Evaluation of distilled water quality and production costs from a modified solar still integrated with an outdoor solar water heater

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Introduction
The world is currently suffering from the depletion of commonly used fossil fuel resources.At the same time, humanity and the environment are being damaged by the effects of using fossil fuels: global warming has negatively changed our environment, causing forest fires, high temperatures, changes in river water levels, and a decrease in drinking water sources, especially in rural and remote areas.In 2016, the Renewable Energy Policy Network for the Twenty-First Century reported that the main source of energy for meeting global demand is fossil fuels.These fuels make up about 78.3% of the total energy, while renewables constitute 19.2%: the remaining 2.5% is represented by nuclear fuel [1].Humanity has resorted to using other forms of energy in order to save our planet, such as alternative renewable energies.Numerous studies have dealt with the use of different renewable energy sources in several industrial applications like solar-powered desalination technology.According to a United Nations Environment report about drinking water levels between 1995 and 2025, water scarcity will increase rapidly by 2025.Therefore, the availability of drinking water has become an important issue and a crucial global goal due to the growth in demand for fresh water [2].
Access to fresh water in remote and rural areas is a real challenge, along with high pollution rates in drinking water due to industrial development [3,4].The availability of drinking water in these areas will lead to reduced insurance and healthcare costs (economic benefits) and save the lives of low-income people in rural or urban areas [5].According to a World Health Assembly report, more than a million people suffer from a lack of safe drinking water, and most of them live in rural areas where fresh water units are difficult to build [6].
Water makes up around 70% of the Earth's surface, of which 2.5% is drinkable.The salinity levels of seawater range from 3500 to 4500 ppm.However, according to the reports and recommendations of the World Health Organization (WHO), dissolved salt in drinking water should generally not exceed 500 ppm (1000 ppm in special cases) [7].Therefore, it is not possible to use seawater directly for human, agricultural, and industrial purposes (except after treatment and desalinization, as is the case in some countries) [8].The average daily production of drinking water from the conventional desalination process worldwide is about 23* 10 6 m 3 [9].However, this production process consumes large quantities of fossil fuels.It is estimated that approximately 130 million tons of oil will be burned annually to produce 13 million cubic meters of potable water per day [10].
Therefore, several studies have been conducted on using renewable energy sources instead of fossil fuels in order to desalinate sea and untreated water, such as solar distillation technology.
The performance of solar stills is usually evaluated in terms of production.Recently, extensive studies have been conducted on solar distillation systems in order to improve their performance and cost effectiveness.Basin-type solar water distillation is a promising renewable technology for producing fresh water.These systems are characterized by their simple construction, low maintenance costs, ease of operation, and safety.However, due to their dependence on solar radiation, the intermittency of which affects performance and reduces production, various studies have been conducted in order to improve their performance and production.
Husham [11] carried out an investigation on combining a passive built-in condenser with a traditional solar still and its impact on the daily accumulation of distilled water.It was noted that this design gives about 16.7% more cumulative distillate water than conventional solar stills.Rai and Pandey [12] investigated the impact on fresh water productivity of adding an external condenser to a traditional solar still (CSS): the results illustrated that the modified solar still (MSS) was 19% more effective than a CSS.
There are many methods for increasing the quantity of solar energy absorbed by stills.Adding coal to the water in the solar still basin increases the thermal energy required to evaporate the water as a result of absorbing more solar radiation energy [13].Abdullah [14] conducted an experimental study to verify the impact on distillate yield of adding various absorbent materials (sponges, metallic and volcanic rocks) to the water in a solar still basin.From the results, it turned out that the most effective materials for absorbing and storing solar energy were black rocks, which improved production by approximately 20%.
Radwan [15] studied the performance (transient type) of a solar drenched still combined with phase changing material in the solar still basin to store thermal energy in order to produce agricultural greenhouse heating and humidification: the study also sought to investigate the effect of PCM thickness and the mass flow rate of air on system performance.The results show that the rate of airflow directly affects the daily yield.Total freshwater productivity was measured at approximately 4.6 L/m 2 , with an efficiency ratio of about 57%.In Naseer [16], PCM was used as a thermal latent material in a solar still.The study included an experimental investigation on the impact of integrating PCM cells with a solar still basin and compared its performance and daily yield to a traditional solar still without PCM cells.The results indicated that the distillate from the proposed solar still was enhanced by approximately 32%.
Liu [17] studied a solar distillation system consisting of a solar still, evacuated tube collectors, and a multi-effect low-temperature solar still.The system configurations were designed to include evacuated tube collectors, flash tanks, heat tanks, a multi-effect solar still, electric heating, and cooling.Mathematical and economic models were developed according to mass and energy conservation, while the economic performance was examined with MATLAB.It was concluded that the cost of distilled water decreases as the area of the evacuated tubes and solar collectors increases and the temperature of the collector outlet water increases.
Another study enhanced the productivity of traditional solar stills by installing a rotating hollow cylinder within the solar still (increasing the area of evaporation).The results show that the rate of improvement in daily yield was 161% with a rotation speed of 0.5 rpm [18].
A study was carried out in Moradabad about the integration of a solar water collector with a solar dish cooker to enhance the distillate.The results show that the cumulative output was 3.66 lit/day: using the combined unit for 3 h per day produced 1.15 L of fresh water while also heating the water (depending on environmental conditions) [19].
Ayoub et al. [20,21] described a new technical modification to improve solar still productivity.This included a simple adjustment of the slowly rotating hollow cylinder inside the solar still, allowing it to produce thin water films on the surface.These films evaporate rapidly and are constantly being generated.The results indicate that there was a significant improvement of more than 200% in the amount of distillate produced.The study concludes that through this improvement, important criteria related to the availability of materials, low cost, simplicity of management, efficiency, space conservation, and safe water quality are met.
Ayoub and Malaeb [22] performed a detailed analysis to evaluate and estimate the operational and economic possibilities of solar distillates.The results indicate that the proposed solar still increased daily yield by about 200-300%.It also showed that the cost of producing 1 L of distilled water from a modified solar still is lower compared to other types of distillation devices.
The productivity of solar stills has been improved [23].The basis for this was the use of a closed hollow cylinder inside the solar distiller.The closed cylinder increased the surface area of evaporation and reduced the thickness of the brine layer.The next stage included adding copper oxide nanoparticles to the basin water.The optimum speed was 0.1 rpm and the productivity about 9.2 lit/m2 from the modified still, as compared to 2 lit/m 2 from a conventional still.The estimated production costs of 1 L of distilled water for the conventional and cylindrical stills was about 0.05 and 0.039 $/lit, respectively.
Another study [24] involved attempts to increase productivity by reducing the thickness of the evaporation water.This included using a rotating metal cylinder inside a transparent cylindrical structure body.The testing was performed in four stages: a closed cylinder, an open cylinder, with a wick, and without a wick.The typical rotation speed for each stage was tested, and it was found that the lowest typical rotation speed was 0.1 rpm without a wick and 0.05 rpm with a wick.Productivity improved by 121% with a closed cylinder and 136% with an open cylinder.At 0.05 rpm with a wick, the improvement ratio was about 140% for a closed cylinder and 175% for an open cylinder.
Surveyed literature presented different techniques of solar still modifications.However, investigation of integrating these techniques as new modification is essential to evaluate the possibility of achieving higher water production rate.An economic analysis of the modified solar still (MSS) and evaluation of the chemical and physical properties are also essential to ensure the fesibility of the suggested solar still utilization.In the current work, a hollow cylinder solar distiller was integrated with a outdoor solar collector in order to study productivity and economic feasibility.The chemical and physical properties of the distilled water output have been included, along with a detailed analysis of the estimated production costs.This study provides reasonable economic estimates that justify the feasibility of the new modified solar still.

