3 Solar Desalination_nc

7/17/2019 3 Solar Desalination_nc

1/41

SOLAR DESALINATIONSOLAR DESALINATION


2/41

WATER DESALINATION TECHNOLOGYWATER DESALINATION TECHNOLOGY

Nature is carrying out the process of water desalinationNature is carrying out the process of water desalinationsince ages.since ages.

Oceanic water due to solar heating converts into vapoursOceanic water due to solar heating converts into vapoursand pours down as precipitation on earth in the form ofand pours down as precipitation on earth in the form offresh water.fresh water.

Water is the most needed substance on the earth forWater is the most needed substance on the earth forsustenance of life.sustenance of life.

Due to rapid expansion of population, accelerated industrialDue to rapid expansion of population, accelerated industrialgrowth and enhanced agricultural production, there is evergrowth and enhanced agricultural production, there is everincreasing demand for fresh water.increasing demand for fresh water.

Demand of fresh water (potable water) has increased fromDemand of fresh water (potable water) has increased from!"#$ litres%person%day to &!"$$ litres%person%day,!"#$ litres%person%day to &!"$$ litres%person%day,

'he ocean covers & recent of the earths surface"$'he ocean covers & recent of the earths surface"$million s*uare miles with a volume of ++$ million cubicmillion s*uare miles with a volume of ++$ million cubicmiles and has an average salt content of +!,$$$ ppm.miles and has an average salt content of +!,$$$ ppm.rac-ish%saline water is strictly defined as the water withrac-ish%saline water is strictly defined as the water withless dissolved salts than sea water but more than !$$ ppm.less dissolved salts than sea water but more than !$$ ppm.


3/41

SOLAR DESALINATION TECHNIQUESSOLAR DESALINATION TECHNIQUES

Potable WaterPotable Water Less than 550 ppmLess than 550 ppm

RequirementRequirement Domestic, Industries andDomestic, Industries andAgricultureAgriculture

Sources of PotableSources of Potable

WaterWaterRiers, La!es, Ponds, Wells etc"Riers, La!es, Ponds, Wells etc"

Demand of PotableDemand of Potable

WaterWater#5$%5 litres & person & da'#5$%5 litres & person & da'

()LD*()LD*

#00$#%5 litres & person & da'#00$#%5 litres & person & da'

(+W*(+W*

-nderground-nderground

Saline WaterSaline Water%,000 . %,500 ppm%,000 . %,500 ppm

Sea WaterSea Water /0,000 . 50,000 ppm/0,000 . 50,000 ppm


4/41

WATER DESALINATION TECHNOLOGYWATER DESALINATION TECHNOLOGY

otable water (fresh water) suitable for humanotable water (fresh water) suitable for human

consumption should not contain dissolved salts moreconsumption should not contain dissolved salts morethan !$$ ppm.than !$$ ppm.

/or agricultural purposes, water containing salt/or agricultural purposes, water containing saltcontent of $$$ ppm is considered as the upper limit.content of $$$ ppm is considered as the upper limit.

otable water is re*uired for domestic, agricultureotable water is re*uired for domestic, agriculture

and industries.and industries.0ome applications in industries li-e cooling purposes,0ome applications in industries li-e cooling purposes,sea water is feasible despite the corrosion problemssea water is feasible despite the corrosion problemswhile other industries use higher *uality water thanwhile other industries use higher *uality water thanis acceptable for drin-ing water. 1odern steam poweris acceptable for drin-ing water. 1odern steam powergeneration plant need water with less than $ ppm.generation plant need water with less than $ ppm.

otable%fresh water is available from rivers, la-es,otable%fresh water is available from rivers, la-es,ponds, wells, etc.ponds, wells, etc.

2nderground saline%brac-ish water contains dissolved2nderground saline%brac-ish water contains dissolvedsalts of about #,$$$"#,!$$ ppm.salts of about #,$$$"#,!$$ ppm.


5/41

METHODS OF CONVERTING BRACKISHMETHODS OF CONVERTING BRACKISH

WATER INTO POTABLE WATERWATER INTO POTABLE WATER

D30456N4'6OND30456N4'6ON7 'he saline water is evaporated using7 'he saline water is evaporated usingthermal energy and the resulting steam is collected andthermal energy and the resulting steam is collected andcondensed as final product.condensed as final product.

