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Double slope fibre reinforced plastic (FRP) multiwick solar still
Solar & Wind Technology Vol. 1, No. 4, pp. 229-235, 1984 Printed in Great Britain.
0741-983X/85 $3.00+ .00 Pergamon Press Ltd.
DOUBLE SLOPE FIBRE REINFORCED PLASTIC (FRP) MULTIWICK SOLAR STILL* G. N. TIWARI a n d G. A. MOHAMED SELIM~ Centre for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India (Received 25 October 1984; accepted 10 November 1984)
Alada'aet--A new design of double slope fibre reinforced plastic (FRP) multiwick solar still has been presented. The working principle of this still is exactly the same as that of the simple muitiwick solar still. The cost and performance of the double slope FRP still has been compared with the cost and performance of the simple multiwick solar still. It is found that a double slope FRP multiwick solar still is more economical and efficient than a simple one. Also, a good agreement between experimental and theoretical results has been observed.
INTRODUCTION Various designs of solar stills (single basin, multiple effect, inclined, greenhouse combination and other designs) have been reviewed by Malik et al. [1]. It is recommended that only single basin and multiwick solar stills be used for large scale installation. Delyannis and Delyannis [2] have reported on some solar distillation plants which are ir~stalled in various countries. The design of solar stills used in these plants was single basin solar still. Recently, Tiwari [3] has presented details of a multiwick solar distillation plant of capacity 70 litres d a y - t which has worked for the last 2 years at the Centre for Energy Studies, Indian Institute of Technology, New Delhi. In this article, an economical and efficient design of a double slope F R P multiwick solar still has been presented. A theoretical analysis has also been developed and it is concluded that there is a good agreement between experimental and theoretical results. The maximum yield per square metre per day reported in the month of May 1984 is 4.5 litres. DESCRIPTION OF DOUBLE SLOPE SOLAR STILL Figure l a schematically represents the cross sectional view and plan of the still. The body (1) with a saline water reservoir (2) is oriented along the east-west
* Work is partiallysupported by DNES, Govt of India, New Delhi and United Nations University Programme on Renewable Sources of Energy at CES, liT Delhi. t Present address: Egyptian Electricity Authority, Studies, Research and Development, Nasr City, Abbassia, Cairo, Egypt 831542.
direction to receive maximum solar radiation, particularly in summer when the sun moves just overhead. The saline water is fed through the inlet (3) while distillate and excess saline water is taken out through the outlets (7) and (8) respectively. The hole (9) is used for thermocouple entry to measure the temperature of the saline water and different jute cloth pieces. One end of the black jute cloth pieces (4a) [separated by black polythene sheet (4b)] is dipped in the reservoir (2) and the other ends are placed over the base of the still (1) with increasing length (the wick dimensions are 110 x 110, l 1 0 x 9 0 , l l 0 x 70, l 1 0 x 5 0 and l 1 0 x 3 0 cm). The inner surfaces of the still are coated black. A window glass (5) of dimensions 114 x 114 cm on each side has been used as a still cover and is fixed on the walls of the still with the help of rubber gaskets (10, 14) and aluminium "L" (13) frame to avoid air leakage. The top of the glass cover is supported by a waterproof wooden bar (12). The fixing arrangement of the glass cover is shown in Fig. lb. The still is placed on a stand and connected to a tank of capacity 50 litres. The saline water comes from another 150-1itre capacity storage tank to ensure continuous supply of water to the still. The whole assembly of the system is shown in Fig. 2. A photograph of the system is shown in Fig. 3.
MATERIAL USED FOR FABRICATION OF THE STILL The body of the still is made up of glass fibre/Bisphenol 'A' polyester composition. First, a wooden die is made according to the required dimensions. With the help of this wooden die, a mould of fibre-reinforced plastic (FRP) is fabricated. Then this 229
230
G . N . TlwnRl and G. A. MOHAMED SELIbl N
E
228cm
.]
(b)
S~ct[onol plon ond sid~
view of lh~
solor
still Fig. l a. Cross sectional view of the plan of double slope FRP multiwick solar still. 0
13
5
~/ 8 f--
10
/ ~2 Sec.
B
Fig. lb. View at "A" of Fig. la.
