NOVELTY - A geothermal power generator comprises fluid filled vessel disposed within downhole environment; double-walled pipe within the vessel; thermotunneling or thermionic converters located between the walls of the pipe, each converter having a first surface in thermal contact with fluid around pipe exterior and a second surface in thermal contact with a fluid around pipe interior; and wiring for connecting the converters together.    USE - For in-situ recovery of underground geothermal energy to generate electricity in a downhole environment (claimed).    ADVANTAGE - The apparatus allows for the efficient production of power in the downhole environment. It utilizes heat found in geothermal sources without needing the introduction of a fluid, thus providing greater efficiency and economical savings and reducing the possibility that the system may eventually become blocked because of the buildup of minerals. It also involves no moving parts and will thus have higher reliability and prolonged lifetime.    DETAILED DESCRIPTION - A geothermal power generator for generating electricity in a downhole environment comprises:    (1) fluid filled vessel (32) disposed within the downhole environment, with the fluid being in thermal contact with heat dissipated by hot strata in the downhole environment;    (2) double-walled pipe (34) disposed within the vessel;    (3) thermotunneling or thermionic converters (38) located between the two walls of the double-walled pipe, each converter comprising a first surface (18) in thermal contact with a fluid around the exterior (28) of the double-walled pipe and a second surface (20) in thermal contact with a fluid around the interior (26) of the double-walled pipe; and    (4) wiring for connecting the thermotunnelling or thermionic converters together and conducting electricity, produced by the thermotunneling or thermionic converters, out of the downhole environment.    An INDEPENDENT CLAIM is also included for a method for producing electricity comprising:    (1) positioning thermotunneling or thermionic converters in close proximity to a geothermal heat source;    (2) thermally contacting a first surface of each of the thermotunneling or thermionic converters to the geothermal heat source;    (3) thermally contacting a second surface of each of the thermotunneling or thermionic converters to a heat sink; and    (4) conducting electricity away from the thermotunneling or thermionic converters.    DESCRIPTION OF DRAWING(S) - The figure is a vertical cross section of Geothermal Power Generator.    First surface (18)    Second surface (20)    Pipe interior (26)    Pipe exterior (28)    Fluid filled vessel (32)    Supporting structure (33)    Double-walled pipe (34)    Thermotunneling or thermionic converters (38)