84O9 :O193006ON84O9 :O193006ON7 ;ere water vapour from boiling7 ;ere water vapour from boilingwater is compressed adiabatically and vapour getswater is compressed adiabatically and vapour getssuperheated. 'he superheated vapor is first cooled tosuperheated. 'he superheated vapor is first cooled tosaturation temperature and then condensed at constantsaturation temperature and then condensed at constantpressure. 'his process is derived by mechanical energy.pressure. 'his process is derived by mechanical energy.

9383903 O01O0609383903 O01O0607 ;ere saline water is pushed at high7 ;ere saline water is pushed at highpressure through special membranes allowing waterpressure through special membranes allowing watermolecules pass selectively and not the dissolved salts.molecules pass selectively and not the dissolved salts.

353:'9OD645


6/41

Solar Distillation

Passi! Distillation A"ti!

DistillationCon!ntiona

l Solar Still

M#lti$!%%!"t

Solar Still

N!& D!si'n

Solar Still

In"lin!(

Solar Still

Hi') T!*+

Distillation

No"t#rnal

Distillation

Wit)R!%l!"tor

Wit)Con(!ns!r

Distillation&it)

"oll!"tor

+an!l

A#,iliar-)!atin'

(istillation

Li%! ra%t S+)!ri"al T#.#lar R!'!n!ration

Classification of Solar Distillation Systems


7/41

Multieffect Solar Still

Diffusion

Still

Cimney

Ty!e Still

Multi effect

"asin Still

Heate# Hea#

Solar Still

Dou$le "asin

Solar Still

Multi!le "asin

Solar Still

Incline# Solar Still

Wic% Solar

Still

Sin&le Wic%

Solar Still

Multi!le effect

tilte# tray Solar

Still

"asin Solar Still

Multi!le Wic%

Solar Still

Tilte# Tray '

ste!!e# Solar

Still

METHODS OF PURIFICATION OF WATER


8/41

Ty!es of Solar StillTy!es of Solar Still

0ingle 3ffect asin 0olar 0till0ingle 3ffect asin 0olar 0till

'ilted 'ray 0olar 0till'ilted 'ray 0olar 0till

1ultibasin 0tepped 0olar 0till1ultibasin 0tepped 0olar 0till

9egeneration 6nclined 0tep 0olar 0till9egeneration 6nclined 0tep 0olar 0till

Wic- 'ype 0olar 0tillWic- 'ype 0olar 0till1ultiple 3ffect Diffusion 0olar 0till1ultiple 3ffect Diffusion 0olar 0till

:himney 'ype 0olar 0till:himney 'ype 0olar 0till

1ulti"'ray 1ultiple 3ffect 0olar 0till1ulti"'ray 1ultiple 3ffect 0olar 0till

Double asin 0olar 0tillDouble asin 0olar 0till;umidification Dumidification Distiller;umidification Dumidification Distiller

1ultistage /lash Distiller1ultistage /lash Distiller

0olar > 4ssisted wiped film 1ultistage /lash Distiller0olar > 4ssisted wiped film 1ultistage /lash Distiller


9/41

MAIN TECHNIQUES FOR DISTILLATIONMAIN TECHNIQUES FOR DISTILLATION

a) /lash Distillationa) /lash Distillation

b) 8apor :ompression rocess.b) 8apor :ompression rocess.c) 3lectrodialysisc) 3lectrodialysis

d) 9everse Osmosis.d) 9everse Osmosis.

e) 0olar Distillation.e) 0olar Distillation.

?26D356N30?26D356N30. @uantity of /resh Water 9e*uired and its 3nd 2se.. @uantity of /resh Water 9e*uired and its 3nd 2se.

#. 4vailable Water 0ources, such as 0ea, onds, Wells,#. 4vailable Water 0ources, such as 0ea, onds, Wells,0wamps etc.0wamps etc.

+. roximity to nearest /resh Water 0ources.+. roximity to nearest /resh Water 0ources.

. 4vailability of 3lectric ower at the 0ite or :loseby.. 4vailability of 3lectric ower at the 0ite or :loseby.!. :ost of 0upplying /resh Water by 8arious 1ethods.!. :ost of 0upplying /resh Water by 8arious 1ethods.

A. :ost and 4vailability of 5abor in the 9egion.A. :ost and 4vailability of 5abor in the 9egion.

&. 1aintenance and Daily Operational 9e*uirements.&. 1aintenance and Daily Operational 9e*uirements.

