Salinity gradient solar ponds - Solar Energy Sciences and Engineering Applications - page 203

Figure 7.3.1 The process of setting up the salinity gradient is shown schematically. Fresh or low salinity
water is injected horizontally into the body of water in the solar pond. Note that the
mixing occurs only in the region above the level of injection.
There are a number of different methods for setting up a salinity gradient in a solar
pond. The method described here is simple and used all over the world. The solar pond
is filled with fresh water up to 1.3 m from bottom; this is approximately equal to the
thickness of the storage zone and half of the gradient layer. Now solid salt crystals are
added to the bottom of the pond and allowed to dissolve to create concentrated brine
with around 25% salinity; this requires about 325 kg of salt per m 2 . The first location
of fresh water injection determines the thickness of the storage zone. If the fresh water
is injected from 1 m above the bottom then the storage zone gets a thickness of 1 m
and so on. The fresh water is injected using a diffuser as shown in Figure 7.3.1 and
Figure 7.3.2 while the diffuser is gradually moved upwards. The mixing of rising fresh
water with the high concentration brine creates the gradient layer. The diffuser should
rise 2 cm for every 1 cm rise in the level of solar pond water (Tabor, 1980; Alagao et al.,
1993).
The salt steadily diffuses upwards due to salinity differences between the storage
zone and top layer. The rate of salt diffusion depends upon the temperature and the
salinity gradient present in the solar pond. As a guideline the rate of diffusion in a
sodium chloride pond can be up to 20 kg/m 2 /year. In order to maintain the difference
in salinity between the bottom and top of the pond, salt must be continuously removed
from the surface zone by surface flushing, and an equal amount added to the storage
zone as shown in Figure 7.3.3.
A 'stability margin' number has been defined to establish an operational safety
limit for the salinity gradient within a pond. Basically the density gradient must be
kept above a certain minimum level to prevent convection currents starting and the