effect of water depth and still orientation on productivity of passive solar still

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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 6340(Print), ISSN 0976 6359(Online) Volume 3, Issue 2, May-August (2012), IAEME

740

EFFECT OF WATER DEPTH AND STILL ORIENTATION ON

PRODUCTIVITY OF PASSIVE SOLAR STILL

Ajeet Kumar RAI*, Ashish KUMAR, Vinod Kumar VERMA

Department of Mechanical Engineering & Applied Mechanics,

SHIATS-DU, Allahabad 211007, India*Email: [email protected]

ABSTRACTIn this communication, an attempt has been made to study the effect of parametric

variations on the performance of a passive solar distillation system. A double slope solar still wasfabricated and investigations were carried out under the open environment of Allahabad, India.

Experiments were conducted by varying water depth in the basin as 1.5 cm, 2.5 cm and 3.5 cm

and for two different still orientations. The heat transfer coefficients are evaluated and theirvariation is studied. Results show a gain of 60 to 65% in distillate output when the still was

oriented towards North-South direction. A maximum loss of 43% has been observed when thebasin water depth was increased from 2.5 cm to 3.5 cm.

Key words: Solar distillation, Double slope solar still, Heat transfer coefficients.

1. INTRODUCTIONWater is the fundamental source for the survival of mankind but its not available in the ready to use

form. According to the study made by the World Health Organization, polluted water and sanitation

deficiency are the cause of 80% of all the diseases which make a person unfit, temporarily or even

permanent. It has been estimated that around 500 million people in the developing countries suffer fromdiseases produced by water [1]. Thus an effective harnessing system is required to produce the water in

consumable form. It is the Salinity of water which makes desalination an important phenomenon. One can

opt for any process available for the same purpose. Out of those various processes here passive solar

distillation method, being cost effective and eco friendly, has been exercised on.

In open environment solar still has to work under some parameters which tremendously affect its

performance and productivity. These parameters can be divided in two categories, metrological

parameters and non-metrological parameters. The former one, which cannot be controlled by human

efforts, constitutes with solar intensity, wind velocity and ambient temperature whereas the later one, also

known as controllable parameters, counts for water-glass temperature difference, free surface area of

water, absorber plate area, temperature of inlet water, glass angle, still orientation and depth of water. In

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND

TECHNOLOGY (IJMET)

ISSN 0976 6340 (Print)

ISSN 0976 6359 (Online)

Volume 3, Issue 2, May-August (2012), pp. 740-753

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the present work, still orientation and depth of water has been selected as the variables for productivity

analysis.

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 heat and mass transfer relation was

given by Dunkle[2]. To predict the hourly and daily distillate output from different designs of solardistillation, numerous empirical relations were developed later on. Most of these relations are based on

simulation studies. Malik et al.[3] has considered the values of C=0.075 & n=0.33 for Gr > 3.2x105

,as

proposed by Dunkle. However, the relation developed by Dunkle has the following limitations:

a) It is valid for a low operating temperature range (45-500C).b) It is independent of the cavity volume, i.e. the average spacing between the condensing andevaporative surfaces.

c) It is valid for cavities that have parallel condensing and evaporative surfaces.Lof et. al[4] have analyzed heat and mass transfer of a solar still in detail and studied the effect of various

design parameter and climatic variables on the performance of solar still. Numerical solution of the heat

balance equations were obtained with the aid of a digital computer. Morse et a[5]l included the thermal

capacity of the system and accordingly carried out a transient analysis. They have expressed various heatfluxes as the functions of the glass cover temperature. Thus the glass temperature has been obtained by agraphical solution. Kumar et al[6] has done thermal and computer modeling for determining heat and

mass transfer coefficient namely C and n for different type of solar still. Sharma et al[7] developed a

method for estimation of heat transfer coefficients upward heat flow and evaporation in still. Calculation

of hourly output was done with a new approach. It was observed that the performance of solar still has an

agreement with the result of an analysis based on Dunkles relation with a factor of 0.65 to account for

instauration.

