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Study of single-effect solar still with an internal condenser
Solar & Wind Technology Vol. 5, No. 6, pp. 637-643, 1988
0741-983X/88 $3.00+.00 Pergamon Press pie
Printed in Great Britain.
S T U D Y OF S I N G L E - E F F E C T SOLAR STILL WITH A N INTERNAL CONDENSER S. T. AHM~ School of Technical Education, University of Technology, Baghdad, Iraq (Received 30 May 1988 : accepted 7 July 1988) Abstract--An experimental study was carried out to evaluate the effect of using an internal condenser on theperformance of a single-effect solar still. The still was tested in two different ways: first, it was used for water vapor condensation without condenser, and second, it was used for water vapor condensation with condenser. The still was single-sloped with a double pass internal condenser. Its effective base area was 0.4 m 2 and its cover slope was 30° to the horizontal. The results showed that combining an internal condenser with basin type solar still caused an improvement in the still performance. The still daily productivity was increased from 5.5 kg/m 2 day for the first test to 5.9 kg/m 2 day for the second test.
NOMENCLATURE a~ absorptivity of transparent cover for radiation aw absorptivity of water to radiation Ca heat capacity of transparent cover and supports per unit basic area, kJ/m 2K Cwb heat capacity of water basin and contents per unit basin area, kJ/m2K lh incident solar radiation on a horizontal surface, kW/m 2 Lh latent heat of evaporation, kJ/kg Pw vapor pressure of water at temperature of water surface, Pa Pws vapor pressure of water at temperature of transparent cover, Pa qb heat flux by conduction from basic area, k~V/m2 qc heat flux by convection from water surface to transparent cover, kW/m 2 q~ond heat lost to the circulated water, kW/m 2 qe heat flux by evaporation]condensation from water surface to transparent cover, kW/m 2 qp heat flux from transparent cover to air, kW/m 2 q, heat flux by radiation from cover to air, kW/m 2 t time, s T~ temperature of transparent cover, K Tw water temperature, K transmissivity of transparent cover for radiation Y still productivity, kg/h m 2 qd, qh daily and hourly efficiency, respectively. INTRODUCTION The single-effect, horizontal-basin solar still is a relatively simple device to construct and operate. The appeal of the simple solar still is its ability to run for m a n y years with negligible power costs and only minor attention to maintenance and repair. Most existing desalination plants use fossil fuels as a source of energy. Although a few techniques such 637
as multi-effect evaporation, multistage flash evaporation, thin film distillation, reverse osmosis and electrodialysis are energy intensive, they are also uneconomical when the supply of fresh water is small. The maximization of water production rate of the basin-type solar still is therefore a prime objective in the development of solar distillation. M a n y attempts have been made to design and study the performance of basin-type solar desalination systerns. Delyannis [1] reviewed the major solar distillation plants around the world. They ranged from a 4460 m 2 glass covered plant built at Las Salina, Chile, to desalinate brackish water, and now abandoned, to a 8690 m 2 plant constructed at Patomas, Greece, to desalinate sea water. L 6 f e t al. [2] presented energy and mass transfer relationships for a basintype solar still. The theoretical work was supplemented with data, from held operations, of a 232 m 2 still. The effect of variations of design parameters on the performance of the solar still was predicted by the aid of digital computer. Distiller productivity was correlated with atmospheric temperature, wind velocity, solar radiation absorptivity and slope of cover, as well as cover temperature. Delyannis and Piperoglou [3] reported on the use of internal condensers as an attempt to improve the performance of the basin-type solar still. They reported a n improvement of the productivity of the still up to 24%, resulting from the addition of the cooler to the deep basin still. Collins and Thompson [4] and G r u n e et ai. [5] made tests on a simple solar still coupled with an external condenser. Malik et al. [6] reported that Bartali et al. [7] developed a simple still coupled with a chimney containing a heat ex-
638
S. T. AHMED
changer and then entered the still. Abed [8] and Khalifa [9] made tests on a simple still coupled with an internal condenser. The condenser was tested in two different ways : first it was used for water preheating and water vapor condensation, and, secondly, it was used for water vapor condensation. However the results showed that combining an internal condenser with the basin-type solar still caused an improvement in the still performance. The still daily productivity was increased from 5 to 5.8 kg/m 2 day for the first test, and to 5.6 kg/m 2 day for the second test. Moustafa et al. [10] reported that the major design factors affecting energy utilization are basin temperature, condensing surface temperature and ambient air temperature. Kanbour and Matloob [11] tested the indoor model single-slopped cover solar still--a computer model was developed for the prediction of performance. In this work, a basin-type solar still with an internal condenser was constructed, and an investigation with the following objectives has been carried out : (1) to study the performance of a basin-type solar still combined with an internal condenser. (2) to explore design modification for increasing the productivity of a desalination unit. (3) to make a computer model to the still which can predict theoretically the effect of basin water temperature and glass cover temperature on the performance of the still.
