Abstract
Vacuum membrane distillation (VMD) is a hybrid membrane-evaporative process which has been shown to be of interest for seawater desalination. The main drawback of this process is the relatively high energy requirement linked to the need to heat the feed water. A way to solve this problem could be the use of a renewable source such as solar energy to provide the heat energy required. Two solutions of solar energy use are investigated in this paper: salinity gradient solar ponds (SGSP) and solar collectors (SC). For each solution, two configurations were studied. The first was based on pre-heating the feed seawater before the membrane process while the second used a membrane module directly coupled with solar energy, i.e. a membrane submerged in an SGSP or an SC integrated at the surface of the membrane module. VMD process simulations were carried out for the four different configurations with VMD modelling software previously developed and adapted to the different combinations. Simulation results showed that immersing the membrane module directly in an SGSP could induce marked concentration and temperature polarisation phenomena that reduced fluxes. Turbulence had to be created in the feed seawater to reduce polarisations and this option was difficult to combine with an SGSP. The most interesting solution seemed to be the use of SC. High fluxes of 140 L h(-1) m(-2) could be reached (for a vacuum pressure of 500 Pa and a membrane with a Knudsen permeability of 1.85 x 10(-5) s mol(1/2) m(-1) kg(-1/2)). (C) 2010 Elsevier B.V. All rights reserved.