DESALINATION

ELSEVIER Desalination 173 (2005) 187-200

www.elsevier.com/locate/desal

Effect of water depth on intemal heat and mass transfer

for active solar distillation
Rajesh Tripathi, G.N. Tiwari*
Centrefor 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@ees.iitd.ernet.in

Received 7 June 2004; accepted 16 August 2004

Abstract
In this communication, an attempt has been made to fred out the convectiveheat transfer coefficient for active solar
distillation system. It is a well-known fact that the distillate output (the yield) decreases significantly with the increase
of water depth in the basin of the solar still. It is also known that more yield is obtained in case of active solar
distillation system as compared to passive solar still due to higher temperature difference between the water and inner
glass cover temperatures in the active mode. For the present study, experiments have been conducted for 24 hours
during winter months for different water depths in the basin (0.05, 0.1 and 0.15 m) for passive as well as active solar
distillation system. The objective of the present paper is to study the effect of different water depths in the basin on the
heat and mass transfer coefficients. It is inferred that the convective heat transfer coefficient between water and inner
condensing cover depends significantly on the water depth in the basin. It is also observed that more yield is obtained
during the off shine hours as compared to daytime for higher water depths in solar still (0.10 m and 0.15 m) due to
storage effect.

Keywords: Solar distillation; Heat and mass transfer coefficients

1. Introduction [2] analyzed the performance o f a multiple-effect

The process o f solar distillation is used to vertical solar still with a flat-plate solar collector.
distill brackish/saline water by using solar Zaki et al. [3] experimentally investigated con-
energy. The systems involved in solar distillation centrator assisted solar stills. Tiwari [4] reviewed
operate under two modes: passive and active. the work on passive as well as active solar stills.
Malik et al. [1] reviewed the work done on Later Kurnar and Tiwari [5] estimated the con-
passive solar stills until 1982. Kiatsiriroat et al. vective mass transfer for passive solar distillation
systems. Recently Tiwari et al. [6] have studied
the convective heat transfer coefficient for a
*Corresponding author. passive/active solar still by using inner glass
0011-9164/05/$- See front matter © 2005 Elsevier B.V. All rights reserved
doi: 10.1016/j.desal.2004.08.03
188 R. Tripathi, G.N. Tiwari / Desalination 173 (2005) 187-200

cover temperature for a limited period of opera-

tion, but they did not consider different water
depths in the basin to calculate the convective
heat transfer coefficient. Further, Tiwari et al. [7] Etstillate / _ __
output . ~ / ~ 4 Ne~
reviewed the present status of research work on
both passive and active solar distillation systems.
Delyannis [8] presented an historical background ~O1 i Ir~;~.a
al.i
on
of desalination and renewable energies. ~':I ~--Blackmed
From earlier published works on solar distilla- .............................

tion, it has been observed that the passive solar

distillation system is a slow process for the
Dram.age
purification of brackish water. This process can
be significantly increased in an active mode of a Fig. la. Cross sectional v i e w o f a single-slope passive
distillation system [6], due to the fact that addi- solar still.
tional thermal energy is fed into the basin from a
Sotar
collector panel [2,3]. Hence there is a strong need
to have a basic knowledge of heat and mass
transfer coefficients in detail for an active solar
still. In this communication, an attempt has been
made to study the effect of different water depths
in a solar still on the heat and mass transfer
coefficients for the passive as well as the active
mode.

