0 Introduction

Water, especially freshwater, is the most valuable and necessary component for human life all over the world. Historically, people have depended on natural resources such as rivers, streams, lakes, and ponds to satisfy their demand for freshwater. However, the demand for portable water is in a continuous increase due to population growth and industrial development alike. In contrast, the increasing rates of sewage and wastes accumulated in lakes and rivers from businesses and industries have noticeably added to the global issue of clean water shortage^[1]. Several relevant reports have stated that in many regions there are millions of individuals who suffer from the lack of drinkable water and as a result, the children there die on daily basis due to various waterborne serious infections.

Oceans and seas are the major reservoirs for water. In numbers, nearly 97.4% of the water has existed as seawater whereas just about 2.1% is stored as ice and glaciers in the polar regions; however, as low as 1% can be accessed by people according to Ref. [2]. Water desalination is the most common and traditional method for yielding drinkable water from brackish water, and desalination has recently gained increasing interest and popularity. The need for desalination is a necessity in highly polluted regions to avoid waterborne diseases. A wide variety of traditional and non-traditional desalination techniques and methods have been lately proposed and developed to solve man-made problems regarding water pollution. Nevertheless, most of the processes related to saline water purification usually utilize classical energy sources and hence leading to other forms of environmental pollution. The substantial two features of the traditional desalination systems are their high energy use and their high cost when used to yield low cubic meters of drinkable water. Several desalination technologies that have gained commercial popularity nowadays such as nano-filtration, reverse osmosis (RO), and multi-effect distillation demand high amounts of electricity to be operated. As a result, such purification technologies are costly and inappropriate to satisfy the need for drinking water in remote areas. The solar-based water distillation process is recognized as a very dependable and environmentally-friendly method in comparison with other common renewable energy techniques. Solar still element is the essential form needed for the process of sun ray-driven distillation technology. It has been an attractive solution because of their numerous distinct features including set-up easiness and simplified fabrication. The operation requires only semi-skilled workers, using naturally available energy (sunlight), and zero-pollution emissions. Solar distillation is of great benefit for remote places where the available water is with a high concentration of salts. It is an effective technology to remove harmful particles from the water, making it qualified for cooking and drinking. Besides, the solar still can aid in substituting the shortage in drinking water during periods of scarcity according to Ref. [3]. This paper focuses on a specific type of modification on conventional solar still that increases the productivity and efficiency via an additional condenser and its progress through the literature.

1 Principle of Operation

A simple solar still consists of a box with a tilted glass cover facing the sun. The base of the still contains the basin which holds the saline water. The solar still simply mimics the hydrological cycle of rainfall. The water was heated by the sun and evaporated due to the greenhouse effect caused by the enclosed space. Then it condenses on the glass cover and is collected by a collecting trough which contains the clean (drinkable) water.

In Ref. [4], research using experimental and theoretical approaches was carried out to explore the influence of using passive condenser on the operational efficiency of a basin-type solar still with a single slope. The basin was made of copper with wooden box housing, and the glass cover had 24° inclinations. The additional condenser was made of 1 mm thick aluminum sheet and connected to the still through a 2 m × 50 mm slot along the still length. The experimental results showed an increase in efficiency by 50% compared with conventional still. The yield was reduced by 70% when the condenser was excluded.

Ref. [5] presents a design for a double condensing chamber. The effective area was 1 m ×1 m. The attachment was split into two chambers that were divided by a partial wall which was 7 cm far from the second condenser cover. The double condenser still gave a higher yield of about 35%-77% compared with conventional solar.

In Ref. [6], a simple solar still was experimentally modified and examined by adding a solar heater and utilizing external and internal condenser. The external condenser was made 1 m long and consisted of two concentric (5 mm thick) glass pipes whose diameters were 55 mm and 75 mm. The size of each internal condenser diameter copper pipes was 1 cm × 120 cm ×120 cm with eight passes. The results show enhancement in the yield and efficiency of the solar still due to the implementation of the above modification compared with conventional solar.

Ref. [7] investigated a parallel double glass solar still with a separate condenser. The basin area was 1 m² iron galvanized sheet with a thickness of 0.35 mm. A separate condenser was attached to the basin via a horizontal slot, whereas the upper glass cover consisted of a parallel double glass. A theoretical analysis of the energy balance of the many components was conducted. The study found out that the efficiency was enhanced from 48% up to 70% when the condenser cover was cooled down.

