a free flow flat plate solar still (boutebila 1991)
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\
. l ~ U G H B ~ O ~ ~ ~ ' ~ ~ ~ \ ~ . .
UNIVERSITY
OF
TECHNOLOGY .
.
LIBRARY
AUTHOR/FILING TITLE
C > 1 3 V ~ I ' - A
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ACCESSION/COPY NO.
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VOL. NO. CLASS MARK
J (/
J
i.o IV 1995
28 ]UM
1996
:
:
JUl 995
.
O A N ~ - ~ K ~
-
M R 992
U
J L E L ' 5 ~ U i O
- 1 JUL
199 t
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A FREE LOW
FL T PL TE SOL R
STILL
by
HICHEM
BOUTEBIL
A Master s Thesis submitted
in
partial fulfilment
of the requirements for the award of
Master of
Philosophy
of
the
Loughborough
University
of Technology
February
987
Supervisor: MR
Leeson
Department of
Mechanical
Engienering
by Hichem Boutebila
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,,.:
~ ~ . ' . f ~ J
...
. ';''-'' : \.Jbrory
1 i o ~ . = J ~ 1 ] . _
{
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-;, .
I\.
1 , . ,
2J (u f'1LYLf. n I . r r:rr s
Lp:
S 1 S it re
i K ~ fEt.tlp: r LI nb HI red RTf r
iAAl:lHf hlT'o-HTPC i L rdt STr s tf:' E"S
.
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I
wish to
express my
grat i tude
to
my
supervisor r M R Leeson whose
guidance
and
encouragement made t
possible for
me
to complete
th i s
work.
y
gra t i tude
i s
a lso
expressed
to
my
Direc tor
o f Research
Mr T
Davies for his assistance during the
research.
I am gra te fu l to Mr Brian Mace
whose
help in designing and
const ruct ing the solar s t i l l was invaluable
and to
Mrs
Janet
Smith
who typed th i s
thesis.
y
grateful thanks
go
to the
s taf f
and students of the
Department
of
Mechanical
Engineer ing
and
to
l l
my
fr iends
whose
help
and
encouragement were valuable to my work.
I am indebted to the
Alger ian
Government
for providing
me with the
opportunity
to c r ry
out th is research
and
for providing
the finance.
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ii
Solar d i s t i l l a t i o n desal inat ion) of s a l t water i s su i t ab le
for
supplying
water
for
drinking
and agr i cu l tu ra l purposes
to smal l
=mmunities
where
the supply of f resh water i s inadequate
or
of poor
qual i ty nd where solar radiation i s abundant. Historical reviews nd
t heore t i ca l developments of
so la r
d i s t i l l a t i on ,
including
the
phys i ca l
and t e c hn i c a l
r e s u l t s
o f t he va r i ous
des igns
and
=nfigurations
are
reported.
This
research
i s
confined
mainly
to
one
type of
so la r
s t i l l , an
inc l ined
f ree f low
f l a t pla t e
so la r
s t i l l . To s tudy the e f f e c t o f
signi f icant
parameters
a
mathematical
two
dimensional flow analysis
was carried
out
based
on =ntinui ty ,
momentum
nd
energy
equations
for l i qu i d and vapour
flow.
t i s presented together wi th an
i t e ra t ive
=mputational pr=edure.
dimensional
se t of
equations
i s
developed
nd
solved
by
the
Runge-Kutta
method.
I t
i s
slx >wn
tha t
the
Signif icant parameters of
the
=mbined
two ph se flow are the
fi lm
thiclmess
the l iquid
flow rate the =l l e c t o r
length nd
inclination
nd
the
solar radiation.
smal l
sca le f ree
flow
f l a t
pla t e so la r still
was designed and
=nstruc ted nd experimental studies conducted
under
laboratory nd
direct solar nd i
ions
to invest igate features
which
would
seem
to
affect
the s t i l l
performance such as solar radiation wind velOCity
ambient a i r temperature l iquid flow
ra te
nd
angle
of inclination.
Fina l ly the t heore t i ca l
and
experimental re su l t s are combined
together
to
form
a
basis for the design of
a
long large solar s t i l l
for
further
study.
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AckncMledgements
Sumnary
List
of Figures
List
of Tables
Ncmenclature
T BLE OF a:Nl ENI S
INTRODUcrION
Introduction
CllAPI ER 1:
1.1
1.2
1.3
Human Population
and
Energy Demand
1.4
1.5
CllAPI ER
2:
2.1
2.2
2.3
2.4
nergy
Sources
1.3.1
Coal
1.3.2 i l
1.3.3 Natural Gas
1.3.4
Nuclear Power
The Sun
and
Solar Energy
References
Solar
desalination
Introduction
Basin type Solar
St i l l
History
of
Solar Desalination
2.3.1
Algeria
2.3.2 Australia
2.3.3
Orile
2.3.4 Egypt
2.3.5 Greece
.
2.3.6 India
2.3.7 Spain
2.3.8 Tunisia
2.3.9
The US
2.3.10 USSR
Results
2.4.1 Effect of
Atmospheric
Parameters
2.4.2 Design Effects
Page No
i
v
x
x
1
1
1
3
3
5
5
6
6
10
11
11
12
12
16
16
16
18
18
18
18
19
19
20
20
22
22
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2 5
2 6
O API'ER 3:
3 1
3 2
3 3
O API'ER
4:
4 1
4 2
4 3
4 4
4 5
iv
2 4 3 Operational Techniques
COst
of Product Water by Solar
t i l l s
References
SOL R DISTILL TION
GENER L THEDRY
Introduction
Theory
3 2 1 Heat
Balance
on the
Absorber
and
Cover Assembly
3 2 2 Heat
Balance
on
the
Absorber
3 2 3 Heat Balance on the Cover
3 2 4 Heat
Flux by Radiation qr
3 2 5
Heat
Flux by Convection
3 2 6
Heat
Flux by Evaporation
3 2 7 Heat
Lcsses qLc
References
A
FREE
FLOW
FL T PL TE SOL R COLLECTOR
THEORETIC L
MODEL
Introduction
Theoretical Model Development
4 2 1 General
Equations
4 2 2 Boundal:y Conditions
Simplification Process
4 3 1
Liquid
and Interface Phases
4 3 2 Solution of the Liquid
Equations
4 3 3 Vapour Phase
4 3 4 Stream Function Approach
4 3 5
Computational
Procedure
4 3 5 1 Method of Solution
Theoretical
Results and Discussion
References
Page No
25
25
26
31
31
33
33
34
34
34
35
35
35
39
40
4
42
42
45
46
46
47
49
50
52
52
54
76
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0iAPI ER 5:
5 1
5 2
5 3
5 4
5 5
5 6
5 7
0iAPI ER
6:
6 1
6 2
6 3
6 4
0iAPI ER 7:
7 1
7 2
v
EXPERll1ENTS ND INSTRUMENT TION
Introduction
t i l l Cbnstruction
5 2 1 The t i l l
5 2 2 The
Tank
5 2 3 The Pump
5 . 2 4
fubes
Outdoor
t i l l
The Laboratory t i l l
Instrumentation
5 5 1
Temperature
Measurements
5 5 2
Solarimeter
5 5 3
Data Logger
5 5 4 Liquid Flow Rate Measurements
Tests
5 6 1 The Outdoor t i l l Tests
5 6 1 1 Principles
5 6 1 2
Tests
5 6 2 Laboratory Tests
5 6 2 1
Principles
5 6 2 2 Tests
References
EXPERIMENTAL RESULTS
Introduction
Laboratory
Results
The Outdoor Tests
Cbnclusions
CONCLUSIONS ND SUGGESTIONS FOR FURTHER
WORK
onclusions
Further
Work: a Long
Large
Scale Solar
t i l l
Plant
Page No
78
78
79
79
79
79
81
81
81
87
87
87
88
90
90
90
90
94
94
94
95
96
97
97
97
99
116
117
117
119
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APPENDICES :
ppendix A l
ppendix 1\2:
ppendix A3:
ppendix A4:
v i
Relative Scale Values
Simplif icat ion
Process
Tabulation o Velcx::ity Vectors
A4 1
Nag Rout ine Programme D 2 BAF to
Solve 2F' + FF ;
A4 2 The
Mathematical
M ldel
Programme
Page
No
123
125
130
131
132
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Q IA Pl ER
1:
Figure 1.1
Figure 1.2
Figure
1.3
Figure 1.4
Figure 1.5
Q IA P l E R
2:
Figure 2.1
Figure
2.2
Figure 2.3
Figure
2.4
Figure
2.5
Q IA P l E R
3:
Figure 3.1
Figure 3.2
Figure
3.3
Figure 3.4
vii
LIST OF FIGUR S
Conversion of energy
from
one
form to
another
GrcMth of world population 1400 2000
Estimated world energy demand 1800-2000
The electranagnetic spectrum
Average
annual so la r r ad ia t ion
horizontal
surface
a t
the
ground
on a
Different
types of
solar s t i l l s used around
the world
Basin
type
solar
s t i l l
Tilted
s t i l l
Inflated
plastic
s t i l l
Geographical
locat ions of the s t i l l s in
North Africa a t
the
end of 1957
Diagrammatic sections of solar s t i l l showing
s igni f icant
energy t ransport
streams
to,
Page No
2
4
4
7
9
13
14
14
17
17
from and
within
the
s t i l l
32
Evaporat ive heat t r ans fe r
qe
vs
cover
temperature
Tg
for
different
values of
brine
temperature Tw .
