*2.2. Recent Findings on SDCs Integrated with Solar Stills*

### 2.2.1. SDCs Integrated with Solar Stills and the Sun-Tracking System (STS)

The design and installation of an SDC that is integrated with a mini single-slope, air-tight solar still is called a 'modified receiver' (boiler) (Figure 4). This approach was presented and tested in a study [80] for the purpose of brackish water desalination under the climate in Egypt, in which the SDC performance was compared experimentally with the performance of a simple CSS. The dish-shaped concentrator (made of aluminum as a point-focus collector with an aperture diameter, depth, and focal length of 100, 20, and 40 cm, respectively) was selected and covered with highly reflective glass mirror strips (with 0.004 m of thickness) to reflect the intensity of the incoming solar insolation into the boiler located at the dish focal point. A tracking system was applied to the SDC to track the sun on two axes by using a 36 VDC tracking motor to move the SDC into the calculated positions. This was carried out throughout the day to maintain the focus of the sun's rays to the boiler in improving its water temperature, thermal efficiency, and distillate yield (Figure 4) [80]. The whole tracking system was powered by a 15 W amorphous silicon solar photovoltaic module, charge controller, battery, and inverter (Figure 4) [80]. Brackish water was preheated by a black hose which was exposed to solar irradiation throughout the day and supplied to both trapezoidal-shaped boiler and CSS. The boiler with a basin surface area of 0.046 m<sup>2</sup> had small dimensions (with the length, width, height from back, and front sides of 27, 16, 17, and 12 cm, respectively) to admit most of the reflected sunlight. The trapezoidal-shaped CSS with the basin area of 0.5 m<sup>2</sup> (with the high side and low side basin walls of 44 and 15 cm, respectively) was covered with a 30◦ inclined glass sheet (Figure 4). The experiments were conducted for nine hours, from 9:00 a.m. to 6:00 p.m. All of the boiler surfaces which were exposed to the sunlight (mainly UV) received some radiation from above as direct solar radiation, and most radiation of the sun's rays was reflected from the SDC surface to the sides and bottom surface of the boiler. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 10 of 28

**Figure 4.** Experimental set-up photograph of the CSS and the solar still coupled with an SDC under Egypt's climate [80]. **Figure 4.** Experimental set-up photograph of the CSS and the solar still coupled with an SDC under Egypt's climate [80].

This process appeared to prevent the growth of bacteria and waterborne pathogens that could cause contamination of the distilled water [58–60] throughout the experiment. This process appeared to prevent the growth of bacteria and waterborne pathogens that could cause contamination of the distilled water [58–60] throughout the experiment.

However, this process is not completely practical in any other types of passive and active solar stills (CSS) which involve early experimental hours (usually in the morning) (Figure

throughout the experiment (Figure 5). This occurred due to the additional concentrated sun's rays hitting the sides and bottom of the boiler as received from the SDC, which were about 230 w/m2 at 9:00 a.m. in the first hour of the experiment, 100 w/m2 at 12:00 p.m., and 600 w/m2 at 5:00 p.m. in the day (Figure 5); it was also because the direct solar radiation intensity received from the top condensing surface of the boiler [80]. Meanwhile, the CSS only received direct intensities of solar radiation from its condensing cover which is located at the top (Figure 4). As can be observed from Figure 5, the starting (initial) brackish water temperature of the boiler, which was integrated with the SDC without preheating by the black hose, immediately reached 70 °C due to absorbing additional rate of solar intensities as reflected from the SDC. This rate was much higher than the temperature of the initial basin water of the CSS (which was recorded at about 45 °C) and solar stills in other studies [30,31,42–54,63–73], as presented in Table 2. The boiler water temperature increased to above 80 °C within an hour, reached approximately 105 °C at 11:00 a.m., and then continued to generate steam with a temperature of 105 °C for a three-and-a-half hour period until 2:30 p.m. Meanwhile, the maximum basin water temperature of the CSS was recorded at 63 °C at 12:00 p.m. (Figure 5) under similar climatic conditions. The produced steam from the boiler moved to a cylindrical tank, referred to as the 'condenser unit', that was filled with cold water which would be con-

verted to fresh water droplets and collected in a graded container (Figure 4) [80].

However, this process is not completely practical in any other types of passive and active solar stills (CSS) which involve early experimental hours (usually in the morning) (Figure 5) [36]. The study results also showed that the water temperatures in the boiler were approximately 36–42 ◦C higher than the water temperatures in the basin of the CSS throughout the experiment (Figure 5). This occurred due to the additional concentrated sun's rays hitting the sides and bottom of the boiler as received from the SDC, which were about 230 w/m<sup>2</sup> at 9:00 a.m. in the first hour of the experiment, 100 w/m<sup>2</sup> at 12:00 p.m., and 600 w/m<sup>2</sup> at 5:00 p.m. in the day (Figure 5); it was also because the direct solar radiation intensity received from the top condensing surface of the boiler [80]. Meanwhile, the CSS only received direct intensities of solar radiation from its condensing cover which is located at the top (Figure 4). As can be observed from Figure 5, the starting (initial) brackish water temperature of the boiler, which was integrated with the SDC without preheating by the black hose, immediately reached 70 ◦C due to absorbing additional rate of solar intensities as reflected from the SDC. This rate was much higher than the temperature of the initial basin water of the CSS (which was recorded at about 45 ◦C) and solar stills in other studies [30,31,42–54,63–73], as presented in Table 2. The boiler water temperature increased to above 80 ◦C within an hour, reached approximately 105 ◦C at 11:00 a.m., and then continued to generate steam with a temperature of 105 ◦C for a three-and-a-half hour period until 2:30 p.m. Meanwhile, the maximum basin water temperature of the CSS was recorded at 63 ◦C at 12:00 p.m. (Figure 5) under similar climatic conditions. The produced steam from the boiler moved to a cylindrical tank, referred to as the 'condenser unit', that was filled with cold water which would be converted to fresh water droplets and collected in a graded container (Figure 4) [80]. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 11 of 28

**Figure 5.** Diurnal hourly variations of direct and reflected intensities of solar radiation and water temperatures in the boiler and CSS and ambient temperature without pre-heating technique [80]. **Figure 5.** Diurnal hourly variations of direct and reflected intensities of solar radiation and water temperatures in the boiler and CSS and ambient temperature without pre-heating technique [80].

The study results showed that by integrating the SDC and small-scale solar still (boiler), the water temperature in the boiler drastically increased to above 70, 80, and 105 °C in the early experimental hours. This approach is seen as the most effective in removing any available waterborne pathogens, bacteria, and viruses—particularly SARS-CoV-2— from the boiler water and produced vapors while preventing the transmission of those impurities into the distillate (Figure 5). This is because the range of temperatures from 50 °C to 70 °C is the limit of the viability of waterborne pathogens and novel coronavirus, as recommended by several studies [74,75]. It was also stated in the study that the increased temperature of brackish water in the boiler by the SDC enhanced the amount of daily fresh water production, ranging from 0.65 and 0.55 L/h at 11:00 a.m. and 4:00 p.m., respectively (Figure 6) [80]. The boiler with SDC produced maximum water of 6.7 L/m2 in a nine-hour period, while the CSS produced only 1.5 L/0.5 m2 within The study results showed that by integrating the SDC and small-scale solar still (boiler), the water temperature in the boiler drastically increased to above 70, 80, and 105 ◦C in the early experimental hours. This approach is seen as the most effective in removing any available waterborne pathogens, bacteria, and viruses—particularly SARS-CoV-2—from the boiler water and produced vapors while preventing the transmission of those impurities into the distillate (Figure 5). This is because the range of temperatures from 50 ◦C to 70 ◦C is the limit of the viability of waterborne pathogens and novel coronavirus, as recommended by several studies [74,75]. It was also stated in the study that the increased temperature of brackish water in the boiler by the SDC enhanced the amount of daily fresh water production, ranging from 0.65 and 0.55 L/h at 11:00 a.m. and 4:00 p.m., respectively (Figure 6) [80]. The boiler with SDC produced maximum water of 6.7 L/m<sup>2</sup> in a nine-hour period, while the CSS produced only 1.5 L/0.5 m<sup>2</sup> within the same period [80].

