**1. Introduction**

In recent years the energy shortage and water pollution have been rising around the world. With the rapid increase in the level of greenhouse gas emissions, the discovery of alternative sources of energy is increasingly gaining importance for the development of a sustainable world. The rising oil price and environmental regulations have dramatically increased the demand for utilizing alternative power sources [1]. In 2015 WHO and UNICEF reported that around 663 million people still use non-potable drinking water. During the emergency conditions, the need for developing efficient and portable techniques to obtain a clean source of water is paramount. One such method is solar distillation using phase change material.

In the twenty-first century, the impact of energy and water on the socioeconomic development of developed and developing nations is significant [1]. Solar energy is one of many renewable energy sources to obtain stable thermal energy

future generations [2]. The process of distillation can be used to get fresh water from brackish or contaminated water. Water is available in different forms, such as seawater, underground water, surface water, and atmospheric water. Clean water is essential for good health. These current conditions serve as a motivational factor for the research conducted, to effectively use Phase Change Material for optimum solar distillation to desalinate the water, abundant in situ around Udupi (near the Indian Ocean) [3].

Single/double-slope solar still is a popular solar device used for converting available brackish or wastewater into potable water. Solar still absorbs the thermal energy solar radiation to distillate polluted water into potable water in an enclosed space—still. The principles of heat transfer and energy balance were the governing equations for the operation of single-slope solar still. Because of its lower productivity, it is not popularly used. Numbers of works are undertaken to improve the productivity and efficiency of the solar still [2, 4].

Several PCMs melt and solidify at an array of temperatures, thus creating a focus on various possible applications. These PCMs are applied for numerous thermal storage systems utilizing latent heat, applications in heat pumps, engineering using solar radiation, and space travel. PCMs have been used for heating and cooling for many years, and the study in this regard has been attracting attention since the past decade. The pragmatic results reckoned in the field of water distillation process with the help of solar energy in the presence of energy storage materials like water and MOFs [2, 5].

Solar still is a latent heat storage system, which uses phase change materials (PCMs). Using PCM is an impactful way of storing thermal energy and has benefits in terms of high-energy storage density and the isothermal nature of the storage process. PCMs have been widely used in latent heat thermal-storage systems for heat pumps, solar engineering, and spacecraft thermal control applications. There are large numbers of PCMs that melt and solidify at a wide range of temperatures, making them attractive in a number of applications [6].

The desalination can provide a 24-hour supply of heat and water in greenhouse-based agricultural projects [7]. In another unsteady state modeling and simulation approach, El-Sebaii and his co-authors presented the transient mathematical models for a single-slope single-basin solar still with and without phase change material under the basin liner [3, 8]. They used stearic acid as PCM and used a computer-based simulation procedure to obtain a better insight of temperatures of the still elements and the PCM. The data were correlated using summer and winter day's temperature data in Jeddah, Saudi Arabia. It was observed that during phase change (liquid to stable) of PCM, the convective heat transfer coefficient from the basin liner to basin water is doubled; thus, the evaporative heat transfer coefficient is increased by 27% upon using 3.3-cm layer of stearic acid beneath the basin liner. Dashtban [9] used paraffin wax as PCM in their theoretical study of PCM-based weir-type cascade solar still. It was expected to obtain enhanced productivity by using PCM, which helps in keeping the temperature of basin high enough to produce the distilled water without interruption, especially after sunset [10]. In this study, water desalination and hot water production using solar still involving PCM are theoretically investigated. The numerical approach is presented to study the performance of desalination units—with and—without phase change materials. The effect of the PCM on the productivity expressed as the amount of water produced is theoretically studied. The following parameters and their effects were theoretically investigated: the type of the PCM, melting point of PCM, solar irradiation. It is hoped to determine the optimum parameter that will result in higher unit productivity. The purpose of a solar distillation system is to clean or purify water within the permissible limit [11, 12]. Besides the

**171**

*Water Desalination Using PCM to Store Solar Energy DOI: http://dx.doi.org/10.5772/intechopen.92597*

energy source [13].

