Accepted Papers - 3.pdf - UNESCO

of solar pond, the hot water is removed late in the
afternoon and stored in insulated reservoirs. Glazing
materials for the solar pond may include Polyvinyl
Chloride (PVC) film and clear acrylic panels. The
panels covering the plastic bags screen out
ultraviolet (UV) radiation and greatly increase the
life of the plastic bags.
Figure 2. Cross section of a shallow solar pond
4.2 Non-convective Solar Ponds
These solar ponds prevent heat losses by
inhibiting the convection to forces caused by thermal
buoyancy. In convective solar ponds, solar radiation
is transmitted through the water to the bottom, where
it is absorbed; in turn, the water adjacent to the
bottom is heated. Natural buoyancy forces cause the
heated water to rise, and the heat is ultimately
released to the atmosphere. The collector of pond
will be much more effective if the convective heat
dissipation is impeded. The non convective solar
pond is similar to the convective one but a layer of
still water is used as an insulator rather than the
normal glazing and air space. The three methods to
accomplish the non-convecting mode of the pond
and to maintain its stability.
Figure 3. Schematic of a solar pond with vertical
concentration gradient
4.2.1 Salt Stabilized Pond
It is a non-convecting body of fluid contained
by an impervious bottom liner. An artificial salt
solution density gradient is achieved by
376
superimposing layers of decreasing salinity. Batches
of brine can be mixed in a small evaporation pond,
and than pumped into the solar pond through a
horizontal diffuser floating on the surface, which is
a non-convective zone. The incident solar radiation
penetrates the water surface and a fraction of it
reaches the bottom after crossing the layers of
varying density. This energy is trapped near the
bottom due to opaqueness of water for far-infrared
gradient. The main advantages of salt-stabilized
ponds are; it serves as an efficient storage,
convective heat dissipation is suppressed without
any additives, membrane etc.
Figure 4a. Cross section of a non-convective salt solar
pond (with tedlar cover)
Figure 4b. Cross section of a prototype economic kind
of non-convective salt solar pond
Figure 4
4.2.2 Partitioned Salt stabilized pond
This pond was proposed for space heating
applications. In these ponds the convecting and nonconvecting
zones are separated by a transparent
membrane or partition. The use of partition allows
a fresh water convecting zone which can reduce the
extraction problems and reduces the salt
requirement. The main advantages of these type of
solar ponds are; they are collection-cum-storage type
ponds, bottom and top insulations are not required,
in-pond heat exchanger may be practical.