SOLAR STILL

Apparatus to use the heat of the sun

directly to purify water by solar distillation.

SOLAR STILL -What’s-it

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

water by solar distillation .

• A solar still is simply a shallow basin with a transparent

glass cover. The sun heats the water in the basin,

causing evaporation. Moisture 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 most other impurities, including germs.

A solar still

Direct Solar Distilled Water

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. continued

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.

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.

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.

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.

Continued… Why use a solar still ?

•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.

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.

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.)

Efficiency is calculated in the following manner:

Efficiency = (Energy required for the

vaporization of the distillate that is

recovered) /

(Energy in the sun's radiation

that falls on the still.)

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.

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.

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.

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.

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.

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.

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

Output per square meter 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 liters (per square meter)

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

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 economical.

Four major components - Basin still

1. a basin;

2. a support structure;

3. a transparent glazing cover; and

4. a distillate trough (water channel)

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.

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.

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.

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.

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.

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.

Glazing Cover is a critical component of any solar still.

It is mounted above the basin and must be able to transmit a

lot of 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.

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.