Improvement principle of the modified solar still
There are two effective methods for improving the productivity of solar stills: increasing the rate of evaporation of the basin water or increasing the rate of vapor condensation.Water depth is an important operational factor affecting the productivity of solar stills.The rate of basin water evaporation increases as its depth decreases [25].As the depth of the water decreases, the heat transfer rate from the basin liner to the basin water increases.So, a pioneering attempt was made to accelerate basin water evaporation by placing a black, hollow, and open-ended rotating cylinder inside the still.A thin water film forms on its surface with each turn inside the solar still.The water film quickly evaporates [26] due to the rapid transfer of heat from the surface of the cylinder to the adjacent water film.
The evaporation of the water film from the hollow cylinder's surface depends on several parameters such as the weather and operating conditions.The most important weather parameter is the intensity of solar radiation, the natural fuel for heating the surface.As for the operational parameters, the most important is the cylinder's speed of rotation per minute: this must be neither too fast nor too slow.If it is too fast, not all the water film on the hollow cylinder surface will evaporate; if it is too slow, the hollow cylinder surface will dry quickly due to the rapid evaporation of the water film.In the present work, four rotational speeds of the hollow cylinder (0.5, 1, 3, and 6 rpm) have been tested [26].From the analysis of the results, it was noted that the best performance of the modified solar still was at 0.5 rpm, because at this rotational speed the water film had sufficient time to evaporate; at other rotational speeds, the evaporation time began to decrease as the speed increased.
The MSS was combined with an outdoor solar collector to overcome the problem of low basin water temperature as a result of the hollow cylinder's shade covering 76% of the basin water's surface [26].