**Figure 6.** Hourly values of distilled water production of CSS and the boiler coupled with SDC

dissolved into each kg of water sample before being fed into the boiler [81].

In another study, a stand-alone point-focus parabolic solar still (PPSS) coupled with the sun-tracking system and a small-scale passive solar still as absorber or (boiler), was designed and fabricated for purification of the seawater and brackish water in Tehran, Iran (Figure 7) [81]. Salt with different masses (from 10 g to 40 g, with 5 g intervals) was

the same period [80].

without the preheating method [80].

the same period [80].

**Figure 5.** Diurnal hourly variations of direct and reflected intensities of solar radiation and water temperatures in the boiler and CSS and ambient temperature without pre-heating technique [80].

The study results showed that by integrating the SDC and small-scale solar still (boiler), the water temperature in the boiler drastically increased to above 70, 80, and 105 °C in the early experimental hours. This approach is seen as the most effective in removing any available waterborne pathogens, bacteria, and viruses—particularly SARS-CoV-2— from the boiler water and produced vapors while preventing the transmission of those impurities into the distillate (Figure 5). This is because the range of temperatures from 50 °C to 70 °C is the limit of the viability of waterborne pathogens and novel coronavirus, as recommended by several studies [74,75]. It was also stated in the study that the increased temperature of brackish water in the boiler by the SDC enhanced the amount of daily fresh water production, ranging from 0.65 and 0.55 L/h at 11:00 a.m. and 4:00 p.m., respectively (Figure 6) [80]. The boiler with SDC produced maximum water of 6.7 L/m2 in a nine-hour period, while the CSS produced only 1.5 L/0.5 m2 within

In another study, a stand-alone point-focus parabolic solar still (PPSS) coupled with the sun-tracking system and a small-scale passive solar still as absorber or (boiler), was designed and fabricated for purification of the seawater and brackish water in Tehran, Iran (Figure 7) [81]. Salt with different masses (from 10 g to 40 g, with 5 g intervals) was dissolved into each kg of water sample before being fed into the boiler [81]. In another study, a stand-alone point-focus parabolic solar still (PPSS) coupled with the sun-tracking system and a small-scale passive solar still as absorber or (boiler), was designed and fabricated for purification of the seawater and brackish water in Tehran, Iran (Figure 7) [81]. Salt with different masses (from 10 g to 40 g, with 5 g intervals) was dissolved into each kg of water sample before being fed into the boiler [81]. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 12 of 28

**Figure 7.** Photograph of the experimental set-up of an auto sun-tracking system SDC-solar still in Tehran, Iran[81]. **Figure 7.** Photograph of the experimental set-up of an auto sun-tracking system SDC-solar still in Tehran, Iran [81].

The above-developed stand-alone system (PPSS) is comprised of several items, including a parabolic dish concentrator, a boiler mounted at the focal point of the dish collector, two plate heat exchangers (to condense the steam generated in the boiler and increase the brackish water temperature before entering the boiler, or the preheating process) and a brackish water level controller in the absorber (Figure 7) [81]. A programmable logic controller (PLC) was used to control the tracking motors to drive the SDC in two axes for tracking the sun based on the calculated positions [81] (Figure 7). The boiler had a receiving surface, which was as small as 0.031 m2 and made of CK45 steel alloy, and the black chrome was coated at its bottom side to increase the absorptivity of the reflected sun's radiations [81]. The reflective area of the SDC was 3.142 m2 with the aperture diameter of 2 m and the focal length of 0.693 m, and it was covered with sil-The above-developed stand-alone system (PPSS) is comprised of several items, including a parabolic dish concentrator, a boiler mounted at the focal point of the dish collector, two plate heat exchangers (to condense the steam generated in the boiler and increase the brackish water temperature before entering the boiler, or the preheating process) and a brackish water level controller in the absorber (Figure 7) [81]. A programmable logic controller (PLC) was used to control the tracking motors to drive the SDC in two axes for tracking the sun based on the calculated positions [81] (Figure 7). The boiler had a receiving surface, which was as small as 0.031 m<sup>2</sup> and made of CK45 steel alloy, and the black chrome was coated at its bottom side to increase the absorptivity of the reflected sun's radiations [81]. The reflective area of the SDC was 3.142 m<sup>2</sup> with the aperture diameter of 2 m and the focal length of 0.693 m, and it was covered with silver-backed glass segments of 0.002 m thickness [81].

ver-backed glass segments of 0.002 m thickness [81]. The study results showed that the initial boiler wall temperature (Ts) increased abruptly to about 70 °C (Figure 8) due to the reflection of solar radiation into the small-scale boiler [81]. All parts of the boiler were exposed to the sunlight in the early hours of the experiment, and the SDC-boiler system performed to produce water at temperatures higher than 70 °C, which is operative to prevent transmission of pathogens, bacteria, and The study results showed that the initial boiler wall temperature (Ts) increased abruptly to about 70 ◦C (Figure 8) due to the reflection of solar radiation into the small-scale boiler [81]. All parts of the boiler were exposed to the sunlight in the early hours of the experiment, and the SDC-boiler system performed to produce water at temperatures higher than 70 ◦C, which is operative to prevent transmission of pathogens, bacteria, and viruses from the brackish water into the vapors and condensed vapors [74,75]. The boiler wall

viruses from the brackish water into the vapors and condensed vapors [74,75]. The boiler wall temperature increased drastically from 70 °C to about 100 °C in a short period of 30

**Figure 8.** Hourly variations of temperature of absorber/or boiler wall of the PPSS versus the distil-

late production of the PPSS [81].

temperature increased drastically from 70 ◦C to about 100 ◦C in a short period of 30 min after the experiment started at 8:00 a.m. (Figure 8). Then it was maintained to reach above 100 ◦C (boiling point) for the remainder of the seven-hour experiment. wall temperature increased drastically from 70 °C to about 100 °C in a short period of 30 min after the experiment started at 8:00 a.m. (Figure 8). Then it was maintained to reach above 100 °C (boiling point) for the remainder of the seven-hour experiment.

The study results showed that the initial boiler wall temperature (Ts) increased abruptly to about 70 °C (Figure 8) due to the reflection of solar radiation into the small-scale boiler [81]. All parts of the boiler were exposed to the sunlight in the early hours of the experiment, and the SDC-boiler system performed to produce water at temperatures higher than 70 °C, which is operative to prevent transmission of pathogens, bacteria, and viruses from the brackish water into the vapors and condensed vapors [74,75]. The boiler

**Figure 7.** Photograph of the experimental set-up of an auto sun-tracking system SDC-solar still in

The above-developed stand-alone system (PPSS) is comprised of several items, including a parabolic dish concentrator, a boiler mounted at the focal point of the dish collector, two plate heat exchangers (to condense the steam generated in the boiler and increase the brackish water temperature before entering the boiler, or the preheating process) and a brackish water level controller in the absorber (Figure 7) [81]. A programmable logic controller (PLC) was used to control the tracking motors to drive the SDC in two axes for tracking the sun based on the calculated positions [81] (Figure 7). The boiler had a receiving surface, which was as small as 0.031 m2 and made of CK45 steel alloy, and the black chrome was coated at its bottom side to increase the absorptivity of the reflected sun's radiations [81]. The reflective area of the SDC was 3.142 m2 with the aperture diameter of 2 m and the focal length of 0.693 m, and it was covered with sil-

ver-backed glass segments of 0.002 m thickness [81].

*Sustainability* **2022**, *14*, x FOR PEER REVIEW 12 of 28

Tehran, Iran[81].

**Figure 8.** Hourly variations of temperature of absorber/or boiler wall of the PPSS versus the distillate production of the PPSS [81]. **Figure 8.** Hourly variations of temperature of absorber/or boiler wall of the PPSS versus the distillate production of the PPSS [81].

