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Solar ponds for trapping solar energy

The sun is the largest source of renewable energy and this energy is abundantly available in all parts of the earth. It is in fact one of the best alternatives to the non-renewable sources of energy.

One way to tap solar energy is through the use of solar ponds. Solar ponds are large-scale energy collectors with integral heat storage for supplying thermal energy. It can be use for various applications, such as process heating, water desalination, refrigeration, drying and power generation.

The solar pond works on a very simple principle. It is well-known that water or air is heated they become lighter and rise upward e.g. a hot air balloon. Similarly, in an ordinary pond, the sun’s rays heat the water and the heated water from within the pond rises and reaches the top but loses the heat into the atmosphere. The net result is that the pond water remains at the atmospheric temperature. The solar pond restricts this tendency by dissolving salt in the bottom layer of the pond making it too heavy to rise.

A solar pond has three zones. The top zone is the surface zone, or UCZ (Upper Convective Zone), which is at atmospheric temperature and has little salt content. The bottom zone is very hot, 70– 85 C, and is very salty. It is this zone that collects and stores solar energy in the form of heat, and is, therefore, known as the storage zone or LCZ (Lower Convective Zone). Separating these two zones is the important gradient zone or NCZ (Non-Convective Zone). Here the salt content increases as depth increases, thereby creating a salinity or density gradient. If we consider a particular layer in this zone, water of that layer cannot rise, as the layer of water above has less salt content and is, therefore, lighter. Similarly, the water from this layer cannot fall as the water layer below has a higher salt content and is, therefore, heavier. This gradient zone acts as a transparent insulator permitting sunlight to reach the bottom zone but also entrapping it there. The trapped (solar) energy is then withdrawn from the pond in the form of hot brine from the storage zone.

Though solar ponds can be constructed anywhere, it is economical to construct them at places where there is low cost salt and bittern, good supply of sea water or water for filling and flushing, high solar radiation, and availability of land at low cost. Coastal areas in Tamil Nadu, Gujarat, Andhra Pradesh, and Orissa are ideally suited for such solar ponds.

Solar Pond at Bhuj

The Bhuj Solar Pond is a research, development, and demonstration project. The construction of the 6000 m2 pond started in 1987 at Kutch Dairy, Bhuj as a collaborative effort between Gujarat Energy Development Agency, Gujarat Dairy Development Corporation Limited, and TERI under the National Solar Pond programme of the Ministry of Non-Conventional Energy Sources. TERI carried out execution, operation, and maintenance of the Bhuj Solar Pond.

The solar pond is 100 m long and 60 m wide and has a depth of 3.5 m. To prevent seepage of saline water, a specially developed lining scheme, comprising locally available material, has been adopted. The pond was then filled with water and 4000 tonnes of common salt was dissolved in it to make dense brine. A salinity gradient was established and wave suppression nets, a sampling platform, diffuses for suction and discharge of hot brine, etc. were also installed. This pond has been successfully supplying processed heat to the dairy since September 1993, and is, at present, the largest operating solar pond in the world.

 

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