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Construction and performance of an assisted solar distiller
DESALINATION Desalination 173 (2005) 249-255
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Construction and performance of an assisted solar distiller C a r m e n E s t e b a n a*, J u d i t h F r a n c o b, A m i l c a r F a s u l o a aUniversidad Nacional de San Luis, Chacabuco y Pedernera, 5700 San Luis, Argentina Tel. +54 424689, ext. 103; email: [email protected] bUniversidad Nacional de Salta, Buenos Aires 177, 4400 Salta, Argentina
Received 30 March 2004; accepted 26 August 2004
Abstract
The design and construction of a new type of assisted solar distiller is presented. The device consists of a basin-type distiller, with the peculiarity that the basin liner is extended as an integrated solar collector accumulator with quite simple characteristics, recently developed at the National University of San Luis (Argentina). The covering is easy to remove for cleansing, and an improvement of the design for future tasks is being considered. The new distiller was compared with a common basin-type distiller and a commonplace basin-type distiller coupled with a fiat solar collector. Daytime, nocturnal, weekly and hourly measurements showed that the daily production of the new assisted solar distiller always surpassed that of the basin-type distiller by approximately 70%, and that of the basin-type distiller coupled with a fiat solar collector by approximately 20%. Results of 2 years of measuring are presented. Keywords: Assisted solar distiller; Basin-type still; Efficiency
I. Introduction The efficiency of basin-type solar distillers (BD) fluctuates around 40% and varies heavily with weather conditions. The main drawback of these distillers is the rapid deterioration of the absorbent black covering at the basin liner, which quickly becomes covered by salt, losing a major portion of its properties and therefore needing regular cleansings. Since water production occurs in coincidence with the hours of sunshine, a way o f augrnenting the output of BDs is thermal assistance. This can *Corresponding author.
be done by incorporation of a flat-plate collector, as done by Kumar and Tiwari [ 1] or Voroupoulos et al. [2]. In the Solar Energy Laboratory of San Luis National University, located at a latitude of 33.27 ° south and a longitude of 66.2 ° W, we have previous experience in construction of BDs [3,4], as well as of flat-plate solar collectors and cylindrical solar collector accumulators [5,6]. This time we have combined all three types and arrived at the construction of a solar distiller assisted by an integrated solar collector storage unit, which we shall name DICS. Integrated solar collector accumulators (ICS) have been recently developed in our laboratory,
001 I-9164/05/$- See front matter © 2005 Elsevier B.V. All rights reserved doi: 10.1016/j.desal.2004.08.033
250
c. Esteban et aL / Desalination 173 (2005) 249-255
with simple characteristics. They are made of a cylindrical stainless-steel tank covered with matte black paint and thermally protected by insulation semi-transparent to solar radiation. Investigation of the ICSs showed that the water that filled the tanks had a high degree of stratification, which was evidenced by their internal temperature readings. Furthermore, it was found that the thermal losses occurred in the upper part of the tanks [7]. This allowed us to consider this type of accumulator as an appropriate component to feed a BD thermally. From these investigations the design and construction of the DICS were developed, which consist of a covering like that used in conventional solar distillers, the basin liner being an ICS. For the purposes of comparison, a BD of equal condensing surface and same kind of insulation as the DICS to be tested was constructed. A systematic and comparative evaluation of both systems was performed, taking into account the environmental physical variables. Comparisons were also performed between the DICS and the BD assisted by a plane collector. Day-time and night-time production was measured.
center of the base) from altering the established thermal stratification. At the upper end of the tank, which is open, a BD was installed. The tray of the distiller is stainless steel, and its dimensions are 1.20 m in length, 0.83 m in width, and 0.08 m in height. At the basin liner of the tray there is a circular opening the same diameter as the collecting tank. Both openings are made to coincide, and the rims are welded. By positioning the distiller with its length in a north-south direction and its surpassing end to the south, the shadow the tray thrown on the tank is minimum. The tray is thermally insulated with expanded polystyrene and is sheltered from rough weather with a metal sheet protection. Fig. 1 shows a schematic representation of the device.