The maximum total daily water production (Pd) was reported on 18 and 22 October 2014 with values of 5.02 and 5.11 kg per 7 h, respectively. Meanwhile, the PPSS system was exposed to higher average solar radiation intensities ((Ib)ave, more than 630 W/m<sup>2</sup> ) in these two days, causing the boiler wall temperatures (Ts)ave to reach maximum average values of about 140 ◦C and 150 ◦C, respectively. The highest average daily efficiency was reported as 36.7% on 22 October 2014 due to the highest average solar insolation, average absorber wall temperature, and total daily productivity [81]. However, it was reported that the average temperatures of air (Tair)ave, wind speed (Vw)ave, and the salinity rates fed in water samples did not affect the daily water production considerably [81]. Water quality parameters of total dissolved solids (TDS) and electrical conductivity (EC) were also measured for feeding salt water into the boiler and discharging brine and distilled water from the boiler after the desalination process for the seven experimental days. As reported, the values of TDS and EC ranges for the distilled water produced by PPSS were the lowest and fell within the acceptable ranges of WHO standards for drinking water purposes [81]. The annual water production of the proposed PPSS system was calculated and stated as 2422.40 kg, while the cost of 1 kg of distilled freshwater produced by the system with an SDC coupled to a boiler was analyzed and reported as USD 0.012; under the Tehran climate, this cost was stated as sufficiently low and cost-effective for rural householders [81]. It was recommended that the photovoltaic modules can be employed as a useful alternative to supply power for the electrical components of the PPSS system, instead of consuming electricity directly in order to reduce the direct electricity consumption per kilogram as well as the operating costs of USD 6187.40 per year for the production of freshwater [81].

In another theoretical and experimental work [82], an SDC which was made from recyclable materials and coupled with a sun-tracking system and a boiler (evaporator) were designed, installed, and experimented for ground water and sea water desalination under the Brazil climate, as depicted in Figure 9. A recycled satellite dish antenna made from galvanized steel with two different aperture diameters (height of 68 cm and width of 62 cm) was selected, mirrored via an electrostatic chroming method, and then used as an SDC in the study [82].

production of freshwater [81].

an SDC in the study [82].

**Figure 9.** Sketch of a solar desalination unit of SDC coupled with a sun-tracking system [82]. **Figure 9.** Sketch of a solar desalination unit of SDC coupled with a sun-tracking system [82].

Based on the experiment results as shown in Figure 10, an intermediate focal point in a focal region of the SDC which was determined between two different focus points of the reflected sun's rays was achieved at the best focal length of 51.5 cm [82]. A two-axis sun-tracking system was mounted on a steel tripod (1) and powered by two motors (2) which rotated the SDC in 64 steps per revolution with 1.8° in each step programmed through the control (3) (Figure 9) [82]. Based on the experiment results as shown in Figure 10, an intermediate focal point in a focal region of the SDC which was determined between two different focus points of the reflected sun's rays was achieved at the best focal length of 51.5 cm [82]. A two-axis suntracking system was mounted on a steel tripod (1) and powered by two motors (2) which rotated the SDC in 64 steps per revolution with 1.8◦ in each step programmed through the control (3) (Figure 9) [82]. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 14 of 28

The maximum total daily water production (Pd) was reported on 18 and 22 October 2014 with values of 5.02 and 5.11 kg per 7 h, respectively. Meanwhile, the PPSS system was exposed to higher average solar radiation intensities ((Ib)ave, more than 630 W/m2) in these two days, causing the boiler wall temperatures (Ts)ave to reach maximum average values of about 140 °C and 150 °C, respectively. The highest average daily efficiency was reported as 36.7% on 22October 2014 due to the highest average solar insolation, average absorber wall temperature, and total daily productivity [81]. However, it was reported that the average temperatures of air (Tair)ave, wind speed (Vw)ave, and the salinity rates fed in water samples did not affect the daily water production considerably [81]. Water quality parameters of total dissolved solids (TDS) and electrical conductivity (EC) were also measured for feeding salt water into the boiler and discharging brine and distilled water from the boiler after the desalination process for the seven experimental days. As reported, the values of TDS and EC ranges for the distilled water produced by PPSS were the lowest and fell within the acceptable ranges of WHO standards for drinking water purposes [81]. The annual water production of the proposed PPSS system was calculated and stated as 2422.40 kg, while the cost of 1 kg of distilled freshwater produced by the system with an SDC coupled to a boiler was analyzed and reported as USD 0.012; under the Tehran climate, this cost was stated as sufficiently low and cost-effective for rural householders [81]. It was recommended that the photovoltaic modules can be employed as a useful alternative to supply power for the electrical components of the PPSS system, instead of consuming electricity directly in order to reduce the direct electricity consumption per kilogram as well as the operating costs of USD 6187.40 per year for the

In another theoretical and experimental work [82], an SDC which was made from recyclable materials and coupled with a sun-tracking system and a boiler (evaporator) were designed, installed, and experimented for ground water and sea water desalination under the Brazil climate, as depicted in Figure 9. A recycled satellite dish antenna made from galvanized steel with two different aperture diameters (height of 68 cm and width of 62 cm) was selected, mirrored via an electrostatic chroming method, and then used as

**Figure 10.** Simulation of the sun's rays (colored lines) as reflected from the two curvatures of the SDC had different diameters from the absorber which is located at the focal length of 51.5 cm [82]. **Figure 10.** Simulation of the sun's rays (colored lines) as reflected from the two curvatures of the SDC had different diameters from the absorber which is located at the focal length of 51.5 cm [82].

The study's experiments were conducted from 9:00 a.m. to 4:00 p.m. for two months (September and October) [82]. As illustrated in Figure 11, a borosilicate glass sphere-shaped absorber called an evaporator absorber with the area of 0.1182 m2 and the storage of 100 mL was filled with crushed basalt and coated with a matte black paint mounted at the focal point of the SDC. Samples of ground water and sea water with similar sea salt concentrations from 0% to 4% were pumped into the absorber from the storage tank (1). Next, the sun's rays' reflections were focused onto the sphere-shaped boiler to heat the sea water. Then, water vapor from the boiler passed through the copper tube with a length of 30 cm for the first phase of the condensing process (2) and was directed from a 1.5 m silicon tube for the second phase of condensation method (3) into the graduated container to store the produced water (4) [82]. The study's experiments were conducted from 9:00 a.m. to 4:00 p.m. for two months (September and October) [82]. As illustrated in Figure 11, a borosilicate glass sphereshaped absorber called an evaporator absorber with the area of 0.1182 m<sup>2</sup> and the storage of 100 mL was filled with crushed basalt and coated with a matte black paint mounted at the focal point of the SDC. Samples of ground water and sea water with similar sea salt concentrations from 0% to 4% were pumped into the absorber from the storage tank (1). Next, the sun's rays' reflections were focused onto the sphere-shaped boiler to heat the sea water. Then, water vapor from the boiler passed through the copper tube with a length of 30 cm for the first phase of the condensing process (2) and was directed from a 1.5 m silicon tube for the second phase of condensation method (3) into the graduated container to store the produced water (4) [82].

(2), and condensed in copper (3) and silicon (4) tubes [82].

these factors were dependent on their diameters [82].

**Figure 11.** Processes of pumping brackish water into the absorber (1), evaporated from the absorber

As shown in Figure 12, two disk-shaped aluminum specimens, Specimen 1 and Specimen 2 (painted matte black with the effective areas of 0.1611, 0.1108 m2, respectively) which were located at the focal region of the SDC acted as solar radiation absorbers and were tested theoretically and experimentally to determine their dynamic heating temperatures [82]. The intercept factors (γ) of Specimens 1 and 2 were experimentally investigated, analyzed, and recorded at 48.64 and 33.45 % respectively, which indicated that

graduated container to store the produced water (4) [82].

**Figure 10.** Simulation of the sun's rays (colored lines) as reflected from the two curvatures of the SDC had different diameters from the absorber which is located at the focal length of 51.5 cm [82].

