In the present work an attempt has been made to study the effect of increasing temperature difference between evaporating surface and condensing surface on the performance of solar distillation system. An indoor simulation study has been performed on a constant temperature bath. In order to increase the temperature difference between evaporating water surface and the condensing surface, the condensing surface temperature has been reduced by putting ice on the glass cover. It is observed that a maximum of 205 % rise in distillate is obtained by 54 % reduction in the condensing surface temperature for constant temperature of the evaporating water at 50 0C.

1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 41-48 © IAEME
41
EFFECT OF LOWERING CONDENSING SURFACE TEMPERATURE ON
THE PERFORMANCE OF SOLAR STILL
Ajeet Kumar Rai*, Vivek Sachan*, Vinay Tripathi#
,
Pramod Kumar*, Abhishek Tripathi*
*Mechanical Engineering Department, SSET, SHIATS –DU Allahabad
#
MED, United College of Engineering and Research, Allahabad
ABSTRACT
In the present work an attempt has been made to study the effect of increasing temperature
difference between evaporating surface and condensing surface on the performance of solar
distillation system. An indoor simulation study has been performed on a constant temperature bath.
In order to increase the temperature difference between evaporating water surface and the
condensing surface, the condensing surface temperature has been reduced by putting ice on the glass
cover. It is observed that a maximum of 205 % rise in distillate is obtained by 54 % reduction in the
condensing surface temperature for constant temperature of the evaporating water at 50 0
C.
INTRODUCTION
Single basin solar still is a very simple solar device used for converting available brackish
water into potable water. This device can be fabricated easily with locally available materials. This
device can be a suitable solution to solve drinking water problems. Because of its low productivity it
is not popularly used. Many theoretical and experimental works were performed by many researchers
to improve the productivity of the still. The performance prediction of a solar distillation unit mainly
depends on accurate estimation of the basic internal heat and mass transfer relations. The oldest,
semi- empirical internal heat and mass transfer relation is given by Dunkle [1]. Then to predict the
hourly and daily distillate output from the different designs of solar distillation units, numerous
empirical relations were developed. Most of these are based on the simulation studies. Malik et al.
[2] has considered the values of C = 0.075 and n = 0.33 for Gr > 3.2 x 105
as proposed by Dunkle.
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2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
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However, the relation developed by Dunkle has the following limitations:
(1) It is valid only for a low operating temperature range (45-50 0
C).
(2) It is independent of the cavity volume, i.e. the average spacing between the condensing and
evaporative surfaces.
(3) It is valid for cavities that have parallel condensing and evaporative surfaces.
Clark [3] developed a model for higher operating temperature range ( ≥ 55 0
C) in a simulated
condition for small inclinations of the condensing surface (β ≤ 15 0
C). Clark [3] has observed that the
coefficient of convective mass transfer becomes half that given by Dunkle [1]. Tiwari et. al. [4]
developed a modified Nusselt number, precisely for a trapezoidal cavity, for evaluation of convective
mass transfer in a solar distillation. A theoretical expression developed was validated by experiments
but only for temperatures greater than 60 0
C. Later on Kumar and Tiwari [5] developed a thermal
model to determine convective mass transfer for different Grashof numbers for solar distillation on a
passive and active solar distillation system for only summer climatic conditions. Then Tiwari and
Tripathi [6] developed a model for a high temperature range of the order of 80 0
C but for an opaque,
metallic, semi-cylindrical condensing cover made of Aluminium, which is not suitable practically for
passive solar distillation in the field. The condensing cover developed may be suitable for either
active solar distillation or for multi-source distillation units. Instead of indoor simulation, many
researchers have performed experimental studies to find real in-situ performance of the passive and
active solar stills. As Heat transfer rate increases with increase in temperature difference between the
evaporating and condensing surfaces, many researchers have attempted to reduce the condensing
surface temperature by spraying water over the glass cover in order to increase the temperature
difference between the evaporating and condensing surfaces, and hence the distillate output. In the
present work an attempt has been made to find the effect of reducing glass cover temperature on the
distillate output of the solar still.
EXPERIMENTAL SET-UP
A constant temperature bath filled with water (having capacity of 40 litres) was used as a
basin for the distillation unit. The temperature range of the bath was from 30 0
C to 110 0
C (lest count
temperature was 5 0
C). The steady state temperature and heating of water was controlled by an
electronic controlled panel.
The condensing chambers were made of GRP sheet with glass as the condensing surface. The
chamber was double walled on four sides with air inside the 2.5 cm thick cavity between the two
GRP surfaces, which acts as an air insulation. The hollow base of the condensing chamber is of the
same size as the constant temperature bath opening, and it can be placed over it to form an air tight
enclosure. Channels have been made along the length and the breadth of condensing chamber in
order to collect the condensate. The inclination of the glass cover with the horizontal β =30 0
. Copper
– constant an thermocouples are used, along with a digital temperature indicator, to recorded the
glass temperature, water temperature and water vapor temperature in the experimental stop. These
thermocouples, over a prolonged usage period, tend to deviate from the actual temperature.
Therefore, they were calibrated with respect to a standard thermometer.
The glass cover (condensing surface for water vapor) is 3mm thick. In order to conduct the
heat released by the condensing water (latent heat) from the inner surface of the glass to the outer
surface, a temperature gradient exists across the thickness of the glass. Therefore, the inner and outer
temperature of the glass cover is not equal, the inner temperature being higher. The average inner
(TG)and outer (Tg) glass cover temperature have been obtained experimentally as well as
theoretically, match with each other within an accuracy of 5%. In our experiment, it was possible to

