**3. Phase change materials**

In an LHSU, energy is kept in the latent heat storage material. The term "Phase Change Materials" refers to the types of substances that may store latent heat (PCMs). Telkes and Raymond [6] were among the first to pioneer the research of PCMs. However, before the energy crises of the 1970s and 1980s, nobody paid any attention to it, when there was intensive research on the use of PCMs in many applications, particularly for solar heating systems. There have been several PCMs found and investigated extensively for their application in LHSU during the last four decades, and these PCMs span a wide range of melting/freezing points. A classification of these PCMs is given in **Figure 1**.

Many factors, including thermodynamic, chemical, kinetic, and economic ones, go into deciding which PCM to use. To be considered as PCMs, a substance must meet the following criteria:

• When choosing a PCM for the first time, it's important to make sure its melting point falls within the system's optimal working temperature range.

**Figure 1.** *Classification of PCMs [5].*


At least three components are needed for an LHSU: a PCM with an appropriate melting point, a heat exchange surface, and a PCM-compatible container. Thus, Phase Change Materials (PCMs), Containers, and Heat Exchangers must be understood to build an efficient LHSU.

Paraffin and nonparaffin organics are organic substances. Organic compounds offer noncorrosiveness, less cooling, chemical stability and thermal stability, harmonious melting ability, and suitability with common construction materials. Because of the broad temperature ranges across which it may melt or solidify and its large latent heat capacity, paraffin is a frequently used PCM for heat storage. During the process of solidification, they do not cause any subcooling effects and experience just a little

*Introductory Chapter: Phase Change Material as Energy Storage Substance DOI: http://dx.doi.org/10.5772/intechopen.108863*

volume change during the phase transition. They are stable, nontoxic, and noncorrosive over time. Lauric, myristic, palmitic, and stearic acids are used as PCMs in nonparaffin organics. Predicting melting and solidification behaviour and eliminating subcooling effects are their advantages. Organic compounds are combustible, have poor heat conductivity, and low phasechange enthalpy. Inorganic substances include salt hydrates, metals, alloys, and salts. Salt hydrates were explored, including sodium sulphate decahydrate (Glauber's salt), calcium chloride hexahydrate, etc. Inorganic compounds have excellent thermal conductivity and volumetric latent heat storage, sometimes double that of organic compounds. Most salt hydrates have problems with super cooling, phase segregation, corrosion, and thermal stability.

#### **3.1 Drawbacks of PCM**

PCM with lower thermal conductivity has a major effect on the efficiency of the device. Relatively large temperature reductions are observed during the energy withdrawal or retrieval process when conductivity values are reduced. Because of this, PCM melting and solidification rate has been slower than predicted, and the deployment of large-scale LHTS units has proved unsuccessful. Incomplete melting/ solidification and a wide temperature disparity inside the PCM are common outcomes of this scenario, and they can contribute to the eventual failure of the material and overheating of the system. Therefore, it is even more crucial to develop certain thermal improvement measures to improve the thermal performance of LHSU.

#### **3.2 Thermal enhancement techniques**

The most studied thermal performance enhancement techniques are as follows: -


#### *3.2.1 Use of extended surface/fins*

Fins implanted in the PCM are the most widely used of all of these approaches. Fin configurations in the PCM-LHTES are classed as longitudinal, circular/annular, plate, and annular/pin fin. The selection was influenced by the benefits of a larger heattransfer surface, simplicity, ease of production, and cheap cost of construction. Fins are utilised in thermal systems to provide greater heat transmission surface. Various academics have looked into the significance of different fin layouts in improving performance in the LHSU.

#### *3.2.2 Micro encapsulation*

Microencapsulated PCMs (MPCMs) may also improve heat transmission between the source and PCM. Microencapsulated PCMs are micro-sized PCMs that are either liquid or solid at the core and are surrounded by a solid shell or wall. There is a vast

range of possible materials that may be used to construct the shell, including synthetic and natural polymers. Such MPCMs can be accomplished using a variety of chemical (e.g., coacervation, complex coacervation, interfacial methods), mechanical, or physical processes (e.g. spray drying method). It is to be anticipated that the thermal performance of MPCMs will outperform that of PCMs that are used traditionally. The heat transmission rate is greater for small PCM particles due to larger heat transfer area per unit volume. There are also advantages to using MPCMs, such as the ability to withstand changes in volume during a phase transition and less PCM reactivity with the container material.

#### *3.2.3 High thermal conductivity nano particles added to the PCM*

In spite of the fact that the impact is heavily dependent on the dispersion of the nanomaterial additions in PCM, carbon nanomaterial additives can enhance the PCM's thermal conductivity. During this process, the nanomaterial is distributed evenly throughout the material, resulting in uniform composite phase change material (CPCM). For the purpose of ensuring that the effectively enhanced thermal conductivity while simultaneously avoiding chemical reactions, the additives need to have a high thermal conductivity and be chemically stable. Metal oxides, carbon nanomaterials such as single- and multi-walled carbon nanotubes, metal nanoparticles, graphite and graphene are commonly used as additives to increase the thermal conductivity of PCM.

#### *3.2.4 Add PCM into the porous metallic foams*

Metal matrices constructed of aluminium, copper, and other metals, as well as naturally occurring porous materials like graphite, can be used to create porous structures. Incorporating high thermal conducting material with a PCM storage system improves heat transfer (latent heat phase). The capillary forces and surface tension in the combination of PCM and ceramic structure keep the molten PCM contained and stabilised inside the micro-porosity of the structure, enabling the use of direct contact heat exchangers. Graphite's higher thermal conductivity, lower density, and chemical resistance make it a promising heat transfer enhancer. The fraction of the composite that is composed of expanded graphite has a significant impact on the effective thermal conductivity of the material, as seen in **Figure 2**. There should be limits placed on the graphite content since increasing it will drive up production costs and reduce the material's volume storage capacity.

#### *3.2.4.1 Closure*

Due to the fact that no one material exists with all the desirable characteristics of an ideal thermal storage media, it is necessary to make do with the material at hand and, if necessary, compensate for its subpar physical characteristics through innovative system design. Thus, selecting a PCM is challenging for researchers. In addition to its usage in the energy storage system, PCMs have a wide range of other potential uses in a variety of fields. A number of researchers have reportedly completed a review on the technical particulars of various PCMs and the applications for which they are used. **Table 1** is a summary of some of the more important uses of PCMs that have been cited by a variety of researchers as having undergone rigorous review.

*Introductory Chapter: Phase Change Material as Energy Storage Substance DOI: http://dx.doi.org/10.5772/intechopen.108863*

#### **Figure 2.**

*Obtained thermal conductivity of PCMs with MgCl2·6H2O/EG for different mass fractions of expanded graphite (EG) [7].*


#### **Table 1.**

*Applications of phase change materials (PCMs).*

Inadequate LHSU performance is caused by the poor heat conductivity of commercially available PCMs, variation in thermo-physical properties after number of thermal cycles, phase separation, volume expansion and high cost [5]. There is a need to devise mechanisms for augmentation in thermal performance of LHSU under these circumstances. Thus, for development of state-of-the-art LHSU, comprehensive investigations are required incorporating the effect of all operating, geometric and design parameters of interest. The results of these analyses can be used as a starting point for further process optimization and the development of design principles.

*Phase Change Materials - Technology and Applications*
