Desalination

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Enhancing the productivity of double-slope
single-basin solar still with internal and external
modifications
S. Joe Patrick Gnanaraj, S. Ramachandran & David Santosh Christopher
To cite this article: S. Joe Patrick Gnanaraj, S. Ramachandran & David Santosh Christopher
(2017): Enhancing the productivity of double-slope single-basin solar still with internal and external
modifications, International Journal of Ambient Energy, DOI: 10.1080/01430750.2017.1340338
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2. INTERNATIONAL JOURNAL OF AMBIENT ENERGY, 2017
https://doi.org/10.1080/01430750.2017.1340338
Enhancing the productivity of double-slope single-basin solar still with internal and
external modifications
S. Joe Patrick Gnanaraja, S. Ramachandrana and David Santosh Christopherb
aFaculty of Mechanical Engineering, Sathyabama University, Chennai, India; bDepartment of Mechanical Engineering, SCAD College of Engineering
and Technology, Tirunelveli, India
ABSTRACT
The major task before us is to increase the performance of solar stills. Researchers have done many mod-
ifications to enhance the productivity of solar stills. In this article, the authors have attempted to improve
the performance of solar stills. Instead of a conventional solar still, a double-slope single-basin solar still
was fabricated. It was internally modified by spreading pebbles at the bottom. External modification
was attempted by fitting an external mirror. Internal modification enhanced the production marginally.
Significant improvement in production was witnessed when both internal and external modifications
were attempted. The performance of the double-slope single-basin solar still with internal and external
modifications was 40.86% higher than that of the solar still without any modification.
ARTICLE HISTORY
Received 18 March 2017
Accepted 3 June 2017
KEYWORDS
Double-slope solar still;
internal modification;
external modification
1. Introduction
Many desalination techniques are available to provide freshwa-
ter to both urban and rural areas. But the challenge is to find
a method suitable to provide uninterrupted supply of drinking
water even for remote areas at a reasonable cost. Solar desali-
nation is one of the cheapest methods to solve this problem.
Solar still is widely used in solar desalination. On the basis of
basin cover, single-basin solar stills can be classified into single-
slope solar stills and double-slope solar stills. To enhance the
yield of the double-slope single-basin solar still, many modi-
fications are being carried out. Murugavel and Srithar (2011)
used different wick materials in basin-type double-slope solar
stills. Rectangular aluminium fin covered with cotton cloth and
arranged in the length-wise direction proved to be more effec-
tive. Dwivedi and Tiwari (2010) theoretically proved that the
energy efficiency of the double-slope active solar still was higher
than the energy efficiency of the double-slope passive solar still.
Rajaseenivasan and Kalidasa Murugavel (2013) found that the
double-basin double-slope solar still was 85% more productive
than the single-basin still. Joe Patrick Gnanaraj, Ramachandran,
and Anbazhagan (2017) proved that a double-slope solar still
connected with a flat plate collector and fitted with a reflecting
mirror was 104.43% more productive compared to a double-
slope solar still without external modifications. Kalidasa Muru-
gavel and Srithar (2014) concluded that the distillate production
of the single-basin double-slope solar still was higher during the
months of March, April, August, November and December and
it was around 4 L/D. The average production was 2.1 L/D/m2.
Vacuum tubes were coupled with a double-basin solar still and
black granite gravels were spread in the basin by Panchal (2015)
and the productivity increased by 65%. Velmurugan et al. (2009)
used fins, sponges and pebbles in the stepped solar still and the
CONTACT S. Joe Patrick Gnanaraj gnanaraj.134@gmail.com
productivity increasedby98%. Nafeyetal. (2001)usedblackrub-
bers and the productivity increased by 20%. Nafey et al. (2002)
used black gravel and the productivity increased by 19%. Vel-
murugan et al. (2008) used sponges, pebbles, black rubbers
and sand in a fin-type single-basin solar still to desalinate the
industrial effluent, and there was considerable increase in pro-
ductivity. The performance of the stepped solar still with internal
and external reflectors (top and bottom) was investigated by
Omara, Kabeel, and Younes (2014). The productivity was 125%
higher than that of the conventional still. Tanaka and Nakatake
(2006) concluded that by using internal and external reflec-
tors in a single-slope basin-type still, the productivity could be
increased throughout the year and the increase in daily distil-
late yield would be around 48%. Tanaka (2009) constructed a
basin-type solar still with internal and external reflectors and the
daily productivity increased by about 70–100%. Omara, Kabeel,
and Younes (2013) fitted mirrors on the vertical side of the steps
of the stepped solar still and the productivity was higher than
that of the conventional still by about 75%. Deshmukh and Kolhe
(2016) used reflectors to increase the temperature of evacuated
tubes that were connected with a double-basin solar still. This
increased the productivity. Omara et al. (2016) proved that water
productivity increased by about 145.5% over the conventional
still in a corrugated single-basin solar still with internal reflec-
tors. Tanaka (2013) adjusted the inclination angle of both the
wick solar still and the flat plate bottom reflector each month,
and the daily distillate yield increased up to 33% compared to
that using a conventional tilted wick solar still. Rajaseenivasan,
Kalidasa Murugavel, and Elango (2015) investigated the effec-
tiveness of wick and energy storage materials. The result showed
that black cotton cloth significantly improved the production
rate in the morning hours and mild steel pieces in the evening
© 2017 Informa UK Limited, trading as Taylor & Francis Group

3. 2 S. J. P. GNANARAJ ET AL.