B. 5ife 0pan of the Water 0upply 0ystem.B. 5ife 0pan of the Water 0upply 0ystem.

C. 3conomic 8alue of the 9egion.C. 3conomic 8alue of the 9egion.


10/41

S")!*ati" o% .asin$t-+! solar still


11/41

COM(ONENTS O) SINGLECOM(ONENTS O) SINGLE

E))ECT SOLAR STILLE))ECT SOLAR STILL

.. asinasin

#.#. lac- 5inerlac- 5iner

+.+. 'ransparent :over'ransparent :over.. :ondensate :hannel:ondensate :hannel

!.!. 0ealant0ealant

A.A. 6nsulation6nsulation&.&. 0upply and Delivery 0ystem0upply and Delivery 0ystem


12/41

MATERIALS FOR SOLAR STILLSMATERIALS FOR SOLAR STILLS

?546N??546N?7 0hould have high transmittance for solar radiation,7 0hould have high transmittance for solar radiation,opa*ue to thermal radiation, resistance to abrasion, longlife,opa*ue to thermal radiation, resistance to abrasion, longlife,

low cost, high wettability for water, lightweight, easy tolow cost, high wettability for water, lightweight, easy tohandle and apply, and universal availability. 1aterials usedhandle and apply, and universal availability. 1aterials usedare7 glass or treated plastic.are7 glass or treated plastic.

56N3956N397 0hould absorb more solar radiation, should be7 0hould absorb more solar radiation, should bedurable, should be water tight, easily cleanable, low cost, anddurable, should be water tight, easily cleanable, low cost, andshould be able to withstand temperature around $$ Deg :.should be able to withstand temperature around $$ Deg :.

1aterials used are7 asphalt matt, blac- butyl rubber, blac-1aterials used are7 asphalt matt, blac- butyl rubber, blac-polyethylene etc.polyethylene etc.

03454N'03454N'7 0hould remain resilient at very low temperatures,7 0hould remain resilient at very low temperatures,low cost, durable and easily applicable. 1aterials used are7low cost, durable and easily applicable. 1aterials used are7putty, tars, tapes silicon, sealant.putty, tars, tapes silicon, sealant.

406N '94


13/41

"ASIC RE*+IREMENTS O) A GOOD"ASIC RE*+IREMENTS O) A GOOD

SOLAR STILLSOLAR STILL

e easily assembled in the field,e easily assembled in the field,

e constructed with locally available materials,e constructed with locally available materials,

e light weight for ease of handling ande light weight for ease of handling andtransportation,transportation,

;ave an effective life of $ to #$


14/41

Cross s!"tion o% so*! t-+i"al .asin t-+! solar still/ 0a1 Solar still &it) (o#.l!

slo+!( s-**!tri"al &it) "ontin#o#s .asin2 0.1 Solar still &it) (o#.l! slo+!(

s-**!tri"al &it) .asin (ii(!( into t&o .a-s2 0"1 Solar still &it) sin'l! slo+!

an( "ontin#o#s .asin2 0(1 Solar still &it) #ns-**!tri"al (o#.l! slo+!( an(

(ii(!( .asin2 0!1 U$tro#') t-+! solar still2 0%1 Solar still &it) +lasti" in%lat!("o!r2 0'1 Solar still &it) str!t")!( +lasti" %il* &it) (ii(!( .asin/


15/41

Scematic of sallo, $asin ty!e solar still


16/41

SOLAR STILL OUTPUT DEPENDSSOLAR STILL OUTPUT DEPENDS

ON MAN3 PARAMETERSON MAN3 PARAMETERS

1.1. Climatic ParametersClimatic ParametersI.I. Solar RadiationSolar Radiation

II.II. Ambient TemperatureAmbient Temperature

III.III. Wind SpeedWind Speed

IV.IV. Outside HumidityOutside Humidity

V.V. Sky ConditionsSky Conditions

.. !esi"n Parameters!esi"n ParametersI.I. Sin"le slope or double slopeSin"le slope or double slope

II.II. #la$in" material#la$in" material

III.III. Water dept% in &asinWater dept% in &asinIV.IV. &ottom insulation&ottom insulation

V.V. Orientation o' stillOrientation o' still

VI.VI. Inclination o' "la$in"Inclination o' "la$in"

VII.VII. Spacin" bet(een (ater and "la$in"Spacin" bet(een (ater and "la$in"

