A review of solar still performance with reflectors

Renewable and Sustainable Energy Reviews 68 (2017) 638–649

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

A review of solar still performance with reflectors a,⁎

b

c

Z.M. Omara , A.E. Kabeel , A.S. Abdullah a b c

crossmark

Faculty of Engineering, Kafrelsheikh University, Egypt Faculty of Engineering, Tanta University, Egypt Faculty of Engineering, Prince Sattam bin Abdulaziz University, Saudi Arabia

A R T I C L E I N F O

A BS T RAC T

Keywords: Reflectors Solar still Solar distillation Review

In numerous respects, the solar still is a perfect wellspring of freshwater for both agriculture and drinking in remote areas or islands. There are numerous sorts of solar stills; the least difficult and most demonstrated is the basin solar still (conventional solar still). Investigations demonstrated that the conventional still has constrained productivity. Scientists have taken efforts to make diverse designs of solar stills to improve the productivity and deduced that solar stills integrated with reflectors are one of the most efficient and effective designs. The reflectors, either external or internal, are a good and cheap modification to increase the solar irradiation directed to the basin liner or the water as well as the distillate efficiency of the still. In this paper, a broad survey for diverse solar stills with reflectors has been conducted.

1. Introduction People living in remote areas or islands, where freshwater supply by transport is expensive, face the problem of water shortage every day. Solar still presents specific advantages to be used in these areas due to its easier construction, minimum skills of operation and maintenance requirements, and friendliness to the environment. The clean free energy and friendly to the environment are two major advantages which strengthen the use of solar stills. The main disadvantage of solar stills is the low yield of freshwater in comparison with the other desalination systems. The production capacity for a simple type still is only between 2–5 l/m2/day. This makes the solar stills uneconomical compared to the other conventional desalination systems [1]. Several researchers have reviewed, thoroughly, the recent work on solar stills such as classification of solar stills [2], design of solar stills [3], improvement techniques of solar stills [4], passive solar stills [5], active solar stills [6], inclined solar stills [7], stepped solar stills [8], wick type solar stills [9], and condensers with solar stills [10]. There is no specific accessible survey on solar stills with reflectors. Along these lines, this work is to make a documental survey on the solar stills combined with external and internal reflectors. 2. Working of conventional solar still with reflectors Internal reflectors are useful tools to concentrate and redirect solar radiation. They are recommended when sunlight is weak or the local temperature is relatively low. External reflectors are preferred to be

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used to change the direction of solar beams to improve the flexibility of the absorber plate configuration such as vertical solar absorber plate which is helpful in recovering vapor latent heat of condensation. External and/or internal reflectors are recommended when sunlight is frail or the local temperature is moderately low, Fig. 1. 3. Classification of reflectors with solar stills To get higher distillate yield, researchers introduced many efforts to make different designs of solar stills. They inferred that the solar still integrated with reflectors is effective and efficient. The reflectors with solar stills can be divided into three types; internal reflectors, external reflectors (top and bottom), and combination between internal and external reflectors. Utilizing external and/or internal reflectors can be an economical approach to expand the solar irradiation incident on the basin liner to make the productivity high as can as possible. A simple comparison of different solar stills with reflectors is illustrated in Table 1. They are compared based on location, daily yield, productivity improvement (%), efficiency, and observations in experiment. 3.1. Internal reflectors (IRs) The reflectors (internal reflectors) inside the solar still affect significantly the output of distilled water, which is credited to the centralization of the reflected solar radiation incident on the water. Besides, reflectors decrease the waste heat energy from the solar still. Tamimi [11] studied experimentally the performance of a single-slope

Corresponding author. E-mail address: [email protected] (Z.M. Omara).

http://dx.doi.org/10.1016/j.rser.2016.10.031 Received 7 December 2015; Received in revised form 25 September 2016; Accepted 20 October 2016 Available online 28 October 2016 1364-0321/ Published by Elsevier Ltd.


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efficiency of the conventional still was only 34%. The performance parameters of the Corrugated Solar Still (CrSS) and Conventional Solar Still (CSS) were investigated experimentally by Omara et al. [18]. The authors' view concerns with using both double layer wick material and reflectors together inside the CrSS, Fig. 8. In addition, the influence of saline water depth (1, 2, and 3 cm) on CrSS performance was also investigated. During experimentations, the product of CrSS with wick and reflectors is about 145.5% higher than the CSS at a brine depth of 1 cm. Besides, the daily efficiencies of CrSS and CSS were approximately 59% and 33%, respectively. In another experimental research by Omara et al. [19], a hybrid solar distillation system comprising of corrugated and wick absorbers of solar stills was integrated with an external condenser and internal reflectors to examine their performance, Fig. 9. They illustrated that the productivity of corrugated wick still with reflectors and external condenser was improved by about 180% over CSS at a brine depth of 1 cm.

