Desalination

1. Enhancing the design to optimize the performance of double basin
solar still
S. Joe Patrick Gnanaraj a
, S. Ramachandran a,
⁎, David Santosh Christopher b
a
Faculty of Mechanical Engineering, Sathyabama University, Chennai, India
b
Department of Mechanical Engineering, SCAD College of Engineering & Technology, Tirunelveli, India
H I G H L I G H T S
• The double basin solar still was fabricated in such a way that even lower basin gets direct sun light.
• To enhance the distillate output, reflectors, FPC and mini solar pond were integrated with the system.
• The contribution of various modifications to productivity was analyzed.
• The relative contribution of lower and upper basin for each modification was studied.
• Theoretical values were compared with the experimental results.
a b s t r a c ta r t i c l e i n f o
Article history:
Received 20 November 2016
Received in revised form 17 February 2017
Accepted 19 February 2017
Available online xxxx
In this study, an attempt was made to optimize the performance of double basin solar still. The dimension of the
lower basin was 100 × 140 cm2
and the dimension of the upper basin was 100 × 100 cm2
. So the lower basin had
100 × 20 cm2
glass cover in both the sides of the still to receive direct sunlight. Further external energy sources
such as reflectors, flat plate collector and mini solar pond were integrated with the double basin still. The produc-
tivity of single basin still, double basin still with no external modifications, double basin still with reflectors, dou-
ble basin still with reflectors coupled with flat plate collector and mini solar pond was 2745, 4333, 5650 and
6249 mL/day respectively. The productivity of double basin still, double basin still with reflectors and double
basin still integrated with flat plate collector and mini solar pond was 57.83%, 105.8% and 127.65% respectively
higher than the single basin still. The above modifications increased the performance of lower basin and upper
basin. But the relative contribution of lower basin improved from 29.75% to 35.22% and to 40.6%.
© 2017 Elsevier B.V. All rights reserved.
Keywords:
Solar energy
Double basin solar still
Reflectors
Solar pond
Flat plate collectors
1. Introduction
Water is very important for the survival of human race in our planet
earth. The plants and animals also need water for their existence. During
recent times, the large scale industrial and agricultural development ex-
erts much pressure on the existing water resources. About 97% of the
water available in our planet is salty sea water. About 2% of water
exist in frozen state in glaciers and polar regions. Only 1% of the water
is available for human consumption. But many sources of the water
are brackish (contains dissolved solids) and or contain harmful bacteria.
So they cannot be directly used for human consumption. In coastal loca-
tions, abundant supply of salt water is available. But potable water is
scarce.
Desalination is the best solution for the above said problems. Desali-
nation refers to the process of removing salt and other minerals from
water. Desalination is done to convert salt water into fresh water and
to make it suitable for human consumption.
Solar stills of the basin type have been used for over 100 years due to
their simple technology, easy operation and low cost. They are also pol-
lution free and environment friendly. The most important drawback of
single basin solar still is its low efficiency because of the loss of latent
heat of condensation through the glass cover of the still. The productive
capacity of a simple type solar still is in the range of 2–5 L/m2
/day. This
makes the system highly uneconomical.
The distillation productivity of a solar still is significantly influenced
by ambient temperature, insulation, wind velocity, dust and cloud am-
bient condition, saline water depth, salt concentration, inlet tempera-
ture of water, water and glass temperature difference, water free
surface area, absorber plate area and glass angle [1,2]. The solar intensi-
ty, wind velocity, ambient temperature and dust and cloud ambient
condition cannot be controlled as they are metrological parameters.
The remaining factors can be varied to enhance the productivity of the
solar still.
Desalination 411 (2017) 112–123
⁎ Corresponding author.
E-mail address: aishram2006@gmail.com (S. Ramachandran).
http://dx.doi.org/10.1016/j.desal.2017.02.011
0011-9164/© 2017 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Desalination
journal homepage: www.elsevier.com/locate/desal

2. In single basin solar still the quantity of distilled water produced per
unit area is very low. So it is not suitable in situation where space is a
limiting factor. Malik and Puri [3] suggested a new design-a double
basin solar still to increase the output per unit area. The main advantage
of the double basin solar still is that the latent heat of vapour, condens-
ing over the lower surface of the lower glass cover is utilized in heating
the upper layer of water, rather than being wasted to the atmosphere
[4]. The lower basin acts as an energy storage chamber and for this rea-
son, it is expected that this type of still would produce more distilled
water. Murugavel et al. [5] used different sensible heat storage materials
like quartzite rock, cement concrete pieces, washed stones and iron
scraps. Quartzite rock proved to be the most effective sensible storage
material. According to Rajaseenivasan et al. [6] the maximum distillate
of 1.27 kg/m2
and 0.46 kg/m2
was collected for glass basin solar still
with charcoal and conventional solar still respectively. Suleiman et al.
[7] analysed the impact of water depth on productivity. The experimen-
tal result showed that a higher productivity of 6.7 L/day was obtained
for low water depth. Murugavel et al. [8] evaluated the performance
of the double slope solar still using different wick materials like light
cotton clothes, sponges, coir mat, waste cotton pieces. The yield was
highest when light black cotton cloth was used. The performance of
stepped solar still with fins, sponges and combination of both fins and
sponges were analysed in terms of productivity by Velumurgan et al.
[9] and the productivity increased by 76%, 60.3% and 96% respectively.
Alaian et al. [10] experimentally proved that the system productivity in-
creased by more than 23% when pin finned wick is applied. Kabeel et al.
[11] compared the performance of conventional single slope solar still
and modified solar still. The influence of depth and width of the trays
on the performance of the solar still was studied. Further wicks were
added in the vertical side of the stepped still. The productivity of the
stepped still was maximum when the tray depth was 5 mm and tray
width was 120 mm. Compared to the conventional still the productivity
was 57.3% higher. El-Agouz [12] investigated the performance of the
modified stepped solar still using cotton absorber and a storage tank.
