An integrated, solar-driven membrane distillation system for water purification and energy generation

Utilising solar thermal energy for membrane distillation desalination represents a green and sustainable solution for building environments in regions with a high correlation between water shortage and high solar irradiance. Today's solar thermal-driven membrane distillation systems are designed with physically separated solar thermal collectors (e.g. flat plate or evacuated solar thermal collectors) and membrane distillation modules. In these systems, a thermal storage tank, a heat exchanger, and complex plumbing arrangements are required to control the heat and mass transfer between the solar collectors and the membrane distillation unit(s). Due to their high complexity and high capital/operational costs, these systems are rarely installed in buildings. To overcome these weaknesses, the present work conducts an experimental and numerical feasibility study of an integrated solar membrane distillation prototype (with the membrane distillation modules built directly into the evacuated solar tubes) for both potable water and/or thermal energy production. To the best of the authors' knowledge, this elegant combination of an evacuated tube solar collector and a membrane distillation unit represents an innovative approach which couples two well-developed technologies into an efficient, yet relatively low cost, hybrid energy-water production system. Our experimental results revealed that 4-10 L per square meter of membrane area per hour of permeate flux is achievable when the feed temperature ranges from 50 to 70 degrees C, achieving a salinity level of 10-200 ppm from a 35,000 ppm (e.g. the salinity of seawater) feed. It was found that a system with a solar absorbing area of 1.6 m(2) integrated with similar to 0.2 m(2) of membranes can produce similar to 4 L of drinkable water and similar to 4.5 kWh of heat energy (at 45 degrees C) per day (with an average daily solar exposure of 4 kWh/m(2)). We envision that this design could be beneficially deployed on the rooftops of residential and commercial buildings-buildings which require a continual supply of both potable water and domestic hot water.