VIII.VIII.Type o' solar stillType o' solar still


17/41

).). Operational parametersOperational parameters

I.I. Water !ept%Water !ept%

II.II. Pre%eatin" o' WaterPre%eatin" o' Water

III.III. Colourin" o' WaterColourin" o' WaterIV.IV. Salinity o' WaterSalinity o' Water

V.V. Rate o' Al"ae #ro(t%Rate o' Al"ae #ro(t%

VI.VI. Input Water supply arran"ementInput Water supply arran"ement

*continuously or in batc%es+*continuously or in batc%es+

SOLAR STILL OUTPUT DEPENDS ONSOLAR STILL OUTPUT DEPENDS ON

MAN3 PARAMETERSMAN3 PARAMETERS Cont#-


18/41

Sin&le slo!e e.!erimental solar still


19/41

Dou$le slo!e# e.!erimental solar still


20/41

E/(ERIMENTS ON SOLAR STILLSE/(ERIMENTS ON SOLAR STILLS

0CLIMATIC (ARAMETERS10CLIMATIC (ARAMETERS1

'he effect of climatic parameters on the still output was'he effect of climatic parameters on the still output wasseen by using two small, single sloped solar stills, each withseen by using two small, single sloped solar stills, each withbasin area e*ual to $.!B s*.m,basin area e*ual to $.!B s*.m,

'hese two solar stills have identical design features except'hese two solar stills have identical design features exceptone with sawdust insulation (#.! cm) in the bottom andone with sawdust insulation (#.! cm) in the bottom and

second without any insulation. ;ourly output and climaticsecond without any insulation. ;ourly output and climaticparameters were determined for one complete year.parameters were determined for one complete year.

'he insulated still gave B percent higher output compared'he insulated still gave B percent higher output comparedto uninsulated solar still.to uninsulated solar still.

'he maximum output was !.#& litres%0*.m. day.'he maximum output was !.#& litres%0*.m. day.

'he still output increased from .&A liters%m'he still output increased from .&A liters%m##day at A.&day at A.&

1F%m1F%m##

day to !. litres%mday to !. litres%m##

day at #&.$B 1F%mday at #&.$B 1F%m##

day.day.4n increase in still output was observed with increase in4n increase in still output was observed with increase inambient temperature. 'he increase in output is about $.B&ambient temperature. 'he increase in output is about $.B&litres%mlitres%m##day for each $G: rise in ambient temperature.day for each $G: rise in ambient temperature.


21/41

Variation o% solar still o#t+#t an( solar insolation %or

(i%%!r!nt &!!4s o% t)! -!ar


22/41

R!lations)i+ .!t&!!n still o#t+#t an( (ail- solar insolation


23/41

EFFECT OF DESIGN PARAMETERSEFFECT OF DESIGN PARAMETERS

'he effect of design variables was studied on four double sloped'he effect of design variables was studied on four double slopedpermanent type solar stills with dimensions of #! x #! x !permanent type solar stills with dimensions of #! x #! x !cm i.e. with a basin area of +.$ mcm i.e. with a basin area of +.$ m##..

0till No. does not contain any bottom insulation while still nos.0till No. does not contain any bottom insulation while still nos.#,+ and each contained #.! cm thic- sawdust insulation.#,+ and each contained #.! cm thic- sawdust insulation.

'he glass angles for stills ,#,+ and are #$,+$,+$ and $'he glass angles for stills ,#,+ and are #$,+$,+$ and $degrees from horiEontal respectively.degrees from horiEontal respectively.

3ach of the still was filled daily with about ! cm of water in the3ach of the still was filled daily with about ! cm of water in themorning and hourly values of distillate was collected andmorning and hourly values of distillate was collected and

measured.measured.0till No.# with base insulation has given a higher output. 'he0till No.# with base insulation has given a higher output. 'heaverage increase is & percent.average increase is & percent.

y comparing stills #", the still with lowest glass angle gavey comparing stills #", the still with lowest glass angle gavehighest output.highest output.

y comparing outputs of stills l and +, it was observed that stilly comparing outputs of stills l and +, it was observed that still

with #$ degree glass inclination and without base insulation, with #$ degree glass inclination and without base insulation,performs better than still + with +$ degree glass inclination andperforms better than still + with +$ degree glass inclination andwith base insulation.with base insulation.

oth the channels of each of the still collect almost e*ualoth the channels of each of the still collect almost e*ualamount of distillate.amount of distillate.


