Keywords

1 Introduction

Clean alternative energy, such as wind energy (Abdelkhalig et al. 2022; Cheng et al. 2021) and solar energy, is sustainable, clean source of energy for sustainable cities and communities (AlMallahi et al. 2022; Elgendi et al. 2022b; Obaideen et al. 2021). Solar energy has multiple uses involving photovoltaics, thermal energy, solar heating, and other typical solar energy-harvesting methods. Solar still technology is a convenient and simple method for acquiring potable water from saline water (Elgendi and Selim 2022). Saline water can be fed manually or automatically (Elgendi et al. 2022a). Water evaporates naturally from the salty water surface and flows to the solar still top cover (El-Gendi 2018a, b; El-Gendi and Aly 2017). Although solar still is an affordable and convenient method, its major drawbacks include low efficiency. Several techniques are applied to enhance the solar still performance and increase its efficiency, such as the wick materials (Khalifa 2011), external reflector (Khalifa and Ibrahim 2010), and solar collector (Sampathkumar and Senthilkumar 2012). A major objective in solar still fields is to enhance the distillate production of drinkable water. Solar still distillate yield is dependent on the availability of solar radiation, which is intermittent. The applicability of a solar still system is determined by meteorological parameters such as solar radiation, wind velocity, and ambient temperature. In addition, using nanomaterials in solar still significantly increases its productivity (Elgendi et al. 2021a). Nanomaterials can be mixed with saline water to form a nanofluid or coat the absorber or the cover (Chamsa-ard et al. 2020). Also, it can be doped in phase change materials to improve its thermal characteristics.

2 Bibliometric Analysis

With the growing attention on the solar stills field, a substantial volume of research literature has evolved. Bibliometric approaches are helpful for statistically analyzing large-scale research publications and determining a field’s development state and research performance. The bibliometric analysis has been widely utilized to reveal the state of research in numerous study sectors. For instance, (Kandeal et al. 2022) reviewed recent water harvesting technologies with the help of bibliometric analysis. Similarly, (Ding et al. 2021) discussed the research status and development in the hybrid thermal energy storage systems field. This section presents a comprehensive bibliometric analysis of solar stills and PCM/nanomaterials. The data in this study were obtained from the Web of Science and Scopus databases. The document type and search language were not restricted, the period span is all years from (2010 to 2022) and data analysis was performed on 24 October 2022. A total of 262 were collected and exported for bibliometric analysis as demonstrated in Table 30.1. Figure 30.1 displays the publication trend of solar stills research from 2010 to 2022.

Table 30.1 Main information about the data obtained
Full size table
Fig. 30.1
The line graph illustrates the number of studies on solar stills based on the number of publications for web science and Scopus databases from 2010 to 2022. The trend of the graph is increasing since 2010 and reaches 65 publications in 2022.

Number of studies on solar stills and PCM/nanomaterials that have been published based on web of science and scopus databases

Full size image

Figure 30.2 portrays the primary authors’ collaborative network. The size of the circle symbolizes the author’s number of publications; the connection between the authors reflects the strength of the collaboration. By examining the collaboration network of high-yield authors, we can rapidly grasp the primary research team in the field of solar stills cooperation and identify the research frontier.

Fig. 30.2
An illustration of the collaboration networks of the most productive authors in this field. The representation of N documents 103, 7, 3, 2, 1 are exhibited for the respective countries

Collaboration networks of the most productive authors in this field

Full size image

The collaboration network among countries and the number of publications are demonstrated in Fig. 30.3. It can be shown that significant levels of collaboration occur between countries located in the same geographical area, such as India, Egypt, and neighboring countries.

Fig. 30.3
An illustration of the country collaboration network that exhibits the levels of collaboration that happens between the same geographical areas.

Country collaboration network

Full size image

The number of citations usually represents the academic significance of the article. Highly cited publications demonstrate that this paper’s research material has been acknowledged by other researchers on the same subject and has a high reference value. Table 30.2 lists the top ten most referenced papers on the topic of solar stills and nanomaterials/PCM. The article with the most total citations (222) was Khanafer and Vafai (2018). Correspondingly, Table 30.3 shows the top 8 most repeated words in the search string set. Because many studies’ end goals are applications, it is normal for researchers to be concerned about productivity. Scholars also appear to be concerned about paraffin wax and nanoparticles in this field.

Table 30.2 Top 10 most cited papers in the search string (2010–2022)
Full size table
Table 30.3 Word repetition frequency
Full size table

The three most relevant global affiliations related to publications on solar stills and PCM/nanomaterials are Tanta University (Egypt), Kafrelshiekh University (Egypt), and Anna University (India). This implies that the majority of relevant research on solar stills comes from Middle East and South Asia countries. Table 30.4 demonstrates the most relevant affiliations and countries in this search string.