hours. Rajaseenivasan, Nelson Raja, and Srithar (2014) used jute
cloth and black gravel in the flat plate collector coupled basin.
Jute cloth enhanced the distillate output during sunshine hours
and black gravel significantly increased the yield during after-
noon hours. Panchal et al. (2017) used different energy storage
materials such as marble pieces and sandstones. It was found
that sandstone was more productive. Tiwari, Dimri, and Chel
(2009) modified the solar still with internal and external mir-
rors, which resulted in an increase in heat transfer inside the
solar still. Al-Hayeka and Badran (2004) fabricated two different
types of solar still: single-basin still with a reflecting mirror and
double-basin still without a reflecting mirror. It was found that
the single-basin solar still with a reflecting mirror had 20% more
efficiency than the double-basin still without a reflecting mir-
ror. El-Bahi and Inan (1999) fabricated a basin-type solar still with
internal and external reflectors. They found out that the external
reflector produced higher efficiency than the internal reflector.
From the above discussion, it is clear that the productivity
of a solar still can be enhanced by using heat storage materi-
als in the basin and fitting internal and external reflectors. In our
study, pebbles, heat storage material in the basin (internal mod-
ification) and external reflectors (external modification) were
used to focus additional rays into the still. The main objective
of this study is to estimate the impact of internal modification
and external modification on the performance of a double-slope
single-basin solar still.
2. Experimental set-up
To compare the performance of double-slope single-basin solar
stills with and without internal and external modification, three
identical double-slope solar stills were fabricated. One such solar
still is shown in Figure 1. The outer wall of the solar still was
fabricated using 1.5 cm thick wood. The outer dimension of the
solarstillwas1 m × 0.6 m × 0.4 m.Abasinmadeupofgalvanised
iron steel was kept inside the wooden box. The dimension of
the basin was 0.9 m × 0.55 m × 0.38 m. The inner surface of the
solar basin was painted black to increase the absorption of solar
radiation. The gap between the outer wooden box and the inner
Figure 1. Double-basin solar still.
Figure 2. Insulation materials in solar still.
Figure 3. Double-slope solar still with internal modification.
basin was filled with materials such as glass wool, thermocol and
sponge, as shown in Figure 2, to restrict the loss of heat from the
basin to atmosphere.
For allowing more radiation into the solar still, a double-slope
glass cover was fabricated. The glass cover consists of transpar-
ent 4 mm thick glass and the slope of the glass is maintained at
30°. Further to collect the distilled water, two collecting pipes
were fitted in the solar still.
2.1. Internal modification
The productivity of the solar still can be enhanced by adapt-
ing some internal modification in the still. Heat storage mate-
rials such as black rubber, black granite, pebbles, aluminium
pieces and iron pieces absorb solar radiation during sunshine
hours and release the heat energy to the basin water during
off-sunshine hours. Therefore, distillate activity is sustained for a
longer period, resulting in higher production. In this experiment,
1 cm size pebbles were spread at the bottom of the solar still. The
internal modification is shown in Figure 3.
2.2. External modification
With the help of external modifications also, the performance
of the solar still can be enhanced. The use of external reflectors

4. INTERNATIONAL JOURNAL OF AMBIENT ENERGY 3
Figure 4. Double-slope solar still with an external reflecting mirror.
increased the solar radiation on the glass cover. This increases
thebasinwatertemperatureanddistillateproductivity.Areflect-
ing mirror of 0.12 m height, 0.6 m width and 6 mm thickness was
fitted on an adjustable steel frame, as shown in Figure 4. The
reflecting mirror was kept opposite to the incidence of solar rays.
