Environmental Engineering Reference
In-Depth Information
in most places is not environmentally acceptable. In order to minimise the heat losses
to the ground it is desirable that the underground water table is 5 metres or more below
the natural ground surface. If the water table is shallower, then insulating the bottom
of the pond may be considered using insulation materials such as sheets of polystyrene.
The liner material should be able to withstand the anticipated maximum pond
temperature, be resistant to ultraviolet radiation, and should not react with salt. Above
all it should be mechanically strong. Failure of liners has been one of the main problems
encountered with working solar ponds. More environmentally friendly liners for solar
ponds can be made from compacted clays. Not all clays are suitable to be used as
natural liners for solar ponds. Hot NaCl brine can cause some clay to flocculate,
making them more porous. Where unlined ponds can be operated effectively, the cost
of solar ponds is lowered significantly since lining is one of the main cost components.
However, in many locations pond lining is necessary, for both environmental as well
as performance reasons (Almanza and Castaneda, 1993).
Different polymeric liner materials can be used for lining the floor of a solar pond.
Very often low-density polyethylene (LDPE) and high-density polyethylene (HDPE) are
used along with natural clay for lining the floor of a solar pond. For small solar ponds
commercially available 10 m wide standard liners can be used. But for large solar ponds
with a surface area of a few hundred hectares it is advisable to make the polymer liners
on site so that they will cost less and also can be made to desired widths, depending
on the available liner-making technology. Liners made from LDPE and HDPE should
be protected against ultraviolet radiation and hence should be covered with a thin
layer of soil or sand. The best liner laying practice is to make sandwich layers of clay
and polymer liners; this will help achieve good leakproofing (Almanza and Castaneda
1993; Akbarzadeh et al., 2005).
Sodium Chloride (NaCl), often called common salt, is the most commonly used
in salinity-gradient solar ponds for construction of a salinity gradient. Magnesium
Chloride (MgCl 2 ), also known as bittern, is the second most common salt used in
the construction of solar ponds. Bittern is a by-product of a NaCl salt production
factory.
The density of sodium chloride solution can be increased up to 1300 kg/m 3 . How-
ever, this density can be increased to more than 1500 kg/m 3 if the salt used is mainly
magnesium chloride. It can be seen from Figure 7.2.1 that the solubility of the sodium
chloride is fairly constant with temperature, while that of magnesium chloride increases
(IUPAC, 2007). It would be possible to construct and maintain stable gradients with
both salts. When setting up a solar pond as an integral part of a commercial salt pro-
duction facility, it makes good economic and environmental sense to set up the solar
pond using bittern, and save the more valuable sodium chloride for salt making.
Due to the large salinity difference between the lower convective zone and the
upper convective zone, there is upward salt diffusion. To maintain the salinity gradient
the top layer of the pond is continuously washed with fresh or low salinity water, while
salt is added to the storage zone. It is important to recycle the salt extracted from the
pond by surface washing to have minimum economic and environmental impact. An
evaporation pond that is at least equal if not twice the total surface area should be
constructed next to the solar pond in order to recycle the washed salt. Alternatively the
flushed salt solution can enter a sequence of evaporation ponds in a salt production
facility.
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