Methods used to improve solar still performance with generated turbulence for water desalination- detailed review

1. Introduction

Despite the fact that water covers more than two-thirds of the Planet, a major lack of drinkable water plagues many countries. Furthermore, the tremendous rise of industry and people around the world have resulted in a significant increase in demand for freshwater. In many ways, solar energy remains a great source of freshwater for drinking and agricultural purposes; it's important and technically viable applications of sun energy. By 2030, freshwater resources are expected to be 40% less abundant [1]. Solar power is the main renewable energy source used in the production of electricity [2], solar collectors [[3], [4], [5], [6], [7]], water desalination [[8], [9], [10]], solar cooking [[11], [12], [13]], solar dryers [14], printed circuits [15], solar-biomass [16], solar chimney [17] and heating of metals [18].

It is fortunate that distillers 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. Hence, using solar panels is a practical technique to harness solar energy. Using solar stills has two key benefits [1]. The first is uncomplicated, safe electricity. The second benefit will be its eco-friendliness. Less fossil fuel and hazardous materials are employed wherever sunshine is utilized more frequently, which lowers global warming emissions. 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 SS's efficiency in sun desalination applications is quite poor when compared to other traditional desalination applications [19]. 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 utilized to efficiently deliver freshwater to these sites.

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), stepped SSs [[20], [21], [22]], half barrel SSs [23,24], tubular SSs [[25], [26], [27]], trays SSs [28,29], and pyramid SSs [[30], [31], [32], [33]], have undergone many design and operational tests. Additionally, reflectors [34,35], heat exchanger [36] fins [37], PCM [38], nanoparticle [39,40], rotating components [[41], [42], [43]], wick material [44] and packed bed [45] have been applied to enhance the functionality of SSs units.

2. The fundamental workings of solar stills

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.

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.

From the previous review, the effect of different methods of creating turbulence on the solar still's performance is not recognized in review article. This study lists numerical and experimental studies on solar stills with different methods of creating turbulence to enhance solar stills productivity in an effort to offer a full picture of current scientific advancements. These methods serve to induce turbulence in basin water and break the thermal boundary layer between still surface and water to enhance the evaporation. The use of turbulence devices converts free evaporation to forced evaporation, which increases the evaporation rate. A metal wire vibrator, air bubbles rotating parts and, ultrasonic fogger were used to created water turbulence. The comparison between the previously reported works related to induced turbulence in basin water was summarized in Table 1.

Table 1. Modified designs utilizing turbulence in basin water.

Reference	Turbulence source	- Results - Cost of distillate	Reference	Turbulence source	- Results - Cost of distillate
Eltawil and Omara [46]	Spraying unit and air bubble injection	- 51–148% higher yield - 0.066 $/L	Kumar et al. [64]	Stirring blades with rotating shaft powered by PV	39.49% higher yield
El-Zahaby et al. [47]	Spraying unit	The productivity and efficiency were 6.36 l/m2and 77.35%.	Abdel-Rehim and Lasheen [65]	Stirring blades with rotating shaft powered by PV	7.5% improvement in yield
Essa et al. [48]	Cracks	12% higher yield, at 10 cracks	Rajaseenivasan et al. [67]	Stirring motors	30% improvement in yield
Pandey [49]	Air bubble injection	14% higher yield	Abdullah et al. [68]	Rotating drum with condenser, heater and nano	- 350% improvement at 0.1 rpm, - 0.039 $/L
Kabeel et al. [50]	Air bubble injection with PCM	108% higher yield	Ayoub and Malaeb [69]	Rotating drum	Output jumped by 200–300%.
Kabeel and Abdelgaied [51]	Air bubble injection	- 21.96% higher yield - 0.014 $/L	Malaeb et al. [73]	Rotating drum	Output jumped by 250%.
Porta-Gándara et al. [53]	Air bubble injection	Daily yield of 6.137 kg/m2	Alqsair et al. [75]	Rotating drum with PCM, condenser and PSC	- 320% higher yield at 0.3 rpm - 72% efficiency - 0.023 $/L
Dumka and Mishra [55]	Ultrasonic fogger	33.36% higher yield 31.04% more thermal efficiency	Alwan et al. [76]	Rotating drum	161% higher yield at revolving speed of 0.5 rpm
Dumka et al. [56]	Ultrasonic fogger and cotton cloth	53.12% higher yield 44.64% more overall efficiency	Amer et al. [79]	Corrugated rotating drum + Ag-black paint	- 318% higher yield at 0.1 rpm - 0.039 $/L
Liu et al. [57]	Ultrasonic fogger and evacuated tubes	- 83.87%, higher yield 18.29%, more thermal efficiency - 0.02 $/L	Essa et al. [80]	Rotating drum	- The output of a tubular SS increased by 175% and 140% on open and closed ends - 0.024 $/L
Emad et al. [58]	Vibratory harmonic effect	- 14.39% higher yield - 0.031 $/L	Essa et al. [81]	Corrugated rotating discs	124% more water production.
Eldalil [59]	Vibration of helical wires	72% higher yield	Younes et al. [42]	4 rotating discs with PCM and reflectors	184% higher productivity - 0.014 $/L
Mohamed et al. [60]	Vibration effect	- Daily of 12.82 kg/day - 0.0178 $/L	Diab et al. [83]	Rotating discs with vertical SS	Yield with/without vacuum fan was increased by660% and 548%,
Kabeel et al. [61]	Stirring fan powered by PV	25% higher yield at 45 rpm and a depth of 3 cm	Haddad et al. [84]	Vertical rotating wick	- Daily productivity rose by about 51% in the winter and by about 14.72% in the summer - 0.011 $/L
Omara et al. [62]	Stirring fan powered by a wind turbine	17% higher yield compared Still efficiency of 39.8%	Abdullah et al. [85]	Vertically and horizontally rotating wick	yield without and with nano was increased by 300% and 315%, - 0.027 $/L

