NOVELTY - The dual thermocline storage tank (2) includes a barrier (13) which comprises a fluid-tight outer shell and a filler arranged inside the outer shell. The filler is made of rigid and compression resistant material and is laid inside the outer shell in loose form, without providing any restriction to thermal growth between different components.    USE - Dual thermocline storage tank for storing two masses of fluid at different temperatures in the thermal energy storage system of a solar thermal power plant.    ADVANTAGE - Provides loose and compression resistant materials as the filler to eliminate any problems related to thermal deformations in the filler. Enables the barrier to easily withstand the pressure load of a stored fluid and maintain a nearly constant volume without having to add a complex and costly structure to its outer shell. Enables dividing the interior filler of the barrier into an insulation layer and a weight adjustment layer to enable effective and easy adjustment of the density of the barrier to a desired value. Provides the outer shell of the barrier with non-planar geometry in one or both of its upper and lower faces, thus increasing its stiffness and reducing its thermal deformations. Enables adding waved or straight circumferential lobes on the outer zone of the barrier shell, so that the connection between the upper and lower faces of the shell is made more flexible and the thermal deformations and stresses are reduced. Enables breaking out the barrier into smaller and independent bodies, arrayed one aside the other to complete a modular barrier, thus reducing problems related to thermal deformations, as well as the manufacturing problems, present in a single bigger component.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic vertical cross-sectional view of dual thermal energy storage tank with a general arrangement of the two masses of fluid and a barrier inside the tank.    Thermal storage system (1)    Dual thermocline storage tank (2)    Hotter fluid (3)    Colder fluid (4)    Barrier (13)