N~
~S
\
@
2. Design and construction of the distiller The covering of the assisted distiller is like that of conventional ones, the only difference being that the basin liner constituent is the abovementioned ICS. The DICS consists of a cylindrical stainless-steel tank, 0.8 m in diameter and 1.2 m in height. The outer surface is painted in a matte black color. The tank is isolated by means of two layers of alveolar polycarbonate, separated from each other and from the tank by 0.03 m. This covering permits the solar radiation to reach the tank and heat the water. A metal plate parallel to the base at approximately 0.05 m was placed inside the tank. This plate prevents the flow of cold water (which enters the tank through a tube located at the
ili
@ Fig. 1. Scheme of the DICS. 1 assemblage of the glass cover with roof gutter for collecting distillate and glass support, 2 stainless steel tray with polystyreneinsulation, 3 protecting foils, 4 membrane of black plastic with holes, 5 stainless-steel collecting tank painted black, 6 polycarbonate insulation, 7 dispersive plate, 8 tube for water inlet, 9 foundation.
C. Esteban et al. / Desalination 173 (2005) 249-255
At the liner basin of the tray a black plastic membrane plastic with holes was placed, which allow the entrance of water from the tank to the distiller. A tube of 0.01 m in diameter and 0.035 m in length leading to the exterior prevents flooding of the water in the tray, which may damage the device isolation. This tube also provides a reference level for comparison of the two distillers by ensuring both are filled to the same level. The cover of the distiller is glass and a design similar to the one used by Follari [4] in order to allow for easy disassembling and cleansing. The design by Follari includes a tray separate from the cover glass so that water entering the tray creates a simple and safe hydraulic seal. The roof gutters are made of stainless steel. At both ends of each of the four roof gutters there is an opening with a tube for the outflow of the distillate. The glass parts are settled on the roof gutters and sealed to each other and to the roof gutters with silicone adherent. The glass cover facing north has an inclination 22 ° in and that facing south has an inclination of 80 ° . The cover sides facing east and west are also glass and are placed vertically. The hermetic closure of the air chamber from outside, between the glass cover and the waterfilled tray, is made by the tray water itself. Consequently, removal of the lid for cleansing of the tray is performed rapidly and without difficulty. The whole device is settled on a foundation. The lower surface of the tank is isolated with an agglomerate foil.
3. Data collection
All measurements were performed in our laboratory. The field (outdoors) experiments can be grouped into four stages: First Stage: The DICS and the BD were run jointly. Measurements were performed at 08:30 and 20:30 h during distillate production. The
251
temperature of the water in the tray, of the covering of each distiller and of the environment was measured. This experiment was run in October 2000. Second Stage: A fiat collector with a 2 m 2 collecting surface was coupled to the BD (BDC). The values of the same variables as in Stage 1 and at the same times were recorded. This experiment was run in November and December of 2000. Third Stage: Weekly measurements of the output of the DICS and the BD were performed. The tests done extended till March 2002, being interrupted in the winter of 2001 to perform a thorough cleansing of the distillers and the accumulating tank of the DICS, at the bottom of which great quantities of salts had precipitated. A second interruption in November and December 2000 was done for performing the measurements for the second stage. Fourth Stage: Measurements of distillate production and temperature of the tray were performed every 2 h from 07:00 till 23:00 during 1 day with the DICS and with the BDC. The temperature measurements inside the distillers were performed with T-type thermocouples. The environmental temperature was measured with an accuracy of 1°C. The distillate production of Stages One and Two were measured with an accuracy of 1%.The weekly output was measured with an accuracy of 5%. Solar radiation on horizontal surfaces and direct radiation are permanently recorded in our laboratory. The global horizontal solar radiation is measured with an Eppley precision pyranometer. Direct normal radiation is measured with an Eppley pyrheliometer mounted on a solar tracker. Both solarimeters are periodically calibrated with an absolute cavity solar radiometer, Eppley model AHF. Filling the trays of the distillers was done manually in the morning, immediately after the first distillate extraction of the day. The trays were filled to a predetermined level of 3 cm.
252
C. Esteban et aL / Desalination 173 (2005) 249-255
4. Data analysis
BD, and vice versa for the points below the line. It can be observed that all points except one are above the equality line. The average DICS production surpassed that of the BD by 69%.