The study's experiments were conducted from 9:00 a.m. to 4:00 p.m. for two months (September and October) [82]. As illustrated in Figure 11, a borosilicate glass sphere-shaped absorber called an evaporator absorber with the area of 0.1182 m2 and the storage of 100 mL was filled with crushed basalt and coated with a matte black paint mounted at the focal point of the SDC. Samples of ground water and sea water with similar sea salt concentrations from 0% to 4% were pumped into the absorber from the storage tank (1). Next, the sun's rays' reflections were focused onto the sphere-shaped boiler to heat the sea water. Then, water vapor from the boiler passed through the copper tube with a length of 30 cm for the first phase of the condensing process (2) and was directed from a 1.5 m silicon tube for the second phase of condensation method (3) into the

**Figure 11.** Processes of pumping brackish water into the absorber (1), evaporated from the absorber (2), and condensed in copper (3) and silicon (4) tubes [82]. **Figure 11.** Processes of pumping brackish water into the absorber (1), evaporated from the absorber (2), and condensed in copper (3) and silicon (4) tubes [82].

As shown in Figure 12, two disk-shaped aluminum specimens, Specimen 1 and Specimen 2 (painted matte black with the effective areas of 0.1611, 0.1108 m2, respectively) which were located at the focal region of the SDC acted as solar radiation absorbers and were tested theoretically and experimentally to determine their dynamic heating temperatures [82]. The intercept factors (γ) of Specimens 1 and 2 were experimentally investigated, analyzed, and recorded at 48.64 and 33.45 % respectively, which indicated that these factors were dependent on their diameters [82]. As shown in Figure 12, two disk-shaped aluminum specimens, Specimen 1 and Specimen 2 (painted matte black with the effective areas of 0.1611, 0.1108 m<sup>2</sup> , respectively) which were located at the focal region of the SDC acted as solar radiation absorbers and were tested theoretically and experimentally to determine their dynamic heating temperatures [82]. The intercept factors (γ) of Specimens 1 and 2 were experimentally investigated, analyzed, and recorded at 48.64 and 33.45 % respectively, which indicated that these factors were dependent on their diameters [82]. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 15 of 28

**Figure 12.** Relationship between absorbers' diameters and their intercept factors [82]. **Figure 12.** Relationship between absorbers' diameters and their intercept factors [82].

The temperature of the smaller absorber (Specimen 2) reached the maximum value of 319 °C in 840 s which was maintained until 1800 s, while the larger absorber (Specimen 1) experienced the maximum temperature of 198 °C at 1800 s which lasted until the end of experiment, i.e., at 3500 s. These results are shown in Figure 13a,b) [82]. It was also reported that the average boiling point temperature of the third absorber (called the 'evaporator absorber', with an optical efficiency of 0.273 and intercept factor of 35.71%) increased from 98.10 °C without sea salt concentration to 99.66 °C with 4% of salt concentration [82] during the desalination experimental works. This result indicated that a disinfection process occurs during the continuous boiling process with the explosion of the solution to the solar radiation ultraviolet waves [83]. It was observed during the study's experiments that all the three absorbers received ultraviolet waves (UV) of the sun's rays from their top sides and as reflected from the parabola of the SDC [82]. Thus, the reviews have proven the feasibility of using SDCs coupled with smaller absorbers of Specimen 2 and evaporator with the sun-tracking system in the study [82] for removing bacteria, waterborne pathogens, and viruses since the high initial temperatures of the absorber water were achieved. The highest yield of 4.95 kg/m2day of distilled water was The temperature of the smaller absorber (Specimen 2) reached the maximum value of 319 ◦C in 840 s which was maintained until 1800 s, while the larger absorber (Specimen 1) experienced the maximum temperature of 198 ◦C at 1800 s which lasted until the end of experiment, i.e., at 3500 s. These results are shown in Figure 13a,b) [82]. It was also reported that the average boiling point temperature of the third absorber (called the 'evaporator absorber', with an optical efficiency of 0.273 and intercept factor of 35.71%) increased from 98.10 ◦C without sea salt concentration to 99.66 ◦C with 4% of salt concentration [82] during the desalination experimental works. This result indicated that a disinfection process occurs during the continuous boiling process with the explosion of the solution to the solar radiation ultraviolet waves [83]. It was observed during the study's experiments that all the three absorbers received ultraviolet waves (UV) of the sun's rays from their top sides and as reflected from the parabola of the SDC [82]. Thus, the reviews have proven the feasibility of using SDCs coupled with smaller absorbers of Specimen 2 and evaporator with the sun-tracking system in the study [82] for removing bacteria, waterborne pathogens, and viruses since the high initial temperatures of the absorber water were achieved. The highest yield of 4.95 kg/m<sup>2</sup> day of distilled water was attained under the average solar irradiances of 791 W/m<sup>2</sup> without adding salt in the sample [82].

attained under the average solar irradiances of 791 W/m2 without adding salt in the

**Figure 13.** Experimental and simulated dynamic heating of Specimens 1 (**a**) and 2 (**b**) versus the

In a study conducted by Chaichan M.T. and Kazem H.A. in Baghdad, a solar distiller (absorber/receiver) was integrated with an SDC to heat the saline water (Figure 14) [93]. Then, hot water was transferred to a conical distiller by a heat exchanger to produce dis-

(**a**) (**b**)

sample [82].

values of solar radiation [82].

sample [82].

**Figure 12.** Relationship between absorbers' diameters and their intercept factors [82].

The temperature of the smaller absorber (Specimen 2) reached the maximum value of 319 °C in 840 s which was maintained until 1800 s, while the larger absorber (Specimen 1) experienced the maximum temperature of 198 °C at 1800 s which lasted until the end of experiment, i.e., at 3500 s. These results are shown in Figure 13a,b) [82]. It was also reported that the average boiling point temperature of the third absorber (called the 'evaporator absorber', with an optical efficiency of 0.273 and intercept factor of 35.71%) increased from 98.10 °C without sea salt concentration to 99.66 °C with 4% of salt concentration [82] during the desalination experimental works. This result indicated that a disinfection process occurs during the continuous boiling process with the explosion of the solution to the solar radiation ultraviolet waves [83]. It was observed during the study's experiments that all the three absorbers received ultraviolet waves (UV) of the sun's rays from their top sides and as reflected from the parabola of the SDC [82]. Thus, the reviews have proven the feasibility of using SDCs coupled with smaller absorbers of Specimen 2 and evaporator with the sun-tracking system in the study [82] for removing bacteria, waterborne pathogens, and viruses since the high initial temperatures of the absorber water were achieved. The highest yield of 4.95 kg/m2day of distilled water was attained under the average solar irradiances of 791 W/m2 without adding salt in the

**Figure 13.** Experimental and simulated dynamic heating of Specimens 1 (**a**) and 2 (**b**) versus the values of solar radiation [82]. **Figure 13.** Experimental and simulated dynamic heating of Specimens 1 (**a**) and 2 (**b**) versus the values of solar radiation [82].

In a study conducted by Chaichan M.T. and Kazem H.A. in Baghdad, a solar distiller (absorber/receiver) was integrated with an SDC to heat the saline water (Figure 14) [93]. Then, hot water was transferred to a conical distiller by a heat exchanger to produce dis-In a study conducted by Chaichan M.T. and Kazem H.A. in Baghdad, a solar distiller (absorber/receiver) was integrated with an SDC to heat the saline water (Figure 14) [93]. Then, hot water was transferred to a conical distiller by a heat exchanger to produce distilled water. The SDC had an aperture diameter of 1.5 m and a depth of 23 cm and the conical distiller was layered with paraffin wax, called 'PCM', as thermal energy storage material to expand the distillation process after daytime [93]. Aluminum foil was adhered to the parabola dish surface for reflecting the sunlight to the absorber. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 16 of 28 tilled water. The SDC had an aperture diameter of 1.5 m and a depth of 23 cm and the conical distiller was layered with paraffin wax, called 'PCM', as thermal energy storage material to expand the distillation process after daytime [93]. Aluminum foil was adhered to the parabola dish surface for reflecting the sunlight to the absorber.

