S M A L L S C A L E W A T E R D I S T I L L A T I O N

U S I N G S O L A R W A R M I N G

SOLAR STILL FOR PURE WATER

SOLAR STILL - WHAT’S IT

• A solar still uses the heat of the sun directly to

purify water by vaporization - condensation.

• A solar still is simply a shallow basin with a

transparent glass cover. The sun heats the water in

the basin, causing evaporation. Vapour rises,

condenses on the cover and runs down into a

collection trough for pure water.

• Left behind is a fraction of input water with salts,

minerals, and other impurities, like germs. 2

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This presentation focuses mainly on small-

scale basin-type solar stills as suppliers of

potable water for families and other small

users.

Of all the solar still designs developed thus

far, the basin-type continues to be the most

simple and economical. 4

DIRECT SOLAR DISTILLED WATER

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How’s -its operation: 1. The sun's energy - short

electromagnetic waves - passes through a clear

glazing surface such as glass. Upon striking a

darkened surface, this light changes wavelength,

becomes long waves of heat- added to the water in a

shallow basin below the glazing. As the water heats

up, it begins to evaporate.

2. The warmed vapor rises to a cooler area. Almost

all impurities are left behind in the basin.

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How’s -its operation: … continued

3. The vapor condenses onto the underside of

the cooler glazing and accumulates into water

droplets or sheets of water.

4. The combination of gravity and the tilted

glazing surface allows the water to run down

the cover and into a collection trough, where it

is channeled into storage.

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SOLAR DISTILLATION: ENERGY REQUIREMENTS

• In this process, water is evaporated, thus

separating water vapor from dissolved matter, the vapor is then condensed as pure water.

• At least 2260 kJ/kg is required to evaporate water.

• To pump a kg of water through 20m head requires only 0.2 kJ/kg.

• Only where there is no local source of fresh water that can be easily pumped or lifted, distillation is therefore normally considered.

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WHEN USE SOLAR DISTILLATION ?

• Solar stills should normally only be considered for

removal of dissolved salts from water.

• If there is no fresh water then the main alternatives

are desalination, transportation and rainwater

collection.

• Unlike other techniques of desalination, solar stills

are more attractive, the smaller the required output.

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WHY USE A SOLAR STILL?

• Solar distillation can be a cost-effective means

of providing clean water -- on a small scale, for

drinking, cooking, washing and bathing--four

basic human needs.

• It can improve health standards by removing

low concentration inorganic impurities from

questionable water supplies.

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•The solar still is also used to purify water for

some business, industry, laboratory, and

green-house applications.

• It also appears able to purify polluted water.

WHY USE A SOLAR STILL ?

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Design objectives for an efficient solar still

• For high efficiency the solar still should

maintain:

• a high feed (un-distilled) water temperature

• a large temperature difference between

feed water and condensing surface

• low vapor leakage.

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Efficiency range:

In most units, less than half the calories of radiant

energy falling on the still are used for the heat of

vaporization necessary to produce the distilled

water.

All commercial stills sold to date have had an

efficiency range of 30 to 45 percent. (The maximum

efficiency is just over 60 percent.)

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Efficiency = (Energy required for the

vaporization of the distillate that is

recovered) /

(Energy in the sun's radiation

that falls on the still.)

Efficiency is calculated in the following manner:

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TO ACHIEVE HIGH EFFICIENCY-1:

A high feed water temperature can be achieved if:

• A high proportion of incoming radiation is absorbed by

the feed water as heat.

• Hence low absorption glazing and a good radiation

absorbing surface are required

• heat losses from the floor and walls are kept low

• the water is shallow so there is not so much to heat.

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TO ACHIEVE HIGH EFFICIENCY-2:

A large temperature difference can be achieved if:

• the condensing surface absorbs little or none of

the incoming radiation

• condensing water dissipates heat which must be

removed rapidly from the condensing surface

• by, for example, a second flow of water or air, or

by condensing at night.

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EFFICIENCY VS. COST OF STILL

• Provided the costs don't rise significantly, an

efficiency increase of a few percent is worth

working for.

• Improvements are principally to be sought in

materials and methods of construction.

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DESIGN TYPES AND THEIR

PERFORMANCE-1

• Single-basin stills have been much studied and their

behavior is well understood. Efficiencies of 25% are

typical.

