**Abstract**

Solar energy is utilizing in diverse thermal storage applications around the world. To store renewable energy, superior thermal properties of advanced materials such as phase change materials are essentially required to enhance maximum utilization of solar energy and for improvement of energy and exergy efficiency of the solar absorbing system. This chapter deals with basics of phase change material which reflects, selection criteria, PCM works, distinguish thermal energy storage system, commercially available PCM, development of PCM thermal properties and durability of PCM. In addition to this chapter focused on PCM in solar water heating system for buildings particularly in India because 20–30% of electricity is used for hot water in urban households, residential and institutional buildings. Discussed Flat plate collectors (FTC) in detail which is suitable for warm water production in household temperature 55 to 70 °C owing to cost effective than the Evacuated Tube collectors (ETC), Concentrated collector (CC) and integration of different methods PCM in solar water heating system.

**Keywords:** Phase change materials, Advanced materials, Solar energy, Renewable energy

#### **1. Introduction**

Renewable energy is a free energy that can impact between energy supply and energy demand. One of the prominent renewable source is solar energy among the wind, rain, waves, tides and geothermal energy [1]. Most of countries receives 5 x 10<sup>15</sup> kWh per annum i.e. incident mean solar energy in between 4 and 7 kWh per m<sup>2</sup> [2]. This can be accomplished in different solar energy fields such as solar water heating systems, desalination, solar-thermal collectors, building heating and daylighting and Photovoltaic (PV) Cells etc. (**Figure 1**) [3]. Technologists and researchers are trying to utilize more renewable energy for distinguish devices/ systems to decrease global energy crisis [4]. Thermal energy storage (TES) systems may assist the renewable energy exploitation for reduction of Green House gases (GHGs) and depletion of fossil fuels [5]. It plays vital role into energy conversation of free energy to reduce energy consumption [6]. TES can be stored in form of sensible heat, latent heat and thermochemical energy [7].

Sensible heat refers to the amount of energy is absorbed without phase change i.e. Solid–solid, liquid –liquid and gas to gas and latent heat refer to the amount of

#### **Figure 1.** *Application of solar energy in different fields.*

energy is absorbed with phase change i.e. solid to liquid, liquid to gas and solid to gas. Thermal chemical energy is energy stored during chemical reaction occurred not only at desired temperature range but also should be reversible reaction and Solid to liquid thermodynamically feasible to solar energy applications [8]. Solid to liquid materials are phase change materials (PCMs) and has the potential to store the energy at constant temperature owing to energy density per unit volume (**Figure 2**) [9].

Avargani et al. [10] installed two consecutive solar collectors with encapsulated paraffin phase change material. The single collector can produce hot water at the temperature of 60°C for 7 hour from midway to midnight. Fazilati and Alemrajabi [11] investigated the effect of PCM in solar water heater. The energy storage density was increased up to 39% and supply of hot water increased 25% as compared to the without PCM. Biwole et al. [12] PCM installed at the back of the solar collector. The solar collector was simulated using CFD model and compared with testing results. Added PCM to the back of solar collector can maintain the hot water temperature under 40°C for 80 min with the constant solar radiation of 1000 W/m<sup>2</sup> . Hasan et al. [13–15] have incorporated different types fatty acids in domestic water heating system. The fatty acids such as myristic acid, palmitic acid and stearic acid, with phase transition between 50–70°C are the most promising PCMs for solar water heating. Manirathnam et al. [16] prepared nano-composite such paraffin wax as PCM with one per cent of Sci and CuO. Nano-composite, PCM and without PCM were studied in evacuated tube solar water heater for thermal energy storage. The energy efficiencies for distinguish cases were found to be 33.8%, 38.3%, and 41.7%, respectively corresponding to without PCM, PCM and nano-composite

*Phase Change Materials for Renewable Energy Storage Applications DOI: http://dx.doi.org/10.5772/intechopen.98914*

respectively. Xie et al. [17] prepared cost effective and eco-friendly form shape stabilized stearic acid with coconut shell. The thermal properties of SA/CSC15 composite were 76.69 J g�<sup>1</sup> and 52.52°C, respectively. The SA/CSC composite has potential for solar water heater energy storage.

This book chapter deals with basics of phase change materials and briefly discussed about selection criteria of PCMs. How these phase change materials are effective for solar water heater domestic uses as well as explained how low thermal conductivity of PCMs can be enhanced using supporting materials to increase efficiency of solar systems for thermal energy storage.

#### **1.1 Working principle of phase change materials**

When surroundings temperature above the PCM melting point, the PCM becomes phase change from solid to liquid and absorbs the heat from water storage tank during night, when surroundings temperature below the PCM melting point, the PCM desorbs heat to ambient/water storage tank, during the material changes phase from liquid to solid. The PCMs are being successfully used as energy storage devices such as heat pumps, solar engineering, space craft etc. [18].

When energy continues from the source, then PCM temperature rises from initial temperature (T1) (K) to final temperature (T2) (K) and during this period energy is riveted due to the sensible heat i.e. solid to solid [19]. The sensible heat can be calculated as per following Eq. (1)

$$\mathbf{Q}\_{\text{sensible heat}} = \mathbf{m}.\mathbf{C}\_{\text{ps}}.(\mathbf{T}\_2 - \mathbf{T}\_1) \tag{1}$$

where Q is the amount of energy stored in the material (J), m is the mas of storage material (kg), Cps is the specific heat of the storage material of solid state (J/kg�K). From temperature (T2) (K), the heat is continuously absorbed until the solid turns into the liquid due to the latent heat. The latent heat of system can be determined as per the following Eq. (2)

$$\mathbf{Q}\_{\text{latent heat}} = \mathbf{m}.\Delta\mathbf{h} \tag{2}$$

**Figure 3.** *Working of phase change material.*

where Q is the amount of heat stored in the material (kJ), m is the mas of storage material (kg), and Δh is the phase change enthalpy (kJ/kg). Further, heat continues heat will be absorbed due to liquid to liquid. It means that, the amount of phase change materials need to be designed as per the application (**Figure 3**) [20].

$$\begin{array}{c} \text{Total amount energy stored by PCM (Q)} = \text{Q}\_{\text{sensible heat}} + \text{Q}\_{\text{latent heat}}\\ + \text{ m C}\_{\text{p}} \text{l } (\text{T}\_{\text{1}} - \text{T}\_{\text{2}}) \end{array} \tag{3}$$

Cpl is the specific heat of the storage material of liquid state (J/kg�K).
