Introduction

A single-basin solar still is a simple device used to purify water using the heat of the sun. It consists of a basin or container that holds the contaminated water and a transparent cover or lid that allows sunlight to enter. The sunlight heats up the water, causing evaporation, and the condensation then collects on the underside of the cover and drips into a separate collection container, leaving behind impurities. The system is simple in manufacturing and with low cost (Prakash and Velmurugan 2015). On the other side, the system suffers from low productivity. The efficiency of a single-basin solar still depends on several factors, including sunlight intensity, ambient temperature, humidity, and design (Sharshir et al. 2020). Generally, these stills have lower efficiency compared to other water purification methods. The output of purified water is relatively low, as the evaporation and condensation processes are slow. It may take several hours or even days to produce a significant amount of purified water, depending on the size and conditions (Feilizadeh et al. 2017).

Researchers have explored various factors to enhance the performance of single-basin solar stills numerically and experimentally. Some of these factors include shape and design optimization (Feilizadeh et al. 2017) [3], materials selection (Prakash and Velmurugan 2015; Keshtkar et al. 2020), heat transfer enhancement techniques (Phadatare and Verma 2007; Bait and Si-Ameur 2018), advanced modeling and simulation(Johnson et al. 2019; Keshtkar et al. 2020; Megahed and El Mahallawy 2022), integration with other renewable energy sources, water flow management (Sathyamurthy et al. 2015), and adding nanomaterials (Elango et al. 2015; Gupta et al. 2016; Kabeel et al. 2017).

Researchers have investigated different shapes and designs of single-basin solar stills to improve their performance. This includes studying the effect of inclined and V-shaped stills, parabolic, as well as different geometries and configurations, to enhance heat transfer and maximize water production. The important conclusion is that increasing the surface area of the basin exposed to solar radiation can enhance heat absorption and evaporation rates, leading to higher productivity (Rajaseenivasan and Srithar 2016). A larger surface area allows for more water to be heated and evaporated, which in turn increases the amount of condensed water collected. There is a total yield increase in productivity from 2.82 to 3.24 L/(m2.day) by changing the shape of the conventional still to a triangular basin solar still (Prasad et al. 2021).

The productivity increased to 4.8 L/m2/day using a pin-finned absorber and wick. Another study includes the enhancement of water residence time using baffles in semicircular stills, as well as the effect of the number of baffles and water flow. Basin water temperature was directly proportional to the number of baffles. The system productivity reached 3 L/m2/day. The design was complex, and the payback period of the present solar still was low in comparison to other conventional methods (Sathyamurthy et al. 2015).

Another study used circular and square fins with wicks integrated into the basin of the solar still (Rajaseenivasan and Srithar 2016). The productivity of the still unit improved by 36% and 45.8% for circular and square fins covered with wick, respectively. The cost of water predicted was 0.024 $/kg for square finned stills with wick material.

Various heat transfer enhancement techniques have been explored to improve the performance of single-basin solar stills. These techniques include the use of fins, ribs, and heat pipes to enhance heat conduction and increase the thermal efficiency of the stills(Omara et al. 2011; Rajaseenivasan and Srithar 2016; Muraleedharan et al. 2019).

Changing the inclination angle of the solar still cover might change the output daily yield and efficiency. Most studies recommend that an inclination angle equal to the latitude angle of the city gives the best yield and efficiency. For Egypt, the best angle of inclination is found to be 30⁰ (Omara et al. 2011).

The water depth effect on the still performance is investigated; as the depth decreases, the productivity increases. The highest performance was at a water depth of 10 mm which is compatible with references (Phadatare and Verma 2007; Rajaseenivasan and Srithar 2016; Kabeel et al. 2019; Wilson et al. 2019). The maximum productivity reached is 4.5 L/(m2.day) at 10 mm water depth for the unit basin area.

The choice of materials for constructing the basin, cover, and insulation layers of the solar still can significantly impact its performance. Researchers have explored the use of different materials, such as glass, plastic, and composite materials, to enhance heat absorption, minimize heat loss, and increase overall efficiency (Megahed and El Mahallawy 2022).

The choice of insulation material in a single-basin solar still can have a significant impact on its productivity and overall efficiency. Insulation helps to reduce heat loss and maintain higher temperatures within the solar still, thereby enhancing evaporation rates and distillation efficiency. Effective insulation reduces heat dissipation and helps maintain higher temperatures within the still. This, in turn, increases the evaporation and condensation rates, resulting in improved productivity (Balachandran et al. 2021; Muthu Manokar et al. 2020). Insulation materials with low thermal conductivity, such as foam, fiberglass, or reflective coatings, help to minimize heat loss from the solar still (Özlüsoylu et al. 2019). When selecting insulation materials for a single-basin solar still, it is important to consider factors such as thermal conductivity, durability, cost, and availability. Additionally, the insulation should be applied to the appropriate areas of the solar still, such as the walls, or base to effectively reduce heat loss and enhance productivity.

