Desalination 227 (2008) 178–189
Effect of condensing cover material on yield of an active solar
still: an experimental validation
Vimal Dimria, Bikash Sarkara, Usha Singhb, G.N. Tiwaria*
a
Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
Tel. +91 (11) 2659-1258/6464; Fax: +91 (11) 2658-1121; email: gntiwari@ces.iitd.ernet.in
b
Department of Physics, University of Rajasthan, Rajasthan, India
Received 27 April 2006; Accepted 21 June 2007
Abstract
An attempt has been made to evaluate inner and outer glass temperature and its effects on yield. Numerical
computations have been performed for a typical day in the month of December, 2005, for the climatic condition of
New Delhi (latitude: 28E35N N; longitude: 77E12N E and an altitude of 216 m above mean sea level). Higher yield
was observed for an active solar distillation system as compared to the passive mode due to higher operating
temperature differences between water and inner glass cover. The parametric study has also been performed to find
out the effects of various parameters, namely thickness of condensing cover, collector absorbing surface, wind
velocity and water depth of the still. It is observed that there is significant effect on daily yield due to change in the
values of collector absorbing surface, wind velocity and water depth. For all the cases, the correlation of coefficients
(r) between predicted and experimental values have been verified and they showed fair agreement with 0.90 < r <
0.99 and root mean square percent deviation 3.22% < e < 22.64%. Effect of condensing cover materials, namely
copper and polyvinyl chloride (PVC), on daily yield have also been investigated and compared.
Keywords: Solar distillation; Active system; Heat transfer coefficient; Parametric studies
1. Introduction
Solar distillation is a process to distill
brackish/saline water by utilizing solar energy. In
general, solar distillation process is carried out in
two modes e.g., passive and active. Malik et al.
*Corresponding author.
[1] reported on passive solar stills until 1982.
Soliman [2] studied the performance of basin type
solar stills integrated with a flat plate collector.
Kiatsiriroat et al. [3] analyzed the performance of
multiple effect vertical solar stills with a flat plate
solar collector. Zaki et al. [4] studied an active
system of conventional single slope solar still
integrated with a flat plate collector under the
0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V.
doi:10.1016/j.desal.2007.06.024
V. Dimri et al. / Desalination 227 (2008) 178–189
thermosyphon mode of operation and found that
the maximum increase in the yield was up to 33%
when the water in the still was preheated in the
collector. Tiwari [5] reviewed the work on
passive as well as active solar stills. Tripathi and
Tiwari [6] carried out an experiment with the
effect of water depth on internal heat and mass
transfer for active system. Tiwari et al. [7]
worked out computer modeling of passive/active
solar stills by using inner glass temperature for
limited period. However, they have not considered this effect in thermal modeling.
Further, Tiwari et al. [8] reviewed the present
status of solar distillation systems for both passive and active modes. In this field a large group
of authors reported that the passive solar distillation system is a slow process for purification
of brackish water. The yield of this still is about
2 L/day per m2 of still area, which is much less
and may not be economically useful. Effect of
shape and size by using plastic condensing cover
for passive solar still has also been carried out by
various scientists [9–15]. They concluded that the
daily yield is decreased due to reduction in top
loss and large surface tension between condensed
water and condensing cover for a same design.
However, there is a method to increase the
yield by integration of solar collector into the
basin. This is generally referred to as active solar
stills. These may be flat plat collector, solar
concentrator or evacuated collector. These collectors may produce temperatures within the
range of 80–120EC depending upon the type of
solar collector. However, the range of temperature within solar stills is reduced to about 80EC
due to high heat capacity of water mass within the
basin. Hence there is a practical application of
such active systems to extract the essence of
medicinal plants placed under the solar still at
about 80EC. The systems used for extraction of
the essence of medicinal plants have become
economical.
Among these, the flat plate collector is becoming more popular due to its easy operation, low
179
cost and easy maintenance. In this system,
additional thermal energy is fed into the basin
from a collector panel [3,4,6]. Therefore the
objectives of the present studies are:
C to investigate the effects of inner and outer
glass temperature on yield;
C to study the effects on various parameters on
yield, viz., thickness of glass cover, collector
absorbing surface, wind velocity and water
depth.
