39201792 solar desalination

7/31/2019 39201792 Solar Desalination

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SOLAR DESALINATION


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WATER DESALINATION TECHNOLOGY

Nature is carrying out the process of water desalinationsince ages.

Oceanic water due to solar heating converts into vapoursand pours down as precipitation on earth in the form offresh water.Water is the most needed substance on the earth forsustenance of life.Due to rapid expansion of population, accelerated industrial

growth and enhanced agricultural production, there is everincreasing demand for fresh water.Demand of fresh water (potable water) has increased from15-20 litres/person/day to 75-100 litres/person/day,The ocean covers 71 recent of the earth's surface-140million square miles with a volume of 330 million cubic

miles and has an average salt content of 35,000 ppm.Brackish/saline water is strictly defined as the water withless dissolved salts than sea water but more than 500 ppm.


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SOLAR DESALINATION TECHNIQUES

Potable Water Less than 550 ppm

Requirement Domestic, Industries andAgriculture

Sources of Potable

Water

Rivers, Lakes, Ponds, Wells etc.

Demand of Potable

Water15-25 litres / person / day

(OLD)

100-125 litres / person / day

(NEW)

Underground

Saline Water

2,0002,500 ppm

Sea Water 30,000

50,000 ppm


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WATER DESALINATION TECHNOLOGY

Potable water (fresh water) suitable for human

consumption should not contain dissolved saltsmore than 500 ppm.For agricultural purposes, water containing saltcontent of 1000 ppm is considered as the upperlimit.

Potable water is required for domestic,agriculture and industries.Some applications in industries like coolingpurposes, sea water is feasible despite thecorrosion problems while other industries use

higher quality water than is acceptable fordrinking water. Modern steam power generationplant need water with less than 10 ppm.Potable/fresh water is available from rivers,lakes, ponds, wells, etc.

Underground saline/brackish water containsdissolved salts of about 2,000-2,500 ppm.


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METHODS OF CONVERTING BRACKISHWATER INTO POTABLE WATER

DESALINATION: The saline water is evaporated usingthermal energy and the resulting steam is collected andcondensed as final product.VAPOR COMPRESSION: Here water vapour from boilingwater is compressed adiabatically and vapour getssuperheated. The superheated vapor is first cooled to

saturation temperature and then condensed at constantpressure. This process is derived by mechanical energy.REVERSE OSMOSIS: Here saline water is pushed at highpressure through special membranes allowing watermolecules pass selectively and not the dissolved salts.ELECTRODIALYSIS: Here a pair of special membranes,perpendicular to which there is an electric field are usedand water is passed through them. Water does not passthrough the membranes while dissolved salts passselectively.In distillation; thermal energy is used while in vapourcompression, reverse osmosis, electrodialysis, etc. some

mechanical and electrical energy is used.


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Solar Distillation

Passive Distillation Active

DistillationConventional Solar Still

Multi-effectSolar Still

New DesignSolar Still

InclinedSolar Still

High TempDistillation

NocturnalDistillation

WithReflector

WithCondenser

Distillationwith

collectorpanel

Auxiliaryheating

distillation

Life raft Spherical Tubular Regeneration

Classification of Solar Distillation Systems


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Multieffect Solar Still

Diffusion

Still

Chimney

Type Still

Multi effect

Basin Still

Heated Head

Solar Still

Double BasinSolar Still

Multiple BasinSolar Still

Inclined Solar Still

Wick Solar

Still

Single WickSolar Still

Multiple effecttilted tray Solar

Still

Basin Solar Still

Multiple WickSolar Still

Tilted Tray /stepped Solar

Still

METHODS OF PURIFICATION OF WATER


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Types of Solar StillSingle Effect Basin Solar StillTilted Tray Solar StillMultibasin Stepped Solar StillRegeneration Inclined Step Solar Still

Wick Type Solar StillMultiple Effect Diffusion Solar StillChimney Type Solar StillMulti-Tray Multiple Effect Solar Still

Double Basin Solar StillHumidification Dumidification DistillerMultistage Flash DistillerSolar Assisted wiped film Multistage FlashDistiller


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MAIN TECHNIQUES FOR DISTILLATION

a) Flash Distillation

b) Vapor Compression Process.c) Electrodialysisd) Reverse Osmosis.e) Solar Distillation.