2. Experimental procedure and observations

A cross sectional view of a single-slope Fig. lb. Schematic diagram of an active solar still
passive solar still made of fibre-reinforced plastic coupled with a flat-plate collector.
(FRP) is shown in Fig. l(a). The bottom surface
of the still was painted black for greater absorpti-
vity and a glass cover 3 mm thick covers the still. with water at different depths: 0.05 m, 0.10 m and
Fig. l(b) shows the schematic diagram of an 0.15 m. This led to the average spacing between
active solar still coupled with a flat-plate collector the water surface and the glass cover to be kept at
(FPC). The area of the still and FPC was taken as 0.255 m, 0.205 m and 0.2 m, respectively. The
1 and 2 m 2, respectively. following parameters were measured every hour
The hot water from the collector was pumped for a period of 24 h for different depths and
into the basin of the still to increase the temp- different modes of experimentation:
erature difference between the glass and water ° water temperature
surface. The pump was operated only during the • inner glass temperature
sunshine hours, i.e., from 9 am to 4 pm, and was • outer glass temperature
kept off during off-sunshine hours to avoid heat ° vapor temperature
losses caused by reverse flow. The storage effect • total radiation on the glass cover and on the
was studied by filling the basin of the solar still collector
 R. Tripathi, G.N. Tiwari/ Desalination 173 (2005) 187-200 189

• vapor temperature is approximately an aver-

age of the water and inner condensing cover
temperature.
• No stratification of temperature along water
depth and vapor column.
• Quasi-steady state condition.
• Vapor leakage proof.

3. T h e r m a l a n a l y s i s
Fig. lc. Photograph of the active solar distillation system.
Following Kumar and Tiwari [5], the rate of
convective heat transfer is described by the
• diffuse radiation on the glass cover and on the general equation
collector
• ambient temperature O=h~'A'(T~-T~)--h~w'A'AT (1)
• distillate output
where hew is the convective heat transfer
Water, glass and vapor temperatures were coefficient.
recorded with the help of calibrated copper- The following relation gives the non-dimen-
constantan thermocouples and a digital tempera- sional Nusselt number related by the convective
ture indicator having a least count of 0.1 °C. The heat transfer coefficient:
ambient temperature and the distillate output
were recorded with the help of a calibrated mer- h Lv
Nu - ~ - C (GrPr) n (2)
cury thermometer having a least count of 0.1 °C Kv
and with a measuring cylinder of a least count
10 ml. The solar intensity was measured with the or
help of a calibrated solarimeter of a least count of
x
2 mW/cm z. The hourly variation of solar inten-
sity, water, glass and ambient temperatures and hcw=EC(G,'e,')"
hourly output for different depths of water in
solar still were used to evaluate average values of where Gr and Pr are the Grashof and Prandtl
each parameter for numerical computation. The numbers, respectively.
average values of each measured parameter are The distillate output (in kg) from the distiller
given in Tables 1-3 for passive as well as active unit can be obtained by the relation
modes. The average values of solar intensity and
ambient temperature have not been used in the
qew Aw t
present paper. However, by using the present rhea, - (3)
L
results, these values will be used in thermal
modeling of passive and active solar stills. The where
photograph of the active solar distillation system
is shown in Fig. 1(c).
The following observations were made on the O~=h~w(T.,-T~i)=O.O1623h~w(Pw-Pci) (4)
basis of the experiments:
190 R. Tripathi, G.N. Tiwari / Desalination 173 (2005) t87-200

Table Ia
Various measured temperatures and yield in passive mode for 0.05 m water depth in the basin for each hour interval

S. no. Time interval, h Solar intensity, W/m2 Tw, °C Wci,°C Teo, °C Tv, °C Ta, °C rhew , 1
1. 9-10 340 25.1 28.95 24.4 27.0 11.0 0.0385
2. 10-11 480 47.05 46.2 33.2 46.0 14.0 0.1335
3. 11-12 560 60.7 56.9 42.35 58.5 16.0 0.32
4. 12-13 590 69.2 58.0 46.65 63.0 17.5 0.54
5. 13-14 550 69.7 56.75 44.9 63.1 19.0 0.64
6. 14-15 450 62.0 53.4 42.4 57.1 20.5 0.57
7. 15-16 310 56.t 45.9 39.1 50.6 20.0 0.45
8. 16-17 110 47.8 34.95 30.1 41.2 18.0 0.345
9. 17-18 0 39.45 27.55 21.4 33.0 16.0 0.2
10. 18-19 0 33.5 23.15 18.05 28.1 14.5 0.11
11. 19-20 0 29.9 19.25 15.45 24.3 13.0 0.085
t2. 20-21 0 25.85 17.0 13.75 21.1 11.5 0.057
13. 21-22 0 22.2 15.05 12.1 18.4 11.0 0.041
14. 22-23 0 19.65 13.55 10.65 16.3 I0.0 0.0335
15. 23-24 0 18.5 12.45 9.7 15.5 9.0 0.026
16. 24-1 0 17.2 11.35 9.0 14.3 8.5 0.0225
17. 1-2 0 t5.95 10.5 8.6 13.1 8.0 0.0195
18. 2-3 0 14.8 9.9 8.45 12.2 7.5 0.016
19. 3--4 0 14.1 9.6 8.15 11.7 7.0 0.0145
20. 4-5 0 13.3 9.35 7.85 11.2 7.0 0.013
21. 5~ 0 12.5 8.8 7.6 10.5 7.0 0.0115
22. 6-7 0 12.05 8.75 7.75 10.3 7.0 0.0105
23. 7-8 0 11.95 9.55 8.55 10.8 7.5 0.01

