Solar collector evacuated tubes collect and convert sunshine energy into thermal energy, which is transferred to the central heat pipes in the tubes through the aluminum fins bridging the tube inner surfaces and the heat pipes. The copper heat pipes have tiny amount of purified water sealed in them at depressurized condition. When heated, the water inside the heat pipes vaporizes at low temperature (about 30˚ C). The vapor (steam) rises to the pipe tips inserted into the water contained in the tank. Vapor is cooled down and becomes condensate falling to the bottom of the pipes. By continuously circulating this way, heat is carried from outside to the water inside the tank.