NOVELTY - The geothermal heat exchanger has a rectangular flexible or semi-flexible reservoir body (10) having width (W), height (H), and length (L). The reservoir body has an inlet port (11) on an upper end portion (10t) and an outlet port (13) on an opposite lower end portion (10b). The reservoir body width is between 4 and 6 inches, while the length is twice greater than the height. The reservoir body is expandable from a pre-installation shape to a liquid-filled post-installation shape and is retained in the post-installation shape during use.    USE - Geothermal heat exchanger for ground trench. Used in geothermal closed loop heat exchange system (claimed) or with other geothermal ground loops including residential and commercial or industrial applications such as industrial buildings, office buildings, multi-dwelling residences, apartments, hotels, motels, or other facilities.    ADVANTAGE - The reservoir body can change in external shape from pre-installation configuration to liquid-filled post-installation configuration, and can be retained in the post-installation configuration during operation in which the body snugly contacts adjacent virgin compacted soil without requiring additional compaction or special fill and provides good heat transfer with earth. The reservoir may also be used with straight and or coiled pipes, ponds, or other geothermal loop heat transfer devices for a hybrid system, allowing greater flexibility in land use and or ground loop configurations.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a geothermal closed loop heat exchange system;    (2) a method of installing a geothermal heat transfer system; and    (3) a method of geothermal heat transfer.    .    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of a geothermal heat transfer reservoir as the geothermal heat exchanger.    Reservoir body (10)    Lower end portion (10b)    Upper end portion (10t)    Inlet port (11)    Outlet port (13)    Height (H)    Length (L)    Width (W)