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Effect of orientation of solar still on the productivity of fresh water
S. Karroute * and A. Chaker
Laboratoire de Physique Energétique, Université des Frères Mentouri Route de Ain El Bey, Constantine, Algeria
Abstract – It is very clear that the world head for investment in renewable and clean energy.
Especially because the industrial development in the previous years is producing a big destruction in the fossil energetic resources; moreover it leaves negative traces on the environment and climate. The use of the solar energy is raising hopes. The qualities of this source of renewable energy are known, free and inexhaustible. It’s moreover the most abounding energy on earth. The solar distillation is the excellent example of economical and ecological use of the solar energy, especially as their design does not present technical difficulties. However their output of fresh water remains insufficient. With a view to ameliorate the yield of the solar still, our study aims at the effect of changing the glass cover geometry on the quantity of absorbed solar energy, we have to compare between three types of solar stills, a single slope, a double slope and a spherical still, in order to select the perfect geometry. After having established the thermal balances of the various solar stills (single slope, double slope and spherical) at in stationary regime, the equations are solved by using 4th order Runge–Kutta method. The numerical results obtained allow showing unambiguously that the yield of the spherical solar still is the more important than that of the double slope and the plat solar stills.
Keywords: Single slope still, double slope still, Spherical still, Productivity.
1. INTRODUCTION
The drinking water needs are increasingly perceptible, especially in arid and/or isolated area. In those sites where the brackish water and solar energy are widely available, desalination using renewable energy will not only help to alleviate this deficit, but also provide a solution to the energy economy and the environment. However the problem is the low yield of this type of process. Malik et al. [1] improved that the conventional solar still can produce average 3 liters/day. For high efficiency, the solar still should maintain a large temperature difference between feed water and condensing surface, choosing a typical geometry of the glass cover play an important role to insure that a high proportion of incoming radiation will be absorbed by the feed water. In the present study a comparison has been made to find out the effect of changing the geometry of the glass cover on the solar still output.
2. THEORITICAL ANALYSIS
A schematic diagram of single and double slope solar stills is shown in figure 1. The bottom surface of the single solar still was painted black for greater absorptivity and a glass cover 3 mm thick covers the still. the bottom section of basin was insulated to reduce thermal losses to the surroundings. This still was oriented a long the south direction to receive maximum solar radiation.
The conventional double slope basin solar still consist of an uninsulated shallow basin painted with black paint holding shallow depth of brackish water and covered
* karrolima@yahoo.fr
with double glass cover of the inverted v type, with long axis of the still facing east- west direction to improve distillate collection process and increase the quantity of distilled water.
Fig. 1: Schematic view of single and double solar stills
Fig. 2: Schematic view of spherical solar stills
A schematic diagram of the spherical solar still is shown in figure 2. The still mainly consists of the circular basin and absorber plate carrying the saline water, the spherical cover. The distillate output from the still was frequently collected using a container placed under the solar still. Due to spherical geometry of the glass cover, this still have not a preferred orientation.
a- Thermal modeling
The energy balance equations in terms of various heat transfer coefficient of solar still are as follows:
Outer and inner glass cover
v cdg go
g g cga
rga Q I
t d T d 2
C m Q
Q
(1)
v cdg gi
g g ev cgw
rgw Q I
t d
T d 2
C m Q Q
Q
(2)
Water mass
w cwb a
w w w ev
w w ev cgw
rgw m C (T T ) Q I
t d
T C d m Q Q
Q (3)
Basin linear
b b b g csb
cwb I
t d
T C d m Q
Q (4)
ICESD’2011: Effect of orientation of solar still on the productivity of fresh water 27
Outer and inner insolent ii cdb i
cdi i Q
t d
T d 2
C
Q m
(5)
b b b g csb
cwb I
t d
T C d m Q
Q (6)
Ig: Solar flux absorbed by glass cover.
Iw: solar flux absorbed by water mass.
Ib: solar flux absorbed by the basin liner.
a1- Heat transfer coefficients
The internal heat transfer between the glass cover and water mass can take place in three ways mainly by convection, radiation and evaporation [2]:
a11- Convective heat transfer
Following Kumar et al. [3], the rate of convective heat transfer is described by the general equation:
w go w cgw
cgw h (T T ) A
Q (7)
Where Aw is the surface of water and hcgw is the convective heat transfer coefficient and it given by [4-6]:
3 / 1
3 w w gi w gi
w
cgw 268.9 10 P
) 273 T
( ) T T ) ( T T ( 884 . 0
h
(8)
Pw, Pgi, are the partial pressures of the vapor of water respectively, in water temperature Tw and the inner glass cover temperature Tgi.
