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Title: Salt-gradient solar ponds
Source:The Energy and Resources Institute (TERI)

Background. In wake of the looming threat from global warming and also the rising scarcity of fossil fuels, there has been an accentuated global call for maximizing the use of renewable energy. TERI researchers foresaw promising results in the indigenously developed salt-gradient solar ponds. The result was the construction of the Bhuj solar pond–an idea mooted by a group of scientists in 1983. The project was sanctioned under the National Solar Pond Programme by the Ministry of Non-conventional Energy Sources in 1987 and completed in 1993 after a sustained collaborative effort by T ERI, the Gujarat Energy Development Agency, and the GDDC (Gujarat Dairy Development Corporation Ltd). TERI provided all technical inputs and took up the complete execution of research, development, and demonstration. TERI operated and maintained this facility until 1996 before handing it over to the GDDC. The solar pond functioned effortlessly till the year 2000 when severe financial losses crippled GDDC. Subsequently, the Bhuj earthquake left the Kutch Dairy non-functional.

Figure. Concept of the salt-gradient solar pond

Scaling new heights. The Bhuj solar pond project stood out in many regards. The first-ever solar pond in India to have connected itself to an end-user – supplying industrial process heat to the Kutch Dairy – this pond, covering an area of 6000 square metres, was, at that time, the largest operating solar pond in the world. Avoiding use of imported membrane lining, the project developed a cost-effective, indigenous lining scheme, using locally mined clay and plastics. While the pond attained a record 99.8 °C under stagnation, stability of the salinity gradient was maintained even at such elevated temperatures. With only one injection diffuser on one side of the pond, the desired salinity profile was achieved even at the farthest end. More important, laboratory scale testing and the success in catering to actual user-demand have paved way for the commercial exploitation of the technology in India. The stove can be easily produced without requiring special materials or tools because intentionally no insulation is provided and the unhindered primary and secondary air flow enables sufficient low temperature for the thin steel sheets.

The entire exercise at the Bhuj solar pond successfully demonstrated the expediency of the technology by supplying 80 000 litres of hot water daily to the plant.

Outcome/Benefits: The Bhuj experiment significantly placed the solar pond option as a comparable alternative to technologies that are in use–fossil-fuel-fired process heating and solar flat plate collectors for water heating (Box 1). The capital and operating costs of the technology are very much site- and application-specific. However, these are expected to be lower than the competing technologies. For example, the cost of heat energy from a solar pond is about 60% of the cost of energy from a flat plate solar water-heating system. Process innovation like clay– plastic–clay lining reduces the construction cost. With suitable government incentives, the technology may soon become one of the most viable solar energy options.

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