SOLAR DESALINATIONSOLAR DESALINATION

WATER DESALINATION TECHNOLOGYWATER DESALINATION TECHNOLOGY

Nature is carrying out the process of water desalination Nature is carrying out the process of water desalination since ages. since ages. Oceanic water due to solar heating converts into vapours Oceanic water due to solar heating converts into vapours and pours down as precipitation on earth in the form of and pours down as precipitation on earth in the form of fresh water. fresh water. Water is the most needed substance on the earth for Water is the most needed substance on the earth for sustenance of life. sustenance of life. Due to rapid expansion of population, accelerated industrial Due to rapid expansion of population, accelerated industrial growth and enhanced agricultural production, there is ever growth and enhanced agricultural production, there is ever increasing demand for fresh water. increasing demand for fresh water. Demand of fresh water (potable water) has increased from Demand of fresh water (potable water) has increased from 15-20 litres/person/day to 75-100 litres/person/day, 15-20 litres/person/day to 75-100 litres/person/day, The ocean covers 71 recent of the earth's surface-140 The ocean covers 71 recent of the earth's surface-140 million square miles with a volume of 330 million cubic million square miles with a volume of 330 million cubic miles and has an average salt content of 35,000 ppm. miles and has an average salt content of 35,000 ppm. Brackish/saline water is strictly defined as the water with Brackish/saline water is strictly defined as the water with less dissolved salts than sea water but more than 500 ppm. less dissolved salts than sea water but more than 500 ppm.

SOLAR DESALINATION TECHNIQUESSOLAR DESALINATION TECHNIQUES

Potable WaterPotable Water Less than 550 ppmLess than 550 ppm

RequirementRequirement Domestic, Industries and Domestic, Industries and AgricultureAgriculture

Sources of Potable Sources of Potable WaterWater

Rivers, Lakes, Ponds, Wells etc.Rivers, Lakes, Ponds, Wells etc.

Demand of Potable Demand of Potable WaterWater

15-25 litres / person / day 15-25 litres / person / day (OLD)(OLD)

100-125 litres / person / day 100-125 litres / person / day (NEW)(NEW)

Underground Underground Saline WaterSaline Water

2,000 – 2,500 ppm2,000 – 2,500 ppm

Sea WaterSea Water 30,000 – 50,000 ppm30,000 – 50,000 ppm

WATER DESALINATION TECHNOLOGY WATER DESALINATION TECHNOLOGY

Potable water (fresh water) suitable for human Potable water (fresh water) suitable for human consumption should not contain dissolved salts consumption should not contain dissolved salts more than 500 ppm. more than 500 ppm. For agricultural purposes, water containing salt For agricultural purposes, water containing salt content of 1000 ppm is considered as the upper content of 1000 ppm is considered as the upper limit. limit. Potable water is required for domestic, agriculture Potable water is required for domestic, agriculture and industries. and industries. Some applications in industries like cooling Some applications in industries like cooling purposes, sea water is feasible despite the purposes, sea water is feasible despite the corrosion problems while other industries use corrosion problems while other industries use higher quality water than is acceptable for drinking higher quality water than is acceptable for drinking water. Modern steam power generation plant need water. Modern steam power generation plant need water with less than 10 ppm. water with less than 10 ppm. Potable/fresh water is available from rivers, lakes, Potable/fresh water is available from rivers, lakes, ponds, wells, etc. ponds, wells, etc. Underground saline/brackish water contains Underground saline/brackish water contains dissolved salts of about 2,000-2,500 ppm. dissolved salts of about 2,000-2,500 ppm.

