1. Introduction
Despite the fact that water covers more than two-thirds of the Earth, a serious shortage of potable water plagues many countries. Additionally, the rapid growth of population and industry around the world has resulted in a significant increase in demand for freshwater. In numerous respects, solar stills remain a perfect supply of potable water for both drinking and agricultural purposes; it is one of the most essential and technically practical sun energyapplications. It is fortunate that stills provide numerous advantages for remote places and islands where the cost of transporting filtered water is currently necessary owing to a freshwater constraint. But it is more economical to create it with easily available resources, with less need for maintenance and operation, in addition with infrastructure that is environmentally friendly. Wherever sunshine (renewable energy) is used more frequently, this lowers global warming emissions. Reverse osmosis, electrolysis, and other primary desalination techniques all require electrical energy as an energy input. Most nations have encountered this significant energy problem in recent decades as a result of a substantial reliance on conventional energy sources. Fossil fuels, carbon, etc.). On these regions, their economic and environmental development has a significant influence. These methods are inappropriate for small, isolated villages. Solar stills can be used to efficiently deliver fresh water to these sites.
In many aspects, sun energy remains a valuable supply of clean water for drinking and agricultural reasons; it's an important and technically viable application of solar energy. Freshwater resources are anticipated to be 40% less abundant by 2030, Kabeel et al. [1]. Solar energy is the most common renewable energy source used in the generation of electricity [2], solar collectors [[3], [4], [5], [6], [7]], water desalination [[8], [9], [10], [11]], solar dryers [12], solar cooking [[13], [14], [15]], solar-biomass [16], printed circuits [17], heating of metals [18] and solar chimney [19].
However, the biggest drawback is that it generates less thoroughly cleansed freshwater than other desalination methods. The capacity in day of the basic version is only 2–5 L/m2. The efficiency of stills in sun desalination applicationsis quite poor when compared to other traditional desalination applications. Many studies have been undertaken for augmenting the thermal efficiency and output of solar stills (SSs) units. A variety of solar stills units system geometries, including conventional solar stills (CSSs), PV/T SSs [[20], [21], [22]], stepped SSs [23], tubular SS [24], half barrel SS [25], pyramid SS [[26], [27], [28]], and trays SS [29,30], have undergone many design and operational tests. Additionally, vibratory harmonic [31], fins [[32], [33], [34]], PCM [35], nanoparticle [36,37], rotating four cylinders [38] and heating coil [39,40] have been applied to enhance the functionality of SSs units.
Investigations suggest that, the yield of solar still is inversely proportional to the depth of the water, as the water temperature rises rapidly as the depth decreases, and thus the rate of evaporation increases. Where, solar still maintain a minimum depth, they can improve distillate productivity. Throughout this in-depth analysis, explained and evaluated the current state of the various methods used to minimize water depth in solar still have been discussed, for instance stepped, cords, wicks, absorber shapes (convex, conical and pyramid absorbers), and rotating parts (drum, disc and wick belt) were used to maintain minimum water depth in solar stills, they can improve distillate productivity. Minimum water depth in solar distiller research is still being conducted and proposals for more research projects have been made in light of the study's findings.
2. The fundamental workings of a solar still
A fundamental solar distiller is comprised of a tank with a small depth of saline-water in the black-painted basin, a glass cover with an angle, a trough for gathering freshwater, insulation on every side but the glass, a drain valve for cleaning the basin of SS, and an intake for contaminated water, as shown in Fig. 1.
Fig. 1. A graph of simple SS.The tank is filled by sunlight passing through its clear glass lid; the saline water inside the tank is heated by convection from the basin, which subsequently evaporates. The vapors rise to the top in the area that is empty. The vapors become liquid water when the heat is absorbed by the glass cover. The vapors turn into liquid water when heat is transferred to the glass lid. The distillation canal on the lower wall of the still receives condensation drops from the tilting lid. Salts and other contaminants from the pool are consequently left behind, where they can subsequently be rejected using the drainage method, as opposed to the incident solar irradiation, which only causes water to evaporate. A CSS can convert saline water into freshwater, but it is ineffective and has a poor capacity for distillation. This study lists numerical and experimental studies on solar stills with different methods used to maintain minimum water depth to enhance solar stills productivity in an effort to offer a full picture of current scientific advancements. The comparison between the previously reported works related to minimum water depth in basin was summarized in Table 1.