Table 30.4 Most relevant affiliations and countries
Full size table

Cluster analysis showed that the terms naturally clustered into six clusters, as shown in Fig. 30.4a. Keywords from the same cluster are frequently found in the same body of literature. Each cluster might identify several research hotspots. On the other hand, word clouds are useful for providing an overview of the literature on any topic during a certain time. The magnitude of the term indicates how frequently authors used it in their publications. Figure 30.4b shows a word cloud of the most frequently used keywords selected by authors. Solar stills, heat storage, productivity, nanoparticles, energy efficiencies, and other words were identified.

Fig. 30.4
2 illustrations. A is the network visualization of the keywords such as phase change materials, solar stills, desalination, and much more. B is the word cloud on publication keywords such as solar stills, productivity, desalination, solar energy, and a few other words are exhibited.

a Network visualization of most used keywords, b word cloud on publications related to “solar stills” AND “phase change materials” OR “nanomaterials”

Full size image

The three-field plot in Fig. 30.5 demonstrates the best-performing country, source (journal), and finally research focus. The thicker rectangle is, the greater frequency is. The connecting nodes, inflows, and outflows show more connections and thicker nodes.

Fig. 30.5
An illustration of three field plots of literature in the field of keywords such as solar still, phase change materials, desalination, p c m, paraffin wax, and nanoparticles for the countries such as India, Egypt, Iran, Iraq, U K, Oman, Malaysia, Mexico, Australia, and China.

Three-field plot of literature in the field of solar stills, PCM and nanomaterials, source (SO), keywords (DE), authors’ countries (AU_CO)

Full size image

3 The Main Components of the Solar Still

The solar still has a simple design (Elgendi et al. 2021b). Figure 30.6 shows the primary features of the solar still:

Fig. 30.6
Two 3-D structures of solar still components are illustrated. A. The labeled parts in the structure are class cover, absorber, base, saline water, and insulation. B. The labeled parts in the structure are the Brine outlet, freshwater pipe collector, saline water Inlet, freshwater outlet, glass cover, and base.

Solar still components: (1) cover, (2) saline water, (3) absorber, (4) insulation, and (5) box

Full size image

A transparent cover, usually made from glass, allows the transmission of solar radiation. Also, the distilled water condenses on its inner surface. Nanomaterials can be applied to its inner surface to improve the wettability properties.

Saline water absorbs solar energy to evaporate. Nanomaterials can be added to saline water to improve its thermal characteristics. Therefore, the mixture will be a nanofluid.

Absorbers are usually coated with black paint and made from non-corrosive materials to hold the saline water and absorb solar energy. In addition, nanomaterials can cover the absorber to increase the absorbed energy.

Insulation hinders the dissipation of thermal energy. Phase change materials (PCM), thermal storage materials, can be added between the absorber and insulation. Nanomaterials can dope the PCM to improve its thermal characteristics.

Box or base usually made from wood includes solar still components.

4 Techniques to Enhance the Solar Still Performance

The productivity of the solar still can be affected by meteorological parameters, operational parameters, and design and fabrication parameters. As illustrated in Fig. 30.7, numerous factors impact solar still performance.

Fig. 30.7
The flow diagram illustrates the factors that influence solar still performance, which classifies into meteorological parameters, operating parameters, design, and fabrication factors. And these follow the further classifications for each factor respectively.

Factors influence solar still performance

Full size image

Several studies have been carried out in the literature to improve solar still performance. For example, a single-slope solar basin with a movable aluminum-suspended absorbent plate is proposed (El-Sebaii et al. 2000). The baffle-suspended absorber boosts the productivity of the solar still by 20% because the preheating time for water evaporation is reduced.

Naim and Abd El Kawi (Naim and Abd El Kawi 2003) proposed a solar still system that utilizes charcoal particles as the absorber medium. The effect of charcoal particle range, saline water flow rate, and glass cover inclination on productivity is examined. As a strong absorber medium, the charcoal particles increase solar still productivity by 15%. Correspondingly, a packed sheet mounted at the bottom of the solar still basin optimizes the performance of the solar still (Addel-Rehim and Lasheen 2005). The compressed layer’s substance has superior thermal properties. The packed coating of glass balls aids in heating saline water during the day and night and raises freshwater productivity to 7.5%.

Heat can be stored in the phase change materials as latent heat. For example, heat is applied to a substance to change its phase from solid-state to liquid by preserving the heat as latent heat of fusion or from liquid to vapor by storing the heat as latent heat of vaporization. Energy storage materials (ESM) improve the efficiency of a solar still (Addel-Rehim and Lasheen 2005). The ESM is inserted in the solar still tray during the still assembly process. The combination of paraffin wax, paraffin oil, and water is an energy storage medium.

Jute fabric has a porous surface that improves its absorption property. Furthermore, jute thread is very solid and one of the least expensive natural fibers available (Sakthivel et al. 2010). It also has high insulating characteristics, low heat conductivity, and a limited capacity to recover moisture. In the regenerative solar still, the air-vapor mixture temperature is lowered and becomes lower than that in the traditional still. The latent heat generated by the glass cover is used to evaporate water from the jute cloth. It was discovered that yield potential was increased by 20%, and performance was increased by 8% compared to the traditional solar still.

