Present Status of Solar Still: A Critical Review - CiteSeerX

IJRREST: International Journal of Research Review in Engineering Science and Technology (ISSN 2278- 6643) | Volume-2 Issue-1, March 2013

Present Status of Solar Still: A Critical Review *Bhavsinh Zala, **Kuldip Dodia, ***Hitesh N Panchal *M.E. (Thermal Engineering) Scholar, Gandhinangar Institute of Technology, Gandhinagar, Gujarat, India **Asst. Professor, Mech. Engg. Dept., Gandhingar Institute of Technology, Gandhingar, Gujarat, India ***Asst. Professor, Mech. Engg. Dept., Gujarat Power Engineering & Research Institute, Mehsana, Gujarat, India

Abstract— People can survive for day, weeks or months without food, but cannot live for more than a week without water. The body use water for digestion, absorption, circulation, transporting nutrients, building tissues, carrying away waste and maintaining body temperature. The average adult consumes about 2.5 to 3 litters of water per day to drink. Solar still is a device, which can produce water more than 2.5 Liter to 3 Liter of water by improvement in it. Hence, this review paper shows researches done by many scientists on solar still to enhance productivity.

1. INTRODUCTION Adequate quality and reliability of drinking water supply is fundamentally needed of all people. Without potable or fresh water, there is no human life. Industries and agricultural need fresh water without which they cannot function or thrive. Water is therefore, the key to man’s prosperity; it is intimately associated with the evolution of civilization from rivers, lakes and ponds in plenty are becoming scarce because of industrialization and population explosion.

2. SOLAR DISTILLATION 2.1. About Distillation The solar distillation method is an easy, small-scale and cost effective technique for providing safe water at homes or in small communities. Distillation is one of many processes that can be used for water purification. This requires an energy input as heat and the solar radiation can be the source of energy. In this process, water is evaporated, thus, separating water vapor from dissolved matter. The vapors get condensed as pure water. A conceptual diagram showing the process of solar still is shown in Figure 1. Most of the conventional water distillation plants discussed earlier are energy-intensive and require scarce electric power or fossil fuel for operation. But solar energy, despite being much lower grade energy, is ideally suited. The technology involved in distillation of saline or brackish water using solar

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energy is relatively simple and semiskilled/unskilled operators can carry out its operation and maintenance.

Fig. 1. Conceptual diagram explaining the process of solar distillation. Thus, for a small scale decentralized production of potable water by using the brackish or saline water of salinity level < 10,000 ppm or mg/l (10 ppm = 10 mg/l = 0.01 g/l). Solar distillation which is capable of removing bacteria also, is best suited.Nature itself provides most of the required fresh water, through a similar hydrological cycle in which a very large-scale process of solar distillation produces fresh water naturally. The essential features of the hydrological cycle can be summarized as the production of vapors above the surface of the liquids, followed by transport of vapors by winds, then cooling of airvapor mixture, condensation and precipitation. This natural process is copied on a small scale in basintype solar stills shown in Figure 2.

Fig. 2. Basin-type single slope passive solar still.


2.2. Basic Principle of double slope solar still A solar still operates using the basic principles of evaporation and condensation. The impure saline feed water goes into the solar still and the sun's rays penetrate a glass surface causing the water to heat up through the greenhouse effect and, consequently, evaporate. When the water evaporates inside the solar still, it leaves all contaminants and microbes behind in the basin. The evaporated and now purified water condenses on the underside of the glass and runs into a collection trough and then into an enclosed container. In this method the salts and microbes that were present in the original feed water to the solar still, are left behind. Additional water fed into the solar still flushes out concentrated waste from the basin of solar still to avoid excessive salt deposition in the basin. A solar still effectively eliminates all water-borne pathogens, salts and heavy metals that other huge methods cannot do. Solar still technologies bring immediate benefits to users by reducing health problems associated with waterborne diseases. For solar still users, there is also a sense of satisfaction in having their own trusted and easy to use water treatment plant on-site at home. As a thumb rule, solar still production is a function of solar energy (insolation) and ambient temperature. Figure 3 diagrammatically shows the principal energy exchange mechanisms in a basin-type double slope solar still. Solar radiation is absorbed on the black bottom of the basin generally known as basin liner. However, there are several reflection losses from the cover surfaces, the water surface, and the bottom itself. There may also be some absorption of solar energy in the cover of the solar still.