Fig. 1. A basin solar still with reflectors [41].

single-basin solar still with mirrors installed on the side walls of a still. He concluded that using mirrors increases the basin efficiency throughout the whole day. El-Swify and Metias [12] used mathematical modeling and conducted experiments to find out the effect of IRs on the back and side walls of a single slope still with its back wall acting as an additional condenser, Fig. 2. They mentioned that the distillate increase of 82.6% and 22% can be obtained by installing reflectors in the winter and summer, respectively. A new design of solar still consisting of a metallic cylindrical parabolic reflector has been studied by Minasian et al. [13]. The reflector was designed to concentrate incident solar radiation on the black outside surface of a tray located on the focal line of the reflector. The tray was lined with blackened wick, representing the evaporative surface of the proposed still, as shown in Fig. 3. They showed that the productivity of the new still was 25–35% greater than that of a conventional basin type still. Abdallah et al. [14] provided modified design of basin solar still. The design modification was fixing interior reflecting mirrors on the internal walls of the still to minimize the amount of energy lost, Fig. 4. The experimental results showed that the use of internal mirrors improved the system thermal performance up to 30%. The still design was modified from flat basin to stepwise basin and the efficiency increased by an average value of 180%. AlHayek and Badran [15] compared the performances of a doubleslope basin solar still and a single-slope basin solar still. The interior surfaces of whole walls were made of mirrors. They performed their experiments during August and concluded that using mirrors on the inside walls of the single slope basin solar still enhanced the production of distilled water by 20% higher than the double basin solar stills, Fig. 5. The effect of an internal reflector (IR) on the productivity of a single-slope solar still (during the summer and winter) was investigated experimentally and theoretically by Karimi et al. [16], Fig. 6. They presented a mathematical model considering the effect of all walls (north, south, west and east) of the still on the amount of received solar radiation to brine. The model was validated with the experimental data. The model can calculate the yield of the still with and without IR on various walls. The results showed that the simultaneous use of IR on front and side walls enhances the still's efficiency by 18%. However, installation of an IR on the back wall can increase the annual efficiency by 22%. The installation of IRs on all walls in comparison to a still without IR can increase the distillate production at winter, summer, and the entire year by 65%, 22%, and 34%, respectively. A modification of the stepped solar still through installing IRs on the vertical sides of steps (Fig. 7) was introduced by Omara et al. [17]. As expected, the productivity of the stepped still with IRs was higher than that without the IRs. The results also indicated that the productivity of the modified stepped solar still with and without IRs is higher than that of the conventional still by approximately 75% and 57%, respectively. Also, the efficiency of the stepped still with and without reflectors was 56% and 53% respectively, whereas, the

3.2. External reflectors (ERs) The external reflectors (ERs) used in the solar still are made up of highly reflective materials such as mirror finished metal plate. The diffuse and direct beams transmitted through the glass cover are improved by using the ERs. Hiroshi Tanaka is the most scientist concerning to study the effects of reflectors on the distillate of stills. 3.2.1. External top reflectors Tanaka et al. [20–22] conducted numerical investigation on a tilted wick solar still (TWSS) with a top mirror (vertical [20], forwards [21] and backwards [22]) extending from the upper edge of the still. They displayed geometrical models to compute the solar irradiation reflected from a top mirror and then absorbed on the evaporating wick. They concluded that the top mirror can increment the absorbed solar radiation by evaporating wick, and further, the amount of solar irradiation reflected from the top mirror and absorbed on the wick can be enhanced by inclining the top mirror to be backwards in summer and forwards in winter. A theoretical analysis was used by Tanaka and Nakatake [20] to study the effect of a vertical flat plate ER on the productivity of a TWSS and showed an average increase of 9% a year, Fig. 10. Another theoretical analysis of a top ER with TWSS on a winter was investigated by Tanaka and Nakatake [21], Fig. 11. Their results indicated that the productivity of a still with an inclined reflector would be around 15% or 27% over that with a vertical reflector when the reflector's length is a half of or the same as the still's length. In addition, a theoretical analysis was made on TWSS with external top reflector with the aim of determining optimum inclination for both reflector and solar still for different seasons by Hiroshi Tanaka [22], Fig. 12. Regarding to the results obtained, 30°N latitude was represented as the best inclination angle either for the still or the reflector monthly. In addition, he concluded that for any season the daily productivity of the solar still can be improved by adjusting the inclination of both the reflector and solar still, thus producing around 21% over the traditional TWSS throughout the year. Tanaka and Nakatake [23] displayed another theoretical investigation of one-step azimuth tracking TWSS with a vertical flat plate reflector, Fig. 13. The TWSS is assumed to be rotated manually just once a day at southing of the sun. They performed numerical analyses of heat and mass transfer in the still to determine the daily productivity of the still on four typical days: the spring and autumn equinoxes and summer and winter solstice days at 30°N latitude. For four days, results indicated that the increase in the productivity of TWSS would be around 41%, and can be accomplished by the simple modification of utilizing a reflector. E1-Bahi and Inan [24] examined a conventional still integrated with an outside condenser, Fig. 14. A reflector made of stainless steel was added to the glass cover to reflect the solar radiation into the still 639


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Table 1 A simple comparison of different solar still with reflectors. Authors

Locations (Latitude)

Reflector material

- Daily yield (kg/m2/d) - Improvement - Efficiency

Reflectors effect (%)

Observations in experiment

El-Swify et al. [12]

Cairo, Egypt (25° N)

Mirrors

82.6% 22%

Minasian et al. [13].