4.1. Daily analysis 4.1.1. Comparison of DICS and BD For this analysis we used a sample of 25 points obtained by measuring the diurnal and the nocturnal production of both distillers, recorded during October 2000. Daily production of both distillers was practically the same, while DICS production was higher at night. The greater output of the DICS at night is accounted for its greater thermal inertia, which causes the tray water to remain at a temperature higher than that of the condensing surface for a longer period than in common distillers. Total daily production can be compared in Fig. 2. The straight line corresponds to equal production of the distillers. Points above the line indicate a higher production of the DICS compared to the
7.00
O
6.oo
,I o
5.00
°
°
4.00
=o 3.00 a. 2.00
4.1.2. Comparison of DICS and BDC For this analysis we used 44 measurement points obtained in November and December of 2000. The diurnal production BDC was 10% above that of the DICS. However, since the DICS nocturnal production markedly exceeded that of the BDC (746%), the DICS daily production was 20% greater in average. 4.2. Weekly analysis Measurements of the weekly output of the DICS and the BD were performed from October 2000 to February 2002, with an interruption in May and June 2001 for cleansing and reconditioning of the distillers and in November and December 2000, for comparison between the DICS and the BDC. A sample of 50 measuring points was obtained, with each point indicating the production of 7 days. The weekly production of both distillers is compared in Fig. 3. As can be observed from the figure, the DICS weekly production was greater than that of the BD, as was to be anticipated considering the results obtained in the daily analysis. Fig.4 shows the temporal development of the weekly production of the distillers. Table 1 shows the production values for both distillers. During the total measuring period, the DICS produced 77% more distillate than the BD.
1.00 0.00 0.00
1.00
2.00
3.00
4.00
......... 5.00
Production BD (I)
Fig. 2. Daily production (1) of the DICS compared to basin type distiller.The straight line correspondsto equal production by the two distillers.
Table 1 Weekly productionof DICS and BD Weekly production, 1
DICS
BD
Summer average Winter average Maximum Minimum
36.5 17.7 46.3 8.6
21.5 7.7 29.3 3
253
C. Esteban et al. / Desalination 173 (2005) 249-255
50
v
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40
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30
•
5
7
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p
CL
10
/
~--
/
10
15
t7
19
21
24
32
Fig. 5. DICS and BDC distillate production every 2 h during one day. ,~ 80
x x X
40
0
13
Time(h)
~
,iO•
11
~/• ~ ~
20
9
20
30
40
a. 20 fi
I-.
Production BD (I)
o o x
]}
t
" z
X
0
X ,
5
Fig. 3. Weekly production (in L) of the DICS with reference to a basin-type distiller.
,DI£~XBEC
,
15
25
35
Time (h)
Fig. 6. DICS and BDC tray water temperature every 2 h during one day.
50 45
° N
40
o
35
° 0000
~_ 30 co 25
~
lanP° as •
o°#
o ep
B
@ 20 0
=,
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17-5-01
Dale
16-9-01
16-1-02
in Fig. 5. As can be observed in this figure, from noon till about 21 h, the DICS distilled a lower quantity of water than the BDC, while during the remaining hours of the day its output surpassed that of BDC. This is consistent with the temperature variation in the trays of both distillers: the BDC has a higher temperature than the DICS between noon and 21 h, as can be observed in Fig. 6.
I DI(~3• BID
Fig. 4. Weekly production of DICS and basin type distiller from October 2000 to March 2002.
4.3. Hourly analysis
Measurements were taken every 2 h during the times of sunshine in order to compare the variations of distilled water production of the DICS and the BDC. These measurements are displayed
4.4. Efficiency
The efficiency of the distillers is defined as the ratio between the heat needed to evaporate the produced water and the energy received by the distiller [8]: ~i = Ah * w i / G i
(1)
where Ah is the enthalpy change from cold water
254
C. Esteban et al. / Desalination 173 (2005)249-255
to vapor (2.5 M J/kg), w is the weight of the distillate produced in the time unit, G is the solar radiation intensity received by the distiller in the time unit, and subscript i indicates the distiller type. The procedure followed for calculating G in each device is reported in the Appendix. To compare DICS and BD, the data of the Third Stage were used. For calculation of the efficiency, the time unit was the week. To compare DICS and BDC, the data obtained during the Second Stage were used, with the day as the time unit for efficiency calculation. 90 80 ~. 70 60 5O 40 3O oq 20 10 0
DICS
,,
50% ....... t0%
BD
5. Conclusions r
0
100
200
300
i
i
400
500
Solar radiation (MJ) Fig. 7. Comparison o f the DICS and the BD. Quantity o f
pure water produced in the week vs. the intensity of the solar radiation on the distiller during the same period of time. The straight lines correspond to constant efficiency at values of 10% and 50%. 10 8 .o "5 -o o
In accordance with this and Eq. (1), it was found that when the daily data of the DICS and the BDC are contrasted, the percent daily efficiencies are 22% and 16%, respectively. On the other hand, comparison of the DICS and BD weekly data of indicates that the percent weekly efficiencies are 25% and 28%. In Figs. 7 and 8 the quantity of pure water produced per area unit of the tray in the time unit is plotted against the intensity of the solar radiation on the distiller during the same period of time. In these figures the straight lines of constant efficiency [as defined by Eq. (1)] at the values 10% and 50% can be observed. Fig. 7 shows the comparison of the DICS and the BD while Fig. 8 compares the DICS with the BDC.
