Moreover, it incorporates its own heat storage, which extends its range
of uses.
A solar pond comprises several layers of salty water, with the saltiest layer
on the bottom (Figure 6.10), at about 1.5m deep. Sunshine is absorbed at
the bottom of the pond, so the lowest layer of water is heated the most.
In an ordinary homogeneous pond, this warm water would then be lighter
than its surroundings and would rise, thus carrying its heat to the air above
by free convection (cf. Section 3.4). But in the solar pond, the bottom layer
was initially made so much saltier than the one above that, even though
its density decreases as it warms, it still remains denser than the layer
above. Thus convection is suppressed, and the bottom layer remains at the
bottom getting hotter and hotter. Indeed there are other liquid solutions
that increase density with increase in temperature, so producing very stable
solar ponds.
Of course, the bottom layer does not heat up indefinitely but settles
to a temperature determined by the heat lost by conduction through the
stationary water above. Calculation shows that the resistance to this heat
loss is comparable to that in a conventional plate collector (Problem 6.3).
Lowest layer equilibrium temperatures of 90C or more have been achieved,
with boiling being observed in some exceptionally efficient solar ponds.
Note that to set up such a solar pond in practice takes up to several months,
because if the upper layers are added too quickly, the resulting turbulence
stirs up the lower layers and destroys the desired stratification.