Baghdad, Lraq, (33.33°N)

Stainless steel

3.05–7.2 82.6% in winter 22% in summer – 1.23–6 25–35% –

3

Abdallah et al. [14]

Amman, Jordan, (32°N)

Mirrors

1.64 30% ~

30%

4

AlHayek and Badran [15]

Amman, Jordan (32°N)

Mirrors

– 20% 11%

20%

5

Karimi et al. [16]

Iran. (30°N)

Mirrors

– 34% –

34%

The cooling effect of the back glass cover, back condenser, is improved due to the higher temperature difference as compared to the ordinary one. The productivity of the conventional basin type solar stil1 has been increased by using a stainless steel cylindrical parabolic reflector. Installing internal side mirrors gave better performance because these mirrors make use of the energy reflected on all sides of the solar still. Performance characteristics of the stills showed that the temperature at the water surface is increased with decreasing water depth, and by the addition of dye. The installation of IRs on all walls in comparison to a still without IR can increase the distillate production at winter, summer and the entire year by 65%, 22% and 34%, respectively. Productivity of stepped still with and without internal reflectors was higher than the conventional still by 75% and 57% respectively. The yield of Corrugated wick solar still with reflectors when providing a vacuum was enhanced to about 180% higher than the conventional still.

No

Categorization

1. Internal reflectors 1

2

Conventional still

25–35%

6

Stepped solar still

Omara et al. [17]

Kafrelsheikh, Egypt, (31.07°N)

Mirrors

6.35 75% 56%

18%

7

Corrugated wick solar still

Omara et al. [18] and [19]


Mirrors

4.1 145.5% 58%

55%

Tanaka and Nakatake [20]

Kurume, Japan (33°N)

Mirrors

– 9% –

9%

9



Mirrors

4.2 15% –

15%

10

Tanaka [22]


Mirrors

– 21% During the year –

21%

2. External reflectors External top reflectors 8 Tilted wick solar still

11

Conventional still coupled with external stainless steel reflector and outside condenser

E1-Bahi and Inan [24]

Ankara, Turkey, (39.6°N)

Stainless steel

–

–

12

Conventional still

Shanmugan et al. [26]

Tamil Nadu, India (11.30°N)

Mirrors

–

13

Conventional still With twin reflector

Srivastava et al. [27]

Rewa India (24.32°N)

Mirrors

4.2 – 45 79

14

“V” type solar still

Selva Kumar et al. [28]

India (22°N)

Mirrors

2.7 7.3 12

7.3%

15

External bottom reflectors Double slope single basin Sebaii [29] solar still

Tanta, Egypt (31° N)

Mirrors

– 19% and 30% summer and winter –

19% and 30% summer and winter

640

79%

The average daily amount of distillate for four days, (spring and autumn equinox and summer and winter solstice days), peaks when the angle of the still is 20° for the still with the reflector, and peaks at 30° for the still without the reflector. The inclined reflector can increase the distillate productivity of the still at any still's inclination, and the reflector's inclination should be set at about 15° from vertical. The distillate can be increased by inclining the reflector backwards in winter and forwards in summer, and the inclination angle of the reflector would be less than 25° throughout the year. The efficiency of the solar still was improved up to more than 70%, and the distilled fresh water was up to 71/m2.d when a solar still coupled with an external stainless steel reflector and outside condenser The efficiency of the still was as high as 35%, and increased to 45% with the mirror booster. The twin reflector booster has to be reoriented only once in a day at midnoon. The main advantage of the “V” type solar still is due to center collection and all the condensation are easily directed to the outlet. The daily productivity of the double slope single basin solar still with mirrors is higher than that without mirrors, due to the increased basin (continued on next page)


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Table 1 (continued) No

16

Categorization

Diffusion-type solar still

17

Authors



Reflector material



Kurume, Japan, (30°N)

Mirrors

34.2 and 39.7 spring and winter – –

–



Mirrors

30 throughout the year except for the winter at 40°N latitude.

–

18

Tilted wick solar still

Tanaka [35]


Mirrors

– 13% –

13%

19

Different designs Inverted absorber solar still

Dev et al. [36]

Muscat, Oman (23.37°N)

Steel

6.3 200% –

200%

20

Double slope solar still

Al-Garni [38]

Dhahran Saudi Arabia (26°N)

Mirrors

4.03 82% in winter

82% in winter

Mirrors

8.10 125% ~61

68%

3. Internal and external reflectors 3.1 Internal and external reflectors (top and bottom) Omara et al. Kafrelsheikh, 21 Stepped solar still (with [39] Egypt, (31.07°N) 5 mm tray depth and 120 mm tray width) 22

Stepped solar still (with 5 mm tray depth and 100 mm tray width)

El-Samadony et al. [40]


Mirrors

7.4 108% ~59%

77%

23

Conventional still (Internal and top reflector)

Tanaka [41]


Mirrors

1.58 75% –

75%

24

Tanaka [42]


Mirrors

48%

25

Tanaka [44]


Mirrors

26

Tanaka [45]


Mirrors

~7 48% – – 12% – – 43% –

27

Khalifa and Ibrahim [46].

Baghdad, Iraq, (33.3°N)

Mirrors

– 32% During the year –

32%

28

Khalifa and Ibrahim [47].