**Figure 14.** Sketch of the SDC with STS integrated with an absorber (receiver) with a conical distiller with PCM [93]. **Figure 14.** Sketch of the SDC with STS integrated with an absorber (receiver) with a conical distiller with PCM [93].

The experiments conducted for four cases consisted of the SDC without STS and PCM as Case 1, the SDC with STS and without PCM as Case 2, the SDC without STS and with PCM as Case 3, and the SDC with STS and PCM as Case 4. Using PCM with STS (Case 4) gave the highest temperatures compared to other three cases, especially for the period after 2:00 p.m. (Figure 15). However, the obtained temperature for Case 4 did not reach the boiling point of brackish water as the reflecting layer adhered to the surface of SDC was made of aluminum foils and had lower sun's rays reflectivity compared to the mirror. It can be seen that the water temperatures of the brackish water reached beyond 65 °C at about 2:00 p.m., and reduced significantly after 2:00 p.m. following the decrease in solar radiation intensity. Although the initial working temperatures of the absorber in Case 4 were between 10 °C and 40 °C in the early hours of the experiment (Figure 15). However, in the results obtained from the experiments conducted in [80–82], the initial temperature of the absorber was above 65 °C and it increased drastically beyond the boiling point immediately in a short period of time due to using glass mirror as the covered layer of the SDC surface. This can be resulted from covering the layer of the surface The experiments conducted for four cases consisted of the SDC without STS and PCM as Case 1, the SDC with STS and without PCM as Case 2, the SDC without STS and with PCM as Case 3, and the SDC with STS and PCM as Case 4. Using PCM with STS (Case 4) gave the highest temperatures compared to other three cases, especially for the period after 2:00 p.m. (Figure 15). However, the obtained temperature for Case 4 did not reach the boiling point of brackish water as the reflecting layer adhered to the surface of SDC was made of aluminum foils and had lower sun's rays reflectivity compared to the mirror. It can be seen that the water temperatures of the brackish water reached beyond 65 ◦C at about 2:00 p.m., and reduced significantly after 2:00 p.m. following the decrease in solar radiation intensity. Although the initial working temperatures of the absorber in Case 4 were between 10 ◦C and 40 ◦C in the early hours of the experiment (Figure 15). However, in the results obtained from the experiments conducted in [80–82], the initial temperature of the absorber was above 65 ◦C and it increased drastically beyond the boiling point immediately in a short period of time due to using glass mirror as the covered layer of the SDC surface. This can be resulted from covering the layer of the surface of the SDC with aluminum foil which

of the SDC with aluminum foil which has a lower solar radiation reflectivity as compared to mirror strips. Thus, it seems that the SDC layered with aluminum foil is unable to in-

teria, waterborne pathogens, and viruses due to the resulting low initial water temperatures in the absorber. Thus, it can be seen from the above results that the reflectivity of the cover layer of the SDC surface has a vital role in increasing the initial temperature of the

brackish water in the absorber significantly.

has a lower solar radiation reflectivity as compared to mirror strips. Thus, it seems that the SDC layered with aluminum foil is unable to increase the absorber water temperature considerably (Figure 15) in order to remove bacteria, waterborne pathogens, and viruses due to the resulting low initial water temperatures in the absorber. Thus, it can be seen from the above results that the reflectivity of the cover layer of the SDC surface has a vital role in increasing the initial temperature of the brackish water in the absorber significantly. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 17 of 28 *Sustainability* **2022**, *14*, x FOR PEER REVIEW 17 of 28

**Figure 15.** Variations of brackish water temperature versus time for the four studied cases [93]. **Figure 15.** Variations of brackish water temperature versus time for the four studied cases [93]. In another study conducted by Bahrami et al., 2019 in Yasouj University, Iran, an

evaporator during the experiment and maintained with the use of a float level controller

In another study conducted by Bahrami et al., 2019 in Yasouj University, Iran, an SDC with an aperture diameter of 2.0 m integrated with an STS to reflect the solar radiation into an evaporator tank mounted on its focal point with a focal length of 1.4 m was designed, installed, and tested to desalinate saltwater (Figure 16) [97]. The evaporator had a base area of 0.2 × 0.2 m and saltwater in the range of 1.0 to 10 kg was fed into the In another study conducted by Bahrami et al., 2019 in Yasouj University, Iran, an SDC with an aperture diameter of 2.0 m integrated with an STS to reflect the solar radiation into an evaporator tank mounted on its focal point with a focal length of 1.4 m was designed, installed, and tested to desalinate saltwater (Figure 16) [97]. The evaporator had a base area of 0.2 × 0.2 m and saltwater in the range of 1.0 to 10 kg was fed into the evaporator during the experiment and maintained with the use of a float level controller (Figure 16). SDC with an aperture diameter of 2.0 m integrated with an STS to reflect the solar radiation into an evaporator tank mounted on its focal point with a focal length of 1.4 m was designed, installed, and tested to desalinate saltwater (Figure 16) [97]. The evaporator had a base area of 0.2 × 0.2 m and saltwater in the range of 1.0 to 10 kg was fed into the evaporator during the experiment and maintained with the use of a float level controller (Figure 16).

**(a) (b) Figure 16.** (**a**) Detailed sketch and (**b**) photograph of the experimental set up of the SDC and **Figure 16.** (**a**) Detailed sketch and (**b**) photograph of the experimental set up of the SDC and evaporator performed in Iran [97]. **Figure 16.** (**a**) Detailed sketch and (**b**) photograph of the experimental set up of the SDC and evaporator performed in Iran [97].

evaporator performed in Iran [97]. It has been stated by Bahrami et al. that the total amount of the produced distilled water increased from 11.5 to 50 kg by increasing the aperture diameter of the SDC from 1.5 to 3.0 m, respectively, and it increased twice while the optical efficiency of SDC increased from 0.5 to 0.8. The amount of produced water was also increased by more than double when the reflectivity of the evaporator base decreased from 0.7 to 0.4 [97]. They It has been stated by Bahrami et al. that the total amount of the produced distilled water increased from 11.5 to 50 kg by increasing the aperture diameter of the SDC from 1.5 to 3.0 m, respectively, and it increased twice while the optical efficiency of SDC increased from 0.5 to 0.8. The amount of produced water was also increased by more than double when the reflectivity of the evaporator base decreased from 0.7 to 0.4 [97]. They have also reported that the saltwater in the evaporator boils at earlier time for an SDC It has been stated by Bahrami et al. that the total amount of the produced distilled water increased from 11.5 to 50 kg by increasing the aperture diameter of the SDC from 1.5 to 3.0 m, respectively, and it increased twice while the optical efficiency of SDC increased from 0.5 to 0.8. The amount of produced water was also increased by more than double when the reflectivity of the evaporator base decreased from 0.7 to 0.4 [97]. They have also reported that the saltwater in the evaporator boils at earlier time for an SDC with larger

with larger aperture diameter. An SDC with an aperture diameter of 3.0 m was able to boil 8 kg of saltwater with a salinity rate of 30 (g salt/kg water) after about 20 min, while

have also reported that the saltwater in the evaporator boils at earlier time for an SDC with larger aperture diameter. An SDC with an aperture diameter of 3.0 m was able to

this took about 40 min for an SDC with a diameter of 2.0 m [97]. As depicted in Figure 17, the SDC with a diameter of 2.0 m was reported to boil 6.15 kg of saltwater with a salinity

aperture diameter. An SDC with an aperture diameter of 3.0 m was able to boil 8 kg of saltwater with a salinity rate of 30 (g salt/kg water) after about 20 min, while this took about 40 min for an SDC with a diameter of 2.0 m [97]. As depicted in Figure 17, the SDC with a diameter of 2.0 m was reported to boil 6.15 kg of saltwater with a salinity rate of 20 (g/kg) in the evaporator in a period of 1.0 h when the distillation process started from 11:40 a.m. and maintained the boiling point until 3:30 p.m. This highlighted that an SDC with larger aperture diameter [97] and smaller absorber area [82] is capable of reflecting more of the sun's rays to the evaporator (absorber) to reach the highest initial temperature and the boiling point in a shorter period. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 18 of 28 rate of 20 (g/kg) in the evaporator in a period of 1.0 h when the distillation process started from 11:40 a.m. and maintained the boiling point until 3:30 p.m. This highlighted that an SDC with larger aperture diameter [97] and smaller absorber area [82] is capable of reflecting more of the sun's rays to the evaporator (absorber) to reach the highest initial temperature and the boiling point in a shorter period.