between the water surface and cover glass can be approximated as that between two infinite parallel planes, which could be approximated by these equations [12, 13, 14] qr = 18.371 × 10-8(T 4 - T4)3600,
(1)
q, = 3.1824(Tw- Tq)
[" pw_PwgTw ~.,3 +[2.65~1~3 ~) (T.,-T.)3600, ( Pw --Pwg) qe = 16.276 x 10- 3qc ( T . , - T g ) "
(2) (3)
The heat flux due to condensation is assumed equal to that by evaporation and is given by q~ = 3600 YL h.
(4)
Heat lost to the circulated water could be calculated from eq. (4). Referring to Fig. 1, heat balance equations could be written for several conditions [11-16] as shown below. (1) Heat balance for basin water:
dT~
awzlh = qb+Cwb~-3600+qe+qr+q,..
(5)
(2) Heat balance for glass cover :
dr~
qe+qr+qc+aglh = qga+ C , ~
3600.
(6)
(3) Heat balance of the basin and cover assembly ENERGY CONSIDERATIONS
The energy absorbed in the basin water and liner of a solar still is dissipated by a variety of modes, as indicated in Fig. 1. These are : (i) unproductive conductive losses to the surroundings ; (ii) combined convection, evaporation and radiation to the inside glass surface. One of the first steps available to the designer for achieving high efficiencies is to minimize the conductive losses from the sides and base by adequate insulation of the still. By lowering the water depth in the basin and hence thermal mass, higher operating temperatures are attained. F o r a given ambient temperature, the driving potential for base and side losses increases as the water temperature increases, so underlining the importance of adequate insulation of the still. The predominant modes of heat transfer in a solar still are convection and evaporation, and radiation
ajh+qw--Zlh = Q~+q~ __.,dT. C dT~ +Cwb dt 3600+ gs dt 3600.
(7)
EXPERIMENTAL CONSIDERATIONS
An "ideal" solar still is defined as one which has no conductive losses and the water depth is sufficiently small so that the sensible heat stored is negligible compared with the energy transfer rates to and from the water. F o r a given ambient temperature, wind velocity (which determines the external convection coefficient) and solar radiation rate, such a still will instantaneously reach a steady state condition. With the above points in mind, a small solar still was designed for experimental operation. Figure 2 shows an isometric of the experimental solar still. A galvanized steel sheet, 1 mm thick, was folded to form a single-sloped tray with a depth of 10 cm, cover slope of 30 ° to the horizontal, and effective base area of 100 × 40 cm. A galvanized steel trough of 100 cm
Single effect solar sell
639
lh
L.eakage I
qgo
qco~
RefLection
[radlotlo~
ond
convec~lon)~
\\\
qe
L ;i'.
Absorption a.rlh \
.~
/,,I '/
qr
SensibLeheat
q¢
RefLection
....
Base ond side losses
Fig. 1. Energy flow diagram for the solar still. length was fixed to the tray for the collection of distillate. The condenser was constructed of a copper tube 1.2 cm in diameter and 230 cm long, and was bent to form a double-pass heat exchanger. The base was painted black, a 4 nun window glass sheet was used as a condensation surface and it was constructed in such
Condenser
a way as to ensure vapor tightness. The base and sides of the still were insulated with 7 cm thick glass wool. Finally the entire structure was encased in a wooden container made of 2.5 cm thick wooden sheet. To collect maximum energy, the still was oriented south. The amounts of condensate from both the glass cover and condenser mass flow rates were measured
( I ) CircuLated wateroutLet. (2) CircuLated water inlet.
