**2. Thermal energy storage**

The storage of energy in thermal form can be carried out according to three physical principles, that is, (i) sensible heat, (ii) phase change or latent heat, and (iii) thermochemical reactions.

#### **2.1 Storage by sensible heat**

Sensible heat storage is undoubtedly common and straightforward, but it is also very inefficient. A material (most often water, stones or thermal oil) is heated to a higher temperature whenever excess heat occurred and cooled when necessary. In general, volume storage capacities remain relatively low. Also, depending on the type of material selected, liquid or solid, the process will be limited in the first case by the chronic tendency to thermal destratification, and in the second by the low thermal conductivity associated with a high porosity rate. In these systems, there

**69**

*Phase Change Materials for Textile Application DOI: http://dx.doi.org/10.5772/intechopen.85028*

*Q* = *m*∫

**2.3 Storage by latent heat or phase change**

enthalpy variation of the phase change ∆H [15].

**2.2 Storage by chemical reactions or evaporation**

The stored energy (Q in J g<sup>−</sup><sup>1</sup>

(Eq. (2)).

is always a need for a tank and also most often an exchange surface [12]. The cost price of these elements is generally the limiting factor in its economic application.

function of the mass of material (m) used and its calorific capacity Cp (Eq. (1)). The primary materials used for this type of storage are water, rocks, earth, and ceramics.

> *Tinitial Tend*

Thermal energy storage by chemical reactions is achieved by using the energy released or required during a chemical reaction. The basic principle is as follows

*AB* + *Heat* ⇌ *A* + *B* (2)

Using the heat provided by the external environment, the AB compound is divided into two components A and B, which can be stored separately. If A is mixed with B, the AB compound is reformed, and the heat is then released. The chemical reactions involved in this process must satisfy the total reversibility of the reaction since the products resulting from the reaction are likely to be separated and can be stored in a solid state and mixed to release heat when energy is required. Adding a catalyst can reduce the reaction temperature, but it is generally high. As a result, research on thermal energy storage by chemical reaction is still in the first stage and cannot be carried out in the short term. Nevertheless, other technologies have developed such as adsorption on activated carbon or zeolite, liquid phase absorption (LiBr), the reaction on chlorides (MnCl2, NiCl2) [13]. Storage systems based on chemical reactions have negligible energy losses, while sensitive heat storage absorbs stored heat from the

environment and needs to be isolated from the outside environment [14].

Storage by latent heat of fusion is carried out with little or no temperature variation since the phase change of a pure body is isothermal but by phase change of material. The case of alloys is different because melting takes place over a limited temperature range between Tsolidus and Tliquidus. Latent heat storage involves a firstorder phase transition (enthalpy variation ∆H). When the transition is essential, the material is called PCM (Phase Change Material). These materials are compounds that can store and release thermal energy through their change of state, most often from solid to liquid, but also from solid to solid. When heated, the material takes calories from the external environment and reaches a temperature, Ttr, transition temperature, then passing from a phase 1 to a phase 2 by heat absorption. If cooled, the reverse transition occurs at Ttr; the material passes from phase 2 to phase 1 and returns all the previously stored energy to the external environment while remaining at the temperature Ttr. The energy involved is the endothermic or exothermic

Since heat is closely linked to temperature (second principle of thermodynamics), this storage method is more interesting than the first because it allows energy to be stored at a given temperature or a given temperature level. Besides, storage is carried out at a small temperature difference, and it offers the possibility of restoring the stored energy at a constant temperature, at least as long as the solid and liquid phases are in equilibrium. The amounts of stored energy are also higher than when

) is dependent on both a temperature variation and a

*Cp*(*T*). *dT* (1)

*Textile Industry and Environment*

commercially available [8].

localized thermal regulation.

coating remains intact.

**2. Thermal energy storage**

(iii) thermochemical reactions.

