New solar still with energy storage: application to the desalination of groundwater in the Bou-Ismail region

Water is essential for the survival and development of humanity, unfortunately, it has become scarce in many regions of the world. It is a basic element for sectors such as agriculture, fisheries, energy production and industry. The lack of potable water is a major problem encountered in the arid and remote regions. Most of the ground and surface water are saline waters, it contains some bacteria depending on the nature of regional geological structures and topography.

The development of desalination technologies combined with the use of renewable energies is a very attractive and promising prospect especially for the remote regions that have the greatest water shortages and higher solar radiation availability (Begrambekov et al. 2011; Kabeel & Emad El-Said 2014; Darwish 2014). Solar distillation is one of the simple techniques for the supply of fresh water to rural areas. It is widely used in water desalination. Many solar stills are developed worldwide (Pages et al. 1989; Dunkle 1961; Fernandez & Chargoy 1990). Different geometries of solar stills have been developed and tested to enhance productivity in order to meet water demand economically (Chaker & Menguy 2001; Eduardo et al. 2002; Velmuruga et al. 2008). Akash et al. (2000) investigated experimentally the efficiency of a solar still for various inclination angles of the cover glass. They found that the maximum water production was obtained for an inclination angle of 35°. A mathematical model is used by Al-Hinai et al. (2002) to calculate the productivity rate of a simple solar still under various climatic parameters of the Oman region. They noted that the optimum design conditions of the solar still give an average annual solar yield of 4.15 kg/m² per day.

Sahoo et al. (2008) developed a new design of solar still with a single basin for the extraction of fluoride in drinking water using a solar still. Tests were conducted to determine the amount of fluoride extracted from the samples, the rate of hourly output, and the efficiency of the distiller. A decrease was observed in the rate of fluoride from 96% to 92%. They showed an increase in efficiency of 11%, by using thermocol as insulation. In addition, the effectiveness of the distiller increases by 4.69% using a blackened absorber only, whereas it was 6.05% when adding polystyrene insulation.

Tanaka (2009) conducted an experimental study to examine the effectiveness of a solar distiller basin modified by internal and external reflectors. The experimental results and theoretical predictions of the daily productivity of the still are in good agreement, especially for clear days. The daily productivity of a conventional distiller obtained with this model is in good agreement with the experimental results of Cooper (1969, 1973). He concluded that the integration of two interiors and outer reflectors can increase daily productivity from 70% to 100%. Other studies related to stills equipped with internal and external reflectors were carried out (Abdul Jabbar & Hussein 2009; Tanaka & Nakatake 2009a, 2009b).

A three dimensional two phase model was developed for a one-stage basin solar still using computational fluid dynamics (CFD) software based on the finite volume method by Setoodeh et al. (2011). Convective and evaporative heat transfer coefficients based on Dunkle's correlation and Kumar and Tiwari model were calculated. It seems that the solar water heater is the best option for increasing the basin water temperature according to the Karuppusamy (2012) work. He showed that coupling a vacuum tube collector with solar energy increased system efficiency up to 49.7%. Likewise, Kargar et al. (2013) integrated a pulsed heat pipe (PHP) in a solar still. They found a marked increase in the production rate of desalted water with a maximum of 875 ml/m² h. It has been found that tube solar collectors have improved performance for high temperature operations compared to the greenhouse distiller. An extensive review paper of solar stills has been published by S. Karuppusamy, (Sampathkumar et al. 2010). Attempts have been made to develop and optimize a basin solar still by adding a magnetic treatment unit (0.12 Tesla) and double glass cover provided with water (Rehman & Al-hilphy 2013).

Several experimental and numerical investigations on multi-stage stills have been reported in previous studies such as the work of Malik et al. (1978), and Kumar et al. (1989). Tigrine et al. (2015) developed a multi-stage water desalination prototype. Different experimental studies have been performed to study its performance. It showed great potential in terms of higher distillation yield per unit area. The multi-stage system at laboratory scale is a research tool to identify the variables and quantify their effects on productivity (Diaf et al. 2012a, 2012b; Darwish 2014). In the Arabic region, solar distillation has been easily adapted in the semi-arid and arid areas when water salinity does not exceed 12 g/l. The development of desalination technologies combined with the use of renewable energies is a very attractive and promising prospect especially for the remote regions that have had the greatest water shortages and higher solar radiation availability (Begrambekov et al. 2011; Darwish 2014; Kabeel & Emad El-Said 2014).

