Experimental study of evaporation in a tubular solar still

INTERNATIONALMechanical Engineering and Technology (IJMET), ISSN 0976 –
International Journal of JOURNAL OF MECHANICAL ENGINEERING
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 4, Issue 2, March - April (2013), pp. 01-09 IJMET
© IAEME: www.iaeme.com/ijmet.asp
Journal Impact Factor (2013): 5.7731 (Calculated by GISI)
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EXPERIMENTAL STUDY OF EVAPORATION IN A TUBULAR
SOLAR STILL

Ajeet Kumar Rai*, Vivek Sachan, Bhawani Nandan
Mechanical Engineering Department
Sam Higginbottom Institute of Agriculture Technology and sciences, Allahabad
*Corresponding author e-mail- raiajeet@rediffmail.com

ABSTRACT

This paper presents the experimental and theoretical work conducted at Allahabad,
Uttar Pradesh, India to analyze the performance of a basin type tubular solar still. The tubular
cover of the still was made of PVC sheet. Design and fabrication of the setup was done in the
premises of SHIATS-DU, Allahabad. Outdoor experimentation has been carried out in the
month of April 2012. The objective of the study was to determine a relation for predicting
convective and evaporative heat transfer coefficients in a tubular solar still. 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 used in the expression
Nu = C (Gr.Pr)n. A good amount of distillate was recorded. It was obtained that the proposed
model gives closer results with the experimental observation than the model given by Dunkle.
The performance of a tubular solar still is improved by 166% than that of a double slope solar
still of the same basin area.

Key words: heat transfer coefficients, tubular solar still.

INTRODUCTION

The availability of potable water is a main problem for the communities who live in
arid new regions or especially for people in deserts. The availability of high intensity solar
radiation in these areas makes the direct use of solar energy a promising option. The solar
energy can be utilized to obtain drinking water from salty or brackish water through the use
of solar still. Solar distillation is one of the available methods for water distillation. Solar

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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

stills of different designs have been proposed and investigated with a view to get greater
distillate output. A basin type solar still is the most common among conventional solar stills.
Many experimental and theoretical studies have been done on single slope solar still [1]. The
oldest, semi- empirical internal heat and mass transfer relation is given by Dunkle [2]. 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. [3] has considered the values of C = 0.075 and n = 0.33 for Gr
> 3.2 x 105 as proposed by Dunkle. Clark [4] developed a model for higher operating
temperature range ( ≥ 55 0C) in a simulated condition for small inclinations of the condensing
surface (β ≤ 15 0C). Clark [4] has observed that the coefficient of convective mass transfer
becomes half that given by Dunkle [1]. Tiwari et. al. [5] 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 0C. Later on Kumar and Tiwari [6] 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 [7] developed a model for a high temperature range of
the order of 80 0C 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.
In this communication, an attempt has been made to evaluate the convective mass transfer by
a modified Nusselt number. The modified Nusselt number has been obtained by regression
analysis using experimental data.

EXPERIMENTAL SET-UP

A prototype solar still having a horizontal tray which acts as absorber of 0.77 m 2
was designed and constructed. Tray was constructed using galvanized iron sheet of
thickness 0.5 mm and later on painted in black. The tray is surrounded by tubular structure
made up of PVC sheet. The total area of the PVC cover is 3.52 m 2. The still is formed by a
tubular transparent surface made up of PVC sheet. Testing was performed by placing the
tubular solar still operating in sunlight for a 24-h period. The work has led to the
development of the tubular solar still and to a technical improvement. In order to achieve the
maximum yield from the system, the still orientation should be the direction at which the
highest average incident solar radiation is obtained. Experimental investigation of the tubular
solar still has shown that the productivity of the system was substantially increased in
comparison with that of the basin type solar still. The present study was concerned with the
design of new TSS and development of theoretical models based on evaporation from the
water surface and based on condensation on the inner surface of the tubular cover.
Copper – constantan thermocouples are used, along with a digital temperature indicator, to
record the glass temperature, water temperature and water vapor temperature in the
experimental setup. These thermocouples, over a prolonged usage period, tend to deviate
from the actual temperature. Therefore, they were calibrated with respect to a standard
thermometer. A view of the condensing chamber and photograph of the experimental set up
are shown in figure1.

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Fig.1. Photograph showing experimental setup Tubular solar still model.

ANALYSIS OF CONVECTIVE MASS TRANSFER

The moist air above the water surface is freely convected 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 ( Qcw ) by
convection in the upward direction through humid fluid can be given by

q cw = hcw (Tw − Tg ) (1)

The coefficient hcw can be determined form the relation

hcw d
Nu = = C (Gr. pr ) n (2)
k
The expression for Gr and Pr are given as
3 2 2
Gr = x 3 .ρ f .g.β '.∆T µ f (3)

C p .µ f
Pr = (4)
kf

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It is clear from the above equation that the value of hcw 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 hcw for normal operating temperature
range,

1/ 3
 ( p w − p g )(Tw + 273) 
hcw = 0.884(TW − Tg ) +  (5)
 (268.9 × 10 3 − p w ) 

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 given hereunder
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,

qew = 0.0163 hcw ( PW − Pg ) => qew = hew (Tw-Tg) (6)

Equation can be further written as
K
qew = 0.0163( PW − Pg )( )C ( Ra ) n (7)
d
where Ra = Gr.Pr

Further, the rate of distillate output is evaluated by
q
m ew = ew × 3600
& (8)
l
Equation (4.6) after substituting qew from Eq. (8) becomes,
K 3600
mew = 0.0163( PW − Pg )( )(
& )C ( Ra ) n (9)
d l
The above equation can be rewritten as,

mew = R C ( Ra ) n
& (10)