Baghdad, Iraq (33.3°N)

Mirrors

145%

29

Boubekri and Chaker [49]

Constantine, Algeria (36°N)

Mirrors

– 145% In winter – 3.5 72.8% in winter –

5 43% – – 42% During the year

43%

30

Portable thermal– electrical solar still

Monowe et al. [50]

Moscow, Russia (55°45′N)

aluminum foil

31

Conventional still (Internal and bottom

Tanaka [51]


Mirrors

641

12%

43%

72.8% in winter

42%


water temperature and water-glass temperature difference. The overall daily productivity decreases with a decrease in reflectivity, and the productivity is about 23% less when reflectivity is 0.6 than when reflectivity is 0.95. The angle of the flat plate reflector should basically be fixed at 10°, and changed to be 0° during the winter season at higher latitudes The distillate would be greatest when the flat plate bottom reflector inclination is about 20° on the winter solstice, 30° on the spring and autumn equinox and 50° on the summer solstice. The maximum optimized water depth can be taken as 0.03 m for the inverted absorber solar still at which the addition of reflector under the basin does not affect its performance considerably in comparison to that of the CSS. Four inclined mirrors were placed around the still to reflect extra solar irradiance onto the solar still.

The productivity of the stepped still with top ER and bottom ER are about 33% and 41% higher than that of the conventional still, respectively. The productivity of the stepped still with both reflectors and a condenser are about 165% higher than that of the conventional still. The daily efficiency is about 66% A very simple modification using IERs can increase the daily productivity of a basin type still in winter by about 70– 100%. Increase in productivity by using IERs is 48% and by adding only internal reflector it was 22%. The benefit of both the IERs would be considerably less in summer than in winter. The daily amount of distillate can be increased by inclining the ER backwards in summer and forwards in other seasons, and the inclination angle of the ER would be less than 25° throughout the year. The average daily yield is increased by the use of IR and/or ER except for summer where the effect of the reflectors is found to be negative. The daily yield of the still with no reflectors would remain almost the same at any glass cover angle. The night production of the still increases when the still is coupled with a storage tank out of the sunshine hours. This increase is about 27.54%, 21% and 23.28% respectively for winter, spring and summer. Using reflectors and external condenser can increase the daily productivity of a still to 68%. Productivity of the still with internal and external reflectors was 41%, 25% (continued on next page)


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Table 1 (continued) No

Categorization

Authors



Reflector material

–

reflector)



and 62% greater than the conventional still on the spring, summer and winter solstices, respectively.

Fig. 2. The double exposure solar still with IRs [12].

Fig. 6. A single-slope solar still with IRs [16].

Fig. 3. A basin solar still with cylindrical parabolic reflector [13].

Fig. 7. Stepped still with IRs [17].

Fig. 4. A basin solar still with IRs [14].

Fig. 5. A basin solar still with IRs [15].

Fig. 8. Corrugated solar still with wick and reflectors [18].

basin through the glass cover and to give a shadow for the condenser. The study led to the conclusion that the efficiency of the solar still was enhanced by about 70% and the daily productivity was up to 7 l/m2 a day. Madhlopa and Johnstone [25] proposed a model to calculate the distribution of solar irradiation inside a conventional still with reflector and external condenser, Fig. 15. The system had one basin (basin 1) in the evaporation chamber and two other basins (2 and 3) in the condenser chamber. It had a glass cover over the evaporator basin and an opaque condensing cover over basin 3. Water vapor from the first basin condenses under the glass cover while the remainder flows

into the condenser by purging, diffusion, and condenses under the liner of basin 2. They found that distilled freshwater was about 2.2 kg/m2 a day. The performance of an acrylic mirror boosted solar distillation unit was studied by Shanmugan et al. [26]. The performance of solar still in terms of distilled water collection has been analyzed and a booster mirror (acrylic) was attached just above the glass cover of solar still. The mirror boosting reflects the excess solar radiation to water and it is possible to adjust the booster mirror for perfect reflection depending upon the sun moving angle as shown in Fig. 16. The maximum amount

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Fig. 9. Corrugated wick solar still with reflectors and vacuum fan [19].

Fig. 12. TWSS with external top reflector [22].

Fig. 13. One step azimuth tracking TWSS with reflector [23].

Fig. 10. Top vertical external reflector with TWSS [20].

Fig. 14. Conventional still integrated with a top reflector [24].

Fig. 11. Top external reflector with TWSS [21].

of water collected per day was about 4.2 L. The efficiency of the still was as high as 35%, and increased to 45% with the mirror booster. Srivastava et al. [27] performed experiments of a modified solar still with multiple low thermal inertia porous absorbers, Fig. 17. The absorbers were made up of jute cloth and they float on the basin water with the help of thermocol insulation. The surfaces of the absorbers were always wet and hence there were no dry spots. The results showed that the productivity was 68% more than the conventional still and 35% more on cloudy days. The basin water below the floating absorber remained warm during off-shine hours, and hence, the distillate was produced even at nights. A twin reflector booster was placed perpendicular to each other on the modified stills. The productivity increased by

Fig. 15. Basin solar still with reflector and external condenser [25].

79% over the still without booster. The thermal performance of a “V” type solar still with a charcoal absorber was analyzed by Selva Kumar et al. [28]. The main advantage of this type of still is that the distilled water collection was directed towards the central water collection channel as shown in Fig. 18. The production of the outlet distillate water had increased. The overall

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Fig. 19. Schematic of double slope single basin solar with ER [29]. Fig. 16. View of solar distillation unit with mirror booster [26].