Table lb
Various measured temperatures and yield in active mode for 0.05 m water depth in the basin for each hour interval

S. no. Time interval, h Solar intensity, W/m2 T,,, °C Tc~,°C Too, °C Tv, °C To, °C rhw, I

1. %10 340 30.85 31.15 28.4 30.5 12.0 0.0325

2. 10-11 500 55.15 47.45 40.75 51.0 14.5 0.2075
3. 11-12 580 68.8 60.05 47.0 64.0 17.0 0.5
4. 12-13 590 73.8 64.7 49.25 69.1 19.0 0.725
5. 13-14 440 73.6 64.75 49.3 69.0 21.0 0.865
6. 14-15 280 70.3 60.2 46.2 65.0 22.5 0.83
7. 15-16 100 62.7 52.4 41.35 57.1 21.5 0.63
8. 16-17 0 53.6 45.15 35.95 49.2 19.5 0.425
9. 17-18 0 46.1 36.25 27.9 41.0 17.0 0.265
10. 18-19 0 38.55 27.4 20.15 33.0 13.5 0.17
11. 19-20 0 32.3 21.7 16.4 27.2 11.0 0.11
12. 20-21 0 27.7 17.95 14.0 23.0 9.5 0.0715
13. 21-22 0 24.5 16.1 12.7 20.1 8.5 0.0485
14. 22-23 0 22.05 14.1 11.35 17.9 8.0 0.04l
15. 23-24 0 19.7 12.5 9.9 16.3 7.5 0.034
16. 24-1 0 17.9 11.3 8.85 14.3 6.5 0.0265
17. 1-2 0 16.5 10.5 8.25 13.2 6.0 0.0215
18. 2-3 0 15.55 9.9 7.95 12.5 6.0 0.0185
19. 3-4 0 14.6 9.4 7.5 11.9 5.5 0.016
20. 4-5 0 13.55 8.95 6.85 11.3 5.0 0.014
21. 5-6 0 12.75 8.6 6.45 10.4 4.5 0.0125
22. 6-7 0 12.25 8.35 6.4 10.0 4.0 0.012
23. 7-8 0 11.85 8.85 7.05 10.4 5.0 0.0115
 R. TripathL G.N. Tiwari / Desalination 173 (2005) 187-200 191

Table 2a
Various measured temperatures and yield in passive mode for 0.1 m water depth in the basin for each hour interval