Shukla et al[8] has recently developed a model, based on regression analysis, to determine the values of C

and n using the experimental data obtained from the stills. This method uses both inner and outer glass

cover temperatures to determine the expressions for internal heat transfer coefficient and does not impose

any limitations.

Singh and Tiwari[9] found that the annual yield of the solar still was maximum when the condensing

glass cover inclination was equal to the latitude of the place. The effect of varying water depths of water

in the solar still is verified by Khalifa and Hamood[10]. Rubio-Cerda et al studied performance of the

condensing covers under two still orientation, east-west and north-south[*****].Their results showed

larger difference in the condensers temperature and higher productivity when the still covers were facing

east-west.

In this paper an attempt has been made to find the most suitable water depth and still orientation for

maximum yield from a double slope solar still. The convective and evaporative heat transfer relations are

also determined for three different water depths of 0.015m, 0.025m and 0.035m and different orientations

for a fixed inclination of 260

at Allahabad in summer climatic conditions. The values of C and n are

determined by the model proposed by Shukla and Rai[11]. The modified Nusselt number has been

obtained by regression analysis.

2. EXPERIMENTAL SET-UP AND PROCEDURE2.1 Set-up

Figure 2.1 shows the photograph and schematic diagram of a double slope solar still. The experimental

setup consists of a passive solar distillation unit with a glazing glass cover inclined at 260

having an area


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of 0.048m x 0.096 m. This tilted glass cover of 3 mm thickness, served as solar energy transmitter as well

as a condensing surface for the vapor generated in the basin. Glass basin, made up of Galvanized Iron, has

an effective area of 0.72 m2. The basin of the distiller was blackened to increase the solar energy

absorption. A distillate channel was provided at each end of the basin. For the collection of distillate

output, a hole was drilled in each of the channels and plastic pipes were fixed through them with an

adhesive (Araldite). An inlet pipe and outlet pipe was provided at the top of the side wall of the still and atthe bottom of the basin tray for feeding saline water into the basin and draining water from still for

cleaning purpose, respectively. Rubber gasket was fixed all along the edges of the still. All these

arrangements are made to make the still air tight. Water gets evaporated and condensed on the inner

surface of glass cover. It runs down the lower edge of the glass cover. The distillate was collected in a

bottle and then measured by a graduated cylinder. The system has the capability to collect distillates from

two sides of the still (i.e. East & West sides and North & South sides). Thermocouples were located indifferent places of the still. They record different temperature, such as inside glass cover & water

temperature in the basin and ambient temperature. In order to study the effect of salinity of the water

locally available, table salt was used at various salinities. All experimental data are used to obtain the

internal heat and mass transfer coefficient for double slope solar still.

2.2

Procedure

The experiments were conducted on different days in the campus of Sam Higginbottom Institute ofAgriculture and Sciences Deemed University, Allahabad, India for three different water depths and two

different orientations. All experiments were started at 09:00 hours by local time and lasted for 8 hours.

Prior to start with the next depth, the still was left idle minimum for a period of one day and the same

procedure was adopted for all three water depths. The following parameters were measured hourly for a

period of 8 hours.

Inner glass temperature Vapor temperature Water temperature

Ambient temperature Distillate output Solar intensity

Water, glass and vapor temperatures were recorded with the help of calibrated copper constant

thermocouples and a digital temperature indicator having a least count of 10C. The ambient temperature is

measured by a calibrated mercury (ZEAL) thermometer having a least count 10C. The distillate output

was recorded with the help of a measuring cylinder of least count 1 ml. The solar intensity was measured

with the help of calibrated solarimeter of a least count of 2mW/cm2. The hourly variation of all above

mentioned parameters were used to evaluate average values of each for further numerical computation.

A Turbo C++ program was used to calculate the values of hcw , hew and the values as proposed by Dunkle.