z ....
BDC 10%
°
D[CS / 50%
/ '
6
,1t
i
4
m
"
2
20
40
60
80
Solar radiation (MJ)
Fig. 8. Comparison of the DICS and the BDC. Quantity of pure water produced per area unit of the tray in the day
vs. the intensity of the solar radiation on the distiller during the same period of time. The straight lines correspond to constant efficiency at values of 10% and 50%.
The following results were obtained from the comparison of the DICS with the BD: • Production of the DICS in diurnal time spans is greater or equal to that of the BD, but is slightly smaller compared to the BDC. This last observation is consistent with the higher temperature reached by the tray part of the tray plus fiat collector, compared to the DICS tray temperature. ° Production of the DICS during nocturnal hours is always superior to that of the other two distillers. ° The daily production of the DICS always surpasses that of the other two: by approximately 70% that of the BD, and by about 20% that of the BDC. • The average daily production of the DICS is of 2.5 L between April and September and 5.2 L between October and March. ° Efficiency of the DICS is slightly inferior to that of the BD due to the larger energy collecting surface. ° When adding the fiat collector to the BD, its efficiency is reduced with respect to that of the DICS.
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C. Esteban et al. / Desalination 173 (2005) 249-255
,
The frequency of cleansing is lower for the DICS than for the tray-type distiller, as in the former the salts are deposited at the bottom of the tank and not in the tray. • The DICS covering is easy to remove for cleansing, and an improvement of the design for future tasks is being considered.
References [1] S. Kumar and G. Tiwari, Estimation of convective mass transfer in solar distillation systems. Solar Energy, 57 (1996) 459--464. [2] K. Voroupoulos,E. Mathiioulakisand V. Belessiotis, Experimental investigation of a solar still coupled with solar collectors. Desalination, 138 (2001) 103110. [3] A. Fasulo, V. Cortinez and L. Odicino, Planta de destilacirn solar de agua para la Facultadde Quimica Bioquimica y Farmacia de la UNSL. Actas de ASADES, 1987. [4] J. Follari,Un destilador solar desmontable.Actas 17~ ASADES, 1 (1994)45--49. [5] A. Fasulo, J. Follari and J. Barral, Comparison between a simple solar collector accumulatorand conventionalaccumulator,Solar Energy,71 (2001) 389401. [6] A. Fasulo, D. Perell6 and J. Follari, Un colector solar aeumulador, Avances en Energlas Renovables y Medio Ambiente, 1 (1997) 93-96. [7] A. Fasulo, The thermal losses of a solar accumulator collector, ISES 2000. [8] E.D. Howe, Fundamentals of Water Desalination, Science and Technology Series, Vol. 1, Marcel Dekker, New York, 1974.