3. Techniques for creating turbulence in solar stills

3.1. Spraying unit

Eltawil et al. [46] linked the SS to an external condenser, a spraying unit, a flat plate solar water heater, and a solar air heater to raise the temperature of the water of basin, as shown in Fig. 2. To improve the rate of evaporation, the circulated water might be sprayed or atomized into the modified SS to break surface tension by pumped from the bottom upward in the shape of a fountain. According to the authors, the freshwater of the MSS was superior to that of the reference SS with the solar heater integrated, as shown in Fig. 3. Depending on the operating conditions, the output of the solar distiller increased by 51–148%. The water solar heater integrated with basin solar still increased the production over CSS by about 56% (3450 for CSS and 5400 mL/m2/day for modified SS) and 82% (3400 for CSS and 6200 mL/m2/day for modified SS) in case of sprayed hot water (passive and active circulations).

El-Zaheby et al. [47] investigated the utilize of a feeding through spraying mechanism to enhance the productivity of corrugated stepped SSs, Fig. 4. A transverse reciprocating spraying mechanism controlled the saline water feeding into the still in the shape of little droplets. The efficiency and total production in 10 h were 77.35% and 6.36 l/m2.

Efficiency is increased and solar still wall losses are decreased with the brand-new SS design known as Tray. In order to create the trays solar still, the conventional SS was altered to contain interior trays as well as exterior and interior mirrors at the top and bottom. TSS vertical walls can be equipped with trays and reflectors to reduce heat loss to the environment and lower their temperature. Besides, Essa et al. [48] tested the performance of TSS with cracks in the trays surface, Fig. 5. As well, the effects of using internal and exterior reflectors as well as crack counts of 7, 10, and 14 were investigated. According to the experimental findings, employing 10 cracks in the trays resulted in higher output than using either 7 or 14 cracks, where the output increased at 10, 7, and 14 cracks, respectively, by 12, 9, and 8%. Additionally, using internal and external reflectors with 7 cracks increased the potable water distillate of the trays SSs by 104% when compared to the CSS.

3.2. Air bubble injection

Hot air bubbles enhance the contact area at the water-air interface in the presence of a bubble injection, which can boost heat and mass transfer between the air and water, resulting in increased productivity. This demonstrates that the bubble injection technology considerably increases the still's performance. Pandey [49] investigated the effects of forced air bubbling, dried and glass cover cooling in an air bubbled SS. He found that combined dry air bubbling and glass cover chilling improves daily distillate more than dry air bubbling alone (Fig. 6). Additionally, he concluded that the increase in distillate yield when compared to a reference was 33.5–47.5%. Thus, the percentage of increase due to air bubbles is 14%.

Kabeel et al. [50] investigated the effect of hot air injection on the behavior of still-contained phase change materials under a basin, as seen in Fig. 7. The results showed that the potable water production reached roughly 9.36 for the MSS while it was 4.5 L/m2 day for the CSS, representing a 108% increase.

Kabeel and Abdelgaied [51] presented experimental results from five operational conditions of the proposed hybrid system. They experimental investigation intends to increase the performance of a PV system via the use of mirrors and cooling, add to the freshwater output of a constructed SS through the use of an air injection unit, Fig. 8. The key outcome was that using air injection inside the constructed solar still improved potable water output by 21.96% compared to not using an air injection system. Where, the freshwater productivity reaches 5490 for modified still and 4500 mL/m2 day for conventional SS, respectively.

