009 023

Document Sample
009 023 Powered By Docstoc
					                    Effect of externals parameters on the
                            working of solar stills
                                                      First A.Chaker, Second O.Sotehi


                                                                            but as a real alternative of energy resources.
    Abstract— Supply drinking water became the most alarming
problem of our time, since the demand for water grows while the
natural resources decrease or tend to disappear. One of the solutions           The operation of a solar distiller is strongly influenced by
which can contribute to the increase in the water potentialities is the
desalination of sea water or brackish water. The most adequate
                                                                             various internal and external parameters.
process to carry out this desalination is solar distillation, considering    In this work our interest will focus on the second and
its simplicity, its lower cost and especially the availability of the        particularly the metrological parameters, which the most
source of energy.                                                            frequently used, are the incident solar radiation, wind speed
   The aim of our work has been to study the effect of externals             and ambient temperature [4].
parameters on the working characteristics of a solar still in order to
improve those parameters and improve the out put and the
production.
                                                                                         II. DESCRIPTION OF SOLAR STILL
   The system of equations of the working of the solar still in                Our study is focused on two types of solar stills.
transition regime has been established. A mathematical simulation
has been carried to get results that show the effect of externals               A. Plane still
parameters, the incident solar radiation which is the most influent             This desalination system (Fig 1) consists essentially of a
factor on the behavior of solar still, wherever the velocity of wind         tight capacity topped with glass. The bottom is covered with
and ambient temperature, on the efficiency and out put of the solar
                                                                             water surface (brackish water or seawater). Under the action of
still.
                                                                             solar radiation IG, transmitted by the transparent cover, the
  Keywords—Externals parameters, Transition regime, Production,              water heats up and some of it evaporates. The steam condenses
Performance, Out put, Solar still.                                           on the inside of the glass and the condensate is recovered by a
                                                                             receiver. Booster water offsets the flow of distillate. To reduce
                          I. INTRODUCTION                                    heat losses to the outside, the side walls and bottom are
                                                                             isolated.
D     rinking water is an essential substance to the survival and
      development of mankind. But what will become this weak
resource?
   In Algeria, the deficit of this blue gold has become
worrisome confirming several expertises and hence hypothesis
using different methodology have concluded that our country
will be between 2010 and 2025 faced to a quasi- endemic
penury [1].
   To this end and for better preserving of the future in terms
of clearance and mobilization of water resources, it is more
efficient to review the planning of water resources in
conventional water in a global vision that incorporates the use
of non-conventional water resources including desalination of
seawater and brackish water [2].
   However, the desalination requires energy that accounts a                                          Fig. 1 Plane still
large part in the water cost [3]. It seems at first to consider the
use of solar energy (solar still) in the process of desalination of
seawater or brackish water, rather than as an ecological factor,                B. Spherical still
                                                                               It is a distiller Plexiglas (Fig. 2), composed of three main
                                                                             parts, an upper half-sphere which is used to transmit solar
  F. A.Chaker, physical energy laboratory, university Mentouri Constantine
25017, algeria (e-mail: chakamine@yahoo.fr)                                  radiation IG on the one hand and plays the role of surface
  S. O.SOTEHI, physical energy laboratory, university, université Mentouri   condensation on the other hand, a half lower-sphere that
Constantine 25017, algeria (e-mail: elwali1@yahoo.fr )                       collects the distillate tray and a horizontal metal blackened
containing distilled water which is in the median plane [5].                     Mg is the mass of the glass (kg), Cpg is specific heat at constant
                                                                                 pressure (j/kg.°C), Ag is the surface of the glass cover (m2),
                                                                                 Tge is the temperature of the external face of glass (°C), Tgi is

                                                                                                                                          c
                                                                                 the temperature of the internal face of glass (°C),    q g a is the
                                                                                 heat flux exchanged by convection between the glass and the
                                                                                                   r
                                                                                 ambient (W),    q g a is   the heat flux exchanged by radiation
                                                                                                                               ev
                                                                                 between the glass and the ambient (W),      q w g is the heat flux
                                                                                 exchanged by evaporation between the glass and the water
                                                                                 (W),  g is the thermal conductivity of the glass (W/m.°C), eg
                                                                                 is the thickness of the glass cover (m) and Pg is the solar power
                              Fig. 2 Spherical still                             absorbed by the glass (W/m2).