Table 1. Modified designs utilizing minimum water depth in basin of SSs.
| Reference | Method |
- Results - Cost of distillate |
Reference | Turbulence source |
- Results - Cost of distillate |
|---|---|---|---|---|---|
| El-Samadony et al. [41] | Stepped solar still tray width = 10 cm step width = 10 cm |
30.4% higher yield 165% with reflectors and condenser - 0.036 $/L |
Essa et al. [42] | Convex tubular SS - with jute wick over the convex absorber |
92.5% higher yield 114% with Nano - 0.012 $/L |
| Kabeel et al. [43] | Stepped solar still tray width = 12 cm step width = 10 cm |
- 57.3% higher yield - 0.039 $/L |
Omara et al. [44] | Dish pyramid SS - with jute wick over the dish absorber |
54% higher yield - 0.015 $/L |
| Omara et al. [45] | Stepped solar still tray width = 12 cm step width = 10 cm |
- 57.3% higher yield 125% higher with reflectors - 0.031 $/L |
Essa et al. [46] | Pyramid SS with conical absorber covered by wick |
69% higher yield 159% with reflectors and condenser - 0.015 $/L |
| Alawee et al. [47] |
Cords solar still 25 Jute cords |
122% higher yield | Essa et al. [48] | Pyramid SS with pyramidal absorber covered by wick |
53% higher yield 142% with reflectors and condenser - 0.017 $/L |
| Essa et al. [49] |
Cords solar still 35 Jute cords |
118% higher yield 195% with reflectors and condenser - 0.01745 $/L |
Ayoub and Malaeb [50] | rotating drum | output jumped by 200–300%. |
| Alawee et al. [51] |
Cords solar still 25 Jute cords |
122% higher yield 176% with baffles and Ag-Nano - 0.01845 $/L |
Abdullah et al. [52] | Rotating drum | 296% higher yield at 0.1 rpm |
| Abdullah et al. [53] |
Cords solar still 25 Jute cords |
195% higher yield with electric heaters | Essa et al. [54], | corrugated rotating discs | 124% more water production. |
| Omara et al. [55] | Wick solar still |
114% higher yield with Crosswise double-layers wick - 0.027 $/L |
Haddad et al. [56], | Vertical rotating wick |
- daily productivity rose by 51% in the winter and 14.72% in the summer - 0.011 $/L |
| Younes et al. [57] | Half-barrel wick solar still |
154% higher yield with vertical sides wick and half-barrel wick - 0.021 $/L |
Abdullah et al. [58] | vertically and horizontally rotating wick |
- yield with/without nanofluids was increased by 300% and 315%, - 0.027 $/L |
| Abdullah et al. [59] | Corrugated wick solar still with vertical sides wick |
139% higher yield 202% with spiral copper water heaters - 0.021 $/L |
Younes et al. [60] | 4 rotating discs with PCM and reflectors |
- 184% higher productivity - 0.014 $/L |
| Omara et al. [61] | Corrugated wick solar still |
90% higher yield 180% with reflectors and condenser |
Alqsair et al. [62] | Rotating drum with PCM, condenser and PSC |
- 320% higher yield at 0.3 rpm - 72% efficiency - 0.023 $/L |
| Abdullah et al. [63] | Convex solar still with jute wick |
54% higher yield 112% with PCM-Ag and black paint-Ag - 0.025 $/L |
Amer et al. [64] | Corrugated rotating drum + Ag-black paint |
- 318% higher yield at 0.1 rpm - 0.039 $/L |
3. Techniques for maintain a minimum depth in solar stills
3.1. Stepped solar stills
The stepped solar stills provide better thermal performance over conventional solar stills for two reasons: less air volume and larger surface area for the same overall dimensions. El-Samadony et al. [41] studied experimentally stepped SS with external and internal reflectors and an external condenser, Fig. 2. A small fan draws water vapor and blows it to an exterior water-cooled heat exchangerthat serves as the condenser. The stepped SS production of with external condenser and reflectors was found to be around 165% higher than that of the CSS.