External resources like adding external reflectors around the solar still can help increase the concentration of sunlight in the basin. Reflectors can be made of materials like mirrors or reflective foils and are positioned to redirect and focus sunlight onto the still. This concentrated solar energy intensifies the heat input, leading to higher evaporation rates and improved productivity. Johnson et al. (2019) used a Linear Fresnel Reflector with different titled angles to focus the solar irradiance on a specific area in the cavity.

In this work, the main aim is to improve the efficiency and increase water production of SBSS to be suitable for mass production of fresh water by changing the depth of water and allowing solar radiation from side walls and backside.

A numerical model was developed to predict the best productivity of the SBSS. The fabrication of a four-inclined SBSS and the subsequent experimental examination based on the numerical model findings add practical validation to the research. The experiments were carried out and the hourly experimental results were compared with the numerical results. By comparing the experimental results with the numerical results, the study demonstrates the effectiveness and reliability of the numerical model in predicting SBSS productivity. The main objective is to address the increasing demand for water, especially in the agriculture sectors and other sectors such as industry, and domestic use, especially in regions with water scarcity or limited access to clean drinking water.

Experimental work

In this investigation, four units of single-basin solar still (SBSS) were tested under different conditions. The setup is installed and constructed at Al Qasr zone, Marsa Matrouh Governorate 31° 21′ 10.44″N, 27°14′14.10″ E, Agricultural Station—Agricultural Research Center (ARC), Egypt. The materials and the geometry of solar units are based on the best-selected parameters findings in Megahed and El Mahallawy (2022) and are shown in detail in Fig. 1.

Fig. 1
figure 1

Unit of single-basin solar still a) isometric view and b) back view

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Materials

The materials used for the installation of a single-basin solar still are an iron frame, tempered glass, a wooden frame for the basin, black paint, Foam for insulation, and an aluminum basin. Extra components are used as a valve for water, tubes for inlet and outlet water, salt water, sink drainage, and water tracks.

Experimental setup

Four solar stills are installed. The main components of the solar still unit are a frame box containing a metallic basin, sides, a cover plate made of tempered glass, and insulation as shown in Fig. 1. The solar collector basin area is 605 cm2 with dimensions 1100 × 550 × 8 cm for length, width, and height, respectively. The basin is made of aluminum sheets. Not only the basin is black matte painted but also the entire inner cavity of the basin has been painted with a matte black color to increase the efficiency of the absorption of solar radiation with an absorptivity ≈ of 0.9. The sides of the solar still frame were structured from pieces of trapezoid-shaped wood with a 15 cm height and thickness of 2.54 cm as shown in Fig. 2.

Fig. 2
figure 2

Schematic drawing for modified solar still including a) elevation and b) side view with dimensions in cm

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In Fig. 3a, 5-mm-thick tempered glass sheets were used to cover the solar flat-plate collector of the solar still unit from all sides front, sides, and back of the solar flat-plate collector. The back side of the solar flat-plate collector was rectangular with dimensions of 112 × 34 cm. As the same, the front side of the solar flat-plate collector was rectangular with dimensions 112 × 66 cm and installed at an angle of inclination of 30° facing the sun. The sides of the solar flat-plate collector were tempered glass right triangles with dimensions of 33 × 55 cm, and the hypotenuse was 64.1 cm.

Fig. 3
figure 3

Solar still with/without insulation glass sides

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A sheet of heat-insulating foam is set at the bottom of the box to preserve the heat inside the solar collector with dimensions 110 × 55 × 5 cm for length, width, and thickness, respectively. The presence of four case studies, two stills with insulation surroundings basin are only as shown in Fig. 3b, and for the other two stills three sheets of insulation material are installed in front of the back, left, and right glass sides of the stills to test the effect of shading on the hourly productivity as shown in Fig. 3c.

The environmental parameters from wind speed and solar irradiance are hourly measured across the day using a wind anemometer (an environment meter, Model: EM-9300SD) and Pyrometers (Model PCE-SPM 1, made in China). Also, the temperature for ambient, tank water, water in the basin, and inner glass cover was hourly measured using a K-type thermocouple.