**Figure 17.** Hourly variation of saltwater temperature in the evaporator tank and the distilled water production of the SDC with aperture diameter of 2.0 m [97]. **Figure 17.** Hourly variation of saltwater temperature in the evaporator tank and the distilled water production of the SDC with aperture diameter of 2.0 m [97].

### 2.2.1. SDCs Integrated with Solar Stills without the Sun-Tracking System (STS) 2.2.2. SDCs Integrated with Solar Stills without the Sun-Tracking System (STS)

In India, several studies investigated the performance of passive solar stills heated by SDCs and additional phase-change materials (PCM) in the still's basin using the cover cooling techniques, without employing the sun-tracking system [98,99]. In one of the studies [98], two passive single-slope solar stills (SSSS) were designed and fabricated, whereby each was mounted on a focal point of a fixed SDC (Figure 18) and stored the heat at their basins using six PCM copper balls filled with paraffin wax (Figure 19a,b); meanwhile, the cold water flow technique was employed at the top cover of one of the solar stills to improve the condensation rate. A black painted hemispherical copper bowl (with a diameter of 0.22 m and a thickness of 4 mm) was separately attached to each basin bottom of the passive SSSS mounted on the focal point of the SDC, which acted as receivers of the sun's rays' reflections to heat the basins water. Six hollow copper balls (each with a thickness of 1.2 mm, as in Figure 19a,b) filled with paraffin wax were used in the absorber of each solar still as the PCM. The balls acted as a heat source for the absorber water to maintain its temperature during the afternoon—i.e., when the solar irra-In India, several studies investigated the performance of passive solar stills heated by SDCs and additional phase-change materials (PCM) in the still's basin using the cover cooling techniques, without employing the sun-tracking system [98,99]. In one of the studies [98], two passive single-slope solar stills (SSSS) were designed and fabricated, whereby each was mounted on a focal point of a fixed SDC (Figure 18) and stored the heat at their basins using six PCM copper balls filled with paraffin wax (Figure 19a,b); meanwhile, the cold water flow technique was employed at the top cover of one of the solar stills to improve the condensation rate. A black painted hemispherical copper bowl (with a diameter of 0.22 m and a thickness of 4 mm) was separately attached to each basin bottom of the passive SSSS mounted on the focal point of the SDC, which acted as receivers of the sun's rays' reflections to heat the basins water. Six hollow copper balls (each with a thickness of 1.2 mm, as in Figure 19a,b) filled with paraffin wax were used in the absorber of each solar still as the PCM. The balls acted as a heat source for the absorber water to maintain its temperature during the afternoon—i.e., when the solar irradiances started to decrease—and then continued to produce fresh water after sunset [98].

diances started to decrease—and then continued to produce fresh water after sunset [98]. The performance of each solar still was strongly dependent on the intensities of solar absorption by the hemispherical copper bowl absorber from the concentrator, and the PCM balls located in the basin [98]. The temperatures of initial basin water temperatures in the early hours of the experiments with the solar stills with PCM and SDC using the top cover cooling techniques (with water flow rates of 40, 50, 60, 80, and 100 mL/min) were observed at 40, 43, 47, 47, and 48 ◦C at 9:00 a.m. and 56, 56, 56, 57, and 56 ◦C at 10:00 a.m. respectively; meanwhile, the temperatures recorded were 43 ◦C and 56 ◦C at 9:00 and 10:00 a.m., respectively, for the experiments without any water flow on the top cover [98].

*Sustainability* **2022**, *14*, x FOR PEER REVIEW 19 of 28

**Figure 18.** Photograph of two solar stills coupled two SDCs, with and without the cover cooling method [98]. **Figure 18.** Photograph of two solar stills coupled two SDCs, with and without the cover cooling method [98]. and 10:00 a.m., respectively, for the experiments without any water flow on the top cover [98].

**Figure 19.** (**a**) Photograph of PCM copper balls used in a hemispherical SSSS; (**b**) sketch of a hemispherical SSSS with PCM balls in bowl-shaped copper basin while receiving the sun's rays from a fixed SDC [98]. **Figure 19.** (**a**) Photograph of PCM copper balls used in a hemispherical SSSS; (**b**) sketch of a hemispherical SSSS with PCM balls in bowl-shaped copper basin while receiving the sun's rays from a fixed SDC [98].

(**a**) (**b**) **Figure 19.** (**a**) Photograph of PCM copper balls used in a hemispherical SSSS; (**b**) sketch of a hemispherical SSSS with PCM balls in bowl-shaped copper basin while receiving the sun's rays from a fixed SDC [98]. As depicted in Figure 18, most parts of the solar still in the study included the top cover, basin bottom, and the sides which were exposed to the UV of solar radiation in the experiment from morning to evening [98], ranging from 580 to 1050 W/m2. However, flowing water on the top cover and water droplets on the inner side of the solar still's cover reduced the inputs of the solar radiation to the basin water. Furthermore, as can be observed, there was a lack of coating of the mirrored layer on the SDC surface and no system to track the directions of the SDC and solar still towards the sun and to use the As depicted in Figure 18, most parts of the solar still in the study included the top cover, basin bottom, and the sides which were exposed to the UV of solar radiation in the experiment from morning to evening [98], ranging from 580 to 1050 W/m2. However, flowing water on the top cover and water droplets on the inner side of the solar still's cover reduced the inputs of the solar radiation to the basin water. Furthermore, as can be observed, there was a lack of coating of the mirrored layer on the SDC surface and no system to track the directions of the SDC and solar still towards the sun and to use the solar still to absorb the reflected sun's rays at a larger scale. As stated by Arunkumar et al., 2015, there were lower initial (ranging from 40 °C to 56 °C) and maximum (ranging from 92 °C to 88 °C) basin water temperatures and lower total yield (ranging from 3.557 to 3.80 L/m2.day) in the solar stills throughout the experiment [98], compared to the use of solar As depicted in Figure 18, most parts of the solar still in the study included the top cover, basin bottom, and the sides which were exposed to the UV of solar radiation in the experiment from morning to evening [98], ranging from 580 to 1050 W/m<sup>2</sup> . However, flowing water on the top cover and water droplets on the inner side of the solar still's cover reduced the inputs of the solar radiation to the basin water. Furthermore, as can be observed, there was a lack of coating of the mirrored layer on the SDC surface and no system to track the directions of the SDC and solar still towards the sun and to use the solar still to absorb the reflected sun's rays at a larger scale. As stated by Arunkumar et al., 2015, there were lower initial (ranging from 40 ◦C to 56 ◦C) and maximum (ranging from 92 ◦C to 88 ◦C) basin water temperatures and lower total yield (ranging from 3.557 to 3.80 L/m<sup>2</sup> .day) in the solar stills throughout the experiment [98], compared to the use of solar stills with SDC, sun tracker system, and mirrored surfaces in other studies [80–82]. Meanwhile, as noted in other studies [36,74,75], it seems infeasible to produce water under low basin water temperatures with the use of solar stills coupled SDC and without the sun-tracking system [98], particularly in terms of removing bacteria, waterborne pathogens, and viruses due to the resulting low initial water temperatures (ranging 40 ◦C to 56 ◦C) in the basins of solar stills.

solar still to absorb the reflected sun's rays at a larger scale. As stated by Arunkumar et al., 2015, there were lower initial (ranging from 40 °C to 56 °C) and maximum (ranging from 92 °C to 88 °C) basin water temperatures and lower total yield (ranging from 3.557 to 3.80 L/m2.day) in the solar stills throughout the experiment [98], compared to the use of solar Another experimental study in India designed and fabricated a triple-basin solar distiller (TBSS) mounted on a focal point of an SDC [99]. Without engaging a sun-tracking system, it was heated by heat storing materials comprised of four triangular hollow fins filled with river sand (RS) and charcoal (CHAR) in the basins of the distiller which were

exposed to the direct solar irradiances [99]. As depicted in Figure 20, a cover cooling (CC) approach using water with different flow rates (20 to 40 mL/s with the intervals of 5 mL/s) was also employed to decrease the still cover temperature and increase the condensation rate [99]. which were exposed to the direct solar irradiances [99]. As depicted in Figure 20, a cover cooling (CC) approach using water with different flow rates (20 to 40 mL/s with the intervals of 5 mL/s) was also employed to decrease the still cover temperature and increase the condensation rate [99].