conduction and convection. A small fraction of the absorbed heat may be lost by conduction into the ground. At the water surface, energy is transferred to the cover by three mechanisms. The principal means is by vaporization of water from the liquid surface. By diffusion and convection, the vapor is transferred to the cover, where its latent heat is liberated by condensation. Some heat is also transferred to the cover by free convection of the air in the distiller. Thirdly, heat is radiated from the water surface to the condensing cover. Figure 4 shows a single slope basin-type solar still.

Fig. 4. Single basin- Single slope solar still. 2.4. Progress of solar still for enhancement of distillate output 2.4.1

Free Surface Area of Water

The evaporation rate of the water in the solar still is directly proportional to the exposure area of the water. Thus the productivity of the solar still increases with the free surface area of the water in the basin. To increase the free surface area of the water, sponges are used at the basin water.

Fig. 3. Double slope Basin type solar still with energy flow. 2.3. Working Principle of single slope solar still Energy absorbed on the basin bottom (basin liner) is then largely transferred to the saline water by

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Fig. 5. Schematic diagram of the solar still with sponge cubes.


Bassam et al. (2003) used sponges to increase the free surface area of the water in the solar still as shown in Fig. 5. Due to capillary action, water is sucked by the sponges. The sponge cubes increase the surface area over which water evaporation occurs. The use of sponge cubes in the basin water resulted in a significant improvement in still production. Experiments were carried out with various sizes of the sponge cubes, sponge to basin water volume ratio and water depth. The optimal combination was: sponge cubes with 6 cm sides, 20% sponge to basin water volume ratio and 7 cm basin water depth. It was found that the increase in distillate production of the still ranged from 18% to 273% compared to an identical still without sponge cubes under the same conditions. Velmurugan et al. (2008, 2009) found that the productivity of the solar still increased by 15.3%, when the sponges are used in single basin solar still and stepped solar still integrated with fin. 2.4.2 Water–Glass Cover Temperature Difference The yield of a solar still mainly depends on the difference between water and glass cover temperatures. The temperature difference between water and glass are acting as a driving force of the distillation process. Regenerative solar still, Yousef et al. (2004), solar still with double glasses, Mink et al. (1998) and triple-basin solar still were used to increase the temperature difference between glass and the water.

vapor condensing on the glass of the first effect to produce more fresh water in the second effect. The performance of the regenerative solar still is evaluated by comparison with the performance of the conventional still under the same weather conditions. The results show that the productivity of the regenerative still is 20% higher compared to the conventional still. 2.4.2.2 Triple-Basin Solar Still A triple-basin solar still was fabricated by El-Sebaii (2005). It consists of three basins namely, lower basin, middle basin and upper basin. Water was made available in all three basins. It was concluded that the productivity of the lower basin is higher than the productivities of the middle and upper basins during the day time, and this behavior is reversed overnight. Further, the total daily productivity of the system is maximum for the least water masses in the lower and middle basins without dry spots over the base of the each effect. 2.4.3

Area of The Absorber Plate

2.4.2.1 Regenerative Solar Still with Double Glass

Fig. 7. Solar still integrated with fins.

Fig. 6. Schematics of the regenerative solar still. The regenerative solar still consists of two basins, with provision for cooling water to flow in and out of the second effect as shown in Fig. 6 This arrangement has the advantages of increasing the temperature difference between water and glass cover in the first effect and utilizes the latent heat of water

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Productivity of the solar still increases with increase in absorber area. Fins are integrated with solar still to increase the absorber plate area. Five rectangular solid fins Velmurugan et al. (2008) are integrated at the basin of the solar still as shown in Fig. 7. The area of the basin increases considerably. Experimental results showed that the average daily production of distilled water in solar still increases by 30%, when fins are integrated in basin of the solar still. 2.4.4

Inlet Temperature of Water

The evaporation rate of saline water increases with the temperature of the saline water. Flat plate collector, storage tank, mini solar pond, multi-source,


multi-use environment, greenhouse and, multisource, multi-use environment heat pipe are integrated with the solar still to increase the temperature of the solar still water. Higher energy may be required to increase the temperature of the entire solar still water. The evaporation rate is proportional with the temperature of the free surface area of the water only. So, baffle suspended absorber plates are used to increase the free surface area of the water.

factors influencing the productivity of a multipleeffect diffusion-type solar still coupled with a flat plate collector and the effect of inclination of external flat plate collector of basin type still in winter were studied by Tanaka and Nakatake (2005, 2007)

2.4.4.1 Flat Plate Collector

Fig. 10. Solar still coupled with tank and collector.

