Heat Pipe
Please refer to picture. Sun's ray will be collected by absorbers, then radiation can be transformed into heat energy by selective absorbing film. Heat energy have few material in heat pipe boiled away and gone up rapidly to condensation end and then become fluid.With the gravity, it goes back to evaporation end. Water can be heated through its continuous circulation. In order to make the fluid go back to evaporation end, the tilt angle should more than 10 degree.