C to study the effects of different condensing
cover materials.
2. Materials and methods
2.1. Experimental set-up
The experiment was carried out at the solar
Energy Park at IIT Delhi (latitude: 28E35NN;
longitude: 77E12NE and an altitude of 216 m
above mean sea level) [6]. The cross sectional
view of an active solar still is shown in Fig. 1.
The bottom surface of the still was painted black
for higher absorptivity and a glass cover of
0.003 m thickness covers the still. The area of the
still was taken to be 1 m2. The still was coupled
by using well insulated pipes to two flat plate
collectors of effective area 4 m2. The depth of
water in the basin has been kept equal to 0.05 m.
2.2. Instrumentation and observations
Parameters such as water, inner glass, outer
glass and ambient temperatures, total and diffuse
radiations on the glass cover and collector and the
yield were measured hourly. Water and glass
temperatures were recorded with the help of
calibrated copper-constantan thermocouples and
a digital temperature indicator having the least
count of 0.1EC. The ambient temperature and the
yield were recorded with the help of calibrated
mercury thermometer having a least count of
0.1EC and with a measuring cylinder of least
count 10 ml, respectively. The solar intensity was
180
V. Dimri et al. / Desalination 227 (2008) 178–189
Fig. 1. Cross sectional view of an active solar still
coupled with a flat plate collector.
measured with the help of calibrated solarimeter
of least count 2 mW/cm2.
During the active distillation process, the hot
water from the collector was pumped into the
basin of the still to increase the temperature
difference between the glass and water surface.
The pump was operated only for the sunshine
hours (9 am to 4 pm) to avoid the heat losses
caused by reverse flow during off-sunshine hours.
Table 1 shows the experimental observations for
particular days for hourly variation of solar
intensity, water, glass and ambient temperature
and yield for 0.05 m depth. Experiments were
conducted from 9 am to 8 am (i.e., 24 h) and
validation was done for a clear typical day —
December 7, 2005 — for 0.05 m water depth. A
computer program in MATLAB was made to
calculate various heat transfer coefficients, the
values of which were then used to calculate the
theoretical values of water, inner, outer glass
temperature and the yield, by providing the initial
values of water and glass temperature and the
effective solar intensity values.
3. Thermal modeling
The energy balance equations in terms of
various heat transfer coefficient of an active solar
still [16] are as follows:
C Inner and outer glass cover:
α′g I effs + h 1w (Tw − Tgi ) =
Kg
Lg
(T
gi
Kg
Lg
(T
gi
− Tgo )
− Tgo ) = h1g (Tgo − Ta )
(1a)
(1b)
Simplifying Eqs. (1a) and (1b), one gets
α′g I effs + h 1w Tw +
Tgi =
and
h 1w +
Kg
Lg
Kg
Lg
Tgo
(2a)
181
V. Dimri et al. / Desalination 227 (2008) 178–189
Table 1
