**3.1 Organic PCMs**

The organic phase change compounds are chemically stable, no super cooling, non-corrosive and nontoxic. Organic PCMs are subdivided in two groups (i) Paraffins (ii) Non paraffins. Paraffins are chemically inert, have low thermal conductivity and large volume change. The non paraffin's such as fatty acids have high heat of fusion than paraffin and small volume change.

### **3.2 Inorganic PCMs**

Inorganic PCMs have high heat of fusion, good thermal conductivity, are cheap and non-flammable. Most of them are corrosive to metals. Most inorganic PCMs are hydrated salt. Hydrated salts have a high energy density and high thermal conductivity. Disadvantage is that undergoes super cooling.

#### **3.3 Eutectic mixture**

Eutectic mixture is a mixing of more than one PCM material. Eutectic mixtures have sharp melting point and energy density is slightly higher than that of organic PCMs. Eutectics are divided in three groups (i) Organic – Organic (ii) Inorganic – Inorganic (iii) Organic – Inorganic [21]. The desired temperature range of eutectic mixture for solar energy applications can be designed according to Schroder's Eq. (3) [22].

$$\ln\,\,\mathbf{X}\_{\rm A} = \frac{\Delta H\_A}{R} \left(\frac{1}{T} - \frac{1}{T\_f}\right) \tag{4}$$

Where XA and ΔHA are the molar fraction and latent heat of fusion kJ/kg of compound A, respectively. T and Tf are the melting temperature °C of the mixture and compound A. R is gas factor 0.8314 kJ/K. mol.

## **3.4 Bio-PCM**

Bio-PCM is bio based materials which are derived from organic - based materials. It is less flammable than the commercial available PCMs. According to various

**Figure 4.** *Classifications of PCMs [24].*


#### **Table 1.**

*Differentiate between raw PCMs.*

weather conditions, the bio-PCM can be prepared from - 22.7°C to 78.33°C. These materials are wraps in sheets as bubble. Bio-PCM has superior thermal properties such as specific heat and latent heat of fusion [23]. Classified PCMs are applying into different fields such as passive systems and active systems (**Figure 4**).

The active systems are waste heat recovery, solar water heater, desalination etc. The passive systems are directly added into the building components such as gypsum, mortar, concrete and brick. These two systems are employing in buildings to reduce energy burden in the buildings. Buildings are more responsible 40% of total energy consumption.

Comparison of different PCMS for renewable storage applications have provided in **Table 1** which helps for selection PCMs for thermal energy storage.
