You heat thermal fluid (oil) in a pipe passing in the focus of a parabolic mirror and use this oil to start a multi-effect evaporator. Another method: Sun Tower: You focus the sun light by many mirror into top of tower where you boil water and use the generated steam to start a multi-effect evaporator. In both cases you will need means to for vacuum in the multi-effect evaporator (ejector). A Thermo-compressor will increase the efficiency of the multi-effect evaporator.