200 DESALINATION
heat source, though reverse osmosis (RO), electrodialysis
(ED) and mechanical vapor compression (MVC) to the elec-
trical power produced from the sun’s radiation.^9
As the incidence of solar radiation varies over the day,
the time of the year, the degree of cloudy weather and the
geographic location, conventional solar evaporation can
never be a steady state operation. Moreover, convectional
solar distillation is a single effect process and is character-
ized by the thermal disadvantages of single stage operation.
The intensity of solar radiation reaching the earth varies
from zero to about 1047 W/m^2. Part of this radiation may
come directly from the sun, but sometimes as much as 10%
of it comes as scattered light, even when the atmosphere is
unobstructed by clouds. In cloudy weather the total radiation
is greatly reduced and most of the light that passes through
may be scattered light.
The solar radiation striking a horizontal surface is great-
est at noon, as the sun’s rays pass through the atmosphere
with a minimum length of passage through the air. In the
morning and the afternoon the rays are subject to increased
absorption and scattering. Considering the latitude, maxi-
mum radiation is at the equator. Hence the radiation inten-
sity depends on the hour of the day, the day of the year, and
the clarity of the atmosphere for a given location, as well
as of the latitude of the earth at the point of observation.
These limitations of the solar radiation render solar distilla-
tion method and solar driven desalination a nonsteady state
operation except if solar energy storage is provided, which
in general increases installation costs.
The daily production of conventional solar distillation
is low, due to low performance of the stills. Depending on
the intensity of solar radiation, the day of the month and the
month of the year the fresh water production ranges from 1.5
to 5.51/m^2 d (0.036 to 0.130 gal/ft^2 d).^10
Increasing feedwater temperature the daily productivity
increases as well. This can be done by connecting a solar still
with a solar collector or by using the condensate from low
pressure steam. Many other methods have been proposed, to
augment the efficiency of solar stills, nevertheless without
any success due to increase of the corresponding costs.
To calculate the efficiency or the daily productivity
of the solar stills have been proposed many mathematical
models. Here two general equations are given: One concerns
the operation of a conventional solar still and the second the
productivity of a solar still connected to a solar collector.
The daily output of a stagnant solar still is given by the
equation:^11M out F 1 H d F 2 (T ad T wd ) F 3 (1)and the daily output of a solar still connected to a solar col-
lector is:^12MFHFoutp d ().swd ad F 2
(2)Both equations depend on construction and operational
parameters.Much material is required to construct a solar still: glass
or plastic for the cover, black basin surface to absorb the
solar radiation, material for the basin, usually concrete or
plastic, pumps and piping—metal or preferably plastic, for
the feed water and the fresh water distribution. 13,14,15
Total cost of installation and operation of solar distillation
plants is not very high if land is given free. They need large
condensing areas and are vulnerable to storms. However,
energy is free, except pumping, operation is simple, and
maintenance cost very low.
Although the advantage of cost-free energy is partly
offset by increased amortization cost and the large installa-
tion area, distillation with solar energy remains a favorable
process for small-capacity water desalting at remote loca-
tions where there is considerable solar radiation. Most solar
distillation plants are being (or will be) erected in less devel-
oped countries or in areas where there are limited mainte-
nance facilities.
Solar energy for evaporation was first used on a major
scale about 1872 in Chile, where a glass-roofed unit had
4,400 m^2 to make 22.4 m^3 /d (6000 gpd) in a mining camp.^16
Today many units, glass covered or plastic ones, are installed
in small capacities world wide, mainly in arid and remote
areas. Figure 3 is the photograph of a solar distillation plant,
glass covered, yet in operation, in Porto Santo (Madeira)
Portugal, with an installation area of 1200 m^2.^17
It seems to be very simple as a method, and really it
is, because theoretically solar energy can replace any other
energy source. From a technical point of view this is not yet
totally feasible because either the corresponding technology
is not fully developed or the market is still very expensive.
Both procedures, solar distillation and solar driven desal-
ination, depend on local insolation rates which vary from site
to site for the same region, from the time of the day, the time
of the year and the cloudy weather making desalination an
unsteady state operation. Heat storage, if possible, improves
productivity by extending operation during the nighttime or
during cloudy days but also affects directly the economics of
the method. However, for certain locations as remote, arid
or semi-arid regions, where the small communities are poor
and where the techniques and tools of water production and
distribution developed in industrialized areas are not always
appropriate to be used, solar desalination is admitted as the
most suitable process.
The other way of using solar energy for desalination
purposes is the collection of solar energy by solar collectors
or concentrators, with subsequent conversion of the solar
energy to heat or electricity. This solar assisted desalination is
expanding rapidly and many installations have been erected
in commercial but as yet small capacity sizes.
The simplest thermal conversion type of collector is a
solar pond. A solar pond is a shallow body of water in which
a stabilizing salinity gradient prevents thermal convection,
thereby allowing the pond to act as a solar trap. The merit
of solar ponds lies in their ability to collect solar energy in
large scale and provide long-term heat storage. This long-
term storage provides also increased flexibility of heat use.
They can operate at all latitudes and are estimated to be lessC004_001_r03.indd 200C004_001_r03.indd 200 11/18/2005 10:18:51 AM11/18/2005 10:18:51 AM