The daily efficiency for modified stepped still was higher than that for
conventional solar still approximately by 20%. Alaudeen et al. [13] con-
structed a stepped tray type basin along with an inclined flat plate col-
lector. Different packing materials such as wooden chips, sand, coal,
coconut coir were added in the inclined flat plate collector to increase
the area of exposure. Rock, sponge and wick combination produced
the maximum productivity of 1745 kg/m2
. The performance evaluation
of a stepped solar still with film cooling was theoretically investigated
by El-Samadony et al. [14]. Water film cooling may increase the stepped
still daily distillate productivity by about 8.2% and this percentage de-
pends on the combination between film cooling parameters. EL-
Samadony et al. [1] theoretically analysed the radiation heat transfer
rate inside a stepped solar still. The productivity of the solar still is
found to be sensitive to the radiation shape factor particularly at low
solar radiation of 200 W/m2
and glass cover inclination angle i.e. latitude
angle of the site and vice versa. Tanaka et al. [15] modified a basin type
solar still with internal and external reflectors. The productivity in-
creased by 70–100% on winter days. Omara et al. [16] compared the per-
formance of modified stepped solar still with the conventional solar still.
The productivity of the modified stepped solar still with internal and ex-
ternal (top and bottom) reflectors is higher than that of the convention-
al solar still approximately by 125%. Abdallah et al. [17] modified the
conventional solar still by installing reflecting mirrors on all interior
sides. Replacing the flat basin by a step-wise basin enhanced the perfor-
mance up to 180% and the coupling of stepwise basin with sun tracking
system improved the production rate of distilled water up to 380%.
Omara et al. [18] investigated the performance of the modified stepped
solar still with mirrors in the vertical side of the steps. The productivity
of modified solar still with and without internal mirrors is higher than
that of conventional still approximately by 75% and 57% respectively.
Tanaka et al. [19] proved that by changing the angle of the external re-
flectors with seasons, the productivity of the solar still could be en-
hanced throughout the year. Kabeel et al. [20] demonstrated that the
fresh water productivity reached approximately 7.54 L/m2
/day for the
solar still with PCM (phase change material). According to Omara et
al. [21] the water productivity of corrugated solar still with wick and in-
ternal reflecting mirrors was 145.5% higher than the conventional still.
Pandey [22] demonstrated that in situation where space is a limitation,
double basin still is most suitable and it is capable of providing 57% more
distilled water than a single basin still. An energy analysis was carried
out to explain the effect of different materials on the energy evaporation
rate and energy efficiency of the single and double basin solar stills by
Rajaseenivasan et al. [23]. Mild steel had a maximum energy efficiency
of 2.072% and 1.412% for double and single basin still, respectively.
Rajaseenivasan and Murugavel [24] concluded that double basin still
production rate was higher than the single basin still by around 85%
for same basin conduction. Al-Karaghouli et al. [25] conducted two
types of experiments-one with the still sides insulated and the other
without. The increase in efficiency in double basin solar still was around
8% for the uninsulated case and around 13% for the side insulated case. A
double basin solar still was coupled to a flat plate collector in the
thermosyphon mode by Yadav et al. [26]. It was more useful for high
temperature distillation than a still using the forced circulation mode,
Nomenclature
English letters
wu Upper basin water mass, kg
wl Lower basin water mass, kg
v Wind velocity, m/s
Ub Heat loss coefficient from basin to ambient, W/m2
°k
Ta Ambient temperature, °k
T Temperature, °k
Q Heat transfer rate, W/m2
P Partial pressure, N/m
m Mass, kg
Hs Solar intensity, W/m2
hfg Latent heat of water, J/kg
h Heat transfer coefficient, W/m2
k
gu Upper glass cover
gl lower glass cover
dT Temperature difference, k
dt Differential time, s
Cp Specific heat capacity, J/kg °k
AB Absorptance
A Area, m2
FPC Flat plate collector
Subscripts
w water
r radiation
loss side loss
g glass
eff effective
e evaporation
c convection
b basin
a ambient air
Greeks
σ Stefan–Boltzmann constant (5.6697 × 10−8
W/m2
k4
)
ρ Density, kg/m3
ε Emmisivity
Δ Difference
α Absorptivity
113S. Joe Patrick Gnanaraj et al. / Desalination 411 (2017) 112–123

3. especially at a location where electrical power is not available. To study
the transient performance of a double basin solar still Kumar et al. [27]
integrated the still with a heat exchanger. The increase in efficiency of
20–25% was obtained when the hot fluid was at an inlet temperature
of 40 °
C and was passed through the heat exchanger (the output de-
pends directly on heat exchanger length). Panchal et al. [28] fabricated
a double slope solar still and connected the lower basin with the vacu-
um tubes. The maximum daily distilled output of 11.064 kg was obtain-
ed when the water depth in the still was 0.03 m. An experimental study
was conducted by Deshmukh et al. [29] on double basin solar still using
evacuated tubes and reflector. The productivity increase was between
50.8% and 62%. A performance analysis of double basin solar still was
done by Panchal et al. [30] when evacuated tubes were coupled with
the still, the performance increased by 56%. The use of black granite
gravel along with evacuated tube improved the performance by 67%.
Shafi et al. [31] studied a solar still equipped with thermoelectric models
and evacuated tube collectors. The total daily efficiency of the system for
cases of full evacuated tubes with/without propeller fan as well as half
full case is calculated as 52%, 46% and 37% respectively. Park et al. [32]
showed that the productivity of the hybrid still increased linearly with
increasing heat input, recording 18.02 kg/m2
at 22.37 mJ/day. According
to Morad et al. [33] the active solar still (solar still integrated with flat
plate solar collector) recorded the highest productivity of
8.52 L/m2
day. Kumar et al. [34] used agitation effect and exhaust fan
in the modified basin still and the output was 39.49% more than the con-
ventional system. Velmurugan et al. [35] integrated a mini solar pond
with solar still and the productivity was 57.8% more than the productiv-
ity of ordinary still. Velmurugan et al. [36] confirmed that integration of
mini solar pond with solar still enhanced the productivity by 59%.
