Refrigeration installations comprising a closed circuit in which a heat-transfer fluid (for example glycol water) is forced to circulate between capsules filled with phase-change material and stacked in a tank (made of steel or of concrete) and then led to the zone that is to be cooled (the technology referred to as “encapsulated PCM”) are known. The thinner the wall of the capsules the better the coefficient of heat transfer from the capsules to the heat-transfer fluid. During the phase referred to as the “store-charging” phase, the heat-transfer fluid, cooled by a refrigeration compressor, circulates through the tank at a temperature lower than the temperature at which the phase-change material contained in the capsules changes state and this has the effect of solidifying the phase-change material contained in the capsules and therefore of storing a certain amount of refrigeration energy. During the phase referred to as the “store discharging” phase, the heat-transfer fluid circulates through the tank and, upon contact with the capsules filled with the solidified PCM, picks up stored refrigeration energy and transfers it to the zone that is to be cooled. This circulation causes the phase-change material in the capsules to melt progressively, which means that the phase-change material has to be returned periodically to the solid state (something which is done during the store-charging phases).
Certain phase-change materials, notably water, occupy a greater volume in the solid state than in the liquid state and it is important for the capsule to be able to absorb this increase in volume without sustaining damage. One immediate solution is to partially fill the capsule with the phase-change material, the remainder of the volume being occupied by air and forming a free volume that can be gradually occupied as the phase-change material solidifies, at the expense of an increase in pressure in the capsule. Although simple to implement, this solution has the disadvantage of causing the thin wall of the capsule to stretch in a zone of weaker strength, and of allowing the casing of the capsule to deform by forming a dished shape in a zone of lesser strength of the casing under the effect of the pressure of the heat-transfer fluid.
The repeated nature of these deformations causes weakening of the casing which may ultimately yield.
Document FR2609536 describes a capsule completely filled with phase-change material and comprising a flexible casing which has hollow dished shapes that can be pushed back by the phase-change material as it solidifies, thereby allowing an increase in the internal volume of the capsule. As before, the repeated nature of the deformations of the casing ultimately weakens the latter.
Document FR2732453 itself describes a capsule with a thin and rigid casing containing a spherical absorption body held at the center of the capsule and occupying part of the internal volume of the capsule. The internal volume of the capsule, which is therefore decreased by the volume of the absorption body itself, is completely filled with phase-change material. The absorption body is compressible and therefore able to be compressed by the phase-change material as it solidifies so as to absorb the increase in volume of said material. The spherical shape of the expansion body associated with the spherical shape of the capsule leads the phase-change material to solidify from the periphery toward the center of the capsule without flowing, thereby avoiding the creation of detrimental internal stresses. However, fitting such an absorption body at the center of the capsule and keeping it there are tricky.