Here is a pond that contains heat in layers of varied temperature—the deeper
you delve into it, the hotter it gets, seemingly defying a simple heat theory
that makes hot-air balloons fly. But this is no wonder pond, nor does it fall
outside any scientific
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| Concept of the salt-gradient solar pond |
premise. The pond is a large saline water body in which solar energy is trapped
in the salt. The salt solution gets denser with depth, making it possible to maintain
a concentrated layer of hot brine at the bottom. T ERI researchers built a salt-gradient
solar pond near Bhuj in Gujarat and put it to a unique use— supply process
heat to an enduser, for the first time in India.
The heat factory
Coming back to the heat theory, air or water, when heated, rise as they lose
weight in the process. In an ordinary water body, when sunlight heats up the
water, it rises to the surface and loses its heat to the atmosphere, keeping
the water at nearly atmospheric temperature. The solar pond technology inhibits
this phenomenon. With depth, the salt concentration increases, thereby creating
a salinity or density gradient at the middle layer—the all-important NCZ
(non-convective zone). This stable ‘gradient zone’ does not allow
the less concentrated salt water from the upper convective zone at the top to
move down and the densely concentrated salt water from the lower convective
zone at the bottom to move up. The NCZ acts as a transparent insulator that
lets sunlight reach the bottom where it remains entrapped, creating a storehouse
of thermal energy in the form of hot brine.
The Bhuj experiment
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| The salt-gradient solar pond near Bhuj |
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 TERI, 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.
Scaling new heights
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| The Bhuj solar pond covered an area of 6000 aquare metres |
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.
Carving a niche
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| Heat exchanger of the solar pond |
The hot brine – extracted from the bottom of the
pond – is pumped through a shell-and-tube heat exchanger where it heats
water up to a temperature of 70 °C. Further, this hot water was delivered
to the Kutch Dairy plant to be used as pre-heated boiler feed water as well
as for cleaning and washing. 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.
Applications/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).
Opening more windows
Other than process heating, thermal energy collected in a solar pond
can be used in many more applications.
 Process heat can
produce hot air for industrial and space heating applications.
Solar pond-based
desalination system offers cost-effective solution for production
of potable water from brackish/sea water.
Using the vapour
absorption refrigeration system, the heat contained in a solar pond
can be used for cold storage of food products and also for air-conditioning.
The hot brine can
also generate electricity, using an organic Rankine cycle engine. |
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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.