METHODS OF CONVERTING BRACKISH METHODS OF CONVERTING BRACKISH WATER INTO POTABLE WATER WATER INTO POTABLE WATER

DESALINATIONDESALINATION: The saline water is evaporated using : The saline water is evaporated using thermal energy and the resulting steam is collected and thermal energy and the resulting steam is collected and condensed as final product. condensed as final product. VAPOR COMPRESSIONVAPOR COMPRESSION: Here water vapour from boiling : Here water vapour from boiling water is compressed adiabatically and vapour gets water is compressed adiabatically and vapour gets superheated. The superheated vapor is first cooled to superheated. The superheated vapor is first cooled to saturation temperature and then condensed at constant saturation temperature and then condensed at constant pressure. This process is derived by mechanical energy. pressure. This process is derived by mechanical energy. REVERSE OSMOSISREVERSE OSMOSIS: Here saline water is pushed at high : Here saline water is pushed at high pressure through special membranes allowing water pressure through special membranes allowing water molecules pass selectively and not the dissolved salts. molecules pass selectively and not the dissolved salts. ELECTRODIALYSISELECTRODIALYSIS: Here a pair of special membranes, : Here a pair of special membranes, perpendicular to which there is an electric field are used perpendicular to which there is an electric field are used and water is passed through them. Water does not pass and water is passed through them. Water does not pass through the membranes while dissolved salts pass through the membranes while dissolved salts pass selectively. selectively. In distillation; thermal energy is used while in vapour In distillation; thermal energy is used while in vapour compression, reverse osmosis, electrodialysis, etc. some compression, reverse osmosis, electrodialysis, etc. some mechanical and electrical energy is used. mechanical and electrical energy is used.

Solar Distillation

Passive Distillation Active Distillation

Conventional Solar Still

Multi-effect Solar Still

New Design Solar Still

Inclined Solar Still

High Temp Distillation

Nocturnal Distillation

With Reflector

With Condenser

Distillation with

collector panel

Auxiliary heating

distillation

Life raft Spherical Tubular Regeneration

Classification of Solar Distillation Systems

Multieffect Solar Still

Diffusion Still

Chimney Type Still

Multi effect Basin Still

Heated Head Solar Still

Double Basin Solar Still

Multiple Basin Solar Still

Inclined Solar Still

Wick Solar Still

Single Wick Solar Still

Multiple effect tilted tray Solar

Still

Basin Solar Still

Multiple Wick Solar Still

Tilted Tray / stepped Solar

Still

METHODS OF PURIFICATION OF WATER

Types of Solar StillTypes of Solar StillSingle Effect Basin Solar StillSingle Effect Basin Solar StillTilted Tray Solar StillTilted Tray Solar StillMultibasin Stepped Solar StillMultibasin Stepped Solar StillRegeneration Inclined Step Solar StillRegeneration Inclined Step Solar StillWick Type Solar StillWick Type Solar StillMultiple Effect Diffusion Solar StillMultiple Effect Diffusion Solar StillChimney Type Solar StillChimney Type Solar StillMulti-Tray Multiple Effect Solar StillMulti-Tray Multiple Effect Solar StillDouble Basin Solar StillDouble Basin Solar StillHumidification Dumidification DistillerHumidification Dumidification DistillerMultistage Flash DistillerMultistage Flash DistillerSolar – Assisted wiped film Multistage Flash Solar – Assisted wiped film Multistage Flash DistillerDistiller

MAIN TECHNIQUES FOR DISTILLATION MAIN TECHNIQUES FOR DISTILLATION

a) Flash Distillation a) Flash Distillation b) Vapor Compression Process. b) Vapor Compression Process. c) Electrodialysis c) Electrodialysis d) Reverse Osmosis. d) Reverse Osmosis. e) Solar Distillation. e) Solar Distillation.

GUIDELINESGUIDELINES1. Quantity of Fresh Water Required and its End Use. 1. Quantity of Fresh Water Required and its End Use. 2. Available Water Sources, such as Sea, Ponds, Wells, 2. Available Water Sources, such as Sea, Ponds, Wells,

Swamps etc. Swamps etc. 3. Proximity to nearest Fresh Water Sources. 3. Proximity to nearest Fresh Water Sources. 4. Availability of Electric Power at the Site or Closeby. 4. Availability of Electric Power at the Site or Closeby. 5. Cost of Supplying Fresh Water by Various Methods. 5. Cost of Supplying Fresh Water by Various Methods. 6. Cost and Availability of Labor in the Region. 6. Cost and Availability of Labor in the Region. 7. Maintenance and Daily Operational Requirements. 7. Maintenance and Daily Operational Requirements. 8. Life Span of the Water Supply System. 8. Life Span of the Water Supply System. 9. Economic Value of the Region. 9. Economic Value of the Region.