Numerical modeling

When conducting numerical analyses for single-basin solar still, it typically focuses on modeling and simulating the heat and mass transfer processes involved. Start by defining the geometry of the single-basin solar still, including the dimensions and shape of the basin, cover, and condenser. Then, consider factors such as the basin area, depth, slope, and the position and size of the condenser. The next step is to establish the governing equations that describe the heat and mass transfer processes occurring in the solar still. This typically involves equations for energy balance, heat conduction, evaporation, condensation, and mass transfer. These equations may be derived from fundamental principles or empirical correlations.

The boundary conditions for each component of the solar still must be specified including the incoming water temperature, the ambient temperature, humidity, solar irradiance, and wind speed. Boundary conditions help simulate the real-world operating conditions of the solar still. Then appropriate numerical algorithms and solvers are used to obtain a numerical solution for the system.

The model in (Megahed and El Mahallawy 2022) is used in numerically estimating the hourly productivity of single-basin solar still using MATLAB software. The model equations not only include heat transfer but also solar flux absorbance of water, basin, and cover plate.

The fraction solar flux absorbance for cover plate \({\stackrel{\cdot}{\alpha }}_{g}\), water \({\stackrel{\cdot}{\alpha }}_{w}\), and basin \({\stackrel{\cdot}{\alpha }}_{b}\), which are given by the following equations (Johnson et al. 2019):

$${\stackrel{\cdot}{\alpha }}_{g}={\alpha }_{g}(1-{R}_{g})$$
(1)
$${\stackrel{\cdot}{\alpha }}_{w}= \alpha_{w} \left( {1 - \alpha_{g} } \right)({1} - R_{g} )({1} - R_{w} )\sum \mu_{j} {\text{Exp}}\eta_{j} d_{w}$$
(2)
$${\stackrel{\cdot}{\alpha }}_{b} = \alpha_{b} \left( {1 - \alpha_{g} } \right)\left( {{1} - R_{g} } \right)\left( {1 - R_{w} } \right)\left( {1 - \sum \mu_{j} {\text{Exp}}\eta_{j} d_{w} } \right)$$
(3)

The top heat losses are the heat losses from the outer cover plate to the atmosphere due to radiation and convection heat transfer. The convection heat transfer loss from the outer glass cover to the atmosphere, QCgoa, can be defined (Lanjewar and Prayagi 2016);

$$Q_{{{\text{cgoa}}}} = h_{{{\text{cgoa}}}} (T_{g} - T_{a} )$$
(4)

As well as, the radiation heat transfer loss from the cover plate’s outer surface to ambient Qrgoa can be defined by (Lanjewar and Prayagi 2016):

$$Q_{{{\text{rgoa}}}} = h_{{{\text{rgoa}}}} (T_{g} - T_{a} )$$
(5)

Heat is also lost from the basin through the insulation and subsequently from the bottom or side surface of the basin. The overall top heat loss coefficient from the water surface to the ambient through the cover plate \(({U}_{t}\)) is given by (Badran and Abu-Khader 2007; Setoodeh et al. 2011; Lanjewar and Prayagi 2016):

$$U_{t} = h_{1} h_{{{\text{Tgoa}}}} /\left( {h_{1} + h_{{{\text{Tgoa}}}} } \right)$$
(6)

The governing equation for the overall bottom loss coefficient (Ub) is given by:

$$U_{b} = {\text{h}}_{w} {\text{h}}_{b} /({\text{h}}_{w} + {\text{h}}_{b} )$$
(7)
$${\text{h}}_{b} = \left( {T_{{{\text{in}}}} /K_{g} } \right) + \left( {h_{{{\text{Tgoa}}}} /\left( {h_{{c{\text{goa}}}} {*}h_{{r{\text{goa}}}} } \right)} \right)^{ - 1}$$
(8)

where the overall all-side losses (\({U}_{s})\) equation is given by:

$$U_{s} = U_{b} \left( {A_{s} /A_{b} } \right)$$
(9)

The overall losses from water to ambient through the top, bottom, and sides (\({U}_{OA})\) is given by:

$$U_{OA} = U_{b} + U_{s} + U_{t}$$
(10)

Natural convection results from the temperature difference between the water surface and the inner cover plate. The rate of convective heat transfer \({(Q}_{cwg})\) and the heat transfer coefficient (\({h}_{cwg})\) between water and the inner glass surface are expressed as follows, respectively (Agrawal et al. 2017).

$${Q}_{\text{cwg}}={h}_{\text{cwg}}\left({T}_{w}-{T}_{g}\right)$$
(11)

Evaporation heat transfer \({Q}_{ewg}\) occurs between water mass and the inner glass surface and is generated when the vapor pressure is lower than the saturation pressure of the liquid, and the governing equation is:

$$Q_{ewg} = h_{ewg} \left( {T_{w} - T_{g} } \right)$$
(12)