The experiment test rig
A CSS and a MSS were constructed with identical dimensions and operating conditions, as shown in Figs. 1 and 2. Made of MDF wood, the structure of the solar still was 103.6 cm in length, 53.6 cm in width, and 1.8 cm in thickness: the height of the big side was 61.8 cm while that of the small side was 26.6 cm.A cover made of plexiglass was placed at an angle of 35 • : its dimensions were 50 cm in width and 0.3 cm in thickness, with the height of the big side at 50 cm and the height of the small side at 14.8 cm.To fix the cover onto the wooden frame and to collect distillate water at the bottom of the solar still in a plastic bottle, an aluminum channel was used.The dimensions of the solar still's base were 103.6 cm in length, 53.6 cm in width, and 1.8 cm in thickness.To install all parts of the system and prevent air leakage, silicone glue was used.The water basin was made of galvanized steel 100 cm long, 50 cm wide, 10 cm high, and 0.1 cm thick.The actual surface area exposed to sunlight was 0.5 m 2 .The basin water plate was painted dull black to increase its ability to absorb solar energy.Black galvanized steel 100 cm long and 100 cm wide was used to form a hollow cylinder 32 cm in diameter, 90 cm in length, and 0.1 cm in thickness.A low carbon steel shaft with a 0.8 cm diameter and 95 cm length was to carry and install the hollow cylinder: on the two ends of the shaft, two bearings 0.8 cm in diameter were installed.
A DC motor (12 V and 0.1 A) was used to drive the hollow cylinder with a rubber belt.A photovoltaic solar panel (110 W) was used to supply the motor with DC power in the daytime, while a battery was used at sunset.The rotation speed of the DC motor (6.8 Nm) was controlled with a speed regulator (PWM type) consisting of a variable resistance volume switch to provide the current and voltage required to operate the DC motor.A mechanical float was used to keep the water depth at 5 cm.To clean the water basin of deposits such as salts and impurities, a globe valve was installed on the bottom.An insulated water tank (50 cm in diameter and 100 cm high) was used to feed water into both solar stills through insulated rubber pipes.
The outdoor solar water heater was integrated into the modified solar still to increase the temperature of the basin water.A DC water pump (12 V and 0.66 A) with a flow rate of 1.2 l/min was used to circulate the water between the flat plate of the solar water heater and the basin water [26].

Experimental procedure and measurement devices
The experimental setup was installed on the roof of a laboratory at the Department of Nuclear Power Plants and Renewable Energy Sources, Ural Federal University.The experimental work was carried out in the weather and operating conditions of Ekaterinburg (latitude 56.84 N, longitude 60.58 E) from June to September 2019 on sunny days.Four typical days were chosen: 19 June, 17 July, 22 August, and 15 September.Over 12 h (8:00 a.m. to 8:00 p.m.), the solar distillation system was directed south.The basin water mass was 21.5 kg, an appropriate weight for a basin water depth of approximately 5 cm.To study the impact of different parameters on the performance of the still, a variety of measuring devices were used, such as a four-channel sd card data logger to measure temperatures at six different points: the basin plate (T pb ), the basin water (T bw ), the inside hollow cylinder surface (T hci ), the outside hollow cylinder surface (T hco ), the inner Plexiglass cover surface (T pgi ), and the outer Plexiglass cover surface (T pgo ).A GM1362 humidity and temperature meter was used to measure the ambient air temperature.To measure the intensity of solar radiation, a TM-207 type solar power meter was used.A UT-363 anemometer was used to measure wind speed.