   A windshield wiper driven by an electric motor will                             -In the brine
maintain constant condensing surface transparent to radiation,                    M w  cpw dTw
and to ensure rapid drainage droplets. The operating principle                                      qw g  qw g  qw g  qb  w  pw
                                                                                                        c      r       ev      c              (4)
                                                                                      Aw        dt
is similar to the plane still [6].
                                                                                 Mw is the mass of the water (kg), Cpw is specific heat at
   C. Writing of equation of heat transfer under transient                       constant pressure (j/kg.°C), Aw is the surface of the water (m2),
regime                                                                                                                         c
                                                                                 Tw is the temperature of the water (°C),    q b  w is the heat flux
  The heat transfer equations are written following the law [7]:                 exchanged by convection between the absorber and the water
                      n
            dT
M i  cpi  i   qij  pi                                  (1)                  (W) and Pw is the solar power absorbed by the water (W/m2).
             dt     i 1

  Mi is the mass of the body with i index ( kg), Cpi is specific                   -In the absorber
heat at constant pressure (j/kg.°C), T is the temperature (°C),                   M b  cpb dTb
                                                                                                   qb  w  qb isi  pb
                                                                                                       c       cd                             (5)
t is time (s),    qi  j is    the heat flux exchanged between the                   Ab        dt
material surfaces indexed i and j (W) and Pi is the solar power                  Mb is the mass of the water (kg), Cpb is specific heat at
absorbed by the body of index i (W/m2).                                          constant pressure (j/kg.°C), Ab is the surface of the water (m2),
                                                                                                                                   cd
We get for plane still                                                           Tb is the temperature of the absorber (°C),     q b isi is the heat
                                                                                 flux exchanged by conduction between the absorber and the
- In the glass                                                                   insulator (W) and Pb is the solar power absorbed by the
   - The outside face                                                            absorber (W/m2).
 M g  cp g dTge                         g
                     q g a  q g a 
                         c        r
                                             (Tgi  Tge )    (2)
   2  Ag        dt                      eg                                      - In the insulating
                                                                                    - Inside face
   Mg is the mass of the glass (kg), Cpg is specific heat at
                                                                                   M is  cpis dTisi g
constant pressure (j/kg.°C), Ag is the surface of the glass cover                                        (Tisi  Tise )  qb isi
                                                                                                                             cd               (6)
(m2), Tge is the temperature of the external face of glass (°C),                       Ais        dt    eg
Tgi is the temperature of the internal face of glass (°C), t is                   With:
               c                                                                 Mis is the mass of the insulator (kg), Cpis is specific heat at
time (s),    q g a is   the heat flux exchanged by convection
                                                                                 constant pressure (j/kg.°C), Ais is the surface of the insulator
                                                         r
between the glass and the ambient (W),                 q g a is the heat flux   (m2), Tisi is the temperature of the inside face of the insulator
exchanged by radiation between the glass and the ambient                         (°C),  is is the thermal conductivity of the insulator
(W),  g is the thermal conductivity of the glass (W/m.°C) and                   (W/m.°C) and eis is the thickness of the insulator (m).
eg is the thickness of the glass cover (m).
                                                                                    - Outside face
- The inside face                                                                M is  cpis dTise                        
                                                                                                     qis  a  qis  a  g (Tisi  Tise )
                                                                                                        c          r                            (7)
 M g  cpg dTgi g                                                                    Ais        dt                       eg
                      (Tgi  Tge )  qw g  qw  g 
                                         c       r