Schematic of basin-type solar still

COMPONENTS OF SINGLE COMPONENTS OF SINGLE EFFECT SOLAR STILL EFFECT SOLAR STILL

1.1. Basin Basin

2.2. Black Liner Black Liner

3.3. Transparent Cover Transparent Cover

4.4. Condensate Channel Condensate Channel

5.5. Sealant Sealant

6.6. Insulation Insulation

7.7. Supply and Delivery System Supply and Delivery System

MATERIALS FOR SOLAR STILLS MATERIALS FOR SOLAR STILLS

GLAZINGGLAZING: Should have high transmittance for solar radiation, : Should have high transmittance for solar radiation, opaque to thermal radiation, resistance to abrasion, longlife, opaque to thermal radiation, resistance to abrasion, longlife, low cost, high wettability for water, lightweight, easy to low cost, high wettability for water, lightweight, easy to handle and apply, and universal availability. Materials used handle and apply, and universal availability. Materials used are: glass or treated plastic. are: glass or treated plastic. LINERLINER: Should absorb more solar radiation, should be : Should absorb more solar radiation, should be durable, should be water tight, easily cleanable, low cost, and durable, should be water tight, easily cleanable, low cost, and should be able to withstand temperature around 100 Deg C. should be able to withstand temperature around 100 Deg C. Materials used are: asphalt matt, black butyl rubber, black Materials used are: asphalt matt, black butyl rubber, black polyethylene etc. polyethylene etc. SEALANTSEALANT: Should remain resilient at very low temperatures, : Should remain resilient at very low temperatures, low cost, durable and easily applicable. Materials used are: low cost, durable and easily applicable. Materials used are: putty, tars, tapes silicon, sealant. putty, tars, tapes silicon, sealant. BASIN TRAYBASIN TRAY: Should have longlife, high resistance to : Should have longlife, high resistance to corrosion and low cost. Materials used are: wood, galvanized corrosion and low cost. Materials used are: wood, galvanized iron, steel, aluminium, asbestos cement, masonary bricks, iron, steel, aluminium, asbestos cement, masonary bricks, concrete, etc. concrete, etc. CONDENSATE CHANNELCONDENSATE CHANNEL: Materials used are: aluminium, : Materials used are: aluminium, galvanized iron, concrete, plastic material, etc. galvanized iron, concrete, plastic material, etc.

BASIC REQUIREMENTS OF A GOOD BASIC REQUIREMENTS OF A GOOD SOLAR STILL SOLAR STILL

Be easily assembled in the field,' Be easily assembled in the field,' Be constructed with locally available materials, Be constructed with locally available materials, Be light weight for ease of handling and Be light weight for ease of handling and transportation, transportation, Have an effective life of 10 to 20 Yrs. Have an effective life of 10 to 20 Yrs. No requirement of any external power sources, No requirement of any external power sources, Can also serve as a rainfall catchment surface, Can also serve as a rainfall catchment surface, Is able to withstand prevailing winds, Is able to withstand prevailing winds, Materials used should not contaminate the distillate, Materials used should not contaminate the distillate, Meet standard civil and structural engineering Meet standard civil and structural engineering standards, and, standards, and, Should be low in cost. Should be low in cost.

Cross section of some typical basin type solar still. (a) Solar still with double sloped symmetrical with continuous basin, (b) Solar still with double sloped symmetrical with basin divided into two bays, (c) Solar still with single slope and continuous basin, (d) Solar still with unsymmetrical double sloped and divided basin, (e) U-trough type solar still, (f) Solar still with plastic inflated cover, (g) Solar still with stretched plastic film with divided basin.

Schematic of shallow basin type solar still

SOLAR STILL OUTPUT DEPENDS SOLAR STILL OUTPUT DEPENDS ON MANY PARAMETERSON MANY PARAMETERS

1.1. Climatic ParametersClimatic ParametersI.I. Solar Radiation Solar Radiation II.II. Ambient TemperatureAmbient TemperatureIII.III. Wind SpeedWind SpeedIV.IV. Outside HumidityOutside HumidityV.V. Sky ConditionsSky Conditions

2.2. Design ParametersDesign ParametersI.I. Single slope or double slopeSingle slope or double slopeII.II. Glazing materialGlazing materialIII.III. Water depth in BasinWater depth in BasinIV.IV. Bottom insulationBottom insulationV.V. Orientation of stillOrientation of stillVI.VI. Inclination of glazingInclination of glazingVII.VII. Spacing between water and glazingSpacing between water and glazingVIII.VIII. Type of solar stillType of solar still