Fig. 17. Twin reflector booster applied to solar still [27]. Fig. 20. Diffusion type solar still with ER [30].

adjusted manually to absorb solar radiation on the first partition effectively according to locations and seasons. Their results showed that the angle of the flat plate reflector should be fixed at about 10° throughout the year. Another design is presented by Tanaka and Nakatake [31] for the conventional still. It was a solar still having vertical multiple-effect diffusion. It was compromising a flat mirror, a vertical parallel partitions contacted with wicks, and small casters. The casters were used for tracking, Fig. 21. Their suggested solar still is easy to construct utilizing common materials and is easy to operate. They analyzed theoretically the dependence of the solar absorption of the first (or heated) partition on the azimuth angle of the still and the mirror angle. They concluded from their research that preparing the still to rotate just one time a day at the south side would increase the amount of solar absorption of the first partition. Besides, the daily solar absorption would be around 99% or 85% of the daily solar irradiation on a horizontal surface on the winter solstice or spring equinox, respectively. The optimum angle of a flat plate reflector and the optimum orientation of a vertical multiple-effect diffusion solar still coupled

Fig. 18. “V” type solar still with ER [28].

efficiency was found to be 24.47% and 11.92% for the still without and with the boosting mirror, respectively. He also conducted similar experiments which were carried out for the charcoal absorber in the still without and with the boosting mirror which yielded 30.05% and 14.10% respectively.

3.2.2. External bottom reflectors The rapid spoiling of the inner mirrors was identified as a disadvantage of using mirrors inside the solar still (Sebaii [29]). Therefore, Sebaii [29] used external mirrors to investigate the performance of a double slope single basin solar (DSSBS). Two mirrors were tilted at an angle and fixed to the outer side of the solar still, Fig. 19. It was reported that the radiation falling on the solar still was increased significantly with the use of these external mirrors. It was found that the daily productivity of the DSSBS with mirrors was about 19% and 30% higher than that of DSSBS without mirrors during the summer and winter, respectively due to the increased basin water temperature and water-glass temperature difference. Tanaka and Nakatake [30] investigated another developed system which consists of a flat plate reflector, casters for manual azimuth tracking, and vertical multiple effect diffusion type still, Fig. 20. The angle of the flat plate reflector and azimuth angle of the still can be

Fig. 21. Diffusion type solar still with ER [31].

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Fig. 24. TWSS with bottom ER [35].

vertical still and was predicted to produce 29.2 and 34.6 l/m2 a day on sunny spring equinox and winter solstice days, respectively at the equator. Another theoretically investigation of TWSS with a bottom mirror on four days (the summer and winter solstices and spring and autumn equinox) at 30°N latitude was studied by Tanaka [35] when the still's inclination is fixed at 30° and the reflector's length is the same as the still's length, Fig. 24. He proposed a geometrical model to calculate the solar irradiation reflected by the bottom mirror and then absorbed by the wick. Results showed that the bottom mirror can reflect sunrays to the evaporating wick and increase distillate freshwater of the TWSS when the reflector's inclination is larger than 15° on the spring, autumn, and winter, and 25° on the summer. Besides, the inclination angle of 35° for the reflector made the four days’ average productivity to have the highest amount of distillate, and then, the yield was higher than that of conventional TWSS by 13%.

Fig. 22. Diffusion solar still with ER [33].

with a flat plate reflector throughout the year were numerically determined by Tanaka and Nakatake [32], with the assumption of being the still located at the equator and at 10°, 20°, 30° and 40° northern latitude, Fig. 20. The optimum orientation of the still which maximizes the distillate productivity of the proposed still was calculated assuming that the orientation of the still is changed once during daytime at southing of the sun. They found that the angle of the flat plate reflector should be basically fixed at 10° from horizontal and changed to be 0° during the winter season (around December) at higher latitudes. The orientation of the still should be adjusted according to month at any latitude. The daily productivity of the proposed still was predicted to be more than 30 kg/m2 a day at any latitude throughout the year except for the winter season (from November to January) at 40°N latitude. Tanaka and Nakatake [33] presented also the results of outdoor experiments for vertical single-effect diffusion solar still, Fig. 22. To know the effect of still orientation deeply, the authors had to make the reflector's inclination angle to be fixed, and change the still orientation just one time a day at the south side. They got the result that during the winter and summer seasons, the optimum reflector angles were 0° and 10° respectively. The optimum orientation was varied according to month. It should be from ± 22.5° (South-Southeast during the morning and South-Southwest during the afternoon) in winter to ± 90° (due East and due West) in summer, when the still is located at 33.2°N latitude. A vertical multiple-effect-diffusion-type solar still coupled with a flat plate reflector (Fig. 23) was also studied by Tanaka [34]. It was concluded that the optimum angle of the flat plate reflector should be 0° during the winter season and 10° during the summer season, when the still is located at 33.2°N latitude. The daily productivity of the system with 10 partitions and 10 mm diffusion gaps between partitions was predicted to be about 5–6 times as large as that of a single-effect

3.2.3. Different designs Dev et al. [15] reported that the daily yields obtained from the basin solar still combined with a curved reflector were 6.3, 5.6 and 4.3 kg/m2 a day at water depths of 0.01, 0.02, and 0.03 m, respectively (Fig. 25). At the same respective water depths, the daily yields obtained from the basin solar still were 2.1, 1.9, 0.8 kg/m2 a day respectively. Nassar et al. [37] conducted experiments of a solar desalination system working on the basis of evacuation, Fig. 26. Concave mirror was used to concentrate the solar energy on the still. The still works under vacuum conditions (25 kPa absolute) to reduce the boiling point of the saline water. A condenser condenses the outlet vapor and the distillate was collected. The productivity of the still was found to be 20 l/m2 a day when using reflector compared to 5 kg/m2 a day for the conventional still. The results showed that the productivity of the still was about 303% compared with the other stills. Al-Garni [38] carried out an experimental work in winter for a double slope solar still with external reflectors, Fig. 27. Experiments

Fig. 23. Diffusion solar still coupled with ER [34].