Trough Wood conic GLosscover
Insu(otion Galvanized steel tray
Fig. 2. The experimental solar still.
640
S.T. A H ~
tO0
a)
(b)
ASolar radiation oBasin temperature • Glass temperature + Ambient airtemperature
I0o [ ~Solar radiation iI o Basin temperature l • GLass temperature O00
eO 8OO
To0
4~
0
I0
o
o
II 12 13 14 Local day time ( h )
+
400
15
16
300
0
9
I0
II 12 13 14 Local day time ( h )
15
16
300
Fig. 3. Solar radiation, basin water temperature, glass cover temperature and ambient air temperature. separately by means of glass bottles with 3 1 capacity each, and with divisions of 10 ml. Temperature of the basin water and glass cover were measured by means of copper constant in thermocouples, using a millivolt meter with divisions of 0.1 mV. Dry bulb air temperature was measured by means of a mercury thermometer with divisions of 0.1 °C. Total incident solar radiation on a horizontal surface was measured in the field using a solar intensity water. The above readings were taken at mid-hour intervals from 8:30 a.m. to 4:30 p.m. (spring time) during the period of February to May, 1986, in Baghdad. Fresh water was used to operate the still through all tests. T E S T A. The still was tested with an initial depth of 1 cm without using the condenser. T E S T B. The still was tested with an initial depth of 1 cm using the condenser, with the cooling water circulated through the condenser from 8:30 a.m. to 4:30 pm. Khalifa [9] investigated the effect of different mass flow rates through the internal condenser ; here only one mass flow rate was used. RESULTS AND DISCUSSIONS The still was continuously operated from February 1986 to May 1986. Total incident solar radiation, ambient air temperature, glass cover temperature and basin water temperature for the still with and without internal condenser are shown in Fig. 3. This figure shows that total incident solar radiation gradually increased to reach its maximum value at the middle of the day and then decreased. Basin water tem-
perature and glass cover temperature behaved in the same way but lagged the total incident solar radiation curve---this is mainly due to storage effects in the still. Variations of production rate of the still and hourly efficiency with local time for the different tests are shown in Fig. 4. Hourly efficiency is calculated using the relation r/h = (LhY)/(Ih) and the daily efficiency by rlaay = (Lh E Y) /(Y,Ih). Figure 4 shows that production rate and hourly efficiency follow solar radiation in their behavior but their maximum is lagging the solar radiation maximum, and again this is due to storage effects. The productivity and hourly efficiency of the still with internal condenser seems to be higher than that without internal condenser; the improvement in the hourly efficiency with internal condenser being about 15%. Theoretical and experimental productivity and hourly efficiency behave in the same way to an acceptable degree but the theoretical maximum lags the experimental maximum. Figure 5 shows the variations of daily production rate and daily efficiency with daily total solar radiation, and also shows : (i) daily productivity and daily efficiency of the still with internal condenser are higher than that of the still without internal condenser ; (ii) daily productivity and daily efficiency increases as the solar radiation intensity increases, so that the productivity of the still in the spring season is greater than that in the winter season, and it is predicted to be even higher in the summer season; (iii) maximum daily efficiency of the still without
Single effect solar still
641
(a)
(b)
iO0[-o S"tilLW i t h o o ~ , lY~rd11906 I e b"titl without conderi~r, Morch 1966 I ~ Experirrmntat 8 o [ - ' " Theoretical
Lo[- o StiLt with condenser, Morc~ 1986 [ • S~itl without condenser, M~'ch 1986 Q8
I - - Theoretical
5= 60-
~m 0.6 --
0.2
•
~/.~e
o1-I'
1
9
I0
I
I
II
I
12
I
13
14
15
0
16
9
mo
ii
=z
~3
I
J
14
m
I
m
Locot doy time (h) Local day time ( h ) Fig. 4. Measured, thereoticai production rate and hourly e~ciency of basin solar still.