GUIDELINES1. Quantity of Fresh Water Required and its End Use.2. Available Water Sources, such as Sea, Ponds, Wells,

Swamps etc.3. Proximity to nearest Fresh Water Sources.

4. Availability of Electric Power at the Site or Closeby.5. Cost of Supplying Fresh Water by Various Methods.6. Cost and Availability of Labor in the Region.7. Maintenance and Daily Operational Requirements.8. Life Span of the Water Supply System.

9. Economic Value of the Region.


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Schematic of basin-type solar still


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COMPONENTS OF SINGLE

EFFECT SOLAR STILL

1. Basin

2. Black Liner

3. Transparent Cover4. Condensate Channel

5. Sealant

6. Insulation7. Supply and Delivery System


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MATERIALS FOR SOLAR STILLS

GLAZING: Should have high transmittance for solar radiation,opaque to thermal radiation, resistance to abrasion, longlife,

low cost, high wettability for water, lightweight, easy tohandle and apply, and universal availability. Materials usedare: glass or treated plastic.LINER: Should absorb more solar radiation, should bedurable, should be water tight, easily cleanable, low cost, andshould be able to withstand temperature around 100 Deg C.Materials used are: asphalt matt, black butyl rubber, blackpolyethylene etc.SEALANT: Should remain resilient at very low temperatures,low cost, durable and easily applicable. Materials used are:putty, tars, tapes silicon, sealant.BASIN TRAY: Should have longlife, high resistance tocorrosion and low cost. Materials used are: wood, galvanizediron, steel, aluminium, asbestos cement, masonary bricks,concrete, etc.CONDENSATE CHANNEL: Materials used are: aluminium,galvanized iron, concrete, plastic material, etc.


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BASIC REQUIREMENTS OF A GOODSOLAR STILL

Be easily assembled in the field,'

Be constructed with locally available materials,

Be light weight for ease of handling andtransportation,

Have an effective life of 10 to 20 Yrs.

No requirement of any external power sources,

Can also serve as a rainfall catchment surface,

Is able to withstand prevailing winds,

Materials used should not contaminate thedistillate,

Meet standard civil and structural engineeringstandards, and,

Should be low in cost.


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Cross section of some typical basin type solar still. (a) Solar still with doublesloped symmetrical with continuous basin, (b) Solar still with double slopedsymmetrical with basin divided into two bays, (c) Solar still with single slopeand continuous basin, (d) Solar still with unsymmetrical double sloped and

divided basin, (e) U-trough type solar still, (f) Solar still with plastic inflatedcover, (g) Solar still with stretched plastic film with divided basin.


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Schematic of shallow basin type solar still


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SOLAR STILL OUTPUT DEPENDSON MANY PARAMETERS

1. Climatic ParametersI. Solar RadiationII. Ambient TemperatureIII. Wind SpeedIV. Outside HumidityV. Sky Conditions

2. Design ParametersI. Single slope or double slopeII. Glazing material

III. Water depth in BasinIV. Bottom insulationV. Orientation of stillVI. Inclination of glazingVII. Spacing between water and glazingVIII.Type of solar still


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3. Operational parameters

I. Water Depth

II. Preheating of Water

III. Colouring of WaterIV. Salinity of Water

V. Rate of Algae Growth

VI. Input Water supply arrangement

(continuously or in batches)

SOLAR STILL OUTPUT DEPENDS ONMANY PARAMETERS Contd


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Single slope experimental solar still


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Double sloped experimental solar still


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EXPERIMENTS ON SOLAR STILLS(CLIMATIC PARAMETERS)