The influence of a bubble injection unit on the portable SS performance was experimentally tested by Rasoul et al. [52], the bubble injection system shown in Fig. 9. Tests were carried out both outside and inside. Air bubbles are injected into the water basin using the bubble injection technology. The effect of the bubble injection unit on the performance of the SS (Fig. 10) demonstrates that employing the bubble injection system boosts the still's productivity. With irradiation of 585 and 210 W/m2, the productivity rate of water decreases from 42 to 26 and 91 to 62 mL/h, respectively, against the system without bubbles injection. This study also studied the influence of depth of water basin on still production by conducting experiments at three various depths of water of 2, 5, and 8 cm. At a water depth of 5 cm, the better freshwater production was obtained. When the water level rises above 5 cm, the bubbles formed by the bubbles injection system must travel a greater distance to reach the water's surface, increasing the likelihood of bubble burst before reaching the surface of the water and, as a result, lowering the still's performance. For a 10-year useful life, an average cost of $0.08/L of freshwater was projected, which compares favorably to other types of SSs.

Gándara et al. [53] studied the possibilities of improving the freshwater production of a shallow conventional SS by inserting air bubbles into the basin water, Fig. 11. The disturbance is accomplished by injecting air bubble into the basin water, causing surface ripples and so enhancing the overall surface area of evaporative and driving the mass transfer coefficient. Generally, freshwater output increases as evaporation enhances. The flowrate and position of the air bubbles injectors appear to have little effect on the amount of evaporation increase. Furthermore, as seen in Fig. 12, rising bubbles approach the surface of water from below and generate a slight bump before bursting, thereby expanding the heat transfer surface. The succession of water droplets propelled upward after they burst severely disturbs a thin micro layer of water.

3.3. Ultrasonic fogger (UF)

An UF is typically made comprised of a piezoelectric ceramic disc that is powered by two electrodes (nickel type). Because the ceramic is piezoelectric, it oscillates at ultrasonic frequency in the presence of an electrical current (As a result, sound-waves formed in saline-water are inaudible). Water attempts to follow the frequency of vibration of the plate as it rises, as shown in Fig. 13. Water, however, fails to match the frequency due to weight and inertia, resulting in water hammers [54]. As a result of the water wave lagging behind the disc wave, a low-pressure zone emerges between them, forming a hollow. This is known as cavitation. When air is dissolved in water, and air moves into the cavity with the production of low pressure, the cavity might be a vacuum or filled with air. When this cavity implodes, a lot of energy is released, and the imploding jet contains a lot of energy as illustrated in Fig. 13. Capillary waves are likewise caused by the vibrating disc. These capillary waves oscillate up and down due to surface tension and gravity. When the hole implodes, a crossed capillary wave forms at the surface, and extremely small water droplets have enough energy to break through the surface tension and escape the water. As soon as these little drops of water leave the surface of the water, they are absorbed into the stream of air above them and depart as mist from the humidifier. With increased frequency, the droplets size would shrink. Furthermore, If the capillary waves oscillate at roughly half the driving frequency, the atomization threshold will be nearly linear with water viscosity [54].

Dumka and Mishra [55] aforementioned the utilize of UF for the improvement of daily production of a conventional SS so that its cost/L can be decreases, Fig. 14. They deployed the UF within the CSS basin water. Fig. 14b shows a photograph of UF with aluminum hood cover. They discovered that the UF increased the surface area of saline water and generated turbulence within the basin water. They discovered that the UF performs best during peak irradiation hours. They have noticed a significant drop in productivity output due to over misting of stills during the hours when sun radiations are quite feeble. In Fig. 14c wakes can be visible on the surface that are propagating from the UF in all directions. They results indicated that, enriched the production of SS by augmenting an UF. Owing to vibration caused by UF the water temperature of modified SS leads over conventional SS, which has resulted in a higher temperatures gradient in modified SS as compared to conventional SS. The UF increased the evaporative heat transfer coefficient about 90% compared to conventional SS which ultimately turned in to more production of around 33%.

Dumka et al. [56] enhanced the SS performance using a UF and cotton cloth, as shown in Fig. 15. To avoid over-misting, a cotton cloth was used in conjunction with a fogger. They demonstrate that the continuous breakdown of water into fine mist (by UF) and capturing that mist by cloth to remain drenched in water (because of capillary action) resulted in a significant increase in the evaporative heat transfer coefficient in modified SS when compared to conventional SS, Fig. 16. MSS's calculated hew value is 46.13% greater than CSS's. When compared to a conventional still, these improvements resulted in a 53.12% higher yield. These changes also increased the still's efficiency by 91 44.6%.