   2  Ag      dt   eg                                                    (3)    Tise is the temperature of the outside face of the insulator (°C),
                                                                                   c
qw g  ph
 ev                                                                              q is  a is the heat flux exchanged by convection between the
                                                                                                                             r
                                                                                 insulator and ambient (W) and            q is  a is the heat flux
exchanged by radiation between the insulator and ambient                       The effect of radiation on production is shown in Figure 4
(W).                                                                           which shows that the increase of the power influences
And for spherical still we have:                                               positively on production. It appears that the production of
- In the glass cover                                                           spherical still is greater than the plane still. The same effect is
   - The out side face                                                         found for the global and internal efficiencies (Fig 5 and 6).
 M g  cp g dTge                          g                                   This was predictable because of the dependence of global
                      q g a  q g a 
                          c        r
                                                (Tgi  Tge ) (8)              efficiency (the ratio of the amount of heat used for evaporation
   2  Ag        dt                       eg
                                                                               and global radiation incident on the still) and internal (the ratio
   - The inside face                                                           of the amount of heat used in evaporation and the amount of
 M g  cpg dTgi g                                                             heat actually received by the mass of water) of solar radiation.
                        (Tgi  Tge )  qw g  qw  g 
                                             c       r
                                                              (9)
   2  Ag       dt    eg                                                       Furthermore, we can note that values of internal efficiency are
                                                                               higher comparing with the global efficiency, which can be
qw g  ph
 ev
                                                                               easily explained by the definitions of those quantities.
  -In the brine
 M w  cpw dTw
                                                                                                         800                                                                      Pg
                   qw g  qw g  qw g  qb  w  pw
                        c        r         ev     c                    (10)
                                                                                                         700
                                                                                                                                                                                  Pw
     Aw        dt                                                                                                                                                                 Pb

                                                                                                         600
  -In the absorber
M b  cpb dTb




                                                                                       Pg,Pw,Pb (w/m )
                                                                                                         500




                                                                                      2
                  qb  w  qb  g  qb  g  pb
                     c        c        r                               (11)
    Ab        dt                                                                                         400


    c                                                                                                    300
q   b g   is the heat flux exchanged by convection between the
                                                                                                         200
                                                     r
absorber and the lower glass (W) and                qb g is   the heat flux                             100

exchanged by radiation between the absorber and the lower                                                            0
glass (W).                                                                                                                         0        200           400           600        800       1000
                                                                                                                                                                                   2
                                                                                                                                                        Incidente power (w/m )
 - At the lower glass
M g  cpg dTg
                 qg  a  qg  a  qb  g  qb  g
                     c       r        c        r                       (12)       Fig .3 Variation of absorbed power according to incident
   Ag        dt
                                                            c                                                                                             power
Tg is the temperature of the lower glass (°C),            q g a is the heat
flux exchanged by convection between the glass and the                                                                                                          Plane still
                          r                                                                                                        10                           Spherical still
ambient (W) and       q   g  a is   the heat flux exchanged by radiation                                                           9
                                                                                                                                    8
                                                                                                         Production (kg/m .jour)




between the glass and the ambient (W).                                                                                              7
                                                                                                                                    6
                                                                                                         2




                                                                                                                                    5
                 III. MATHEMATICAL SOLUTION
                                                                                                                                    4
                                                                                                                                    3
    For the transient regime, the more rigorous simulation is a                                                                     2
                                                                                                                                    1
'' step by step '', which reflects the changing of the temperature                                                                  0
of all components of the still in time and space.
                                                                                                                                        0         200      400         600        800      1000
Meteorological data are obtained from files (wind speed and
                                                                                                                                                                                       2
ambient temperature) or approximate formulas (sky                                                                                                       Incidente power (w/m )

temperature) [8].
                                                                               Fig. 4 Variation of production depending of the incident power
                 IV. RESULTS AND DISCUSSION
                                                                                                                                            for the two solar stills
    A. Incident radiation

  The curves in Fig 3 illustrate the change in power absorbed
by the glass, the brine and the absorber, it is clear that the
increase in power will lead to an increase in power absorbed
by these three components.
  The power absorbed by the glass is not considerable and is
still less than that absorbed by the brine, this is due to its
geometrical characteristics (low thickness) and optical (low
absorption coefficient and high coefficient of transmission).
                                                     Spherical still                                                                                        Ta= 20°C
                                                     Plane still                                                     0,28                                   Ta= 25°C
                                                                                                                                                            Ta= 30°C
                                   28                                                                                0,26
                                                                                                                                                            Ta= 35°C
                                   26                                                                                0,24
                                   24                                                                                0,22
     Global efficiency (%)