natural process.

in a solar distillation system [6].

cancer, including India, Mexico, and UAE.

residue salt [16]. The 2-cm increment is to study the effect.

problem of water shortage, process energy constitutes another problem area. Due to the high cost of conventional energy sources, which are also environmentally harmful, renewable energy sources have gained more attraction since their use in distillation plants will save conventional energy for other applications, reduce environmental pollution and provide a free, continuous, and low maintenance

The objective of this thesis is to study how solar distillation is used by nature to produce rain, which is the primary source of freshwater supply and replicate the process using knowledge of engineering. Solar radiation falling on the surface of the sea is absorbed as heat and causes evaporation of the water. The vapor rises above the surface and is moved by winds [7]. When this vapor cools down to its dew point, condensation occurs, and freshwater precipitates as rain. All available artificial distillation systems are small-scale duplications of this

Solar distillation differs from other forms of desalination that are more energyintensive, such as methods such as reverse osmosis, or simply boiling water due to its use of free energy. A very common and, by far, the most significant example of

The novelty of the present study is an in-depth and multifaceted comparison of two solar distillation methods, that is, one utilizing PCM to discover an effective way to bring about solar distillation and another technique reflecting the natural phenomenon of distillation. Two methods can accompany the distillation of water using solar energy. The first method utilizes the so-called greenhouse effect to evaporate saltwater enclosed in a simple solar still (direct collection). A typical basin type containing the saline water is covered with a transparent airtight top. The top is mounted sloping downward toward sweet water collecting troughs. Solar energy is absorbed by the basin, causing the water to evaporate and condense on the inside of the cover and slides down into the collecting trough. The second method or indirect collection system often involves more than one subsystem, one for collecting and another for energy storage and the third system for energy utilization in the distillation process, multi-stage flash evaporation mar offer good potential when utilized

To emphasize the scope of this research, consider rural areas in and around Udupi district and several such underdeveloped regions around the tropic of cancer. In these areas, the solar distillation process yields drinking water tantamount to the process of rain generation in the water cycle. The solar radiation causes water to evaporate, segregating the water vapor from salt or impurities. The vapor formed from the process of evaporation condenses on the still for collection [14]. The fundamental operation is evaporation. As the temperature of water increases, vaporization starts at the surface of the liquid. The vapor then rises from the surface of the water and gets condensed on the top cover. The condensed vapor is free from minerals and impurities and thus separated through some distillate channel [15]. The application of finding from this thesis encompasses solar energy applications, primarily supported by government initiatives in the countries along tropic of

We are choosing two heights—5 and 7 cm— of PCM to understand and relate this distillation over real application in salt pans. The average height of humanmade salt pans near the coast of Karnataka is in the range of 25 cm. Since the height of PCM is less than the height of the salt pan, its implementation is possible. If implemented, this method of distillation can obtain not only potable water but also

**Figure 1** below represents the solar still to be designed as intended, upon which

experimental work has been performed to yield the results as presented below.

solar distillation is the natural water cycle that the Earth experiences [4].

#### *Water Desalination Using PCM to Store Solar Energy DOI: http://dx.doi.org/10.5772/intechopen.92597*

*Thermodynamics and Energy Engineering*

productivity and efficiency of the solar still [2, 4].

making them attractive in a number of applications [6].

Ocean) [3].

and MOFs [2, 5].

future generations [2]. The process of distillation can be used to get fresh water from brackish or contaminated water. Water is available in different forms, such as seawater, underground water, surface water, and atmospheric water. Clean water is essential for good health. These current conditions serve as a motivational factor for the research conducted, to effectively use Phase Change Material for optimum solar distillation to desalinate the water, abundant in situ around Udupi (near the Indian

Single/double-slope solar still is a popular solar device used for converting available brackish or wastewater into potable water. Solar still absorbs the thermal energy solar radiation to distillate polluted water into potable water in an enclosed space—still. The principles of heat transfer and energy balance were the governing equations for the operation of single-slope solar still. Because of its lower productivity, it is not popularly used. Numbers of works are undertaken to improve the