Double slope fibre reinforced plastic (FRP) multiwick solar still
231
so that it is smooth and glass comes at the other side of gel coated surface. The column is put in an oven at 80°C for heat treatment and the following values result: conductivity = 0.04 W m - 2 °C- 1 ; thickness = 0.005 m. MEASUREMENTS OF VARIOUS PARAMETERS
Assembly of exper{ment Fig. 2. Assemblyof the complete system.
FRP mould is used to prepare the actual tray of required thickness. The material has the following specification : Glass: (i) one layer of 0.45 kg m -2 chopped strand mat (ii) one layer of if6 kg m -2 woven roving. Resin: Bisphenol 'A' polyester resin in which ultraviolet retardant is added and used to keep the glass to resin ratio 1 : 2.5. The composite has an upper coat of gel with a minimum of 0.4 kg m - 2 coverage. The lay-up is such that there is no air entrainment. The columns are supplied in light sea green colour which have pigmented gel coat. The subsequent lamination work has also been done in the same colour. The laminationis finally covered with extra Bisphenol 'A' polyester resin
Temperature measurements: The copper-constantan wire thermocouple was used to measure water and glass cover temperature. The junction of the thermocouples were welded by fusion using oxyacetylene oxidizingflame. The cold junction was kept in an ice box at 0°C. A millivoltmeterwas used to measure the e.m.f, produced in the thermocouple through a selector switch. The thermocouples were calibrated by direct comparison with a standard thermometer in hot water. A check for the fitted curve was made. The ambient air temperature has been measured by using a mercury-in-glassthermometer having a range of 100°C with a division of 0.1°C. Radiation measurements A pyranometer was used to measure total radiation received on a horizontal surface. It was kept nearby the still but away from any source of reflection at the height of 2 m from the ground. Wind speed measurements The wind speed was measured in m s- 1 by means of an anenometer placed near the still. It was noticed that the average wind speed for a typical day in May was 4 m s -1. Yield measurement Yield was measured by graduated cylinder.
THEORETICAL ANALYSIS
Fig. 3. Photograph of the double slope FRP multiwick solar still.
In this case, orientation of the solar still is in the eastwest direction. The working principle of double slope FRP still is the same as that of a simple multiwick solar still. Solar radiation, after transmission from the glass cover, is absorbed by the blackened jute cloth and the water in the jute cloth is heated. Due to temperature difference between the water and glass cover, water is evaporated and condensed on the inner surface of the glass cover, giving out its latent heat of vaporisation. Since the glass cover is inclined, condensed water trickles down into the drain and is collected in the bottles placed below the still. The energy balance equations for the glass and water layer can be written as
zgH~"F hl(rw-- T#) = h2(Tg--T~
(1)
232
G. N. TIWARI and G. A. MOHAMEDSELIM
and
zwns = hl(T~- To)+ hb(Tw - T.)
(2)
where hb is the conductive heat transfer coefficient between the water layer and the ambient air through F R P body from bottom as well as sides. It is assumed constant throughout calculation and its expression is given by hb=
+
h2 = the convective and radiative heat transfer coefficient from the glass cover to the ambient air and hi = h,w + hew+ hew is assumed to be constant during one set of observations (in this case it is 1 h) otherwise it is temperature dependent quantities through h,w, hew and hew and is given by
In writing the above equations, the following assumptions have been made : (i) there is no vapour leakage in the still; (ii) the water level in the still is maintained in such a way that the amount of excess water is negligible i.e. heat carried out by excess water is very much smaller in comparison to the heat utilized for evaporation; (iii) there is no temperature gradient along the thickness of glass cover and the water layer. Equations (1) and (2) can be solved for Tw and To for given experimental values of Twand To.The hourly yield can be calculated from
rhe
L#
EXPERIMENTAL A N D THEORETICAL OBSERVATIONS
h,~ = ~tr[(7"w+ 273) 4 - ( ~ + 273) 4]
(:rw- ~)
hew(Tw- TO) x 3600.
