- Email: [email protected]
Simulation and experimental study for an inverted trickle solar still
DESALINATION Desalination 164 (2004) 77-85
ELSEVIER
www.elsev~er..eom/Ioealehlesal
Simulation and experimental study for an inverted trickle solar still Ali A. Badran*, Loay M. Assaf, Khaled S. Kayed, Fadi A. Ghaith, Mohammad I. Hammash Department of Mechanical Engineering, Faculty of Engineering and Technology, University of Jordan, Amman, Jordan 11942 Fax: +962 (6)535-5588; email: badran@/u.edu.jo Received 16 April 2003; accepted 13 August 2003
Abstract An experimental study for an inverted trickle solar still was performed. The still was basically composed of an inclined absorber plate painted black on the top. Saline water flowed at the backside of the plate and was kept attached to the plate. The water flow rate was kept low so that its temperature was raised to produce vapor. Condensation took place in another compartment where a heat exchanger was placed to provide heat recovery. The still was tested using brackish water of 6000 ppm salinity during the month of November at 47 ° and 32 ° tilt angles. The condensate obtained was 2.8 and 2 L/d at the above angles, respectively. This represents an 18% increase in this kind of output over previous work, which is due to reduction in the salinity of feed water. However, the intermediate header production, which is saline water of reduced salinity (3600 ppm), was also reduced by 13%. A computer simulation program was developed to predict the performance of the still.
Keywords: Distillation; Simulation; Trickle; Still
1. I n t r o d u c t i o n The inverted trickle concept was first utilized [1 ] for solar desalination b y allowing the flow o f saline water on the back side o f an absorber plate. The water was remained on the plate with the help o f a porous material fixed on its back. A low water flow rate was maintained such that its
*Corresponding author.
temperature was raised enough to produce vapor. Condensation occurred in another compartment where condensate was collected and another product was obtained at the same time, which was water o f reduced salinity. This was obtained at a location called the intermediate header and its production was at a rate o f 4.89 L/d. The saline water feed in that experiment was about 32,000 ppm, the condensate was 356 ppm and the intermediate was 9327 ppm. The latter product
0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved PII: S 0 0 1 1 - 9 1 6 4 ( 0 4 ) 0 0 1 5 8 - 4
78
A.A. Badran et al. / Desalination 164 (2004) 77-85
was considered useful because, in principle, it was reduced from a saline water feed of three times its salinity. This product is useful because it is suitable for irrigation of some desert trees and plants. The productivity of the still was improved [2] via heat recovery by adding a heat exchanger in the condenser. The condensate production was increased up to 2.3 L/d and the intermediate to 5.9 L/d. The saline water feed in that experiment was at 35,000 ppm (seawater), the condensate was at 417 ppm and the intermediate was at 9,742 ppm. If the two products were mixed together, a product of 8.2 L/d at 7,130 ppm was obtained. The idea of an inverted trickle still was also investigated [3], but with a slightly different design that does not have the insulating layer that separates the evaporator from the condenser. The total productivity of the device is not explicitly reported, but it can be estimated to be about 3.2 L/mZ.d. In a similar work [4], the absorber plate was replaced with charcoal particles that operate as an absorber and a wick. One of the shortcomings of the design that the new still tried to avoid is the condensation that occurs on the lower face of the glass, causing considerable heat loss to the outside. The concept of reduced salinity was further investigated in this work by using brackish water feed at 6000 ppm instead of seawater. Various tests were performed during the fall in the month of November at two different tilt angles: 47 ° and 32 °. The condensate obtained was 2.8 and 2 L/d at the above two angles, respectively. A computer simulation program was also developed to predict the performance of the still.
\ A~so~=E~ PLA,E--,.
"--
INTERMEDIATE
.
T
.
.
.
.
.
.
.
...... ISLE T
Tji
-:
""j/k'/.z~'."'
OUT ET<:,2// SALINE
A,s'~y///
....
CONDENSER PLATE
~'¢//~"
CONDENSATE
Fig. 1. Schematic diagram of the still.
for easy reference and the interconnection with the simulation work is shown later. Fig. 1 shows a schematic of the still with heat recovery (heat exchanger) where various energy quantities are shown. Before conducting energy balances on various still components, the following assumptions are made: (1) steady-state operation, (2) constant solar radiation over the time period within which the energy balance is made and (3) low thermal resistance of still material. Now, with the previous assumptions, the following energy balances are made for the instantaneous conditions: • Energy balance on the still absorber plate: (~o:), G : q'e+qp_~+q,+(rh/A,)Cpw AT'
•
Energy balance on the glass cover (2)
qp-~ = q,-a
•
Energy balance on the lower condenser plate
q"~ : q'b
•
(1)
(3)
Energy balance on the exchanger
2. Theoretical analysis
q.=
The analysis used in this work is the same as that of a previous study [2]. It is summarized here
And since
% ( r - r.,)
(4)
A.A. Badran et al. / Desalination 164 (2004) 77-85 qex = q'e - q"e
(5)
then it can be shown that an energy balance on the whole still yields
(r.o~)e G=q~,~+q'b+qc_a+q,+(rh/A~)CpwAT '
(6) Eq. (6) is analogous to Eq. (1) with the exception that a new term, qex, appears on the right-hand side and that the heat loss from the bottom is modified to be q'b (q'b is expected to be less than %), and the temperature difference along the absorber plate is modified to be AT' (which is also expected to be less than AT). The reduction in qb and ATis compensated for by the term qex" The rest of the heat quantities may be calculated as follows. The heat lost from the lower condenser plate is given by:
q'b : h0(r'w- to)
79
Compare this expression with a similar one [1] for the case of no heat recovery:
m : q ' b A /hfg
(12)
It is clear that an additional quantity, qex, is added to the numerator, which increases the productivity, m, provided that q'6 does not change. In fact, even if q'b changes and moreover it decreases, the increase in q~x is so large such that a net increase occurs in the numerator, which increases the productivity. The efficiency o f the still may be expressed as~
rl =(qex+q'b)/G
(13)
This equation shows that, assuming q'b remains constant, an additional amount, qex, is added in the numerator, which increases the efficiency.