Fig. 8. Solar still coupled with collector.

A single-stage basin-type solar still, a storage tank and a conventional flat-plate collector were connected together in order to study the effect of augmentation on the still as shown in Fig. 10. The still inlet was connected to a locally made, fin-type collector such that its outlet was fed to the still basin instead of the common storage tank. It was found that the mass of distilled water production using augmentation was increased by 231% in the case of tap water as feed and by 52% in the case of salt water as a feed. 2.4.4.2 Storage Tank

Fig. 9. Solar still coupled with flat plate collector. A flat plate collector is integrated by Voropoulos et al. (2001) with the solar still, to increase the temperature of the basin water. The preheated water from the solar collector was circulated by a tube through the basin water. The tube is acting as a heat exchanger and exchanging heat from the preheated water to the basin water as shown in Fig. 8. Thus the basin water gets heated. In an another arrangement, as shown in Fig. 9, the basin water is directly circulated through the flat plate collector. The various

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Fig. 11. Solar still coupled with a hot water storage tank. An experimental evaluation of the behavior of a solar still is presented by Voropoulos et al. (2003) in which a thermal storage tank with hot water is integrated as shown in Fig. 11. The integration of the storage tank is done in such a way that a compact solar distillation system is formed. This design leads to higher distilled


water output due to higher basin water temperatures as a result hot storage tank water. 2.4.4.3 Mini Solar Pond

Fig. 12. Solar still integrated with a mini solar pond. Velmurugan et al. (2007, 2008) integrated a mini solar pond with single basin solar still as shown in Fig. 12. The mini solar pond supplied hot water to the solar still. Thus evaporation rate of the saline water in the solar still was augmented. The productivity of the solar still increases by 27.6% than the conventional solar still. 2.4.4.4 Baffle Suspended Absorber

Fig. 13. Solar still with baffle suspended absorber. As shown in Fig. 13, a single slope single basin solar still with baffle suspended absorber El-Sebaii et al. (2000) was designed and fabricated, to decrease the preheating time of the basin water. The effect of vent area and water depth of upper and lower water column on the daily productivity of the solar still was studied. It was found that, the daily productivity of the solar still is about 20% higher than that of conventional solar still when it is used with baffle suspended absorber.

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2.4.4.5 Floating Perforated Plate Productivity of the solar still increases with the saturation pressure of the water. The saturation pressure is determined by the temperature of the basin water. To increase temperature of the free surface area of the water, initially porous wick material was used. During the long run of experiments, the porous wick material would be clogged by accumulated salt. Therefore, floating perforated plates are used to maintain film evaporation the still. The effect of using floating perforated black plate on the productivity process is investigated theoretically and experimentally at different brine depth under the same operating condition. It was found that using floating perforated aluminum plate in the solar still increase the solar still productivity by 15% for a brine depth of 3 cm and 40% for a brine depth of 6 cm. 2.4.5

Glass Angle

Singh and Tiwari (2004) found that the annual yield of the solar still was maximum when the condensing glass cover inclination is equal to the latitude of the place. 2.4.6

Depth of Water

It has been reported that the yield is maximum for the least water depth. While maintaining minimum depth in the solar still, dry spot may occur. So, it is very difficult to maintain minimum depth in the solar still. Wick type solar stills, a plastic water purifier and stepped solar still were developed. The effect of various depth of water in the solar still is verified by Khalifa et al. (1990) Extreme operating condition in shallow solar stills was studied by Porta et al. (1997). Murugavel et al. (2007, 2010) studied an experimental study on single basin double slope simulation solar still with thin layer of water in the basin. 2.4.6.1 Wick Type

Fig. 14. Wick type solar still.