Mousa & Arabi (2013) designed a solar still coupled with a cooling water flow rate combined with an external solar collector in order to improve its productivity. They used a double glass cover through which cooling water flows and increases vapor condensation. The experimental results showed that the production rate is proportional to the solar irradiation, ambient temperature, and cooling water flow rate. The system was capable of distilling 0.4 l/h of saline water of low salinity (2–6 ppm), the temperature of the hot water could reach 87 °C.

Sheeba et al. (2015) studied and evaluated the performance of a solar still–flat plate collector system under the environmental conditions in Trichy with fresh tap water and saline water. A flat plate collector system was used as a still basin. It was found that the combined solar still–flat plate collector daily efficiency was 20.4 and 23.6% more than the still alone for fresh water and saline water systems, respectively.

Over recent years, to improve the efficiency of solar stills using an external power sources such as collectors or reflectors many approaches have been developed (Bapeshwar & Tiwari 2007; Chan et al. 2013).

Bapeshwar & Tiwarit (2007) analyzed a solar still with water flow over a glass cover coupled with a flat plate solar collector to increase the difference between water and glass temperatures which increased the yield of the apparatus. This system showed the best performance compared with that of a simple single basin solar still. W. Wilco and colleagues (Chan et al. 2013) examined the energy performance of two types of solar collectors. Their evaluation revealed that the heat output of the two considered solar collectors was promising. In comparison with the existing solar systems, the results provided reliable and independent data on the economic viability and energy performance.

To improve the productivity of the solar still, Khare et al. (2017) studied the effect of different materials such as black rubber and black gravel that were used in the evaporator. They found that the efficiency of the solar still was 32, 29 and 27% respectively for a quantity of water of 5 L, 10 L and 15 L. Hassan & Abo-Elfadl (2017) examined the influence of the condenser types (glass plate, aluminum plate, aluminum heat sink with pin fins and aluminum plate covered with an umbrella) and saline water on the performance of single slope solar still.

They indicated an increase in daily productivity of up to 35% by using a glass condenser with black steel fiber compared with sand.

To enhance the evaporation rate and productivity of the solar still, different new absorber configurations (flat, grooved, and fined shaped) were used by Samuel and Kalidasa Murugavel (2017). They reported an enhancement efficiency of the integrated still with a fin-shaped absorber was 40.9%, which was 34.1% higher than that of a conventional inclined still with a flat absorber.

Recently, researchers have widely investigated the use of micro/nanoparticles to enhance the water evaporation as well as solar still yield (Balachandran et al. 2019; Elsheikh et al. 2019; Kabeel et al. 2019; Balachandran et al. 2020). In fact, nanoparticles increase water thermal characteristics by increasing surface area for heat transfer.

Several effective approaches and ideas have been proposed to take full advantage of nanotechnology. Muraleedharan et al. (2019) used Al₂O₃–Therminol-55 as heat transfer nanofluid to increase the heat transfer rate in the solar collector loop. The performance of the modified active solar distillation system with 0.1% of nanoparticles to the obtained efficiency of 53.55%. Sharshir et al. (2020) employed black paint blended with nanomaterial tested in three types of stills. They used nanoferric oxide and micro ferric oxide. Maximum efficiency was recorded for nano absorbent layer solar still on both the experimental days with values at 68 and 55%, respectively on the consecutive days, which are higher than a micro-absorbent layer, and conventional solar still.

In this paper, a concrete solar still coupled with a flat plate solar collector was developed and tested under the climatic conditions of Bou-Ismail town. The main advantages of this system destined for water desalination applications and the wastewater treatment by using solar energy are its economy, low maintenance, no operation requirements and solar energy use. The main target of the present experimental study was to introduce a developed concrete solar still and enhance its performance by coupling it with two solar heating collectors.

Design parameters can have an important effect on the performance of the concrete solar still such as the geometry, water depth in the evaporator, the temperature difference between the water in the boiler and bottom side of the glass, the material of the evaporator and the energy storage system. Two flat-plate solar collectors are used for water heating and energy storage in the concrete mass. This technique is more advantageous due to its greater reduction in heat loss and large heating surface area. They have improved performance compared to solar stills (distiller greenhouse) because they provide temperature operations relatively high therefore heat transfer is increased. The performance of greenhouse solar distillers and active solar stills is predicted by measuring the yield for each considered case. The experimental results showed a significant improvement in the system yield due to coupling the system with a solar thermal collector which increases the temperature in the solar still basin in the day and the heat stored in the concrete is used during the night. Physical and chemical analysis of well water and distilled water produced by equipment were evaluated.