Moreover, the influence of coated and uncoated metallic wiry sponges, as well as volcanic pebbles, on solar still production was explored. Compared to the traditional solar still, The yield of coated metallic wiry sponges, uncoated metallic wiry sponges, and volcanic rocks is improved by 28%, 43%, and 60%, respectively (Abdallah et al. 2009).

The glass cover inclination parameter mainly influences the efficiency of the solar still. In winter, increasing the inclination raises the yield, and vice versa in the summer (Khalifa 2011). On the other hand, increasing the inclination decreases the evaporation rate in the summer and winter. Moreover, an increased tilt angle increases heat losses from the cover while decreasing yield by increasing reflected radiation from the absorber.

Water quickly evaporates at much lower temperatures by creating a vacuum within the solar still, using less energy than traditional techniques. The productivity of a solar desalination device depends on the solar still pressure (Nassar et al. 2007). The experimental model is an elliptical, metallic container positioned in the focus of the concave reflector. The saline water is subjected to an absolute vacuum pressure of 25 kPa, and in a condenser, the distilled water is collected. Applying vacuum pressure improves the efficiency by 303% and the output ratio by 900% (Nassar et al. 2007). Table 30.5 presents a summary of the papers reviewed and their objectives.

Table 30.5 Objectives of the papers reviewed in brief
Full size table

5 Applications of Nanomaterials in Solar Still

Nanomaterials coat the absorber to increase the thermal conductivity and, thus, the hourly productivity of coated absorber using nano-Fe2O3 particles (Balachandran et al. 2020). The basin water temperature peak is 66 °C for nano-Fe2O3 coated solar still, which is 3 and 5 °C higher than those of micro-coated and conventional solar stills, respectively. The maximum efficiency of nano-Fe2O3 solar still was 68% and 55% for water depths equal to 5 mm and 10 mm, respectively (Balachandran et al. 2020).

Graphene oxide (GO) decreases the melting temperature of paraffin as phase-change materials (PCMs) (Safaei et al. 2019). Paraffin is an organic material and is considered the most futuristic PCMs because of its ideal characteristics, such as remarkable latent heat. Graphene oxide nanomaterial is applied to improve the thermal efficiency of PCM. The absorbed heat transfers to the paraffin/graphene oxide during the daytime. At a temperature of 45 ℃, graphene oxide is dispersed into melted paraffin to maximize the solar still’s yeild. Furthermore, the stored heat in the PCM as latent heat boosts solar efficiency at night. The solar still performance with GO/paraffin solar still has higher productivity by 25% than solar performance with PCM only. The daily yield of solar still was 5 kg/m2 and 6.25 kg/m2 for GO/PCM and PCM solar still, respectively (Safaei et al. 2019).

Because of the outstanding durability and inexpensive cost of oxide nanoparticles, oxide nanofluids are the most often utilized nanofluid in solar still (Elango et al. 2015). Al2O3 is a useful oxide nanoparticle among many others in passive solar stills. Al2O3, SnO2, and ZnO nanofluids had thermal conductivities of 0.63556 W/(m2˖K), 0.6215 W/(m2˖K), and 0.6105 W/(m2˖K), respectively. The performance of the solar still is enhanced by 29.5%, 18.63%, and 12.67%, by Al2O3, SnO2, and ZnO nanofluids, respectively.

Black paint blended with titanium oxide nanoparticles is applied to the absorber of a solar still, a 0.25 m2 mild steel basin formed into an equilateral triangle (Kabeel et al. 2019). To stop escaping heat to the outside environment, insulation was added on the bottom and sidewalls of the solar still. Black paint called TiO2 nanoparticles raises water temperature by 1.5 °C compared to an absorber without them. As a result, TiO2 nanoparticles enhance the solar still’s production by 12% at the maximum water depth. For the absorber plate with and without a TiO2 nanoparticle coating, the cumulative yield from the solar still is 6.6 and 6.2 kg/m2, respectively (Kabeel et al. 2019).

The standard solar still was modified using flake graphite nanoparticles (FGN), phase change material (PCM), and film cooling (Sharshir et al. 2017). The still’s productivity is 50.28%, 65%, 56.15%, and 73.8%, with FGN (modification A), FGN and PCM (modification B), FGN and film cooling (modification C), and FGN, PCM, and film cooling (modification D), respectively. As the water depth grows, the solar still performance of modificatoins A and B declines (Sharshir et al. 2017).

6 Conclusions

In this chapter, the main elements of the solar still are illustrated and the most common solar still systems are summarized and compared. The importance of utilizing nanotechnology to improve solar still efficiency using different techniques is discussed. Nanomaterials and saline water can be combined to create nanofluids. Some techniques include coating the absorber, coating the cover, or dispersing PCM.

The suggestions for future work can be summarized as follows:

  • Combine two or three nanomaterials

  • Combine new PCM with nanomaterials

  • Coat the glass cover to decrease the temperature (previous research affects the wettability)

  • Combine a nanomaterial and a sensible storage material (previous with PCM)

  • Investigate the effect of nanomaterial morphology

  • Coat both the absorber and glass.