Internal-integral sodium return line for sodium heat engine

In a thermoelectric generator adapted to convert heat energy from a heat source to electrical energy and which comprises:

• (1) means for enclosing a first reaction zone;
• (2) means for enclosing a second reaction zone;
• (3) a reaction zone separator which:
• (a) separates and substantially completes enclosure of said first reaction zone and said second reaction zone, and
• (b) comprises a cationically conductive, solid electrolyte that is essentially impermeable to elemental alkali metal and compounds thereof and ionically conductive with respect to cations of said alkali metals;
• (4) elemental alkali metal within said first reaction zone and in fluid communication with said solid electrolyte;
• (5) an electrode within said second reaction zone in electrical contact with said solid electrolyte and sufficiently permeable to permit alkali metal to pass therethrough;
• (6) means for conducting electron flow between the elemental alkali metal within the first reaction zone and the electrode;
• (7) means for maintaining a temperature in said first reaction zone at least 100.degree. C. in excess of the lowest temperature in said second reaction zone;
• and
• (8) means for returning condensed elemental alkali metal from said second reaction zone to said first reaction zone, which means comprise a return line for said condensed alkali metal and a means for pumping said condensed alkali metal through said return line;
• the improvement comprising that at least a portion of said return line is located within said second reaction zone, so that said portion of said return line receives sufficient heat from the heat source during operation of said generator to maintain said alkali metal within said portion of said return line at a temperature above the melting point of said alkali metal.