<p>Phase-change materials (PCMs) are capable of storing or releasing significant thermal energy as a result of a phase transition, which is the reason that they find application in temperature stabilization systems inside residential and industrial premises and in complex computer systems, as well as for accumulating thermal energy for use during periods of peak demand. The problem with using PCMs is associated with the low thermal conductivity of these materials, which causes high inertia in systems based on them. To overcome this problem, an additive of nanocarbon particles (carbon nanotubes, graphene, and graphene oxide) is used, whose thermal conductivity coefficient is 4 to 5 orders of magnitude higher than the corresponding value for PCMs. The high cost of nanocarbon particles requires careful attention to determining the optimal amount of nanocarbon additive that provides the maximum intensity of heat exchange between the PCM and the working fluid, taking into account the phase transition. The report sets up and solves a model problem of heat exchange between a fluid pumped through a pipe and a PCM enclosed in an external pipeline. The dependences of the characteristic heat exchange time on the composite’s thermal conductivity, calculated using the Comsol Multiphysics software package, have a decreasing nature and saturate at thermal conductivity values approximately 25 times higher than the initial value for the PCM. This behavior is due to the contribution of convection of the liquid PCM to heat transfer.</p>