Fig. 15. Multi-wick type solar still. Minasian and Al-Karaghouli (1995) connected a wick type solar still with a conventional basin-type solar still. The hot waste brine leaving the wick-type would feed the basin type in order to extract the energy contained in the lost hot drainage water and thus increase the productivity of the wick-type solar still. Fig. 15 shows the schematic diagram of the experimental set-up. Two types of solar stills were investigated. These were: a conventional basin type and wick-basin type solar still. The total yearly amounts of distilled water indicate that the wickbasin type could produce 85% more than the basin type and 43% more than the wick type solar still. In multi-wick solar still, jute cloths with different lengths are used. One end of a wet jute cloth pieces (porous fiber) are dipped into a water reservoir while the other ends are spread over the absorber basin as shown in Fig 15. The jute cloth pieces are blackened and placed one upon the other, separated by polythene sheets. Here the wet piece of jute cloth forms the water surface in the still. It could attain a higher temperature due to its negligible heat capacity. This leads to rapid evaporation of water. Surfaces used for evaporation and condensation phenomena play important roles in the performance of basin type solar still.

As shown in Fig. 16, a concave wick surface by Kabeel AE (2009) was used for evaporation, whereas four sides of a pyramid shaped still were used for condensation. Use of jute wick increased the amount of absorbed solar radiation and enhanced the evaporation surface area. A concave shaped wick surface increases the evaporation area due to the capillary effect. Results show that average distillate productivity in day time was 4.1 l/m2 and a maximum instantaneous system efficiency of 45% and average daily efficiency of 30% were recorded. The maximum hourly yield was 0.5 l/hm2 after solar noon. An estimated cost of 1 l of distillate was 0.065$ for the presented solar still. 2.4.6.2 Plastic Solar Water Purifier Ward (2003) designed a solar water purifier, which consists of a plastic sheet and a glass window. The author formed the plastic into an array of interconnected square cells which contain impure water. In their experimental set-up there were no filters, no electronics, no moving parts and cleaning was also rarely needed. It was light weight, cheap strong, durable and cheap. It can be used in any sunny location on earth. 2.4.6.3 Stepped Solar Still

Fig. 17. Schematic diagram of a stepped solar still.

Fig. 16. Schematic diagram of concave wick solar still.

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While maintaining minimum depth in the solar still dry spots are formed. So, maintaining minimum depth in the solar still is very difficult. To overcome this problem, a solar still with stepped structured absorber plate was designed and fabricated by Velmurugan et al. (2009, 2008) As shown in Fig. 17, there are 50 trays in the solar still. The sizes of the trays are 99mm×99mm cross-section with different depth. Experiments were carried out with fins and


sponges in the stepped solar still to enhance its productivity. 2.4.7

Other Methods

Reflectors, condenser, vacuum technology, combined stills, asphalt basin liner and sprinkler and sun tracking systems are used to maximize the yield of the solar still. Also energy storage materials like black rubber, gravel and metallic wiry sponges are used for enhancing the productivity of the solar still. The effect of using different designs of solar stills on water distillation was studied by Al-Hayek and Badran (2004).

An external condenser Tiwari et al. (2003) is attached with a solar still as shown in Fig. 19 to enhance the productivity of the solar still. The condensation occurs due to the temperature difference not only on the glass surface but also on the four sidewalls, which can be cooled by water circulation through tubes attached on the wall surface for efficiency enhancement. Such an arrangement Khalifa et al. (1999)is made to enhance the productivity of the solar still as shown in Fig. 20.

2.4.7.1 Reflectors

Fig. 20. Solar still with internal condenser.

Tanaka and Nakatake (2006) proposed a geometrical method to calculate the solar radiation reflected by the internal and external reflectors and then absorbed on the basin liner. The schematic diagram for the set is shown in Fig. 18. They found that the internal and external reflectors remarkably increase the distillate productivity by 48%.