The hourly difference in water and inner glass temperature, i.e. T is also shown in figs for all concerned

water depth. It is explicit that the fluctuation in water temperature decreases with increase of water depthsdue to storage effect as expected. Further the maximum of this temperature shifted to later hours for

higher depths.

3. GOVERNING EQUATIONS

Convective heat transfer is given by:

Qcw=hcw.A.(Tw-Tg) (3.1)


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Where hcw is convective heat transfer coefficient. Since the convection inside the solar still is only due to

the difference in temperatures between the water surface and the condensing cover, so this falls under the

category of free convection. So hcw can be determined by:

Nu=hcw*Lv/= C(Gr.Pr)n (3.2)

Or, hcw=k.C.(Gr.Pr)n/Lv (3.3)

Gr=g2Lv

3T/

2(3.4)

Pr=Cp/ (3.5)

The unknowns C and n constants, given in Eq.(3.2) can be determined by regression analysis using

experimental data and following the Shukla and Rai model (2008).

Convective heat transfer coefficient can also be calculated by a relation as proposed by Dunkle:

hcw=.884[(Tw-Tg)+(Pw-Pg)(Tw+273)/268.9x103-Pw]

1/3 (3.6)

Evaporative heat transfer is given by:

Qew=hew.A.(Tw-Tg) (3.7)

Where hew is known as evaporative heat transfer coefficient. It can be evaluated as:

hew=Qew/(Tw-Tg) (3.8)

Alternatively,

hew=.01623.hcw.(Pw-Pg)/(Tw-Tg) (3.9)

It is worth mentioning here that only evaporative heat transfer causes and contributes to water distillation.

Thus mass of water distilled can be calculated by knowing the evaporative heat transfer rates:

mew= Qew.A.t/hv (3.10)

from eq (3.3), (3.7) and (3.9);

mew=.01623..A.t.(Pw-Pg).C(Gr.Pr)n/hv.Lv (3.11)

eq (3.11) can be rewritten as :

mew=R.C(Gr.Pr)n (3.12)

or,

mew/R=C(Gr.Pr)n (3.13)

where,

R=.01623..A.t.(Pw-Pg)/hv.Lv (3.14)

Taking the logarithm to both sides of eq. (3.13) and comparing it with the straight line equation,

y=mx+c (3.15)


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744

We get,

y= ln(mew/R), Co=lnC, x=ln(Gr.Pr) and m=n (3.16)

By using linear regression analysis, the coefficient in eq(3.15); m, and Co can be obtained by the

following expression:

=

=

Where N is number of experimental observations for steady state condition and become N+1 in quasi

steady condition as in the case of this experiment.

The constant m and Co can be evaluated with the help of eq. (3.17) and (3.18). Further, the value of m and

Co is used evaluate constants C and n by using following eqs:

C= exp(Co) (3.19)

n=m (3.20)

4. RESULTS AND DISCUSSIONFigure 4.1 shows the daily yield on different water depths and still orientations. It is evident from the

graph that higher yield is obtained when still was placed in North-South direction. Again, graph depicts

that the lowest yield is obtained at the maximum selected depth of 0.035m in both the orientations of still.

The overall higher yield is obtained for 0.025m of water depth while still was oriented towards North-

South direction. For water depth of 0.015 m a 65.05% rise in yield is recorded when the still is oriented in

North-South direction to that of East-West direction. Again, for water depth 0.025m a rise in yield is

recorded as 65.40% in North-South orientation of still as compared to the East-West orientation. Thisgain is reduced to 59.36% when the depth is increased to 0.035m. The effect of orientation is found to be

minimal for higher water depth.

Fig: 4.1 Variation of daily yield with respect to all water depth and both orientations.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.015 0.025 0.035

Dailyyield(Kg)

Day Hours

Variation of daily yield

Mew (E-W) Mew (N-S)


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745

Heat transfer between the water and the glass cover also depends on their temperature difference. Figure

4.2 shows variation in glass surface temperature for different depth of the basin water and for different

orientations of the still (East-West and North-South). It is evident from graphs that higher temperature is

attained by the glass cover facing sunrays directly. East facing glass, when still was oriented towards

North-South direction, attains maximum temperature where as the south facing glass of East-West

oriented still has the maximum temperature throughout the day.