                                   22




                                                                                            Production (kg/m . hr)
                                                                                                                     0,20
                                   20                                                                                0,18
                                   18




                                                                                          2
                                                                                                                     0,16
                                   16                                                                                0,14
                                   14                                                                                0,12
                                   12
                                                                                                                     0,10
                                   10
                                    8                                                                                0,08
                                    6                                                                                0,06
                                    4                                                                                0,04
                                    2                                                                                0,02
                                    0                                                                                0,00

                                        0   200      400     600       800   1000                                    -0,02
                                                                                                                              0    200       400      600           800   1000
                                                                         2                                                                                       2
                                                  Incidente power (w/m )                                                                  Incidente power (w/m )


                                                                                    Fig. 7a.Variation of the production a function of the ambient
 Fig. 5 Variation of global efficiency in terms of the incident
                                                                                                    temperature for the plane still
                                                     power

                                                       Spherical still                                                                                Ta=20° C
                                                                                                                                                      Ta=25° C
                                                       Plane still                                                   0,30                             Ta=30° C
                                   40                                                                                0,28                             Ta=35° C
                                                                                                                     0,26
                                   35
         Internal efficiency (%)




                                                                                                                     0,24
                                                                                                                     0,22

                                                                                            Production (kg/m .hr)
                                   30                                                     2                          0,20
                                                                                                                     0,18
                                   25                                                                                0,16
                                                                                                                     0,14
                                   20                                                                                0,12
                                                                                                                     0,10
                                   15                                                                                0,08
                                                                                                                     0,06
                                   10                                                                                0,04
                                                                                                                     0,02
                                    5                                                                                0,00
                                        0   200      400     600       800   1000                                    -0,02
                                                                                                                              0    200       400      600           800   1000
                                                                         2                                                                                       2
                                                  Incidente power (w/m )                                                                  Incidente power (w/m )


                                                                                       Fig. 7b Variation of the production as a function of the
 Fig. 6 Variation of internal efficiency in terms of the incident
                                                                                              ambient temperature for the spherical still
                                                     power
B. Ambient temperature
                                                                                                                                                       Ta=20°C
                                                                                                                     18                                Ta=25°C
   The analysis of the curves in Figure 7a shows that the                                                                                              Ta=30°C
                                                                                                                     16                                Ta=35°C
increase in ambient temperature leads to increased production
                                                                                                                     14
of distillers plan. The same effect is observed for the spherical
                                                                                          Global efficiency (%)




                                                                                                                     12
still (Fig 7b). In fact, the ambient temperature affects the
                                                                                                                     10
different components of still. Indeed, the reduction in
temperature causes a drop in temperature of the brine.                                                                8

Moreover, the figures 8a, 8b, 9a and 9b show clearly that the                                                         6

increase in temperature leads to an increase in global                                                                4

efficiency and effectiveness of the two distillers.                                                                   2

                                                                                                                      0

                                                                                                                          0       200      400       600         800      1000
                                                                                                                                                                2
                                                                                                                                         Incidente power (w/m )


                                                                                    Fig. 8a Variation of the global efficiency as a function of the
                                                                                              ambient temperature for the plane still
                                                                                           C. Effect of wind speed
                                                                Ta=20°C
                                                                Ta=25°C
                                 32
                                                                Ta=30°C                      The figure 10 shows the effect of wind on the hourly
                                 30
                                 28
                                                                Ta=35°C                    production of solar plan. The different curves show clearly that
                                 26                                                        the increase in wind speed influences positively on production.
                                                                                           Indeed the increase in wind speed resulted a drop in
       Internal efficiency (%)