3.3. Operational parametersOperational parametersI.I. Water DepthWater Depth

II.II. Preheating of WaterPreheating of Water

III.III. Colouring of WaterColouring of Water

IV.IV. Salinity of WaterSalinity of Water

V.V. Rate of Algae GrowthRate of Algae Growth

VI.VI. Input Water supply arrangement Input Water supply arrangement (continuously or in batches)(continuously or in batches)

SOLAR STILL OUTPUT DEPENDS ON SOLAR STILL OUTPUT DEPENDS ON MANY PARAMETERSMANY PARAMETERS Contd…

Single slope experimental solar still

Double sloped experimental solar still

EXPERIMENTS ON SOLAR STILLS EXPERIMENTS ON SOLAR STILLS (CLIMATIC PARAMETERS) (CLIMATIC PARAMETERS)

The effect of climatic parameters on the still output was The effect of climatic parameters on the still output was seen by using two small, single sloped solar stills, each with seen by using two small, single sloped solar stills, each with basin area equal to 0.58 sq.m, basin area equal to 0.58 sq.m, These two solar stills have identical design features except These two solar stills have identical design features except one with sawdust insulation (2.5 cm) in the bottom and one with sawdust insulation (2.5 cm) in the bottom and second without any insulation. Hourly output and climatic second without any insulation. Hourly output and climatic parameters were determined for one complete year. parameters were determined for one complete year. The insulated still gave 8 percent higher output compared The insulated still gave 8 percent higher output compared to uninsulated solar still. to uninsulated solar still. The maximum output was 5.271 litres/Sq.m. day. The maximum output was 5.271 litres/Sq.m. day. The still output increased from 1.76 liters/mThe still output increased from 1.76 liters/m22 day at 16.74 day at 16.74 MJ/mMJ/m22 day to 5.11 litres/m day to 5.11 litres/m22 day at 27.08 MJ/m day at 27.08 MJ/m22 day. day. An increase in still output was observed with increase in An increase in still output was observed with increase in ambient temperature. The increase in output is about 0.87 ambient temperature. The increase in output is about 0.87 litres/mlitres/m22 day for each 10°C rise in ambient temperature. day for each 10°C rise in ambient temperature.

Variation of solar still output and solar insolation for different weeks of the year

Relationship between still output and daily solar insolation

EFFECT OF DESIGN PARAMETERS EFFECT OF DESIGN PARAMETERS

The effect of design variables was studied on four double The effect of design variables was studied on four double sloped permanent type solar stills with dimensions of 245 x sloped permanent type solar stills with dimensions of 245 x 125 x 15 cm i.e. with a basin area of 3.0 m125 x 15 cm i.e. with a basin area of 3.0 m22. . Still No. 1 does not contain any bottom insulation while still Still No. 1 does not contain any bottom insulation while still nos. 2,3 and 4 each contained 2.5 cm thick sawdust nos. 2,3 and 4 each contained 2.5 cm thick sawdust insulation. insulation. The glass angles for stills 1,2,3 and 4 are 20,30,30 and 40 The glass angles for stills 1,2,3 and 4 are 20,30,30 and 40 degrees from horizontal respectively. degrees from horizontal respectively. Each of the still was filled daily with about 5 cm of water in Each of the still was filled daily with about 5 cm of water in the morning and hourly values of distillate was collected and the morning and hourly values of distillate was collected and measured. measured. Still No.2 with base insulation has given a higher output. The Still No.2 with base insulation has given a higher output. The average increase is 7 percent. average increase is 7 percent. By comparing stills 2-4, the still with lowest glass angle gave By comparing stills 2-4, the still with lowest glass angle gave highest output. highest output. By comparing outputs of stills l and 3, it was observed that By comparing outputs of stills l and 3, it was observed that still 1 with 20 degree glass inclination and without base still 1 with 20 degree glass inclination and without base insulation, performs better than still 3 with 30 degree glass insulation, performs better than still 3 with 30 degree glass inclination and with base insulation. inclination and with base insulation. Both the channels of each of the still collect almost equal Both the channels of each of the still collect almost equal amount of distillate. amount of distillate.

EFFECT OF OPERATIONAL PARAMETERSEFFECT OF OPERATIONAL PARAMETERS1. The effect of operational parameters was studied 1. The effect of operational parameters was studied

on five single sloped solar stills each with a basin on five single sloped solar stills each with a basin area of 0.58 Sq.m. All are of identical construction area of 0.58 Sq.m. All are of identical construction except still 5 had 5 cm thick sawdust insulation. except still 5 had 5 cm thick sawdust insulation.

