Solar-heated submerged vacuum membrane distillation system with agitation techniques for desalination

Abstract

Submerged membrane distillation (S-MD) has been proposed as an alternative to conventional cross-flow MD systems to desalinate hypersaline water. In conventional S-MD systems, the hydrophobic membrane is submerged in the feed water tank that is directly heated by an electric heating element, eliminating the need for feed pumping. In this study, a solar-heated submerged vacuum membrane distillation (S-VMD) system that uses an evacuated tube collector as the feed container is proposed. Indoor tests under steady-state operating conditions and daily outdoor tests under ambient weather conditions were conducted to investigate the system’s functionality. The effect of two agitation techniques (aeration and internal circulation) to reduce temperature and concentration polarizations were studied. The daily performance tests revealed that the solar-heated S-VMD system can achieve a permeate flux of 5.9–11.1 kg·m−2·h−1 depending on solar intensity. The permeate flux was enhanced by 9% under aeration and by 22% under circulation in the outdoor tests. The water production per solar absorbing area can be as high as 0.96 kg·m−2·h−1. The system maintained a stable permeate flux and excellent water quality over a long-term operation. The small-scale system can provide fresh water in remote areas with limited natural resources.