Cover heat
loss vs
ver temperature
g
for
values of
ambient temperature
Ta
and
wind
ve loc i ty
O1aracteristic char t
for
thermal performance
37
37
of a
solar
s t i l l
. . . . . . . . . . . . . .
38
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Qi l \P l ER 4:
Figure 4.1
Figure
4.2
Figure 4.3
Figure 4.4
Figure 4.5
Figure
4.6
Figure 4.7
Figure
4.8
v i i i
Solar = l l e c t o r
A
two-dimens iona l vapour behav i our in
re la t ion to Hquid-vapour in terface
DiInensionless longi tudinal vap:>ur v e l = i
y
DiInensionless
t ransversa l
vap:>ur v e l = i y
Variat ion o f
dF/dB
with u t
Var ia t ion
o f
l i qu i d
ve loc i ty
with f i lm
thickness
for
various
i nc l ina t ions
Var ia t ion o f
the
l iqu id th ickness
wi th
the
= l l ec t o r length
Film thickness var ia t ion with heat flux
for
various incl inat ions
Figure 4.9a EV versus Y
Lr
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CliAPl ER
6:
Figure 6.1
Figure
6.2
Figure 6.3
Figure 6.4:
Figure
6.5
Figure 6.6
Figure 6.7
Figure
6.8
Figure
6.9
Figure
6.10
Figure
6.11
Figure 6.12
Figure 6.13
Figure
6.14
Figure 6.15
CliAPl ER
7:
Figure
7.1
Figure 7.2
e
Water
production versus
pl te
temperature
Water
production
versus
l iqu id flow r te
Water production versus s t i l l
inc l ina t ion
Daily s t i l l
output
Average
dai ly insolat ion
Average d i ly wind speed
Average
d i ly
ambient i r temperature
Environmental data of
day
1
Environmental data of
day
2
Environmental
data
of
day 3
Environmental
data
of
day
4
Environmental data of
day
5
Environmental
data
of
day
6
Environmental data of day 7
Environmental
data of day 8
A
long large
scale sol r s t i l l
Evaporated water
of a
long
scale sol r s t i l l
Page
No
98
100
101
104
105
106
107
108
109
110
111
112
113
114
115
121
122
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0IAPl ER 2:
Table
2 1
Table 2 2:
AI PFNDIX A3:
Table
Al:
Table A :
LIST
OF
TABLES
Data on the most
impor t an t
so l a r
dis t i l l a t ion
plants tha t have been bui l t
from 1872
to
1980
.
Lis t of
the s t i l l components
tha t have
proved to be reasonably sa t i s fac tory
in
solar s t i l l s around
the r l d .
Tabulation
of
l iquid velocities
Tabulation
of vapour
velocities
Page
No
21
24
130
130
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x i
A Film th ickness
a* Dimensionless fi lm thickness = A/YLr
2
Br Brinkman
number =
J
U
r
/
A 1
Tr
Cp
Heat
capaci ty
Fe External forces
9
Gravitat ional acceleration
I Ehthalpy
l\..
Kutateladze
number
= f :, TLr/L
L Heat of vaporization
M )Jc;I)JL
m Variation of
viscosi ty
m Mass
t r nsfer
per
unit area nd per uni t
of
t ime
-
n
Normal uni t
vector
P
Pressure
p*
Dimensionless pressure
m Dimensionless m::xlified
pressure
=
P-P
r
)/6P
r
Pe
Pr
Jw
Re
t
T
-
t
U
-
V
V
Peclet
number
=
U
r
YrP Cp/A)
Prandtl number =
)JCp/A
Plate heat flux
Reynolds number = U
r
YrP/)J
Time
Temperature
Tangential uni t vector
Longitudinal
veloc i ty
Dimensionless longitudinal veloci ty
= U/Ur
Dimensionless
interf ci l
longitudinal
veloc i ty
difference
UGi - Vr,i)
= ----"'=-,Z\crO,------:=-
r
Velocity vector
Transversal
veloc i ty
Dimensionless
t ransversal
veloci ty
V/V
r
Dimensionless
i n t e r f c i l
t ransversal velocity difference
VGi
- VLi)
= - - - - ' - ' - - - . , 6 . V T r ~ ~
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x
We
X
2
Weber number = J/PL U
r
Y
Lr
x
Y
Longitudinal
CXJOrdinate
Dimensionless longi tudinal CXJOrdinate = X ~
Transversal CXJOrdinate
y
Dimensionless t ransversal
CXJOrdinate =
Y/Yr
Greek symbols:
i
Variat ion o f the densi ty
y pg/Pr.
r
Viscous
s t r e s s tensor
P Density
f
Stress
tensor
Dynamic viscosi ty
A
Thennal conductiv i y
8
Dimensionless
temperature = T-T
r
)/6T
lj Stream funct ion
n Angle o f inc l ina t ion of
the pla te
to the
oorizontal
Indices:
G
Vapour
i
nter face
L Liquid
P
Par t ic le
r Scale
S
Saturat ion
W Pla te
Z G,L)
Dimensionless
term
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What
i s
energy?
I s
it f lash of l igh t? A burs t of heat?
Not
rea l ly . These are j u s t
two
among many
forms
o f
energy:
e l e c t r i c
chemical biochemical nuclear kinetic gravitational magnetic and
forms
we
have
not
ye t
discovered.
That
i s
why
the re
can
never
be
a
t rue
energy
shortage. As Einste in demonstrated in hi s
famous
formula
E =
2
,
tha t
everything
in the
Universe i s
energy.
Light
heat
matter
-
i s j u s t energy
in
one form
or i n
t r an s i t between
different .forms.
Figure 1.1 shows
the
conversion
of energy from one
form
to arnther.
nergy surrounds us in
inconceivably
vast quanti t ies .
However, while
the ear th
i t s e l f
i s
composed
o f
so
much
energy
t ha t
we
can
never
complain about i t
we
are
still
concerned about
harnessing
energy
supplies
mainly
in the 20th and 21st centuries
which
i s due
to
ever
increasing energy demand. This increas ing demand i s due to human
populat ion
growth
and r i s i ng
indus t r i a l i za t ion
and s tandards o f
l iving.
1 2 lM\N roPULATICN AND
ENERGY
DEMI\NI
The
ut i l i sa t ion
of
power
by man
in
the
past
followed a s imilar
t rend
to
the growth of the population.
Figure 1.2
shows
some estimates
of
the
t o t a l
world popula t ion
with pro jec t ions to
the end o f
t h i s
century
Brinkworth
[ ]
As
can be seen from Figure 1.2, up to
the
19th
century the
t o t a l
populat ion
had
remained
in
the i n t e rva l o f
300
to 1000
million. The
ra te of
increase in
that
period
was
nearly
zero
to
0.75
per
year.
The
big
change
occurred
in
the
20th
century
passing
from
approximately
1500 mil l ion a t the
beginning
of the
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nuclear e n ~ r g )
radiant
n e r ~ r
/ - ~ - - - - - - - - - - - - - - - - - - - - - - ~ ~ ~ ~ ~
V
electrical enerGY
c n c q ~ 1
FIGURE
1 1 :
ONVERSION
OF ENERGY FROM ONE FORM
TO
NOTHER
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3
century to about 5000
mil l ion
a t the end of i t , with
a
ra te increase
o f 2
per
year .
The
curve
for energy demand steepens
more
rapidly than tha t fo r t he
world population see Figure 1.3), mainly from the mid-20th century
where
the ra te increase
was
higher
than
5
per
year. This was
caused
by t he ext raord ina ry
technology
development
and
soc ia l changes ,
espec ia l ly
a f t e r the
Second
World
War.
This
development
has
dramat ica l ly
inc reased t he demand fo r energy
to
the ex t en t o f
reaching
a
c r i t i ca l s ta te .
The
future
dem8f .ds
for
energy
are
l ike ly
to
go
up
both
on
account
o f
increasing
population and owing to
a bet te r
standard o f l iv ing in
a l l
parts
of the World.
What, therefore,
are
the
energy al ternat ives to
meet these
demands?
To
answer th i s
quest ion
a br i e f discussion
on energy sources
wil l be
given.