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

3. Higher distillate output was observed with lower 3. Higher distillate output was observed with lower water depth. water depth.

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

studied. The output got increased by colouring studied. The output got increased by colouring the water. the water.

6. The effect of use of waste heat for heating the 6. The effect of use of waste heat for heating the saline water in still was also studied. One still was saline water in still was also studied. One still was filled with water at 30°C and the other with water filled with water at 30°C and the other with water at 45°C. Higher output was observed in a still at 45°C. Higher output was observed in a still using water at higher temperature. using water at higher temperature.

Different empirical correlations for daily yield Different empirical correlations for daily yield from a solar stillfrom a solar still

S.N.S.N. Performance Relations (l/mPerformance Relations (l/m22 d) d) ReferencesReferences

1.1.

2.2.

3.3.

4.4.

5.5.

6.6.

7.7.

8.8.

9.9.

10.10.

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

Mw = 0.0172 I(t) – 1.1668Mw = 0.0172 I(t) – 1.1668

Mw = 0.000369 I(t)Mw = 0.000369 I(t)1.641.64

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

Mw = 1.18 x 10Mw = 1.18 x 10-4-4 I(t) I(t)1.641.64

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

Mw = 0.013 I(t) – 3.5969Mw = 0.013 I(t) – 3.5969

Mw = 0.1323 WMw = 0.1323 W0.30.3 (T (Tinin – T – Taa) – 1060) – 1060

Mw = 0.00354 I(t)Mw = 0.00354 I(t)

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

Grunne et al (1962)Grunne et al (1962)

Lawand & Boputiere Lawand & Boputiere (1970)(1970)

Battele (1965)Battele (1965)

Zaki et al (1983)Zaki et al (1983)

Madani and Zaki Madani and Zaki (1989)(1989)

Garg and Mann (1976)Garg and Mann (1976)

Garg and Mann (1976)Garg and Mann (1976)

Malik et al (1982)Malik et al (1982)

Maum et al (1970)Maum et al (1970)

Natu et al (1979)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)

PROBLEMS ENCOUNTERED WITH PLASTIC COVERSPROBLEMS ENCOUNTERED WITH PLASTIC COVERS

Fragility and short service life of plastic sheets. Fragility and short service life of plastic sheets. Leakage of water vapor and the condensate. Leakage of water vapor and the condensate. Over-heating, and hence melting, of the plastic Over-heating, and hence melting, of the plastic bottom of the still due to the development of dry bottom of the still due to the development of dry spots in course of time. In the extreme case the spots in course of time. In the extreme case the black polyethylene sheets used as the basin liner black polyethylene sheets used as the basin liner may get heated beyond its melting point. may get heated beyond its melting point. The plastic cover surface does not get wetted and The plastic cover surface does not get wetted and this leads to reduced transmission of incoming this leads to reduced transmission of incoming solar energy and also to dripping of distilled solar energy and also to dripping of distilled water back into the brine liquid. water back into the brine liquid. Susceptibility to damage by wind and other Susceptibility to damage by wind and other elements of nature. elements of nature. Occasional unforeseen mixing of brine and Occasional unforeseen mixing of brine and distilled water in some of the designs. distilled water in some of the designs.

Energy transfer in a single effect basin solar still

Major heat fluxes for a solar still

The performance of solar still can be predicted by writing energy balance equations on various components of the still. A steady state analysis of solar still is described here.Referring to the figure the instantaneous heat balance equation on basin water can be written as :

dt

dTCqqqqI wwbcrew

PERFORMANCE PREDICTION OF BASIN-TYPE SOLAR STILL

(1)

Where I is the solar radiation on horizontal surface; w is absorptivity of water and basin liner, is transmittance of glass cover; qe, qr, qc are the evaporative, radiative and convective heat losses from water to the transparent cover respectively; qb is the conductive heat loss from water basin; Cw is heat capacity of water and basin; Tw is water temperature; and t is the time. Similarly the instantaneous heat balance equation on glass cover will be :

cregg

gga qqqIdt

dTcq ….(2)

Where qga (=qca + qm) is the heat loss from cover to atmosphere, Cg is the heat capacity of glass cover, Tg is glass temperature, g is the absorptivity of glass cover, qca is the heat loss by convection from cover to atmosphere, and qra is heat loss by radiation from cover to atmosphere.