The effect of climatic parameters on the still output wasseen by using two small, single sloped solar stills, each withbasin area equal to 0.58 sq.m,These two solar stills have identical design features exceptone with sawdust insulation (2.5 cm) in the bottom andsecond without any insulation. Hourly output and climatic

parameters were determined for one complete year.The insulated still gave 8 percent higher output comparedto uninsulated solar still.The maximum output was 5.271 litres/Sq.m. day.The still output increased from 1.76 liters/m2 day at 16.74MJ/m2 day to 5.11 litres/m2 day at 27.08 MJ/m2 day.An increase in still output was observed with increase inambient temperature. The increase in output is about 0.87litres/m2 day for each 10C rise in ambient temperature.


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Variation of solar still output and solar insolation fordifferent weeks of the year


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Relationship between still output and daily solar insolation


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EFFECT OF DESIGN PARAMETERS

The effect of design variables was studied on four doublesloped permanent type solar stills with dimensions of 245 x125 x 15 cm i.e. with a basin area of 3.0 m2.Still No. 1 does not contain any bottom insulation while stillnos. 2,3 and 4 each contained 2.5 cm thick sawdustinsulation.The glass angles for stills 1,2,3 and 4 are 20,30,30 and 40degrees from horizontal respectively.

Each of the still was filled daily with about 5 cm of water inthe morning and hourly values of distillate was collectedand measured.Still No.2 with base insulation has given a higher output.The average increase is 7 percent.By comparing stills 2-4, the still with lowest glass anglegave highest output.By comparing outputs of stills l and 3, it was observed thatstill 1 with 20 degree glass inclination and without baseinsulation, performs better than still 3 with 30 degree glassinclination and with base insulation.Both the channels of each of the still collect almost equalamount of distillate.


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EFFECT OF OPERATIONAL PARAMETERS

1. The effect of operational parameters was studiedon five single sloped solar stills each with a basinarea of 0.58 Sq.m. All are of identicalconstruction except still 5 had 5 cm thick sawdustinsulation.

2. The effect of water depth was studied by filingstills with 2.0, 4.0,6.0,8.0 cm water foruninsulated stills and 4.0 cm for insulated still.

3. Higher distillate output was observed with lowerwater depth.

4. The insulated still gave higher output.5. The effect of dye on water output was also

studied. The output got increased by colouringthe water.

6. The effect of use of waste heat for heating thesaline water in still was also studied. One still wasfilled with water at 30C and the other with waterat 45C. Higher output was observed in a still

using water at higher temperature.

Diff t i i l l ti f d il i ld


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Different empirical correlations for daily yieldfrom a solar still

S.N. Performance Relations (l/m2 d) References

1.2.

3.

4.

5.

6.

7.

8.

9.

10.

Mw = 0.216 + 0.00385 I(t)Mw = 0.0172 I(t) 1.1668

Mw = 0.000369 I(t)1.64

Mw = 4.132 x 10-3 I(t) [1+{I(t) / 110}]

Mw = 1.18 x 10-4 I(t)1.64

Mw = 0.0086 I(t) + 0.0636Ta+0.0633V

Mw = 0.013 I(t) 3.5969

Mw = 0.1323 W0.3 (Tin Ta) 1060

Mw = 0.00354 I(t)

Mw = 2.295 x 10-4 I(t) 0.0139Ta+0.0185V 0.433

Grunne et al (1962)Lawand & Boputiere(1970)

Battele (1965)

Zaki et al (1983)

Madani and Zaki

(1989)Garg and Mann (1976)

Garg and Mann (1976)

Malik et al (1982)

Maum et al (1970)

Natu et al (1979)

Where

I = Solar Intensity W/m2; t= time, s; mw = Daily Distillate Output, kg/m2;

T = Temperature, C; W = Humidity Ratio; V = Wind Speed (m/s)


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PROBLEMS ENCOUNTERED WITH PLASTIC COVERS

Fragility and short service life of plastic sheets.Leakage of water vapor and the condensate.