24/41

E))ECT O) O(ERATIONAL (ARAMETERSE))ECT O) O(ERATIONAL (ARAMETERS

. 'he effect of operational parameters was studied. 'he effect of operational parameters was studiedon five single sloped solar stills each with a basinon five single sloped solar stills each with a basin

area of $.!B 0*.m. 4ll are of identicalarea of $.!B 0*.m. 4ll are of identicalconstruction except still ! had ! cm thic- sawdustconstruction except still ! had ! cm thic- sawdustinsulation.insulation.

#. 'he effect of water depth was studied by filing#. 'he effect of water depth was studied by filingstills with #.$, .$,A.$,B.$ cm water forstills with #.$, .$,A.$,B.$ cm water foruninsulated stills and .$ cm for insulated still.uninsulated stills and .$ cm for insulated still.

+. ;igher distillate output was observed with lower+. ;igher distillate output was observed with lowerwater depth.water depth.

. 'he insulated still gave higher output.. 'he insulated still gave higher output.

!. 'he effect of dye on water output was also!. 'he effect of dye on water output was also

studied. 'he output got increased by colouringstudied. 'he output got increased by colouringthe water.the water.

A. 'he effect of use of waste heat for heating theA. 'he effect of use of waste heat for heating thesaline water in still was also studied. One still wassaline water in still was also studied. One still wasfilled with water at +$G: and the other with waterfilled with water at +$G: and the other with water

at !G:. ;igher output was observed in a stillat !G:. ;igher output was observed in a stillusing water at higher temperature.using water at higher temperature.

Di%% i i l l i % ( il i l(Di%% t i i l l ti % ( il i l(


25/41

Di%%!r!nt !*+iri"al "orr!lations %or (ail- -i!l(Di%%!r!nt !*+iri"al "orr!lations %or (ail- -i!l(

%ro* a solar still%ro* a solar still

S.,.S.,. Per'ormance Relations *l-mPer'ormance Relations *l-md+d+ Re'erencesRe'erences

1.1...

).).

..

/./.

0.0.

..

2.2.

3.3.

14.14.

5( 6 4.10 7 4.44)2/ I*t+5( 6 4.10 7 4.44)2/ I*t+5( 6 4.41 I*t+ 8 1.10025( 6 4.41 I*t+ 8 1.1002

5( 6 4.444)03 I*t+5( 6 4.444)03 I*t+1.01.0

5( 6 .1) 9 145( 6 .1) 9 14:):)I*t+ ;17I*t+ ;17

5( 6 1.12 9 145( 6 1.12 9 14::I*t+I*t+1.01.0

5( 6 4.4420 I*t+ 7 4.40)0Ta74.40))V5( 6 4.4420 I*t+ 7 4.40)0Ta74.40))V

5( 6 4.41) I*t+ 8 )./3035( 6 4.41) I*t+ 8 )./303

5( 6 4.1)) W5( 6 4.1)) W4.)4.)*T*Tinin8 T8 Taa+ 8 1404+ 8 1404

5( 6 4.44)/ I*t+5( 6 4.44)/ I*t+

5( 6 .3/ 9 145( 6 .3/ 9 14::I*t+ 8 4.41)3I*t+ 8 4.41)3

TTaa74.412/V 8 4.))74.412/V 8 4.))

#runne et al *130+#runne et al *130+?a(and @ &oputiere?a(and @ &oputiere

*134+*134+

&attele *130/+&attele *130/+

aki et al *132)+aki et al *132)+

5adani and aki5adani and aki

*1323+*1323+#ar" and 5ann *130+#ar" and 5ann *130+

#ar" and 5ann *130+#ar" and 5ann *130+

5alik et al *132+5alik et al *132+

5aum et al *134+5aum et al *134+

,atu et al *133+,atu et al *133+

W)!r!

I 5 Solar Int!nsit- W6*78 t5 ti*!2 s8 *&5 Dail- Distillat! O#t+#t2 4'6*78

T 5 T!*+!rat#r!2 C8 W 5 H#*i(it- Ratio8 V 5 Win( S+!!( 0*6s1


26/41

PROBLEMS ENCOUNTERED WITH PLASTIC COVERSPROBLEMS ENCOUNTERED WITH PLASTIC COVERS

/ragility and short service life of plastic sheets./ragility and short service life of plastic sheets.