Now the heat balance equation on the still is :

dt

dTC

dt

dTCqqqII w

wg

gbracagw ….(3)

The parameters like (1 - g - ) I and (I-w) I are not included in equations since these do not add to evaporation or condensation of water.

The heat transfer by radiation qr from water surface to glass cover can be calculated from the equation

)( 44gwr TTFq ……(4)

Where F is the shape factor which depends on the geometry and the emissivities of water and glass cover, and is the Stefan Boltzmann constant. For the basin type solar still and for low tilt angles of glass cover, the basin and glass cover can be assumed as two parallel infinite plates. The shape factor can be assumed to be equal to the emissivity of the water surface which is 0.9. Hence Eq. 4 will be:

)(9.0 44gwr TTq ……(5)

The convective heat loss from hot water surface in the still to the glass cover can be calculated from the following 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

gwgwc T

P

PPTTh …(7)

Where Pw and Pg are the saturation partial pressures of water vapour (N/m2) at water temperature and glass temperature respectively.The evaporative heat loss qe from water to the glass cover can be calculated by knowing the mass transfer coefficient and convective heat transfer coefficient. The empirical expression for qe as give by Dunkle is given as :

)(28.16 gwce PPhq ….(8)

Heat loss through the ground and periphery qb is difficult to compute since the soil temperature is unknown. Moreover, the heat conducted in the soil during daytime comes back in the basin during night time. However, it can be computed from the following simple relation :

)( awbb TTUq ….(9)

Where Ub is the overall heat transfer coefficient from bottom.

The convective heat loss qca from glass cover to ambient air can be calculated from the following expression :

)( 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 be determined provided the radiant sky temperature Ts is known, which very much depends on atmospheric conditions such as the presence of clouds etc.

Generally for practical purposes the average sky temperature Ts can be assumed to be about 12 K below ambient temperature, i.e. Tg = Ta - 12. Thus radiative heat loss qra from glass cover to the atmosphere is given as:

)( 44sggra TTq …. (12)

Where g is the emissivity of glass cover.The exact solution of the above simultaneous equations is not possible and hence iterative technique is employed to find the solution. The digital simulation techniques for solving the above equations for a particular set of condition can also be adopted. Even charts are given by Morse and Read and Howe which can be used for performance prediction of solar stills for a particular set of conditions.

Main Problems of Solar StillMain Problems of Solar Still

Low distillate output per unit areaLow distillate output per unit area

Leakage of vapour through jointsLeakage of vapour through joints

High maintenanceHigh maintenance

Productivity decreases with time for Productivity decreases with time for a variety of reasonsa variety of reasons

Cost per unit output is very highCost per unit output is very high

CONCLUSIONS ON BASIN- TYPE SOLAR STILLCONCLUSIONS ON BASIN- TYPE SOLAR STILL

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

solar radiation on a horizontal surface. The distillate solar radiation on a horizontal surface. The distillate output increases linearly with the solar insolation for a output increases linearly with the solar insolation for a given ambient temperature. If the ambient temperature given ambient temperature. If the ambient temperature increases or the wind velocity decreases, the heat loss increases or the wind velocity decreases, the heat loss from solar still decreases resulting in higher distillation from solar still decreases resulting in higher distillation rate. It is observed for each 10rate. It is observed for each 10C rise in ambient C rise in ambient temperature the output increases by 10 percent. temperature the output increases by 10 percent.

2.2. The depth of water in the basin also effects the The depth of water in the basin also effects the performance considerably. At lower basin depths, the performance considerably. At lower basin depths, the thermal capacity will be lower and hence the increase in thermal capacity will be lower and hence the increase in water temperature will be large resulting in higher water temperature will be large resulting in higher output. However, it all depends on the insulation of the output. However, it all depends on the insulation of the still. If there is no lnsulatlon, increase in water still. If there is no lnsulatlon, increase in water temperature will also increase the bottom heat loss. It temperature will also increase the bottom heat loss. It has been observed that if the water depth increases from has been observed that if the water depth increases from 1.2 cm to 30 cm the output of still decreases by 30 1.2 cm to 30 cm the output of still decreases by 30 percent. percent.

CONCLUSIONS ON BASIN- TYPE SOLAR CONCLUSIONS ON BASIN- TYPE SOLAR STILL (contd.)STILL (contd.)