Radiation heat transfer from basin water to glass cover is produced by the emission of internal energy between two bodies having different temperatures which are, in this case, water mass and the inner glass surface. The governing equation is:

$$Q_{{{\text{rwg}}}} = \sigma \varepsilon_{{{\text{eff}}}} \left( {T_{w}^{4} - T_{g}^{4} } \right)$$
(13)
$$\varepsilon_{{{\text{eff}}}} = { 1}/ \, (({1}/E_{w} ) + (1/E_{g} ) - {1})$$
(14)

From the relation between heat and mass transfer, the mass transfer rate \(\dot{{\text{M}}_{\text{w}}}\) can be written as (Badran and Abu-Khader 2007; Lanjewar and Prayagi 2016)

$$\dot{M}_{w} = 3600{*}Q_{{{\text{ewg}}}} /{\text{L}}_{{{\text{ev}}}}$$
(15)

The cumulative productivity \({C}_{{M}_{w}}\) can be calculated using the following equation:

$$C_{{M_{w} }} = \mathop \sum \limits_{i}^{t} \dot{M}_{w}$$
(16)

The overall efficiency \({\eta }_{o}\) represents the heat evolved divided by the (Bait 2020):

$$\eta_{o} = \frac{{\sum \dot{M}_{w} {\text{*L}}_{{{\text{ev}}}} }}{{\sum I\left( t \right)*A_{t} *\eta_{\alpha } *3600}}$$
(17)

\({\eta }_{\alpha }\) is the absorption efficiency of the solar still in absorbing solar radiation (typically assumed to be between 60 to 80%). The absorption efficiency can be calculated based on the transmittance (T) and reflectance (R) of the solar still’s cover material. The transmittance refers to the fraction of solar radiation that passes through the cover material, while the reflectance represents the fraction of solar radiation that is reflected off the cover material.

The absorption efficiency (\({\eta }_{\alpha }\)) can be determined using the following formula:

$$\eta_{\alpha } = 1 - T - R$$
(18)

Through the numerical model, the input data are geometry parameters, optical, and thermal properties of tempered glass, basin, and thermal resistance of insulating material shown in Table 1. In addition to the hourly ambient temperature, wind speed, and solar irradiance shown in Table 2, the output data are the hourly water and glass temperature and productivity of the still.

Table 1 The input data for glass, insulation, and water for a numerical model (Megahed and El Mahallawy 2022)
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Table 2 Variation in ambient temperature, wind speed, and solar irradiance for the 8th, 9th, and 10th of August
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Results and discussion

The productivity of a single-basin solar still can be influenced by various factors, including water depth and insulating material. In this research work, the productivity of single-basin solar still is investigated for water depths 1 and 2 cm with and without insulation around the glass cover for three different days 8th, 9th, and 10th in August 2023.

From the experimental measurement, it was found that the average ambient temperature was 31.7 °C across the three days. The wind speed varies across the day with averages of 2.4, 2.35, and 2 m/s on day 1, day 2, and day 3, respectively. The values of solar irradiance increase till 1 pm for the three days and then start to decrease. The highest average solar irradiance was on day 8th and 9th of August with a maximum solar irradiance reaching 920 and 920.3 W/m2.

The choice of tempered glass as a cover plate was because of its transparency and ability to transmit around 91% of the sun’s rays to the water and basin. Additionally, when heat or solar energy is absorbed by the cover plate, it is either convection away by moving air or reradiated by the plate surface.

In the case of a completely insulated solar still from the sides, back, and basin area, the model results have been compared to the experimental results for three days including the hourly change in water temperatures and cumulative productivity at water depths 1 and 2 cm. Figure 4a–c indicates a good agreement between the model and experimental results for the trend of water temperature variation across the three days. The average variation in water temperature ranges from 10–13%. With an average 3% increase in water temperature by decreasing water depth from 2 to 1 cm, the change in water temperature can vary based on several factors, including the water depth. With natural convection and heat transfer, deeper layers of water tend to have lower temperatures compared to shallower layers. This is because heat from the surface can be transferred to the deeper layers, leading to a decrease in temperature with increasing depth.

Fig. 4
figure 4figure 4

Comparing experimental and numerical results: a–c water temperature and d–f cumulative productivity, for SBSS with cross-section area 0.606m2 at water depth 1 and 2 cm with insulation surrounding sides and basin

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The maximum productivity reached is on day 3, where the cumulative productivity for 13 h with cross section 0.606 m2 is 2.24 L for a water depth of 2 cm and 2.7 L for a water depth of 1 cm. Figure 4d-f illustrates the hourly variation in cumulative productivity across three days for water depths 2 cm and 1 cm.