The maximum daily production of the solar still was about 1.4 l/m2/day, and its efficiency was about 30% with corresponding average solar insolation of 28 MJ/day. The condensate water quality was analysed and was found to be comparable with water quality standards and against rainwater and mineral water. Increased cooling on the wall surface was observed enhancing the condensation process. To comprise in a nutshell, conclusions can be drawn that the utilization of solar energy in purifying water offers a good recommendation not only on the environmental aspect but also on the sanitation. Per- formance study on solar still with enhanced condensation was studied by Kumar and Bai (2008).

2.4.7.2 Condenser

2.4.7.3 Energy Storing Materials

Fig. 18. Solar still with reflectors.

To store the thermal energy in solar still, some energy storing materials are used. Black rubber, gravel, metallic wiry sponges and surfactant additives are some of the energy storing materials used in solar still. 2.4.7.3.1 Black rubber

Fig. 19. Solar still with external condenser.

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The influence of the variation of the rubber sheet thickness on the still productivity is investigated by Nafey et al. (2001). The experimental results showed that black rubber of 10mm thick improves the productivity by 20% at the conditions of 60 l/m2 brine volume and 15o glass cover angle.


2.4.7.3.2 Gravel

(SLS) in relatively small dosages of concentration with a small size unit of solar water distillation process is investigated. 2.4.7.4 Vacuum Technology

Fig. 21. Solar still with gravel. Nafey et al. (2001) studied the influence of black gravel on the productivity of the solar still. The solar still with gravel is as shown in Fig. 21. Using black gravel of 20–30mm size improves the productivity by 19% at the condition of 20 l/m2 brine volume and 15◦ glass cover angle. Also Velmurugan et al. (2001) added pebbles in the solar still and found that the productivity increased by 20% than the conventional solar still.

The effect of vacuum inside the still is to avoid any heat transfer due to convection in the still. The heat loss from the water in an insulated still is due to evaporation and radiation only. In the presence of vacuum, the effect of the non-condensable gas, which reduces the rate of condensation, was also avoided. Hussaini and Smith (1995) studied the effect of applying vacuum inside the solar still. It was found that the solar still’s water productivity increased by 100% when vacuum was applied. 2.4.7.5 Combined Stills

2.4.7.3.3 Metallic Wiry Sponges

Fig. 23. Schematic diagram of combined stills. Fig. 22. Solar still with metallic wiry sponges. Different types of absorbing materials like metallic wiry sponges (coated and uncoated) and black volcanic rocks are used to study their effect on the yield of solar stills as shown in Fig. 22. The results showed that the uncoated sponge has the highest water collection during day time, followed by the black rocks and then coated metallic wiry sponges. On the other hand, the overall average gain in the collected distilled water taking into the consideration the overnight water collections were 28%, 43% and 60% for coated and uncoated metallic wiry sponges and black rocks, respectively. 2.4.7.3.4 Surfactant Additives The presence of surfactant additives in water is found to enhance the boiling heat transfer significantly. The effect of using a surfactant as sodium lauryl sulfate

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Velmurugan et al. (2009) integrated amini solar pond, a stepped solar still and a single basin solar still as shown in Fig. 23. They were used this experimental set-up for effluent desalination process. Initially, water from the effluent settling tank is allowed to pass through the mini solar pond. While passing through the mini solar pond, it is getting heated. The hot effluent water enters into stepped solar still and the distilled water is produced. The excess water from the stepped solar still enters into the single basin solar still. Thus, distilled water is produced in both single basin solar still and stepped solar still.

3. SUMMARY The orientation of glass cover is very important for increase in productivity. Hence, north latitude places south facing glass cover should be used and south


latitude places north facing glass cover should be used. • Lower latitude places double slope solar still and higher latitude places single slope solar still should be used. • Lower thickness glass cover is preferred compared with higher thickness glass cover due to its higher absorptions. • Glass is most suitable material for the solar still cover because of higher transmittance and higher thermal conductivity. • Day time as well as night time productivity of solar still is greatly depends on the heat capacity and water capacity inside the basin. • To improve the higher absorption of the water, black die is most preferable compared with another dies. • For higher radiation intensity places deep basin solar still should be prefer for nocturnal production and lower radiation intensity places shallow basin solar still should be prefer. • Rubber, gravel, saw dust and sponge cubes are good material to store the solar energy and increase the productivity.

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