Over-heating, and hence melting, of the plasticbottom of the still due to the development of dryspots in course of time. In the extreme case theblack polyethylene sheets used as the basin linermay get heated beyond its melting point.

The plastic cover surface does not get wetted andthis leads to reduced transmission of incomingsolar energy and also to dripping of distilledwater back into the brine liquid.Susceptibility to damage by wind and other

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


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Energy transfer in a single effect basin

solar still


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Major heat fluxes for a solar still

PERFORMANCE PREDICTION OF


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The performance of solar still can be predicted by writingenergy

balance equations on various components of the still. A steadystate analysis of solar still is described here.Referring to the figure the instantaneousheatbalanceequationon basin water can be written as :

dtdTCqqqqI wwbcrew

PERFORMANCEPREDICTION OFBASIN-TYPE SOLAR STILL

(1)

Where I is the solar radiation on horizontal surface; w isabsorptivity of water and basin liner, is transmittance of glasscover; qe, qr, qc are the evaporative, radiative and convectiveheat losses from water to the transparent cover respectively; qbis the conductive heat loss from water basin; Cw is heat capacityof water and basin; Tw is water temperature; and t is the time.Similarly the instantaneous heat balance equation on glass

cover will be :


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creg

g

ggaqqqI

dt

dTcq .(2)

Where qga (=qca + qm) is the heatlossfromcoverto atmosphere,Cg is the heat capacity of glass cover, Tg is glass temperature, gis the absorptivity of glass cover, qca is the heat loss byconvection from cover to atmosphere, and qra is heat loss byradiation from cover to atmosphere.

Now the heatbalanceequation on the still is :

dt

dT

Cdt

dT

CqqqII

w

w

g

gbracagw

.(3)

The parameters like (1 - g -) I and (I-w) I are not included inequations since these do not add to evaporation or condensation

of water.


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The heat transfer by radiation qr from water surfaceto glass covercan be calculated from the equation

)( 44 gwr TTFq (4)

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

cover, and is the Stefan Boltzmann constant. Forthe basin type solar still and for low tilt angles ofglass cover, the basin and glass cover can beassumed as two parallel infinite plates. The shape

factor can be assumed to be equal to the emissivityof the water surface which is 0.9. Hence Eq. 4 willbe:

)(9.044

gwrTTq (5)

The convective heat loss from hot water surface in the


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The convective heat loss from hot water surface in thestill to the glass cover can be calculated from thefollowing expression :

)( gwcc TThq (6)

Where hc is the convective heat transfer coefficient, the

value of which depends on many parameters like

temperature of water and glass, density, conductivity,specific heat, viscosity, expansion coefficient of fluid,and spacing between water surface and glass cover.

Dunkle suggested an empirical relation for the

convective heat transfer coefficient as given below :3/1

3109.268

)(884.0

w

w

gw

gwc TP

PPTTh (7)


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Where Pw and Pg are the saturation partial pressures of watervapour (N/m2) at water temperature and glass temperaturerespectively.The evaporative heat loss qefrom water to the glass covercan be

calculated by knowing the mass transfer coefficient andconvective heat transfer coefficient. The empirical expression forqe as give by Dunkle is given as :

)(28.16gwce

PPhq .(8)

Heat loss through the ground and periphery qb is difficult tocompute since the soil temperature is unknown. Moreover, theheat conducted in the soil during daytime comes back in the

basin during night time. However, it can be computed from thefollowing simple relation :

)( awbb TTUq .(9)

Where Ub is the overallheattransfer coefficient from bottom.


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The convective heat loss qca from glass cover toambient air can be calculated from the followingexpression :

)(agcaca

TThq (10)

Where hca is the forced convection heat transfer

coefficient and is given by :

Vhca 8.38.2 (11)

Where V is the wind speed in m/s.The radiative heat loss qra from glass to sky can bedetermined provided the radiant sky temperature Ts isknown, which very much depends on atmospheric

conditions such as the presence of clouds etc.