'
In order to appreciate the numerical results, calculations have been made for one of the typical hot days i.e. 7 May 1984 on which the experiment was conducted. Corresponding to this day, the hourly variation of solar intensity (H~), reservoir water temperature (Tb) and ambient air temperature (Ta) is shown in Fig. 4. The following parameters are used to evaluate water
(/~w_/~,)(Tw + 2 7 3 ) i , / 3
hew = 0"884 7'w - L + 2 6 T . 9x ~ - ] and hew = 16"273 x 10 -3 x /~w--/Sg
Tw-~
1000
70 800
l
6O 600
o
E u
50
4O0
0
E 40 2OO 30 I 8
I 9
I 10
l ~
I 12
I 1 Time
I 2
I 3
I z.
I S
I 6
I
00
7
(hr s ) ~.-
Fig. 4. Hourly variation of solar intensity (Hs),reservoir water temperature (Tb)and ambient air temperature (T,).
Double slope fibre reinforced plastic (FRP) multiwick solar still
233
80
7O LU CE -~ 60 <~ LU Q.
,,,
50
c," LU
Dun c k l e ' s
correlation
t0 o
30
o
o
Experimental
values
for east
,,
",
Experimental
values
for
I 8
I 9
I 10
I 11
L 12
I, 1
side
west I 2
side I 3
I 4
I 5
I 6
TIME ( h r s )
Fig. 5. Hourly variation of water temperature ofeast and west side of the still.
8O
I 70 60
o
so
E tO ;n
o 30
a
correlot
ton
o
o
Experimental
Duncle),~
values
for
4
~" E x p e r i m e n t a l
values
for
east
side
west
side
I
I
I
I
I
I
l
I
I
I
I
8
9
10
11
12
1
2
3
t
5
6
T~me
(hrs)
Fig. 6. H o u r l y variation of glass cover temperature o f east and west side of the still
234
G. N. TIWAgl and G. A. MOHAMEDSELIM
and glass temperature and the hourly yield from the still : r 0 = 0"05
e = 0"9
~w = 0'80
~ = 5'7x 10-SWm-ZK
L = 0"005 m
k = 0.04 W m - 1 ° C - 1
hi = 2 2 W m - 2 ° C
4
F r o m these figures, it is also clear that :
1
h 1 and h 2 have been calculated separately for every set of observations. The calculated hourly variation of water and glass temperatures and yield m - z from the still is shown in Figs. 5-7; corresponding experimental observations are also shown in the same figures. F r o m these figures, it is clear that there is good agreement between the theoretical and experimental observations. The small discrepancy between these observations is due to : (i) mixing ofvapour from one side to the other side and vice versa which has not been considered in analysis. Since the difference is too small, it can be neglected ;
Date
(ii) there is some amount of heat lost through excess water ; (iii) a small amount of solar energy is absorbed in the water of the reservoir ; (iv) there may be some vapour leakage.
(i) in the morning, the temperature and yield of the east side is more than half that of the west side because of high insolation received by the east side ; (ii) about noon, the temperature and yield from both sides is the same ; and (iii) this situation is just the reverse of case (i). Table 1 gives the daily performance of the double slope and single slope F R P multiwick solar still respectively. The results of double slope F R P still has been compared with the two units of simple F R P solar still. F r o m this table, it can be concluded that : (a) the total yield from the double slope still is
7. 5 .1984
Theoretical
for
each
s~de
East 0 7
x---x
West
0.6
I
05
E
~
0.~
_5 -
0.3
o
0.2
/ 0.1 I
8
9
I
I
I
I
I
I
I
I
10
11
'~2
1
2
3
z.
5
Time
(hrs)
Fig. 7. Hourly variation of yield m- 2 from the east and west side of the still.
Double slope fibre reinforced plastic (FRP) multiwick solar still
235
Table 1. Sample data of daily observations from the double sloped muitiwick of each side and one simple multiwick solar still Daily yield (iitres)
S.N.