The place and angle of the mirror were adjusted periodically to
reflect maximum solar radiation into the basin.
3. Experimental procedure
Three double-slope solar stills were fabricated and placed under
identical climatic conditions.
(1) Solar still-I → Double-slope solar still without any internal
and external modification.
(2) Solar still-II → Internal modification was attempted to
enhance the yield. Pebbles were spread at the bottom of the
basin.
(3) Solar still-III → Both internal and external modifications
were attempted. Along with pebbles at the bottom of the
basin, an external mirror was used to reflect solar rays into
the basin.
The experiment was conducted in Villianur, Pondicherry. The
experiment was started at 8 a.m. and 5 L of brine water was
poured in each still. The experiment was repeated for seven days
and the average was calculated.
4. Measurement of performance
The performances of the solar stills were evaluated by using the
following mathematical ratios.
4.1. Daily productivity (in percentage)
Daily productivity was calculated by dividing the output of the
solar still by the input of the solar still.
Daily productivity =
Output
Input
× 100.
4.2. Increase in productivity (in percentage)
It is derived by dividing the increase in output due to modifica-
tion by output without modification.
Increase in productivity
=
Increase in production after modification
Production before modification
× 100.
5. Accuracies and ranges of measuring instruments
The accuracy and range of measuring instruments used in the
experiment are given below.
5.1. Anemometer
Type: Vane type
Accuracy: ±0.14 m/s
Range: 0–37 m/s
Weight: 0.9 kg
Purpose: To measure the wind speed
5.2. Thermometer
Type: Digital thermometer
Accuracy: ±2°C
Range: −48–120°C
Weight: 56 g
Length: 19 cm
Purpose: To measure ambient temperature
5.3. Measuring jar
Type: Transparent plastic jar
Accuracy: ±9 mL
Range: 0–800 mL
Purpose: To measure output water
6. Results and discussion
The maximum temperature of the day and total distillate pro-
duction of the still at the end of the day were recorded for all the
three stills.
6.1. Double-slope solar still alone (solar still-I)
The distillate productivity of the double-slope solar still with-
out any internal and external modification was investigated. The
maximum ambient temperature varied between 32°C and 38°C.
As a consequence, distillate production ranged between 1.4 and
2.1 L/D. The maximum output of 2.1 L/D was obtained when the
ambient temperature was 38°C. The average output per day was

5. 4 S. J. P. GNANARAJ ET AL.
Table 1. Performance of solar still-I.
S. no Date
Max.
temperature
(°C)
Water
input (L)
Output
(L)
Daily
efficiency
(%)
1 29 February 2016 32 5 1.4 28
2 4 March 2016 33 5 1.6 32
3 9 March 2016 34 5 1.7 34
4 13 March 2016 35 5 1.9 38
5 16 March 2016 34 5 1.7 34
6 19 March 2016 37 5 2 40
7 21 March 2016 38 5 2.1 42
Avg 5.00 1.77 35.43
1.77 L. The daily efficiency of the still ranged within 28% and
42%, in accordance with variations in ambient temperature. The
average daily efficiency of the double-slope solar still without
any external modification was 35.43%. The performance of solar
still-I is shown in Table1.
6.2. Double-slope solar still with internal modification
(solar still-II)
To enhance the productivity of the double-slope solar still,
pebbles were spread at the bottom of the basin. The inter-
nal modification attempted in the still marginally improved
the performance. The distillate output ranged between 1.5 and
2.2 L/D, and the maximum output of 2.2 L/D was recorded when
the maximum ambient temperature was 38°C. On an average,
1.89 L/D was collected and it was higher than that of solar still-I
by 0.12 L. In other words, the internal modification improved the
performance by 6.58%. The average daily efficiency of solar still-
II was 37.71% and it was higher than that of solar still-I by 2.28%.
This is shown in Table2.
6.3. Solar still with internal and external modifications
(solar still-III)
In this experimental set-up, both internal and external
modifications were attempted. Internally, pebbles were spread
and externally a reflector was used. As a consequence, there was
significant improvement in the performance of the solar still. The
maximum output produced increased to 2.7 L/D. On an average,
2.47 L/D was collected. The average output of solar still-III was
higher than that of solar still-II by 0.58 L and solar still-I by 0.7 L.
In other words, the average daily efficiency of solar still-III was
higher than that of solar still-I by 40.86% (Table 3).