Fig. 25. Inverted absorber solar still with curved ER [36].

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Fig. 29. Stepped still integrated with IERs and condenser [40].

Fig. 26. Solar desalination system with concave ER [37].

equals to the step width of 100 mm, Fig. 29. Stepped still was compared with conventional still to evaluate the performance well. Results showed that installing internal and external reflectors led to increase the glass temperature (about 9 °C over conventional still's) and thus led to resistance of condensation. In addition, the temperature of saline water for stepped still was over that of conventional still by around 16 °C. That is why using the external condenser was important. So, the daily amount of stepped still distillate increased by about 165% over that of conventional still, when both of external condenser and reflectors were used. Tanaka [41] designed a basin still with IERs, Fig. 30, consisting of a basin liner with internal reflectors, a glass cover, and an external top reflector. Compared to the conventional basin solar still, more solar radiation was introduced into the still by the reflectors. The daily productivity could be increased by 70–100% on winter days. In addition, Tanaka et al. [42] presented a theoretical analyses of a basin type solar still with IERs, Fig. 31. It was found that the productivity increased considerably throughout the year except for the summer season. During the summer season, the external reflector made shadow on the basin liner in the morning and evening and hence the productivity decreased. The increase in productivity for one year by adding IERs was 48%. While, with adding only internal reflector, the increase in productivity was around 22%. In 2007, Tanaka and Nakatake [43] predicted theoretically the performance of conventional still with IERs on a winter day at 30° N latitude, Fig. 32. They proposed another geometrical method for calculating the solar irradiation reflected by the inclined external mirror and then absorbed by the basin liner. They showed that the benefits from the vertical external mirror would be littler or even insignificant for a still with a bigger worth for the glass cover angle.

Fig. 27. Double slope solar with ER [38].

were performed in Dhahran (a city in the eastern province of Saudi Arabia at latitude 26°N). Four inclined mirrors were placed around the still to reflect extra solar irradiance onto the solar still. Numerical analysis was also carried out using heat and mass transfer inside the solar still and was validated with the experimental results. The results showed that the external reflectors significantly enhanced the productivity of the solar still by approximately 82% as observed. 3.3. Internal and External reflectors (IERs) In another paper, the performance of the stepped still with IERs was evaluated by Omara et al. [39]. Fig. 28 shows a photograph of the stepped still with external (top and bottom) and internal mirrors. They found that the accumulative distillate of the stepped still with IERs was over the conventional still by about 125%. An experimental study of stepped solar still with external condenser and IERs was conducted by El-Samadony et al. [40] with a tray width

Fig. 28. Stepped still with IERs [39].

Fig. 30. A Conventional still with IERs [41].

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Fig. 31. A basin type solar still with IERs [42].

Fig. 34. A basin solar still with IERs [45].

the range of 10–25° from vertical when the glass cover slant is from 10° to 40°. Also, he showed that the increase of the daily productivity by inclining the external mirror from vertical would be about 7% or 12% when the length of the external mirror is a half of or the same as the still's length. In 2010, Tanaka [45] studied theoretically the effects of IERs on both the amount of solar irradiation absorbed by the basin liner and as well as the daily productivity of a basin still, Fig. 34. The external reflector was inclined backwards or forwards according to the month. Results indicated that the optimum external mirror slant for each month for a still with a glass cover slant of 10–50°. The increase in the average daily productivity during the year of a still with inclined external mirror with optimum slant in addition to an internal mirror, compared to a basin still was anticipated to be 29%, 43% or 67% when the glass cover inclination is 10°, 30° or 50° and the length of external mirror is half the length of still. Khalifa and Ibrahim [46] examined experimentally the performance of a basin-type solar still with IRs and ERs (inclined at angles 0° (vertical), 10°, 20° and 30°) from June to December, Fig. 35. They concluded that using IRs increased the yield by an average of 19.7% during this period. In addition, the increase in the output of the still with ERs is averaged at 34.5%, 34.4%, 34.8%, and 24.7% at 0°, 10°, 20°, and 30°, respectively. In a separate study, Khalifa and Ibrahim [47] performed an experimental investigation (latitude 33.3°N) of a solar still with IERs tilted at 0° (vertical), 10°, 20°, and 30° for cover slope angles 20°, 30°, and 40°. They found the productivity to be highest for the still having slope angle 20°, internal and external reflector inclined at 20° and its daily distillate will be 145% over simple still, Fig. 36. In another work, using IERs, Khalifa and Ibrahim [48] did an

Fig. 32. A conventional still with IERs [43].