Fig: 4.2 variations in glass temperature for various water depths.

Figure 4.3 depicts variation in the difference of water and glass temperature throughout the day for all

water depths under consideration. This difference is the main driving potential to cause evaporation, sothe higher the difference the better is productivity of still. From figure, it is clear that the west and north

0

10

20

30

40

50

60

10 11 12 13 14 15 16 17

GlassTemp.(

0C)

Day Hours

DPW=0.015m

Tg(s)

Tg(N)

Tg(W)

Tg(E)

0

10

20

30

40

50

60

70

10 11 12 13 14 15 16 17

GlassTemp.

(0C)

Day Hours

DPW=0.025m

Tg(S)

Tg(N)

Tg(W)

Tg(E)

0

10

20

30

40

50

60

10 11 12 13 14 15 16 17

GlassTemp.

(oC)

Day Hours

DPW=0.035M

Tg(S)

Tg(N)

Tg(W)

Tg(E)


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746

facing glasses have maximum T, but since it is not the only factor responsible for heat transfer rate, the

heat transfer coefficients are also important for the same basin area.

Fig: 4.3 Variation in difference between water and glass temperature for different water depth and

different still orientation.

Figure 4.4a to 4.4c show the variation of convective heat transfer coefficient obtained from present model

and Dunkle model, these differences are because of assumptions made by Dunkle. Maximum values of

the convective heat transfer coefficient were obtained for 0.015 m of basin water depth when the still is

oriented towards North-South. Deviation of convective heat transfer coefficient obtained from present

model to Dunkle model is found to be higher in case of the still oriented in North-South direction. The

maximum variation of hcw obtained from present model and that of Dunkles for 0.015 m water depth andEast-West orientation is 47.61 % where as for North-South orientation and same depth, maximum

variation is 79.18%. For 0.025 m water depth and East-West orientation is 53.57% whereas for North-

South orientation, it is 70.45%. For 0.035 m water depth and East-West orientation is -11.70% whereas

for North-South orientation, it is 47.30%.

0

2

4

6

8

10

12

10 11 12 13 14 15 16 17

Tw-Tg(0C)

Day Hours

Still Axis: East-WestSouth,DPW=.015m

North,DPW=.015mSouth, DPW=.025m

North DPW=.025m

South,Dpw=.035m

North,DPW=.035m

0

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17

Tw-Tg(0C)

Day Hours

Still Axis: North-SouthWest DPW=.015m

East DPW=.015mWest DPW=.025m

East DPW=.025m

West DPW=.035mEast DPW=.035m


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Fig 4.4(a) Variation of convective heat transfer coefficient hcw for .015 m of water depth and both

orientations.

Fig: 4.4(b) Variation of convective heat transfer coefficient hcw for 0.025 m of water depth and both

orientations.

0

0.5

1

1.5

2

2.5

3

10 11 12 13 14 15 16 17

Conv.H.T.

Coeff.W/m2k

Day Hours

Still Axis:East-West

hcw[PM] hcw[DUNKL]

0

1

2

3

45

6

7

10 11 12 13 14 15 16 17

Conv.H.T.C

oeff.W

/m2k

Axis Title

Still Axis: North-South

hcw[PM] hcw[DUNKL]

00.5

1

1.5

2

2.5

3

10 11 12 13 14 15 16 17Conv.H.T.

Coeff

.W/m2k

Day Hours

Still Axis: East westhcw[PM] hcw[DUNKL]

01

2

3

4

5

6

10 11 12 13 14 15 16 17COnv.H

.T.C

oeff

.W/m2k

Day hours

Still axis: North-South

hcw[PM] hcw[DUNKL]


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Fig: 4.4(c) Variation of convective heat transfer coefficient hcw for 0.035 m of water depth and both

orientations.