Appendix each device
Procedure for calculating Gt in
BD: the tray surface, Sb, receives horizontal global solar radiation I (M j/m2). Therefore,
GBD
=
I * Sb
BDC: The radiation collecting surface is the tray surface, Sb, plus the collecting plane surface, Sp. The tray receives the global radiation on the horizontal surface,/, and the collecting plane receives the normal radiation on a 45 ° sloped surface facing north (I45N). Then GBDc = 1 * Sb + I45N * Sp
DICS: The surface that receives radiation is the surface of the tray, Sb, plus the collecting tank surface, Sc. The tray receives the global radiation on the horizontal surface, L The half of the cylinder facing the sun receives the normal radiation on the vertical surface, on an area S c / n (which is the vertical projection of the cylinder half exposed to the sun). The tank surface receives 0.5 o f the diffuse radiation Id. GDICS = f *Sb +Id90 * Sc + 0.5] d * Sc TC
ELSEVIER
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Construction and performance of an assisted solar distiller C a r m e n E s t e b a n a*, J u d i t h F r a n c o b, A m i l c a r F a s u l o a aUniversidad Nacional de San Luis, Chacabuco y Pedernera, 5700 San Luis, Argentina Tel. +54 424689, ext. 103; email: [email protected] bUniversidad Nacional de Salta, Buenos Aires 177, 4400 Salta, Argentina
Received 30 March 2004; accepted 26 August 2004
Abstract
The design and construction of a new type of assisted solar distiller is presented. The device consists of a basin-type distiller, with the peculiarity that the basin liner is extended as an integrated solar collector accumulator with quite simple characteristics, recently developed at the National University of San Luis (Argentina). The covering is easy to remove for cleansing, and an improvement of the design for future tasks is being considered. The new distiller was compared with a common basin-type distiller and a commonplace basin-type distiller coupled with a fiat solar collector. Daytime, nocturnal, weekly and hourly measurements showed that the daily production of the new assisted solar distiller always surpassed that of the basin-type distiller by approximately 70%, and that of the basin-type distiller coupled with a fiat solar collector by approximately 20%. Results of 2 years of measuring are presented. Keywords: Assisted solar distiller; Basin-type still; Efficiency
I. Introduction The efficiency of basin-type solar distillers (BD) fluctuates around 40% and varies heavily with weather conditions. The main drawback of these distillers is the rapid deterioration of the absorbent black covering at the basin liner, which quickly becomes covered by salt, losing a major portion of its properties and therefore needing regular cleansings. Since water production occurs in coincidence with the hours of sunshine, a way o f augrnenting the output of BDs is thermal assistance. This can *Corresponding author.
be done by incorporation of a flat-plate collector, as done by Kumar and Tiwari [ 1] or Voroupoulos et al. [2]. In the Solar Energy Laboratory of San Luis National University, located at a latitude of 33.27 ° south and a longitude of 66.2 ° W, we have previous experience in construction of BDs [3,4], as well as of flat-plate solar collectors and cylindrical solar collector accumulators [5,6]. This time we have combined all three types and arrived at the construction of a solar distiller assisted by an integrated solar collector storage unit, which we shall name DICS. Integrated solar collector accumulators (ICS) have been recently developed in our laboratory,
001 I-9164/05/$- See front matter © 2005 Elsevier B.V. All rights reserved doi: 10.1016/j.desal.2004.08.033
250
c. Esteban et aL / Desalination 173 (2005) 249-255
with simple characteristics. They are made of a cylindrical stainless-steel tank covered with matte black paint and thermally protected by insulation semi-transparent to solar radiation. Investigation of the ICSs showed that the water that filled the tanks had a high degree of stratification, which was evidenced by their internal temperature readings. Furthermore, it was found that the thermal losses occurred in the upper part of the tanks [7]. This allowed us to consider this type of accumulator as an appropriate component to feed a BD thermally. From these investigations the design and construction of the DICS were developed, which consist of a covering like that used in conventional solar distillers, the basin liner being an ICS. For the purposes of comparison, a BD of equal condensing surface and same kind of insulation as the DICS to be tested was constructed. A systematic and comparative evaluation of both systems was performed, taking into account the environmental physical variables. Comparisons were also performed between the DICS and the BD assisted by a plane collector. Day-time and night-time production was measured.
center of the base) from altering the established thermal stratification. At the upper end of the tank, which is open, a BD was installed. The tray of the distiller is stainless steel, and its dimensions are 1.20 m in length, 0.83 m in width, and 0.08 m in height. At the basin liner of the tray there is a circular opening the same diameter as the collecting tank. Both openings are made to coincide, and the rims are welded. By positioning the distiller with its length in a north-south direction and its surpassing end to the south, the shadow the tray thrown on the tank is minimum. The tray is thermally insulated with expanded polystyrene and is sheltered from rough weather with a metal sheet protection. Fig. 1 shows a schematic representation of the device.