While, an inclined external mirror could improve the still production rate at any angle of glass cover, and the external mirror angle should be set at about 15° from vertical on a winter solstice day. For a winter day, the results revealed that the still output yield was increased by 16% when using the inclined mirror than that when using external vertical mirror. Besides, it was 2.3 times greater than that of conventional still. Tanaka [44] presented theoretical analyses to obtain the effects of slant external mirror on daily productivity of a conventional solar still with IERs on the summer at 30°N latitude, Fig. 33. They found that the productivity of the still increased by inclining the external reflector slightly backwards. In addition, the daily productivity of solar still with both the external and internal mirrors was increased with decreasing the slant of glass cover. Also, an optimum slant backwards from vertical of the external mirror depends on the glass cover slant, and would be in



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Fig. 36. A conventional solar still with IERs [47].

Fig. 39. The combined reflector with thermal–electrical solar still [50].

made of aluminum foil glued to some hard boards. Their results showed that the daily productivity was 5.0 l, and the efficiency of the still increased to 43%. A conventional solar still with an external bottom and internal (back and two side walls) mirrors was presented theoretically on three days (winter, spring, and summer) by Tanaka [51] at 30°N latitude, Fig. 40. He indicated that the external reflector can reflect sunrays to the basin liner and improve daily productivity. The productivity of the still with external bottom and internal mirror was anticipated to be 41%, 25%, and 62% over a basin still on the spring, summer, and winter, respectively, by setting the external mirror's slant to the best possible qualities as per the seasons when the glass cover's inclination angle is fixed at 20° from horizontal and the length of the external mirror is the same as the length of the still.

Fig. 37. A basin solar still with IERs [48].

experimental study (latitude 33.3°N) on a solar still for summer, autumn, and winter seasons, Fig. 37. When only an internal reflector was used, the productivity increased by 19.9%. Whereas the combination of internal and external reflectors increased the productivity by 35.5%. The use of reflectors was concluded to have an insignificant effect on productivity for summer season. Boubekri and Chaker [49] reported that using IERs had the effect of increasing the productivity by up to 72.8% in the winter, 40.33% in the spring, and only 7.54% in the summer, Fig. 38. They also found that integrating a thermal storage tank with the solar still increased the productivity by 27.5%, 21%, and 23.2% in the winter, spring, and summer respectively. A new configuration of a portable thermal–electrical solar still with an outside condenser and an external reflecting booster was proposed by Monowe et al. [50], Fig. 39. The angle of the booster reflector was adjusted each hour starting from 8:00 am to 4:00 pm. The IERs were

4. Scope for further research There are still some problems and challenges on the design of solar stills not being clearly analyzed for performance improvement, including solar stills with reflectors. This paper reviews the studies and developments of solar stills with reflectors. The results from the preceding work done clearly show that the enhancement of solar stills with reflectors performance differs greatly for different techniques due to different experimental conditions. The review represents specific inferences drawn from the analyses of solar stills with reflectors by various authors to pave way researchers to grasp the previous designs and to fabricate new designs with optimum design parameters for higher distillate output.

Fig. 38. Active solar still with IERs [49].

Fig. 40. A Conventional still with external bottom and internal reflectors [51].