The experiments were carried out on a concrete solar single slope still coupled with a thermal collector designed and realized by our team in the Development Unit of Solar Equipments UDES/Algeria. The distillation equipment consists of two components which are the distillation basin and a lower part realized in concrete mass. The experimental set up of the solar distillation is shown in Figure 1. All outside walls of the distillation unit and basin still are made of galvanized steels sheet of 2 mm. The solar still basin is rectangular in shape with an area of 1.08 m² and a total capacity of saline water to be treated estimated 110 L. The parameters of the concrete solar still prototype are given in Table 1.

As mentioned above, the sides of the still were manufactured with the same material and are closed in order to make the still airtight. The solar still is enclosed by a transparent top glass cover which is 5 mm thick and which allows the passage of the incident solar radiation. The cover glass is inclined at an angle of 13° with respect to the horizontal plane to increase the solar radiation by catching it head on. In the present study, the inner surface of the basin is not blackened in order to study the efficiency of the solar collector on the productivity of the system by enhancing heat transfer in the saline water. To increase the heat transfer in the basin of the solar still, we have mounted this last one on a slab of 7 cm height realized in concrete where a heat exchanger is embedded. This system is intended to recover the energy stored during sunshine. Figure 2 shows the copper serpentine exchanger resting on the plate before casting the slab.

The operating principle of a solar still greenhouse used to desalinate all waters types involves change phase water – steam. The process can separate the constituents of water in the salts deposit form and distilled water. The solar flux transmitted through the glass cover reaches the bottom basin, which in turn emits thermal radiation that heats the water by the greenhouse. In addition, the heat exchanger integrated into the basin still provides energy by convection. Heat evaporates part of this water, whose water molecules escape, leaving the salts dissolved as a deposit and all other residues in seawater salts. The obtained steam is condensed on the inclined glass surface and the distillate trickles runs down into a gutter from where it is fed to a storage tank Figure 3.

Furthermore, the amount of solar heat received during the day is stored in the concrete mass that is transferred to the still chamber by conduction. This energy storage is used to ensure continuous production of the equipment during the nocturne period.

The global solar radiation received on the ground is measured by the pyranometer type CMP11 « Kipp & ZONEN/ 7–14 μV/W/m² » of unit development of solar equipment (UDES) situated in the city of Bou-Ismail with an altitude of 33 meters which is located at Latitude 36°38′33″ North and Longitude 2°41′24″ East. Thermocouples type K (Chromel/Alumel) are used for measuring temperatures because of their good stability over time, good sensitivity (40 μV/°C) and good linearity of temperatures considered in the range 0 °C to −100 °C. Thermocouples are also placed at various points of the slab located in the same horizontal plane T_s and in the basin solar still namely the temperature of the saline water T_w, basin still T_b, interior and exterior glass cover T_ig,T_eg and ambient temperature T_amb. The data logger allows data acquisition (Figure 4). It is controlled by a PC running the software Hydra with a data acquisition card. To limit the size of the stored files during 24 hours, measurement intervals of a quarter of an hour are selected. The data are processed in Origin to plot curves.

The measurements of the distilled water characteristics that determined water quality (salinity, electrical conductivity, oxygen dissolved in water, pH, TDS) were performed using Inolab laboratory conductivity meter (Cond level 1).

The experimental setup consists mainly of a solar still basin where brine water will be distilled and a concrete slab in which energy is stored via a heat exchanger.

The experiments were divided into four series of tests namely: Water heating, Energy storage, Energy storage and water heating, and Greenhouse solar effect. Each test was conducted for four days separately. We recorded the following measurements for each day and for a specific time interval:

• 1.

  Distillate productivity (ml).

• 2.

  Temperatures of each still element (water, slab, and absorber, glass exterior and interior and ambient).

• 3.

  Flow rate of saline water was adjusted for each morning at 09 AM.

• 4.

  Solar radiation strength was measured by the meteorological station of the Research Center.

• 5.

  Water depth inside the basin still is kept constant.

• 6.

  Orientation of the solar still southward.

The experiments were performed using two flat solar collectors in order to conduct four different tests as presented below:

Case 01: Water heating; only one solar collector is connected with heater exchanger immersed in the brackish water of the solar distillation basin. This technique allows enhancing water evaporation in the basin. The salt water in the evaporator is heated by direct solar irradiation and by hot water flowing in the heat exchanger heated by the flat plan solar collector.

Case 02: Energy storage; only the other solar collector is connected to the heat exchanger that was integrated in slab concrete (for storing heat in the concrete base).

Case 03: Energy storage as well as water heating; the two solar collectors are connected in parallel (at same time) to heat water in the basin and concrete slab.

Case 04 (reference case): solar greenhouse; test without solar collectors (passive solar still).