M.Esen and H.Esen [37] conducted a study to determine the thermal
performance of thermosyphon heat pipe solar collector and concluded
that prototype heat pipe solar water heater performed satisfactory.
Esen [38] concluded that the cooking time in a vacuum tube integrated
solar cooker depends on the thermosyphon properties of the refrigerant
used in the heat pipe. El-Sebaii et al. [39] estimated that the daily pro-
ductivity and efficiency of the stills with shallow solar pond were
found to be higher than those obtained without shallow solar pond by
52.36% and 43.80% respectively.
From the above literature study, we come to know that productivity
of the solar still can be increased by properly utilizing the latent heat of
condensation, increasing the absorber plate area and adding different
energy storage materials in the basin. For further enhancement of pro-
ductivity the solar still can be linked with external energy sources
such as external reflectors, flat plate collector and mini solar pond.
The distillate output of the solar still depends on the water to glass
cover temperature difference and the absolute value of the basin
water temperature. The high temperature solar distillation could be
achieved by connecting a solar energy collector to the still [39]. Rai
and Tiwari [42] coupled a flat plate collector with the single basin still
and the distilled water production increased by 24%. The solar basin
coupled to a collector can operate in two modes- forced circulation
mode and natural circulation mode. Electrical power is not required
for thermosyphon mode. So thermosyphon mode is preferred over cir-
culation mode [26].
The main objective of this work was to maximize the distilled water
output per unit area by constructing a double basin solar still and incor-
porating various modifications in the still. The upper basin was designed
as a stepped basin to increase the absorber plate area. Sensible heat stor-
age materials were used in the basin to improve the evaporation rate
and heat capacity of the still. External heat energy sources such as exter-
nal reflectors, flat plate collector and mini solar pond were integrated
with the system for further enhancement of production.
To achieve the above objective, the following methodologies were
adopted in the experimental and theoretical investigation. A conven-
tional single basin solar still was fabricated. Then a double basin solar
still with stepped tray in the upper basin was constructed. Sensible
heat storage materials were used in the basin. Detailed experimental in-
vestigations were carried out to assess the performance of the solar stills
under the following conditions.
1. Conventional single basin solar still.
2. Double basin solar still without any external modification.
3. Double basin solar still with external reflectors in the upper and
lower basin.
4. Double basin solar still with external reflectors, coupled with flat
plate collector in the upper basin and mini solar pond in the lower
basin. The photograph of conventional still and double basin solar
still with all modifications is shown in Fig. 1.
2. Experimental setup
First of all, it is important to evaluate the performance of the tradi-
tional solar still design as a reference. This helps to measure the im-
provement due to new modifications. The absorber area of the single
basin solar still is 140 × 100 cm2
. The solar still is fabricated using
2 mm thick galvanized steel. Low side wall height is 61 cm and high
side wall height is 119 cm. The inner surface of the whole basin is
painted black. This is to increase the absorption of solar radiation. The
basin is placed inside a wooden frame and a gap of 2 cm is maintained
between the basin and wooden frame. The gap is filled with saw dust
to reduce the heat loss from the still to the ambient. The still is covered
by 4 mm thick clear glass sheet. The angle of glass is adjusted to be 30°
with the horizontal. Pebbles and black rubber cubes are spread at the
bottom of the still to attract more solar radiation and to retain the
heat for longer period. Arrangement is made in the top glass cover to
drain the distilled water. The schematic diagram is given in Fig. 2(a).
To enhance the productivity of conventional single basin solar still, it
is modified as double basin solar still. The schematic diagram of the dou-
ble basin solar still is given in Fig. 2(b). It consist of two basins- the lower
basin and the upper basin. The dimension of the lower basin is the same
as conventional single basin solar still, i.e. 100 × 140 cm2
. The bottom of
the lower basin is designed as a double walled one. A wooden sheet is
placed at the bottom of the double walled lower basin and glasswool
is packed between the basin and wooden sheet. The inner surface of
the basin is painted black and pebbles and black rubber cubes are put
in the base. The basin is covered by three sheets of glasses, i.e.
100 × 100 cm2
glass in the centre and two 20 × 100 cm2
glass at the
two sides. The two 20 × 100 cm2
glasses are fitted in a easily removable
manner. In the double basin solar still, the upper basin is placed just
above the lower basin. The upper basin has a dimension of
100 × 100 cm2
. The depth of the basin is 41 cm. To increase the absorber
area, a stepped absorber plate is fitted into the basin. The absorber plate
is fixed at an angle of 30° with the horizontal. The stepped basin pro-
vides an extra contact surface area of 33%. This increases the heat and
mass transfer area.
Fig. 1. Photograph of conventional and double basin solar still.
114 S. Joe Patrick Gnanaraj et al. / Desalination 411 (2017) 112–123

4. The stepped absorber plate has five steps. The steps are arranged in a
step-wise fashion and they extend from one side wall of the basin to the
other side wall. The width of each tray is 19 cm and the depth is 7 cm.
The trays are made of GI sheet of 2 mm thickness because of easy fabri-
cation and low cost. The tray in each step is partioned into five seg-
ments. To enhance the productivity the following modifications are
made in the segments. In one segment, sensible heat storage material,
pebbles are spread at the bottom [13,40]. Another heat storage material,
black granite gravel [28] is used in another segment. Fins [9,41] are used
in another segment. To improve the capillary effect and to increase the
exposed area, sponges [9,13] are placed in one segment. In another seg-
ment mild steel pieces [23] are used to increase the solar absorption.
