Abstract: One or a number of subsoil cavities are employed for the storage or recovery of hot liquids. At the time of heat storage, a heat-transporting liquid is extracted at low temperature at the lower end of each cavity, heated by heat exchange with an external fluid and reinjected at the upper end. At the time of heat recovery, the heat-transporting liquid is extracted at high temperature at the upper end of each cavity, cooled by heat exchange with an external fluid and reinjected at the lower end. The system comprises flow-arresting means for maintaining a minimum operating pressure at the top of each cavity, air or gas purge means, an expansion reservoir and a safety unit for ensuring one-way flow of the heat-transporting liquid from the expansion reservoir to each cavity.

Abstract: A method for storing fluid underground at maximum pressure p, comprising the steps of digging, at a depth at which the lithostatic pressure generated by the weight of the abovejacent soil formations is at least p, an underground cavity in which is made a tight deformable casing anchored to the cavity wall at some places only, while it can freely expand or contract in every direction between said anchoring places, then of injecting pressured fluid into said casing in order that the latter be fully applied against the cavity wall, the possible movements of which said casing subsequently follows by gliding, the pressure of said fluid being, at every moment, counterbalanced by the lithostatic pressure of the cavity walls transmitted to said fluid by the thus expanded casing.

Abstract: Two or more reversible wells open respectively into a "hot" zone and a "cold" zone of a reservoir or captive layer. The water of the layer is circulated within a primary circuit in a first direction corresponding to circulation within an external loop from the "cold zone" wells to the "hot zone" wells and in a second direction corresponding to circulation from the "hot zone" wells to the "cold zone" wells. Indirect heat transfer takes place between the primary circuit and a secondary heating circuit in the first direction or between the primary circuit and a secondary utilization circuit in the second direction, the flow rate within the primary circuit being regulated in both directions.