NOVELTY - The device (1) has a geothermal probe (10) provided with an inlet piping (11) and an outlet piping (12) for circulation of heat exchange fluid e.g. water. An external pipe (20) is closed at opposite ends to shape a thermal chamber (21) axially carrying in an inner side of the geothermal probe and inserted in a drilling made in a ground. The external pipe is filled with heat exchange medium. A regulation unit (40) of an exchange surface of the inlet and/or outlet piping varies heat exchange capacity of the inlet and/or outlet piping. A control unit (41) controls the regulation unit.    USE - Geothermal heat exchanger device for use in an air-conditioning system that is used for exploiting geothermal energy as an alternative for a fuel powered system.    ADVANTAGE - The device is easy to install and maintain, and has high functioning reliability and versatile use with low economic cost, and can be adapted to any type of thermal pump in an easy manner. The device can vary thermal impedance of the geothermal probe in a passive way such that capacity of releasing or intercepting heat from the ground, the thermodynamic condition of the ground and the thermal impedance in which the probe works through the control of the active exchange surface, so that high efficiency level can be obtained in any season and climatic zone. The device ensures that a control circuit manages the thermal power exchanged by the geothermal plant in all probes in a distributed way, while maintaining four elements working in the period corresponding to the increase of conditioning demand and six elements working in half of probes of the geothermal plant in the period corresponding to the heating demand if the geothermal plant is installed in a warm temperate climate in which the balance of the annual heat demand is 2/3 in air conditioning and 1 /3 in heating, so that the control system is able to reduce the thermal drift to only half of the geothermal plant exchanging periodically each year the portion of the plant that is affected, thus preventing distributed drifts and keeping constant average operating conditions of efficiency of the heat pump even under conditions of unbalanced thermal demand.    DETAILED DESCRIPTION - The external pipe is made of thermal conductive material. The outlet piping is made of multiple tubular tracts axially associated with one another. The heat exchange medium is selected from a group consisting of water, snow and bentonite dispersed in water. An INDEPENDENT CLAIM is also included for a method for operating a geothermal heat exchanger device.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a geothermal heat exchanger device in axial cross section.    Geothermal heat exchanger device (1)    Geothermal probe (10)    Inlet piping (11)    Outlet piping (12)    External pipe (20)    Thermal chamber (21)    Regulation unit (40)    Control unit (41)