Hourly variation of solar intensity, water, glass, ambient temperature and yield for 0.05 m water depth in the basin
Time (h)
Ieffs (W/m2)
Ieffc (W/m2)
Tw (EC)
Tgi (EC)
Tgo (EC)
Ta (EC)
ṁew (kg/m2 h)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
1
2
3
4
5
6
7
8
369.55
570.47
680.96
747.05
660.00
433.34
318.00
94.04
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
533.0
779.0
895.0
967.0
862.0
589.0
438.0
170.0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
19.1
40.6
50.0
56.0
67.9
62.6
60.5
49.4
42.6
37.4
32.5
29.5
26.3
24.1
22.4
20.8
19.4
18.4
17.5
16.8
16.1
15.6
15.2
15.1
23.0
32.6
40.8
48.3
57.7
56.9
50.8
41.6
32.9
27.7
23.6
21.3
19.4
18.1
16.9
15.6
14.7
14.0
13.4
13.1
12.8
12.6
12.5
14.9
14.0
18.0
22.0
23.0
24.0
25.0
25.0
26.0
26.0
26.0
26.0
26.0
24.0
20.0
15.0
11.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
9.0
14.0
18.0
22.0
23.0
24.0
25.0
25.0
26.0
26.0
26.0
26.0
26.0
24.0
20.0
15.0
11.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
9.0
0.000
0.068
0.281
0.495
0.590
0.570
0.450
0.340
0.180
0.120
0.090
0.060
0.053
0.047
0.030
0.027
0.021
0.018
0.016
0.014
0.013
0.011
0.011
0.011
Tgo =
α′g I effs h k + U woTw + h 1g Ta
h 1g + U wo
C Water mass:
(2b)
+α′ (1- α′ ) I + h ( T − T )
Q
u
w
g
effs
w
b
w
= ( M C )w
C Basin liner:
α′b (1- α′g ) (1- α′w ) I effs = hw ( Tb − Tw )
+ hb ( Tb − Ta )
α′− b I effs + hwTw + hbTa
hw + hb
(4)
where
(3a)
After simplifying the above equation, one gets
Tb =
d Tw
+ h1w ( Tw − Tgo )
dt
(3b)
= A F ⎡( ατ ) ⎤ I − U ( T − T )
Q
u
c R ⎣
LC
w
a
c⎦ c
(5)
Substituting the values of Tgi and Tb from
Eqs. (2) and (3b) in Eq. (4) and after simplifying,
we get
182
V. Dimri et al. / Desalination 227 (2008) 178–189
d Tw
+ aTw = f ( t )
dT
(6)
UAeff
( MC )w
Ut =
f (t ) =
;
h1w h1g
( IA)eff
;
hw hb
;
hw + hw
= NAc FR ( ατ )effc I effc + ( ατ )effs I effs
h1w
U wo =
( IA)eff + (UA)eff Ta
( MC )w
; Ub =
h1g + U wo
Kg
h1w +
(UA)eff
Lg
Kg
qew = hew (Tw − Tg )
(8)
and the hourly output is given by
h ew (T w - T g ) × 3600
kg/ m 2h )
(
L
24
ew = ∑ m
ew
M
4. Statistical analyses
U LS = U b + U t
Kg
h1w +
Kg
;
hw =
Kw
n
C ( Gr Pr )
L
Lg
α′− b = (1 − α′g ) (1 − α′w )
= α′b
(10)
i =1
= U LS + NAC FRU LC
Lg
(9)
The daily yield is given by
Lg
−1
( ατ )eff
where Tw0 is the temperature of basin water at t =
0 and f (t ) is the average value of f(t) for the
time interval between 0 and t.
Inner, outer glass and basin temperatures in
terms of water temperature can be calculated
using Eqs. (2a), (2b) and (3b), respectively.
Now the rate of evaporation is given by:
ew =
m
⎡L
1 ⎤
hb = ⎢ 1 +
⎥ ;
⎣⎢ K1 h1g ⎥⎦
hk =
f (t )
⎡1 − exp ( −a Δt )⎤⎦ + Tw 0 exp ( -a Δt )
a ⎣
(7)
where
a=
Tw =
hw
h1w
hk
+ α′w + α′g
hw + hb
h1g + U wo
4.1. Coefficient of correlation (r)
When predicted values are validated with the
experimental data, then correlation between predicted and experimental values is presented with
a coefficient known as coefficient of correlation.
The coefficient of correlation can be evaluated
with the following expression [17].