a12- Radiative heat transfer
In the same of the convective heat transfer, the rate of radiative heat transfer can be determined from the relation:
w gi w rgw
rgw h (T T ) A
Q (9)
Where hr is the radiative heat transfer coefficient and it given by [6, 7]:
(T 273) (T 273) T T 546
1 1 F 1
h w 2 gi 2 w g
w w 12
rgw
(10)
a13- Evaporative heat transfer
The evaporation heat transfer from basin water to condensing cover is described by the relation [8]:
w gi w ev
ev h (T T ) A
Q (11)
Where hev is the evaporative heat transfer coefficient and it given by [3, 9]:
) T T (
) P P ( h 10 273 . 16 h
gi w
gi w 3 cgw
ev
(12)
The hourly yield per unit area can be evaluated from known values of water and glass temperatures, and is given by [10]:
v gi w ev
ev L
3600 ) T T (
m h
(13)
Where Lv is the latent heat of vaporization, dependent of temperature [11]:
3 v(T) 3408 5.21 T 0.01 T2 1.194 T
L (14)
3.NUMERICALRESOLUTION
Equations are solved by using 4th order Runge-Kutta method. The Computer programs have been developed in ‘Fortran’ language to predict the hourly variations of water temperature, glass temperature, distillate output and the various heat transfer coefficients of solar stills.
4.RESULTATSANDDISCUSSION
The figure 3 show that the tree solar stills don’t absorbed the incoming radiation in the same way. The single slope solar still that was oriented in the south, absorbed the major of solar energy between 11h and 15h, when the sun was in the south, however the double slope solar still absorbed the major of incoming radiation in the morning and in the evening, we can explain this result by the orientation east-west of the double slope solar still, where one side of the glass cover is extended to the sun end the other side is under shadow, who keeping a low temperature of the glass cover and arising the rate of evaporation.
Fig. 3: Hourly variation of the absorbed incoming radiation of the stills (single slope, double slope and spherical solar still)
m 03 . 0
ew , V3m/h, day17july
The solar radiations are the main factors affecting the productivity of the solar still, that why the hourly productions, as shown in figure 4, have the same trend of the hourly absorbed energy (Fig. 3). We can observe that the hourly production of the spherical solar still is the higher than that of the others solar stills all the time of day.
ICESD’2011: Effect of orientation of solar still on the productivity of fresh water 29 The theoretical results obtained in this work indicated that the yield of a single slope solar still (The hours ranges were between 6hand19h) is 2.99 l/m2.day, however the yield of the double slope solar still is about 3. 97 l/m2.day, but if we use a spherical lass cover, the yield increase to 5.05 l/m2.day (Fig. 5).
Fig. 4: Hourly production of the stills (single slope, double slope and spherical solar still)
m 03 . 0
ew , V3m/h, day17july
Fig. 5: Comparative variation of still productivity (single slope, double slope and spherical solar still)
m 03 . 0
ew , V3m/h, day17july
Fig. 6 compare hourly temperatures of saline water obtained for single slope solar still, double slope solar still and spherical solar still additionally to the ambient temperature. The highest temperatures occurred between the hours of 11h and 15h.
The increasing of temperature difference between the brine and the condenser surface leads to a better production (Fig. 7). In the case of single slope solar still the difference of temperatures between water and glass cover reached 10 °C, however
cooling of one side of the double slope solar still by shadow arise this different to 10.61
°C, the spherical geometry allow to obtain a difference of temperatures of 11.89 °C.
Fig. 6: Hourly variation of water temperatures (single slope, double slope, spherical solar still and ambient)
m 03 . 0
ew , V3m/h, day17july
Fig. 7: Comparative variation of still productivity (single slope, double slope and spherical solar still)
m 03 . 0
ew , V3m/h, day17july
5. CONCLUSION
On the basis of present studies the following are the conclusions drawn:
The solar radiations are the main factors affecting the productivity of the solar still.
The daily yield of single slope solar still is about 2.99 l/m2, however the yield of double solar still is 3.97 l/m2.
In our case, a better production has been obtained by using spherical solar still with average 5.05 l/m2.
ICESD’2011: Effect of orientation of solar still on the productivity of fresh water 31
Cooling the glass cover of double slope solar still by extending one side of the still on the shadow arise the output.
NOMENCLATURE
A: Area, m2 C: Heat capacity per unit, J/kg.K h: Heat transfer coefficient, W/m2.K I: Absorbed solar radiation, W Lv: Latent heat of vaporization, J/kg Q : Heat flux (W)
m: Mass (kg) mev: Distillate rate, kg/s
P: Partial pressure, Pa t: Time, s T: Absolute temperature, K : Emissivity
a : Ambient, b : Basin linear : Stefan-Boltzmann constant c : Convection, cd : Conduction ev : Evaporation, g: Glass cover gi: Inner glass cover, i: Insolent go: Outer glass cover, r: Radiation ii: Inner insolent, s : Sky io : Outer insolent, w : Water
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