Tarig, you are right about ...

Tarig, you are right about that. With what you have mentioned, a canopy over a greenhose effect could double the amount of sun light used for heating the water. Like wise, the clean water could be cooled by preheating the brackish water preparing it for cleaning. There is a free energy concept that I have posted which if it works, it could be used to generate electricity. It could either assist in heating the water or for using other methods of water purification. With the free energy principle, it is something that could help in impoverished areas. And with what you have mentioned, by combining different idea's, some help might be able to be given to those who do need it.