(7) 3. Simulation
where T' w is the wall temperature of the condenser plate in the heat recovery case. The heat lost from the cover to the ambient is given by qc o : v
(rc-ro)
(s)
and the heat lost from the sides of the still is given by q : Ue(Tp-7")
(9)
The productivity of the still is defined by
m :{ q'ed c) / hfgo
(10)
Substituting for q'e from Eq. (5) and replacing q"e with q'b from Eq. (3) yields:
m :(qex+q'b)Ac/hfg
(11)
A computer simulation was performed to predict the performance of the still using the MATLAB software. The main features of this simulation are the prediction of: • productivity and efficiency vs. time at various conditions • solar radiation vs. time at various tilt angles • productivity and efficiency vs. time for various wind speeds. A clear sky model for solar radiation was assumed, and various heat losses were calculated following the steps of Duffle and Beckman [5] along with equations previously mentioned in this work. The algorithm of the computer pro-gram is shown in Fig. 2.
4. Experimental The experimental work used in this work is the same as the previous one [2], with the
80
A.A. Badran et al. / Desalination 164 (2004) 77-85
/
.....
/
DA TA INPUTS
/ /
~.,,~.t.
/
~l,rzrlal'bl¢.~*Day ,Time+ ~,..3d~3Uff~al!l~|e. 'l*~ lill~|¢ of" ~llcctor, ,.:kral~lel~ttcmprt~t~re, ,Tcn~fwl~dm~c~of olt~let saline water, ~l~ortmr plate at~d outlet ~ a t e r frr?rr~ t:e~t exeh:m~t.r .~3,~w rate, (:*Ak-c~or a~,a, Emitt:mee of
/ / /
/ ~--~_
/ i
t f
/
1~ th~ day ~atered not wi:thJn365 ?
tl ........................
Equati~r~ oflhne ,3f~ ~ohtr thin,, Apparent ~l,t~r tirr~'., llour ~:~gl~~LatiluLi{~t~gle . Derl~l~tlon mimic ~&,l{L{.tlde~mgh~.
"F'~e sun t'~ I ~low~ I~
I~ altitude angle Nt~i~ e ?
NO
MMify tN, ]
/
Is the deelinathm
NO I ~ t a r ~|tt|utl~ ~t~d I~'tdent~ngles. D~x~'t ttormal Imam ra41~tlo~ Total so~,~r P~dtatlolt. T o ~ ~lde, a;~d Im~i~m ll~s cocmctetlia, fleat | ~ r s frcm~ place ~o mnb|e~{, |Ie~t ]os-~esfrom h ~ sidc~ ~ mxd bot*~ml. Heat ~aha du~ |~t |;eat ~xchan~er,
Disp|av O u t p u t ~ti
Fig. 2. Simulation program algorithm.
81
A.A. Badran et al. / Desalination 164 (2004) 77-85 INLEI HEADER--
ANEMOMEIER" "-
A ~xS~'~ i ~ / / ~
-...'~iq~
CO"ffR 0LFLoWVALVF ~4E[ER-.
- - ! bSALINE ILE T
SAI]f~IE "\.
GLASS COVER . . . . . .~ ABSORBER PLAIE-. ~ WIRE SCREE N ~... SALINE ~ / ~ 0 UTLE 1_ x . . ~ ~ . ~ / ~
"<,../,j INTERMED1ATE--~ ~IAIrE--~ ) CONDENSATE
SALINE WATER FRON CONSTA~ HEAD SUPP~
HE .,",T ............EXCHANGER
",~ ~ \ - - - - B A C K PLATE ~ [NSULATION ~'~ UPPER • "~CONDENSER PLATE "".. L(,~.OWUR " ~ ..............CONDENSER PLA,r E SECTION B_B
SOLAR S]]LL ~ -..
SALINE OUTLE'[
"....... EX,2~ANGF,~
<
IN[ERMEDIATE..............
