## 1 Introduction  
One of the many ways of storing heat is by phase change. A phase transition is an isothermal thermodynamic process caused by a change in ambient temperature or pressure. As a result of the change in state aggregation (e.g., melting-solidification), phase-change materials (PCMs) can absorb, store, and release energy in the form of latent heat at a constant temperature, called the phase transition temperature. The amount of heat absorbed (and stored) during the heating of a typical material (non-PCM) is much less than the amount of heat absorbed (and accumulated) during the phase transformation (melting) of this material. When analyzing the possibility of using a given type of PCM as a material that absorbs and accumulates heat, four key essential properties should be taken into account: (1) _Heat capacity_, i.e., the ability to accumulate heat of PCM materials, depending primarily on the heat of phase transition; (2) _Phase transition temperature_, i.e., the temperature at which the PCM absorbs the largest amount of energy (and accumulates and releases it to the environment). The type of phase-change material should be selected so that its phase-change temperature is within the temperature range of the environment in which it will be used; (3) _Thermal conductivity_, i.e., the ability of PCMs to effectively absorb and release heat, even in conditions of small temperature differences between the system and the environment. This condition is met by PCMs with high thermal conductivity; (4) _Thermal property stability_. For the PCM to work effectively multiple times, it should retain its thermal properties over many melt-solid cycles [1; 2; 3]. For decades, PCMs, due to their useful properties, have been widely used as heat accumulators in installations using renewable energy sources (in particular solar radiation energy-photovoltaic modules) [4; 5; 6; 7]; heat insulators in structural elements of buildings [8; 9; 10; 11; 12; 13]; temperature stabilizers of food products or medicines (isothermal containers) [14; 15; 16]; temperature stabilizers of road surfaces [17; 18; 19]. The use of these materials in multilayer wall structures results in a reduction in daily temperature amplitudes inside the building and a delay in releasing the stored heat. In the case of building applications, the PCMs used possess a phase-change temperature within the temperature range outside the building (extreme temperatures in a daily cycle) and the temperature range of thermal comfort inside the building. PCMs are also commonly and successfully used in heat-insulating clothing, designed especially for work in extreme thermal conditions. Both in very low and very high ambient temperatures, an important problem is to simultaneously ensure effective protection against the harmful effects of the cold/heat and ensure the ergonomics of clothing [20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30]. This clothing is usually made of thick, multi-layered textile systems, which significantly reduces the user's freedom of movement and prevents him from effective and precise work [31]; in addition, during intense physical exertion, these garments often become wet due to the secretion of sweat by the wearer, which in turn reduces their thermal insulation and increases the feeling of discomfort [32; 33; 34; 35; 36; 37]. Figure 1 shows a schematic comparison of the PCM vs. non-PCM heating process. The process starts at time \(t_{0}\), when the initial temperature of both materials is \(T_{0}\). At time \(t_{1}\), both materials reach the temperature \(T_{1}\), which for the PCM is the phase transition temperature. At time \(t_{1}\), melting of the PCM starts and continues until time \(t_{3}\) is reached. During this time, the temperature of the PCM does not rise because all the heat supplied to the material is converted into a change in state of the PCM. At time \(t_{3}\), the PCM is completely melted, and its temperature starts to rise again to reach the value of \(T_{2}\) at time \(t_{4}\).