Several PCMs melt and solidify at an array of temperatures, thus creating a focus

on various possible applications. These PCMs are applied for numerous thermal storage systems utilizing latent heat, applications in heat pumps, engineering using solar radiation, and space travel. PCMs have been used for heating and cooling for many years, and the study in this regard has been attracting attention since the past decade. The pragmatic results reckoned in the field of water distillation process with the help of solar energy in the presence of energy storage materials like water

Solar still is a latent heat storage system, which uses phase change materials (PCMs). Using PCM is an impactful way of storing thermal energy and has benefits in terms of high-energy storage density and the isothermal nature of the storage process. PCMs have been widely used in latent heat thermal-storage systems for heat pumps, solar engineering, and spacecraft thermal control applications. There are large numbers of PCMs that melt and solidify at a wide range of temperatures,

The desalination can provide a 24-hour supply of heat and water in greenhouse-based agricultural projects [7]. In another unsteady state modeling and simulation approach, El-Sebaii and his co-authors presented the transient mathematical models for a single-slope single-basin solar still with and without phase change material under the basin liner [3, 8]. They used stearic acid as PCM and used a computer-based simulation procedure to obtain a better insight of temperatures of the still elements and the PCM. The data were correlated using summer and winter day's temperature data in Jeddah, Saudi Arabia. It was observed that during phase change (liquid to stable) of PCM, the convective heat transfer coefficient from the basin liner to basin water is doubled; thus, the evaporative heat transfer coefficient is increased by 27% upon using 3.3-cm layer of stearic acid beneath the basin liner. Dashtban [9] used paraffin wax as PCM in their theoretical study of PCM-based weir-type cascade solar still. It was expected to obtain enhanced productivity by using PCM, which helps in keeping the temperature of basin high enough to produce the distilled water without interruption, especially after sunset [10]. In this study, water desalination and hot water production using solar still involving PCM are theoretically investigated. The numerical approach is presented to study the performance of desalination units—with and—without phase change materials. The effect of the PCM on the productivity expressed as the amount of water produced is theoretically studied. The following parameters and their effects were theoretically investigated: the type of the PCM, melting point of PCM, solar irradiation. It is hoped to determine the optimum parameter that will result in higher unit productivity. The purpose of a solar distillation system is to clean or purify water within the permissible limit [11, 12]. Besides the

**170**

problem of water shortage, process energy constitutes another problem area. Due to the high cost of conventional energy sources, which are also environmentally harmful, renewable energy sources have gained more attraction since their use in distillation plants will save conventional energy for other applications, reduce environmental pollution and provide a free, continuous, and low maintenance energy source [13].

The objective of this thesis is to study how solar distillation is used by nature to produce rain, which is the primary source of freshwater supply and replicate the process using knowledge of engineering. Solar radiation falling on the surface of the sea is absorbed as heat and causes evaporation of the water. The vapor rises above the surface and is moved by winds [7]. When this vapor cools down to its dew point, condensation occurs, and freshwater precipitates as rain. All available artificial distillation systems are small-scale duplications of this natural process.

Solar distillation differs from other forms of desalination that are more energyintensive, such as methods such as reverse osmosis, or simply boiling water due to its use of free energy. A very common and, by far, the most significant example of solar distillation is the natural water cycle that the Earth experiences [4].

The novelty of the present study is an in-depth and multifaceted comparison of two solar distillation methods, that is, one utilizing PCM to discover an effective way to bring about solar distillation and another technique reflecting the natural phenomenon of distillation. Two methods can accompany the distillation of water using solar energy. The first method utilizes the so-called greenhouse effect to evaporate saltwater enclosed in a simple solar still (direct collection). A typical basin type containing the saline water is covered with a transparent airtight top. The top is mounted sloping downward toward sweet water collecting troughs. Solar energy is absorbed by the basin, causing the water to evaporate and condense on the inside of the cover and slides down into the collecting trough. The second method or indirect collection system often involves more than one subsystem, one for collecting and another for energy storage and the third system for energy utilization in the distillation process, multi-stage flash evaporation mar offer good potential when utilized in a solar distillation system [6].