Ref. [8] experimentally investigated the effect of adding water circulation through tubes attached to the wall surface to improve the rate of condensation and efficiency. The basin was constructed of cement with an rectangular area of 0.5 m². To invoke the condensation effect on all sidewalls, the inner walls were constructed with a mild steel sheet with a 67 cm × 67 cm dimension coated with mat black paint. Moreover, a 72 cm long aluminum tube with a diameter of 0.5 cm and three turns was attached to the walls of the iron surface by welding for cooling channel arrangement. The glass cover was tilted by 15°. The maximum daily yield was 1.4 L/m² per day and the efficiency was 30%.

Ref. [9] designed and fabricated passive solar still with the detached condenser. The still contained one basin in the evaporation chamber and two basins in the condenser chamber. The top part of the condenser was shielded from solar radiation. All basins were made of galvanized steel but only the basin in the evaporation chamber was painted black at the bottom to increase its solar absorbance. A 0.004 m thick glass cover was fitted on the evaporation chamber. Basins of saline water and the bottom insulation layer were all housed in a box (plywood). Results showed that the yield was 62% higher than the traditional solar still. The contributions of the first, second, and third effects were found to be 60%, 22%, and 18% of the total productivity, respectively.

Ref. [10] experimentally designed and evaluated a portable single basin solar still with an external reflecting booster and an outside condenser. The basin was made of 1 m² steel inside a wood housing. The results proved a still efficiency of around 77% can be reached.

Ref. [11] experimentally studied a portable solar still. The evaporation area was 1200 cm²(40 cm × 30 cm).The horizontal roof region had an aluminum plate cooled with a thermoelectric module (12 V DC) and the 1.9-watt fan was used to cool down the heat pipes powered with PV panels. The experiments were carried out in 5 days. The maximum daily efficiency was 7% and there was a reduction in productivity with the increase of wind speed.

Ref. [12] experimentally investigated the possible advantage of attaching an external condenser to solar still using nanofluids. Basin area was 5000 cm² (100 cm × 50 cm), the high and low sidewall was 45 cm and 16 cm respectively, and was made of the galvanized iron sheet (1.5 mm thick) and black coated bottom surface. It was insulated with 5 cm fiberglass, and glass cover sheet of 3 mm thick inclined with nearly 30° on horizontal direction. The unit of condensation contained a 3 m copper tube of 3.81 cm diameter on polyethylene tank housing (40 cm × 40 cm × 50 cm) filled with cold water. The vacuum was created using axial flow type fan (2 A, 12 V, 1440 r/min). The analysis revealed that the external condenser increased the yield by 53.2% and using nanofluids improves the productivity by 116% when the still was integrated with the external condenser.

Ref. [13] designed and constructed a solar still with the external condenser. The net basin area was 0.6 m² galvanized steel with 1.4 mm thickness with a 5 mm glass cover and 23° inclinations. A cylindrical condenser attached to the passive solar still was made of PVC pipe with a diameter of 6 inches. Each condenser had a diameter of 15 cm and 50 cm height fixed at the back of the still. Results show the comparison of higher productivity for the advanced solar still and the conventional solar still. Also, the productivity increased with the decreasing water depth of the basin.

Ref. [14] experimentally tested the effect of adding a natural circulating loop unit attached to the rear side of the solar still. The experiments were carried out during the summertime (June to July). Basin area was constructed of a galvanized plate of 0.35 m² (0.5 m × 0.7 m) with 1 mm thickness and wooden box housing with double glazing cover (3 mm thick). It was insulated with glass wool of 100 mm thick. The condenser was a horizontal tubular heat exchanger fabricated from three parallel aluminum tubes with a length of 70 cm, a thickness of 0.1 cm, and a diameter of 5 cm. The results showed a good effect for natural circulation on still performance, where the still output was 3.72 kg/(m²·day) and the highest attained efficiency was 45.15%.