5ea-age of water vapor and the condensate.5ea-age of water vapor and the condensate.

Over"heating, and hence melting, of the plasticOver"heating, and hence melting, of the plasticbottom of the still due to the development of drybottom of the still due to the development of dryspots in course of time. 6n the extreme case thespots in course of time. 6n the extreme case theblac- polyethylene sheets used as the basin linerblac- polyethylene sheets used as the basin linermay get heated beyond its melting point.may get heated beyond its melting point.

'he plastic cover surface does not get wetted and'he plastic cover surface does not get wetted andthis leads to reduced transmission of incomingthis leads to reduced transmission of incomingsolar energy and also to dripping of distilled watersolar energy and also to dripping of distilled waterbac- into the brine li*uid.bac- into the brine li*uid.

0usceptibility to damage by wind and other0usceptibility to damage by wind and other

elements of nature.elements of nature.Occasional unforeseen mixing of brine andOccasional unforeseen mixing of brine anddistilled water in some of the designs.distilled water in some of the designs.


27/41

En!r'- trans%!r in a sin'l! !%%!"t .asin

solar still


28/41

Ma9or )!at %l#,!s %or a solar still

O C C O O


29/41

'he performance of solar still can be predicted by writing

energy balance e*uations on various components of the still. 4steady state analysis of solar still is described here.9eferring to the figure the instantaneous heat balance e*uationon basin water can be written as 7

dtdTCqqqqI wwbcrew ++++=

PBROR5A,CB PRB!ICTIO, O&ASI,:TDPB SO?AR STI??

()

Where 6 is the solar radiation on horiEontal surface= wis

absorptivity of water and basin liner, is transmittance of glass

cover= *e, *r, *care the evaporative, radiative and convectiveheat losses from water to the transparent cover respectively= *b

is the conductive heat loss from water basin= :wis heat capacity

of water and basin= 'wis water temperature= and t is the time.

0imilarly the instantaneous heat balance e*uation on glasscover will be 7


30/41

creg

g

gga qqqIdt

dTcq +++=+ H.(#)

Where *ga(I*caJ *m) is the heat loss from cover to atmosphere,

:gis the heat capacity of glass cover, 'gis glass temperature, g

is the absorptivity of glass cover, *ca is the heat loss by

convection from cover to atmosphere, and *ra is heat loss by

radiation from cover to atmosphere.Now the heat balance e*uation on the still is 7

dt

dT

Cdt

dT

CqqqII

w

w

g

gbracagw ++++=+

H.(+)

'he parameters li-e ( " g" ) 6 and (6"w) 6 are not included in

e*uations since these do not add to evaporation or condensation

of water.


31/41

'he heat transfer by radiation *r from water surface

to glass cover can be calculated from the e*uation

)( 44 gwr TTFq = HH()

Where / is the shape factor which depends on thegeometry and the emissivities of water and glass

cover, and is the 0tefan oltEmann constant. /orthe basin type solar still and for low tilt angles ofglass cover, the basin and glass cover can beassumed as two parallel infinite plates. 'he shape

factor can be assumed to be e*ual to the emissivityof the water surface which is $.C. ;ence 3*. willbe7

)(9.0 44gwr

TTq = HH(!)

'he convective heat loss from hot water surface in the


32/41

'he convective heat loss from hot water surface in thestill to the glass cover can be calculated from thefollowing expression 7

)( gwcc TThq = H(A)

Where hcis the convective heat transfer coefficient, the

value of which depends on many parameters li-e

temperature of water and glass, density, conductivity,specific heat, viscosity, expansion coefficient of fluid,and spacing between water surface and glass cover.Dun-le suggested an empirical relation for the

convective heat transfer coefficient as given below 73/1

3109.268

)(884.0

+= w

w

gw

gwc TP

PPTTh H(&)


33/41

Where w and g are the saturation partial pressures of watervapour (N%m#) at water temperature and glass temperaturerespectively.'he evaporative heat loss *efrom water to the glass cover can be

calculated by -nowing the mass transfer coefficient andconvective heat transfer coefficient. 'he empirical expression for*eas give by Dun-le is given as 7

)(28.16gwce

PPhq = H.(B)

;eat loss through the ground and periphery *b is difficult to

compute since the soil temperature is un-nown. 1oreover, theheat conducted in the soil during daytime comes bac- in the

basin during night time. ;owever, it can be computed from thefollowing simple relation 7

)( awbb TTUq = H.(C)

Where 2bis the overall heat transfer coefficient from bottom.