Results and discussion
For all the tested days, the MSS gave much higher yields than the CSS.Fig. 3 illustrates the results from 17 July of 2019 for the cumulative distillate water produced by both still systems.From this figure, it can be observed that the average increase in daily yield ranged from 296% at 8:00 p.m. to about 300% at 8:00 a.m. on the next day.The average increase for the other months ranged between 281% and 400% (the results for the other months are not included in the current work).
Therefore, it is evident that the hollow cylinder greatly contributed to improving the daily yield.This is because the rotating hollow cylinder increased the surface area of absorption and evaporation inside the MSS [27].Equally, the transfer of heat from the surface of the cylinder to the adjacent water film was more rapid compared to the heat transferred from the basin plate to the water in the CSS.
By comparing the cumulative production rate of the stills, it was observed that the cumulative output of distilled water from the CSS at 8:00 p.m. was 2800 ml/m 2 and about 3100 ml/m 2 at 8:00 a.m. the next day: the MSS produced about 11100 ml/m 2 and 12500 ml/ m 2 , respectively.The productivity for the other four months (data not shown) ranged between 700 ml/m 2 to 2840 ml/ m 2 for the CSS and 3500 ml/m 2 to 11100 ml/m 2 from the MSS, depending mainly on climatic conditions.Although the intensity of solar radiation decreased after about 8:00 p.m. (as shown in Fig. 4), the production of distilled water from the MSS and CSS at night continues because the basin water at a depth of 5 cm has enough potential thermal energy to discharge it at sunset.In addition, the plexiglass temperature decreases due to a lower ambient air temperature.It was also noted that the hourly distilled water output from the MSS was higher than CSS due to the higher temperature of the basin water: this was a result of preheating the basin water with the outdoor solar water collector, as shown in Fig. 4 (A and B).
Fig. 5 illustrates the change per hour in thermal efficiency for the CSS and the MSS on 17 July 2019.It can be noted that thermal efficiency continues to increase over time due to the increase in the rate of evaporation of the basin water and the intensity of solar radiation.Therefore, the highest value of thermal efficiency was recorded at 4:00 p.m. due to continuous productivity with the decrease in the intensity of solar radiation and the ambient temperature (the latent heat of evaporation).Therefore, the heat loss from the plexiglass cover to the ambient air at this time increases the condensation of the water vapor.The average optimal hourly efficiency of the CSS was 52%, which was lower than the thermal efficiency of the MSS at about 72.58%.This was due to the rotational hollow cylinder, which accelerated the evaporation rate of the basin water, and the preheating of the basin water by the outer solar collector.

Analysis of production costs
In solar distillation systems, researchers are trying to achieve two goalsthe first is to increase the yield of fresh water and the second is to reduce production costs.Various studies have included a detailed economic analysis of the main factors involved in the  cost of producing 1 L of distilled water [28].The economic analysis includes the following: • Estimating the capital cost (CS) of manufacturing and installation.
• The capital recovery factor (CRF) was calculated with the following equation: (1) • The first annual cost (FAC) was calculated with the following formula: • The sinking fund factor (SFF) was calculated with the following equation: • The salvage value of the distillation system (S) was taken as a percentage of the initial cost (20% of CS) [29].
• The annual salvage value (ASV) was calculated with the following formula [29]: • The annual maintenance cost (AMC) of the distillation system (S) was taken as a percentage of the first annual cost (15% of FAC) [30].• The annual cost of the distillation system (AC) was calculated with the following formula [30]: • The annual cost of producing 1 L of distilled water (ACP) was calculated with the following formula [30]: Where m y is the yearly yield from the solar distillation system (l /m 2 .year), m d is the daily yield from the solar distillation system (l /m 2 .day).It is assumed that the number of operating days (j) for the distillation system per year according to the climatic conditions in Ekaterinburg is 180 days.Table 1 includes the price of all components for both stills on the local Russian market.The capital cost of the traditional and modified solar stills was $ 82 and $ 315, respectively.Table 2 provides an analysis of the production cost per liter from the two solar stills.The cost of producing 1 L of distilled water from both solar stills was about $ 0.0282 and $ 0.0268, respectively, if they run for about 180 days a year (average number of sunny days per year in Ekaterinburg).A comparison of the cost of production was compiled from earlier studies (shown in Table 3).It is noted that the cost of producing a liter of distilled water with the MSS in the current study is compatible with estimates from previous studies.Single slope solar still India 1.91 0.14 [32] Single slope solar still Pakistan 3.25 0.063 [33] Single slope solar distiller with ultrasonic humidifiers and cotton cloth India 2.5 0.014 [29] Single slope solar distiller unit integrated with a solar water heater and ultrasonic humidifiers Egypt

