Introduction

Water is the basic requirement for human being to live on earth. The water available on earth is in higher proportion compared to surface. Most of the amount of water is saline water which is available in ocean. Small amount of fresh water which is available in the form of rivers, lakes etc. Also there is a rapid growth of industries, population, agriculture production and many more such community demands of the pure water increasing.morever most of the diseases to the man are arise due to drinking impure water. Due to decreasing the available fresh water and rapidly increasing the demand of fresh water it is required to obtained fresh water from impure water. For this solar desalination is one of the important option to obtain pure water from impure water with the help of abundant available solar radiation heat energy and the instrument which is required to do this process is known as solar still.

Glass cover is very necessary component in the solar still. Panchal and Shah [1] found that, lower glass cover thickness, enhanced the performance of solar still. Panchal and Shah [2] also carried out the comparison of CFD results and experimental results on solar still glass cover thickness and found good agreement. Kaushal and Varun [3] prepared an analysis for the different types of solar still. The objective of this review paper is to present the investigation of different design, climatic & operational parameter that affects the performance of solar still.

Working Principal of Simple Solar Still

The working of the actual solar still is similar to the nature’s hydrological cycle. In natural distillation solar radiation is incident on the ocean water and causes water to evaporate and the evaporated water rises above the earth’s surfaces due to low density and moved by current of air. When this steam is cools down to its dew point temperature condensation procedure start and pure water comes down as rain. The primary process is responsible for the nature’s hydrological cycle. This similar hypothesis evaporation and condensation is utilized for the separation of saline water and clean water in any solar distillation system.

The simple sketch of solar still showing its working is shown in Fig. 1. It consists of a shallow blackened basin fill with saline water and whole structured enclosed by a sloping transparent cover. Solar radiation transmit through the transparent cover and incident on the basin water causes it to evaporate then evaporated vapours condensed on the inner side of the glass cover and fall down due to gravity gathered in a distillate channel. According to the research done solar still are classified into two categories name it as active distiller and passive distiller. In active solar still supplementary collector, mechanical or electrical energy is used to improve the output. In passive solar still straightforward modification is done inside the basin to improve its output.

Fig. 1
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Simple sketch of solar still showing its working

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Factors Affecting the Performance of Solar Still

The distillate output of solar still depends on the climatic parameters such as solar radiation intensity, wind velocity, ambient temperature etc. Also the design parameters such as brine depth, insulation, cover material & its thickness, inclination of glass cover and operational parameters such as preheating of feed water, salinity of water that affect the yield of solar still. Hence the main aim of this review paper is to shows the investigation done on the different design, climatic and operational parameters that affect the yield of solar still.

Investigation of the Design Parameters of Solar Still

Water Depth

One of the most important design parameters that affect the yield of solar still is water depth. Water depth affects the rate of evaporation and ultimately the yield of still.So that many scientists investigated the effect of water depth on the yield of different configuration of solar still at different place and concluded that minimum water depth gives maximum output.

Badran and Abu-Khader [4] has carried out experimental and theoretical investigation to examine the effect of water depth of 2cm & 3.5cm on a single slope single basin solar still. Test were done on 1st April 2004 and result shows that output of 25.7 % increase by decreasing water depth from 3.5cm to 2cm. Figure 2 shows the hourly distillate output with time for different water depth.

Fig. 2
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Hourly distillate output vs time [4]

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Experimental Test were conducted by Kandasamy et al. [5] to inspect the performance of double slope solar still for six different water depth of 1 to 6 cm.also performance correlation was developed and compared with experimental result. Figure 3 shows deviation in the yield of still for different brine depth. It was observed from the graph that maximum yield obtained at minimum brine depth due to rapid evaporation.

Fig. 3
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Shows deviation in the yield of still for different brine depth [5]

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Condensing Cover Material, Thickness & its Inclination

Tiwari et al. [6] investigated the impact of different condensing cover material like copper, glass & plastic on the daily yield and conclude that copper gives higher yield due to higher thermal conductivity compared to glass & plastic as shown in Fig. 4.

Fig. 4
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Daily yield for different condensing cover material for a still connected with flat plate collector [6]

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Tiwari et al. [6] also investigated the impact of different thickness of glass cover ranging from 2 to 6mm for passive solar still & solar still joined with flat plate collector and reports that with increasing glass cover thickness there is a reduction on the yield is noted for both active and passive mode as shown in Fig. 5.