3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 41-48 © IAEME
43
record the outer glass temperature only. This was changed to the average inner glass temperature by
using the previously obtained steady state values for the inner outer glass temperature. A view of the
condensing chamber and photograph of the experimental set up are shown in figure1.
Fig.1: Photograph showing experimental work
Table 1: Observation Table
Serial
No.
Tw Tv Tg Tw-Tg mew (p) Operating condition
(Glass Cover)
1 40 36 30 10 0.036 open to atmosphere
2 45 41 33 12 0.046 open to atmosphere
3 50 45 37 13 0.053 open to atmosphere
4 55 49 41 14 0.070 open to atmosphere
5 60 52 44 16 0.090 open to atmosphere
6 40 30 15 25 0.085 Covered with ice
7 45 35 16 29 0.095 Covered with ice
8 50 40 17 33 0.162 Covered with ice
9 55 42 18 37 0.212 Covered with ice
10 60 50 22 38 0.240 Covered with ice

4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 41-48 © IAEME
44
ANALYSIS OF CONVECTIVE MASS TRANSFER
The moist air above the water surface is freely converted to the condensing cover by the
action of a buoyancy force caused by density variation due to the difference between the water
surface and condensing cover. This process within the unit always happens in natural mode.
However the external heat transfer from condensing cover to the atmosphere takes place outside the
still and can either be under the natural or forced mode depending on ambient conditions
The rate of heat transfer from the water surface to glass cover ( cwQ ) by convection in the
upward direction through humid fluid can be given by
)( gwcwcw TThq −= (1)
The coefficient hcw can be determined form the relation
ncw
prGrC
k
dh
Nu ).(== (2)
The expression for Gr and Pr are given as
223
'....3 ffr TgxG µβρ ∆= (3)
f
fp
r
k
C
P
µ.
= (4)
It is clear from the above equation that the value of cwh depends upon the values of two
coefficients namely, C and n. It had been observed from the different values of C and n for given
models, for a particular range of Grashof number, that experimental and theoretical values closely
agree with a reasonable accuracy only for indoor simulation. However, for outdoor experiments the
deviation was more prominent between theoretical and experimental values.
Dunkle (1961), gave following expression for cwh for normal operating temperature range,
3/1
3
)109.268(
)273)((
)(884.0 





−×
+−
+−=
w
wgw
gWcw
p
Tpp
TTh (5)
The expression for hcw cannot be use for the situations not fulfilling conditions i.e. for
operating temperature of 75 °C, spherical, conical and higher inclined solar stills etc. Hence new
values of C and n need to be developed.
In the present work, a thermal model will be developed and methodology is proposed to
evaluate values of C and n. These are found by using experimental data of distillate output (mw),
water temperature (Tw) and glass temperature (Tg).
Malik et. al. (1982) have assumed that water vapour obeys the perfect gas equation and have
given the expression for evaporative heat transfer rate (qew) as,
=ewq )(0163.0 gWcw PPh − (6)

