Experimental study of thermal performance in air gap membrane distillation systems, including the direct solar heating of membranes

Membrane distillation (MD), a thermally driven membrane technology which runs at a relatively low pressure and withstands high salinity feed streams, has shown potential as a means of desalination and water purification. This paper focuses on the air gap MD (AGMD) process experimentally with the goal of demonstrating and predicting means of improving the energy efficiency of AGMD systems. In particular, a novel configuration which delivers solar radiation directly to the membrane is investigated using a composite solar-absorbing membrane. The use of reduced pressure in the air gap, for lower diffusion resistance, was also explored. A parameter to relate the performance of a bench-scale experiment with similar membrane and gap size to a production system was developed through the application of previously developed models. Small scale experiments were conducted to verify performance for the novel solar powered configuration and the effect of reduced gap pressure. Experiments demonstrated the efficacy of a solar absorbing membrane to improve the thermal performance of the cycle beyond heating an opaque surface in contact with the feed stream. The results also establish a benefit from the deformation of the membrane into the air gap as a result of hydraulic pressure. •Novel heating configuration whereby solar flux is absorbed directly onto the membrane.•Experimental evaluation of reducing pressure of the air gap in AGMD.•Non-dimensional scaling parameter to compare systems of different sizes.