NOVELTY - A U-tube heat exchanger comprises a working fluid, which may be water, coolant, or cryogen or cryogenic vapor that absorbs thermal energy in the form of heat from geothermal source. The cryogen will transform into cryogenic vapor as the cryogen boils. The cryogenic vapor will increase in enthalpy as it receives thermal energy. This vapor can transfer heat to geothermal thermoelectric generator.    USE - For geothermal power generation.    ADVANTAGE - The method introduces a very clean geothermal technology that does not interact directly with the ground water of the earth and does not produce corrosive substances and mineral deposits. It is capable of producing electricity from a geothermal source that may be used in almost any location worldwide. It reduces the risks associated with oil and gas exploration.    DETAILED DESCRIPTION - A U-tube heat exchanger comprises a working fluid, which may be water, coolant, or cryogen or cryogenic vapor, that absorbs thermal energy in the form of heat from geothermal source, such as producing or depleted oil or gas wells, and geothermal water wells. The cryogen that absorbs thermal energy will transform into cryogenic vapor as the cryogen boils. The cryogenic vapor will increase in pressure and enthalpy as it receives thermal energy for geothermal heat source. This vapor can transfer heat absorbed at the bottom of the geothermal heat source to a geothermal thermoelectric generator located in the upper portion of the geothermal well or located at the surface.    INDEPENDENT CLAIMS are also included for:    (a) a geothermal thermoelectric generator constructed of the quantum well technique of micro-thin layers that is 10-100 Angstrom thick of such materials as silicon/silicon germanium (Si/SiGe), or boron carbon alloys (B4C/B9C);    (b) an injection well;    (c) a solid-oxide electrolyzer to electrolyze high temperature steam produced from heated water air-lift pumped to the surface into hydrogen and oxygen; and a catalyst for enhancing the electrolysis.    The generator may be constructed in alternating layers of any P-type and N-type materials, known as bismuth telluride, aluminum and nickel, Si/SiGe, or B4C/B9C.    DESCRIPTION OF DRAWING(S) - The figure describes a U-tube shaped geothermal heat exchanger in which a cryogen is injected.