Fig. 5
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Variation of yield on different thickness of glass covers [6]

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Tiwari and Tiwari [7] have conduct an experiment for three different condensing cover inclination namely 15∘, 30∘ & 45∘ for a passive solar still through the year of clear sky daily over 24hr under climatic condition of New Delhi and conclude that 15∘ is the best inclination angle for single slope passive solar still for the yield and efficiency for annual performance and said that minimum angle is preferred in summer and rainy seasons and maximum angle is preferred in winter.

Types of Solar Still

Single Basin Solar Still-Passive

An experimental work has been carried out by Badran [8] to predict the performance of single slope solar still with the enhancers like asphalt basin liner and sprinkler. Figure 6 Shows pie chart of contribution of daily and overnight productivity which conclude that night production is 16 % than daily productivity. Also it can be conclude that when combined enhancers such as asphalt basin liner and sprinkler is used, the still productivity is increased by 51 % and impart 16 % of daily production to night production.

Fig. 6
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Daylight and overnight output of still [8]

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Abdallaha et al. [9] have design and fabricated four solar still units with same dimensions and studied the effect of different absorbing material like coated metallic wiry sponge, uncoated metallic wiry sponge and black rocks and compared with reference still having no absorbing material and conclude that black rocks absorbs, store and realese the incident solar energy better than remaining absorbing material and its productivity is nearly 20 % more than coated metallic & uncoated metallic wiry sponge. Also shows that overnight productivity of black rocks is 60 % followed by 43 % of uncoated wiry sponges and 28 % of coated wiry sponges. Figure 7 Shows overnight productivity variation for three different days of experiment with and without absorbing materials.

Fig. 7
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Variation of overnight productivity for three days of experiment [9]

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Experimental study has been performed for a single slope single basin solar still with different floating absorber plate by El-Bialy [10] and compared with conventional solar still in the identical weather situation of Tanta, Egypt during summer days. He had used mica, aluminum; copper and stainless steel as floating absorber plate and concludes that the improvement ratio of overnight productivity increase and daily and daylight productivity decrease with increasing the mass of water above the absorber plate. Figure 8 shows the comparison of daylight, overnight and daily productivity between single basin solar still with and without floating absorber

Fig. 8
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Comparison of daylight, daily and overnight productivity of SBDFA with SBD [10]

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Single Basin Solar Still-Active

Dwivedi and Tiwari [11] reports Experimental and theoretical study for a double slope active solar still under natural circulation mode on the basis of energy balance of east and west glass cover with collector facing to south with water depth of 0.03m. The result of experimental and theoretical data shows that double slope solar still joined with flat plate collector under natural circulation mode giving 51 % higher output compared to conventional distiller. Figure 9 shows Comparison of yield for experimental test and theoretical of double slope distiller connected with flat plate collector.

Fig. 9
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Comparison of yield for experimental test and theoretical of double slope distiller connected with flat plate collector [11]

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The effect of coupling a flat-plate solar collector on the yield of solar still was carried out by Dimri and its co worker [6]. As shown in Fig. 10 it was observed from the experimental result that Yield of active solar still is noted more than passive solar still.

Fig. 10
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Comparision of yield for active and passive solar still [6]

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The efficiency of a simple basin solar still and similar one attached to a flat plate collector was investigated by Bouker and Harmin [12] under the effect of desert climatic condition of adrar, an Algeria Saharan site. Three month experiment shows that its yield is strongly depend on the solar radiation and ambient temperature. Panchal et al. [20] used heat pipes attached with solar still and found 30 % increment in distillate output.

Multi basin solar still- passive

Due to small efficiency of single effect solar still some researchers modified a little bit on its construction with a concept of multi basin because in single effect still latent heat of vaporizations release by the exterior side of glass cover is wasted to the surroundings whereas the multi basin have a advantage that the latent heat of vaporization of lower basin is used to heat the upper basin water through transparent glass cover.

A double basin double slope solar still was made-up by Rajaseenivasan and Kalidasa Murugavel [13] and experimentally & theoretically study with different basin condition was also done. Study of different water depth in both upper and lower basin on yield of double basin still was carried out and the result of double basin still was compared with single basin still. Figure 11 indicates the comparison of productivity with different brine depth for single and multi basin with both theoretically and experimentally. It was observed that double basin still efficiency is higher than the conventional solar still by 85 % for the same basin condition. Highest water output of 2.99 litre/day (4.75 l/m2.day) for double basin still was noted.

Fig. 11
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Experimental and theoretical productivity for single and multi basin still vs different brine depth [13]

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Performance study conducted by Elango [14] for different water depth from 1 to5 cm under both insulated and uninsulated conditions.comparision of yield for single basin ad double basin solar still for insulated and uninsulated condition is shown in Fig. 12. The experimental result shows that at 1cm water depth 17.38 % output is noted for double basin insulated still and uninsulated still gives 8.12 % output more than conventional still.