Various factors may affect the performance of the solar distillation system where the productivity of distilled water via this equipment depends on design parameters (construction materials, absorber, water depth and temperature) and also meteorological parameters (solar radiation, ambient temperature, wind speed, humidity and weather).

The solar collector converts solar radiation into heat energy, usually by means of a heat transfer fluid (water, air…). We used a flat plate solar collector DUROTHERM type. The characteristics of flat plate solar collector are given in Table 2. It is well optimized, and developed according to the most modern manufacturing processes. It is installed on metal supports in galvanized steel 2 and 3 mm in thickness and inclined at 40°, it is painted with a special material to protect it from corrosion (Figure 5). The solar collector operates by indirect thermosiphon which protects it against freezing and plugging and thus increases the lifetime of the system.

Moreover, tempered solar glazing with low iron content provides maximum transparency, protection against extreme weather conditions and more effective use of solar energy through its microprisms that prevent the dissipation of reflected sunlight. The coolant fluid is composed of water, sodium molybdate, trisodium phosphate, sodium silicate and propylene glycol; its physicochemical characteristics are antifreeze and corrosion protection. Physical and dimensional characteristics of flat plate solar collector are shown as flows:

• 1.

  Overall dimensions: 1,990 × 990 × 88 mm

• 2.

  Overall surface: 1.97 m²

• 3.

  Input area: 1.78 m²

• 4.

  Maximum allowable pressure: 10 bars

• 5.

  Vacuum weight: 35 kg

• 6.

  Water content of the absorber: 2.15 liters

• 7.

  Optical efficiency of the collector: 0.78

• 8.

  Loss coefficient 1st degree: 5.6 W/K/m²

• 9.

  Loss coefficient of 2nd degree: 0.011 W/K/m².

The following tables show the components and characteristics of a solar collector design used in our experimental work.

In this work, the objective is to study the dual heat exchanger system effects which provide heat energy via solar collector either in the concrete slab or in the basin of the still on the performance of the distillation equipment. We attempt to quantify the temperature effect of the water to be distilled in the basin still using a heat exchanger driven by a flat plate solar collector during the day and nocturnal period. In our experiment, we used Bou-Ismail well water of electrical conductivity 2,085 μS/cm, salinity 0.9 g/l and pH = 7.43 at T = 22.1 °C.

Firstly, the quantity of water in the basin of the solar still was maintained at 35 kg. The experiments were carried out for different days under climatic conditions of Bou-Ismail city. The single basin solar still was coupled with a flat plate collector; commercialized water was supplied to the solar collectors for their operations in natural circulation mode ‘thermosiphon circulation’ by checking their working temperatures and heat transfer. For each experiment, the glass cover was cleaned to avoid dust deposition over the outer layer of the glass. All temperature values were recorded for 24 hours.

The experimental apparatus was studied during the sunshine and night period through a series of tests. Daytime period is considered between sunrise until sunset (6AM to 7PM) and the night period between 7 PM to 5 AM.

The performance tests were conducted from July to September 2013 in order to determine experimental parameters conditions and find the effect of coupling the solar collector and the solar still.

In this investigation, we evaluate the performance system driven by solar energy two times in order to enhance daily productivity. To improve the global efficiency of the distillation system, a heat exchanger has been used to accelerate the heating process in the morning and a second heat exchanger has been placed in the concrete slab to store energy which is transferred by conduction at the end of the day (after the sunset) to enhance heat transfer. Experiments were conducted to understand the performance of the solar still coupled with the flat plate solar collector and compare it with the conventional solar still. For all experiment days, we have measured different temperatures as mentioned below. The temperature is an important parameter that influences strongly the system yield. The experimental system is conducted on a summer's day where the operating period of the system is longer in comparison to the winter period. It is very important to measure the evolution of temperature in the concrete slab and different temperature inside the distillation chamber specifically water temperature to be desalinated, basin temperature, glass cover temperature and ambient temperature. It is well known in the distillation process that water temperature and the inner glass cover temperature were the most influential parameters on the system yield. Figure 6 shows different temperature profiles for three thermal energies used in the present work namely greenhouse solar effect, basin water heating and stored energy in the concrete base.

It presents the results of the experimental data in 24 h time period. It can be observed that the temperatures increase versus time until maximum value at about 16 PM and then decrease again. Temperature difference between the glass and the water is relatively important and it is maintained until 18 PM. This temperature difference (25 °C) helps the vapor to condensate on the lower glass surface and hence condensing more vapors and the collected water increases during daytime. We observe that water temperature obtained using the combined effect of energy storage and water heating system is greater than the system with greenhouse solar effect. In addition, interior glass temperature is more important than that of water temperature which explains the low rate of production during the daytime. After 18 PM this difference of temperature becomes reversed and by consequence the night production was considerable for all case studies.