S. no. Time interval, h Solar intensity, W/m2 Tw, °C T~,, °C Too, °C Tv, °C To, °C rhew, 1

1. 9-10 310 19.3 26.55 23.95 22.0 12.0 0.0235

2. I0-11 440 27.4 33.6 31.75 30.1 15.0 0.0885
3. 11-12 510 40.9 39.0 37.4 39.5 17.0 0.225
4. 12-13 540 48.7 44.25 40.0 46.2 18.5 0.39
5. 13-14 480 52.1 48.0 39.6 50.0 20.5 0.49
6. 14-15 370 56.6 52.85 37.15 54.5 21.5 0.545
7. 15-16 250 54.7 46.8 33.65 50.3 20.5 0.465
8. 16-17 90 50.15 38.95 28.8 44.6 19.5 0.37
9. 17-18 0 45.4 32.1 25.05 38.4 18.0 0.275
10. 18-19 0 42.2 29.1 22.1 35.8 16.0 0.185
11. 19-20 0 38.3 26.4 19.6 32.5 14.5 0.17
12. 20-2l 0 34.6 23.35 17.85 29.0 13.5 0.13
13. 21-22 0 32.0 21.05 16.6 26.3 12.0 0.095
14. 22-23 0 29.45 19.35 15.3 24.2 11.0 0.09
15. 23-24 0 27.6 17.6 13.8 22.5 10.5 0.08
16. 24-1 0 25.9 16.35 12.7 21.2 10.0 0.059
17. 1-2 0 23.8 15.35 12.2 19.6 9.5 0.048
18. 2-3 0 22.35 14.35 11.75 18.5 9.0 0.0455
19. 3-4 0 21.3 t3.65 11.15 17.5 8.5 0.04
20. 4-5 0 20.1 12.95 10.4 16.3 8.0 0.0355
21. 5-6 0 18.9 12.25 9.9 15.6 8.0 0.0285
22. 6-7 0 17.9 11.6 9.75 14.9 8.0 0.025
23. 7-8 0 17.4 12.0 10.6 14.8 8.5 0.022

Table 2b
Various measured temperatures and yield in active mode for 0.1 m water depth in the basin for each hour interval

S. no. Time interval, h Solar intensity, W/m 2 Tw, °C Tci, °C Tco, °C Tv, °C T., °C me~", 1

1. 9-10 350 26.75 29.7 25.8 28.1 13.0 0.03

2. 10-11 510 44.95 39.8 31.6 42.0 15.5 0.115
3. 11-12 620 58.5 49.75 38.5 54.3 18.5 0.24
4. 12-13 650 66.0 57.9 43.0 62.0 20.5 0.485
5. 13-14 610 70.0 62.85 44.0 66.1 22.5 0.64
6. 14-15 510 70.2 61.05 42.5 65.5 24.0 0.685
7. 15-16 330 65.9 55.75 39.5 61.0 23.0 0.65
8. t6-17 110 60.5 50.75 37.0 55.5 21.5 0.52
9. 17-18 0 54.0 43.0 30.75 48.4 19.0 0.44
10. 18-19 0 47.2 33.95 24.0 41.0 15.0 0.32
I 1. 19-20 0 42.8 29.7 21.25 36.0 12.0 0.205
12. 20-21 0 39.05 26.9 18.65 33.1 10.5 0.t6
13. 21-22 0 35.4 23.15 16.5 30.1 9.5 0.14
14. 22-23 0 32.45 20.0 14.85 26.3 8.5 0.115
15. 23-24 0 29.8 18.15 13.5 24.0 8.0 0.095
16. 24-I 0 27.25 16.7 12.5 22.0 7.5 0.0735
17. 1-2 0 25.15 15.4 11.75 20.4 6.5 0.0615
18. 2-3 0 23.6 13.95 11.5 19.0 6.0 0.053
19. 3-4 0 22.3 12.9 9.9 17.5 5.5 0.045
20. 4-5 0 21.0 12.3 9.15 16.5 5.0 0.035
21. 5-6 0 19.6 11.7 8.8 15.5 5.0 0.028
22. 6-7 0 18.4 11.2 8.3 14.9 5.0 0.0255
23. 7-8 0 17.5 11.8 9.2 14.7 6.0 0.024
192 R. Tripathi, G.N. Tiwari / Desalination 173 (2005) 187-200

Table 3a
Various measured temperatures and yield in passive mode for 0.15 m water depth in the basin for each hour interval