cooling water through the internal condenser. His results showed an improvement in daily efficiencies while increasing the mass flow rates; this is mainly due to the increase in evaporation heat flux and condensation heat flux in the condenser, and no significant effect was observed on the convection and radiation heat fluxes. Theoretical results from a computer program are presented in Fig. 7, which shows predicted behavior of different types of heat fluxes and production rates at different basin water temperature and glass cover temperature, i.e. "different temperature difference". The figure shows that all types of heat fluxes and
condenser is about 50%, with about a 10% increase with internal condenser, but the daily efficiency of the still with internal condenser never exceeds 60%. Figure 6 shows how different types of heat fluxes vary through the daytime. It is clear that evaporation and condensation heat fluxes are dominant, convection and radiation heat fluxes are small, and conduction losses can be neglected for a good insulating solar still. In this work, only one mass flow rate was circulated through the internal condenser, but Khalifa [9] investigated the effect of different mass flow rates of the
(¢)
(b)
gell without m
getLMth cendemer
9
~7
_
o
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a
April May
r-~
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7o~
°
,-T=-
~lt
sey
~
eC
~112l
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.." ,t~%.,''^ . ,
~D
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~o
J
I0 Toter ~
i .
15 mdl~lon
l
!
2~ 25 intently (kd m'2day-I)
Fig. 5. Variation of daily production rate and daily solar efficiencywith daily total solar radiation.
642
S . T . Am,lED
b)
o)
x
ion
• Convection o Evaporation • Radiation
--
"1-
• Radiation 3
~ l ) ~
~-e
II
.....
12
e " - " "
e
13
14
- e--,.
15
3 m
II
12
13
14
15
Local day time ( h )
Locat day time (h)
Fig. 6. Variation of heat fluxes for a solar still through daytime hours.
Ca) 9OO x.
/
/ ---
Radiative Convective
7~
~E
/ ,/
1"
t
d
Basin w a t e r t.emper~ure
(=C)
Fig. 7. Theoretical variation of different types of heat fluxes and distillated water production rate of solar still with basin water temperature and glass cover temperature.
Single effect solar still production rate of the solar still increases as the basin water temperature increases and the difference between basin water temperature and glass cover temperature increases.
CONCLUSIONS (1) Using an internal condenser in a simple basin solar still improves daily efficiency about 10% and the productivity of the still from 5.5 kg/m 2 day to 5.9 kg/m 2 day, (2) The major design factors affecting energy utilization are basin temperature, condensing surface temperature, ambient air temperature, solar radiation and depth of water layer in the basin. Smaller water depth makes it easier to heat the water layer, so increasing the basin temperature which increases the rate of evaporation. Lower air temperature means lower condensing temperature which causes a high production of condensate. (3) Daily efficiency of the still increases, as incident solar radiation increases, up to 57% but it will never reach 60%.
REFERENCES
I. A. A. Delyannis and E. A. Delyannis, Water desalting. Chem. Engng 136-140 (1970). 2. G. O. G. L6f, J. A. Eibling and J. W. Bloemer, Energy balance in solar distillers. A.LCh.E.J. 7(4) (1961).
643
3. A. Delyannis and E. Piperoglou, Solar distillation in Greece. 1st Int. Syrup. Water Desalination, Washington, D.C. (Oct 1965). 4. R. A. Collings and T. Thompson, Forced convection multiple-effect solar still for desalting sea and brackish water. Proc. U.N. Conf. New Source of Energy, Rome. 6, 205-217 (1961). 5. W. N. Grune, R. B. Hughes and T. Thompson, Operating experiences with natural and forced convection solar stills. Water Sewage Works 108, 378-383 (1961). 6. M. A. S. Malik, G. N. Tiwari, A. Kumar and M. S. Sodha, Solar Distillation. Pergamon Press, Oxford (1982). 7. Bartali et al. 8. M. K. Abed, An experimental study for improving the performance of the simple solar stills. M.Sc. Thesis, University of Technology, Baghdad, lraq (1983). 9. Abdul-Jahbar N. Khalifa, Evaluation and energy balance study of a solar still with an internal condenser, d. Sol. Energy Res. 3, 1-11 (1985). 10. S. M. A. Moustafa, G. H. Brnswitz and D. M. Farmel, Direct use of solar energy for water desalination. Sol. Energy 22, 141-148 (1979). 1 I. S. I. Kanbour and H. S. Mafloob, Simple solar still performance synthesis and predictions. J. Sol. Energy Res. 3, 11-33 (1985). 12. G. O. G. L6f, Design and operating principles in solar distillation, Adv. Chem. Ser., No. 27 Saline Water (conversion) 156-165 (1960). 13. P. I. Cooper, The maximum efficiency of single effect solar stills. Sol. Energy 15, 205-217 (1973). 14. A. A. M. Sayigh (ed), Solar Energy Engineering, Ch. 20, pp. 431-464. Academic Press, New York (1977). 15. G. O. G. Lff, J. A. Eiblin~ and J. W. BIomer, Energy balance solar distiller. A.I. Ch. E. J. 7, 64 1-649 ( 1961). 16. R. N. Morse and W. R. Read, A rational basis for the engineering of a solar still. Sol. Energy 12, 5-17 (1968).
