Apparatus for temperature- controlling a space to be temperature-controlled with a space limitation separating the space to be temperature-controlled from a surrounding area , comprising: a primary heat pump circuit with an evaporator , a condenser , a compressor , and an expansion element , wherein the primary heat pump circuit comprises a natural primary working fluid, wherein the evaporator , the liquefier , the compressor , and the expansion element are suitable to be arranged outside of the space to be temperature-controlled; a secondary circuit thermally coupled to and fluidically decoupled from the evaporator or the condenser via a heat exchanger and comprising a temperature-controlling element configured to be arranged in the space to be temperature-controlled and configured to be connected to the heat exchanger via a line arrangement comprising a secondary fluid that differs from the primary working fluid, wherein the line arrangement is suitable to penetrate the space limitation , wherein the secondary circuit is configured as a thermosiphon cycle, and wherein a controllable pump configured to reverse a conveying direction in the secondary circuit as a response to a control signal is arranged in the secondary circuit; wherein the apparatus is configured to cool the space by means of the temperature-controlling element in a refrigeration mode; and a controller configured to cause, as a response to a control signal , a heat pump cycle reversal in the primary heat pump circuit so that, in the refrigeration mode, energy is dissipated from the heat exchanger through the primary heat pump circuit, and so that, in a defrosting mode, energy is supplied to the heat exchanger through the primary heat pump circuit in order to defrost the temperature-controlling element , and the conveying direction in the secondary circuit is reversed by the controllable pump . a primary heat pump circuit with an evaporator , a condenser , a compressor , and an expansion element , wherein the primary heat pump circuit comprises a natural primary working fluid, wherein the evaporator , the liquefier , the compressor , and the expansion element are suitable to be arranged outside of the space to be temperature-controlled; a secondary circuit thermally coupled to and fluidically decoupled from the evaporator or the condenser via a heat exchanger and comprising a temperature-controlling element configured to be arranged in the space to be temperature-controlled and configured to be connected to the heat exchanger via a line arrangement comprising a secondary fluid that differs from the primary working fluid, wherein the line arrangement is suitable to penetrate the space limitation , wherein the secondary circuit is configured as a thermosiphon cycle, and wherein a controllable pump configured to reverse a conveying direction in the secondary circuit as a response to a control signal is arranged in the secondary circuit; wherein the apparatus is configured to cool the space by means of the temperature-controlling element in a refrigeration mode; and a controller configured to cause, as a response to a control signal , a heat pump cycle reversal in the primary heat pump circuit so that, in the refrigeration mode, energy is dissipated from the heat exchanger through the primary heat pump circuit, and so that, in a defrosting mode, energy is supplied to the heat exchanger through the primary heat pump circuit in order to defrost the temperature-controlling element , and the conveying direction in the secondary circuit is reversed by the controllable pump . <EOS>