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&
mew
= C ( Ra ) n (11)
R

K 3600
Where, R = 0.0163( PW − Pg )( )( ) (12)
d l
Equation (11) can be rewritten in the following form

Y = aX b (13)

&
mew
Where Y = ; X = Ra ; a = C ; b = n ,
R

Equation (13) can be reduced to a linear equation by taking log on both the side

ln(Y ) = ln(a ) + b ln( X ) (14)

Y ' = a ' + b' X ' (15)

Y ' = ln(Y ); a ' = ln(a ); b ' = b; X ' = ln( X ) (16)

From Eq. (16), the values of coefficients a′ and b′ are calculated using regression analysis.
The expressions for a′ and b′ are given by:

N (ΣX ' Y ' ) − (ΣX ' )(ΣY ' )
b′ = (17)
N (ΣX ' 2 ) − (ΣX ' ) 2

ΣY ' ΣX '
a′ = − b' (18)
N N

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)

The experimental method used is an indirect approach for estimating the convective heat
transfer coefficient based on the mass of distillate collected from the still.

RESULTS AND DISCUSSION

Fig. 2 shows the solar intensity on a particular day in the month of April 2012. The
maximum intensity of 1190 W/m2 is received at 13:45 hrs. Water and cover temperatures are
nearly equal at the start of the experimentation, but due to green house effect water
temperature is higher than the cover temperature which is shown in fig 3.

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Fig. 2 Variation of solar intensity on 18-04-12

Fig. 3 Variation of Temperature with time

Fig.4 shows the variation of convective heat transfer coefficient obtained from present model
and than that of Dunkle model. hcw obtained from present model is higher than that of Dunkle
model.

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Fig.4. Variation in convective heat transfer coefficient

Evaporative heat transfer coefficient is plotted in fig 5. hew obtained from present model is
higher than that of Dunkle model. It increases with time of heating and starts decreasing as
solar flux declines after certain period of time. Maximum value of hew is at 13: 45 hrs. The
values of constants calculated for the present model are c = 1.017 and n = 0.210.

Fig. 5 Variation in evaporative heat transfer coefficient
Distillate output calculated by present model is having their values more closure to the
practical one than calculated using Dunkle model. This difference is due to the assumptions
made by Dunkle. This is shown in fig.6. So the present model is more accurate to predict the
performance of a tubular solar still than using Dunkle model. Earlier work on double slope
solar still [9,10] has given a maximum daily distillate collected from the same size of basin
area is in the range of 1-2 kg/m2. Whereas from this tubular solar still total distillate collected
from 24 hour is 3.2 kg. Instantaneous efficiency is also plotted in fig 7. Maximum value of
instantaneous efficiency is obtained as 79.63%. The overall efficiency the system is obtained
as 46.35%.

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Fig. 6. Variation in distillate output

Fig. 7. Variation of instantaneous efficiency

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CONCLUSION

An experimental work has been conducted to find the performance of a Tubular solar
still. It is observed that the Dunkle model is not accurate in estimating the performance of a
solar still, because of the assumptions made by Dunkle. From the present work it is therefore
inferred that the evaporative heat transfer coefficients are important for designng solar
distillation systems. It is also observed that tubular solar still of the proposed design gives
better results than the double slope slope solar still of the same basin area.

REFERENCES

1. Tiwari G. N., Tiwari A, Solar Distillation Practice for Water Desalination Systems,
Anamaya, New Delhi.
2. Dunkle, R.V., Solar water distillation: The roof type still and multiple effect diffusion
still. Int. Development in Heat Transefer, ASME, Proc. Int. Heat Transfer. Part V,
University of Colorado. 1961, P.895.
3. Malik, M.A.S., Tiwari. G.N., Kumar, A. and Sodha, M.S.,Solar Distillation.
Pergamon Press Ltd, UK, 1982.
4. Clark, J. A., The steady state performance of a solar still. Solar Energy, 1990, 44, 43.
5. G. N. Tiwari, A. Minocha, P. B. Sharma and K. M. Emran, Simulation of convective
mass transfer in a solar distillation process, Energy conv. Mgment., 38(8) (1977) 761-
770.
6. S. Kumar and G.N. Tiwari, Estimation of convective mass transfer in solar distillation
system, Solar Energy, 57 (1996) 459-464.
7. G. N. Tiwari and R. Tripathi, Study of heat and mass transfer in indoor condition for
distillation, Disalination, 154(2003) 161-169.
8. B. C. Nakra and K. K. Chaudhary, Instrumentation Measurements and Analysis, Ist
ed., Tata McGraw Hill, New Delhi, 1985. P. 33.
9. Shukla S.K., and Rai A. K. 2008, Analytical Thermal Modelling of Double Slope
Solar Still by Using Inner Glass Cover Temperature, Thermal Science: 12(3) 139-152.
10. Ajeet Kumar Rai, Ashish Kumar and Vinod Kumar Verma, “Effect of water depth
and still orientation on productivity of passive solar still”, International Journal of
Mechanical Engineering and Technology (IJMET), Volume 3, Issue 2, 2012,
pp 740-753. Published by IAEME.
11. Ajeet Kumar Rai, vivek Sachan and Maheep Kumar, “Experimental Investigation of
a double slope solar still with a latent heat storage medium”, International Journal of
Mechanical Engineering and Technology (IJMET), Volume 4, Issue 1, 2013,
pp 22-29. Published by IAEME.
12. Hitesh N Panchal, Dr. Manish Doshi Anup Patel and Keyursinh Thakor,
“Experimental Investigation on Coupling Evacuated Heat Pipe Collector on Single
Basin Single Slope Solar Still Productivity”, International Journal of Mechanical
Engineering and Technology (IJMET), Volume 2, Issue 1, 2011, pp 1 - 9. Published
by IAEME.

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Experimental study of evaporation in a tubular solar still

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