r=
N ∑ xi yi − ∑ ( xi ) ∑ ( yi )
N ∑ xi 2 − ( ∑ xi )
2
N ∑ y i 2 − ( ∑ yi )
2
(11)
4.2. Root mean square of percent deviation (e)
The solution of Eq. (6) is given as
The prediction is done with the help of
183
V. Dimri et al. / Desalination 227 (2008) 178–189
experimental values. The predicted values are
validated with experimental data. The closeness
of predicted values and experimental data can be
presented in terms of root mean square of percent
deviation. The expression used for this purpose is
as follows [17]:
e=
∑(e )
2
i
(12)
n
where ei =
X pred − Yexp
X pred
5. Results and discussion
The values of design parameters for validation
are given in Tables 2 and 3, which are used as
input parameters to calculate the theoretical basin,
water, inner and outer glass temperature. Fig. 2
shows the hourly variation of solar radiation on
condensing cover and flat plate collector. The
hourly variation of ambient air temperature has
also been shown in the same figure. Ambient air
proportionately increases with increase of solar
intensity and decreasing trends was noticed
during off-sunshine hours. Eq. (7) has been evaluated for water temperature in the basin for a
given design (Tables 2 and 3) and climatic parameters (Fig. 2). After knowing the water
temperature, the inner and outer glass cover and
the basin liner temperatures have been evaluated
from Eqs. (2) and (3b). The hourly variation of
basin, water, inner and outer glass temperature
and yield is shown in Fig. 3. For comparison, the
hourly yield has also been shown in the same
figure. It is observed from the figure that the yield
increases with an increase of temperature as
expected, but a declining trend was observed
during off-sunshine hours due to lower temperature differences. The range of temperature
decreases in the order of Tb > Tw >Tgi >Tgo as
expected. It is further to be noted from Fig. 3 that
Table 2
Design parameters for a single slope solar still
Ab, m2
Ag, m2
As, m2
Aw, m2
C
Cw, J/kgEC
g, m/s
Kc, W/mEC
Kg, W/mEC
Kp, W/mEC
Kw, W/mEC
K1, W/mEC
L
L1 , m
1
1
1
1
0.54
4190
9.81
385
0.78
0.16
0.614
0.38
1
0.003
Lc/Lg/Lp, m
Mw, kg
n
v, m/s
αNb
αNg
αNw
(ατ)s
geff
μ, kg/m s
ρ, kg/m3
σ
αNp
0.003
50
0.25
0–3
0.8
0.05
0.05
0.8
0.82
8.6×10!4
995.8
5.67×10!8
0.05
Table 3
Design parameters for flat plate collectors
Ac, m2
Cf, J/kg EC
FN
ṁ, kg/s
N
(ατ)c
ULC, W/m2 EC
2
4190
0.8
0.035
2
0.8
6
there is marginal difference between the basin
and water temperature due to a high value of
convective heat transfer from the basin liner to
the water mass as expected. Similarly, the inner
glass cover temperature is higher about 0.11–
2.70EC than the outer glass cover temperature
due to higher value of wind velocity. This difference becomes significant in the case of PVC as
a condensing cover due to low value of its
thermal conductivity.
The validation of inner and outer glass cover
temperature, water temperature and yield are
shown in Figs. 4. It can be observed that there is
a fair agreement between theoretical and experimental values of these parameters. In all the cases
the coefficient of correlation are in the range of
184
V. Dimri et al. / Desalination 227 (2008) 178–189
Fig. 2. Hourly variation of ambient air and solar radiation on still and collector.
Fig. 3. Hourly variation of basin, water, inner and outer glass temperature.
r = 0.97–0.99 and root mean square percent
deviation (e) lies between 11.46–36.98%, respectively. The higher value of root mean square
deviation is due to very low value of output
during the night.
Figs. 5a and 5b show the hourly variation of
internal heat transfer coefficient, namely convective and evaporative evaluated by using Dunkle’s
relation [18]. The figure indicates that the convective heat transfer coefficient was lower
V. Dimri et al. / Desalination 227 (2008) 178–189
185
Fig. 4a. Hourly variation of theoretical and experimental
water temperature and yield.
Fig. 4b. Hourly variation of theoretical and experimental
inner glass temperature and yield.
Fig. 4c. Hourly variation of theoretical and experimental
outer glass temperature and yield.
Fig. 4d. Hourly variation of theoretical and experimental
yield at 0.05 m water depth.
between 13:00 to 20:00 hours due to decrease in
the temperature differences between the water
and inner glass cover. Higher evaporative heat
transfer coefficient was noticed at 12:00 due to
prevailing higher temperature differences and at
night time decreasing trend was observed due to
lower temperature differences. The hourly variation of radiative heat transfer coefficient is shown
in Fig. 5c. Convective heat transfer coefficient
mainly depends on wind velocity; it increases
with the increase of wind velocity and vice-versa.