These arrangements are done in a mixed manner. As a result there
will be five segments under each modification. This arrangement is
shown in Figs. 3 & 4. Further, more evaporation is ensured due to low
depth in stepped trays. Moreover the sensible storage materials absorb
more heat and sustain production for a longer period. The upper basin is
covered by 4 mm thick glass. The glass cover is fixed at an angle of 30° to
the horizontal. The backside wall of stepped absorber plate is insulated
by 30 mm polyurethane foams [10] (thermal conductivity of
0.021 W/m k).
To enhance the distillation two external reflecting mirrors (top and
bottom) are fixed in the upper basin. The sizes of the reflecting mirrors
are equal to the size of the top glass cover. The top reflecting mirror is
inclined slightly towards the back and fixed at an inclined angle of
25°. The bottom reflecting mirror is inclined upward from the horizon-
tal and optimum reflector angle is 15°.
In the same manner, additional solar rays are focused into the bot-
tom basin through the top glass cover (20 × 100 cm2
extensions on
both the sides). Two external mirrors fitted in an adjustable stand are
kept on the two sides of the still. The angle of the mirrors are adjusted
and kept in positions that attract maximum solar radiation into the
lower basin still.
To enhance the productivity, flat plate collector is linked with upper
basin. Metal pipes that run the entire length of each tray are connected
with flat plate collector. This arrangement serves as an exchanger of
heat from flat plat collector to the upper basin. Further the lower
basin is integrated with Lower Converting Zone of mini solar pond.
The bottom of the lower basin is designed as a double walled one. The
hot saline water of LCZ is permitted to flow into the double walled
base of the lower basin. This arrangement enables the exchange of
heat energy from LCZ of solar pond to the bottom still.
Fig. 2. (a,b) schematic diagram of conventional and double basin solar still.
Fig. 3. Top View of double basin solar still. Fig. 4. Arrangement of heat storage materials in the tray.
115S. Joe Patrick Gnanaraj et al. / Desalination 411 (2017) 112–123

5. Small gap is maintained at the low side and high side of the upper
basin. This gap is linked with lower basin through a hole in the top
cover of the lower basin. This permits exchange of heat energy between
the upper and lower basin.
3. Experimental procedure
The present study was conducted at Villianur in Pondicherry, India,
(latitude 11.9089°N, 79.7589°E). Experimental setup was placed in
north-south direction. For daily experiment, the upper and lower
glasses were cleaned in the morning to remove dust deposition. Re-
quired saline water was poured into the two basins. Top up of saline
water was done every 2 h. The experiment was started at 9 a.m. and
continued up to 7 p.m. For every 1 h, the ambient temperature, glass
temperature and basin water temperature were recorded. Further dis-
tilled water output was collected, measured and recorded for every
1 h. At the end of the experiment day, the total distilled water collected
was measured. The collected data were analysed and inferences were
drawn.
4. Theoretical analysis
Theoretical analysis is carried out to find the daily productivity of
conventional solar still and double basin solar still. Initial temperatures
are assumed as ambient temperature.
4.1. Energy balance equations of double basin solar still
The transient energy balance equations for the basin, upper and
lower water mass, upper and lower glass are derived as follows.
4.1.1. Energy balance equation for basin (Eb) is written [24] as Eqs. (1) and
(2)
Eb ¼ Hs  Ab  ABbð Þ−Qloss−Qc;b−w ð1Þ
Eb ¼ m  cp Â
dT
dt
b
ð2Þ
4.1.2. Energy balance equation for lower basin water mass (Elw) is written
[24] as Eqs. (3) and (4)
Elw ¼ Hs  Alw  ABlwð Þ þ Qc;b−w−Qc;w−g;l
−Qe;w−g;l−Qr;g−w;l ð3Þ
Elw ¼ m  cp Â
dT
dt
lw
ð4Þ
4.1.3. Energy balance equation for lower glass cover (Elg) is written [24] as
Eqs. (5) and (6)
Elg ¼ Hs  Alg  ABlg
À Á
þ Qc;w−g;l þ Qe;w−g;l þ Qr;w−g;l−Qc;g−w;u ð5Þ
Elg ¼ m  cp Â
dT
dt
lg
ð6Þ
4.1.4. Energy balance equation for upper basin water mass (Euw) is written
[24] as Eqs. (7) and (8)
Euw ¼ Hs  Auw  ABuwð Þ þ Qc;g−w;u−Qe;w−g;u−Qr;w−g;u−Qc;w−g;u ð7Þ
Euw ¼ m  cp Â
dT
dt
uw
ð8Þ
4.1.5. Energy balance equation for upper glass cover (Eug) is written [24] as
Eqs. (9) and (10)
Eug ¼ Hs  Aug  ABug
À Á
þ Qe;w−g;u þ Qr;w−g;u
þ Qc;g−w;u−Qr;g−sky−Qc;g−sky ð9Þ
Eug ¼ m  cp Â
dT
dt
ug
ð10Þ
4.2. Energy balance equations for conventional still
The transient energy balance equations can be written for still basin
plate, water mass and glass cover, are derived as follows [24,9 35].
4.2.1. For basin plate
Eb ¼ Hs  Ab  ABbð Þ−Qloss−Qc;b−w ð11Þ
Eb ¼ m  cp Â
dT
dt
b
ð12Þ
4.2.2. For water mass
Ew ¼ Hs  Aw  ABwð Þ þ Qc;b−w−Qe;w−g−Qr;w−g−Qc;w−g ð13Þ
Ew ¼ m  cp Â
dT
dt
w
ð14Þ
4.2.3. For glass cover
Eg ¼ Hs  Ag  ABg
À Á
þ Qe;w−g þ Qr;w−g
þ Qc;g−w−Qr;g−sky−Qc;g−sky ð15Þ
Eg ¼ m  cp Â
dT
dt
g
ð16Þ
4.3. Calculation of absorptance values (AB)
The different absorptance (AB) values used in Eqs. (1) to (10) are
calculated using Eqs. (17) to (22). In the same way the absorptance
(AB) values for the conventional still used in Eqs. (11) to (16) are calcu-
lated using Eqs. (23) to (25).