1 . 3
ENERGY SOORCES
There are two types
o f
energy
sources: those which
are exhaust ible
c l a s s i f i ed as non- renewable such as
fo s s i l
fue l s
(Le . coal ,
o i l ,
gas) ,
nuc lea r fue l s (Le . uranium)
and
geothermal power; and the
inexhaust ible sources
c l a s s i f i ed as renewable
l i k e so l a r , wind and
hydro power.
Today man
re l i es
on
f ive
main sources of energy. The
foss i l fue ls -
coal , o i l
and na tu ra l
gas
-
account fo r no l e s s
t han 95
o f wor ld
wide consumption, the remainder coming from hydroelectr ic with
3
and
nuclear power
s ta t ions
with
2 ,
Garg [2].
1.3 .1 bal
Coal,
which i s
a
combustible
sedimentary rock
formed
from
the
remains
o f p lan t l i f e , s
the
most p len t i fu l o f
the
e a r t h s fo s s i l fue l s .
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4
6
I
I
I
5
E
04
x
.c
~
3
a.
a.
:g2
1800 1900
2
year A D
FIGURE 1 2 :
GROWTH OF WORLD
POPULATION
1400-2000
Afte r 1)
50
40
18
19
year A.D.
I
I
I
I
I
2000
FIGURE
1 3 :
ESTIMATED
WORLD
ENERGY
DEMAND,
1800-2000
After 1)
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5
Ninety three
per cent of
CDal
reseIVes
are concentrated
in
only three
countr ies the
USSR,
USA
and China
[2] . This
uneven dispar i ty
in
coal reserve di s t r ibu t ions add
to
t ha t the pol lu t ion which can be
caused
by the re lease of
carbon
dioxide through combustion and
the
r i s k o f e x h a u s t i b i l i t y
in
t he
nea r
fu tu re
make coa l no t
recommendable as a solution for the 21st century.
1.3.2
i l
Oil,
which
i s fonned from
marine
l i f e by the de=mposition
of
living
matter ,
i s
the
most used energy
source
in
the
world.
While
coal
i s
concentra ted in c e r t a in places, o i l i s widely dis t r ibu ted in
the
world
The
exploration of
oi l
which i s
also exhaustible and
causes
atmospheric pollution, i s
more uncertain
than tha t
for
coal and t i s
bel ieved t ha t o i l production wi l l reach i t s peak in the 1990's and
s t a r t to
decl ine a f t e r tha t .
That
i s
why the re are
growing
d i f f i c u l t i e s in mainta in ing an equi l ibr ium i n o i l demand.
Add
to
these disadvantages, the
pol i t i ca l
manoeuvres in o i l
pr ices
which
have
dramatically
worsened
the
s i tuat ion
to
the
extent of
reaching
an
energy cris is .
t can be
sa id then,
from the
actual infonnat ion
that
oi l cannot be
a
solution to meet the energy
demands
in
the
future.
1.3.3
Natural Gas
Natural
gas, which
always
accompanies
crude
oi l
i s
a hydrocarbon
composed essent ia l ly of 90
methane
(0l4).
he
remainder i s ethane,
propane and butane . 80 o f
gas
r e se rve s in t he wor ld are
concentrated
in
a few regions Garg [2]:
Middle
East and Africa: 31 ,
Russia: 39.9 ,
USA: 8.3 .
This uneven
dis t r ibu t ion
of reserves
ensures t ha t na tura l gas wi l l never
become
ava i lab le
to
meet any
generalised world energy
demand
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6
1.3 .4 Nuclear
I\ Jwer
Nuclear power i s one o f
t he
new energy
sources
which
have
been
extens ive ly developed
in
the
l a s t 30 years. The
power can be
obtained
by
two contrasting types of nuclear reaction:
i )
The
fissioning of
heavy
atonic isotopes (Uranitnn 235)
ii
The
fusion of
the
isotopes of
hydrogen
in to heavier
helitnn.
It can
be sa id
tha t it
i s a
promising source o f
energy
to meet
the
energy
demands
but
the
cos t o f
ins ta l l a t ion ,
the
use
of
nuclear
weapons a t
Hiroshima
in
1945)
and
mainly the
pol lu t ion
which
threatens
l i f e on
ear th
the
las t
example
of th i s pollut ion was
the
CheITlObyl
disas ter
in Russia, which
caused
the
death
of many
people
and polluted
eveIything round
the
s i te , reaching
many neighbouring
=untr ies )
are
to the
disadvantage of th is
source of
power.
There i s no
need to
speculate
fa r
beyond the non-renewable
sources
for
each
one has
sh:>wn
a
lack
n
meeting the
demand for
energy
n
the
future.
The
only solution for us
i s to develop the
renewable sources
such as solar , wind, oceans, agr i cu l tu ra l wastes , and solve the
technical problems
which
face them.
Unl ike
the
non-renewable
sources ,
so l a r energy causes no
environmental pollut ion and it i s the only
one
for
which
technology
i s
available n many applications.
1.4 ' HE SUN AND SOLAR ENERGY
The sun i s responsible
for most of our
energy
resources , including
foss i l fuels , so la r ,
wind, hydroe lec t r ic power
and also
food.
In
other words,
without
the
sun t he re would
be no l i f e on ear th .
The
so la r radia t ion which
i s an inexhaus t ib le energy source, comes
to
ear th as
l ight ,
which
i s
a
form
of
electromagnetic
radiation.
Most
solar radiat ion
fa l l s
between
0.15
and 120 m but the
practical
one
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COSMIC
GAMMA
RAYS
RAYS
REACHiNC
.a.ATH
(MITTED
FROM
SKY RADIUM
X
RAYS
_l
1
H'GH
FR.EQUENCY
DSCllL rlONS
PR ODUCED
BY
X - ~ A Y
rUBES
7
WAvELENGTH [MICRONS)
.
,.
lOl.r.7 0
ULT'RA
IN FRARED
VIOjLET
pR.QOUCEO
ay
HE T
0
,
I
I
I
I
I
I I
~
PIIDOUCEO By
I
IElECTlItC LAMPS
I
I I
,
10
RADIO
WAVES
PP QOUCEO BY
HIGH-FAEQUENCY G(NEIlATOR.
ELECTRIC
WAVES
PROOUCED er
ELECflUC
GENERATO S
r - - - - - - - - - - - - - - - - - - - - ~
~ - - - - - - - - - - - - - - - - - - ~ - -
I ,
I I
~ F ~ : : ~ - - ~ ~ - - ~ ; _ I r _ ~ ~ : J ~ ~ r - ~ ~ : = : : ~ ~ ~ ~ ~ ~ ' ' > 4 r r - - - - l r - - - - l r - - - - l r - - - - l :
. l. 100
' ~ ~ RO
MOOLE NEAQ THE [UTI-
60
w
,0
~
l
uo
VISIBLE
SHORT WAVE
INFRARED
DISCQIMINATlO"l
f
HEAT THER Py - DRYING
O ~ ~ ~ ~ ~ ~ ~ ~ ~ I ~ L J ~ ~ L J ~ 2 L ~ U U L ~ ~ ~ ~ ~ = = ~ ~ 1
0.) 0.J.5.
0
0 5 05 055 06
0.65
0-7 0-75 10
l
)0
0
50
FIGURE 1.4 :
WAVELENGTH (MICRONS)
THE ELECTROMAGNETIC
SPECTRUM
Af te r
(3)
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8
f a l l s between
0.15
and ~ m
Figure
1.4 shows
the e lec t romagnet ic
spectrum
Giedt
[3].
The
ear th
receives annual
energy from
the sun amounting to 10
18
kWh
Garg [2] . This i s
equivalent
to
more
than 500 000 bi l l ion barre ls of
o i l o r about 1000
t imes
the energy o f
the
known reserves of o i l o r
more
than
20 000 t imes
the
present annual consumption
of energy o f
the
whole world.
The most favourable s i t e s
for exploiting solar energy
are
oonfined to
areas
between
la t i tudes
35
deg
north
and
south
of
the
Equator which
rece ive some 2000-3500
hours of
sunshine per
year
[2] .
Figure
1.5
shows
the average annual solar
radiat ion on a
horizontal surface a t
the
ground
Sellers
[4].
The
major technical
obstacles
which
face
solar energy are:
i so l a r
energy
s
a
d i f fu se energy form
i .e . ~ l i t o u t
ooncentration
i i the
short
term
varia t ion of solar energy.
hese obstacles
imply things:
i
large
areas
and s tructures are necessary
to
provide t he
needed
energy
i i energy must
be
stored
for
time wh n t i s not available.
Unlike other
sources
o f energy so la r
energy
has several unique
fea tures which
place t in
an advantageous
posi t ion. Most o f
the
materials
required for
making solar
apparatus are
eas i ly ava i lab le
and are not very =mplex t design and solar energy
can
be used for
a
variety of
applications
such as for:
heat ing
water
for
domest ic
i ndus t r i a l
and
agr icul tura l
purposes;
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> :.