It was found that the model results are higher than the experimental results, and the average variation in cumulative productivity ranges from 19–26% for a water depth of 2 cm which is a high deviation if compared to a water depth of 1cm with a deviation of 10–15%. Since increasing water depth increases the turbulence inside the still, decreasing water depth increases the cumulative productivity of single-basin solar still by around 21% experimentally and 7% numerically (Fig. 5).

Fig. 5
figure 5

Three days cumulative productivity experimental versus numerical results for SBSS with cross-section area 0.606m2 at: a water depth 2 cm, b water depth 1cm

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Comparing experimental results to numerical results, Johnson et al. (2019) showed a deviation in cumulative productivity of around 37.2%. In this study, the variation is very low where the maximum variation is only 24%.

As shown in Fig. 6a-c, it was found that the average variation in experimentally and numerically water temperatures is 7%, for SBSS without insulation surrounding sides. The variation is lower if compared to the variation with insulation. Figure 6d-f represents the hourly cumulative productivity. The maximum productivity reached is 3 L for 13 h working SBSS with a cross-section area of 0.606m2 at a water depth of 1 cm on day 3. The deviation in numerical results for cumulative productivity ranges from 2 to 12%.

Fig. 6
figure 6figure 6

Comparing experimental and model results: a–c water temperature and d–f cumulative productivity for water depth 1 and 2 cm for SBSS with cross-section area 0.606m2 without insulation surrounding sides and with insulated basin area

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Furthermore, a comparison between the effect of water depth with and without insulation surrounding the glass cover is shown in Fig. 7. For a water depth of 10mm with full insulation for the basin area and sides of the glass cover, the maximum cumulative productivity reached was 2.7 L for 13 h working still with an efficiency of 83% on the 3rd day “10th of August”. Compared to the still, without insulation surrounding the glass cover, the maximum cumulative productivity reached was on the 2nd day with a productivity of 3 L for a 13-h working system. The system efficiency is 96% as shown in Table 3.

Fig. 7
figure 7

Hourly cumulative productivity of single-basin solar still for three days in August at 1 and 2 cm water depth with and without insulation surrounding side glass

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Table 3 Efficiency of different systems with and without insulation surrounding sides at 1 and 2 cm water depth
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Conclusions and recommendations

This study evaluated four SBSS to examine the effect of water depth and the surrounding glass cover on performance and freshwater productivity. The novelty of this research aimed to enhance the efficiency of using passive water desalination systems to exploit solar energy and employ it effectively to reduce dependence on traditional energies and thus reduce the carbon footprint of these systems to confront severe climate changes. Also, the novelty in exploring the insulation techniques to improve the SBSS performance. The study investigates the impact of insulation on water temperature, shading area, and cumulative productivity. The findings suggest that strategically adding insulation to certain parts of the SBSS can enhance its efficiency and productivity. Based on the numerical and experimental results, the main conclusion could be summarized as follows:

  • Using glass surrounding all sides allows more solar radiation to increase the evaporation rate and enhance productivity. In addition to, the appropriate location to install solar stills. The advantage of Marsa Matrouh is the availability of solar irradiance at a high rate especially in summer and the presence of seawater.

  • The basin water depth between the water surface and glass cover has a considerable effect on the distillate freshwater yield; the highest collected distilled yield could be obtained at the lowest water depth due to the rapid evaporation and condensation rate. The best water depth is 1cm, and the front side length is 10 cm.

  • Despite the simplicity of the setup, the change in geometry parameters and selection of materials led to an increase in productivity of around 3 L for a basin area 0.6 m2 with efficiency 94% for only 13 h working system.

  • The addition of insulation in front of the sides and back of tempered glass increased the shading area of the SBSS unit. This increased shading area resulted in reduced water temperature and decreased productivity. The insulation effectively reduced the heat transfer to the water, thereby lowering its temperature.

Confuting the experimental work at Marsa Matrouh, Egypt, provides context-specific insights into the performance of SBSS in that region. The results for modified SBSS design and performance optimization can be achieved in similar geographical and environmental conditions.

More research is required for the night period and how to enhance the desalination rate in the absence of solar radiation by implementing phase change materials (PCM) and storage materials with high thermal capacities on the basin.

Further numerical simulations or experimental attempts based on nanofluids to increase the water’s thermal conductivity should be conducted including cost and applicability. It is also recommended to study the effect of Nano coating with different concentrations and thicknesses on increasing the basin absorptivity.