**2.1 Storage by sensible heat**

between 2 and 15°C, they are generally used for air conditioning, that is, comfort applications [1]. Higher transition temperatures allow these materials to be used for solar energy storage [2], in agriculture [3], electronic equipment protection [4], or textiles [5–7]. Thermal energy storage by solid-liquid phase change has been the subject of a census by Zalba on more than 150 existing PCMs, 45 of which are

The thermal properties of phase-change materials allow them to be perceived as the material of choice for thermal insulation of the human body. The possibility of keeping the wearer as long as possible in his thermal comfort zone, and at the same time to reduce the thickness of the garment, is a conceivable objective, taking into account the thermal insulation capacity of textile support containing PCMs. Indeed, these are active during the phase change period and stop when the phase change of all PCMs is complete. This insulating effect is generally referred to as effective thermal insulation. The choice of PCMs to be used is therefore based on the feeling of comfort that the user can feel, regardless of his metabolic activity and external conditions. The effectiveness of PCMs inserted in a textile structure will depend mainly on the temperature differential between the body temperature and that of the surrounding environment. Therefore, a product ideally having a thermal window from 19°C (vasoconstriction temperature) to 37°C should contribute to

The conventional phase change material formulations have some disadvantages related to their chemical structure [9], justifying that they cannot be used without being contained in a capsule or trapped by capillarity in a graphite matrix [10] or chemical gel [11]. Moreover, since these materials can be in a liquid state, they cannot easily be incorporated into a textile carrier without being contained in capsules. Besides, they must be as small as possible in order to facilitate their integration and thermoregulation regarding heat exchange surface, thus helping to compensate for their low thermal conductivity. Regardless of the physical state of the microencapsulated material (solid, liquid, or both), it remains trapped inside. This allows it to be integrated into a textile coating and thus to keep its functionality as long as the

Microcapsules incorporated in commercial textile composites contain only one active ingredient, usually paraffin. These systems are thus limited by the thermal properties, thermal window, and enthalpy of phase change, of the microencapsulated paraffin. The objective of this work is to develop new materials with improved

The storage of energy in thermal form can be carried out according to three physical principles, that is, (i) sensible heat, (ii) phase change or latent heat, and

Sensible heat storage is undoubtedly common and straightforward, but it is also very inefficient. A material (most often water, stones or thermal oil) is heated to a higher temperature whenever excess heat occurred and cooled when necessary. In general, volume storage capacities remain relatively low. Also, depending on the type of material selected, liquid or solid, the process will be limited in the first case by the chronic tendency to thermal destratification, and in the second by the low thermal conductivity associated with a high porosity rate. In these systems, there

thermal properties before their incorporation on textile support.

**68**

is always a need for a tank and also most often an exchange surface [12]. The cost price of these elements is generally the limiting factor in its economic application. The stored energy (Q in J g<sup>−</sup><sup>1</sup> ) is dependent on both a temperature variation and a function of the mass of material (m) used and its calorific capacity Cp (Eq. (1)). The primary materials used for this type of storage are water, rocks, earth, and ceramics.

$$\mathbf{Q} = m \int\_{T\_{\text{Tantal}}}^{T\_{\text{rad}}} \mathbf{C}\_{p}(T) \, dT \tag{1}$$

#### **2.2 Storage by chemical reactions or evaporation**

Thermal energy storage by chemical reactions is achieved by using the energy released or required during a chemical reaction. The basic principle is as follows (Eq. (2)).

$$AB \star Heat \rightleftharpoons A \star B\tag{2}$$

Using the heat provided by the external environment, the AB compound is divided into two components A and B, which can be stored separately. If A is mixed with B, the AB compound is reformed, and the heat is then released. The chemical reactions involved in this process must satisfy the total reversibility of the reaction since the products resulting from the reaction are likely to be separated and can be stored in a solid state and mixed to release heat when energy is required. Adding a catalyst can reduce the reaction temperature, but it is generally high. As a result, research on thermal energy storage by chemical reaction is still in the first stage and cannot be carried out in the short term. Nevertheless, other technologies have developed such as adsorption on activated carbon or zeolite, liquid phase absorption (LiBr), the reaction on chlorides (MnCl2, NiCl2) [13]. Storage systems based on chemical reactions have negligible energy losses, while sensitive heat storage absorbs stored heat from the environment and needs to be isolated from the outside environment [14].