Another experimental study in India designed and fabricated a triple-basin solar distiller (TBSS) mounted on a focal point of an SDC [99]. Without engaging a sun-tracking system, it was heated by heat storing materials comprised of four triangular hollow fins filled with river sand (RS) and charcoal (CHAR) in the basins of the distiller

stills with SDC, sun tracker system, and mirrored surfaces in other studies [80–82]. Meanwhile, as noted in other studies [36,74,75], it seems infeasible to produce water under low basin water temperatures with the use of solar stills coupled SDC and without the sun-tracking system [98], particularly in terms of removing bacteria, waterborne pathogens, and viruses due to the resulting low initial water temperatures (ranging 40 °C

*Sustainability* **2022**, *14*, x FOR PEER REVIEW 20 of 28

to 56 °C) in the basins of solar stills.

**Figure 20.** Photograph of a triple basin solar distillation still coupled with an immobile SDC [99]. **Figure 20.** Photograph of a triple basin solar distillation still coupled with an immobile SDC [99].

As shown in Figure 21, the TBSS performed as an absorber whereby its three basins consisted of three basins (lower, middle, and upper basins) acting as an evaporator in the study [99]. Meanwhile, a trapezoidal-shaped glass casing (made of 4 mm thick window glass) was used as the condenser cover and placed at the top of the SDC (with a focal length of 50 cm) with no sun-tracking system to absorb the reflected sun's rays and direct solar intensities. The SDC had a diameter of 1.25 m and was made from a polished aluminum sheet with a thickness of 1 mm. The TBSS evaporator had an overall size of 0.3 × 0.36 m with a height of 0.33 m, while the three basins had a vertical gap of 0.12 m from each other to allow the water vapor to be directed into the inner surface of the condensing cover, as illustrated in Figure 21 [99]. The TBSS cover was constructed with the size of 0.4 × 0.46 m2, heights of 0.4 m and 0.47 m at two different sides, and a 10° incline at the top. A plastic pipe with a diameter of 0.032 m and length of 0.46 m was punctured at regular intervals and then installed at the top of the outer surface of the condensing cover in order to cool the cover and maintain a uniform flow of water that was pumped over the outer glass of the condensing cover surface [99]. As shown in Figure 21, the TBSS performed as an absorber whereby its three basins consisted of three basins (lower, middle, and upper basins) acting as an evaporator in the study [99]. Meanwhile, a trapezoidal-shaped glass casing (made of 4 mm thick window glass) was used as the condenser cover and placed at the top of the SDC (with a focal length of 50 cm) with no sun-tracking system to absorb the reflected sun's rays and direct solar intensities. The SDC had a diameter of 1.25 m and was made from a polished aluminum sheet with a thickness of 1 mm. The TBSS evaporator had an overall size of 0.3 × 0.36 m with a height of 0.33 m, while the three basins had a vertical gap of 0.12 m from each other to allow the water vapor to be directed into the inner surface of the condensing cover, as illustrated in Figure <sup>21</sup> [99]. The TBSS cover was constructed with the size of 0.4 <sup>×</sup> 0.46 m<sup>2</sup> , heights of 0.4 m and 0.47 m at two different sides, and a 10◦ incline at the top. A plastic pipe with a diameter of 0.032 m and length of 0.46 m was punctured at regular intervals and then installed at the top of the outer surface of the condensing cover in order to cool the cover and maintain a uniform flow of water that was pumped over the outer glass of the condensing cover surface [99]. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 21 of 28

**Figure 21.** Photograph of a triple basin with triangular hollow fins [99]. **Figure 21.** Photograph of a triple basin with triangular hollow fins [99].

The water temperatures in solar still basins of the TBSS, which was filled with The water temperatures in solar still basins of the TBSS, which was filled with charcoal and coupled with SDC without the sun-tracking system, were found to be 36 ◦C and

charcoal and coupled with SDC without the sun-tracking system, were found to be 36 °C and 57 °C at the early experimental hours of experiment—i.e., at 9:00 a.m. and 10:00 a.m.,

experiment, as illustrated in Figure 22 [99]. These values are within the critical ranges for the transmission of pathogens and viruses in the produced water, as reported by several studies [36,74,75]. However, all parts of the TBSS were exposed directly to the reflected UV radiation of the sun. Hence, the competency of the TBSS which was coupled with SDC without a sun-tracking system as used in the study [99] seems to be impractical to remove bacteria, waterborne pathogens, and viruses. This was due to the water production at low basin water temperatures—i.e., ranged between 36 °C to 57 °C—as stated in

**Figure 22.** Hourly water temperatures and productivities of the TBSS (filled with and without heat

*2.3. Cost Per Liter (USD) of Small-Scale Passive Solar Stills (Absorbers) Integrated with SDCs*

In order to evaluate the economic benefits of passive solar stills (absorbers) integrated with SDCs for the remote and rural communities, it is essential to consider the cost per liter of the SDC distillation systems and their comparison against other passive and active solar stills. Previous studies revealed that the cost per liter (USD) of the small-scale

storage materials in the basins) were affected by the use of SDC [99].

other studies [36,74,75].

57 ◦C at the early experimental hours of experiment—i.e., at 9:00 a.m. and 10:00 a.m., respectively. As a result, about 0.30 kg/m<sup>2</sup> water was produced in the first hour of the experiment, as illustrated in Figure 22 [99]. These values are within the critical ranges for the transmission of pathogens and viruses in the produced water, as reported by several studies [36,74,75]. However, all parts of the TBSS were exposed directly to the reflected UV radiation of the sun. Hence, the competency of the TBSS which was coupled with SDC without a sun-tracking system as used in the study [99] seems to be impractical to remove bacteria, waterborne pathogens, and viruses. This was due to the water production at low basin water temperatures—i.e., ranged between 36 ◦C to 57 ◦C—as stated in other studies [36,74,75]. experiment, as illustrated in Figure 22 [99]. These values are within the critical ranges for the transmission of pathogens and viruses in the produced water, as reported by several studies [36,74,75]. However, all parts of the TBSS were exposed directly to the reflected UV radiation of the sun. Hence, the competency of the TBSS which was coupled with SDC without a sun-tracking system as used in the study [99] seems to be impractical to remove bacteria, waterborne pathogens, and viruses. This was due to the water production at low basin water temperatures—i.e., ranged between 36 °C to 57 °C—as stated in other studies [36,74,75].

The water temperatures in solar still basins of the TBSS, which was filled with

charcoal and coupled with SDC without the sun-tracking system, were found to be 36 °C and 57 °C at the early experimental hours of experiment—i.e., at 9:00 a.m. and 10:00 a.m., respectively. As a result, about 0.30 kg/m2 water was produced in the first hour of the

**Figure 21.** Photograph of a triple basin with triangular hollow fins [99].

*Sustainability* **2022**, *14*, x FOR PEER REVIEW 21 of 28

**Figure 22.** Hourly water temperatures and productivities of the TBSS (filled with and without heat storage materials in the basins) were affected by the use of SDC [99]. **Figure 22.** Hourly water temperatures and productivities of the TBSS (filled with and without heat storage materials in the basins) were affected by the use of SDC [99].

