination systems. In solar driven systems, the reported thermal performance has not been much better than a simple solar still. This paper examines the energy efficiency of single-stage MD-based desalination cycles in each of the MD configurations commonly used for desalination (direct contact, air gap, and vacuum) and compares the gained output ratio, or GOR, of each configuration across the range of membrane module geometries, and operating conditions. Limitations of each configuration are identified. Direct contact MD and air gap MD, in particular, have potential for high GOR.