Localized heating to improve the thermal efficiency of membrane distillation systems

Membrane distillation (MD) is a thermal-based membrane operation with high potential for the treatment of aqueous streams. However, its implementation is limited and only few examples of MD pilots can be found in desalination. One of the reasons behind this is that MD requires thermal energy for promoting the evaporation of water, which implies higher energy consumption with respect to pressure-driven membrane operations, like reverse osmosis (RO). Recently, among the different methods investigated to improve the thermal efficiency of MD, attempts for obtaining a localized heating of the feed, close to the membrane surface, were carried out. This work reviews experimental activities on the topic, dealing with both modified membranes, used under solar irradiation or coupled to an electric source, and specifically designed heated modules. The main results are reported and points of action for further optimization are identified. In particular, although at an early stage, this type of approach led to improvements in membrane flux and to a reduction of energy consumption with respect to conventional MD. Nevertheless, long tests to ensure a stable performance time, the optimization of operating conditions, the development of methods to control fouling issues, and the identification of the best module design, together with the scale-up of membranes/ modules developed, represent the main research efforts needed for future implementation of localized heating strategy.