Ref. [15] experimentally investigated a solar still with an external air-cooled condenser with the use of a vacuum pump. The evaporation chamber was constructed of a 3 mm thick carbon steel sheet with a top surface of 15 mm thick security glass sheet (transmittance of 0.65). The chamber was with a square shape of 0.25 m² cross-section area. The condenser chamber was fabricated using a copper tube of 10 cm diameter, 60 cm length, and 1 mm thickness in addition to 10 squared fins of 25 cm length and 1 mm thickness. Results show maximum efficiency of 40% and a rise in productivity and thermal efficiency of 16.2% and 29.7%, respectively.

Ref. [16] experimentally evaluated the effect of using a separate condenser on the performance of solar still. Experiments were conducted from April to May. Basin was constructed from 1.4 mm galvanized steel with an area of 1 m² and 4 mm thick glass cover at 26° inclination. A separate condenser was attached to the backside of the still. The daily productivity was found to be 19% more than the conventional still and the efficiency increased by 5% due to the addition of the separate condenser.

Ref. [17] carried out an experimental investigation of portable solar still with the thermoelectric module. The testing took place over six typical summer days. The bottom of the solar still was made of black color Plexiglas, while other walls were made of Plexiglas with 6 mm thickness. The wall in which the thermoelectric was mounted consisted of an aluminum plate. The minimum and maximum yields were found to be approximately 225 mL and 500 mL, respectively.

Ref. [18] designed and tested inflatable plastic solar still with the passive condenser. The basin area was 1.8 m² with black bottom. The still was tested indoor to avoid the possible effects of the climatic factors. The study showed that the yield was 0.75 L/h at 73℃ water temperature and increased to 0.95 L/h after using airflow over the passive condenser.

Ref. [19] numerically and experimentally investigated the effect of the external condenser with natural circulation loop on the passive solar still. The experiment was conducted on a typical summer day. The numerical simulation was carried out using Matlab software to solve the basic equations of the natural circulation loop. It was found that the maximum daily production can reach 2.71 kg/m² and 4.73 kg/m² under winter and summer conditions, respectively.

Ref. [20] experimentally studied the effect of the built-in condenser on the performance of the solar still. Their work involved examining four forms of condensers: glass (3 mm thick), aluminum plate (1 mm thick), aluminum heat sink with pin fins (1 mm thickness + 15 mm rod diameter and 40 mm length), aluminum plate covered with an umbrella. The dimension of shallow basin was 80 cm ×80 cm ×2 cm with 3 mm glass cover of 27° inclination, facing to the south. Results show an increase of daily productivity by 35% when glass condenser with black steel fibers was utilized inside the basin, an increase of 31% when heat sink was used in case of using saline water only, a decrease of 26% when 20 cm umbrella was used at the top of the aluminum condenser, and a decrease of 31% when 40 cm umbrella was used.

Ref. [21] experimentally studied the effect of adding pin fin absorber and condenser in single basin-single slope solar still. The experiments lasted for 3 days. A water yield attainment of about 32% was achieved for the still with the condenser (using airflow and external condenser) concerning the original solar. For the pin fin absorber, the gain was only 14.53% for water production. The effect of the external condenser on the yield was central in comparison with the effect of the pin fin absorber.

Ref. [22] numerically investigated nanofluid and external condenser. Aluminum oxide Al₂O₃ and cuprous oxide Cu₂O were used with the external condenser. A mathematical model was developed and written in FORTRAN to evaluate the performance. The results showed that the modified still had an efficiency of 84.16% and 73.85% for cuprous oxide and aluminum oxide respectively with an operating fan. The conventional still efficiency was 33%.

Ref. [23] designed a solar still with a separate condenser chamber. The 1 m² basin was made of galvanized iron. A separate component was attached to the back of the still. Glass cover was 4 mm thick with 26° inclinations. The experimental results show that the productivity of the new still was 36.9% higher than the conventional still and 37% more efficient. The energy efficiency was 60.8% higher than the conventional still.