For Double basin solar still,
ABug ¼ αg 1−ρg
À Á
ð17Þ
ABcg ¼ αg 1−ρg−ABu;g
À Á
ð18Þ
ABuw ¼ αw 1−ρg−ABu;g−ABc;g
ð19Þ
ABlg ¼ αg 1−ρg−ABu;g−ABc;g−ABu;w
ð20Þ
ABlw ¼ αw 1−ρg−ABu;g−ABc;g−ABu;w−ABl;g
ð21Þ
116 S. Joe Patrick Gnanaraj et al. / Desalination 411 (2017) 112–123

6. ABb ¼ αb 1−ρg−ABu;g−ABc;g−ABu;w−ABl;g−ABl;w
ð22Þ
For conventional solar still
ABg ¼ αg 1−ρg
À Á
ð23Þ
ABw ¼ αw 1−ρg−ABg
À Á
ð24Þ
ABb ¼ αg 1−ρg−ABg−ABw
À Á
ð25Þ
4.4. Calculation of daily productivity
The amount of distillate output of the modified solar still is given by
[24].
me ¼
ΔT Â Qe;w−g
hfg
ð26Þ
Qe;w−g ¼ Qe;w−g;l þ Qe;w−g;u for double basin solar stillð Þ ð27Þ
The daily production of modified solar still is given by [24].
Me ¼ ∑
19
9 me ð28Þ
Initially the upper, lower water temperature, basin temperature and
upper, lower glass temperature are taken as ambient temperature. The
change in upper water temperature (ΔTw)u, lower water temperature
(ΔTw)l, basin plate temperature (ΔTb), change in upper glass cover tem-
perature (ΔTg)u and lower glass cover temperature (ΔTg)l are noted.
The solar radiation and ambient temperature are measured hourly on
the corresponding days. These values are substituted in Eqs. (1) to
(16). By using the MATLAB program, the corresponding equations are
solved.
Energy gained by the double basin still = The transient energy rise
from direct heating from sun + Energy got from reflectors + Energy
got from pond + Energy got from FPC.
The different parameters used in the above equations, Eqs. (1) to
(28) are calculated using the different formulas given in Appendix 1
and the different thermo parameters used during calculations are listed
in Table 1.
5. Results and discussion
In this study the performance of conventional single basin solar still
was compared with double basin solar still without any modifications,
double basin still fitted with external reflectors and double basin still
with external reflectors coupled with FPC and solar pond. The glass
cover temperature, basin water temperature and distillate yield of
solar stills were taken for discussion and analysis.
5.1. Performance of the single basin solar still
As the ambient temperature changes, the glass temperature and
basin water temperature also changes. The ambient temperature
reached the maximum at 2 p.m. As a consequence, the glass cover tem-
perature and basin water temperature also increased slowly and
reached the maximum of 55 °C and 75 °C respectively at 2 p.m. The
glass temperature ranged above 50 °C from 12 noon to 3 p.m. The
water temperature was above 70 °C during the period 12 noon to
3 p.m. The difference between glass temperature and basin water tem-
perature which is mainly responsible for distillate yield widened up to
2 p.m. and then the difference narrowed. As a consequence, the distillate
yield increased from 9 a.m., reached the maximum at 2 p.m. and then it
declined. The distillate output from 9 a.m. to 7 p.m. was 2745 ml/day.
The trend of change in ambient temperature, glass temperature and
basin water temperature are given in Fig. 5.
5.2. Performance of double basin solar still
In the double basin solar still, the performance of the upper basin
and lower basin was recorded separately. The glass cover temperature
of the upper basin was 35 °C at 9 a.m. It increased slowly and reached
the maximum of 54 °C at 2 p.m. and then it declined. The upper basin
water temperature was within the range of 70 °C to 77 °C between 12
noon and 4 p.m. It was above 60 °C between 11 a.m. and 5 p.m. The
glass and water temperature difference of 21 °C, 23 °C, 23 °C and 25 °C
was maintained at 12 noon, 1 p.m., 2 p.m. and 3 p.m. respectively. On
the other hand, the glass temperature and basin water temperature of
the lower basin was lower than that of the upper basin. The lower
basin glass temperature reached the maximum of 50 °C at 2 p.m. It
was 4 °C lower than that of the upper basin. The basin water tempera-
ture was only 70 °C at 2 p.m. It was lower than the upper basin by
7 °C. As a consequence, the distillate output was also low.
The distillate output of the upper basin was 3044 ml/day and it was
1289 ml/day for the lower basin. The yield of the upper basin was
1755 ml/day higher than the lower basin. The combined output of the
two basins was 4333 ml/day. The distillate yield of double basin solar
still was 1588 ml/day higher than the conventional solar still.
In the double basin solar still, the contribution of upper basin was
significant. Out of the total yield, 70.25% was contributed by the upper
Table 1
Parameters used for theoretical study.
S. no Various parameter used Values
1 Mass of basin (mb) 8.6 kg
2 Area of upper basin (Aub) 1.4 m2
3 Area of lower basin (Alb) 1.0 m2
4 Absorptivity of basin (αb) 0.97
5 Absorptivity of water (αw) 0.06
6 Absorptivity of glass (αg) 0.0476
7 Reflectivity of the glass (ρg) 0.09
8 Specific heat of basin (Cpb) 471 J/kg °C
9 Specific heat of water (Cpw) 4181 J/kg °C
10 Specific heat of glass (Cpg) 798 J/kg °C
11 Heat loss coefficient from basin to ambient (Ub) 12 W/m2
k
12 Convective heat transfer between basin and water hb-w 133 W/m2
k
13 Convective heat transfer between glass and upper
waterhg-w,u
97 W/m2
k
Fig. 5. Temperature variation in conventional still.
