Definition:
Solar energy is an abundant
and renewable energy source. The annual solar energy incident at the ground in
India is about 20,000 times the current electrical energy consumption. The use
of solar energy in India has be solar energy is a dilute energy source (average
daily solar energy incident in India is 5 kWh/m 2 day) and hence energy must be
collected over large areas resulting in high initial capital investment; it is
also an intermittent energy source. Hence solar energy systems must incorporate
storage in order to take care of energy needs during nights and on cloudy days.
This results in further increase in the capital cost of such systems. One way
to overcome these problems is to use a large body of water for the collection
and storage of solar energy. This concept is called a solar pond.
Principle of a Solar Pond:
In a clear natural pond about
30~ solar radiation reaches a depth of 2 metres. This solar radiation is
absorbed at the bottom of the pond. The hotter water at the bottom becomes
lighter and hence rises to the surface. Here it loses heat to the ambient air
and, hence, a natural pond does not attain temperatures much above the ambient.
If some mechanism can be devised to prevent the mixing between the upper and
lower layers of a pond, then the temperatures of the lower layers will be
higher than of the upper layers. This can be achieved in several ways. The
simplest method is to make the lower layer denser than the upper layer by
adding salt in the lower layers. The salt used is generally sodium chloride or
magnesium chloride because of their low cost. Ponds using salts to stabilize
the lower layers are called 'salinity gradientponds'. There are other ways to
prevent mixing between the upper and lower layers.
One of them is the use of a
transparent honeycomb structure which traps stagnant air and hence provides
good transparency to solar radiation while cutting down heat loss from the
pond. The honeycomb structure is made of transparent plastic material. Ortabasi
& Dyksterhuis (1985) have discussed in detail the performance of a
honeycomb-stabilized pond. One can also use a transparent polymer gel as a
means of allowing solar radiation to enter the pond but cutting down the losses
from the pond to the ambient.
The steady-state analysis of a
solar pond is useful in the sizing of the pond for a specific application.
There will, however, be strong seasonal variation in the performance of the
pond on account of seasonal variations in solar insolation, wind and
temperature. Srinivasan (1990) has proposed a simple two-zone model for the
simulation ofthe storage zone temperature of the pond. How does this simple
two-zone model predict the observed features of the seasonal variation of
storage zone temperature in the Bangalore solar pond? The observed values of
solar radiation, heat extraction and gradient zone thickness in the Bangalore
solar pond were used in the simulation. The predictions of the storage zone temperature
are compared with the observations . The predicted storage zone temperatures
are in good agreement with observation. Predictions based on climatological
variation of solar radiation show higher deviation. This is because solar
radiation in September 1986 was much lower than predicted by climatology.
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