In Ref. [24], stepped solar still was modified by adding an external condenser to enhance the performance. The basin area was 1 m² (0.5 m × 2 m). The absorber plate involved steps, whose dimension was 200 cm ×10 cm with a tray of 0.5 cm deep and 12 cm wide. The absorber was painted with black color and fabricated from galvanized steel sheet. It also consisted of internal reflectors. The glass cover was 3 mm thick. The internal and external reflectors had dimensions of 0.05 m × 2 m and 0.6 m ×2 m, respectively. The external condenser was made of galvanized iron with a dimension of 0.4 m × 2 m. The basin plate was also modified to obtain five fins with a dimension of 2 m×0.035 m×0.001 m. The analysis revealed that the evaluation parameters were with higher hourly values after modification than before modification. Also, the daily yield increased from 6.9 kg/m² to 8.9 kg/m² (i.e., 29% enhancement).

Ref. [25] investigated the performance of a single-slope basin-type still equipped with a finned condensing chamber in addition to several photovoltaic cells that are submerged in the basin. The condenser chamber contained a wall with fins constructed on both sides. The total area of saline water surface was 1.2 m², the base area of fin condenser wall was 0.6 m², and all constructed fin surface area of the condenser was 1.8 m². Productivity, still efficiency, and system efficiency increased up to 27%, 21% and 28%, respectively.

Ref. [26] experimentally investigated the effectiveness of adding copper-oxide nanofluid into a solar still with inclined cover. The still was also provided with an external thermoelectric glass cover cooling channel. The conventional and modified still were fabricated using galvanized steel with a thickness of 1.5 mm. The still basin area was 0.5 m² (0.8 m long and 0.625 m wide). The basin surface was painted black for high absorptivity. The glass cover had a thickness of 4 mm with an inclination of 35°. For the modified still, four thermoelectric cooling modules were attached on the walls of the galvanized external channel. The results show that the highest increase in yield, energy, and energy efficiencies were 81%, 80.6%, and 112.5% by adding 0.08% volume fraction of copper oxide nanoparticles to the modified solar still.

Ref. [27] evaluated theoretical modeling for solar still with the integrated built-in condenser. The effective area of the basin was 1 m² (1 m ×1 m) with 4 mm thick glass sheet for the upper front side of the still and galvanized sheet for the built-in condenser cover. Comparison of the theoretical study and the experiment showed good agreement with the same trends.

Ref. [28] improved the condensation rate and productivity using a special design condenser. Basin dimensions were 0.5 cm ×0.5 cm ×0.15 cm with black coated bottom. The condenser was constructed using aluminum. Plexiglas (transparent acrylic) were used for inside walls. The outer surface of the condenser was cooled with cooling water at different rates. The cotton wick was used inside the basin. It was found that the maximum yield was achieved when both jute and the wick material were installed and when the condenser was cooled by water at a flow rate of 10 L/h.

Ref. [29] experimentally investigated solar still with an enhanced condenser. The proposed sun light-based thermosiphon active still involved the use of an improved condenser. The basin was fabricated from stainless steel and semi-cylinder with a 0.65 m² evaporation area. It was shielded by polyurethane foam with a thickness of 10 cm. The V-shaped condenser was fabricated using an aluminum sheet and was with a total area of 0.65 m² with a 30° slope. The results show that an effective fill to the condenser can lead to a decrease in its temperature and an increase in the yield by 46%. A still yield of up to 66% was attained when the condenser and the basin were filled simultaneously.

Ref. [30] studied the operational characteristics of single-slope solar still integrated with semitransparent photovoltaic module and passive condenser. Solar still basin area was 1 m × 0.6 m and the body was made of glass fiber reinforced plastic, with the PV module placed as a top cover with 30° inclination. The results showed that the overall thermal efficiency of the suggested system was 57.5% for a higher value of the packing factor (βc=0.85). From the review it can be summarized that this type of modification has good potential to increase the productivity of the solar still due to several reasons: 1) adding another condenser improves the vapor flow pattern (i.e., increases the natural convection buoyancy forces inside the still). The additional condenser has a lower temperature than the primary condenser since it is insulated from the solar radiation, which leads to a better condensation rate. Hence, a higher temperature difference was achieved causing higher distilled yield and enhanced efficiency. More research needs to be done to explore this possibility. 2) Adding a condensing chamber creates a low-pressure area (diffusion and purging). The still leads to various vapor flow patterns, which enhances the heat transfer coefficient and the overall productivity.