34/41

'he convective heat loss *ca from glass cover to

ambient air can be calculated from the followingexpression 7

)( agcaca TThq = H($)

Where hca is the forced convection heat transfer

coefficient and is given by 7

Vhca 8.38.2 += H()

Where 8 is the wind speed in m%s.'he radiative heat loss *ra from glass to s-y can be

determined provided the radiant s-y temperature 'sis

-nown, which very much depends on atmospheric

conditions such as the presence of clouds etc.

? ll f l h -


35/41

?enerally for practical purposes the average s-ytemperature 's can be assumed to be about # K

below ambient temperature, i.e. 'gI 'a " #. 'hus

radiative heat loss *ra from glass cover to theatmosphere is given as7

)( 44 sggra TTq = H. (#)

Where gis the emissivity of glass cover.

'he exact solution of the above simultaneous e*uationsis not possible and hence iterative techni*ue isemployed to find the solution. 'he digital simulationtechni*ues for solving the above e*uations for aparticular set of condition can also be adopted. 3vencharts are given by 1orse and 9ead and ;owe whichcan be used for performance prediction of solar stills for

a particular set of conditions.


36/41

Main (ro$lems of Solar StillMain (ro$lems of Solar Still

5ow distillate output per unit area5ow distillate output per unit area

5ea-age of vapour through Loints5ea-age of vapour through Loints

;igh maintenance;igh maintenanceroductivity decreases with time forroductivity decreases with time for

a variety of reasonsa variety of reasons

:ost per unit output is very high:ost per unit output is very high


37/41

CONCLUSIONS ON BASIN$ T3PE SOLAR STILLCONCLUSIONS ON BASIN$ T3PE SOLAR STILL

.. 'he solar still output (distillate) is a strong function of'he solar still output (distillate) is a strong function of

solar radiation on a horiEontal surface. 'he distillatesolar radiation on a horiEontal surface. 'he distillateoutput increases linearly with the solar insolation for aoutput increases linearly with the solar insolation for agiven ambient temperature. 6f the ambient temperaturegiven ambient temperature. 6f the ambient temperatureincreases or the wind velocity decreases, the heat lossincreases or the wind velocity decreases, the heat lossfrom solar still decreases resulting in higher distillationfrom solar still decreases resulting in higher distillationrate. 6t is observed for each $rate. 6t is observed for each $: rise in ambient: rise in ambient

temperature the output increases by $ percent.temperature the output increases by $ percent.

#.#. 'he depth of water in the basin also effects the'he depth of water in the basin also effects theperformance considerably. 4t lower basin depths, theperformance considerably. 4t lower basin depths, thethermal capacity will be lower and hence the increase inthermal capacity will be lower and hence the increase inwater temperature will be large resulting in higherwater temperature will be large resulting in higher

output. ;owever, it all depends on the insulation of theoutput. ;owever, it all depends on the insulation of thestill. 6f there is no lnsulatlon, increase in waterstill. 6f there is no lnsulatlon, increase in watertemperature will also increase the bottom heat loss. 6ttemperature will also increase the bottom heat loss. 6thas been observed that if the water depth increases fromhas been observed that if the water depth increases from.# cm to +$ cm the output of still decreases by +$.# cm to +$ cm the output of still decreases by +$percent.percent.

CONCLUSIONS ON BASIN T3PE SOLARCONCLUSIONS ON BASIN T3PE SOLAR


38/41

CONCLUSIONS ON BASIN$ T3PE SOLARCONCLUSIONS ON BASIN$ T3PE SOLAR

STILL 0"ont(/1STILL 0"ont(/1

+.+. Number of transparent covers in a solar still doNumber of transparent covers in a solar still donot increase the output since it increases thenot increase the output since it increases thetemperature of the inner cover resulting intemperature of the inner cover resulting inlower condensation of water vapour.lower condensation of water vapour.

.. 5ower cover slope increases the output. /rom5ower cover slope increases the output. /rom

practical considerations a minimum cover slopepractical considerations a minimum cover slopeof $ deg. is suggested.of $ deg. is suggested.

!.!. 'he maximum possible efficiency of a single'he maximum possible efficiency of a singlebasin solar still is about A$ percent.basin solar still is about A$ percent.