One can concluded that there is a reasonable
agreement between heat transfer coefficient
evaluated by experimental observation and theoretical value with the correlation coefficient and
root mean square deviation of 0.90, 0.99 and 0.98
and 18.81, 15.26 and 3.22%, respectively. Similar
agreement has been observed for total internal
heat transfer coefficient as shown in Fig. 5d.
Theoretical total heat transfer coefficients were
verified with the experimental values in term of
coefficient of correlation and root mean square
percent deviation. Coefficients of correlation and
root mean square percent deviation are r = 0.99
and e = 10.57%, respectively.
Fig. 6 shows the variations of daily yield for
186
V. Dimri et al. / Desalination 227 (2008) 178–189
Fig. 5a. Hourly variation of theoretical and experimental
convective heat transfer coefficient at 0.05 m depth.
Fig. 5b. Hourly variation of theoretical and experimental
evaporative heat transfer coefficient at 0.05 m depth.
Fig. 5c. Hourly variation of theoretical and experimental
radiative heat transfer coefficient at 0.05 m depth.
Fig. 5d. Hourly variation of theoretical and experimental
total heat transfer coefficient (convective, radiative and
evaporative) at 0.05 m depth.
different thickness of glass cover in active and
passive mode. It is clearly indicated from the
figure that daily yield decreases with the increase
of glass cover thickness due to reduction in the
top loss coefficient [Uwo of Eq. (2b)]. This reduction is significant for both passive and active
solar stills. Fig. 7 indicates the variation of daily
yield for five numbers of collectors in active
mode. From this figure, it is observed that yield
increases with increase of collector surface area
due to increasing high operating temperature
range. However, the increase in yield is marginal
due to higher value of inner glass cover temperature. This indicates that one collector is optimum for present design parameters of an active
solar still [19]. Effect of wind on daily yield is
shown in Fig. 8. It clearly indicates that wind
blowing over the glass cover causes faster evaporation. As the wind velocity increases, the convective heat transfer coefficient from the glass
cover to ambient air increases and simultaneously
the glass cover temperature decreases which
increases the water-glass cover temperature differences and ultimately increased the overall
V. Dimri et al. / Desalination 227 (2008) 178–189
187
Fig. 9. Variation of yield on effects of 0.05 m water
depth.
Fig. 6. Variation of yield on different thickness of glass
cover.
Fig. 10. Effect of condensing cover material on the
performance of an active solar still.
Fig. 7. Variation of yield on effects of collector absorbing
surface.
Fig. 8. Variation of yield on effects of wind velocity.
yield. Similar observations were also found [20].
The effect of water depth on daily yield in active
and passive mode is shown in Fig. 9. It is seen
from the figure that the yield decreases with
increase of water mass from 20 to 150 kg. The
decrease in yield may be attributed to higher
specific heat capacity of water by the increased
water mass. This is in accordance with the results
previously reported [20].
The effect of different condensing cover
material on daily yield is shown in Fig. 10. It is
observed that the yield is maximum in the case of
copper condensing cover due to fast release of
heat available to it. It is due to the high value of
thermal conductivity of copper, which gains
188
V. Dimri et al. / Desalination 227 (2008) 178–189
higher overall heat loss coefficient [Uwo of
Eq. (2b)]. In case of PVC, the daily yield is
decreased due to reduction in top loss coefficient
(Uwo). This attribute is due to the low thermal
conductivity of PVC condensing material. These
results are in accordance with results reported
earlier by various scientists. This strongly justifies using glass as a condensing cover for use of
solar energy with cost effectiveness of the
system.
6. Conclusions
The present study indicates the importance of
the active distillation at lower water depth. Inner
glass temperature plays a key role to determine
the yield. The daily yield is more for active
distillation as compared to passive mode using
inner glass temperature. Yield is also directly
related to thermal conductivity of condensing
cover materials; copper gives a greater yield compared to glass and plastic due to higher thermal
conductivity.