0741-983X/88 $3.00+.00 Pergamon Press pie
Printed in Great Britain.
S T U D Y OF S I N G L E - E F F E C T SOLAR STILL WITH A N INTERNAL CONDENSER S. T. AHM~ School of Technical Education, University of Technology, Baghdad, Iraq (Received 30 May 1988 : accepted 7 July 1988) Abstract--An experimental study was carried out to evaluate the effect of using an internal condenser on theperformance of a single-effect solar still. The still was tested in two different ways: first, it was used for water vapor condensation without condenser, and second, it was used for water vapor condensation with condenser. The still was single-sloped with a double pass internal condenser. Its effective base area was 0.4 m 2 and its cover slope was 30° to the horizontal. The results showed that combining an internal condenser with basin type solar still caused an improvement in the still performance. The still daily productivity was increased from 5.5 kg/m 2 day for the first test to 5.9 kg/m 2 day for the second test.
NOMENCLATURE a~ absorptivity of transparent cover for radiation aw absorptivity of water to radiation Ca heat capacity of transparent cover and supports per unit basic area, kJ/m 2K Cwb heat capacity of water basin and contents per unit basin area, kJ/m2K lh incident solar radiation on a horizontal surface, kW/m 2 Lh latent heat of evaporation, kJ/kg Pw vapor pressure of water at temperature of water surface, Pa Pws vapor pressure of water at temperature of transparent cover, Pa qb heat flux by conduction from basic area, k~V/m2 qc heat flux by convection from water surface to transparent cover, kW/m 2 q~ond heat lost to the circulated water, kW/m 2 qe heat flux by evaporation]condensation from water surface to transparent cover, kW/m 2 qp heat flux from transparent cover to air, kW/m 2 q, heat flux by radiation from cover to air, kW/m 2 t time, s T~ temperature of transparent cover, K Tw water temperature, K transmissivity of transparent cover for radiation Y still productivity, kg/h m 2 qd, qh daily and hourly efficiency, respectively. INTRODUCTION The single-effect, horizontal-basin solar still is a relatively simple device to construct and operate. The appeal of the simple solar still is its ability to run for m a n y years with negligible power costs and only minor attention to maintenance and repair. Most existing desalination plants use fossil fuels as a source of energy. Although a few techniques such 637
as multi-effect evaporation, multistage flash evaporation, thin film distillation, reverse osmosis and electrodialysis are energy intensive, they are also uneconomical when the supply of fresh water is small. The maximization of water production rate of the basin-type solar still is therefore a prime objective in the development of solar distillation. M a n y attempts have been made to design and study the performance of basin-type solar desalination systerns. Delyannis [1] reviewed the major solar distillation plants around the world. They ranged from a 4460 m 2 glass covered plant built at Las Salina, Chile, to desalinate brackish water, and now abandoned, to a 8690 m 2 plant constructed at Patomas, Greece, to desalinate sea water. L 6 f e t al. [2] presented energy and mass transfer relationships for a basintype solar still. The theoretical work was supplemented with data, from held operations, of a 232 m 2 still. The effect of variations of design parameters on the performance of the solar still was predicted by the aid of digital computer. Distiller productivity was correlated with atmospheric temperature, wind velocity, solar radiation absorptivity and slope of cover, as well as cover temperature. Delyannis and Piperoglou [3] reported on the use of internal condensers as an attempt to improve the performance of the basin-type solar still. They reported a n improvement of the productivity of the still up to 24%, resulting from the addition of the cooler to the deep basin still. Collins and Thompson [4] and G r u n e et ai. [5] made tests on a simple solar still coupled with an external condenser. Malik et al. [6] reported that Bartali et al. [7] developed a simple still coupled with a chimney containing a heat ex-
638
S. T. AHMED
changer and then entered the still. Abed [8] and Khalifa [9] made tests on a simple still coupled with an internal condenser. The condenser was tested in two different ways : first it was used for water preheating and water vapor condensation, and, secondly, it was used for water vapor condensation. However the results showed that combining an internal condenser with the basin-type solar still caused an improvement in the still performance. The still daily productivity was increased from 5 to 5.8 kg/m 2 day for the first test, and to 5.6 kg/m 2 day for the second test. Moustafa et al. [10] reported that the major design factors affecting energy utilization are basin temperature, condensing surface temperature and ambient air temperature. Kanbour and Matloob [11] tested the indoor model single-slopped cover solar still--a computer model was developed for the prediction of performance. In this work, a basin-type solar still with an internal condenser was constructed, and an investigation with the following objectives has been carried out : (1) to study the performance of a basin-type solar still combined with an internal condenser. (2) to explore design modification for increasing the productivity of a desalination unit. (3) to make a computer model to the still which can predict theoretically the effect of basin water temperature and glass cover temperature on the performance of the still.