                                 24
                                 22
                                                                                           temperature of the glass, which increases the temperature
                                 20
                                 18
                                                                                           difference between the brine and the glass leading to increased
                                 16                                                        production. Experimental results obtained by COOPER
                                 14                                                        showed that the increase in wind speed from 0 to 2.5 m / s
                                 12
                                                                                           increases the production of 11.5%, beyond this speed increase
                                 10
                                  8
                                                                                           is only 1.5% [9]–[10]. SOLIMAN found that increasing the
                                  6                                                        wind speed increases the production of high temperatures of
                                  4                                                        the brine, however, when the coverage sun is less, the wind
                                      300   400   500   600   700       800   900   1000
                                                                          2                becomes unfavorable [11]. This decrease was observed in the
                                                  Incidente power (w/m )
                                                                                           afternoon. Similar results were obtained for the spherical still
                                                                                           (Fig 10b). In addition, we will notice that the effect of wind on
Fig. 8b Variation of the internal efficiency as a function of the                          the internal efficiency (Fig 11a and 12a) for plane and
            ambient temperature for the plane still                                        spherical still, and the global effectiveness (Fig 11b and 12b)
                                                                                           is similar to that on production, because of the relationship
                                 22
                                                              Ta=20°C                      related the production and the efficiencies.
                                                              Ta=25°C
                                 20                           Ta=30°C
                                                              Ta=35°C                                                                                          v=1 m/s
                                 18
                                                                                                                           0,30                                v=1.5 m/s
       Global efficiency (%)




                                 16                                                                                                                            v=2 m/s
                                                                                                                                                               v=3 m/s
                                 14
                                                                                                                           0,25
                                 12
                                                                                                   Production (kg/m .hr)

                                 10                                                                                        0,20
                                                                                                  2




                                  8

                                  6                                                                                        0,15

                                  4

                                  2                                                                                        0,10

                                  0
                                      300   400   500   600   700       800   900   1000                                   0,05
                                                                         2
                                                  Incidente power (w/m )
                                                                                                                           0,00
                                                                                                                                  0   200     400       600          800   1000
Fig. 9a Variation of the global efficiency as a function of the                                                                                                  2
                                                                                                                                            Incidente power (w/m )
         ambient temperature for the spherical still
                                                                                           Fig. 10a Variation of the production as a function of the speed
                                                                                                             of wind for the plane still
                                                                Ta=20°C
                                                                Ta=25°C
                                 34                             Ta=30°C                                                                                  v=1 m/s
                                 32                             Ta=35°C                                                    0,30                          v=1.5 m/s
                                 30                                                                                                                      v=2 m/s
                                 28
                                                                                                                           0,25
                                                                                                                                                         v=3 m/s
                                 26
       Internal efficiency (%)




                                                                                                   Production (kg/m .hr)




                                 24
                                                                                                  2




                                                                                                                           0,20
                                 22
                                 20
                                 18                                                                                        0,15

                                 16
                                 14                                                                                        0,10
                                 12
                                 10
                                                                                                                           0,05
                                 8
                                 6
                                                                                                                           0,00
                                 4
                                                                                                                                  0   200     400       600          800   1000
                                      300   400   500   600   700       800   900   1000
                                                                                                                                                                 2
                                                                         2                                                                  Incidente power (w/m )
                                                  Incidente power (w/m )


     Fig. 9b Internal efficiency according to the ambient                                    Fig. 10b Production according to the speed of wind for the
              temperature for the spherical still                                                                  spherical still
                                                    22
                                                                                         v=1 m/s                                                                                           v=1 m/s
                                                    20                                   v=1.5 m/s                                                            34                           v=1.5 m/s
                                                                                         v=2 m/s                                                                                           v=2 m/s
                                                    18                                   v=3 m/s                                                              32                           v=3 m/s
                         Global efficiency (%)


                                                    16                                                                                                        30




                                                                                                                                    Internal efficiency (%)
                                                    14
                                                                                                                                                              28

                                                    12
                                                                                                                                                              26
                                                    10
                                                                                                                                                              24
                                                        8
                                                                                                                                                              22
                                                        6
                                                                                                                                                              20
                                                        4
                                                                                                                                                              18
                                                        2
                                                            300    400     500     600    700    800           900   1000
                                                                                                                                                                   300   400   500   600      700      800   900   1000
                                                                                                     2
                                                                           Incidente power (w/m )                                                                                                        2
                                                                                                                                                                               Incidente power (w/m )


Fig .11a Variation of the global efficiency as a function of the                                                             Fig .12b Variation of the internal efficiency as a function of
               speed of wind for the plane still                                                                                       the speed of wind for the spherical still