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design of a single slope solar still. Desalination 2008;219:222–30. [15] Al-Hayek I, Badran OO. The effect of using different designs of solar stills on water distillation. Desalination 2004;169:121–7. [16] Karimi Estahbanati MR, Ahsan Amimul, Feilizadeh Mehrzad, Jafarpur Khosrow, Ashrafmansouri Seyedeh-Saba, Feilizadeh Mansoor. Theoretical and experimental investigation on internal reflectors in a single-slope solar still. Appl Energy 2016;165:537–47. [17] Omara ZM, Kabeel AE, Younes MM. Enhancing the stepped solar still performance using internal reflectors. Desalination 2013;314:67–72. [18] Omara ZM, Kabeel AE, Abdullah AS, Essa FA. Experimental investigation of corrugated absorber solar still with wick and reflectors. Desalination 2016;381:111–6. [19] Omara ZM, Kabeel AE, Essa FA. Effect of using nanofluids and providing vacuum on the yield of corrugated wick solar still. Energy Convers Manag 2015;103:965–72. [20] Tanaka H, Nakatake Y. Improvement of the tilted wick solar still by using a flat plate reflector. Desalination 2007;216:139–46. [21] Tanaka H, Nakatake Y. Increase in distillate productivity by inclining the flat plate external reflector of a tilted-wick solar still in winter. Sol Energy 2009;83:785–9. [22] Tanaka H. Tilted wick solar still with external flat plate reflector: optimum inclination of still and reflector. Desalination 2009;249:411–5. [23] Tanaka H, Nakatake Y. One step azimuth tracking tilted-wick solar still with a vertical flat plate reflector. Desalination 2009;235:1–8. [24] E1-Bahi A, Inan D. A solar still with minimum inclination, coupled to an outside condenser. Desalination 1999;123:79–83. [25] Madhlopa A, Johnstone C. Computation of solar radiation distribution in a solar still with internal and external reflectors. Sol Energy 2011;85:217–33. [26] Shanmugan S, Rajamohan P, Mutharasu D. Performance study on an acrylic mirror boosted solar distillation unit utilizing seawater. Desalination 2008;230:281–7. [27] Pankaj SK, Agrawal SK. Experimental and theoretical analysis of single sloped basin type solar still consisting of multiple low thermal inertia floating porous absorbers. Desalination 2013;311:198–205. [28] Selva Kumar B, Kumar Sanjay, Jayaprakash R. Performance analysis of a V type solar still using a charcoal absorber and a boosting mirror. Desalination 2008;229:217–30. [29] Sebaii AA. Effect of wind speed on some designs of solar stills. Energy Convers Manag 2000;41(6):523–38. [30] Tanaka H, Nakatake Y. Factors influencing the productivity of a multiple effect diffusion-type solar still coupled with a flat plate reflector. Desalination 2005;186:299–310. [31] Tanaka H, Nakatake Y. A simple and highly productive solar still: a vertical multiple-effect diffusion-type solar still coupled with a flat-plate mirror. Desalination 2005;173:287–300. [32] Tanaka H, Nakatake Y. Numerical analysis of the vertical multiple-effect diffusion solar still coupled with a flat plate reflector: optimum reflector angle and optimum orientation of the still at various seasons and locations. Desalination 2007;207:167–78. [33] Tanaka H, Nakatake Y. Outdoor experiments of a vertical diffusion solar still coupled with a flat plate reflector. Desalination 2007;214:70–82. [34] Tanaka H. Experimental study of vertical multiple-effect diffusion solar still coupled with a flat plate reflector. Desalination 2009;249:34–40. [35] Tanaka H. Tilted wick solar still with flat plate bottom reflector. Desalination 2011;273:405–13. [36] Dev R, Abdul-Wahab SA, Tiwari GN. Performance study of the inverted absorber solar still with water depth and total dissolved solid. Appl Energy 2011;88:252–64. [37] Fathi Nassar Yasser, Saib Yousuf A, Awidet Salem Abubaker. The second generation of the solar desalination systems. Desalination 2007;209:177–81. [38] Al-Garni AZ. Effect of external reflectors on the productivity of solar still during winter. J Energy Eng 2014;140(1). [39] Omara ZM, Kabeel AE, Younes MM. Enhancing the stepped solar still performance using internal and external reflectors. Energy Convers Manag 2014;78:876–81. [40] El-Samadony YAF, Abdullah AS, Omara ZM. Experimental study of stepped solar still integrated with reflectors and external condenser. Exp Heat Transf 2015;28:392–4. [41] Tanaka H. Experimental study of a basin type solar still with internal and external reflectors in winter. Desalination 2009;249:130–4. [42] Tanaka H, Nakatake Y. Theoretical analysis of a basin type solar still with internal and external reflectors. Desalination 2006;197:205–16. [43] Tanaka H, Nakatake Y. Effect of inclination of external flat plate reflector of basin type still in winter. Sol Energy 2007;81:1035–42. [44] Tanaka H. Effect of inclination of external reflector of basin type still in summer. Desalination 2009;242:205–14. [45] Tanaka H. Monthly optimum inclination of glass cover and external reflector of a basin type solar still with internal and external reflectors. Sol Energy 2010;84:1959–66. [46] Khalifa AJN, Ibrahim HA. Effect of inclination of the external reflector on the performance of a basin type solar still at various seasons. Energy Sustain Dev 2009;13:244–9. [47] Khalifa AJN, Ibrahim HA. Effect of inclination of the external reflector of simple solar still in winter: an experimental investigation for different cover angles. Desalination 2010;264(1–2):129–33. [48] Khalifa AJN, Ibrahim HA. Experimental study on the effect of internal and external reflectors on the performance of basin type solar stills at various seasons. Desalination. Water Treat 2011;27(1–3):313–8. [49] Boubekri M, Chaker A. Yield of an improved solar still: numerical approach. Energy Procedia 2011;6:610–7. [50] Monowe P, Masale M, Nijegorodov N, Vasilenko V. A portable single-basin solar still with an external reflecting booster and an outside condenser. Desalination 2011;280:332–8. [51] Tanaka H. A theoretical analysis of basin type solar still with flat plate external bottom reflector. Desalination 2011;279:243–51.

The following factors may be taken into consideration for further research of solar stills with reflectors: 1. For active solar still: 11. Combine solar still with solar water heater in order to increase the basin temperature. 12. For active solar stills, more research may be carried out with reflectors. 13. Further studies should be conducted to improve the solar stills productivity, especially in the fields of coupling with heat storage and various waste heat sources. 2. For tracking system: 21. Solar still with azimuth sun tracking and just an adjustment of the optimum elevation of the day. 22. Combine the solar still with sun tracking and the solar water heater. 23. The sun tracking system is more effective than fixed still and it is capable of improving the distillate output of the still with reflectors. 5. Conclusions Reflectors are used to enhance the daily amount of distillate of the solar still. The accompanying conclusions can be inferred on the basis of above discussions:

• • • • • • •

Installing reflectors is more practical in places where solar radiation is weak and the ambient temperature is relatively low. Installation angle of the external reflector should be changed with the seasons to enhance the productivity through all the year. The external reflector is inclined backwards or forwards according to the month. The daily productivity can be increased by adjusting the inclination of both the still and reflector mirror in any season. The benefits of both the inclined external top and internal reflectors would be considerably less in summer than in winter. For the still with larger angle of the glass cover, the effect of the external top reflector would be smaller. The vertical flat plate external reflector would be less effective for the tilted wick solar still than for the conventional still.

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