Experimental Investigation Of A Solar Driven Membrane Distillation Desalination System

Combined by a solar heat collection unit and a membrane distillation （MD） desalination unit, solar MD isa new technology to produce fresh water from seawater. A critical review of the current research on the systemwas provided. Based on the experimental set-up built, a series of experiments were carried out separately onthe vacuum MD unit, the solar heating unit and the combined solar MD system, with the experimental resultsreported and discussed in this study.The performance of the vacuum MD unit was examined first, using electricity as heat source and NaClsolution as feed. The effect of the feed temperature, the feed flow rate, the feed concentration and the cold-sidevacuum pressure on the trans-membrane flux, the retention rate, the specific energy consumption and therecovery rate was studied. The results show that, the feed temperature and the cold-side vacuum pressure arethe main factors influencing the performance, and feed temperature over65℃and vacuum pressure over0.08MPa are favorable for high water flux. The maximum water flux obtained was14.4kg·m^-2·h^-1. Thehydrophobic polypropylene membrane showed good separation performance, with the retention rate higherthan99.9%and the electricity conductivity of the produced water lower than30μs·cm^-1.Two solar heat collection units, one based on a full glass vacuum tube collector and the other on aconcentrated flat plate collector, were examined experimentally. The collector efficiency, and the daily heatgain and the daily energy efficiency of the collection system, were studied. The vacuum tube collector hadmuch higher efficiency, and the maximum efficiencies of the two collectors were72%and36%, respectively.Although the aperture area of the vacuum tube collector was4.2times of the concentrated one, the heat storedin the water tank was not substantially higher, and the result was that the daily energy efficiency of theconcentrated system was very low, lower than10%.Using the hot water from the water tank of the two solar heat collection units as heat source respectively,the performance of the combined solar MD system was tested. The system ran for3hours and produced3.07Lof distilled water when the driving water was from the vacuum tube collection unit, and ran for2hours andproduced2.14L of distilled water when driven by the concentrated unit. The average trans-membrane flux waslow, only4kg·m^-2·h^-1, mainly due to the low temperature of the driving water from the solar heating system.