~..t~ ................................... /~-- GLASS COVER .J,_ . ~ ' , ~ A B S O R B E R PLAT[:: LNSULATION.....,.\ 8~'~- / / / . - ' - B A C K PLATE
Fig. 4. Solar still testing facility. " ~f ~/ , d-l l - U ~ P .. P E R COND. PL. HEAl EXCHANGER-.~/B"q "~" "'~ LOWER COND. PL SECTj.QN A~ A
Fig. 3. Cross-sections in the still.
exception that the inclination angle of the still was variable and that the salinity of the water feed was 6000 ppm instead of 35,000ppm. The experimental work is summarized here for easy reference. As shown in Fig. 3, the still is basically composed of one glazing surface and four plates of sheet metal. The glazing is 4 mm ordinary glass, 1.71 x0.74 m and all sheet metal plates are 1 mm thick. With the exception of the absorber plate, the parts are galvanized black steel. Saline water is introduced via the inlet header such that it is uniformly spread beneath the plate's lower surface. A wire screen-jute sandwich is fixed to that surface to keep saline water on it. The condensate header collects the condensate and saline water that m a y drip on the back plate. The heat exchanger is composed of a copper tube 12 mm in diameter and 6.5 m length bent into ten equal sections spaced 70 mm from each other. The still is installed at a slope ranging from 47 ° to 17°; and the still was tested during November. F e e d water is provided from a constant-head tank, as shown in Fig. 4. Brackish water o f about 6000 ppm was fed from the tank at various flow rates ranging from 0.7 to 2 g/s. A float-type flow m e t e r in the range 10-80 cm3/min
with a built-in control valve was used to control the flow. A dynamometer (Kipp and Zonen type CM 11) was installed in the same plane of the still and the wind speed was measured by a turbinetype anemometer. Temperature measurements were made at the indicated locations in Fig. 1 using type T thermocouples connected to a microprocessor.
5. Results and discussion
Fig. 5 shows the experimental results of condensate productivity vs. time for a slope angle of 32 ° and various flow rates. Fig. 6 shows efficiency vs. time for the same parameters. Both figures indicate that productivity and efficiency increase as flow rate decreases; a similar trend has been previously found [6]. The intersection of the two curves of 0.7 and 0.95 g/s flow rates may be attributed to the fact that they are taken at two different days, November 1 and 4, respectively, where actual solar radiation followed the same pattern, as shown in Fig. 7. The following figures s h o w simulation results. Fig. 8 shows productivity vs, time for a slope angle of 32 ° and various f l o w rates. Fig. 9 shows efficiency vs. time for the s a m e parameters. Both figures indicate that productivity and efficiency increase as flow rate decreases. Fig. 10 shows solar radiation vs. time for various tilt angles.
A.A. Badran et al. / Desalination 164 (2004) 77-85
82
350 15 •
,
k
5d
9
~O
11 (hr)
12
13
9
14
10
11 {hr}
12
13
14
Fig. 5. Condensate productivity vs. time for various flow rates.
Fig. 6. Efficiency vs. time for various flow rates. 700
..............................................................................
~-::3;;-.%;-T 2 :i
......................................
~---~~ -2-! I .i ~7~ ~::!. ~: , ';:I~O~I~'~.i~: "~ t'*";flowrate[~:'
600
i
i :!DO •
. [: 2.091~ ',:~?? [
~E~ ~:~ 400"500 },~oo i
15
Time
Time
::=::
....
"~ 300 "E
,
,/
200
,tc~
,oo
200
/
!
\
/
\
/
\
'\
! i
l i I1
9
10
~I Time (hr)
12
13
M
Fig. 7. Solar radiation vs. time for November 1 and 4.
Figs. 11-13 show a comparison of productivity, efficiency and solar radiation, respectively. To avoid congestion, only the results of one set of data (at 0.7 g/s) were taken. There is about 40% over-prediction by simulation over the experimental productivity and about 35% overprediction in efficiency. The reason may be because solar radiation was calculated using a clear sky model, which under-predicts the actual by about 13% at noon. The difference in productivity between the experimental and simulation may be because the simulation was based on steady-state conditions, which was not the case in the actual experimental work.
8,'00
9;00
!0.00 Time
11.00 (
hour~
12,00
13"00
1400
!500
)
Fig. 8. Productivity vs. time at various flow rates (simulation).
The experimental productivity of the still reached 2.8 L/d at a 47 ° tilt angle and about 2 L/d at 32 °. This was obtained at a feed water flow rate of 0.7 g/s. These figures compare well with previous work [2] where the condensate productivity was 2.5 L/d. The reason for the improvement was that the salinity of the feed water was greatly reduced (from 35,000 ppm to 6000 ppm). The productivity of the intermediate header was 5.12 L/d, which is 13% less than that of the previous work [2]. The salinity of the output saline water was 6272 ppm, while for the
A.A. Badran et al. / Desalination 164 (2004) 7~85
83
1200
........................................
/ :.~.+, ,)~i,!~0z :
1000
800
'F +/
500
/
/
]"
/
"..+
\
/
/,
\
F
s
\
/
400
",
/
\. \
/
i
2O0
\
/
\
0
'
9:90
B:O0
I
1000
1!00
"~200
1300
1,100
!5.00
Time (hours) o
~. . . . . . . . . . . . . . . . . . ,......................................... r................................,. . . . . . . . . . . . . . . . . . . . . . . .
8;00
9"00
10:00
1100
1200
I3;00
i
14:00
15:00
Time (h~ur5)
Fig. 9. Efficiency vs. time at various flow rates (simulation). 700
Fig. I0. Solar radiation vs. time at various tilt angles (simulation). 35
+T+I
....................................