N~
~S
\
@
2. Design and construction of the distiller The covering of the assisted distiller is like that of conventional ones, the only difference being that the basin liner constituent is the abovementioned ICS. The DICS consists of a cylindrical stainless-steel tank, 0.8 m in diameter and 1.2 m in height. The outer surface is painted in a matte black color. The tank is isolated by means of two layers of alveolar polycarbonate, separated from each other and from the tank by 0.03 m. This covering permits the solar radiation to reach the tank and heat the water. A metal plate parallel to the base at approximately 0.05 m was placed inside the tank. This plate prevents the flow of cold water (which enters the tank through a tube located at the
ili
@ Fig. 1. Scheme of the DICS. 1 assemblage of the glass cover with roof gutter for collecting distillate and glass support, 2 stainless steel tray with polystyreneinsulation, 3 protecting foils, 4 membrane of black plastic with holes, 5 stainless-steel collecting tank painted black, 6 polycarbonate insulation, 7 dispersive plate, 8 tube for water inlet, 9 foundation.
C. Esteban et al. / Desalination 173 (2005) 249-255
At the liner basin of the tray a black plastic membrane plastic with holes was placed, which allow the entrance of water from the tank to the distiller. A tube of 0.01 m in diameter and 0.035 m in length leading to the exterior prevents flooding of the water in the tray, which may damage the device isolation. This tube also provides a reference level for comparison of the two distillers by ensuring both are filled to the same level. The cover of the distiller is glass and a design similar to the one used by Follari [4] in order to allow for easy disassembling and cleansing. The design by Follari includes a tray separate from the cover glass so that water entering the tray creates a simple and safe hydraulic seal. The roof gutters are made of stainless steel. At both ends of each of the four roof gutters there is an opening with a tube for the outflow of the distillate. The glass parts are settled on the roof gutters and sealed to each other and to the roof gutters with silicone adherent. The glass cover facing north has an inclination 22 ° in and that facing south has an inclination of 80 ° . The cover sides facing east and west are also glass and are placed vertically. The hermetic closure of the air chamber from outside, between the glass cover and the waterfilled tray, is made by the tray water itself. Consequently, removal of the lid for cleansing of the tray is performed rapidly and without difficulty. The whole device is settled on a foundation. The lower surface of the tank is isolated with an agglomerate foil.
3. Data collection
All measurements were performed in our laboratory. The field (outdoors) experiments can be grouped into four stages: First Stage: The DICS and the BD were run jointly. Measurements were performed at 08:30 and 20:30 h during distillate production. The
251
temperature of the water in the tray, of the covering of each distiller and of the environment was measured. This experiment was run in October 2000. Second Stage: A fiat collector with a 2 m 2 collecting surface was coupled to the BD (BDC). The values of the same variables as in Stage 1 and at the same times were recorded. This experiment was run in November and December of 2000. Third Stage: Weekly measurements of the output of the DICS and the BD were performed. The tests done extended till March 2002, being interrupted in the winter of 2001 to perform a thorough cleansing of the distillers and the accumulating tank of the DICS, at the bottom of which great quantities of salts had precipitated. A second interruption in November and December 2000 was done for performing the measurements for the second stage. Fourth Stage: Measurements of distillate production and temperature of the tray were performed every 2 h from 07:00 till 23:00 during 1 day with the DICS and with the BDC. The temperature measurements inside the distillers were performed with T-type thermocouples. The environmental temperature was measured with an accuracy of 1°C. The distillate production of Stages One and Two were measured with an accuracy of 1%.The weekly output was measured with an accuracy of 5%. Solar radiation on horizontal surfaces and direct radiation are permanently recorded in our laboratory. The global horizontal solar radiation is measured with an Eppley precision pyranometer. Direct normal radiation is measured with an Eppley pyrheliometer mounted on a solar tracker. Both solarimeters are periodically calibrated with an absolute cavity solar radiometer, Eppley model AHF. Filling the trays of the distillers was done manually in the morning, immediately after the first distillate extraction of the day. The trays were filled to a predetermined level of 3 cm.
252
C. Esteban et aL / Desalination 173 (2005) 249-255
4. Data analysis
BD, and vice versa for the points below the line. It can be observed that all points except one are above the equality line. The average DICS production surpassed that of the BD by 69%.