117S. Joe Patrick Gnanaraj et al. / Desalination 411 (2017) 112–123

7. basin alone. Fig. 6 shows the relationship between the glass tempera-
ture, water temperature and ambient temperature.
5.3. Performance of the double basin solar still with external reflectors
To enhance the productivity, the external reflectors were fitted in
the upper basin and lower basin. As a consequence, the upper basin
glass temperature was above 60 °C between 12 noon and 3 p.m. and it
reached the maximum of 67 °C at 2 p.m. The basin water temperature
reached 81 °C at 2 p.m. and it ranged from 70 °C to 81 °C between
11 a.m. and 4 p.m. The fitting of external mirror increased the maximum
basin water temperature from 77 °C to 81 °C.
The performance of the lower basin too improved. The glass temper-
ature was 50 °C or above between 11 a.m. and 4 p.m. and touched 58 °C
at 2 p.m. There was improvement in the basin water temperature too. It
ranged above 70 °C between 12 noon and 3 p.m. and reached the max-
imum of 75 °C at 2 p.m.
The distillate yield of the upper basin was 3660 ml/day. It was
616 ml/day higher than the double basin still without modifications.
On the other hand, the distillate yield of the lower basin was
1990 ml/day, i.e. 701 ml/day higher than the still without reflectors. In
other words the fitting of external reflectors increased the yield of
upper basin by 20.24% and lower basin by 54.38%.
The lower and upper basin together contributed 5650 ml/day which
was 1317 ml/day higher than the double basin still without modifica-
tion. The share of the upper basin to the yield was 64.78% and the
share of the lower basin was only 35.22%. The fitting of external reflec-
tors increased the share of lower basin to the total contribution from
29.75% to 35.22%. Fig. 7 shows the relationship between the glass tem-
perature, water temperature and ambient temperature.
5.4. Performance of the double basin solar still fitted with external reflec-
tors, linked with flat plate collector and mini solar pond
The upper and lower basins of the double basin solar still were fitted
with external reflectors. Now the upper basin was linked with flat plate
collector and the lower basin was integrated with mini solar pond.
There was remarkable improvement in the performance of the upper
and lower basin. The upper basin glass temperature reached the maxi-
mum of 54 °C at 2 p.m. in double basin solar still without modifications.
It was 67 °C when external mirrors were fitted and it reached 69 °C
when linked with FPC and mini solar pond. The water temperature
which was 77 °C at 2 p.m. in double basin still without modification,
reached 81 °C in reflectors fitted still and 84 °C in FPC and mini solar
pond linked still. Distilled water production of upper basin was
3712 ml/day. In the same manner the performance of the lower basin
too improved. The lower basin glass temperature of the double basin
still was 50 °C at 2 p.m. It reached 58 °C in reflectors fitted still and fur-
ther increased to 66 °C in FPC and solar pond linked still. In the same
way, the basin water temperature increased from 70 °C to 75 °C and fur-
ther to 80 °C at 2 p.m. 2537 ml/day distilled water was collected from
lower basin. The lower basin and upper basin together contributed
6249 ml/day of distillate. This was 1916 ml/day higher than the double
basin still without modification and 599 ml/day higher than the exter-
nal reflectors fitted still. The contribution of the upper basin to total
yield was 59.4% and the contribution of lower basin was 40.6%. Fig. 8
shows the relationship between the glass temperature, water tempera-
ture and ambient temperature.
5.5. Comparative performance
The performances of the solar stills are given in Table 2. The produc-
tion of fresh water was only 2745 ml/day in a conventional single basin
solar still. It increased to 4333 ml/day in a double basin still. When ex-
ternal reflectors were fitted in the double basin still, the production in-
creased to 5650 ml/day. The production reached 6249 ml/day, when
FPC was linked with the upper basin and solar pond with the bottom
basin. The performance of the double basin solar still was 57.83% higher
than the performance of the conventional still. When external reflectors
Fig. 6. Temperature variation in double basin still.
Fig. 7. Temperature variation in double basin still with reflectors. Fig. 8. Temperature variation in double basin still with reflectors, pond and FPC.
118 S. Joe Patrick Gnanaraj et al. / Desalination 411 (2017) 112–123

8. were fitted the productivity increased by105.8%. An increase of 127.65%
was recorded when it was linked with FPC and solar pond. The perfor-
mance of the four solar stills is shown in Fig. 9.
In double basin solar still, the contribution of upper basin to the total
production was always higher than lower basin. The contribution of
upper basin was 70.25% and that of lower basin was 29.75%. The modi-
fications made in the solar still improved the relative performance of
lower still. It increased from 29.75% to 35.22% in external reflectors
fitted still and further to 40.6% in flat plate collector and mini solar
pond linked still. This shows that the modification added to the double
basin solar still increased the yield of both the upper basin and lower
basin. But their role in enhancing the performance of the lower basin
was significant. The relative performance of the lower basin and upper
basin is shown in Fig. 10.
A comparison between this research work and earlier research
works is given in Table 3.
5.6. Distribution of distillate production
The production rate of distillate is not uniform thought out the day.
The distillate slowly increased from 9 a.m. and reached the peak level
between 1 p.m. and 3 p.m. Nearly 36% to 38% of the distillation was
obtained during this period (1 p.m. to 3 p.m). Hourly production of
the solar stills in all the four conditions are given in Table 4 and Fig. 11.
5.7. Cost evaluation
It is very important to produce fresh water at minimum cost. The
fabrication cost of conventional still and modified solar still is given in
Table 5.
5.7.1. Double basin solar still
The fabricating cost of double solar basin is Rs. 9480. It is as-
sumed that the still can function for 10 years and further it is as-
sumed that the variable cost is Rs.1000 per year. So the variable
cost for 10 years =Rs.10000. The total cost incurred during
10 years = 9480 + (1000 × 10) = Rs.19480.
Further it is assumed the solar still will be in operation for 300 days
in a year. So the total distilled water produced during 10 years =
6.249 × 300 × 10 = 18747 l. The cost of 1 l of fresh water is =
(19,480/18747) = Rs.1.039 (0.015$).