• Daily output is a function of solar irradiation and is

greatest in the early evening when the feed water is

still hot but when outside temperatures are falling.

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DESIGN TYPES AND THEIR

PERFORMANCE-2

• Material selection is very important. The

cover can be either glass or plastic. Glass

is considered to be best for most long-term

applications, whereas a plastic (such as

polyethylene) can be used for short-term

use.

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DESIGN TYPES AND THEIR

PERFORMANCE-3

• Sand concrete or waterproofed concrete are

considered best for the basin of a long-life

still if it is to be manufactured on-site,

• but for factory-manufactured stills,

prefabricated ferro-concrete is a suitable

material.

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OUTPUT OF A SOLAR STILL

• Q = [E x G x A] / 2.3

where:

• Q = daily output of distilled water (litres /day)

• E = overall efficiency

• G = daily global solar irradiation (MJ/m²)

• A = aperture area of the still i. e, the plan areas

for a simple basin still

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OUTPUT PER SQUARE METRE

OF AREA IS:

• The average, daily, global solar irradiation is typically

18.0 MJ/m² (5 kWh/m²).

• A simple basin still operates at an overall efficiency of

about 30%.

• daily output = [0.30 x 18.0 x 1] / 2.3

= 2.3 litres (per square metre)

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DESIGN VARIATIONS

• concentrating collector stills

• multiple tray tilted stills

• tilted wick solar stills

• and basin stills

• 95 percent of all functioning stills are of the

basin type

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FOUR MAJOR COMPONENTS -

BASIN STILL

1. a basin;

2. a support structure;

3. a transparent glazing cover; and

4. a distillate trough (water channel)

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ANCILLARY COMPONENTS

1. insulation (usually under the basin);

2. sealants;

3. piping and valves;

4. facilities for storage;

5. an external cover to protect the other

components from the weather and to

make the still esthetically pleasing; and

6. a reflector to concentrate sunlight.

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PHYSICAL DIMENSIONS

• If the only glazing available is one meter at its

greatest dimension, the still's maximum inner

width will be just under one

meter. And the length of the still will be set

according to what is needed to provide the

amount of square meters to produce the

required amount of water. It is generally best to

design an installation with many small modular

units to supply the water.

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COMMUNITY AND RESIDENTIAL

SIZE STILLS

• Most community size stills are 1/2 to 2 1/2 meters wide,

with lengths ranging up to around 100 meters.

• Their lengths usually run along an east-west axis to

maximize the transmission of sunlight through the

equatorial facing sloped glass.

• Residential, appliance type units generally use glass about

0.65 to 0.9 meter wide with lengths ranging from two to

three meters.

• A water depth of 1.5 to 2.5 cm is most common.

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DEPTH OF WATER: THE SHALLOWER THE DEPTH, THE BETTER.

• Note that solar heat can evaporate about 0.5 cm of water on a clear day in summer.

• By setting the initial charge at about 1.5 cm depth, virtually all of the salts remain in the solution, and can be flushed out by the refilling operation.

• Of course, if the basin is too shallow, it will dry out and salts will be deposited, which is not good.

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TWO GENERAL TYPES OF BASINS

• material that maintains its own shape and provides

the waterproof containment by itself / with the aid of a

surface material applied directly to it

• uses one set of materials (such as wood or brick) to

define the basin's shape; Into this is placed a second

material that easily conforms to the shape of the

structural materials and serves as a waterproof liner.

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One alternative is ordinary aluminum coated

with silicone rubber. The durability of basins

made with this material increased into

the 10- to 15-year range. For the hundreds of

stills one company sold using this material,

the coating was all done by hand. With

production roll coating equipment, the basin's

durability could probably be increased even

more.

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Glazing Cover is a critical component of a solar

still. It is mounted above the basin and must be

able to transmit light in the visible spectrum yet

keep the heat generated by that light from escaping

the basin.

Exposure to ultraviolet radiation requires a material

that can withstand the degradation effects or that is

inexpensive enough to be replaced periodically.

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Since it may encounter temperatures

approaching 95 [degrees] Celsius , it must

also be able to support its weight at those

temperatures and not undergo excessive

expansion, which could destroy the

airtight seals.