S. no. Time interval, h Solar intensity, W/m 2 Tw, °C Tc~, °C Too, °C rv, °C T,,, °C mew", l

1. 9-10 300 26.2 31.4 27.5 28.5 15.0 0.0225

2. 10-11 470 33.1 39.0 33.75 36.2 18.0 0.0545
3. 11-12 580 43.05 47.55 37.75 45.0 21.0 0.112
4. !2-13 580 54.75 52.85 41.4 53.5 23.0 0.205
5. 13-14 560 61.85 56.25 43.9 59.2 24.5 0.31
6. 14-15 450 63.9 57.15 43.8 60.7 26.0 0.405
7. 15 16 245 60.6 54.9 41.45 58.0 25.5 0.43
8. 16-17 75 55.5 48.4 37.15 52.0 23.5 0.368
9. 17-18 0 51.5 42.4 33.15 46.7 20.5 0.298
10. 18-19 0 48.9 38.2 29.95 44.0 17.0 0.27
11. 19-20 0 46.2 34.2 26.9 40.6 15.0 0,24
I2. 20-21 0 43.5 32.15 24.1 38.2 13.5 0.205
!3. 21-22 0 40.4 29.75 21.7 35.4 12.5 0.18
14. 22-23 0 37.25 27.3 20.0 32.5 l 1.5 0.16
t5. 23-24 0 35.25 25.65 1%2 31.0 11.0 0.125
16. 24-I 0 33.65 24.15 18,25 29.0 11.0 0.105
17. 1 2 0 32.05 22,95 16,75 27.8 i0.5 0.095
18. 2-3 0 30.6 21.9 15,5 26.5 t0.0 0.085
19. 3-4 0 28.9 20.55 14.8 25,0 10.0 0.075
20. 4-5 0 27.35 19.4 14.1 23.8 9.5 0.065
21. 5-6 0 26.05 18.4 13.4 22.5 8.5 0.06
22. 6-7 0 24.7 17.55 12.9 21.0 8.5 0.06
23. 7-8 0 23.6 18.15 13.55 20.5 9.5 0.055

Table 3b
Various measured temperatures and yield in active mode for 0.15 m water depth in the basin for each hour interval

S. no. Time interval, h Solar intensity, W/m z T,,, °C Tci, °C T~o, °C Tv, °C T,, °C m', 1

1. 9-10 360 30.0 32.0 24.6 30.5 16.5 0.0225

2. I0-11 520 39.65 41.3 32.2 40.0 19.5 0.0595
3. 11-12 650 51.85 49.45 40.4 50.0 22.5 0.137
4. ~2-13 690 61.1 55.75 45.8 58.1 24.0 0.29
5. 13-14 610 65.4 59.6 46.5 52.0 25.0 0.45
6. 14-15 480 67.6 60.1 45.55 63.5 26.5 0.55
7. i5-16 320 66.4 57.55 42.75 61.5 26.0 0.565
8. 16-17 120 61.65 52.8 39.5 57.5 24.5 0.505
9. 17-18 0 56.0 46.5 36.0 51.0 23.0 0.435
10. 18-19 0 51.1 41.25 30.5 46.0 21.0 0.335
i 1. 19-20 0 48.6 38.05 27.55 42,9 19.0 0.245
12, 20-21 0 46,0 34.8 26.55 40,0 17.5 0,215
13. 21-22 0 42.3 31.5 25.3 37.0 16.0 0.19
14, 22-23 0 39.2 28.9 24.0 34,2 14.0 0.16
15. 23-24 0 36.6 26.8 21.4 32.0 13.0 0.13
16. 24-1 0 34.4 25.5 18.45 30.0 13.0 0.115
17. 1-2 0 32.7 24.35 16.85 28.0 13.0 0.105
18. 2-3 0 31.25 23.1 16.05 27.0 12.5 0.095
19. 3-4 0 29.75 21.9 15.75 26.0 12.0 0.085
20. 4-5 0 28.25 21.0 15.3 25,0 12.0 0.075
21. 5-6 0 26.75 20.35 14.65 24.0 l 1,5 0.065
22. 6-7 0 25,5 20.0 14.8 23.0 I 1.5 0.055
23. 7-8 0 24.5 20.55 16.7 22.5 t2.5 0.045
 R. TripathL G.N. Tiwari / Desalination 173 (2005) 187-200 193

