As per Dr.Rafik Karaman Solar radiation and vaccum pressure in the flash evaporator. But in the evacuated tube collector system increase the temp, in the range of 90 to 95 degree C, and in case of compound paraboilc concentrators increase the temp?
Shridhar, if you are evaporating at 90C or more be careful that you have a large flow of water through the evaporator or you will get a lot of salts deposited as the water boils off. These may eventually block the water flow altogether and hard deposits like that might not be easily removed.