FIGURE 1 5 :
AVERAGE ANNUAL
SOLAR RADIATION
ON
A HORIZONTAL
SURFACE
AT
THE GROUND
THE UNITS
ARE
KILO-
LANGLEYS
PER YEAR
VALUES S O w ~
IN PARENTHESES
ARE
IN kWh/rn
2
YEAR
f ter
4)
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1
dxying agr icul tural and indus t r ia l products;
space heat_ing and
cooling;
ref r igerat ion for preservation of
food;
desalination
and d is t i l l a t ion of water;
cooking
of food; and
e lec t r ic i ty production
The appl i ca t ion
o f
s o l a r energy i s wider than any other form and
there fore to ob ta in t echnologica l progress , only a
spec ia l i s ed
applicat ion should be
oonsidered. In
th i s
work we have
addressed
our
a t tent ion to
the
so la r desal inat ion
process.
1.5
REFEREN ES
1
B J
Brinkworth:
Solar
energy
for
man . The
Canpton
Press,
1972.
2
H P
Garg:
Treat ise on s o l a r energy:
Vol. 1,
fundamentals o f
so la r
energy .
A
Wiley-Interscience
Publication,
1982.
3. W H Giedt:
Pr inc ip les
o f enginee r ing
heat
t ransfe r . Van
Nostrand,
New
Jersey,
USA
1961.
4.
W D Sellers:
Physical Climatology . University of Chicago Press,
1965.
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2.1
:INl RaXX:I ICN
11
OIAPI ER
2
SOLAR DESALINATICN
Solar
desa1inat ion
or
d i s t i l l a t i on
of
s a l t
water i s su i t ab le fo r
supplying f resh
water
to
smal l
communit ies where the supply of
potab le water i s inadequate o r o f poor
qua l i ty
and where s o l a r
radia t ion
i s abundant.
' 'he basic approach of
d is t i l l ing
sa l ine water by solar
energy
i s
the
natural hydrologic
cycle which
consists of :
i
The absorpt ion o f
s o l a r
rad ia t ion as hea t
by
oceans,
r ive rs ,
lakes, causes evaporation
of
water;
i i The vapour produced s
t ranspor ted as humidi ty
of
the a i r to
cooler regions by
means
of winds;
i i i hen the
a i r
vapour
mixture
i s cooled, the condensation occurs
and causes
i t s precipitat ion
as ra in
and
S 'DN.
This process
i s motivated by
solar
energy which penetra tes the water
surface, warms t and causes i t s evaporation. The t ransport of the
vapour to the cooler
regions where
t condenseS are caused by winds
which are also produced by solar
energy.
By analogy to this, man has reproduced, on a small scale,
the
natural
cycle. As a resul t the following process by which
pure
water
can
be
produced n a solar
s t i l l
is :
i The
production of
vapour fran the solut ion;
i i The
t ransport
of
th is vapour by convection to
the
t ransparent
cover where
t i s
cooled
and condensed; and
)
The
col lec t ion
o f
the
condensed
water.
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12
There
are
several
so la r
st ll designs which use t h i s
process.
They
may dif fe r
from
one another in shape and mater ials used, but a l l use
the same
pr inciples and
serve the same functions. Figure
2.1 shows
different types
of
solar
s t i l l .
2 .2
BASIN-TYPE
SOL R STILL
The
most
=mmonly
used
solar s t i l l i s
the
basin-type s t i l l
which
i s
also Irnown as a
greenl Duse-type,
r=f- type simple-type. The design
=mprises a
horizontal
blackened
surface
which i s f i t t ed
with
sal ine
o r brackish water in
a
shal low o r deep dish and covered
with a
t ransparen t
s loping
surface
on which
water
can
be
condensed.
The
CXJVer which
can be ei ther glass
or
plast ic, i s sloped towards troughs
where
t i s
col lec ted and
then
stored.
Such a
st ll i s
shown in
Figure 2.2.
In operat ion,
so la r
energy
which
i s
t ransmit ted by
the
cover
i s
absorbed by the solut ion (30 ) and the basin (70 ), ooper [1]. Heat
which i s =nducted from the
black
surface to the solution, ir lcreases
water
temperature
and thereby causes
evaporation.
The t ransparen t
cover which
i s
=o l e r than the brine, condenses the warm air-vapour
mixture
which has
been carried
by
convection
currents. The =ndensed
moisture s l ides down the slope to the =l lec t ing troughs from
which
t passes to storage.
To increase
the
product iv i
ty
above t ha t achieved
in
the-hor izontal
basin
s t i l l
t i l t e d
o r
inc l ined
so la r
s t i l l s
have
been used.
The
reasons
for th i s improvement
are
that the t i l t ed
surfaces
in tercept
more energy
per
square metre of co l l ec tor area
and
t ha t covers
re f lec t less
sunlight because
of
a
more
dire t
angle
of
incidence.
A
t i l t ed s t i l l
i s
i l lus t ra ted in
Figure 2.3.
2 .3
HIS'lORY OF SOL R DES LIN TICN
Severa l
repor ts
and
hi s to r i ca l
reviews
of
so la r
desa l ina t ion are
available
in the
l i t e ra ture
review, Telkes
[2,
3]; Daniels [4]; Howe
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3
BASIN-TYPE PLASTIC COVERS
TILTED
WICK
PREFABRICATED
TRAY
''''
mm
c:c::J
DOUBLE
: TUBE
:=:
EXTERNAL CONDENSING
l ~ ~ i o l ; ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
olar
Disliliotion
l o n t ~ _
Transparent
Distillate
t
BASIN-TYPE GLASS
COVER
FIGURE 2 1 : DIFFERENT TYPES OF
SOL R
ST LLS USED ROUND
THE
WORLD
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Angle of glass
10-20)
14
Glass cover
FIGURE
2.2:
BASIN
TYPE SOLAR
STILL
Distillate outlet
:::: -
Brine outlet
FIGURE
2.3:
TILTED
STILL
Insulation
Condensate trough and
F e e d
water
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5
[5] , United Nations [6], the t r e a t i s e
o f
Talber t
e t
a l [7] and the
l a tes t
book
by
Malik
e t al
[8].
The ea r l i e s t solar dis t i l la t ion plant on record was the large basin
type
s o l a r still designed in
1872
by
a
Swedish engineer , Car los
Wilson
in Las Sal inas in the
province
o f
Antofagasta
in Northern
Chile .
t
had
a
co l l ec tor area o f 4700
m
2
and produced
20
m
3
o f
d i s t i l l e d
water
per day during the Summer.
t
has been reported,
[2 ] , t h a t t he still
worked
u n t i l
1910,
t h a t is 30 ye a r s
approximately.
A
detai led descripti.on of the design and
operation
o f
th is
f i r s t s t i l l
was
reported
by Harding
[9]
in 1883.
t has been
repor ted
[7]
tha t in the ear ly
1930 s,
a t i l t ed-wick
design had
been proposed by
Trofimov in
Russia.
During the
Second
World War,
a
new i n t e r e s t
in so la r
d i s t i l l a t i o n
emerged
with
the
invent ion
by Dr
Maria
Telkes of i n f l a t ed p l a s t i c
stills to be used in emergency
l i f e
r a f t s of
the US Navy
and Air
Force, [10].
These uni t s
cons i s t ed o f
an
i n f l a t a b l e smal l
p l a s t i c
envelope containing
a black
absorbent pad
made
of
cellulose sponge
to
be sa tu ra ted
with
sea water before
in f la t ion , and a d i s t i l l a t e
co l l ec tor b o t t l e connected to the bottom of the envelope. Vapour
which would be produced by so la r energy on
s t r i k ing
the absorbent
pad, would
ondense
on the plast ic envelope and dr ip in to t he bot t le .
t was
reported
tha t over 200,000 o f these uni ts were
produced
during
World War I . Such
a
still i s
shown
in Figure 2.4. After the war,
she
inves t iga ted
glass-covered
s t i l l s
and
in
1951,
she
designed
a
glass
greenhouse-type
s t i l l .
She
has
also
reported
[3] experiments
on
t i l ted-wick s t i l l s where 20
of
them were constructed in 1960-6l.
During
the decades
fol lowing
World War 11,
sus ta ined
drought
condi t ions
i n many pa r t s
o f the
world caused problems
in water
supply.
The
use
o f
s o l a r desa l ina t ion seemed to
give a
so lu t ion to
t h i s
problem
by
producing
f resh
water.
All
over
the
world,
wOl k
on
so la r d i s t i l l a t i o n began.
Amongst the countries which experimented
with
so la r
desal inat ion
were:
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6
2.3.1
Algeria
In 1953
Cyri l
Gomella developed
various t ray- type so la r
stills
in
Algeria [11].