Using Eqs. (2)-(4), one gets, interval has been used to evaluate C and n by
regression analysis [Eq. (5)].
rh Fig. 2 shows the effect of the inner and outer
c (Grer) ° (5) condensing cover temperature on the internal
R
convective heat transfer coefficient for a parti-
where cular water depth in the basin. It can be seen that
there is a significant difference in convective and
evaporative heat transfer coefficients due to inner
R - 0.01623 K~ A~ t~°w-Pci) and outer glass cover temperatures. This differ-
L Lv
ence is more dominant in the active mode of
After calculating the coefficients C and n in operation due to a large difference between inner
Eq. (5) by using linear regression analysis, the and outer glass cover temperatures for a 0.15 m
convective heat transfer coefficient (hcw)is evalu- water depth.
ated from Eq. (2), which is free from various From now on all results for convective and
limitations as contained in the relation obtained evaporative heat transfer coefficients will be dis-
by Dunkle [9], which is given by cussed by considering the inner condensing cover
temperature.
+ ~ ~1T +27 ~x]l/3 The results for internal convective and
hcw-'-0.884 rw-Tci ~w-F_-_ci)_~lw_ZlS)
I evaporative heat transfer coefficients for passive
268.9x103-Pw ] (6) and active solar stills are presented in Figs. 3-5.
The effect of water depth on the internal convec-
The evaporative heat transfer coefficient is given tive heat transfer coefficient for the passive and
by the following expression: active modes is shown in Figs. 3 and 4, respect-
ively. For comparison, the results for the con-
vective heat transfer coefficient obtained by
h =0.01623 ~ C(GrPr)"( Pw-Pci (7) Dunkle's relation [Eq. (6)] are also shown in the
ew Lv ~ Tw- Li same figures. These figures indicate that the inter-
nal convective heat transfer coefficient decreases
with water depth due to decrease in water
temperature. The trends are the same in both the
4. Results and discussion
passive and active mode. Further, it is important
From Tables I-3, it can be verified that the to note that the fluctuation in the internal con-
vapor temperature is approximately an average of vective heat transfer coefficient decreases with an
water and inner condensing cover temperature for increase of water depth due to storage effect.
the passive as well as the active mode of opera- The results of Figs. 3 and 4 for the convective
tion of the solar still. Further, from the same heat transfer coefficient have been used in Eq. (5)
tables it is clear that the hourly water temperature to evaluate evaporative heat transfer coefficient
and yield are also a strong function of water for different water depths in the basin. The
depth for both modes of operation. Hence it is variations of evaporative heat transfer coefficient
important to note that inner glass cover temp- with time of the day for a passive and active solar
erature should be taken into account for studying still for different water depths are shown in
internal convective heat transfer coefficient for a Fig. 5a and 5b, respectively. The value of the
given water depth. evaporative heat transfer coefficient is greater in
The average values of yield, water, inner and the active mode due to a higher operating temp-
outer glass cover temperatures for each hour erature range for a given water depth as expected.
194 R. Tripathi, G.N. Tiwari / Desalination 173 (2005)]87-200

2.2 I I I I i i I ] J ~ I I I 35
hew(Tel)
hcw(Tco)
2.1
O + hew(To i) 30

E
v "e---e.... e ~.

O 19

20
8
--1
C

"~ 1.7
03 o_.
r--
Ill
> ~6
~d
t-
O

/ 2 3 4 5 6 7 8 9 18 11 12 t3 14 15 16 17 18 19 20 21 22 23 24

Time (h)

Fig. 2a. Hourly variation of convective and evaporative heat transfer coefficient in passive mode for inside and outside
glass cover temperature at 0.15 m depth.