2 Discussion/Advantage

Solar still is a simple device intended to provide fresh drinkable water at low cost and use green renewable energy. One drawback of this device is its low productivity. As a result, researchers investigated how to improve the overall yield and maintain its simplicity in design. The most important part is how to keep its cost as low as possible. The passive type solar still is very attractive for research study due to its low productivity, thus there are many ways to improve it and its simple operation does not need skilled labor to build or to operate.

The passive type also costs less than the active type of solar stills. According to the review that has been presented in this work, the two main categories can be used to improve the productivity of the solar still, i.e., increasing the evaporation rate, increasing the condensation rate, or both. The parameters under control are the design parameter and the operating parameter, while the climate parameter is not under control. Consequently, researchers tried to optimize and implement different design and operating aspects to increase the yield of the solar still.

There are many ways to optimize the design of the solar still. The evaporation rate can be increased by the following: (all measures aim to increase the evaporation rate by either increasing the surface area or increasing the initial inlet temperature)

1) Increasing the inlet water temperature with flat plate collector, evacuated tubes, or concentrators;

2) Adding an absorbing material to the basin such as porous material, cotton materials, phase change materials, and so on;

3) Reducing the condenser's glass cover thickness;

4) Decreasing the water level in the basin;

5) Using reflective material inside and outside the still;

6) Using concentrators to concentrate the solar rays in a specified area.

The condensation rate can be increased by the following methods:

1) Changing the wettability of the glass cover;

2) Reducing the temperature of the glass cover by cooling;

3) Using an additional condenser.

Methods of investigation:

Most researchers prefer the experimental investigation because it demonstrates the real system and gives the true performance of the device. The drawback of experimental investigation is the cost and time.

An alternative is to use the simulation method to optimize new designs without any fee and with less time. Different simulation approaches are available regarding the mathematical modeling of the solar still and can be summarized as follows:

1) Building the mathematical model using already existing simulation programs such as ANSYS-FLUENT and ANSYS-CFX. The advantage of this approach is that most of the modeling can be done using the program interface from drawing the model to meshing to calculation.

2) Write the mathematical model using the equations (continuity, momentum, energy, etc.) and then convert them to code with a coding program such as Matlab.

The advantages of their approach are the modeling following the exact steps for the programmed model (i.e., all the solution step has been constructed by the programmer compared with the fluent.) The disadvantage of this approach is its need for programming skills as well as in-depth knowledge of the mathematical equation that governs the modeled case.

In summary, passive solar still can be improved and enhanced in terms of productivity and efficiency. A combination of different parameter designs needs to be implemented at the same time in one design to investigate the effect of multiple enhancement factors at the same time and under the same weather conditions.

3 Conclusions

Based on the above review, adding condenser has a good potential to enhance both the solar still productivity as well as the efficiency. From the review it can be concluded that this type of modification has good potential to increase the productivity of the solar still, due to several reasons:

1) Adding another condenser improves the vapor flow pattern (i.e., increasing the natural convection buoyancy forces inside the still).

2) Increasing the temperature difference between the water temperature and the glass cover temperature, which is the key method to increase the condensation rate.

3) Adding a different possibility to modify the still furthermore. For example, by adding shading to the additional condenser since it is not the primary condenser and does not need to be under sun rays.

4) This type of modification needs to be further explored, especially in terms of the experimental studies, due to its possibility to enhance the solar still yield.

The following recommendations could serve as future topics of research within the field:

1) Optimizing the design of the solar still by increasing the inlet water temperature. This can be achieved by using solar still coupled with flat plate collector, evacuated tubes, or concentrators.

2) Adding an absorbing material to the basin such as porous material, cotton materials, and phase change materials.

3) Reducing the glass cover thickness of the condenser (For example, use a glass cover with a thickness of 3 mm instead of 4 mm or 6 mm).

4) Decreasing the water level in the basin, which can make the water quantity inside the still reduce to the minimum amount for faster evaporation rate.

5) Using reflective material inside and outside the still. The specific outcome of this increases the reflection of solar rays inside the still.

6) Using concentrators to concentrate the solar rays in a specified area.

The condensation rate can be increased by the following:

a) Changing the wettability of the glass cover;

b) Reducing the temperature of the glass cover by cooling;

c) Using an additional condenser.