between the water surface and cover glass can be approximated as that between two infinite parallel planes, which could be approximated by these equations [12, 13, 14] qr = 18.371 × 10-8(T 4 - T4)3600,
(1)
q, = 3.1824(Tw- Tq)
[" pw_PwgTw ~.,3 +[2.65~1~3 ~) (T.,-T.)3600, ( Pw --Pwg) qe = 16.276 x 10- 3qc ( T . , - T g ) "
(2) (3)
The heat flux due to condensation is assumed equal to that by evaporation and is given by q~ = 3600 YL h.
(4)
Heat lost to the circulated water could be calculated from eq. (4). Referring to Fig. 1, heat balance equations could be written for several conditions [11-16] as shown below. (1) Heat balance for basin water:
dT~
awzlh = qb+Cwb~-3600+qe+qr+q,..
(5)
(2) Heat balance for glass cover :
dr~
qe+qr+qc+aglh = qga+ C , ~
3600.
(6)
(3) Heat balance of the basin and cover assembly ENERGY CONSIDERATIONS
The energy absorbed in the basin water and liner of a solar still is dissipated by a variety of modes, as indicated in Fig. 1. These are : (i) unproductive conductive losses to the surroundings ; (ii) combined convection, evaporation and radiation to the inside glass surface. One of the first steps available to the designer for achieving high efficiencies is to minimize the conductive losses from the sides and base by adequate insulation of the still. By lowering the water depth in the basin and hence thermal mass, higher operating temperatures are attained. F o r a given ambient temperature, the driving potential for base and side losses increases as the water temperature increases, so underlining the importance of adequate insulation of the still. The predominant modes of heat transfer in a solar still are convection and evaporation, and radiation
ajh+qw--Zlh = Q~+q~ __.,dT. C dT~ +Cwb dt 3600+ gs dt 3600.
(7)
EXPERIMENTAL CONSIDERATIONS
An "ideal" solar still is defined as one which has no conductive losses and the water depth is sufficiently small so that the sensible heat stored is negligible compared with the energy transfer rates to and from the water. F o r a given ambient temperature, wind velocity (which determines the external convection coefficient) and solar radiation rate, such a still will instantaneously reach a steady state condition. With the above points in mind, a small solar still was designed for experimental operation. Figure 2 shows an isometric of the experimental solar still. A galvanized steel sheet, 1 mm thick, was folded to form a single-sloped tray with a depth of 10 cm, cover slope of 30 ° to the horizontal, and effective base area of 100 × 40 cm. A galvanized steel trough of 100 cm
Single effect solar sell
639
lh
L.eakage I
qgo
qco~
RefLection
[radlotlo~
ond
convec~lon)~
\\\
qe
L ;i'.
Absorption a.rlh \
.~
/,,I '/
qr
SensibLeheat
q¢
RefLection
....