Liu et al. [57] conducted experimentally an investigation to enhance the thermo-economic pyramid SS performance by enhancing the processes of condensation and evaporation. Ultrasonic foggers and evacuated tubes were combined into the design to achieve high vapor generation, as shown in Fig. 17. As a result of including all of the suggested additives, the DPSS may increase production of fresh water, exergy efficiency and energy efficiency, by 83.8%, 38.86%, and 18.29%. Where, the daily yield reached 6270 mL/m2 per day for modified PSS, and 3410 mL/m2 per day for PSS with 83.87% improvement. Also, the cost of manufacturing per L of freshwater was decreased by 11.6%.

3.4. Vibratory harmonic effect

The vibrator is used to break down the surface tension and boundary layer of saline water, increasing vaporization rate and heat transfer. Emad et al. [58] use a porous packed media made of steel wire mesh screen to improve the effectiveness of absorbing, transmitting, and storing heat in tubular SS, Fig. 18. A vibrator is also connected to the wire mesh screen to provide forced vibration, which is a harmonic movement in the transverse plane. Experimental research, as well as economic and thermodynamic analysis, is carried out. The MSS yield was 4.2 L/m2 with a 34% increase over the standard yield. The MSS produced freshwater at a cost of roughly 0.031 US$/L.m2 and a 14.4% savings over the regular one.

Eldalil [59] putting helical spin wires in the conventional SS. As seen in Fig. 19, the bundled layer has a worthwhile influence when harmonically agitated. The helical wires oscillation has a positive influence on the thermal efficiency of the modified SS due to an increase in Nusselt number. This raises the overall heat transfer coefficient. Condensation on the polycarbonate vibrating glass cover increases as well. The SS production was 3.8 L/m2 per day, a 35% improvement with helical wires, and a 5.8 L/m2 per day and 60% improvement in the presences of vibration. The output increased by 72% as a result of the vibratory stimulation influence. The distiller efficacy of helical wires is 35% higher than that of a simple still and increases to 132% when vibration is included.

Mohamed et al. [60] investigated how the CSS performance may be improved by combining thermoelectric cooling, TC, an electric heater, vibration motion, Fig. 20. The investigation looked at the influence of combining two of these parts, then all of them, on water production. The vibration motion and electric heater are employed to increase the rate of evaporation, while TC are utilized to increase the rate of condensation, resulting in high freshwater productivity. The maximum daily water production obtained with vibration motion, thermoelectric coolers, the solar heater, and was 12.8 L/day, with a maximum projected cost of 0.0178 $/L/m2.

3.5. Stirring turbulence

A smart idea was used to increase distiller profitability by rotating the film of water in the SS basin utilizing paddles or blades powered by various mechanisms such as a DC-motors. Others also discovered a connection between 2 kinds of renewable energy. To boost movement in the basin, a tiny wind turbine can power the water fan. Hence, by combining wind and solar energy, we may obtain cleansed water. In the presence of rotating fan, the productivity increases as evaporation and condensation improve; this could be attributed to the following:

1.
Because the moving water fan disrupts the water surface boundary layer, condensation and evaporation rise.
2.
By utilizing a rotating fan, free evaporation is converted to forced evaporation, which increases the evaporation rate.
3.
Convection heat exchanges between basin water and the basin's base continue to improve as the rotation of the fan improves the water molecules turbulence intensity.
4.
The saline water is heated by the revolving water fan.
5.
Because the operation of the water fan and turbine causes some vibration in the system, the condensate distillate flows faster downward towards the output.
6.
Waves are formed as a result of the movement of the water fan through the water, increasing the absorption area of solar irradiation and the evaporation area.

Kabeel et al. [61] used a vertical shaft spinning water fan to investigate distiller production efficiency. A photovoltaic unit was utilized to power the DC-motor that turned the fan. As demonstrated in Fig. 21, the fan speed (ranging from 30 to 45 rpm) and salty water level (ranging from 1 to 7 cm) had an effect on the efficiency of this still. It was discovered that employing a water fan increases the gain by roughly 25% at depth of 3 cm and 45 rpm. However, there were certain issues with this design, such as the possibility of leaking of water through the ball bearing and the expense of PV-unit.

Omara et al. [62] attempted to resolve the preceding flaws. As a result, a water fan powered by a wind turbine, WT, increases the distillate. To prevent leakage, the rolling shaft passes through a SS rear wall. Moreover, maintenance and start-up costs are reduced. As seen in Fig. 22, the new model has a minor advantage over the previous system. At 3 cm and 30 rpm, using a water fan improved daily yield by roughly 17%. Nevertheless, the fan distiller's daily efficiency was approximately 39.8%.