~ .....................
p;:~o,-~i,X,~qZ
25
oE.: ~,
soo
2O
~ 500
o
400
i
300
.E 5 200 8,'00
9;00
10:00
11,00
f2:00
1300
14:00
1500
Time (hours) !00
0
................ , ..............................,. . . . . . . . . . . . . 8"00
9,00
10;00
...............
1100
12'00
,
....................
13:00
14;00
I i
Fig. 12. Efficiency simulation).
vs.
time (experimental
and
15;00
Time (hour~)
Fig. 11. Productivity vs. time (experimental simulation).
and
intermediate header it was 3600 ppm, which is considered brackish water. The productivities and concentrations of various water types are shown in Table 1. The unique feature o f this method is that starting with brackish water o f 6000 ppm, one can obtain 7.92 L/d o f a product o f reduced salinity, 2424 ppm, by mixing the yield o f the condensate and the intermediate headers. A water
Table 1 Concentration and flow rates of various types of water flows in and out from the still at 0.7g/s Water type
Concentration, ppm
Amount, L/d
Saline inlet Saline outlet Intermediate Condensate
6000 6272 3600 257
17.65 9.72 5.12 2.8
o f such quality is suitable for a wide range of crops, such as barley (Hordeus vulgare) and palm trees [7].
84
A.A. Badran et al. / Desalination 164 (2004) 77-85 ;200 I. . . . . . . . . . . . . . . . . . . . . .
.'~o~r,
~ooo ~oo 800 ]I] !
i
1,~t
by mixing the yield of the condensate and the intermediate headers.
[~,,.m2~,!~:~::
Symbols A c o
Cpw G
4O0 ]
200
--
-
]'/o 0
................................. ~3:00
9:00
10'00
........................................................................ 11"00 12:00 13:00 ~4:00 15:00
Time ( hours}
Fig. 13. Solar radiation vs. time (experimental and simulation).
m
q'b
m
qc-~
An uncertainty analysis was performed and it was found that, based on an accuracy in flow rates within + 2.2%, the uncertainty in efficiency is +0.5%.
6. Conclusions 1. The productivity of the inverted trickle solar still is moderately improved by using brackish water. The productivity increased from 2.5 to 2.8 L/d when the salinity of the water was reduced from that of seawater (35,000 ppm) to brackish water (6000 ppm). 2. Simulation results over-predict the experimental by about 35-40% due to the use of a clear sky model. Also, simulation is based on steadystate performance and does not take into account the transient behavior of the still. 3. Decreasing the feed water flow rate down to 0.7 g/s substantially increases the productivity. This suggests that this is the optimum flow rate at which to operate the still. 4. A large amount of production (7.92 t/d) of water of reduced salinity (2424 ppm) is possible
q'e q"e qex
qp.• q,
---
L
L
--
L,¢
Tsi ~s,o
Tt W
--
Collector/still area, m 2 Specific heat, J/kg°C Specific heat of water, J/kg°C Solar irradiation, W/m 2 Heat of vaporization, kJ/kg Heat transfer coefficient between the condenser wall and the outside, W/m 2 °C Mass flow rate, kg/s Productivity of the still, ml/h Heat loss from the lower condenser plate, W/m 2 Heat loss from cover to ambient, W/m 2 Heat carried out by evaporation from absorber plate, W/m 2 Heat carried out by vapor to condenser plate, W/m 2 Heat transferred in the exchanger, W/m 2 Heat loss from plate to cover, W/m 2 Heat loss from sides per unit still area, W/m 2 Ambient temperature, °C Back plate temperature, °C Cover temperature, °C Exit saline water temperature (from exchanger), °C Inlet saline water temperature, °C Outlet saline water temperature, °C Condenser wall temperature, °C Overall heat transfer coefficient from the sides (edges) of the still, W/m 2 °C Combined convection and radiation heat loss coefficient from the cover to the ambient, W / m 2 °C
A.A, Badran et al. / Desalination 164 (2004) 77-85 Greek
85
References
c~ AT
---
AT'
--
r1 "c (~:c0e
----
Absorptance Difference b e t w e e n inlet and outlet temperatures o f saline water before recovery, °C Difference between inlet and outlet temperatures o f saline w a t e r after recovery, °C Efficiency Transmittance Effective transmittance - absorbant product
Acknowledgement
The authors thank the University o f Jordan and the National Center for Energy Research in Jordan for their support o f this work.
[ 1] A.A. Badran and M.A. Hamdan, The inverted trickle solar still, Int. J. Solar Energy, 17 (1995) 51-60. [2] A.A. Badran, Inverted trickle solar still: the effect of heat recovery, Desalination, 133 (2001) 167-173. [3] B. Bouchekima, B. Gros, R. Oaches and M. Diboun, The performance of the capillary film solar still installed in South Algeria, Desalination, 137 (2001) 31-38. [4] M. Naim and M. Abd E1 Kawi, Non-conventional stills with charcoal particles as absorber medium, Desalination, 153 (2002) 55-64. [5] J. Duffle and W. Beckman, Solar Engineering of Thermal Processes, 2nd ed., Wiley, New York, 1991. [6] K.K. Matrawi, Design and experimental study of an inclined wick type solar still-comparative study with the basin type, Proc. Int. Conference of Energy Systems, (ICES 2K), Amman, 2000, pp. 95-114. [7] T. Abu-Sharar, On the application of Arabian Gulf water in irrigation: a future possibility, Qatar University Sci. Bull., 7 (1987) 331.