5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 41-48 © IAEME
45
Equation (4.4) combined with Eq. (4.2) can be written as
ewq n
gW RaC
d
K
PP )())((0163.0 −= (7)
where Ra = Gr.Pr
Further, the rate of distillate output is evaluated by
3600×=
l
q
m ew
ew& (8)
Equation (4.6) after substituting ewq from Eq. (8) becomes,
n
gWew RaC
ld
K
PPm )()
3600
)()((0163.0 −=& (9)
The above equation can be rewritten as,
ewm& = R n
RaC )( (10)
or,
new
RaC
R
m
)(=
&
(11)
Where,
)
3600
)()((0163.0
ld
K
PPR gW −= (12)
Equation (3.9) can be rewritten in the following form
Y = b
aX (13)
Where ,;;; nbCaRaX
R
m
Y ew
====
&
Equation (13) can be reduced to a linear equation by taking log on both the side
)ln()ln()ln( XbaY += (14)
or,
''''
XbaY += (15)
Where,
)ln(;);ln();ln( ''''
XXbbaaYY ==== (16)

6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 41-48 © IAEME
46
From Eq. (16), the values of coefficients a′ and b′ are calculated using regression analysis.
The expressions for a′ and b′ are given by:
22
)'()'(
)')('()''(
XXN
YXYXN
b
Σ−Σ
ΣΣ−Σ
=′ (17)
N
X
b
N
Y
a
'
'
' Σ
−
Σ
=′ (18)
Where N is number of experimental observations.
Knowing a′ and b′ from Equation (17) & (18), the value of C and n can be obtained by the
following expressions
C=exp (a′) and n =b′ (19)
RESULTS AND DISCUSSION
The measured distillate output for different operating conditions is given in table 1. First five
readings are taken with condensing cover open to atmosphere. Next five taken with condensing
cover covered with ice to create the maximum temperature difference and to study the effect of
atmospheric temperature on the distillate output. It is clear that the distillate output increases with
increase in ∆ T. It is also clear that the convective heat transfer coefficient strongly depends on the
operating temperature range. It goes on increases with rise in temperature
The experiment was conducted at bath temperature of 40, 45, 50, 55, and 60 0
C respectively.
Number of readings was taken, and average values for each operating temperature range are reported
in table 1. Graphs are plotted against different fixed values of water temperature for two different
conditions of condensing cover (i) when glass cover is open to atmosphere and (ii) when condensing
cover is covered with ice. It is clear from fig.2 that when glass cover is covered with ice, its
temperature decreases thereby increasing the temperature difference between evaporating water
surface and condensing glass cover Fig.3. It is shown in fig 4 that tremendous rise in distillate output
is obtained and this increases with rise in water temperature.
Fig.2: Variation of glass temperature in different conditions w.r.t. water temperature for different
conditions of condensing cover
0
5
10
15
20
25
30
35
40
45
50
40 45 50 55 60
Temperature(GlassCover)0C
Tw (Water Temperature) 0C
Tg (open)
Tg(covered)

7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 41-48 © IAEME
47
Fig.3: Variation of temperature difference with respect to water temperature for different conditions
of condensing cover
Fig.4: Variation of distillate output in with respect to water temperature for different conditions of
condensing cover
CONCLUSIONS
An indoor simulation experimentation has been carried out over a typical operating
temperature range (40 0
C to 60 0
C) by using constant temperature bath. The condensing surface was
maintained at different environmental conditions, (a) by exposing it to ambient air temperature and
(b) by keeping ice on the condensing surface. The temperature dependent physical properties of
enclosed vapor were considered. The temperatures and yields obtained were used to determine the
values of constants C and n in expression Nu = C (Gr.Pr)n
. For a given water temperature of 40 0
C, if
glass cover temperature is reduced by half, 136% rise in distillate output is observed, whereas its
value increases to 166 % for evaporating water temperature of 60 0
C.
0
5
10
15
20
25
30
35
40
40 45 50 55 60
Temperaturedifference0C
Tw (Water Temperature) 0C
Tw-Tg(open)
Tw-Tg(cover)
0
0.05
0.1
0.15
0.2
0.25
0.3
40 45 50 55 60
Distillateoutputinkg
Tw (Water Temperature) 0C
mew (open)
mew (cover)

8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 41-48 © IAEME
48
REFERENCES
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Ltd, UK,1982.
3. Clark, J. A., The steady state performance of a solar still. Solar Energy, 1990, 44, 43.
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