7. Symbols
A
Ac
Aw
C
—
—
—
—
Cf
—
Cw
—
e
—
FN
FR
g
Gr
hb
—
—
—
—
—
Surface area, m2
Area of collector, m2
Area of water surface, m2
Constant in Nusselt number expression
Specific heat of working fluid,
J/kgEC
Specific heat of water in solar still,
J/kgEC
Root mean square of percent deviation
Collector efficiency factor
Heat removal factor
Acceleration due to gravity, m/s2
Grashof number
Overall heat transfer coefficient
from basin liner to ambient air
through bottom and side insulation,
W/m2 EC)
h1g
—
h1w
—
hcw
—
hew
—
hrw
—
hw
—
Ieff
K1
—
—
Kg /Kc /
—
Kp
L
—
—
L1
Lg /Lc /
—
Lp
Mw
n
—
—
—
—
—
N
Pgi
—
—
Pr
Pw
—
—
r
t
T
Ti
—
—
—
—
ṁ
ṁew
ew
M
Convective and radiative heat transfer coefficient from glass cover to
ambient, W/m2 EC
Total heat transfer coefficients from
water surface to glass cover,
W/m2EC
Convective heat transfer coefficient
from water surface to glass,
W/m2EC
Evaporative heat transfer coefficient from water surface to glass,
W/m2EC
Radiative heat transfer coefficient
from water surface to glass,
W/m2EC
Convective heat transfer coefficient
from basin liner to water, W/m2EC
Effective solar intensity, W/m2
Thermal conductivity of insulation
material, W/mEC
Thermal conductivity of glass, copper and PVC, W/mEC
Latent heat of vaporization, J/kg
Thickness of insulation material, m
Thickness of glass, copper and
PVC, m
Flow rate of pump, L/s
Hourly output of still, kg/m2 h
Daily output of still, kg/m2 h
Mass of water in basin, kg
Constant in Nusselt number expression
Number of observations
Partial vapor pressure at inner glass
temperature, N/m2
Prandtl number
Partial vapour pressure at water
temperature, N/m2
Coefficient of correlation
Time, s
Temperature, EC
Average of water and inner glass
temperature, EC
V. Dimri et al. / Desalination 227 (2008) 178–189
Tb
Tgi
Tgo
Ub
—
—
—
—
ULC
—
Ut
—
v
Xi
Yi
—
—
—
Temperature of basin water, EC
Inner glass temperature, EC
Outer glass temperature, EC
Overall bottom heat loss coefficient, W/m2EC
Overall heat transfer coefficient for
collector, W/m2EC
Overall top loss coefficient from
water surface to ambient air,
W/m2EC
Wind velocity, m/s
Theoretical or predicted value
Experimental value
Greek
Α
αN
(ατ)
—
—
—
β
ΔT
γ
μ
σ
—
—
—
—
—
Absorptivity
Fraction of energy absorbed
Effective absorptance-transmittance
product
Expansion factor, K!1
Temperature difference, EC
Relative humidity
Viscosity of humid air, N.s/m2
Stefan Boltzman constant
Subscripts
a
b
c
eff
g
s
w
—
—
—
—
—
—
—
Ambient
Basin liner
Collector
Effective
Glass cover
Solar still
Water
References
[1] M.A.S. Malik, G.N. Tiwari, A. Kumar and M.S.
Sodha, Solar Distillation, Pergamon Press, Oxford,
UK, 1982.
[2] H.S Soliman, Solar still coupled with a solar water
heater, Mosul University, Mosul, Iraq, 1976, p. 43.
[3] T. Kiatsiriroat, P. Wibulswas and S.C. Bhattacharya,
Performance analysis of multiple effect vertical solar
still with flat plate solar collector. J. Solar Wind
189
Technol., 4(4) (1987) 451.
[4] G.M. Zaki, A. Al-Turki and M. AI-Fatani, Experimental investigation on concentrator assisted solar
stills, J. Solar Energy, 11(1992) 193–199.
[5] G.N. Tiwari, Recent Advances in Solar Distillation,
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