ELSEVIER
www.elsev~er..eom/Ioealehlesal
Simulation and experimental study for an inverted trickle solar still Ali A. Badran*, Loay M. Assaf, Khaled S. Kayed, Fadi A. Ghaith, Mohammad I. Hammash Department of Mechanical Engineering, Faculty of Engineering and Technology, University of Jordan, Amman, Jordan 11942 Fax: +962 (6)535-5588; email: badran@/u.edu.jo Received 16 April 2003; accepted 13 August 2003
Abstract An experimental study for an inverted trickle solar still was performed. The still was basically composed of an inclined absorber plate painted black on the top. Saline water flowed at the backside of the plate and was kept attached to the plate. The water flow rate was kept low so that its temperature was raised to produce vapor. Condensation took place in another compartment where a heat exchanger was placed to provide heat recovery. The still was tested using brackish water of 6000 ppm salinity during the month of November at 47 ° and 32 ° tilt angles. The condensate obtained was 2.8 and 2 L/d at the above angles, respectively. This represents an 18% increase in this kind of output over previous work, which is due to reduction in the salinity of feed water. However, the intermediate header production, which is saline water of reduced salinity (3600 ppm), was also reduced by 13%. A computer simulation program was developed to predict the performance of the still.
Keywords: Distillation; Simulation; Trickle; Still
1. I n t r o d u c t i o n The inverted trickle concept was first utilized [1 ] for solar desalination b y allowing the flow o f saline water on the back side o f an absorber plate. The water was remained on the plate with the help o f a porous material fixed on its back. A low water flow rate was maintained such that its
*Corresponding author.
temperature was raised enough to produce vapor. Condensation occurred in another compartment where condensate was collected and another product was obtained at the same time, which was water o f reduced salinity. This was obtained at a location called the intermediate header and its production was at a rate o f 4.89 L/d. The saline water feed in that experiment was about 32,000 ppm, the condensate was 356 ppm and the intermediate was 9327 ppm. The latter product
0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved PII: S 0 0 1 1 - 9 1 6 4 ( 0 4 ) 0 0 1 5 8 - 4
78
A.A. Badran et al. / Desalination 164 (2004) 77-85
was considered useful because, in principle, it was reduced from a saline water feed of three times its salinity. This product is useful because it is suitable for irrigation of some desert trees and plants. The productivity of the still was improved [2] via heat recovery by adding a heat exchanger in the condenser. The condensate production was increased up to 2.3 L/d and the intermediate to 5.9 L/d. The saline water feed in that experiment was at 35,000 ppm (seawater), the condensate was at 417 ppm and the intermediate was at 9,742 ppm. If the two products were mixed together, a product of 8.2 L/d at 7,130 ppm was obtained. The idea of an inverted trickle still was also investigated [3], but with a slightly different design that does not have the insulating layer that separates the evaporator from the condenser. The total productivity of the device is not explicitly reported, but it can be estimated to be about 3.2 L/mZ.d. In a similar work [4], the absorber plate was replaced with charcoal particles that operate as an absorber and a wick. One of the shortcomings of the design that the new still tried to avoid is the condensation that occurs on the lower face of the glass, causing considerable heat loss to the outside. The concept of reduced salinity was further investigated in this work by using brackish water feed at 6000 ppm instead of seawater. Various tests were performed during the fall in the month of November at two different tilt angles: 47 ° and 32 °. The condensate obtained was 2.8 and 2 L/d at the above two angles, respectively. A computer simulation program was also developed to predict the performance of the still.
\ A~so~=E~ PLA,E--,.
"--
INTERMEDIATE
.
T
.
.
.
.
.
.
.
...... ISLE T
Tji
-:
""j/k'/.z~'."'
OUT ET<:,2// SALINE
A,s'~y///
....
CONDENSER PLATE
~'¢//~"
CONDENSATE
Fig. 1. Schematic diagram of the still.
for easy reference and the interconnection with the simulation work is shown later. Fig. 1 shows a schematic of the still with heat recovery (heat exchanger) where various energy quantities are shown. Before conducting energy balances on various still components, the following assumptions are made: (1) steady-state operation, (2) constant solar radiation over the time period within which the energy balance is made and (3) low thermal resistance of still material. Now, with the previous assumptions, the following energy balances are made for the instantaneous conditions: • Energy balance on the still absorber plate: (~o:), G : q'e+qp_~+q,+(rh/A,)Cpw AT'
•
Energy balance on the glass cover (2)
qp-~ = q,-a
•
Energy balance on the lower condenser plate
q"~ : q'b
•
(1)
(3)
Energy balance on the exchanger
2. Theoretical analysis
q.=
The analysis used in this work is the same as that of a previous study [2]. It is summarized here
And since
% ( r - r.,)
(4)
A.A. Badran et al. / Desalination 164 (2004) 77-85 qex = q'e - q"e
(5)
then it can be shown that an energy balance on the whole still yields
(r.o~)e G=q~,~+q'b+qc_a+q,+(rh/A~)CpwAT '
(6) Eq. (6) is analogous to Eq. (1) with the exception that a new term, qex, appears on the right-hand side and that the heat loss from the bottom is modified to be q'b (q'b is expected to be less than %), and the temperature difference along the absorber plate is modified to be AT' (which is also expected to be less than AT). The reduction in qb and ATis compensated for by the term qex" The rest of the heat quantities may be calculated as follows. The heat lost from the lower condenser plate is given by:
q'b : h0(r'w- to)
79
Compare this expression with a similar one [1] for the case of no heat recovery:
m : q ' b A /hfg
(12)
It is clear that an additional quantity, qex, is added to the numerator, which increases the productivity, m, provided that q'6 does not change. In fact, even if q'b changes and moreover it decreases, the increase in q~x is so large such that a net increase occurs in the numerator, which increases the productivity. The efficiency o f the still may be expressed as~
rl =(qex+q'b)/G
(13)
This equation shows that, assuming q'b remains constant, an additional amount, qex, is added in the numerator, which increases the efficiency.