Fig. 12
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Comparisons of yield for single basin ad multi basin solar still for insulated and uninsulated condition [14]

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Multi Basin Solar Still-Active

The still efficiency mostly depends on the temperature difference between basin water and interior side condensing glass cover temperature. As the water temperature increase the temperature difference between evaporating and condensing surface increase. One of the major drawbacks of multi basin still is that lower basin collect lower radiation than upper basin because of Transmitivity introduced by adding multi basin. Hence it is necessary to increase the thermal energy from external sources.

Experimental analysis was reported by Panchal [15] to examine the effect of coupling evacuated tubes to the lower basin of double basin single slope solar still with and without using black granite gravel as energy absorbing material. the performance of double basin still alone and with still attached with evacuated tubes investigated with and without black granite gravel. Result shows that distillate output is increase by 56 % with coupling evacuated tubes and 65 % with evacuated tubes and black granite gravel. Also economic analysis carried out and shows that within 195days cost of still is recovered. Panchal and Shah [19] used double basin solar still with different mass of water and found 4 cm depth enhances the distillate output (Fig. 13).

Fig. 13
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Shows change in temperature with respect to time for double basin still [15]

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Investigation of the Climatic Parameters of Solar Still

Wind Velocity

Variation of Wind speed affects the production of solar still. As the wind speed increase the convective heat transfer co-efficient from glass cover to the surrounding increase and temperature difference between water and inner side glass cover increase and eventually performance of solar still increase [6].

Indoor test was carried out by Khalifa and Ali [16] to inspect the impact of wind speed on still performance and efficiency. The test was carried out at four different wind speeds of 1.14, 2.06, 2.92 and 4.01m/s and compared with no wind. It was found that performance of solar still increase with increasing wind speed and high wind speed give less development in yield than reasonable wind speed. Figure 14 shows change in yield of solar still vs wind speed. it was found that maximum yield obtained at 2.92m/s wind speed. So he concludes that moderate wind speed increase the output of solar still.

Fig. 14
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Shows change in yield of solar still vs wind speed [16]

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Ambient Temperature

Ambient Temperature also affects the still productivity. Increase in ambient temperature still productivity is increase [17].

Solar Radiation

Distillation require heat energy to evaporate the water and this heat energy is obtained by solar Radiation which is available abundant at any place, never lasting and pollution free. But it’s intensity varies day by day, season to season Which affect the daily yield of solar still.

Badran and Abu-Khader [4] reported an effect of solar intensity on the yield of still with respect to time Fig. 15 variations in intensity of solar radiation vs time in hours. It was observed that maximum solar radiation intensity obtained in the period of 12 to 2pm which lead to increase in output of still.

Fig. 15
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Variations in intensity of solar radiation vs time in hours [4]

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Figure 16 shows that solar radiation intensity recorded on the suez city in month of May 1998. It was found that maximum yield was obtained during afternoon hours due to high intensity.

Fig. 16
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Variation of solar radiation intensity with time in hours [18]

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Investigation of the Operational Parameters of Solar Still

Salinity of Water

Akash et al. [17] has studied the effect of salinity of water on solar distillation and conclude that clean water production decrease with increasing salinity.

Outcome

Table 1 shows the summary of different design, climatic and operational parameter cited in this article. Also the following key point summarizes the above discussion:

  • It is recommended to use minimum brine depth for increasing daily yield. Because at higher brine depth evaporation rate is decreasing.

  • Performance of still is associated with thermal conductivity of cover material.

  • Lower thickness of condensing glass cover is pre- ferred.

  • Inclination of glass cover is very important for increase in productivity. It is suggested that larger cover tilt angle should be preferred in winter and smaller angle is preferred in summer and it is also observed that finest cover tilt angle is near about the latitude angle of the place.

  • Black granite gravel and black rocks are good energy absorbing material to store the solar energy and increase the productivity.

  • Use of Asphalt Basin liner increase the productivity of solar still due to higher absorbency compared to black paint.

  • Integrating the flat plate collector with distiller increase the output of still.

  • Output of Multi-basin solar still is more than the single basin solar still.

  • Double-basin solar still coupled with vacuum tubes gives higher yield.

  • Productivity of still increase with increasing wind speed but performance of still little bit decrease with higher wind speed.

  • The salinity of water affects yield even at low concentration it decrease with increasing salinity.

Table 1 Summary of different design, climatic and operational parameters cited in this review paper
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