2.0 ,5
- 0 - hew(Tci)
2.6I -El- hcw(Tco) 40 m

2.4 i ----~ ....~..~.__o _..e..._~....e...~ ~ hew(Tco) 35 -o

2. 30 ~"

25

1. ~ 208
=~ , ~.
1.5 15
j

q) i "E3
5

i 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (h)
Fig. 2b. Hourlyvariationof convective and evaporativeheattransfercoefficient in active mode for inside and outsideglass
cover temperatureat 0.15 m depth.
 R. Tripathi, G.N. Tiwari / Desalination 173 (2005) 187-200 195

-k:5-" hew (PM)

-43- hcw(DM)
G
E 2,5
g

o 2
0

c
m
s.~_~_.~ s "h9
~ 1.5
r-

t-
O 1

0.5;
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (h)

Fig. 3a. Hourly variation of convective heat transfer coefficient in passive mode at 0.05 m depth.

_0_ hcw(PM)
""G....~.._~. -{3-- hcw(DM)
2.4

& 2.2

0
o 1.8 -"E&--.ta....tp....~

i l.6 %
r-
• 1.4
.>_

~ 1.2

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (h)

Fig. 3b. Hourly variation o f convective heat transfer coefficient in passive mode at 0.10 m depth.
196 R. Tripathi, G.N. Tiwari / Desalination 173 (2005) 187-200

2.6 ~IIIIIII~I~FILII~jlII
-ff~ hcw(PM)
2.4
-E3- hew(DM)

&
E 2.2

~ 1.8~
e-
~ 1.6

~ 1.4

g 1.2
0
1

0.~ It IIIIIII1}111111~1111
2 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (h)
Fig. 3c. Hourly variation of convective heat transfer coefficient in passive mode at 0.15 m depth.

hew(PM)
-El- hew(DM)

~ 2.5
&0
E

E 2

8
~ 1.5

o
(.J
0.5

01 2I 3I 4I 5I 6I 7I 3I 9I 110 111 112 113 I~4 115 I16 117 118 119 210 211 212 23 24

Time (h)

Fig. 4a. Hourly variation of convective heat transfer coefficient in active mode at 0.05 m depth.
 R. Tripathi, G.N. Tiwari / Desalination 173 (2005) 187-200 197

2.6

2.4 -43- hcw(DM)

(,,3

~E 2,2 "~E3"--E~

~o 1.8

i 1.6

e--

"5 1.2
>
t-
O
0 1

0.,8
[
0.6 I I I t I I I I I L I i I I I I I I I i I l
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (h)
Fig. 4b. Hourly variation of convective heat transfer coefficient in active mode at 0.10 m depth,

3[ ~ i i i ~ : ( i ~ i i i i i i i i I i i i I i

hcw(PM)
hc~(DM)

2.5 r~ "~-"-G

~ (- ~

~ 1.5
e-

0g 1

0.5 E i I m
i i i i p ii0 i18 t3
2 3 4 5 6 "7 8 9 11 12 13 14 15 16 17 19 20 21 22 2 24
Time (h)
Fig. 4c. Hourly variation of convective heat transfer coefficient in active mode at 0.15 m depth.
198 R. Tripathi, G.N. Tiwari / Desalination 173 (2005) 187-200

4 ~ l l l l l l l l l l l l l l l l l l 1 I I I

--e,- hew(5 era)

hew(10 cm)
3 40
& hew(15 cm)
E
~ 35
v

~ 30

o 25 /

~ 2o

~~ 15 ~

0i1 2 ; ,~ 5 6 ; ; ; 110 1~1 lk2 1~3 1'4 115 116 117 118 119 210 211 2'2 2~3 24
Time (h)
Fig. 5a. Hourly variation of evaporative heat transfer coefficient in passive mode for different depths.

60 I I I I I T I I I I I [ I I I I l I I I I

-e- %w[5 cm)

~=- %w[10 cm)
~" -0- hew(15 cm)
& 5o

~ 40

~ 3o
E

x: 20
.I

O
e~

> lO

t 1 i I I P 1 I I F
2 3 4 5 6 7 8 9 10 1'1 1'2 13 114 115 1'6 117 118 19 210 211 22 23 24
Time (h)
Fig. 5b. Hourly variation of evaporative heat transfer coefficient in active mode for different depths.
 R, TripathL G.N. Tiwari / Desalination 173 (2005) 1 8 7 - 2 0 0 199

2.8 ~ _ E ~ " " 45

hew(passive)
2.7 hcw(active) : 40
--e- hew(passive) m
~E 2.6 ~ -~- hew(aCtive) •<

2.5 --~9~
._~ \ 30

2.4
--1EL._El 25 =
0~
/ \ 20 8
"~ 2.2
m :15 ~'
~= 2.1
e-
0
!10
2 c;
1.9, ~,~ :5

1.8 ~ 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (h)
Fig. 5c. Hourly variation of convective and evaporative heat transfer coefficient in passive and active mode for 0.15 m
depth.