4.1. Daily analysis 4.1.1. Comparison of DICS and BD For this analysis we used a sample of 25 points obtained by measuring the diurnal and the nocturnal production of both distillers, recorded during October 2000. Daily production of both distillers was practically the same, while DICS production was higher at night. The greater output of the DICS at night is accounted for its greater thermal inertia, which causes the tray water to remain at a temperature higher than that of the condensing surface for a longer period than in common distillers. Total daily production can be compared in Fig. 2. The straight line corresponds to equal production of the distillers. Points above the line indicate a higher production of the DICS compared to the
7.00
O
6.oo
,I o
5.00
°
°
4.00
=o 3.00 a. 2.00
4.1.2. Comparison of DICS and BDC For this analysis we used 44 measurement points obtained in November and December of 2000. The diurnal production BDC was 10% above that of the DICS. However, since the DICS nocturnal production markedly exceeded that of the BDC (746%), the DICS daily production was 20% greater in average. 4.2. Weekly analysis Measurements of the weekly output of the DICS and the BD were performed from October 2000 to February 2002, with an interruption in May and June 2001 for cleansing and reconditioning of the distillers and in November and December 2000, for comparison between the DICS and the BDC. A sample of 50 measuring points was obtained, with each point indicating the production of 7 days. The weekly production of both distillers is compared in Fig. 3. As can be observed from the figure, the DICS weekly production was greater than that of the BD, as was to be anticipated considering the results obtained in the daily analysis. Fig.4 shows the temporal development of the weekly production of the distillers. Table 1 shows the production values for both distillers. During the total measuring period, the DICS produced 77% more distillate than the BD.
1.00 0.00 0.00
1.00
2.00
3.00
4.00
......... 5.00
Production BD (I)
Fig. 2. Daily production (1) of the DICS compared to basin type distiller.The straight line correspondsto equal production by the two distillers.
Table 1 Weekly productionof DICS and BD Weekly production, 1
DICS
BD
Summer average Winter average Maximum Minimum
36.5 17.7 46.3 8.6
21.5 7.7 29.3 3
253
C. Esteban et al. / Desalination 173 (2005) 249-255
50
v
• •
40
--
•t ~1)
0 4)
"o
4 /
30
•
5
7
"o
~ *
p
CL
10
/
~--
/
10
15
t7
19
21
24
32
Fig. 5. DICS and BDC distillate production every 2 h during one day. ,~ 80
x x X
40
0
13
Time(h)
~
,iO•
11
~/• ~ ~
20
9
20
30
40
a. 20 fi
I-.
Production BD (I)
o o x
]}
t
" z
X
0
X ,
5
Fig. 3. Weekly production (in L) of the DICS with reference to a basin-type distiller.
,DI£~XBEC
,
15
25
35
Time (h)
Fig. 6. DICS and BDC tray water temperature every 2 h during one day.
50 45
° N
40
o
35
° 0000
~_ 30 co 25
~
lanP° as •
o°#
o ep
B
@ 20 0
=,
10 5 0 15-9-00
15-1-0I
17-5-01
Dale
16-9-01
16-1-02
in Fig. 5. As can be observed in this figure, from noon till about 21 h, the DICS distilled a lower quantity of water than the BDC, while during the remaining hours of the day its output surpassed that of BDC. This is consistent with the temperature variation in the trays of both distillers: the BDC has a higher temperature than the DICS between noon and 21 h, as can be observed in Fig. 6.
I DI(~3• BID
Fig. 4. Weekly production of DICS and basin type distiller from October 2000 to March 2002.
4.3. Hourly analysis
Measurements were taken every 2 h during the times of sunshine in order to compare the variations of distilled water production of the DICS and the BDC. These measurements are displayed
4.4. Efficiency
The efficiency of the distillers is defined as the ratio between the heat needed to evaporate the produced water and the energy received by the distiller [8]: ~i = Ah * w i / G i
(1)
where Ah is the enthalpy change from cold water
254
C. Esteban et al. / Desalination 173 (2005)249-255
to vapor (2.5 M J/kg), w is the weight of the distillate produced in the time unit, G is the solar radiation intensity received by the distiller in the time unit, and subscript i indicates the distiller type. The procedure followed for calculating G in each device is reported in the Appendix. To compare DICS and BD, the data of the Third Stage were used. For calculation of the efficiency, the time unit was the week. To compare DICS and BDC, the data obtained during the Second Stage were used, with the day as the time unit for efficiency calculation. 90 80 ~. 70 60 5O 40 3O oq 20 10 0
DICS
,,
50% ....... t0%
BD
5. Conclusions r
0
100
200
300
i
i
400
500
Solar radiation (MJ) Fig. 7. Comparison o f the DICS and the BD. Quantity o f
pure water produced in the week vs. the intensity of the solar radiation on the distiller during the same period of time. The straight lines correspond to constant efficiency at values of 10% and 50%. 10 8 .o "5 -o o
In accordance with this and Eq. (1), it was found that when the daily data of the DICS and the BDC are contrasted, the percent daily efficiencies are 22% and 16%, respectively. On the other hand, comparison of the DICS and BD weekly data of indicates that the percent weekly efficiencies are 25% and 28%. In Figs. 7 and 8 the quantity of pure water produced per area unit of the tray in the time unit is plotted against the intensity of the solar radiation on the distiller during the same period of time. In these figures the straight lines of constant efficiency [as defined by Eq. (1)] at the values 10% and 50% can be observed. Fig. 7 shows the comparison of the DICS and the BD while Fig. 8 compares the DICS with the BDC.