Fig. 2. Schematic diagram of stepped and conventional solar stills [41].Kabeel et al. [43] investigated the effect of increasing tray width (13, 12, 11, and 10 cm) and depth (2, 1 and 0.5 cm) on the performance of the stepped SS. The step width remained constant at 10 cm. The modified stepped SS, shown in Fig. 3, is evaluated and compared to a CSS under the identical conditions. According to the findings, the maximum production of stepped SS is attained at a tray depth of 0.5 cm and a tray width of 12 cm, which is approximately 57.3% greater than that of the CSS. In this scenario, the thermal efficiency and anticipated cost of 1 L of freshwater for stepped SS and CSS are 53%-0.039 $ and 33.5%-0.049 $, respectively.
Fig. 3. Stepped solar still with various trays width and depth [43].Omara et al. [45] examined the performance of the modified stepped SS with internal and exterior (bottom and top) reflectors, with 0.5 cm depth and 12 cm width of trays. Using external and internal reflectors can be a cost-effective way to boost solar radiation incident on the basin liner and keep production as high as feasible. Fig. 4 depicts an image of a modified stepped SS with reflectors. During the experiments, the productivity of the modified stepped SS with reflectors was roughly 125% higher than that of CSS.
Fig. 4. Photos of stepped SS with internal and external reflectors [45].A modified stepped SS designated as the enhanced stepped solar still (ISSS) by Amiri [65]. The ISSS compared with a conventional stepped solar still (SSS). In the ISSSS, the stepped solar still body is divided into two parallel chambers; the bottom and upper chambers. The upper one is an evaporation chamber, and the bottom one is located on the under of the absorber assembly and called the condensation chamber, Fig. 5. Results showed that, the maximum fresh water production was achieved as 2335 and 3960 mL/m2/day for SSS and ISSS, respectively. The maximum efficiency of the SSS and ISS is about 21% and 36%, respectively.
Fig. 5. Sectional view of the stepped solar still [65].3.2. Cords solar stills
A new mechanism of utilizing cords wicks inside the PSS was provided by Alawee et al. [47]. This was accomplished by developing a specific number of cracks on a parallel top basin absorber with a 3 cm offset from the solar still's original basin absorber, Fig. 6. The saline water in the basin was preserved at 2.5 cm above the original basin absorber. A wick substance was also used to cover the upper basin liner. Wick cords were also hanged from the cracks and corners of the upper basin absorber to pull saltwater and keep the wick surface wet at all times. Because the cords withdraw an amount of saltwater equal to the evaporated quantity without excessing or withdrawing hot-water, this design saves sun energy and optimizes the utilize of wick inside the SS. As a result, there is no loss of hot water. In this work, the various kinds of wicks (jute and cotton cloths) and the number of cords (35, 25, 16 and 9 cords) were evaluated, Fig. 7. Experimental results obtained that at 25 cords the best performance of cords pyramid SS was achieved, where the increment in daily production of cords pyramid SS higher than the traditional pyramid SS was 118 and 122% when utilizing cotton cloth and jute wicks. Also, at 25 cords the efficacies of jute cords wick pyramid SS and conventional pyramid SS was about 53% and 35%, respectively.
Fig. 6. Schematic view of the experimental setup [47].
Fig. 7. Front and back view of cotton cloth 25 cords [47].In extension paper of pyramid solar stills with cords wick, Essa et al. [49] employed internal and external mirrors to enlarge the sun energy input to the SS, Fig. 8, Fig. 9. In addition, the effect of vapor withdrawal from the cords wick pyramid solar stills to be condensed in internal and exterior cooling cycle was investigated. The experimental results showed that the maximum performance of cords wick PSS was achieved when utilizing the reflectors and fan at wick cords equal 35, where the production rise reached 195% higher than that of the traditional pyramid SS and the daily efficiency was 53%.
