Materials for energy conversion in membrane distillation localized heating: Review, analysis and future perspectives of a paradigm shift

Despite its ability to treat high salinity feeds and its integration readiness with renewable energy, membrane distillation (MD) is still facing many challenges. Intrinsically, the process suffers from low water fluxes and high thermal energy input, further aggravated by the temperature polarization phenomenon. Recent progress in MD design to improve its efficiency has taken the process to the heart of the materials-energy nexus. The use of advanced materials for efficient heat delivery has led to the concept of localized heating. Here, after emphasizing the main challenges that still hinder MD from reaching the industry arena, a compilation of the energy sources used in localized heating with concomitant materials is presented. Whether by coating, or incorporation, or brought close to the membrane, materials are grouped following the energy sources they respond to and their level of integration in the MD system is discussed accordingly. An energy analysis is carried out for cases reported in the literature. Results are assembled following different criteria to highlight the performance achieved with each energy source, the material integration strategy and the MD variant used with an adjustment if photovoltaics research cells are to power these energy sources in the future. Particular emphasis is put on process scale-up opportunities when localized heating is used compared to classical MD configurations. It is shown that, although localized heating provides a significant improvement, process and module design need to be included in the material energy development loop for MD to fully penetrate the desalination and water treatment industry.