Abstract: A unit for recovering thermal energy which utilizes a plurality of unique heat pipes, and the method and apparatus for fabricating the heat pipes is disclosed. The heat pipes are disposed horizontally and are filled with a volume of working fluid sufficient to cause the liquid phase to travel in either direction by gravity. Circumferential capillary grooves in the side walls of the heat pipes transport the liquid phase vertically above the liquid level to increase the area of the liquid-vapor interface. Additionally, the solid metal strips which form the grooves provide a low impedance thermal path from the walls of the heat pipe to the liquid-vapor interface where evaporation and condensation occur. These two factors significantly increase the efficiency of the system. A divider plate having an X-shaped cross section separates the liquid phase from the high velocity vapor phase to prevent slugging under high energy transfer conditions.