### *2.3. Cost Per Liter (USD) of Small-Scale Passive Solar Stills (Absorbers) Integrated with SDCs*

*2.3. Cost Per Liter (USD) of Small-Scale Passive Solar Stills (Absorbers) Integrated with SDCs* In order to evaluate the economic benefits of passive solar stills (absorbers) integrated with SDCs for the remote and rural communities, it is essential to consider the cost per liter of the SDC distillation systems and their comparison against other passive and active solar stills. Previous studies revealed that the cost per liter (USD) of the small-scale In order to evaluate the economic benefits of passive solar stills (absorbers) integrated with SDCs for the remote and rural communities, it is essential to consider the cost per liter of the SDC distillation systems and their comparison against other passive and active solar stills. Previous studies revealed that the cost per liter (USD) of the small-scale solar stills (absorbers) coupled with SDCs and the sun-tracking system were USD 0.028 and 0.012 [80,81] and without the sun-tracking system were USD 0.0085 and 0.084 [98,99], respectively. As indicated in Table 3, these values were lower than the cost per liter of some conventional passive, and also active, solar stills used in several other studies [40,46,100–105]. Subsequently, the maximum water yields of the solar stills with both SDC and the sun-tracking system [80] and without the tracking system [99] were found to be higher than the maximum water production of passive and active solar stills tested in other studies [40,46,100–105] (Table 3).


**Table 3.** Cost per liter (USD) and maximum daily yield of the passive solar absorbers coupled with SDCs and the sun-tracking system in comparison with some other passive and active solar stills.

### **3. Discussion**

The limit of water temperatures ranging from 50 ◦C to 70 ◦C was reported as appropriate for the viability of some waterborne pathogens, bacteria, and viruses—particularly SARS-CoV-2—in water bodies [74,75]. The vast public health concern was pertaining to the existence of the aforementioned impurities during the pandemic, particularly SARS-CoV-2, in distilled water produced by passive and active solar stills, recently highlighted in previous studies [31,36,39,40,42–44,46–51,53,63–73], in which the solar stills were found to be able to generate the distillate in low initial operating water temperature. As can be observed from the reviews, using SDCs coupled with small-scale passive solar stills (i.e., absorbers or boilers) [80–82,97] and the sun-tracking systems could lead to drastic and instant increases in the initial water temperatures in the boilers until above 70, 80, and 105 ◦C in the early experimental hours. This was due to the high rates of the reflected sun's rays and heat from the SDC's mirrored surfaces onto the boiler outer surfaces, which is recommended as one of the most effective ways for removing any available waterborne pathogens, bacteria, and viruses—particularly SARS-CoV-2—from the absorber water in order to prevent the transmission of those impurities into the distillate.

However, initial basin water temperatures in the absorbers—which are coupled with SDCs, but without the sun-tracking systems as experimented in several studies [98,99]—were lower than 50 ◦C in the beginning of the experiments. Such conditions are an important factor for the viability and survival of water borne pathogens and viruses in the basins water and distillates, as noted by other studies [36,56–60].

Furthermore, as seen from the experimental works on the SDCs integrated with absorbers and sun tracking devices [80–82,97], all parts of the absorbers were exposed to the sunlight (mainly ultraviolet waves (UV)) and received direct radiation at the top surfaces and the reflected sun's radiation at the bottom and sides from the parabola surface of the SDCs throughout the experiments. Exposing all parts of the solar stills to the sun's rays is an efficient technique to prevent the growth of bacteria and pathogens in the distillate [58–60]. However, this method was not completely practical in the use of any other types of passive and active solar stills because the sun's rays were only received from the top condensing cover surfaces in the early experimental hours [21,29,31,39,40,42–44,46–51,53,63–73].

It was also stated in another theoretical and experimental study [80–82] that the absorbers with smaller surfaces areas and lower water capacity have experienced greater water temperatures, as compared to those with larger surfaces areas [97–99], when the SDCs and the boiler were used under the hourly sunlight periods. The water temperature of the small-scale absorbers coupled with SDCs and the sun-tracking systems as reported in several studies [80–82] increased drastically from about 70 ◦C to above 100 ◦C (i.e., the boiling point). The maximum values were achieved at 105, 150.7, and 319 ◦C, respectively, within a few minutes in the early morning after the daily experiments began, and then the condition was maintained for several hours until the evening. This indicated that a disinfection process occurred during the continuous boiling processes in the absorbers due to the explosion of the solution onto the solar radiation ultraviolet waves [83]. It was obtained from the results of the above studies [80–82,93,97] that an SDC with largest aperture diameter, greatest optical efficiency, and reflectivity with STS integrated with an absorber with smallest area and lowest reflectivity had a vital role in increasing the initial temperature of the brackish/saline water in the absorber around 70 ◦C, maintaining the water temperature beyond the boiling point and enhancing the amount of distillate significantly.

On the other hand, other studies [87,88,98,99] reported that solar stills with immobile solar reflectors were unable to significantly improve basin water temperature to reach the boiling point.

Furthermore, as reported in several studies [80–82], small-scale absorbers coupled with SDCs and dual-axis sun-tracking systems had better performance and were more effective in obtaining higher productivities with lower cost per liter, compared to passive and active solar stills investigated by others [40,46,100–105]. This was due to the resulting higher average water temperatures of the absorbers. Nevertheless, solar stills integrated with immobile SDCs and heat storage materials in their basins [98,99] had higher water productivities and lower costs per liter compared to the mobile SDCs—distillation systems [80–82]. Despite this, low initial absorber temperatures of the absorbers are highlighted as a public health concern in terms of preventing the transmission of pathogens and viruses into the distillate.

### **4. Conclusions**

Based on the above reviews and discussions, SDCs with mirrored surfaces and suntracking systems were seen as capable of increasing the initial water temperature of the integrated small-scale absorbers until exceeding 70 ◦C. Furthermore, continuous increase in the absorbers' wall temperatures beyond the boiling point until the end of the operation is also recommended as another efficient technique to demolish the waterborne pathogens and viruses, especially SARS-CoV-2, at the same time to prevent transmitting these impurities to the produced water during the pandemic. Smaller scale absorbers were found to be more effective in terms of the SDC's surfaces' ability to absorb more heat from the reflected sun's rays, compared to those with larger areas. SDCs with and without the sun-tracking systems (STS) produced greater amounts of freshwater at a lower cost compared to the other previous passive and active solar stills. An SDC with larger aperture diameter, greater optical efficiency, and reflectivity with the STS integrated with an absorber with smaller area and lower reflectivity was perceived to be more operative in increasing the initial temperature of the brackish/saline water in the absorber around 70 ◦C, maintaining the water temperature at the boiling point during sunshine hours and enhancing the amount of distillate significantly. SDCs with the STS were more effective than the immobile and non-sun tracking SDCs in terms of obtaining higher operating

absorber temperatures. Therefore, SDCs which are integrated with small-scale absorbers and sun-tracking systems are recommended as a cost-effective and reliable alternative of an impure water treatment system that can produce hygienic and pathogen-free fresh water, particularly during the SARS-CoV-2 pandemic, for the benefit of the communities in remote and rural areas—including those located in the Middle East, South-East Asia, and Africa—which are suffering from water scarcity and have abundant annual bright sunshine hours.

**Author Contributions:** M.F.Y. and M.R.R.M.A.Z. wrote the original draft of the manuscript; A.R., N.A.Z., A.V.S., S.S., M.S.A.A., M.H.Z., P.V., M.R.R.M.A.Z., N.M.N., and J.I. edited the manuscript, data curation, validation, and prepared the technical aspects of the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the "Ministry of Higher Education (MOHE), Malaysia", grant number "FRGS/1/2021/TK0/USM/02/17" and by TUIASI Internal Grants Program (GI\_Publications/2021), financed by the Romanian Government. APC was funded by the "River Engineering and Urban Drainage Research Centre (REDAC)".

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Informed consent was obtained from all subjects in the study.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors greatly appreciate the support and funding provided by the Ministry of Higher Education (MOHE), Malaysia.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