Table 2
Accumulated production for some experiment days.
S·no Date Daily Production (ml/day)
Conventional still Double basin still alone Double basin still with reflectors Double basin still with reflectors,
FPC and pond
Upper Basin Lower Basin Total Upper basin Lower basin Total Upper basin Lower basin Total
1 14-4-2016 2170 2940 1260 4200 –
2 15-4-2016 2820 – 3480 1990 5470
3 16-4-2016 3000 – 3615 2185 5800
4 19-4-2016 3090 3100 1340 4440
5 20-4-2016 2870 – 3410 2470 5880
6 22-4-2016 2920 – 3675 1975 5650 –
7 25-4-2016 2970 – 3535 2375 5910
8 30-4-2016 2870 – 3810 2740 6550
9 13-5-2016 2770 2975 1275 4250
10 15-5-2016 2720 – 3935 2798 6733
11 16-5-2016 2820 – 3800 1999 5799 –
12 20-5-2016 2720 3260 1305 4565 –
13 23-5-2016 2770 – 3730 1800 5530 –
14 24-5-2016 2270 2947 1263 4210 –
15 26-5-2016 2400 – 3870 2304 6174
Average 2745 3044 1289 4333 3660 1990 5650 3712 2537 6249
% increase in productivity over conventional still 57.83 105.80 127.65
Percentage contribution 70.25 29.75 100 64.78 35.22 100 59.4 40.6 100
Fig. 9. Distillate output. Fig. 10. Relative share of lower basin and upper basin.
119S. Joe Patrick Gnanaraj et al. / Desalination 411 (2017) 112–123

9. 5.7.2. Single basin solar still
The fabricating cost is Rs.4780. The variable cost for 10 years =
(1000 × 10) = Rs.10000. The cost for 10 years =
4780 + (1000 × 10) = Rs.14780.
The fresh water produced during 10 years = 2.745 × 300 × 10 =
8235. The cost of 1 l of fresh water is = (14,780/8235) = Rs.1.79
(0.026$).
Table 3
Comparison with earlier research works.
Sl. no Name of the authors Modification done
Productivity (kg/m2
/day)
Daily productivity rise (%)Double basin solar still Conventional solar still
1. V. Velmurugan and K. Srithar [35] Solar still integrated with mini solar pond 3.7 2.82 27.6
2. A.A. El-Sebaii [39] Solar still integrated with a shallow solar pond 4.036 2.64 52.36
3. G.C. Pandey [22] Double basin solar still 1.23 1.925 57
4. T. Rajaseenivasan, K.K. Murugavel [24] Double basin double slope solar still 4.75 2.56 85
5. Z.M.Omara et al. [18] Stepped solar still with internal and
external mirror
8.1 3.6 125
6. This work Double basin still with reflectors, FPC and pond 6.249 2.745 127.65
Table 4
Distribution of distillate output.
HRS 9–10 10–11 11–12 12–13 13–14 14–15 15–16 16–17 17–18 18–19
Conventional still
Hourly production (ml/day) 49 162 277 390 519 513 280 222 195 137
Percentage (%) 1.80 5.90 10.10 14.20 18.90 18.70 10.20 8.10 7.10 5.00
Cumulative production (ml/day) 49 211 489 878 1397 1911 2191 2413 2608 2745
Cumulative percentage (%) 1.80 7.70 17.80 32.00 50.90 69.60 79.80 87.90 95.00 100.00
Double basin still alone
Hourly production (ml/day) 65 221 355 602 832 828 451 377 334 269
Percentage (%) 1.50 5.10 8.20 13.90 19.20 19.10 10.40 8.70 7.70 6.20
Cumulative production (ml/day) 65 286 641 1244 2076 2903 3354 3731 4064 4333
Cumulative percentage (%) 1.50 6.60 14.80 28.70 47.90 67.00 77.40 86.10 93.80 100.00
Double basin still with reflectors
Hourly production (ml/day) 90 322 514 893 1062 1079 576 463 412 237
Percentage (%) 1.60 5.70 9.10 15.80 18.80 19.10 10.20 8.20 7.30 4.20
Cumulative production (ml/day) 90 412 927 1819 2882 3961 4537 5000 5413 5650
Cumulative percentage (%) 1.60 7.30 16.40 32.20 51.00 70.10 80.30 88.50 95.80 100.00
Double basin still with reflectors, FPC and pond
Hourly production (ml/day) 87 325 594 881 1225 1181 694 537 437 287
Percentage (%) 1.40 5.20 9.50 14.10 19.60 18.90 11.10 8.60 7.00 4.60
Cumulative production (ml/day) 87 412 1006 1887 3112 4293 4987 5524 5962 6249
Cumulative percentage (%) 1.40 6.60 16.10 30.20 49.80 68.70 79.80 88.40 95.40 100.00
Fig. 11. Production of distillate output.
120 S. Joe Patrick Gnanaraj et al. / Desalination 411 (2017) 112–123

10. 5.8. Comparison of theoretical and experimental values
Theoretical values for the four types of solar stills were calculated
and they were compared with the experimental values. There was
close agreement between the two. The maximum deviation was only
15%. This is shown in Fig. 12.
5.9. Error analysis
By using the calibrated constant copper thermocouples, the temper-
ature of glass, water and basin were measured. The water output was
measured with the help of measuring jar. The solar intensity was calcu-
lated by a Calibrated solarimeter. The least counts and ranges of measur-
ing instruments are shown in Table 6.
6. Conclusion
1. To enhance the production of distilled water per unit area, double
basin solar still was recommended. The double basin solar still
proved to be more effective and productive than the conventional
solar still. The single basin still was able to contribute only 2745 ml
of distilled water per day. On the other hand, the productive capacity
of double basin still enhanced to 4333 ml/day.