The variation of the radiation received by an inclined surface of 13° during the experimental tested glass solar radiation is shown in Figure 7. Three solar radiations are given as a function of time 04/08/2013, 26/08/ 2013 and 03/09/2013. It can be noted from Figure 7 that the variation in solar radiation incident values during the three days of the testing is insignificant. For example, the maximum values of solar incident radiation around midday were around, 885.875 and 783 W/m² for the three days. Observations show that the solar radiation becomes most intense between 10 AM to 15 PM whereas it decreased from 16 PM this represents a long time exposure during the month of July and August. Measurements of solar radiation obtained show a slight difference between 03/09/2013, 26/08/ 2013 and 04/08/2013 days from 11 AM and 15 PM which explains the increase in the daily production. It has been found that the production of distilled water strongly depends on the incident solar energy.

In Figure 8, the results obtained from the experiments depict the impact of operating parameters which are greenhouse solar distillation, distillation by water heating and storage energy production of fresh water. The performances of the distillation system for the considered cases are represented. We compared the daytime production and that of the overnight production that were recorded for fixed volume and different energy sources. It is clearly noticeable that the total daily production is higher using the two solar collectors with respect to the production obtained by the greenhouse solar effect. The excess of the estimated production rate was about three times more.

We found that night water productivity is higher than during the day due to the storage of solar energy, which is stored in the base allowing us to extend the operating time of our solar still well beyond sunset. The operation of the solar still at night is a unique and very advantageous characteristic because the relative coolness of the night offers thermodynamic parameters favoring the increase of the efficiency of the still.

For different energy sources, we observe that night production is more important than production recorded during the daytime. For example, on the day of 04/08/2013, the night production was equal to four times the daily production using water heating and energy storage. This is due to weak values of interior glass temperatures in the time range 18 PM to 24 PM where it exceeded 55 °C at 14 PM to 17 PM. For cons, basin temperature becomes higher than the temperature of the lower surface glass after 18 PM. Figure 9 displays the total output recorded during four days for each case study. It can be summarized that the performance of the equipment is more significant when two solar collectors were used simultaneously. In general, water quality is determined according to the intended use. Physical and chemical testing of source waters is required to determine the level of treatment in order to obtain the required water quality.

In this work, the physical characteristics of the raw water source were evaluated before and after water desalination. We have carried out chemical analysis of well water and distilled water obtained from the solar still pH, TDS, EC and salinity (Table 3). These chemical analyzes were performed on 24/05/2013.

The quality control of distillate was ensured by measuring electrical conductivity and pH for each distilled water collection. Figure 10 shows the evaluation of the physical and chemical properties of distilled water produced by the concrete solar still for different days.

The measured electrical conductivities were lower than 45 μS/cm this permits good quality water to be obtained according to the international norms and the pH values averaged about 6. Following the results and evaluation of these parameters, the distillation process proved to be a good option for water treatment.

Table 4 presents a comparison of the product cost of solar stills between the present study and the published literature. The result indicates that the cost of the present solar still (0.11$/L) is more than the indicated references solar stills. This is due to the additional cost of the commercial model of flat plate solar collector used in the present study. To minimize the distilled water cost, the authors recommended the designing of a new solar collector using simple and economical local materials in order to improve the thermal performance of the solar water as well as the production rate.

Solar water distillation is a very simple and effective technology that can be used to provide fresh water in many remote areas of arid and semi-arid developing countries. Therefore, salted water represents the greater part of the water available on Earth, its desalination becomes a key solution that contributes to the sustainable development.

The present study aimed to enhance the daily productivity of the concrete solar still coupled with two solar collectors. The performance of the developed solar still is compared with that of the conventional solar still (reference case). The solar still with or without a flat plate solar collector permits the use of solar thermal desalination technologies and produces clean water.

Some thermal and operational parameters that affect strongly the performance of the solar still have been examined and optimized. The experimental test results showed that the increase in distilled water through the heater system is very important in comparison with the productivity of the conventional solar still using the greenhouse effect. A significant improvement in the productivity rate is achieved. the obtained results indicated that the use of energy storage increased productivity by fifty percent in the nocturnal period. Indeed, the daily productivity of the concrete solar still is strongly affected by the heat transfer produced from concrete to the evaporator water are presented in this study. It was found that the average daily distillate output of the concrete solar still with energy storage is more significant than the one of a solar still without a collector.

All relevant data are included in the paper or its Supplementary Information.