Abstract: A device for heating and/or cooling a medium contained in a compartment, includes a reactor, a reservoir containing a fluid able to change states in a determined temperature and pressure range, and elements for circulating gaseous fluid between the reactor and the reservoir, a rigid enclosure that contains a reactive medium able to absorb and desorb the fluid in the gaseous state through a thermochemical reaction, elements for diffusing the gaseous fluid in and from the reactive medium, heat transfer elements between the reactive medium and the outside medium. The enclosure of the reactor is formed from two bodies assembled by a peripheral flange, at least the first of which adopts the general shape of a cap with a circular or quasi-circular straight section that contains the reactive medium in the form of at least one block hugging the general shape of the cap.