(Fig. 5c). Further, it is important to note that the C -- Unknown constant in Nusselt
evaporative heat transfer coefficient becomes number expression
more for a 0.15 m depth due to the increased Cp -- Specific heat, J/Kg°C
value of solar intensity on a particular day of g -- Acceleration due to gravity, m/s 2
experimentation (Fig. 5a and b). Gr -- Grashofnumber
hew Convective heat transfer coeffi-
cient from water to condensing
5. Conclusions cover, W / m 2, °C
h ew Evaporative heat transfer coeffi-
The present study indicates the importance of
cient, W/m 2 °C
a variation of the convective heat transfer coeffi-
Thermal conductivity of humid
cient with water depth in the basin. This will be
air, W / m 2 °C
useful to design efficient passive as well as active
L Latent heat of vaporization of
solar distillation systems.
water, J/kg
Lv Characteristic dimension of con-
densing cover, m
6. Symbols
ffl ew Distillate output, kg
A Surface area, m 2 1"1 Unknown constant in Nusselt
Aw Area of water surface, m 2 number expression
200 R. TripathL G.N. Tiwari / Desalination 173 (2005) 187-200

G Partial saturated vapor pressure References

at condensing cover temperature,
[1] M.A.S. Malik, A. Kumar and M.S. Sodha, Solar
N/m 2
Distillation, Pergamon Press, Oxford, UK, 1982.
PF Prandtl number [2] T. Kiatsiriroat, P. Wibulswas and S.C. Bhattacharya,
Pw Partial saturated vapor pressure Performance analysis of multiple effect vertical solar
at water temperature, N/m2 still with a fiat plate solar collector, J. Solar Wind
0 Rate of heat transfer by con- Technol., 4(4) (1987) 451.
vection, W [3] G.M. Zaki, A. A1-Turki and M. AI-Fatani, Experi-
Rate of evaporative heat trans- mental investigation on concentrator assisted solar
fer, W/m 2 stills. J. Solar Energy, 11 (1992) 193-199.
t Time, s [4] G.N. Tiwari, Recent Advances in Solar Distillation,
Inner temperature of condensing Wiley Eastern, New Delhi, India, 1992, Chap. 2.
L [5] S. Kumar and G.N. Tiwari, Estimation of convective
cover, °C
mass transfer in solar distillation systems, Solar
rco Outer temperature of condensing Energy, 57 (1996) 459.
cover, °C [6] G.N. Tiwari, S.K. Shukla and I.P. Singh, Computer
r, Evaporative surface temperature, modeling of passive/active solar stills by using inner
°C glass temperature, Desalination, 154 (2003) 171.
rv Vapor temperature, *C [7] G.N. Tiwari, H.N. Singh and R. Tripathi, Present
rw Water temperature, °C status of solar distillation, Solar Energy, 75(5) (2003)
1". Temperature of the boundary 367-373.
layer away from the evaporating [8] E. Delyannis, Historic background of desalination
surface, °C and renewable energies, Solar Energy, 75(5) (2003)
357-366.
[9] R.V. Dunkle, Solar water distillation; the roof type
Greek still and multiple effect diffusion still, Intemational
Developments in Heat Transfer, ASME, in Proc.
Coefficient of volumetric ther- International Heat Transfer, Part V, University of
mal expansion, K- Colorado, 1961.
9 Density of humid air, kg/m3
Dynamic viscosity of humid air,
N.S/m z
AT Temperature difference, °C