04-05-2012, 01:48 PM
Solar pond technology
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Introduction
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 been very limited. This is because
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.
Thermal performance
The thermal performance of a solar pond can be represented in a form similar to
that used for conventional flat plate collectors. Assuming a steady state condition,
Q,=Q,-Qe,
where Q, = useful heat extracted, Qa = solar energy absorbed, Qe = heat losses.
The thermal efficiency of a solar pond can be defined as r/= (Q,/I) where I is the
solar energy incident on the pond. Thermal effÉciency can be written as r/= % - Qe/1,
where % is called the optical efficiency of the pond (Qa/I). We express Qe = Uo (Ts - T,),
where T~ is the pond storage-zone temperature, Ta is the ambient temperature and
Uo is the overall heat-loss coefficient.