Date (May/June 1984)
1 2 3 4 5 6 7 8 9 10
21 May 22 May 23 May 24 May 25 May 29 May 7 June 8 June 9 June 10 June
Simple multiwick Double slope FRP still solar still East (1 m 2) West (1 m 2) (1 m 2) litres 4-58 4.50 4.20 4.60 3.85 4-0 3.2 3.5 4.4 4-35
4.09 4'05 3-95 4"15 3.65 . 3.8 3"0 3.3 4.25 3.95
3"6 3.21 3'30 3-75 3-65 3-50 2"50 3.8 3.95 3"85
Remarks clear clear clear clear clear maintenance* semi cloudy clear clear clear
* During maintenance, the glass has been broken and broken glass replaced by new.
significantly higher t h a n t h a t from the simple multiwick solar still; (b) the total material used in this still is less t h a n t h a t used for two simple stills, hence the cost of this still will be lower t h a n t h a t of a simple one. NOMENCLATURE hi h2 hb hew hew hl h,w Hs k
total heat transfer coefficient from water surface to the glass cover ( W m -2 °C- x) total heat transfer coefficient from glass cover to ambient air temperature (W m-2 oc-1) bottom heat transfer coefficient from water to ambient through insulation (FRP) (W m-2 °C- 1) convective heat transfer coefficient from water surface to the glass cover ( W m -2 °C -1) evaporative heat transfer coefficient from water surface to the glass cover ( W m -2 °C-t) bottom heat transfer from the bottom of the still to ambient (W m-2 oC-1) radiative heat transfer coefficient from water surface to the glass cover (W m -2 °C -1) solar radiation (W m-2) thermal conductivity of FRP material (W m - 1 °C- t)
L .W the Pw pg T~ Tb T~ Tw zg zw e
thickness of FRP body of the still latent heat of vaporisation, kJ kg- t hourly yield from the still, kg m - 2 h - 1 partial pressure of water vapour at water temperature (Pa) partial pressure of water vapour at glass temperature (Pa) ambient air temperature (°C) reservoir water temperature (°C) glass cover temperature (°C) water temperature (°C) fraction of solar energy absorbed by the glass cover fraction of solar energy absorbed by water surface emissivity from the water surface Stefan-Boltzman constant REFERENCES
1. M.A.S. Malik, G. N. Tiwari, A. Kumar and M. S. Sodha, Solar Distillation. Pergamon Press, Oxford (1982). 2. A. Delyannis and E. Ddyannis, Solar distillation plant of high capacity. Proc. 4th Int. Syrup. on Fresh Waterfrom Sea 4, 487 (1973). 3. G.N. Tiwari, Demonstration plant of multiwick solar still. Energy Conversion and Management (in press) (1984).
0741-983X/85 $3.00+ .00 Pergamon Press Ltd.
DOUBLE SLOPE FIBRE REINFORCED PLASTIC (FRP) MULTIWICK SOLAR STILL* G. N. TIWARI a n d G. A. MOHAMED SELIM~ Centre for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India (Received 25 October 1984; accepted 10 November 1984)
Alada'aet--A new design of double slope fibre reinforced plastic (FRP) multiwick solar still has been presented. The working principle of this still is exactly the same as that of the simple muitiwick solar still. The cost and performance of the double slope FRP still has been compared with the cost and performance of the simple multiwick solar still. It is found that a double slope FRP multiwick solar still is more economical and efficient than a simple one. Also, a good agreement between experimental and theoretical results has been observed.
INTRODUCTION Various designs of solar stills (single basin, multiple effect, inclined, greenhouse combination and other designs) have been reviewed by Malik et al. [1]. It is recommended that only single basin and multiwick solar stills be used for large scale installation. Delyannis and Delyannis [2] have reported on some solar distillation plants which are ir~stalled in various countries. The design of solar stills used in these plants was single basin solar still. Recently, Tiwari [3] has presented details of a multiwick solar distillation plant of capacity 70 litres d a y - t which has worked for the last 2 years at the Centre for Energy Studies, Indian Institute of Technology, New Delhi. In this article, an economical and efficient design of a double slope F R P multiwick solar still has been presented. A theoretical analysis has also been developed and it is concluded that there is a good agreement between experimental and theoretical results. The maximum yield per square metre per day reported in the month of May 1984 is 4.5 litres. DESCRIPTION OF DOUBLE SLOPE SOLAR STILL Figure l a schematically represents the cross sectional view and plan of the still. The body (1) with a saline water reservoir (2) is oriented along the east-west
* Work is partiallysupported by DNES, Govt of India, New Delhi and United Nations University Programme on Renewable Sources of Energy at CES, liT Delhi. t Present address: Egyptian Electricity Authority, Studies, Research and Development, Nasr City, Abbassia, Cairo, Egypt 831542.