Base ond side losses
Fig. 1. Energy flow diagram for the solar still. length was fixed to the tray for the collection of distillate. The condenser was constructed of a copper tube 1.2 cm in diameter and 230 cm long, and was bent to form a double-pass heat exchanger. The base was painted black, a 4 nun window glass sheet was used as a condensation surface and it was constructed in such
Condenser
a way as to ensure vapor tightness. The base and sides of the still were insulated with 7 cm thick glass wool. Finally the entire structure was encased in a wooden container made of 2.5 cm thick wooden sheet. To collect maximum energy, the still was oriented south. The amounts of condensate from both the glass cover and condenser mass flow rates were measured
( I ) CircuLated wateroutLet. (2) CircuLated water inlet.
Trough Wood conic GLosscover
Insu(otion Galvanized steel tray
Fig. 2. The experimental solar still.
640
S.T. A H ~
tO0
a)
(b)
ASolar radiation oBasin temperature • Glass temperature + Ambient airtemperature
I0o [ ~Solar radiation iI o Basin temperature l • GLass temperature O00
eO 8OO
To0
4~
0
I0
o
o
II 12 13 14 Local day time ( h )
+
400
15
16
300
0
9
I0
II 12 13 14 Local day time ( h )
15
16
300
Fig. 3. Solar radiation, basin water temperature, glass cover temperature and ambient air temperature. separately by means of glass bottles with 3 1 capacity each, and with divisions of 10 ml. Temperature of the basin water and glass cover were measured by means of copper constant in thermocouples, using a millivolt meter with divisions of 0.1 mV. Dry bulb air temperature was measured by means of a mercury thermometer with divisions of 0.1 °C. Total incident solar radiation on a horizontal surface was measured in the field using a solar intensity water. The above readings were taken at mid-hour intervals from 8:30 a.m. to 4:30 p.m. (spring time) during the period of February to May, 1986, in Baghdad. Fresh water was used to operate the still through all tests. T E S T A. The still was tested with an initial depth of 1 cm without using the condenser. T E S T B. The still was tested with an initial depth of 1 cm using the condenser, with the cooling water circulated through the condenser from 8:30 a.m. to 4:30 pm. Khalifa [9] investigated the effect of different mass flow rates through the internal condenser ; here only one mass flow rate was used. RESULTS AND DISCUSSIONS The still was continuously operated from February 1986 to May 1986. Total incident solar radiation, ambient air temperature, glass cover temperature and basin water temperature for the still with and without internal condenser are shown in Fig. 3. This figure shows that total incident solar radiation gradually increased to reach its maximum value at the middle of the day and then decreased. Basin water tem-
perature and glass cover temperature behaved in the same way but lagged the total incident solar radiation curve---this is mainly due to storage effects in the still. Variations of production rate of the still and hourly efficiency with local time for the different tests are shown in Fig. 4. Hourly efficiency is calculated using the relation r/h = (LhY)/(Ih) and the daily efficiency by rlaay = (Lh E Y) /(Y,Ih). Figure 4 shows that production rate and hourly efficiency follow solar radiation in their behavior but their maximum is lagging the solar radiation maximum, and again this is due to storage effects. The productivity and hourly efficiency of the still with internal condenser seems to be higher than that without internal condenser; the improvement in the hourly efficiency with internal condenser being about 15%. Theoretical and experimental productivity and hourly efficiency behave in the same way to an acceptable degree but the theoretical maximum lags the experimental maximum. Figure 5 shows the variations of daily production rate and daily efficiency with daily total solar radiation, and also shows : (i) daily productivity and daily efficiency of the still with internal condenser are higher than that of the still without internal condenser ; (ii) daily productivity and daily efficiency increases as the solar radiation intensity increases, so that the productivity of the still in the spring season is greater than that in the winter season, and it is predicted to be even higher in the summer season; (iii) maximum daily efficiency of the still without
Single effect solar still
641
(a)
(b)
iO0[-o S"tilLW i t h o o ~ , lY~rd11906 I e b"titl without conderi~r, Morch 1966 I ~ Experirrmntat 8 o [ - ' " Theoretical
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Locot doy time (h) Local day time ( h ) Fig. 4. Measured, thereoticai production rate and hourly e~ciency of basin solar still.