(7) 3. Simulation
where T' w is the wall temperature of the condenser plate in the heat recovery case. The heat lost from the cover to the ambient is given by qc o : v
(rc-ro)
(s)
and the heat lost from the sides of the still is given by q : Ue(Tp-7")
(9)
The productivity of the still is defined by
m :{ q'ed c) / hfgo
(10)
Substituting for q'e from Eq. (5) and replacing q"e with q'b from Eq. (3) yields:
m :(qex+q'b)Ac/hfg
(11)
A computer simulation was performed to predict the performance of the still using the MATLAB software. The main features of this simulation are the prediction of: • productivity and efficiency vs. time at various conditions • solar radiation vs. time at various tilt angles • productivity and efficiency vs. time for various wind speeds. A clear sky model for solar radiation was assumed, and various heat losses were calculated following the steps of Duffle and Beckman [5] along with equations previously mentioned in this work. The algorithm of the computer pro-gram is shown in Fig. 2.
4. Experimental The experimental work used in this work is the same as the previous one [2], with the
80
A.A. Badran et al. / Desalination 164 (2004) 77-85
/
.....
/
DA TA INPUTS
/ /
~.,,~.t.
/
~l,rzrlal'bl¢.~*Day ,Time+ ~,..3d~3Uff~al!l~|e. 'l*~ lill~|¢ of" ~llcctor, ,.:kral~lel~ttcmprt~t~re, ,Tcn~fwl~dm~c~of olt~let saline water, ~l~ortmr plate at~d outlet ~ a t e r frr?rr~ t:e~t exeh:m~t.r .~3,~w rate, (:*Ak-c~or a~,a, Emitt:mee of
/ / /
/ ~--~_
/ i
t f
/
1~ th~ day ~atered not wi:thJn365 ?
tl ........................
Equati~r~ oflhne ,3f~ ~ohtr thin,, Apparent ~l,t~r tirr~'., llour ~:~gl~~LatiluLi{~t~gle . Derl~l~tlon mimic ~&,l{L{.tlde~mgh~.
"F'~e sun t'~ I ~low~ I~
I~ altitude angle Nt~i~ e ?
NO
MMify tN, ]
/
Is the deelinathm
NO I ~ t a r ~|tt|utl~ ~t~d I~'tdent~ngles. D~x~'t ttormal Imam ra41~tlo~ Total so~,~r P~dtatlolt. T o ~ ~lde, a;~d Im~i~m ll~s cocmctetlia, fleat | ~ r s frcm~ place ~o mnb|e~{, |Ie~t ]os-~esfrom h ~ sidc~ ~ mxd bot*~ml. Heat ~aha du~ |~t |;eat ~xchan~er,
Disp|av O u t p u t ~ti
Fig. 2. Simulation program algorithm.
81
A.A. Badran et al. / Desalination 164 (2004) 77-85 INLEI HEADER--
ANEMOMEIER" "-
A ~xS~'~ i ~ / / ~
-...'~iq~
CO"ffR 0LFLoWVALVF ~4E[ER-.
- - ! bSALINE ILE T
SAI]f~IE "\.
GLASS COVER . . . . . .~ ABSORBER PLAIE-. ~ WIRE SCREE N ~... SALINE ~ / ~ 0 UTLE 1_ x . . ~ ~ . ~ / ~
"<,../,j INTERMED1ATE--~ ~IAIrE--~ ) CONDENSATE
SALINE WATER FRON CONSTA~ HEAD SUPP~
HE .,",T ............EXCHANGER
",~ ~ \ - - - - B A C K PLATE ~ [NSULATION ~'~ UPPER • "~CONDENSER PLATE "".. L(,~.OWUR " ~ ..............CONDENSER PLA,r E SECTION B_B
SOLAR S]]LL ~ -..
SALINE OUTLE'[
"....... EX,2~ANGF,~
<
IN[ERMEDIATE..............
