Experimental investigation of a membrane distillation system using solar evacuated tubes

Membrane distillation (MD) has been studied as a promising solution in the desalination industry but it has not been widely accepted or commercialized due to energy and cost concerns. MD is considered as a hybrid method that involves phase-change thermal processes and the use of membrane separation. Unlike conventional pressure-driven membrane methods such as reverse osmosis (RO), MD does not require intensive pre-treatment and can operate at lower pressure with higher salinities; but more importantly, it can utilize low-grade heat sources such as solar energy or waste heat for its operation. In this paper, an innovative MD module to directly employ solar thermal energy to assist in desalination is studied. MD systems that use solar energy as an external heater is investigated experimentally and theoretically. The proposed system, however, integrates hollow-fiber distillation membranes inside evacuated tubes solar collectors. As a result, the temperature is more uniformly distributed, minimizing the effect of temperature polarization, one of the key challenges of MD operation, thus can enhance the MD performance. The technical performance of the system is measured experimentally. The results of the proposed system are compared with a conventional MD process to investigate improvements in water production.