Fig. 8. Experimental setup of cords wick SS with mirrors [49].
Fig. 9. Schematic view of the experimental setup of cords SS [49].In another modified paper, Alawee et al. [51] employed six baffles inserted in the cords PSS to reduce the amount of water in the basin and thus raise water temperature of still, Fig. 10. Cotton and jute wick were also examined. Furthermore, three kinds of Nano were studied: TiO2, CuO, and Ag. The Nano were combined with black paint before being applied to the CPSS. Economic and environmental assessments were carried out to validate the CPSS results. The CPSS with baffles and Ag enhanced production by 176% above the PSS with thermal efficiency of 60.4%. CPSS with baffles and Ag had an environmental parameter of 28.71 tonnes CO2 per year.
Fig. 10. Photographs of the experimental test rig (a) setup, (b) side view of baffles and wick cords, and (c) front view [51].Abdullah et al. [53] investigated studied the influence of several burlap wicks made of silk, cotton, jute, and plush clothes on the performance of cords pyramid SS (CPSS). Additionally, three electric heaters powered by a PV-panel were employed to boost the temperature of water of the CPSS basin, increasing its production. The results showed that, the accumulated production of conventional PSS and cords pyramid SS with heaters were 3650 and 10,750 mL/m2, representing a 195% productivity boost. Furthermore, the thermal efficiency the cords pyramid SS with electric heaters and jute wick was 63.5%.
Sharon et al. [66] investigated slanted solar still performance, environmental benefits, distillate quality, and economic viability in depth. As shown in Fig. 11, capillary action assisted in wetting the examined black blended woolen wick sheet via the blackened aluminum tray filled with salty water. Findings showed that the main annual production of the tilted wicks SS was 19.76% lower than that of the tilted SS without wicks. Where, the main production was of 3.94 and 3.29 L/day for the tilted SS without and with wick. The thermal efficiency and exergy efficiency were achieved as 41.06 and 3.06% for the still basin without wick versus 33.8 and 32.88% for the solar still with jute wick.
Fig. 11. Schematic view of tilted wick SS [66].3.3. Wick solar stills
To begin, the water tap is turned on to supply the wicks-type stills. The upper surface of the wicks becomes wet as a result of the capillary action (CA). The CA contributes to the formation of a saturated saline water layer on the entire wicks inside the solar still basin. The porous wicks surface area stretched throughout the basin absorber collects as much solar energy as possible from the still glass cover and surrounding walls. Due to natural convection, the air enclosed by the basin liner, still walls, and the more saturated water particles will be carried by the glass cover, at the higher saline water surface area. It should be noted that the wick-type still has a larger absorbing-evaporating surface area than the typical basin, allowing it to evaporate more basin water than the conventional still. The created saltwater layer through the material of wick is then heated and evaporated. Due to the capillary effect, another water layer is formed, and so on.
Omara et al. [55] evaluated the experimental and theoretical consequences of a single geotextile wick SS coupled with an evacuated solar water collector to assure freshwater continuity from the SS, as shown in Fig. 12. The delicate linen woven materials were stitched (crosswise and longitudinally) into the wicks. They experimented with single-layer wick and double-layer wick, plane wicks without and with lengthwise/crosswise linen, 2 wick stills base slopes (20 and 30°), and giving warm water throughout the night. The results showed that the tilted double-layers wick SS (slope of 30°) generated a distillate that was 114% better than the regular still. Furthermore, the thermal efficiency of the double-layer wicks remained at 71.5%. While, through the night times providing hot water increased the production by about 215%.
