Improvement of condensation step of water vapor in solar desalination of seawater and the development of three-ply membrane system

Seawater desalination using sunlight is a sustainable freshwater production, which contributes the solution of global water shortage. This solar desalination has generally three steps; conversion of solar energy to heat, vaporization of seawater by the heat and condensation of water vapor to form freshwater. We have studied a seawater desalination using two-ply membrane system, where a dye modified hydrophilic membrane is put on a hydrophobic one. When seawater loaded on the system is exposed to sunlight, desalinated water is obtained under the hydrophobic membrane by the membrane permeation of water vapor. This paper reports the improvement of the third step, condensation of water vapor, of our process, which is indispensable for the efficient production of freshwater. When Al petri dish, heatsink and cooling plate were equipped under the twoply membrane system, the recovery of the vaporized water was remarkably improved by promoting the condensation of water vapor using those devices. Next, capillary action with hydrophilic membrane wick was applied for long term operation of the process with continuous supply of seawater. Finally, a three-ply membrane system, where black light-absorbing sheet, hydrophilic membrane wick and PTFE membrane were stacked, was found to be effective for the mass production of freshwater.