                                                                                                                                                    V. CONCLUSION
                                                                                         v=1 m/s                               In this work particular attention is paid to the influence of
                                                    32                                   v=1.5 m/s                          external parameters on the operation of a solar still. The results
                                                                                         v=2 m/s
                                                    30                                   v=3 m/s                            show unambiguously that the increase in global solar radiation,
                                                                                                                            which is the most influential parameter, leads to an increase in
                          Internal efficiency (%)




                                                    28
                                                                                                                            its characteristics (production, internal and global efficiencies).
                                                    26                                                                      In addition, it is clear that wind speed favors the exchange by
                                                    24                                                                      convection and leads to a drop in temperature of the glass,
                                                                                                                            which increases the amount of water condensed. So the results
                                                    22
                                                                                                                            are to increase production, global and internal efficiencies.
                                                    20


                                                    18                                                                                                                         REFERENCES
                                                    16
                                                            300    400     500     600    700    800           900   1000   [1] R. Hadef, A. Hadef, Le déficit d’eau en Algérie: une
                                                                                                     2
                                                                           Incidente power (w/m )                               situation alarmante. Desalination vol 13, pp 215–218,
                                                                                                                                2001.
Fig .11b Internal efficiency according to the speed of wind for                                                             [2] Salim Kehal. Rétrospective et perspectives du dessalement
                          the plane still                                                                                       en Algérie. Desalination vol 136, pp 35–42, 2001.
                                                                                                                            [3] Raymond Desjardins, Le traitement d’eaux, 2ème édition de
                                        24
                                                                                            v=1 m/s
                                                                                                                                l’Ecole Polytechnique de Montréal, pp 6, 1990.
                                        22                                                  v=1.5 m/s                       [4] R. Bernard, G.Merguy, M.schwartz. Le rayonnement
                                                                                            v=2 m/s
                                        20
                                                                                            v=3 m/s
                                                                                                                                solaire: conversion thermique et application. Technique et
                                        18                                                                                      documentation, deuxième édition, 1980.
     Efficacité globale (%)




                                        16                                                                                  [5] Chaker A, Boukerzaza. N. Caractéristiques de
                                        14                                                                                      fonctionnement d’un distillateur solaire. 12èmes Journées
                                        12                                                                                      Internationales de Thermique. Tanger, Maroc du 15 au 17
                                        10                                                                                      Novembre 2005.
                                                    8                                                                       [6] Chaker A., Bellel N., et Menguy G., Pertes thermiques
                                                    6
                                                                                                                                dans un distillateur sphérique. Revue Internationale
                                                    4
                                                                                                                                D’Héliothermie, vol 28, pp46-49, 2003.
                                                    2
                                                                                                                            [7] Kaabi Abdenacer, Smakdji Nafila. Impact of temperature
                                                    0
                                                        0         200        400         600             800         1000
                                                                                                                                difference (water-solar collector) on Solar-still global
                                                                         Puissance incidente (w/m )
                                                                                                          2                     efficiency. Desalination vol 209, pp 309–316, 2007.
                                                                                                                            [8] Michel Daguenet. Les séchoirs solaires: théorie et
                                                                                                                                pratique, Unisco, paris, 1985.
Fig .12a Variation of the global efficiency as a function of the                                                            [9] G.N.Tiwari, Solar energy: Fundamentals, Design,
             speed of wind for the spherical still                                                                              Modelling and Application. Alpha science, England.
                                                                                                                                2006.
                                                                                                                            [10] P.I. Cooper, W.R.W. Read, Design philosophy and
    generating experience for solar stills, Australia, Solar
    energy, vol 16, pp1-8,1989
[11] Siaka Toure, Pierre Meukam. A numerical model and
     experimental investigation for a solar still in climatic
     conditions in abidjan (Côte D’ivore), Renewable
     Energy vol 11, pp 319-330,1996.




  For the transient regime, the more rigorous simulation is a ''
step by step '', which reflects the changing of the temperature
of all components of the still in time and space.
Meteorological data are obtained from files (wind speed and
ambient temperature) or approximate formulas (sky
temperature) [8].

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:14
posted:11/26/2011
language:English
pages:7