~..t~ ................................... /~-- GLASS COVER .J,_ . ~ ' , ~ A B S O R B E R PLAT[:: LNSULATION.....,.\ 8~'~- / / / . - ' - B A C K PLATE
Fig. 4. Solar still testing facility. " ~f ~/ , d-l l - U ~ P .. P E R COND. PL. HEAl EXCHANGER-.~/B"q "~" "'~ LOWER COND. PL SECTj.QN A~ A
Fig. 3. Cross-sections in the still.
exception that the inclination angle of the still was variable and that the salinity of the water feed was 6000 ppm instead of 35,000ppm. The experimental work is summarized here for easy reference. As shown in Fig. 3, the still is basically composed of one glazing surface and four plates of sheet metal. The glazing is 4 mm ordinary glass, 1.71 x0.74 m and all sheet metal plates are 1 mm thick. With the exception of the absorber plate, the parts are galvanized black steel. Saline water is introduced via the inlet header such that it is uniformly spread beneath the plate's lower surface. A wire screen-jute sandwich is fixed to that surface to keep saline water on it. The condensate header collects the condensate and saline water that m a y drip on the back plate. The heat exchanger is composed of a copper tube 12 mm in diameter and 6.5 m length bent into ten equal sections spaced 70 mm from each other. The still is installed at a slope ranging from 47 ° to 17°; and the still was tested during November. F e e d water is provided from a constant-head tank, as shown in Fig. 4. Brackish water o f about 6000 ppm was fed from the tank at various flow rates ranging from 0.7 to 2 g/s. A float-type flow m e t e r in the range 10-80 cm3/min
with a built-in control valve was used to control the flow. A dynamometer (Kipp and Zonen type CM 11) was installed in the same plane of the still and the wind speed was measured by a turbinetype anemometer. Temperature measurements were made at the indicated locations in Fig. 1 using type T thermocouples connected to a microprocessor.
5. Results and discussion
Fig. 5 shows the experimental results of condensate productivity vs. time for a slope angle of 32 ° and various flow rates. Fig. 6 shows efficiency vs. time for the same parameters. Both figures indicate that productivity and efficiency increase as flow rate decreases; a similar trend has been previously found [6]. The intersection of the two curves of 0.7 and 0.95 g/s flow rates may be attributed to the fact that they are taken at two different days, November 1 and 4, respectively, where actual solar radiation followed the same pattern, as shown in Fig. 7. The following figures s h o w simulation results. Fig. 8 shows productivity vs, time for a slope angle of 32 ° and various f l o w rates. Fig. 9 shows efficiency vs. time for the s a m e parameters. Both figures indicate that productivity and efficiency increase as flow rate decreases. Fig. 10 shows solar radiation vs. time for various tilt angles.
A.A. Badran et al. / Desalination 164 (2004) 77-85
82
350 15 •
,
k
5d
9
~O
11 (hr)
12
13
9
14
10
11 {hr}
12
13
14
Fig. 5. Condensate productivity vs. time for various flow rates.
Fig. 6. Efficiency vs. time for various flow rates. 700
..............................................................................
~-::3;;-.%;-T 2 :i
......................................
~---~~ -2-! I .i ~7~ ~::!. ~: , ';:I~O~I~'~.i~: "~ t'*";flowrate[~:'
600
i
i :!DO •
. [: 2.091~ ',:~?? [
~E~ ~:~ 400"500 },~oo i
15
Time
Time
::=::
....
"~ 300 "E
,
,/
200
,tc~
,oo
200
/
!
\
/
\
/
\
'\
! i
l i I1
9
10
~I Time (hr)
12
13
M
Fig. 7. Solar radiation vs. time for November 1 and 4.
Figs. 11-13 show a comparison of productivity, efficiency and solar radiation, respectively. To avoid congestion, only the results of one set of data (at 0.7 g/s) were taken. There is about 40% over-prediction by simulation over the experimental productivity and about 35% overprediction in efficiency. The reason may be because solar radiation was calculated using a clear sky model, which under-predicts the actual by about 13% at noon. The difference in productivity between the experimental and simulation may be because the simulation was based on steady-state conditions, which was not the case in the actual experimental work.
8,'00
9;00
!0.00 Time
11.00 (
hour~
12,00
13"00
1400
!500
)
Fig. 8. Productivity vs. time at various flow rates (simulation).
The experimental productivity of the still reached 2.8 L/d at a 47 ° tilt angle and about 2 L/d at 32 °. This was obtained at a feed water flow rate of 0.7 g/s. These figures compare well with previous work [2] where the condensate productivity was 2.5 L/d. The reason for the improvement was that the salinity of the feed water was greatly reduced (from 35,000 ppm to 6000 ppm). The productivity of the intermediate header was 5.12 L/d, which is 13% less than that of the previous work [2]. The salinity of the output saline water was 6272 ppm, while for the
A.A. Badran et al. / Desalination 164 (2004) 7~85
83
1200
........................................
/ :.~.+, ,)~i,!~0z :
1000
800
'F +/
500
/
/
]"
/
"..+
\
/
/,
\
F
s
\
/
400
",
/
\. \
/
i
2O0
\
/
\
0
'
9:90
B:O0
I
1000
1!00
"~200
1300
1,100
!5.00
Time (hours) o
~. . . . . . . . . . . . . . . . . . ,......................................... r................................,. . . . . . . . . . . . . . . . . . . . . . . .
8;00
9"00
10:00
1100
1200
I3;00
i
14:00
15:00
Time (h~ur5)
Fig. 9. Efficiency vs. time at various flow rates (simulation). 700
Fig. I0. Solar radiation vs. time at various tilt angles (simulation). 35
+T+I
....................................
~ .....................
p;:~o,-~i,X,~qZ
25
oE.: ~,
soo
2O
~ 500
o
400
i
300
.E 5 200 8,'00
9;00
10:00
11,00
f2:00
1300
14:00
1500
Time (hours) !00
0
................ , ..............................,. . . . . . . . . . . . . 8"00
9,00
10;00
...............