Fig. 12. Schematic view of the experimental setup [55].Significant energy losses to ambient were caused by the high temperature area created by the sun radiation striking the SS vertical walls. The vertical walls of the half-barrel solar still (BWSS) and corrugated solar still (CWSS), shown in Fig. 13, were treated with a jute wick by Younes et al. [57]. The surface area is increased by the wick material, where water evaporates and blocks direct sunlight from hitting the distiller's vertical wall sides. The result is the rate condensation and evaporation rate improved while the rate of heat losses decreased. To further enhance the BWSS and CWSS performances underneath the absorber, paraffin wax mixed with CuO-Nano were utilize. According to the findings, CWSS and BWSS had productivity 139% and 154% over traditional SS.
Fig. 13. Corrugated and half barrel absorbers with vertical wick stills [57].Abdullah et al. [59] conducted an experimental study to improve the performance of corrugated wick SS (CWSS) using external spiral copper water heaters (SCWH), and PCM mixed with Ag-Nano, Fig. 14. The corrugated absorber wick SS was planned and built with a SCWH installed vertically on the back wall of the CWSS to warm the feedwater before it reaches the CWSS absorber. Additionally, the vertical wicks are installed, to block the solar radiation falling on the walls, as a result, the temperature of the side walls reduces, reducing the quantity of heat that escapes into the surroundings. The consequences of using a PCM bed and Nano Ag particles beneath the corrugated absorber are also addressed. According to the findings, the CWSS with SCWH and PCM-Ag is 202% more production than conventional SS with thermal efficiency about 63%.
Fig. 14. Spiral water collector with Corrugated wick SS [59].Omara et al. [61] examined the impact of making use of outside condenser with the corrugated absorber SS performance in different experimental research. As seen in Fig. 15, an exhaust fan to the solar still's back has been employed to transfer steam from the SS to the outside condenser. The researchers examined the still performance utilizing various nano in saline water with different depth of water (3, 2, and 1 cm), also using vertical reflectors over the interiors of the 4 vertical sides. The experiment's findings showed that each parameter under consideration had a significant impact on the final water production. As a consequence, the production of the corrugated-wick SS with reflectors was 180% higher than that of CSS while establishing vacuum inside the corrugated absorber SS at 1 cm of basin saline water. Adding cuprous oxides and aluminum-Nano also resulted in increases in the output distillate of 255% and 285%, respectively, under the same circumstances.
Fig. 15. Corrugated SS wick image with mirrors and a vacuum [61].3.4. Absorber shape
3.4.1. Convex absorber
To improve the surface areas of vaporization and exposure inside the SS, a convex absorber was employed instead of the flat absorbers. The convex SS is a traditional SS with a convex absorber covered by wick materials. Furthermore, using convex wick material inside the SS can increase the area of evaporation, reduce water thickness, eliminate dry spots, and reduce the heat capacity of the distillers. Finally, the proposed design aids in improving solar performance while requiring no additional horizontal areas or components.
Abdullah et al. [63] presented an experimental investigation for new solar still called convex solar still, Fig. 16. The convex still tested at various wick materials, various convex heights, with Ag-Nano mixed with black paint, and with PCM-Ag. The results indicated that, at convex height of 15 cm and jute wick the convex still showed 54% superior productivity than CSS. At 15 cm height, the efficiency of convex still indicated around 40.8% and 41.2% for cotton and jute wicks. Additionally, using Ag-Nano into the black paint utilized to coat the surface of convex absorber enhanced daily productivity by 72% over CSS. Furthermore, combining paraffin wax coupled with Ag-Nano and mixed with black paint increased the production of the convex still by 112% over CSS.
Fig. 16. Schematic view of convex and conventional SSs [63].Because increasing the solar still surface area is a key characteristic of its performance, Essa et al. [42] set out to improve the performance of tubular SS (TSS), Fig. 17. Thus, a new-designed convex absorber was used rather than the flat plate absorber to increment both of the exposure surface area of sun radiation and vaporization surface area inside the tubular SS. Furthermore, the impact of graphene and TiO2 nanocomposites on the performance of convex TSS was investigated. It was concluded that utilizing the convex base incremented the surface area of vaporization by about 21.3%. Thus, using the jute wick with the convex TSS enhances the yield by 92.5% and 114% without and with nanocomposites.