2. To augment the productivity of double basin still, some modifications
were attempted. Additional solar radiations were focused into the
lower and upper basin using external reflectors. As a consequence,
the distilled water production increased from 4333 ml/day to
5650 ml/day.
3. For further enhancement of production, upper basin was connected
with the FPC and the lower basin with the mini solar pond. The
productivity of the double basin still with external reflectors and in-
tegrated with FPC and mini solar pond increased from 4333 ml/day
to 6249 ml/day.
4. The productivity of the double basin still was 57.83% higher than the
single basin conventional still. The productivity was 105.8% higher
when the external reflectors were used. The integration of FPC and
mini solar pond enhanced the productivity by 127.65%.
5. In the double basin solar still, the upper basin being a stepped basin
was always more productive than the lower basin. But the relative
contribution of the upper basin declined with modifications. In the
double basin solar still, the contribution of lower basin was only
29.75%. It increased to 35.22% in reflectors fitted double basin still.
It further increased to 40.6%, when FPC and mini pond were linked
with the still. In other words the modifications added with the dou-
ble basin still increased the relative contribution of lower basin. The
modifications influenced the productivity of the both upper and
lower basin. But their contribution is more significant in the lower
basin.
6. Theoretical values were compared with experimental values. There
was good agreement between this two. The maximum deviation
was 15%.
7. Finally, we can conclude that double basin still is recommended over
single basin still.
Appendix 1
The following equations are used for the theoretical calculation.
They are referred from References. [24,40 41]
For double basin still
Qc;b−w;l ¼ hc;b−w;l  Tb−Twð Þl  Ab
Qloss ¼ Ub  Tb−Tað Þ Â Ab
Qc; w−g;l ¼ hc;w−g;l  Tw−Tg
À Á
l
 Aw
hc;w−g;l ¼ 0:881 Twl−Tgl þ
Pwl−Pgl
À Á
Twl þ 273ð Þ
2:77 Â 103
−Pw
l
2
6
4
3
7
5
1=3
Qr;w−g;l ¼ hr;w−g;l  Twl−Tgl
À Á
 Aw
hr; w−g;l ¼ εeff  0:884
Twl þ 273ð Þ4
− Tgl þ 273
À Á4
Twl−Tgl
À Á
#
 σ
Qe;w−gl ¼ he;w−gl  Twl−Tgl
À Á
 Aw
he;w−g;l ¼
PT  Mw  hfg
Ma  cpa Ptl−Pwlð Þ Ptl−Pgl
À Á hc;w−g;l
Qc;g−w;u ¼ hc;g−w;u  Tgu−Twu
À Á
 Ab
Qc;w−g;u ¼ hc;w−g;u  Twu−Tgu
À Á
 Awu
Table 5
Cost analysis of conventional still and double basin still in the present work.
Sl. no Unit Cost(INR)a
Conventional still Double basin solar still
1 Glass cover (transparent) 500 900
2 Mild steel plate 1200 2500
3 Installation materials 900 1300
4 Paint 250 450
5 Labour charge 1880 4000
6 Handling charge 50 330
Fabrication cost 4780 9480
1a
(American dollar) = 63.13 INR(Indian rupee).
Fig. 12. Comparison of experimental value and theoretical value.
Table 6
Ranges and accuracy.
Sl. no. Instrument Accuracy Range Error (%)
1. Thermocouple 0.1 °C 0 to 100 °C 0.10
2. KippZonen solarimeter ±1 W/m2
0 to 4000 W/m2
0.10
4. Measuring jar 1 ml 0–1000 ml 5
6. Thermometer ±1 °C 0–100 °C 0.2
7. Anemometer ±0.1 m/s 0–15 m/s 10
121S. Joe Patrick Gnanaraj et al. / Desalination 411 (2017) 112–123

11. hc;w−g;u ¼ 0:881 Twu−Tgu þ
Pwu−Pgu
À Á
Twu þ 273ð Þ
0:265:5 Â 103
−Pwu
2
4
3
5
1=3
Qr;w−g;u ¼ hr;w−g;u  Twu−Tgu
À Á
 Aw
Qe;w−g;u ¼ he;w−g;u  Twu−Tgu
À Á
 Aw
he; w−g;u ¼
PT  Mw  hfg
Ma  cpa Ptu−Pwuð Þ Ptu−Pgsu
À Á hc;w−g;u
Qr; g−sky ¼ εg  Tg þ 272:72
À Á4
− Tsky þ 272:72
À Á4
h i
 σ  Ag
Qc;g−sky ¼ hc;g−sky  Tg−Tsky
À Á
 Ag
hc;g−sky ¼ 2:742 þ 3:4V
P ¼ 7225−431:45T þ 10:77T2
hfg ¼ 2513:4−2:393Tð Þ1000
cpa ¼ 999:1 þ ð0:14229 Â Tav
þ 0:0001201 Â T2
av
− 0:000000068482 Â T3
av
For conventional solar still
Qc; b−w ¼ hc;b−w  Tb−Twð Þ Â Ab
Qloss ¼ Ub  Tb−Tsky
À Á
 Ab
Qc;w−g ¼ hc;w−g  Tw−Tg
À Á
 Aw
hc; w−g ¼ 0:882 TW−Tg
À Á
þ
Pw−Pg
À Á
Tw þ 273ð Þ
0:261:5 Â 103
−Pw
!1=3
Qr;w−g ¼ hr;w−g  Aw  Tw−Tg
À Á
hr; w−g ¼ εeff  0:988
Tw þ 273:22ð Þ4
− Tg þ 273:22
À Á4
Tw−Tg
À Á
#
 σ
Qe;w−g ¼ he;w−g  Tw−Tg
À Á
 Aw
he; w−g ¼ hc;w−g
PT  Mw  hfg
Ma  cpa Pt−PWð Þ Â Pt−Pg
À Á
∈eff ¼
1
1
∈ug
þ
1
∈lg
−1
Tsky ¼ Ta−6
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