More
t han twenty s t i l l s of
ten
different designs
were
t e s t ed and
some
o f
them
were so ld
comm ercia lly throughout
North
Africa ,
Senegal, Cyprus and Aust ra l i a . Figure 2.5 shows the
geographical
loca t ions
o f
the s t i l l s
in
North
Afr ica
a t the
end o f
1957. Savornin
and
Lejeune [12] inves t iga ted f ive other types,
including
th ree
t r ays and
one t i l t e d . These
designs
at tempted
to
improve convection within the s t i l l .
2.3.2 Austral ia
In
1953,
the
SIRO
(Commonwealth
Scient i f ic and Industr ial
Research
Organisation) in
Austral ia ,
s tar ted invest igat ing so la r s t i l l s . hey
developed
a uni t
s imi la r t the Gomela s t ray
and
from
963
t 1967,
CSIRO bu i l t
more
than 8 glass-covered s t i l l s . The aim of these
experiments
was t improve the efficiency of so la r s t i l l s by studying
the
e f f e c t o f
some parameter s
such as wind
ve loc i ty ambient
a i r
temperature, cover incl inat ion,
water
depth,
thermal
capacity,
base
and
edge losses;
and
by
operat ing the
stills
under a range o f
conditions with sa l ine water supplies varying from brackish water t
sea
water , Morse
and
Read
[13 and
14].
Also a
va r i e t y of
mate r ia ls
were used
in
s t i l l
construction in an
attempt
t evaluate the i r
l i f e
expectancy and re l iab i l i ty
[15,
16, 17].
2.3.3 0li1e
t was mentioned above t ha t the f i r s t
so la r
still in the world
was
bui l t
in Chile in 1872.
In
1969/70,
two
so la r
s t i l l
pi lot
plants were
bui l t a t
Quilagua
by Santa Maria Technical University [18]. In 1972
a t the Por t
o f
Pisagua,
four inCl ined so la r
stills
were i n s t a l l e d
[19]. The purpose of the work was t make theoret ica l
predict ions
of
s t i l l
character i s t ics
under
different environmental conditions.
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- bl-cJ.:
1 0 .0 .. JNd
t -
P< d
ppori
]
t r ' I U ~ t
...... r o p
..
- .,1 '-
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18
2.3 .4
Egypt
During 1960
several
smal l
so la r s t i l l s were
t es ted
by the National
Research
Cent re Hafez e t a l [19] . It has a l so been
repor ted
[7]
tha t in 1966
a
plas t ic
= ver ed
still
was
developed
and
tes ted
on
the
Red Sea
coast .
2.3.5
reece
From
1964
to 1973 several
la rge
s t i l l s located in di f fe ren t i s lands
were b u i l t fo r a
t o t a l
area o f 28891 m
2
. A V-shaped
cover
made o f
g l a s s
and
p l a s t i c was t r e a t e d w i th
a va r i e ty o f
cons t ruc t ion
mater ia l s . They have
a l so
in t roduced new
concepts i n des ign ing a
l a rge
still to inc rease the da i l y
product ion
Delyannis
e t
a l [20] .
These two concepts
are:
i a s tronger
=ns t ruc t ion of
the = ve r , and
ii making the s t i l l surface
including s ide
walls as a whole area.
2.3.6
India
Five
small t ray- type s t i l l s were =ns t ruc ted
in 1957 by
the
National
Phys ica l
Labora tory
in
New Delhi Khanna e t a l [21]. To
asse s s
the
per formance o f var ious
m ate r i a l s
and g la s s cover des igns , severa l
other experimental so la r
s t i l l s
were tested, Gomkale e t a l [22].
They
a lso s tudied the
ef fec t
of
different
parameters such as
atmospheric
variables
=ns t ruc t ion
mater ia ls
and
operational
techniques
on
the
performance of solar
s t i l l s ,
Ahmed e t a l [23]. Two of the =nclus ions
obtained
were
tha t :
20
degree
incl inat ion was the bes t angle for the
covers
and t h a t t he average e f f i c i e nc y was about 30
o f
energy
ut i l i sa t ion in the so la r s t i l l .
2.3.7
Spain
It
has
been
r epor ted
[7]
t h a t
dur ing
1958
two
smal l
t r a y s o l a r
stills were cons t ruc ted
to s tudy t he
e f f e c t
o f var ious glass
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9
incl inat ions nd construction techniques.
I t was found
that a
glass-
covered st ll
with
a
shal low bas in and
a low inc l ina t ion cover was
the bes t
design.
In 1966, an
869 m
2
st ll
was completed a t
Las
Marinas. The i n s t a l l a t i on
was
des igned to
provide
a vi l l age of 300
persons
with
fresh
water.
2.3.8
' l \mis ia
Since 1962, the so la r
energy group
o f
the
Tunis ian
Atomic
Energy
Authority has
been
act ively studying solar
dis t i l la t ion [24]. More
than a dozen
were bu i l t and
a t the
beginning
o f
1967, th ree la rge
solar
dis t i l l a t ion
sta t ions
were
constructed
[25].
2.3.9 ' be
US
After
the Second World
War,
many
research
cent res
in the
US
amducted work
on
solar
d i s t i l l a t i on . The
Universi ty o f
Cal i fo rn i a
s t a r t e d ts inves t iga t ion in 1952 and
cont inued
fo r more than
20
years. Various configurations
for
simple
solar
s t i l l s were bu i l t nd
tes ted
in
t rying
t
reduce
capi ta l
cos ts
nd
improve
eff iciency.
The
work aimed t
study the
features
which
would
seem t
affect
the s t i l l
efficiency,
such
as various geometrical configurations,
batch-feeding
versus continuous-feeding,
means
of
recircu1ation
of
air ,
nd kinds
nd
thicknesses
of
insulat ion.
I t was concluded that the
conditions
which
seemed
to lead
to maximum eff ic iency are:
i
a low heat capacity of the s t i l l nd the water contained in i t ;
i i a low
incl inat ion of
the vapour-tight
t ransparent
cover;
nd
i i i
good
insulat ion
of the
bottom of
the s t i l l .
A
summary of
th is
work can
be found
in
Howe nd Tleimat [26]. From
1958 to
1965,
the
Office
o f
Saline Water
planned
a
solar dis t i l l a t ion
programme nd financed the Bat tel le Mem=ia1 Inst i tu te to
build
nd
t e s t
severa l
types of s o l a r
st lls
a t Daytona Beach Sta t ion in
Florida
[27].
Many
other
unive rs i t i e s
and
research cent res
in
the
US invest igated
solar dis t i l la t ion .
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20
2.3.10
USSR
I t has been reported
[7]
tha t during 1956 the Solar nergy Laboratory
in
Krzhizharovsky
in
Moscow began
invest igat ing
so la r
s t i l l s
as
a
means
o f
supplyirq water to ar id and
semi-ar id
lands
in
Russia.
In
1962,
a so la r s t i l l was designed and t e s ted a t Tashkent University.
From 1961 to 1965, experimental s t i l l s
were
t es t ed in Turkemenian and
based on th i s work, =nst ruc t ion of a large s t i l l began in Ashkhabad
in 1969
[28] .
t has been r e p o r t e d
i n
t he
l i t e r a t u r e
rev iews t h a t s o l a r
d is t i l l a t ion
has
also
been
invest igated
in
the
following
= m t r i e s :
I t a ly Japan, Taiwan, South Afr ica , Libya, France, Morocco,
Kenya,
ew
Caledonia, West
Indies,
Pakistan,
Cyprus, Iran,
Senegal,
Mexi=,
China etc.
Data on
the
most
important
solar d is t i l l a t ion plants
tha t have been
bu i l t
fIOm
1872 to 1980 are shown in Table 2.1 [29].
2 .4
RESULTS
Here
are
the
conclus ions obtained in
a
carefu l s tudy o f
the
s ign i f i can t r e s u l t s
presented
by
many inves t iga to rs a l l over the
world.
Solar d is t i l l a t ion should be =nsidered a
possible
method for water
supply under
the
following
circumstances:
1. Natural fresh water
i s not available
and sa l ine or brackish water
i s available;
2. The
cl imate i s
good i .e.
solar radiat ion leve ls a re high);
3. The potable water needs are below
200
m
3
per
day;
4.
The
land i s avai lable
for solar
s t i l l
s i tes ;
and
5. Such land i s in i sola ted
locations where other
sources of
energy
are non-exis tent .