Figure 4.5(a) to 4.5(c) shows variation in the evaporative heat transfer between the water mass and theglass cover with time for different water depths and for different still orientations. It increases with time

of heating and then starts decreasing as solar flux declines after a certain period of time. Maximum valuesof evaporative heat transfer coefficient was obtained at 0.015 m of water depth when still was oriented

towards North-South direction. The trend of graph showcases higher values for the North-South

orientation as compared to the East-West orientation of the still. The maximum variation of evaporative

heat transfer coefficient obtained from present model and that of Dunkles for 0.015 m water depth and

East-West orientation is 47.61 % where as for North-South orientation and same depth, maximum

variation is 79.18%, for 0.025 m water depth and East-West orientation is 53.57% whereas for North-South orientation, it is 70.51%, for 0.035 m water depth and East-West orientation is -11.70% whereas for

North-South orientation, it is 46.87%.

Fig: 4.5(a) Variation of evaporative heat transfer coefficient hew for 0.015 m of water depth and bothorientations.

0

0.5

1

1.5

2

10 11 12 13 14 15 16 17Conv.H.T.

Coeff.W/m2K

Day Hours

Still Axis: East-West

hcw[PM] hcw{DUNKL]

0

1

2

3

4

10 11 12 13 14 15 16 17COnv.H.T.C

oeff.W/m2K

Day Hours


hcw[PM] hcw[DUNKL]

0

5

10

15

20

25

30

10 11 12 13 14 15 16 17

Evap.H.T.C

oeff

.W/m2K

Day Hours


hew[PM] hew[DUNKL]

0

10

20

30

40

50

60

70

80

90

10 11 12 13 14 15 16 17

Evap.H.T.C

oeff

.W/m2K

Day Hours


hew[PM] hew[DUNKL]


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Fig: 4.5(b) Variation of evaporative heat transfer coefficient hew for 0.025m of water depth and for both

orientations

Fig: 4.5(c) Variation of evaporative heat transfer coefficient h ew for 0.035m of water depth and for bothorientations

The actual distillate collected during the experiment through the drainage channels at bottom of the two

inclined glass covers of the solar still for various conditions have been plotted and shown in figure 4.6.From the graphs, it is clear that the output for the maximum depth is the lowest. The maximum distillate

collected were .070 kg, .076 kg and .050 kg for the water depths of .015m, 0.025m and 0.035m,

respectively at the East-West orientation of the still. However, it was .234 kg, .232 kg and 0.140 kg for

water depths of 0.015m, 0.025m and 0.035m, respectively at the North-South orientation of still.

0

5

10

15

20

25

30

35

10 11 12 13 14 15 16 17

Evap.H.T.C

oeff

.W/

m2K

Day Hours


hew[PM] hew[DUNKL]

0

10

20

30

40

50

60

70

80

90

10 11 12 13 14 15 16 17

Evap.H.T.C

Oeff

.W/

m2K

Day Hours


hew[PM] hew[DUNKL]

0

2

4

6

8

10

12

14

16

10 11 12 13 14 15 16 17

Evap.H

.T.C

oeff

.W/m2K

Day Hours


hew[PM] hew[DUNKL]

0

5

10

15

20

25

30

35

10 11 12 13 14 15 16 17

Evap.H.T.C

oeff

.W/m2K

Day Hours


hew[PM] hew[DUNKL]


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Fig 4.6 variation in the measured distillate output for different water depths and still orientation.

Fig: 4.7(a) Comparison of calculated and measured distillate output at 0.015m water depth.

Fig: 4.7(b) Comparison of calculated and measured distillate output at 0.025m water depth.