Physical and chemical tests of the distillate water
Physical and chemical properties are important in determining the validity of water produced by solar stills.These properties are particularly important when determining the suitability of water for human use.Each variable was compared with the approved specifications listed in Table 4 as follows:  1.Total dissolved solids (TDS): Fig. 6shows the following results from the total dissolved solution test: • The concentration values of TDS of all examined water samples were found to range from 13 to 230 ppm.
• The distillate water produced from the solar still gives the best results among all the tested samples (13 ppm).2. pH meter: the pH values ranged between 6 and 8.5, as shown in Fig. 7.The results are under the standard specifications for all water characteristics.The distillate water gave the best results.3. Electrical conductivity (EC): Electrical conductivity is defined as a digital expression of the potential of a water solution to transfer electricity and depends on the concentration and quality of ions in the water.Fig. 8 indicates that the electrical conductivity of the water from the solar water stills had a value of 8.7 μs/cm.This is due to the improved water treatment system.

Conclusion and suggestions
This work suggested a new type of modified solar still, one that combines a single slope solar still with a hollow cylinder and is integrated with an outer solar collector.This greatly increased daily fresh water production.Based on what is presented in the experimental study, the analysis of the costs, and the characteristics of the distillate water, the following is concluded: 1.The integration of the suggested solar still with a hollow cylinder and outdoor solar heater significantly improved the yield of distillate water compared to a traditional solar still due to an increased surface area for evaporation and the reduction of the dimension between the surface of evaporation (water surface) and the surface of condensation (plexiglass cover).The MSS productivity increased by 281%-300% compared to the CSS, depending on weather parameters.2. The improvement in fresh water productivity is low in cost and exceeds the improvements produced by other modified solar stills.3. The distillate water produced by the modified still gave the best results on the TDS, pH, and EC tests.4. To further improve results, the surface area for absorption and evaporation in a rotating cylinder should be increased with a fin or surface corrugation.5. To further improve results, the wetness of the surface of the rotating cylinder should be guaranteed by placing a wick on its surface.6.To further improve results, the cylinder surface should be continuously heated by installing phase-changing material like paraffin wax on the inner surface.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 1 .
Fig. 1.Photographic view of the modified solar still and the traditional solar still.

Fig. 3 .
Fig. 3. Cumulative distillate water from the MSS and the CSS and the improvement ratio.

Fig. 4 .
Fig. 4. Hourly variation of solar radiation and temperatures for different points of the (A) CSS and (B) the MSS.

Fig. 5 .
Fig. 5. Hourly variation in the thermal efficiency of the CSS and the MSS.

Fig. 7 .
Fig. 7.A compression of the pH values of different water samples from Ekaterinburg, 2019.

Fig. 8 .
Fig. 8.A compression of the EC of different water samples from Ekaterinburg, 2019.

Table 1
Manufacturing and installation capital costs of solar stills, $.

Table 2
Unit cost analysis for water produced in dollars.

Table 3
The production cost comparison with other studies.

Table 4
The highest permissible values: WHO and Russian standards.
Fig. 6.Compression of the TDS content in different water samples from Ekaterinburg, 2019.