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21
Country Loc.ation
Year
,,
Feed Cover RCIIl Drke
Australia Huresl
1
196)
372
Brackish l au Rebuil
t
Huresk l
1966
)72
Brackish Class
Operating
Coaber
Pedy
1966
)160
Bnui5h
Class
Operating
Caiguna
1966
372
Bracki . b
Clau Operating
Hamelin Pool
1966
557
Brackish
Clan Operac.in&
Griffit.h
J ~ 7
413 Buck.iah Class
Operat.ing
Cape
Verde
Santa Y..aria
1965
743
Seilv.ater
Pl.astic
1 1 Santoll Haria
1968
Abandoned
Chile
Las 5aliDalO
1872 4460
Brad.ish Class
Ab.mdoned
QuillaE'ua
1968 1 S ~ l J a t e r
Clan Operating
Greece SyDi I 1964 2686 Sea1Jater
Planic Rebuil t
Symi I l
1968
2600 Se.vater
StT. Plas. Disn-..antled
Aegic.a. I 1965 1490 Sea\r.lter
J. lastic
Rebuilt
Aegina 11 1968 10486 Se31Jat er St.r. PIa&:. Abandoned
Salamis 1965 388
Se.auat.er
Plastic
Abandoned
Patmos 1967
8600
5uuater
Class
OperOlting
Kimolos 1968
2508
5eawater
Clau Operating
lHsyros
1969 2 5
5uv.ater
Class
Operating
Fiskardo 1971
22
Se0l10l3t.cr
Class
Operating
Kionioc
1971 2400
Seav.lt.er
C l . a s ~ Operating
Hegist i 1973 2528
S e ~ l 1 . : a t e r Class Operating
India Bhavnagar 1965 377
Se.avater
C ass
Operaticg
A\',rania
1978
1866
Bru.kish Class Operating
Mexico
Natividad Is l
1969 95
Seavatcr Class Operating
Pakistan
C\.7adar
1969 306
Sea\.73ter
0011;,
Operating
G'uadar
1I
1972
9072
Seavater
C l a s ~
Operating
Spain
Las Marinas
1966
868
Se.Jnlater Glass Operating
Tunis ia
Chakmou
1967
Brackish
Class
Operating
l- .ahd
ia
1968
13
Brackish Class Operating
U.
S.A.
Daycona Bead> 1959 228 Se.avater
Cb.sl;
Rebuilt
Daytona
B z :::h
1961 246
Seavater Class Disoantled
.Daytoo.a
Beach
1961 216
Sem.:-ater
Plas t i c
DiSI:iantled
Daytona
Beach
1963
148 Se.avate:r
Ph st ic
Dismantled
USSR
Bakb.arden
1969
600
Brackisb
Class Operating
\.Jest
Indies Pot i t 1967
171 Seavater Plast ic
Operating
St.Vincent
Rai t i
19.9
223
Seavater
Clau
Operating
hldia
Bilra
1980
Brackisb
Class
Gperot.ing
(capacity
2000
l /day)
Kult: .i,
198
Bracitisb Class
Operating
(c:apaci ty
:;000
l/da} )
China
\..'uzhi
j 9 ) ~
385
Se.al.1ater Class
Operat.ing
Zbungjian
1979
50
Sea ater
Class
Operating
TABLE
2.1
D T ON
THE MOST
PLANTS T F ~ T H VE
After 29)
IMPORT NT
SOL R
BEEN BUILT FROM
DISTILLATION
1872 TO
1980
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22
I t h s also been found
tha t the
number
of
variables
inf luencing
the
produc t i v i t y o f s o l a r stills i s very h igh and t hey a re
o f t e n
independent. Among
t he
most impor tant are:
atmospheric
variab les
so lar
radiat ion,
wind
velocity
nd
ra infal l ) ;
design brine
depth,
insu la t ion , vapour
t igh tness , cons t ruc t ion
mater ia ls , maintenance)
nd operational techniques.
2.4 .1
f fec ts o f Atln::>s[tleri c Paraneters
1 . Ambient temperature:
Solar
s t i l l product iv i ty
increases s l igh t ly
as ambient a i r t empera ture increases . For each lOoF r i s e
in
ambient t empera ture ,
the
magni tude o f the
output i nc rease
averages 5 .
2.
Solar
r ad ia t ion :
As
f a r as the
atmospher ic
va r i ab le s
are
concerned,
the
solar s t i l l
product iv i ty
depends
almost ent i re ly
upon the
s o l a r
r ad ia t ion
in tens i ty .
It
w i l l
depend
to
some
extent
upon how
the
radiat ion i s dis t r ibuted throughout the day;
but it i s usual ly
suff ic ient
to consider
only
the to ta l rad ia t ion
received
each
day.
3.
Thermal
capac i ty : The thermal capac i t y o f a still has a smal l
ef fec t
on
i t s
performance.
4. Wind velOCity: As long as t he
still
is w e l l
sea led
to prevent
v p o ~ r leakage,
produc t i v i t y
i s
s l i g h t l y a f fec t ed
by wind.
However, if t he
product iv i ty to
a
still
is poor ly
sea led , wind
can lower
t he
great
extent.
5.
Rainfa l l :
S t i l l
produc t i v i t y c:;an
be
increased
by ca tch ing t he
ra infa l l .
2.4.2
Design f fec ts
1.
Brine-depth:
I t
h s been
concluded
tha t
the
shal lower
the
brine
depth o f a still t he higher t he t o t a l d a i l y product iv i ty , but
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23
the
small water depth requires a=ura te levell ing
o f the
absorber
surface , s ince any
humps
in
the surface could cause
dry spots
thus
decreasing
the water surface area available for evaporation
as well as deterioration
of
the s t i l l .
2.
Materials
of
oonstruction:
s fa r as
construction i s
ooncerned,
useful information on materials i s yielded, part icular ly cover,
absorber and
sealants.
The t ransparen t cover
can
be e i t he r
glass or p las t i c but
glass
i s prefe rred to plas t i c because o f ts high t ransmiss ivi t :y
for
s o l a r
r a d i a t i on low t r a n s m i s s i v i t y for low temperature
rad ia t ion
high
we t t a b i l i t y
for
water
and
re l a t ive ly
high
s tabi l i ty of properties
over
a
long
period of
time.
The absorber
must
absorb so la r r ad ia t ion read i ly must be
water t ight and
shoUld
be
capable
of supporting high temperatures
without deleterious effects .
To prevent
vapour
leaks, cover
sea l ing i s most
important s ince
leakage can dramatically
decrease
the
production rate.
The Office
o f
Sal ine Water USA)
in the
r epor t [7] l i s t ed st ll component
materials
that
have proved t be reasonably sat i sfactory in
solar
s t i l l s
around the
world,
see
Table
2.2.
The
mater ia l s
are
l i s ted
in order of preference from a durabi l i ty standpoint).
3.
Insulation:
To ra ise
br ine temperature and reduce
heat
losses,
the
bottom and s ides o f
the st ll should be
insula ted. In some
cases, the annual productivity of an insulated s t i l l
i s 15
over
the uninsulated version.
4. Cover
design: The most practicable
cover for
large ins ta l la t ions
i s a glass se t a t
an
angle
of
10-20 degrees from the oorizontal.
The cover should
also
be placed a t no greater
distance
above the
br ine surface.
When a long l i f e up
to
a
maximum
of 5 years)
i n s t a l l a t i on i s
envisaged, o r in
i so la ted
locat ions and
where
glass t ranspor ta t ion could be d i f f i c u l t and expensive, glass
covers
can be
replaced
by p las t i c
ones
which should
be
t rea ted
for wet tabi l i ty to
prevent dropwise
oondensation.
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Component
BJ
.
in
liner
Support
slruc.:ture
. Distillate
trough
Sealant
Piping and .valves
\ . ala 1 r a g e
rcscrvtlirs
24
Materials
Butyl
rubber
(OOI}-()OJO-in.
thick):
B:o;ph:llt
mats (O12-0lS-in. thick): black
p o l y e l h y l e n ~
O. ) )8in.
thick): roofing OlSph:l1t (o er con
crele. etc.)
Window
sla
(0-10
or
OI2-in. thick); cttablc
Tcdlar plastic IO-OO4-in.
thick)
Concrete:
concrete
block: aluminum: galvanized
metal:
redwood-
StainJess steel: butyl rubber (lining): black poly
. t:thylcne (lining)
Silicone rubber: asphalt caulking
compound:
butyl
rubber extrusions
PVC (polyvinylchloride): asbestos cement (for
saline water):
SS
(acrylonitrilc-butadicnc
slyrene)
Concreh:: masonry
Rl. lalively
short lifetimes.
TABLE.2.2:
LIST OF THE STILL COMPONENTS
THAT
HAVE
PROVED TO
BE REASONABLY SATISFACTORY
IN
SOLAR STILLS
AROUND THE
WORLD
f ter
(7)
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5
5. Condensate
leakage:
The
leakage of the
condensate
from troughs
i s another
reason
for
the decrease of
still output . To prevent
the oondensate spi l l ing or overflowing, the troughs must
be
deep
and narrow
ElO IUgh to
minimise shaoowing of the brine.