3.3. Number of transparent covers in a solar still do Number of transparent covers in a solar still do not increase the output since it increases the not increase the output since it increases the temperature of the inner cover resulting in temperature of the inner cover resulting in lower condensation of water vapour. lower condensation of water vapour.

4.4. Lower cover slope increases the output. From Lower cover slope increases the output. From practical considerations a minimum cover slope practical considerations a minimum cover slope of 10 deg. is suggested. of 10 deg. is suggested.

5.5. The maximum possible efficiency of a single The maximum possible efficiency of a single basin solar still is about 60 percent. basin solar still is about 60 percent.

6.6. For higher receipt of solar radiation and For higher receipt of solar radiation and therefore the higher yield the long axis of the therefore the higher yield the long axis of the solar still should be placed in the East-West solar still should be placed in the East-West direction if the still is installed at a high latitude direction if the still is installed at a high latitude station. At low latitude stations the orientation station. At low latitude stations the orientation has no effect on solar radiation receipt. has no effect on solar radiation receipt.

ADDITIONAL CONCLUSIONS DRAWN FROM ADDITIONAL CONCLUSIONS DRAWN FROM EXPERIMENTAL STUDIES ON SOLAR STILLSEXPERIMENTAL STUDIES ON SOLAR STILLS

7.7. The main problem in a solar still Is the salt deposition of The main problem in a solar still Is the salt deposition of calcium carbonate and calcium sulphate on the basin liner calcium carbonate and calcium sulphate on the basin liner which are white and insoluble and reflect solar radiation which are white and insoluble and reflect solar radiation from basin water and basin liner and thereby lowering the from basin water and basin liner and thereby lowering the still output. It is difficult to stop the salt deposition. still output. It is difficult to stop the salt deposition.

8.8. The physical methods suggested to prevent the salt The physical methods suggested to prevent the salt deposition are Frequent flushing of the stills with complete deposition are Frequent flushing of the stills with complete drainage & Refilling or continuous agitation of the still water drainage & Refilling or continuous agitation of the still water by circulating it with a small pump. by circulating it with a small pump.

9.9. Once the salt gets deposited then the only way is completely Once the salt gets deposited then the only way is completely draining the still and then scrubbing the sides and basin liner draining the still and then scrubbing the sides and basin liner and then refilling the still. and then refilling the still.

10.10. Another serious observation made in Australia is the Another serious observation made in Australia is the crystalline salt growth which takes place on the sides of the crystalline salt growth which takes place on the sides of the basin and into the distillate trough effecting the purity of basin and into the distillate trough effecting the purity of distilled water. distilled water.

11.11. Some success in preventing the crystalline salt growth is Some success in preventing the crystalline salt growth is achieved in Australia by pre-treating the feed water with a achieved in Australia by pre-treating the feed water with a complex phosphate compound which reduces the rate of complex phosphate compound which reduces the rate of nucleation of salt crystals. nucleation of salt crystals.

ADDITIONAL CONCLUSIONS DRAWN FROM ADDITIONAL CONCLUSIONS DRAWN FROM EXPERIMENTAL STUDIES ON SOLAR STILLSEXPERIMENTAL STUDIES ON SOLAR STILLS

12.12. Saline water in the still can be supplied either Saline water in the still can be supplied either continuously or in batches. continuously or in batches.

13.13. In Australia continuous supply of saline water in the solar In Australia continuous supply of saline water in the solar still is preferred at a rate of about 1.70 I/sq.m hr which Is still is preferred at a rate of about 1.70 I/sq.m hr which Is twice the maximum distillate rate. twice the maximum distillate rate.

14.14. This helps in reducing the salt deposition from the salt This helps in reducing the salt deposition from the salt solution. solution.

15.15. From thermal efficiency point of view, batch filling i.e. From thermal efficiency point of view, batch filling i.e. filling of saline water when the basin water is coolest filling 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 labour costs and special plumbing arrangements. costs and special plumbing arrangements.

16.16. Algae growth within the solar still also effects the Algae growth within the solar still also effects the performance to a little extent but its growth must be performance to a little extent but its growth must be checked since its growth is unsightly and may finally block checked since its growth is unsightly and may finally block the basin and contaminate the distillation troughs. the basin and contaminate the distillation troughs.

17.17. The algae growth can be checked by adding copper The algae growth can be checked by adding copper sulphate and chlorine compounds in the saline water in sulphate and chlorine compounds in the saline water in the still.the still.