direction to receive maximum solar radiation, particularly in summer when the sun moves just overhead. The saline water is fed through the inlet (3) while distillate and excess saline water is taken out through the outlets (7) and (8) respectively. The hole (9) is used for thermocouple entry to measure the temperature of the saline water and different jute cloth pieces. One end of the black jute cloth pieces (4a) [separated by black polythene sheet (4b)] is dipped in the reservoir (2) and the other ends are placed over the base of the still (1) with increasing length (the wick dimensions are 110 x 110, l 1 0 x 9 0 , l l 0 x 70, l 1 0 x 5 0 and l 1 0 x 3 0 cm). The inner surfaces of the still are coated black. A window glass (5) of dimensions 114 x 114 cm on each side has been used as a still cover and is fixed on the walls of the still with the help of rubber gaskets (10, 14) and aluminium "L" (13) frame to avoid air leakage. The top of the glass cover is supported by a waterproof wooden bar (12). The fixing arrangement of the glass cover is shown in Fig. lb. The still is placed on a stand and connected to a tank of capacity 50 litres. The saline water comes from another 150-1itre capacity storage tank to ensure continuous supply of water to the still. The whole assembly of the system is shown in Fig. 2. A photograph of the system is shown in Fig. 3.
MATERIAL USED FOR FABRICATION OF THE STILL The body of the still is made up of glass fibre/Bisphenol 'A' polyester composition. First, a wooden die is made according to the required dimensions. With the help of this wooden die, a mould of fibre-reinforced plastic (FRP) is fabricated. Then this 229
230
G . N . TlwnRl and G. A. MOHAMED SELIbl N
E
228cm
.]
(b)
S~ct[onol plon ond sid~
view of lh~
solor
still Fig. l a. Cross sectional view of the plan of double slope FRP multiwick solar still. 0
13
5
~/ 8 f--
10
/ ~2 Sec.
B
Fig. lb. View at "A" of Fig. la.
Double slope fibre reinforced plastic (FRP) multiwick solar still
231
so that it is smooth and glass comes at the other side of gel coated surface. The column is put in an oven at 80°C for heat treatment and the following values result: conductivity = 0.04 W m - 2 °C- 1 ; thickness = 0.005 m. MEASUREMENTS OF VARIOUS PARAMETERS
Assembly of exper{ment Fig. 2. Assemblyof the complete system.
FRP mould is used to prepare the actual tray of required thickness. The material has the following specification : Glass: (i) one layer of 0.45 kg m -2 chopped strand mat (ii) one layer of if6 kg m -2 woven roving. Resin: Bisphenol 'A' polyester resin in which ultraviolet retardant is added and used to keep the glass to resin ratio 1 : 2.5. The composite has an upper coat of gel with a minimum of 0.4 kg m - 2 coverage. The lay-up is such that there is no air entrainment. The columns are supplied in light sea green colour which have pigmented gel coat. The subsequent lamination work has also been done in the same colour. The laminationis finally covered with extra Bisphenol 'A' polyester resin
Temperature measurements: The copper-constantan wire thermocouple was used to measure water and glass cover temperature. The junction of the thermocouples were welded by fusion using oxyacetylene oxidizingflame. The cold junction was kept in an ice box at 0°C. A millivoltmeterwas used to measure the e.m.f, produced in the thermocouple through a selector switch. The thermocouples were calibrated by direct comparison with a standard thermometer in hot water. A check for the fitted curve was made. The ambient air temperature has been measured by using a mercury-in-glassthermometer having a range of 100°C with a division of 0.1°C. Radiation measurements A pyranometer was used to measure total radiation received on a horizontal surface. It was kept nearby the still but away from any source of reflection at the height of 2 m from the ground. Wind speed measurements The wind speed was measured in m s- 1 by means of an anenometer placed near the still. It was noticed that the average wind speed for a typical day in May was 4 m s -1. Yield measurement Yield was measured by graduated cylinder.
THEORETICAL ANALYSIS
Fig. 3. Photograph of the double slope FRP multiwick solar still.