6.4. Comparison of performance – increase in production
The performance of solar stills-I, II and III is depicted in Figure 5.
The distillate output of solar stills-I, II and III was 1.77, 1.89 and
2.47 L/D, respectively. The performance of the solar still without
any modification was very low. The internal and external mod-
ifications added in the still significantly improved the distillate
output.
6.5. Comparison of performance – increase in productivity
The increase in productivity of solar stills-II and III in comparison
with that of solar still-I is shown in Figure 6. There was a marginal
increase in productivity when internal modification alone was
1.77 1.89
2.47
0.00
0.50
1.00
1.50
2.00
2.50
3.00
Still- I Still- II Still- III
Production,liters
Figure 5. Comparative performance – increase in production.
Table 2. Performance of solar still-II.
S. no Date
Max.
temperature (°C) Input water (L) Output (L)
Daily
efficiency (%)
Increase in
efficiency (%)
1 29 February 2016 32 5 1.5 30 7.14
2 4 March 2016 33 5 1.7 34 6.25
3 9 March 2016 34 5 1.8 36 5.88
4 13 March 2016 35 5 2 40 5.26
5 16 March 2016 34 5 1.9 38 11.76
6 19 March 2016 37 5 2.1 42 5.00
7 21 March 2016 38 5 2.2 44 4.76
Avg 5.00 1.89 37.71 6.58
Table 3. Performance of solar still-III.
S. no Date
Max.
temperature (°C) Input water (L) Output (L)
Daily
efficiency (%)
Increase in
efficiency (%)
1 29 February 2016 32 5 2.2 44 57
2 4 March 2016 33 5 2.4 48 50
3 9 March 2016 34 5 2.4 48 41
4 13 March 2016 35 5 2.5 50 32
5 16 March 2016 34 5 2.5 50 47
6 19 March 2016 37 5 2.6 52 30
7 21 March 2016 38 5 2.7 54 29
Avg 5.00 2.47 49.43 40.86

6. INTERNATIONAL JOURNAL OF AMBIENT ENERGY 5
6.57
40.86
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
Still-II Still-III
Increaseinproductivity,%
Figure 6. Comparative performance – increase in productivity.
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
0.6 0.65 0.67 0.7
Production,Liters
Wind Velocity, m/s
Figure 7. Wind velocity vs. production.
attempted. The inclusion of internal and external modification
achieved remarkable improvement in productivity.
6.6. Impact of wind velocity
Wind velocity has an impact on the performance of solar stills.
So the performance of solar still-III was recorded under varying
wind velocity conditions. When the wind velocity was 0.6 m/s,
the output was 2.6 L. When the wind velocity increased to
0.65 m/s, the output declined to 2.1 L. A further increase in
wind velocity to 0.67 m/s reduced the output to 1.9 L. When
the wind velocity was 0.7 m/s, the output declined to 1.5 L. The
above finding reveals that wind velocity influences the produc-
tion and productivity. The higher the wind velocity, the lower
is the production, and vice versa, and the relationship is shown
in Figure 7.
7. Conclusion
Anattemptwasmadetoenhancetheproductivityofthedouble-
slope single-basin solar still. Internal modification (spreading
of pebbles) and external modification (external reflector) were
added to the still. To study the comparative performance of
these modifications, three solar stills were fabricated: solar still-I
(without any modification), solar still-II (with internal modifica-
tion)andsolarstill-III(withinternalandexternalmodifications).
(1) The distillate production of solar stills-I, II and III was 1.77,
1.89 and 2.47 L/D, respectively.
(2) The distillate output of solar still-II was higher than that of
solar still-I by 0.12 L. The performance of solar still-III was
higher than that of solar still-II by 0.58 L and solar still-I by
0.7 L.
(3) The productivity of solar stills-II and III was higher than that
of solar still-I by 6.58% and 40.86%, respectively.
(4) The internal modification marginally increased the perfor-
mance of the solar still. The internal and external modifi-
cations added to the still significantly improved the perfor-
mance.
(5) The distillate output varied in response to ambient temper-
ature of the day. In other words, there is a direct relationship
between the ambient temperature and performance of a
solar still.
(6) Wind velocity also influences the performance of the solar
still. When the wind velocity is low, the distillate yield is high.
In other words, there is an indirect relationship between
wind velocity and productivity.
To conclude, the performance of the double-slope single-
basin solar still can be enhanced by adding internal and external
modifications.
Disclosure statement
No potential conflict of interest was reported by the authors.
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