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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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