In this case, orientation of the solar still is in the eastwest direction. The working principle of double slope FRP still is the same as that of a simple multiwick solar still. Solar radiation, after transmission from the glass cover, is absorbed by the blackened jute cloth and the water in the jute cloth is heated. Due to temperature difference between the water and glass cover, water is evaporated and condensed on the inner surface of the glass cover, giving out its latent heat of vaporisation. Since the glass cover is inclined, condensed water trickles down into the drain and is collected in the bottles placed below the still. The energy balance equations for the glass and water layer can be written as
zgH~"F hl(rw-- T#) = h2(Tg--T~
(1)
232
G. N. TIWARI and G. A. MOHAMEDSELIM
and
zwns = hl(T~- To)+ hb(Tw - T.)
(2)
where hb is the conductive heat transfer coefficient between the water layer and the ambient air through F R P body from bottom as well as sides. It is assumed constant throughout calculation and its expression is given by hb=
+
h2 = the convective and radiative heat transfer coefficient from the glass cover to the ambient air and hi = h,w + hew+ hew is assumed to be constant during one set of observations (in this case it is 1 h) otherwise it is temperature dependent quantities through h,w, hew and hew and is given by
In writing the above equations, the following assumptions have been made : (i) there is no vapour leakage in the still; (ii) the water level in the still is maintained in such a way that the amount of excess water is negligible i.e. heat carried out by excess water is very much smaller in comparison to the heat utilized for evaporation; (iii) there is no temperature gradient along the thickness of glass cover and the water layer. Equations (1) and (2) can be solved for Tw and To for given experimental values of Twand To.The hourly yield can be calculated from
rhe
L#
EXPERIMENTAL A N D THEORETICAL OBSERVATIONS
h,~ = ~tr[(7"w+ 273) 4 - ( ~ + 273) 4]
(:rw- ~)
hew(Tw- TO) x 3600.
'
In order to appreciate the numerical results, calculations have been made for one of the typical hot days i.e. 7 May 1984 on which the experiment was conducted. Corresponding to this day, the hourly variation of solar intensity (H~), reservoir water temperature (Tb) and ambient air temperature (Ta) is shown in Fig. 4. The following parameters are used to evaluate water
(/~w_/~,)(Tw + 2 7 3 ) i , / 3
hew = 0"884 7'w - L + 2 6 T . 9x ~ - ] and hew = 16"273 x 10 -3 x /~w--/Sg
Tw-~
1000
70 800
l
6O 600
o
E u
50
4O0
0
E 40 2OO 30 I 8
I 9
I 10
l ~
I 12
I 1 Time
I 2
I 3
I z.
I S
I 6
I
00
7
(hr s ) ~.-
Fig. 4. Hourly variation of solar intensity (Hs),reservoir water temperature (Tb)and ambient air temperature (T,).
Double slope fibre reinforced plastic (FRP) multiwick solar still
233
80
7O LU CE -~ 60 <~ LU Q.
,,,
50
c," LU
Dun c k l e ' s
correlation
t0 o
30
o
o
Experimental
values
for east
,,
",
Experimental
values
for
I 8
I 9
I 10
I 11
L 12
I, 1
side
west I 2
side I 3
I 4
I 5
I 6
TIME ( h r s )
Fig. 5. Hourly variation of water temperature ofeast and west side of the still.
8O
I 70 60
o
so
E tO ;n
o 30
a
correlot
ton
o
o
Experimental
Duncle),~
values
for
4
~" E x p e r i m e n t a l
values
for
east
side
west
side
I
I
I
I
I
I
l
I
I
I
I
8
9
10
11
12
1
2
3
t
5
6
T~me
(hrs)
Fig. 6. H o u r l y variation of glass cover temperature o f east and west side of the still
234
G. N. TIWAgl and G. A. MOHAMEDSELIM
and glass temperature and the hourly yield from the still : r 0 = 0"05
e = 0"9
~w = 0'80
~ = 5'7x 10-SWm-ZK
L = 0"005 m
k = 0.04 W m - 1 ° C - 1
hi = 2 2 W m - 2 ° C
4
F r o m these figures, it is also clear that :
1
h 1 and h 2 have been calculated separately for every set of observations. The calculated hourly variation of water and glass temperatures and yield m - z from the still is shown in Figs. 5-7; corresponding experimental observations are also shown in the same figures. F r o m these figures, it is clear that there is good agreement between the theoretical and experimental observations. The small discrepancy between these observations is due to : (i) mixing ofvapour from one side to the other side and vice versa which has not been considered in analysis. Since the difference is too small, it can be neglected ;
Date
(ii) there is some amount of heat lost through excess water ; (iii) a small amount of solar energy is absorbed in the water of the reservoir ; (iv) there may be some vapour leakage.