1100
12'00
,
....................
13:00
14;00
I i
Fig. 12. Efficiency simulation).
vs.
time (experimental
and
15;00
Time (hour~)
Fig. 11. Productivity vs. time (experimental simulation).
and
intermediate header it was 3600 ppm, which is considered brackish water. The productivities and concentrations of various water types are shown in Table 1. The unique feature o f this method is that starting with brackish water o f 6000 ppm, one can obtain 7.92 L/d o f a product o f reduced salinity, 2424 ppm, by mixing the yield o f the condensate and the intermediate headers. A water
Table 1 Concentration and flow rates of various types of water flows in and out from the still at 0.7g/s Water type
Concentration, ppm
Amount, L/d
Saline inlet Saline outlet Intermediate Condensate
6000 6272 3600 257
17.65 9.72 5.12 2.8
o f such quality is suitable for a wide range of crops, such as barley (Hordeus vulgare) and palm trees [7].
84
A.A. Badran et al. / Desalination 164 (2004) 77-85 ;200 I. . . . . . . . . . . . . . . . . . . . . .
.'~o~r,
~ooo ~oo 800 ]I] !
i
1,~t
by mixing the yield of the condensate and the intermediate headers.
[~,,.m2~,!~:~::
Symbols A c o
Cpw G
4O0 ]
200
--
-
]'/o 0
................................. ~3:00
9:00
10'00
........................................................................ 11"00 12:00 13:00 ~4:00 15:00
Time ( hours}
Fig. 13. Solar radiation vs. time (experimental and simulation).
m
q'b
m
qc-~
An uncertainty analysis was performed and it was found that, based on an accuracy in flow rates within + 2.2%, the uncertainty in efficiency is +0.5%.
6. Conclusions 1. The productivity of the inverted trickle solar still is moderately improved by using brackish water. The productivity increased from 2.5 to 2.8 L/d when the salinity of the water was reduced from that of seawater (35,000 ppm) to brackish water (6000 ppm). 2. Simulation results over-predict the experimental by about 35-40% due to the use of a clear sky model. Also, simulation is based on steadystate performance and does not take into account the transient behavior of the still. 3. Decreasing the feed water flow rate down to 0.7 g/s substantially increases the productivity. This suggests that this is the optimum flow rate at which to operate the still. 4. A large amount of production (7.92 t/d) of water of reduced salinity (2424 ppm) is possible
q'e q"e qex
qp.• q,
---
L
L
--
L,¢
Tsi ~s,o
Tt W
--
Collector/still area, m 2 Specific heat, J/kg°C Specific heat of water, J/kg°C Solar irradiation, W/m 2 Heat of vaporization, kJ/kg Heat transfer coefficient between the condenser wall and the outside, W/m 2 °C Mass flow rate, kg/s Productivity of the still, ml/h Heat loss from the lower condenser plate, W/m 2 Heat loss from cover to ambient, W/m 2 Heat carried out by evaporation from absorber plate, W/m 2 Heat carried out by vapor to condenser plate, W/m 2 Heat transferred in the exchanger, W/m 2 Heat loss from plate to cover, W/m 2 Heat loss from sides per unit still area, W/m 2 Ambient temperature, °C Back plate temperature, °C Cover temperature, °C Exit saline water temperature (from exchanger), °C Inlet saline water temperature, °C Outlet saline water temperature, °C Condenser wall temperature, °C Overall heat transfer coefficient from the sides (edges) of the still, W/m 2 °C Combined convection and radiation heat loss coefficient from the cover to the ambient, W / m 2 °C
A.A, Badran et al. / Desalination 164 (2004) 77-85 Greek
85
References
c~ AT
---
AT'
--
r1 "c (~:c0e
----
Absorptance Difference b e t w e e n inlet and outlet temperatures o f saline water before recovery, °C Difference between inlet and outlet temperatures o f saline w a t e r after recovery, °C Efficiency Transmittance Effective transmittance - absorbant product
Acknowledgement
The authors thank the University o f Jordan and the National Center for Energy Research in Jordan for their support o f this work.
[ 1] A.A. Badran and M.A. Hamdan, The inverted trickle solar still, Int. J. Solar Energy, 17 (1995) 51-60. [2] A.A. Badran, Inverted trickle solar still: the effect of heat recovery, Desalination, 133 (2001) 167-173. [3] B. Bouchekima, B. Gros, R. Oaches and M. Diboun, The performance of the capillary film solar still installed in South Algeria, Desalination, 137 (2001) 31-38. [4] M. Naim and M. Abd E1 Kawi, Non-conventional stills with charcoal particles as absorber medium, Desalination, 153 (2002) 55-64. [5] J. Duffle and W. Beckman, Solar Engineering of Thermal Processes, 2nd ed., Wiley, New York, 1991. [6] K.K. Matrawi, Design and experimental study of an inclined wick type solar still-comparative study with the basin type, Proc. Int. Conference of Energy Systems, (ICES 2K), Amman, 2000, pp. 95-114. [7] T. Abu-Sharar, On the application of Arabian Gulf water in irrigation: a future possibility, Qatar University Sci. Bull., 7 (1987) 331.