Solar Ponds - A Novel Way of Energy Collection

Description and Advantages

A relatively new technology known as the Solar Pond technology is gradually beginning to emerge. Ideally speaking, these types of ponds are effective systems for a large area energy collection and storage. The predominant constituent of a solar pond is a body of saline water, within which, the concentration of the dissolved salts increases with depth. This causes a relative increase in the water density as well. Further, as the water body captures the solar radiation, the water at the bottom gets heated. This way, the density gradient suppresses the convention current, following which, the heat is trapped at the base of the pond at high temperatures.

The heat thus generated can be put to an effective use for a variety of end use applications, using the heat exchanger. This is the basic underlying technology of a solar pond, though a pond can be either shallow or deep. In principle, there are three different layers or regions in a solar pond. The top layer is the surface zone, also known as the upper convecting zone at an atmospheric temperature and with marginal salt content.

In contrast, the bottom zone is quite hot, roughly between 70-85oC and quite salty as well. In fact, this zone is responsible for the collection and storage of solar energy in the form of heat. Due to this capability, it is also known as the storage zone or lower convective zone. Finally, acting as a barrier between the two zones, is an all important gradient zone or the non-covecting zone. In this particular zone, the salinity increases, as one goes down. This difference in the salinity between the two layers creates a salinity gradient. In this way, the stable gradient zone acts as a transparent insulator, which enables the sunlight to reach the bottom zone and get trapped there. Ultimately, it is this useful energy that is derived from the pond in the form of hot brine from the storage zone.

Beyond doubt, there is no geographical constraint with adopting the solar pond technology anywhere, but locations with a large scale availability of cheap salt, water/sea water and importantly high solar radiation intensity surely make a hot choice to do so. It is for these reasons, Bhuj, a coastal location in Gujarat, today boasts of having the largest operating solar pond in the country. This 6000 m2 pond is a successful operation and has been supplying the process heat to the Kutch dairy. This specific solar pond project was developed at a total outlay of approximately Rs. 9 million, with the collaborative efforts of Gujarat Energy Development Agency (GEDA), Tata Energy Research Institute (TERI) and the Gujarat Dairy Development Corporation (GDDC).

Of course, the design of a solar pond of this magnitude involves a fair amount of civil construction work as well, apart from incorporating the solar thermal technology principles. The Bhuj solar pond is about 100 m long and 6 m wide and has a depth of about 3.5 m. To complement this facility, evaporation ponds (for recycling salt), a 225 m2 test solar pond and a salt mixing pond were also put in place. Moreover, a specially designed lining scheme comprising of an effective combination of the China clay and polyethylene films was used as a preventive measure, against any seepage of saline water. Following which, the pond was filled with water and about 400 tonnes of common salt was dissolved in it to make a dense brine.

Area Of Application

The pond has been supplying the process heat to the Kutch dairy. About, 80,000 liters of hot water at a temperature of about 70oC is being made available to this dairy on a daily basis. However, this is not the sole use of the solar pond technology, as there are numerous other applications possible as well. These include producing the hot air for industrial and space heating applications. This hot air can be subsequently used for drying the agriculture produce, fish and chemicals, as well as for seasoning of the timber. On the other hand, the solar pond based desalination system also offer a viable solution, for yielding the sweet water from the brackish/seawater. Also, the vapour absorption system is ideally suited for operating with the solar pond. Importantly, the thermal energy stored in a pond can be used for producing the electrical power.

Source: PTI Science Service, March, 2001