A thermal energy storage device comprising a heat exchanger arrangement, which is configured for guiding a flow of a heat transfer medium between a first end of the heat exchanger arrangement and a second end of the heat exchanger arrangement, and a heat storage material, which surrounds the heat exchanger arrangement, wherein the heat exchanger arrangement is adapted to (a) transport the heat transfer medium from the first end to the second end, if the thermal energy storage device is in a first operational mode, in which the heat storage material is supposed to receive thermal energy from the heat transfer medium and (b) transport the heat transfer medium from the second end to the first end, if the thermal energy storage device is in a second operational mode, in which the heat storage material is supposed to release thermal energy to the heat transfer medium, characterized in that the heat exchanger arrangement comprises a first heat exchange section being associated with the first end, a second heat exchange section being associated with the second end, a first connecting section connecting the first heat exchange section with the second heat exchange section and a second connecting section (117, 118) connecting the first heat exchange section with the second heat exchange section parallel to the first connecting section, wherein each one of the connecting sections (116, 117, 118) comprises a valve (116a, 117a, 118a) for controlling the flow of the heat transfer medium through the respective connecting section (116, 117, 118) and by opening one valve and closing one or more of the other valves (117a, 118a) the energy storage capacity of the thermal energy storage device may be decreased or increased. a heat exchanger arrangement, which is configured for guiding a flow of a heat transfer medium between a first end of the heat exchanger arrangement and a second end of the heat exchanger arrangement, and a heat storage material, which surrounds the heat exchanger arrangement, wherein the heat exchanger arrangement is adapted to (a) transport the heat transfer medium from the first end to the second end, if the thermal energy storage device is in a first operational mode, in which the heat storage material is supposed to receive thermal energy from the heat transfer medium and (b) transport the heat transfer medium from the second end to the first end, if the thermal energy storage device is in a second operational mode, in which the heat storage material is supposed to release thermal energy to the heat transfer medium, characterized in that the heat exchanger arrangement comprises (a) transport the heat transfer medium from the first end to the second end, if the thermal energy storage device is in a first operational mode, in which the heat storage material is supposed to receive thermal energy from the heat transfer medium and (b) transport the heat transfer medium from the second end to the first end, if the thermal energy storage device is in a second operational mode, in which the heat storage material is supposed to release thermal energy to the heat transfer medium, a first heat exchange section being associated with the first end, a second heat exchange section being associated with the second end, a first connecting section connecting the first heat exchange section with the second heat exchange section and a second connecting section (117, 118) connecting the first heat exchange section with the second heat exchange section parallel to the first connecting section,