G ll f l h k


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Generally for practical purposes the average skytemperature Ts can be assumed to be about 12 Kbelow ambient temperature, i.e. Tg = Ta - 12. Thusradiative heat loss q

rafrom glass cover to the

atmosphere is given as:

)(44

sggra TTq . (12)

Where g is the emissivity of glass cover.The exactsolution of the above simultaneous equations

is not possible and hence iterative technique isemployed to find the solution. The digital simulation

techniques for solving the above equations for aparticular set of condition can also be adopted. Evencharts are given by Morse and Read and Howe whichcan be used for performance prediction of solar stills for

a particular set of conditions.


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CO C S O S O S SO S


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CONCLUSIONS ON BASIN- TYPE SOLAR STILL

1. The solar still output (distillate) is a strong function of

solar radiation on a horizontal surface. The distillateoutput increases linearly with the solar insolation for agiven ambient temperature. If the ambient temperatureincreases or the wind velocity decreases, the heat lossfrom solar still decreases resulting in higher distillationrate. It is observed for each 10C rise in ambienttemperature the output increases by 10 percent.

2. The depth of water in the basin also effects theperformance considerably. At lower basin depths, thethermal capacity will be lower and hence the increase inwater temperature will be large resulting in higheroutput. However, it all depends on the insulation of the

still. If there is no lnsulatlon, increase in watertemperature will also increase the bottom heat loss. Ithas been observed that if the water depth increases from1.2 cm to 30 cm the output of still decreases by 30percent.

CONCLUSIONS ON BASIN- TYPE SOLAR


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CONCLUSIONS ON BASIN- TYPE SOLARSTILL (contd.)

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

4. Lower cover slope increases the output. From

practical considerations a minimum cover slopeof 10 deg. is suggested.5. The maximum possible efficiency of a single

basin solar still is about 60 percent.6. For higher receipt of solar radiation and

therefore the higher yield the long axis of thesolar still should be placed in the East-Westdirection if the still is installed at a highlatitude station. At low latitude stations theorientation has no effect on solar radiationreceipt.


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ADDITIONAL CONCLUSIONS DRAWN FROMEXPERIMENTAL STUDIES ON SOLAR STILLS

7. The main problem in a solar still Is the salt deposition ofcalcium carbonate and calcium sulphate on the basin linerwhich are white and insoluble and reflect solar radiationfrom basin water and basin liner and thereby lowering thestill output. It is difficult to stop the salt deposition.

8. The physical methods suggested to prevent the salt

deposition are Frequent flushing of the stills withcomplete drainage & Refilling or continuous agitation ofthe still water by circulating it with a small pump.

9. Once the salt gets deposited then the only way iscompletely draining the still and then scrubbing the sidesand basin liner and then refilling the still.

10. Another serious observation made in Australia is thecrystalline salt growth which takes place on the sides ofthe basin and into the distillate trough effecting the purityof distilled water.

11. Some success in preventing the crystalline salt growth isachieved in Australia by pre-treating the feed water with

a complex phosphate compound which reduces the rateof nucleation of salt crystals.

ADDITIONAL CONCLUSIONS DRAWN FROM


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ADDITIONAL CONCLUSIONS DRAWN FROMEXPERIMENTAL STUDIES ON SOLAR STILLS

12. Saline water in the still can be supplied eithercontinuously or in batches.

13. In Australia continuous supply of saline water in the solarstill is preferred at a rate of about 1.70 I/sq.m hr whichIs twice the maximum distillate rate.

14. This helps in reducing the salt deposition from the saltsolution.

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

16. Algae growth within the solar still also effects theperformance to a little extent but its growth must be

checked since its growth is unsightly and may finally blockthe basin and contaminate the distillation troughs.17. The algae growth can be checked by adding copper

sulphate and chlorine compounds in the saline water inthe still.


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39201792 solar desalination

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