z ....
BDC 10%
°
D[CS / 50%
/ '
6
,1t
i
4
m
"
2
20
40
60
80
Solar radiation (MJ)
Fig. 8. Comparison of the DICS and the BDC. Quantity of pure water produced per area unit of the tray in the day
vs. the intensity of the solar radiation on the distiller during the same period of time. The straight lines correspond to constant efficiency at values of 10% and 50%.
The following results were obtained from the comparison of the DICS with the BD: • Production of the DICS in diurnal time spans is greater or equal to that of the BD, but is slightly smaller compared to the BDC. This last observation is consistent with the higher temperature reached by the tray part of the tray plus fiat collector, compared to the DICS tray temperature. ° Production of the DICS during nocturnal hours is always superior to that of the other two distillers. ° The daily production of the DICS always surpasses that of the other two: by approximately 70% that of the BD, and by about 20% that of the BDC. • The average daily production of the DICS is of 2.5 L between April and September and 5.2 L between October and March. ° Efficiency of the DICS is slightly inferior to that of the BD due to the larger energy collecting surface. ° When adding the fiat collector to the BD, its efficiency is reduced with respect to that of the DICS.
255
C. Esteban et al. / Desalination 173 (2005) 249-255
,
The frequency of cleansing is lower for the DICS than for the tray-type distiller, as in the former the salts are deposited at the bottom of the tank and not in the tray. • The DICS covering is easy to remove for cleansing, and an improvement of the design for future tasks is being considered.
References [1] S. Kumar and G. Tiwari, Estimation of convective mass transfer in solar distillation systems. Solar Energy, 57 (1996) 459--464. [2] K. Voroupoulos,E. Mathiioulakisand V. Belessiotis, Experimental investigation of a solar still coupled with solar collectors. Desalination, 138 (2001) 103110. [3] A. Fasulo, V. Cortinez and L. Odicino, Planta de destilacirn solar de agua para la Facultadde Quimica Bioquimica y Farmacia de la UNSL. Actas de ASADES, 1987. [4] J. Follari,Un destilador solar desmontable.Actas 17~ ASADES, 1 (1994)45--49. [5] A. Fasulo, J. Follari and J. Barral, Comparison between a simple solar collector accumulatorand conventionalaccumulator,Solar Energy,71 (2001) 389401. [6] A. Fasulo, D. Perell6 and J. Follari, Un colector solar aeumulador, Avances en Energlas Renovables y Medio Ambiente, 1 (1997) 93-96. [7] A. Fasulo, The thermal losses of a solar accumulator collector, ISES 2000. [8] E.D. Howe, Fundamentals of Water Desalination, Science and Technology Series, Vol. 1, Marcel Dekker, New York, 1974.
Appendix each device
Procedure for calculating Gt in
BD: the tray surface, Sb, receives horizontal global solar radiation I (M j/m2). Therefore,
GBD
=
I * Sb
BDC: The radiation collecting surface is the tray surface, Sb, plus the collecting plane surface, Sp. The tray receives the global radiation on the horizontal surface,/, and the collecting plane receives the normal radiation on a 45 ° sloped surface facing north (I45N). Then GBDc = 1 * Sb + I45N * Sp
DICS: The surface that receives radiation is the surface of the tray, Sb, plus the collecting tank surface, Sc. The tray receives the global radiation on the horizontal surface, L The half of the cylinder facing the sun receives the normal radiation on the vertical surface, on an area S c / n (which is the vertical projection of the cylinder half exposed to the sun). The tank surface receives 0.5 o f the diffuse radiation Id. GDICS = f *Sb +Id90 * Sc + 0.5] d * Sc TC