(i) in the morning, the temperature and yield of the east side is more than half that of the west side because of high insolation received by the east side ; (ii) about noon, the temperature and yield from both sides is the same ; and (iii) this situation is just the reverse of case (i). Table 1 gives the daily performance of the double slope and single slope F R P multiwick solar still respectively. The results of double slope F R P still has been compared with the two units of simple F R P solar still. F r o m this table, it can be concluded that : (a) the total yield from the double slope still is
7. 5 .1984
Theoretical
for
each
s~de
East 0 7
x---x
West
0.6
I
05
E
~
0.~
_5 -
0.3
o
0.2
/ 0.1 I
8
9
I
I
I
I
I
I
I
I
10
11
'~2
1
2
3
z.
5
Time
(hrs)
Fig. 7. Hourly variation of yield m- 2 from the east and west side of the still.
Double slope fibre reinforced plastic (FRP) multiwick solar still
235
Table 1. Sample data of daily observations from the double sloped muitiwick of each side and one simple multiwick solar still Daily yield (iitres)
S.N.
Date (May/June 1984)
1 2 3 4 5 6 7 8 9 10
21 May 22 May 23 May 24 May 25 May 29 May 7 June 8 June 9 June 10 June
Simple multiwick Double slope FRP still solar still East (1 m 2) West (1 m 2) (1 m 2) litres 4-58 4.50 4.20 4.60 3.85 4-0 3.2 3.5 4.4 4-35
4.09 4'05 3-95 4"15 3.65 . 3.8 3"0 3.3 4.25 3.95
3"6 3.21 3'30 3-75 3-65 3-50 2"50 3.8 3.95 3"85
Remarks clear clear clear clear clear maintenance* semi cloudy clear clear clear
* During maintenance, the glass has been broken and broken glass replaced by new.
significantly higher t h a n t h a t from the simple multiwick solar still; (b) the total material used in this still is less t h a n t h a t used for two simple stills, hence the cost of this still will be lower t h a n t h a t of a simple one. NOMENCLATURE hi h2 hb hew hew hl h,w Hs k
total heat transfer coefficient from water surface to the glass cover ( W m -2 °C- x) total heat transfer coefficient from glass cover to ambient air temperature (W m-2 oc-1) bottom heat transfer coefficient from water to ambient through insulation (FRP) (W m-2 °C- 1) convective heat transfer coefficient from water surface to the glass cover ( W m -2 °C -1) evaporative heat transfer coefficient from water surface to the glass cover ( W m -2 °C-t) bottom heat transfer from the bottom of the still to ambient (W m-2 oC-1) radiative heat transfer coefficient from water surface to the glass cover (W m -2 °C -1) solar radiation (W m-2) thermal conductivity of FRP material (W m - 1 °C- t)
L .W the Pw pg T~ Tb T~ Tw zg zw e
thickness of FRP body of the still latent heat of vaporisation, kJ kg- t hourly yield from the still, kg m - 2 h - 1 partial pressure of water vapour at water temperature (Pa) partial pressure of water vapour at glass temperature (Pa) ambient air temperature (°C) reservoir water temperature (°C) glass cover temperature (°C) water temperature (°C) fraction of solar energy absorbed by the glass cover fraction of solar energy absorbed by water surface emissivity from the water surface Stefan-Boltzman constant REFERENCES
1. M.A.S. Malik, G. N. Tiwari, A. Kumar and M. S. Sodha, Solar Distillation. Pergamon Press, Oxford (1982). 2. A. Delyannis and E. Ddyannis, Solar distillation plant of high capacity. Proc. 4th Int. Syrup. on Fresh Waterfrom Sea 4, 487 (1973). 3. G.N. Tiwari, Demonstration plant of multiwick solar still. Energy Conversion and Management (in press) (1984).