A.A. /or higher receipt of solar radiation and/or higher receipt of solar radiation and

therefore the higher yield the long axis of thetherefore the higher yield the long axis of thesolar still should be placed in the 3ast"Westsolar still should be placed in the 3ast"Westdirection if the still is installed at a high latitudedirection if the still is installed at a high latitudestation. 4t low latitude stations the orientationstation. 4t low latitude stations the orientationhas no effect on solar radiation receipt.has no effect on solar radiation receipt.


39/41

ADDITIONAL CONCLUSIONS DRAWN FROMADDITIONAL CONCLUSIONS DRAWN FROM

E:PERIMENTAL STUDIES ON SOLAR STILLSE:PERIMENTAL STUDIES ON SOLAR STILLS

&.&. 'he main problem in a solar still 6s the salt deposition of'he main problem in a solar still 6s the salt deposition ofcalcium carbonate and calcium sulphate on the basin linercalcium carbonate and calcium sulphate on the basin linerwhich are white and insoluble and reflect solar radiation fromwhich are white and insoluble and reflect solar radiation frombasin water and basin liner and thereby lowering the stillbasin water and basin liner and thereby lowering the stilloutput. 6t is difficult to stop the salt deposition.output. 6t is difficult to stop the salt deposition.

B.B. 'he physical methods suggested to prevent the salt'he physical methods suggested to prevent the salt

deposition are /re*uent flushing of the stills with completedeposition are /re*uent flushing of the stills with completedrainage M 9efilling or continuous agitation of the still waterdrainage M 9efilling or continuous agitation of the still waterby circulating it with a small pump.by circulating it with a small pump.

C.C. Once the salt gets deposited then the only way is completelyOnce the salt gets deposited then the only way is completelydraining the still and then scrubbing the sides and basin linerdraining the still and then scrubbing the sides and basin linerand then refilling the still.and then refilling the still.

$.$. 4nother serious observation made in 4ustralia is the4nother serious observation made in 4ustralia is thecrystalline salt growth which ta-es place on the sides of thecrystalline salt growth which ta-es place on the sides of thebasin and into the distillate trough effecting the purity ofbasin and into the distillate trough effecting the purity ofdistilled water.distilled water.

.. 0ome success in preventing the crystalline salt growth is0ome success in preventing the crystalline salt growth isachieved in 4ustralia by pre"treating the feed water with aachieved in 4ustralia by pre"treating the feed water with acomplex phosphate compound which reduces the rate ofcomplex phosphate compound which reduces the rate of

nucleation of salt crystals.nucleation of salt crystals.



40/41


E:PERIMENTAL STUDIES ON SOLAR STILLSE:PERIMENTAL STUDIES ON SOLAR STILLS

#.#. 0aline water in the still can be supplied either0aline water in the still can be supplied either

continuously or in batches.continuously or in batches.+.+. 6n 4ustralia continuous supply of saline water in the solar6n 4ustralia continuous supply of saline water in the solarstill is preferred at a rate of about .&$ 6%s*.m hr whichstill is preferred at a rate of about .&$ 6%s*.m hr which6s twice the maximum distillate rate.6s twice the maximum distillate rate.

.. 'his helps in reducing the salt deposition from the salt'his helps in reducing the salt deposition from the saltsolution.solution.

!.!. /rom thermal efficiency point of view, batch filling i.e./rom thermal efficiency point of view, batch filling i.e.filling of saline water when the basin water is coolestfilling of saline water when the basin water is coolest(early morning) is the best but it involves greater labour(early morning) is the best but it involves greater labourcosts and special plumbing arrangements.costs and special plumbing arrangements.

A.A. 4lgae growth within the solar still also effects the4lgae growth within the solar still also effects theperformance to a little extent but its growth must beperformance to a little extent but its growth must be

chec-ed since its growth is unsightly and may finally bloc-chec-ed since its growth is unsightly and may finally bloc-the basin and contaminate the distillation troughs.the basin and contaminate the distillation troughs.

&.&. 'he algae growth can be chec-ed by adding copper'he algae growth can be chec-ed by adding coppersulphate and chlorine compounds in the saline water insulphate and chlorine compounds in the saline water inthe still.the still.


41/41

3 Solar Desalination_nc

Documents

Transcript of 3 Solar Desalination_nc