0

0.02

0.04

0.06

0.08

10 11 12 13 14 15 16 17Measureddistilla

te(Kg)

Day Hours

Still Axis: East-WestDPW=.015m DPW=.025m DPW=.035m

0

0.05

0.10.15

0.2

0.25

10 11 12 13 14 15 16 17Measureddistilla

te(Kg)

Day Hours

Still Axis: North-SouthDPW=.015m DPW=.025m DPW=.035m

0

0.02

0.04

0.06

0.08

0.1

10 11 12 13 14 15 16 17Distillateoutput(Kg)

Day Hours

Still Axis: East-WestDPW=.015m C

DPW=.015m M

0

0.05

0.1

0.15

0.2

0.25

10 11 12 13 14 15 16 17Distillateoutput(Kg)

Day Hours

Still Axis: North-SouthDPW=.015m c

DPW=.015m M

0

0.02

0.04

0.06

0.08

0.1

10 11 12 13 14 15 16 17Distillate

output(Kg)

Day Hours

Still Axis: East-WestDPW=.025m C

DPW=.025m M

0

0.050.1

0.15

0.2

0.25

0.3

10 11 12 13 14 15 16 17Distillate

output(Kg)

Day Hours

Still Axis: North-SouthDPW=.025m C

DPW=.025m M


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752

qew Rate of evaporative heat transfer (W/m2)

t Time (s)

Tg Glass temperature (0C)

Tv Vapor temperature (0C)

Tw Water temperature (0C)

hv Enthalpy of evaporation of water (J/kg)

Greek Symbols

Thermal conductivity of the humid air (W/m0C)

Dynamic viscosity of humid air (N.S/m2)

Density of humid air (kg/m3)

Coefficient of volumetric thermal expansion (1/K)

REFERENCES[1] Tiwari, G.N., Tiwari, A.(2007), Solar Distillation Practice for Water Desalination Systems, Anamaya,New Delhi,.[2] Dunkle, R.V.(1961), Solar water distillation: The roof type still and multiple effect diffusion still,

International Development in Heat Transfer, ASME, Proceedings of International Heat Transfer, Part v,

University of Colorado, , pp.895.

[3] Malik, M.A.S., et al. (1982), Solar Distillation, Pergamon Press Ltd, UK,.

[4] Lof,G.O.G., Eibling, J.A., Blomer, J.W. (1961), Energy Balances in Solar Distillation, J. Am. Inst.Chem. Eng. 7, 4, pp.641.

[5]Morse R.N. and Read W.R.W. (1968), A rational basis for the engineering development of the solarstill, Solar Energy 12: 5.

[6] Kumar, Sanjay and Tiwari, G.N.(1996), Estimation of Convective Mass Transfer in Solar Distillation

System, Solar Energy, 57,459.

[7] Sharma V.B. and Mullick S.C.(1991), Estimation of heat transfer coefficients, the upward heat flowand evaporation in a solar still, Transaction of the ASME 113,pp. 36-43.

[8] Shukla S.K. and Sorayan V.P.S. (2005), Thermal modeling of solar stills: An experimental validation,

Renewable Energy , 30, , pp 683-699.

[9] Singh, H.N., Tiwari, G.N. (2004), Monthly Performance of Passive and Active Solar Stills for

Different Indian Climatic Conditions,Desalination, 168, pp. 145-50.

[10] Khalifa, A.J.N., Hamood, A.M. (2009), Verification of the Effect of Water Depth on thePerformance of Basin Type Solar Still, Solar Energy, 83, pp. 1312-21.

[11] Shukla, S.K., Rai, A.K. (2008), Analytical Thermal Modeling of Double Slope Solar Still by using

Inner Glass Cover Temperature, Thermal Science, 12, pp. 139-52.


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AUTHORS ADDRESSES:

Ajeet Kumar Rai

Department of Mechanical Engineering

SHIATS-DU

Post office - AAI(formerly AAI-DU), Allahabad

PIN 211007 (UP) INDIA

Ashish Kumar

Department of Mechanical Engineering

SHIATS-DUPost office- AAI

(formerly AAI-DU), Allahabad


Vinod Kumar Verma

Department of Mechanical EngineeringSHIATS-DU

Post office-AAI

(formerly AAI-DU), Allahabad


effect of water depth and still orientation on productivity of passive solar still

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