cooling water through the internal condenser. His results showed an improvement in daily efficiencies while increasing the mass flow rates; this is mainly due to the increase in evaporation heat flux and condensation heat flux in the condenser, and no significant effect was observed on the convection and radiation heat fluxes. Theoretical results from a computer program are presented in Fig. 7, which shows predicted behavior of different types of heat fluxes and production rates at different basin water temperature and glass cover temperature, i.e. "different temperature difference". The figure shows that all types of heat fluxes and
condenser is about 50%, with about a 10% increase with internal condenser, but the daily efficiency of the still with internal condenser never exceeds 60%. Figure 6 shows how different types of heat fluxes vary through the daytime. It is clear that evaporation and condensation heat fluxes are dominant, convection and radiation heat fluxes are small, and conduction losses can be neglected for a good insulating solar still. In this work, only one mass flow rate was circulated through the internal condenser, but Khalifa [9] investigated the effect of different mass flow rates of the
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Fig. 5. Variation of daily production rate and daily solar efficiencywith daily total solar radiation.
642
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ion
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Locat day time (h)
Fig. 6. Variation of heat fluxes for a solar still through daytime hours.
Ca) 9OO x.
/
/ ---
Radiative Convective
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~E
/ ,/
1"
t
d
Basin w a t e r t.emper~ure
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Fig. 7. Theoretical variation of different types of heat fluxes and distillated water production rate of solar still with basin water temperature and glass cover temperature.
Single effect solar still production rate of the solar still increases as the basin water temperature increases and the difference between basin water temperature and glass cover temperature increases.
CONCLUSIONS (1) Using an internal condenser in a simple basin solar still improves daily efficiency about 10% and the productivity of the still from 5.5 kg/m 2 day to 5.9 kg/m 2 day, (2) The major design factors affecting energy utilization are basin temperature, condensing surface temperature, ambient air temperature, solar radiation and depth of water layer in the basin. Smaller water depth makes it easier to heat the water layer, so increasing the basin temperature which increases the rate of evaporation. Lower air temperature means lower condensing temperature which causes a high production of condensate. (3) Daily efficiency of the still increases, as incident solar radiation increases, up to 57% but it will never reach 60%.
REFERENCES
I. A. A. Delyannis and E. A. Delyannis, Water desalting. Chem. Engng 136-140 (1970). 2. G. O. G. L6f, J. A. Eibling and J. W. Bloemer, Energy balance in solar distillers. A.LCh.E.J. 7(4) (1961).
643
3. A. Delyannis and E. Piperoglou, Solar distillation in Greece. 1st Int. Syrup. Water Desalination, Washington, D.C. (Oct 1965). 4. R. A. Collings and T. Thompson, Forced convection multiple-effect solar still for desalting sea and brackish water. Proc. U.N. Conf. New Source of Energy, Rome. 6, 205-217 (1961). 5. W. N. Grune, R. B. Hughes and T. Thompson, Operating experiences with natural and forced convection solar stills. Water Sewage Works 108, 378-383 (1961). 6. M. A. S. Malik, G. N. Tiwari, A. Kumar and M. S. Sodha, Solar Distillation. Pergamon Press, Oxford (1982). 7. Bartali et al. 8. M. K. Abed, An experimental study for improving the performance of the simple solar stills. M.Sc. Thesis, University of Technology, Baghdad, lraq (1983). 9. Abdul-Jahbar N. Khalifa, Evaluation and energy balance study of a solar still with an internal condenser, d. Sol. Energy Res. 3, 1-11 (1985). 10. S. M. A. Moustafa, G. H. Brnswitz and D. M. Farmel, Direct use of solar energy for water desalination. Sol. Energy 22, 141-148 (1979). 1 I. S. I. Kanbour and H. S. Mafloob, Simple solar still performance synthesis and predictions. J. Sol. Energy Res. 3, 11-33 (1985). 12. G. O. G. L6f, Design and operating principles in solar distillation, Adv. Chem. Ser., No. 27 Saline Water (conversion) 156-165 (1960). 13. P. I. Cooper, The maximum efficiency of single effect solar stills. Sol. Energy 15, 205-217 (1973). 14. A. A. M. Sayigh (ed), Solar Energy Engineering, Ch. 20, pp. 431-464. Academic Press, New York (1977). 15. G. O. G. Lff, J. A. Eiblin~ and J. W. BIomer, Energy balance solar distiller. A.I. Ch. E. J. 7, 64 1-649 ( 1961). 16. R. N. Morse and W. R. Read, A rational basis for the engineering of a solar still. Sol. Energy 12, 5-17 (1968).