2 4 3
Opera t ia la l
Techniques
t has been concluded t ha t
a long
term opera t ion
o f s t i l l s does
not
r qu i r
clearu.ng
in the
case of glass covers. However
plast ic covers
which
a t t rac t dust because
of the i r
eleclLostatic properties,
have
to
be
washed periodically. I t has been recommended t ha t a solar s t i l l
should
operate
cont inuously
throughout
the
year
and
feed water
preheat ing and
f lushing
methods can
be
used.
2 5
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26
P
= annual i n t e res t
and
amor t iza t ion r a t e (percentage o f
investment)
MR =
annual maintenance
and
repa i r ,
labour and mater ia l s
costs
(percentage of
investment)
TI
=
annual
taxes and insurance charges (percentage o f
investment)
L
= annual operating
labour
costs
W
=
operating labour wages, $/man hours
Y
D
=
annual
unit yield
of dis t i l l ed water (gallons/m
2
)
Y
R
= annual unit yield
of =l lec ted
rainwater (gallons/m2)
n = area
of d i s t i l l e r
on which d i s t i l l a t e yie ld i s
based
m
2
)
AR = area of dis t i l le r on
which
ra infa l l
=l lec t ion
i s based
m
2
)
S = t o t a l cos t ( f ixed and operat ing) o f s a l t water supply
($/1000 gallons of product).
1.
P I Cooper:
The m ~ x m u m eff ic iency of s ing le e f fec t so la r
s t i l l s ,
Solar
Energy,
Vol
15, pp 205-217 (1973).
2. Maria
Telkes:
Fresh water from sea water by solar dist i l lat ion ,
Indust r ia l
and
Engineering Chemistry,
45 (5) pp
1108-1114 May
1953).
3. Maria Telkes:
Solar
s t i l l s ,
Proceedings of World Symposium on Applied Solar
Energy, Phoenix, Arizona,
pp 73-79
(November
1955).
4. Daniel Farrington:
Direct use
of the
sun 's
energy ,
Yale
Univers i ty
Press,
New
Haven,
374 pages
(1964)
[Chapter
10, 'Dist i l lat ion of
Water', pp
167-195].
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27
5. Everett Howe:
Review o f still
t ypes ,
Chap t e r prepa red
fo r
U S o l a r
Dist i l la t ion Panel Meeting,
34
pages
October 14-18, 1968).
6.
Solar d i s t i l l a t i o n as
a
means o f meet ing sma l l - s ca le
water
demands , United Nations Publication (1970).
7. S G
Talbert;
J A
Eibling
and G G
Lof:
Manual of
so la r
d is t i l l a t ion of
sal ine
water , Office of Saline
Water, US Department
of
the
Inter ior ,
Res
and
Dev,
report
No
546
(1970).
8. M A S Malik, G N
Tiwari,
A Kumar
and
M S
Sodha:
Solar dist i l la t ion , Pergamon
Press,
Oxford, England (1982).
9.
Josiah
Harding:
Appara tus
f o r
s o l a r
d i s t i l l a t i o n ,
Proceed ings
o f t he
Inst i tut ion
of
Civil Engineers,
Vol 73, pp
284-288
(1883).
10.
Maria
Telkes:
Solar d i s t i l l e r
for
l i f e raf t s , US Off ice o f Science,
Report
No
525,
PB
21120, 24 pages
(19 June, 1945).
11.
C
Gcrnella:
Contribution
a
l 'e tude
de
l a d i s t i l l a t ion
so la i re
les
resu l ta ts
i ndus t r i e l s acquis en Alger ie apercu
s u r
l ' importance de
l ene r g i e t he rmique , Colloques in te rna t ionaux du Centre
National de
la
Recherche Scient i f ique [Applications thermiques
de
l ' energ ie
s o l a i r e dans le domaine de l a recherche e t de
l ' indus t r ie]
France,
pp 601-620 (1961).
-
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28
12.
Savornin
and G Lejeune:
Etude
su r
1 ' evaporat ion e t
l a
condensat ion de 1 ' eau dans
1es
d is t i11a teu rs
sola i res , Co11oques
in te rna t ionaux
u cen t r e
National
de la Recherche Scient i f ique [Applications
thermiques
de
l ' energ ie so la i r e
dans l e domaine
de
l a
recherche
e t de
l ' industr ie] France, pp 589-600
(1961).
13. R N M:>rse and W R W Read:
A r a t iona l bas is for
the
engineer ing development o f a s o l a r
s t i l l , Solar Energy, Vol 12, pp
5-17
(1968).
14. P I Cooper:
The maximum
e f f i c iency o f s ing le -e f fec t
solar
s t i l l s , Solar
Energy,
Vol 15,
pp 205-217 (1973).
15. P I
ooper
and J A Appleyard:
The
construction and performance
of a
th ree effec t , wick
type,
t i l t e d so la r s t i l l , Sun a t
Work,
Vo1 12
(1),
pp
4-8
( f i r s t
quar te r , 1967).
16.
R W M:>rse:
The c ons t ruc t ion
and i n s t a l l a t i o n o f s o l a r st lls in
Australia ,
Desa1ination, vo1 5,
pp
82-89 (1968).
17. P I ooper and W R W Read:
Design
philosophy and operating
experience
for Austral ian
solar
s t i l l s ,
Solar
Energy, Vo1 16,
pp
1-8 (974).
18. German Frick and Jul io Hirschmann:
Theory and exper ience
with
s o l a r st lls in
Chile ,
Solar
Energy,
Vol
14, pp 405-413,
(1973).
19. M M Hafez and M K
Elnesh:.
Deminera l iza t ion o f sa l ine
wate r by
s o l a r
rad ia t ion in the
United
Arab
Republic ,
UN
Conference
on
New
Sources
of
Energy,
Paper 35/S/63,
Rome
10 pages (August
1961).
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29
20. A Delyannis
and
E Piperoglou:
The
Patmos s o l a r
d i s t i l l a t i o n
plant , Technical
paper, Solar
Energy, Vol
12,
pp
113-115
(1968)
21.
M L
hanna
and
K N
Mathur:
Experiments
on deminera l i za t ion
of
water
in
North
India , UN
Conference on
New
Sources
of Energy, paper 35/S/115,
Rome
11
pages (August 1961).
22.
S D Gomkale S Y
Ahmed
R L Datta
and
D S
Datar:
Fresh
water from sea by so la r s t i l l , Paper presented a t the
Annual Meeting o f
the
Indian
In s t i t u t e
o f Olemical Engineers,
Bangalore,
India (Dec
1964).
23
S
Y
Ahmad S D
Gcmkale,
R
L
Datta
and
D S
Datar:
Scope and
development
of solar s t i l l s for
water desal inat ion
n
India ,
Desalination,
Vo1
5, pp
64-74
(1968).
24. Tunisian Atonic Energy
Ccmnission:
Report
o f
a c t i v i t i e s
1966-67,
Chapter
10,
Solar
energy ,
pp
53-76,
Solar
Dist i l la t ion,
pp
54-64,
n French (1967).
25.
Tunisian
Atonic
Energy Ccmnission:
Brochures descr ib ing so la r d i s t i l l a t i o n s t a t i ons
a t
Olibou,
Chekmou and
Mahdia, Tunisia ( in French
and
Arabic) ,
8
pages
each (1968).
26.
E D
Howe
and B W Tleimat:
Twenty
years
of
work
on
solar
di s t i l l a t ion
a t
the University of
California ,
Solar
Energy Vol 16, pp 97-195 (1974).
27.
Bat t e l l e Memorial Ins t i tu te : J
W
Bloemer, J R I rwin and J A
Eibling:
Final th ree years
progress
on s tudy and f i e ld evalua t ion o f
so la r
sea water
s t i l l s ,
June
1965,
OSW
Report
No
190
87
pages,
(May 1966).
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3
8
V Batnn and R
Bairarrov:
Prospects
o f so la r st lls in Turkmenia ,
Solar
Energy,
Vol
16
(1), pp
38-40
(1966).
29. Delyarmis and
E
Delyarmis:
Sola r d i s t i l l a t i o n p lan t o f high capaci ty , Proceedings o f
Fourth
International
Symposium on Fresh Water from
the
Sea,
Vol
4, p
487
(1973).
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31
QlAPl ER 3
SOL R DISTlLIATICN GENER L llIEDRY
3 .1 INl RCIXCl ICN
The bas ic p r incfp les
of opera t ion o f
s o l a r s t i l l s have been s t a t e d
and
developed by
many authors
[1
2
3 4]
to
the point
where
the
numerical signiUcance of the various
parameters
may be determined in
re la t ion
to
performance
and
fresh
water
production.
The work
which
was developed by
Dunkle [1]
in 1961
and
which
reviewed some o f
the
work on roof type s o l a r s t i l l s analysed
and
discussed the heat
and
mass t ransfer relat ionships and indicated the
ef fec t of temperature
and
pressure
on
the performance. That work
was
s l igh t ly modif ied by Morse and Read [2] in 1968 who considered the
heat and mass t ransfer relat ionships which
govern
the operation of a
so la r still
in
the unsteady s t a t e and
expressed
the
var ious
hea t
fluxes as
functions of the
= v e r
temperature.