To emphasize the scope of this research, consider rural areas in and around Udupi district and several such underdeveloped regions around the tropic of cancer. In these areas, the solar distillation process yields drinking water tantamount to the process of rain generation in the water cycle. The solar radiation causes water to evaporate, segregating the water vapor from salt or impurities. The vapor formed from the process of evaporation condenses on the still for collection [14]. The fundamental operation is evaporation. As the temperature of water increases, vaporization starts at the surface of the liquid. The vapor then rises from the surface of the water and gets condensed on the top cover. The condensed vapor is free from minerals and impurities and thus separated through some distillate channel [15]. The application of finding from this thesis encompasses solar energy applications, primarily supported by government initiatives in the countries along tropic of cancer, including India, Mexico, and UAE.

We are choosing two heights—5 and 7 cm— of PCM to understand and relate this distillation over real application in salt pans. The average height of humanmade salt pans near the coast of Karnataka is in the range of 25 cm. Since the height of PCM is less than the height of the salt pan, its implementation is possible. If implemented, this method of distillation can obtain not only potable water but also residue salt [16]. The 2-cm increment is to study the effect.

**Figure 1** below represents the solar still to be designed as intended, upon which experimental work has been performed to yield the results as presented below.

**Figure 2** represents the schematic of that implementation. The angle of Glass Cover is kept at 32° following the laws of reflection and refraction, to explain that let us consider Snell's law of refraction and law of reflection.

**Figure 1** schematic is a focused view of glass cover in **Figure 2**, say μ2 = 1.003 and μ1 = 1.517 are refractive indices of air and glass cover [17];

**we have,**

$$
\mu\_1 \star \text{Sin } \theta\_2 = \mu\_2 \star \text{Sin } \theta\_1 \tag{1}
$$

And so assuming all rays are incoming perpendicular, thus,

$$
\mathfrak{S}\mathfrak{in}\mathfrak{\theta}\_1 \, = \, \mathbf{1},
$$

we get

$$\theta\_2 \quad = \text{ Sin}^{-1}(\mu\_2/\mu\_1) \quad = \text{ 41.14}^o,\tag{2}$$

which is the ultimate critical angle of the glass cover, and the angle we chose is 32° signifying maximum refraction [4].

Giving as of **Figure 1**, say we have θ1 = 32° and θ2 = 58°

**Figure 1.**

*Schematic of glass cover and sunrays.*

**173**

**Figure 3.**

*enthalpy [15].*

*Water Desalination Using PCM to Store Solar Energy DOI: http://dx.doi.org/10.5772/intechopen.92597*

similarities between the two compounds [20].

and being researched upon [14].

**2.2 Experimental synopsis**

*2.2.1 Experimental setup*

**Figure 1**, and the following phase change material was used:

**Figure 3** shows PCM classified according to their commonalities as per the melting point and the enthalpy of fusion. It follows that the two vital characteristics of phase change material, relating to their semantics "phase" and "change," are derivates of temperature and heat released during the phenomenon of phase change [18, 19].

In this pragmatic study, the experimental setup is similar to the one described in

• Water and water solutions with eutectic compositions are used below the triple

As we can observe in **Figure 2**, both of these compounds are on the left corner of the graph with melting temperature near zero or below zero and enthalpy of fusion

As far as the solar distillation goes, the following **Figure 4** summarizes the various substances used for an optimized solar distillation setup. Each material novel

Double-slope solar still shown schematically in **Figure 5** was used to conduct the experiments [21]. Concerning **Figures 1** and **2**, we have designed this schematic

*Classes of materials that can be used as PCM and their typical range of melting temperature and melting* 

. Hence, the comparison is rather challenging owning to the

**2. Methodology**

point [5].

around 300 MJ/m3

• Phenol.

**2.1 Materials**

**Figure 2.** *Solar distillation still with PCM: schematic [4].*