The analysis
was
then
used to find the effects
on
output of changes in various
parameters
such as wind velOCity
ambient
temperature and heat
loss
from
the
base.
From t ha t
work
Cooper and Read [3] s tud ied both t heore t i ca l ly
and
pra c t i c a l l y the opera t ion
of
a
so la r
s t i l l .
They
showed t ha t
success fu l
development
of s o l a r
stills i s
dependent upon
a desfgn
ph i lo sophy i nvo l v i ng a
working
knowledge of t he
the rmal
character i s t ics of solar s t i l l operation. The design philosophy led
to
t h e establishment
of thermal
and
oost i e r i a for the selection
of
materials
and
design
of
component parts .
Final ly Malik e t a l
[4] in
t he i r
l a t e s t book on so la r d i s t i l l a t i o n
reviewed
the
work
which had
been
carr ied
out
up
to
that
time.
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3
/ - o .q
T
s
Cwg
FIGURE 3 .1 : DIAGRAMMATIC SECTIONS
OF
SOLAR
STILL
SHOWTNG SIGNIFICANT ENERGY
TRANSPORT
STREAMS
TO
FROM
AND
WITHIN
THE
STILL
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33
3.2
THEDRY
The relat ionships
which
govern the operation
of
a
so la r s t i l l
in
the
steady
s ta te condition, are the heat and
mass
t rans fe r ra te and the
energy
balances.
A diagrammat ic
cross - sec t ion
o f
a solar still
on
which
are indicated
the
heat and energy
f luxes
and the i r direct ions,
i s
shown in
Figure
3.1.
It
was
demonstrated t ha t a
se t
of e i gh t equat ions suf f i ces
to
describe the system.
3.2.1
Heat Balance
en
t he
Absorber and
Caller Assembly
The
energy balance
for
the
s t i l l
requires
tha t
th
to ta l solar energy
absorbed must
be equal
to the energy t ransferred from the CtJ
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4
d
w
wb dt:
energy s to r ed
in
the system as the water temperature Tw
changes with t ime t ;
energy s to r ed
in
the cover
as
the cover temperature Tg
changes with t ime t
This
term can
be
neglected.
3 2 2 Heat
Balarx e
en the Als n ber
The hea t balance on
the
bas in which
was
given by
Morse and
Read [2]
i s as
follows:
d
< w
=
qlo
wb d qr
where i s
the
heat flux
by
evaporation
and condensation
qr
i s the heat
flux
by
radiation, and
i s
the
heat flux
by convection.
All the
terms are expressed in
SI uni ts
3 2 3 Heat Balarx e en the
Cover
(3.2)
he heat
t ransfer
between
the er
and the
s a l twa te r i s the
sum of
q r
qc qe
while the hea t flow to the su=oundings
i s
th i s to ta l
heat flux,
plus
,the
so la r
energy absorbed
by the
glass:
+
(3.3)
3 2 4 Heat Flux by Radia t i cn Ir
The
hea t
f lux by rad ia t ion
between
the cover and
the
water surface
was
given by
Dunkle
[1] and i s equal
in
SI system to:
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35
(3.4)
where
EW i s the
emissivi ty of the
water
surface
and i s
usual ly taken
as 0.9
i s the Stefan-BoI zmann
constant
and i s equa l
to
5.6697 10-
8
m
1
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36
where
h
lo
i s equivalent
to
heat t r an s f e r
coe f f i c ien t
base to the
surroundings
and
ground. This quant i ty i s
d i f f i cu l t
to
est imate because the temperature of
the
ground i s general ly
l.Il lkrn-m.
ArDther equation
which relates
the
heat dissipation from
the
oover
to
the ambient temperature Ta can be
added to
describe
the
system more
precisely.
This equation which was given by ooper and
Read
[3]
n SI
un ts i s :
3.8)
where
Ts
i s t he
sky temperature
and i s
equal, Sayigh
[5]
to:
3.9)
and
hga i s the convect ive heat t r ans fe r
coeff ic ien t
which i s
dependent on wind velocity and a= r d i ng to MacAdarns [6] ~ i s equal
to:
~
; 5.7
+
3.8
w
3.10)
where w
which
i s the a i r velocity, i s 0
i\
I
. < - ~ . -,
,
. ,
I'; - \ \ \ , , ~
o
40
60
80
100 120 140
I
COVEr
TMPRATUR . 1 9
180
FIGURE 3.4: CHARACTERISTIC
CHART FOR T H E R ~ L
PERFOR-
MANCE OF A SOLAR
STILL
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39
From equat ion 3.6 and us ing
the
l a t en t
heat o f vapor iza t ion ,
the
evaporation
mass t ransfer
ra te which was given by
ooper
and
Read [3]
i s :
(3 .11)
3.3
REFEREN ES
1. R V Dunkle:
Solar water d i s t i l l a t i o n : the
roof- type
st ll and a mul t ip le
e f f e c t
d i f fus ion
s t i l l . In te rna t iona l Developments
i n
Heat
Transfer,
ASME,
pp
895-902
(1961).
2.
R N M >rse
and
W R W
Read:
A ra t iona l
bas i s fo r
t he engineer ing
development o f a s o l a r
s t i l l .
Solar Energy, Vol.
12,
pp 5-18
(1968).
3.
P I
Cooper and
W R W
Read:
Design
philosophy and operating experience for Austral ian
solar
s t i l l s .
Solar Energy,
Vol. 16,
pp 1-8
(1974).
4.
M A S Malik, G N
Tiwari,
A Kumar
and
M S Sodha:
Solar
dis t i l l a t ion .
pergamon Press, Oxford, England
(1982).
5. A A M Sayigh:
Solar energy engineering . Academic Press Inc., New YO:r:K (1977,
[pp 431-464
by E D
Howe and
B W T1eimat
' fundamentals o f wate r
desa l ina t ion ' ] .
6.
William
H MacAdams
Heat transmission . McGraw-Hill
Inc. ,
p 249 (1954).
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4
rn PrER
4
A FllliE FLCM FLAT PLATE
SOLAR CXlLLEx IDR
llIEOREl ICAL
MDEL
4.1
l:Nl RCIXCl ICN
In
orde r
to
maximise the heat
absorption the = l l e c t o r
i s
incl ined.
The f l u i d
which
i s pumped
to
the
top
of the still f lows
f r ee ly
downward
n a t h in
f i lm and
subsequently the heated
surface
ra i ses
the
moving f lu id t empera ture and
evapora t ion
w i l l s t a r t . The
evaporated water which i s condensed on the
inner
s ide of the
=ver
i s
recovered
in a
s imi l a r
arrangement
to the bas in still. Such a
co l l ec t o r
i s
shown
in
Figure
4.1.
Depending
on
the
in tended
appl icat ion of the t i l t e d
= l l e c t o r under =nsidera t ion
[1-7]
many
theore t ica l analyses have been reported invest igat ing the f luid flow
and heat
t ransfer
character i s t ics of t h i s type [2 3 4 5 7].
An
ea r l y work by Col l i e r [2] cons idered
a descending
f l u i d
in
an
open f l a t
plate solar
= l l e c t o r to be
used
n a
refr igerat ion
system.
From his analysis which =mmenced
from
the energy equation n which
it
was assumed that the
flow
was steady he
developed
an expression
fo r
the
vapour
mass
f low and showed
t ha t the
performance
o f
the
=l l e c t o r was
in i t i a l ly
dependent
on environmental
=ndi t ions.
A s imi la r type o f study n
which Peng
and Hawell [3] endeavoured
to
improve
the
accuracy of the previous work was based on the
mass
and
heat balance equat ions . In order
to
obta in the tempera ture
dist r ibut ion
along
the
=l l e c to r
the
authors
assumed
the
flow
ra te
and
hea t
capac i ty o f
the
f lu id to
be
constant .
These
are
inva l id
assumpt ions s ince the evapora t ion could
be
cons i de r ab l e and
fur thermore the tempera ture change in the f lu id
could
be l a rge
mainly
for
a long. l a rge
plan t .
Johannsen
and Grossman
[4]
carr ied
out a study on a regenerating type
s o l a r co l l e c t o r
for
an a i r -condi t ion ing
system.
Sta r t ing
from
the
mass and hea t balance
equa t ions
they der ived
a genera l
formula
to
simulate the i r system.
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\P
\ \
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42
Aoc>ther invest igat ion
relevant to the
present
work
was presented by
Vaxman
and Sokolov [5]. The autlxlrs star ted the i r analysis from the
energy equation for both the
f lu id
f i lm and the black pla t e
neglecting evaporation
ra te and
assuming
steady
s ta te
ful ly
developed
f low