With the current emphasis on marketization in renewable energy utilization development of indigenous technology at the local and regional levels is being neglected. Solar pond is one such technology, which seems to be both relevant and promising for India. The solar pond is a natural energy collector and also a renewable source of energy. The first recorded use of solar ponds was made about 2500 years ago when Roman baths were heated by sun. Salt gradient pond research started in Israel in 1958,the design given by Tabor and Bloch. Solar pond research in India was initiated at the Central Salt and Marine Research Institute (CSMRI), Bhavnagar in 1971 by constructing a 1210 square meter pond. A 100 square meter pond at the field research unit of TERI, Pondicherry and a 240 square meter solar pond at The Indian Institute of Science (IIS), Bangalore, were constructed and operated around the same time. Gujarat Development Agency (GDA), Gujarat Diary Development Corporation Ltd (GDDC) and Tata Energy Research Institute (TERI) jointly executed a solar pond of area 6000 square meters. Solar ponds are environmentally friendly, since they produce energy with out creating air or water pollution or emitting harmful gases. They can also be used in winter season. However, unless sealed properly a salt gradient pond can produce pollution of land and ground water, which can be particularly harmful in agricultural area. Salt gradient solar pond can actually have a short-term beneficial effect that is utilization of unwanted salt. An essential requirement of the solar pond is its requirement of large areas of land. Hence inexpensive land, salt and water resources are essential for economic viability. However, the use of solar pond is an effective solution for the energy crisis we are facing at present.

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A solar pond is large-scale solar thermal energy collector with integral heat storage for

supplying thermal energy. A solar pond can be used for various applications, such as process

heating, desalination, refrigeration, drying and solar power generation.

Description

A solar pond is simply a pool of saltwater which collects and stores solar thermal energy.

The saltwater naturally forms a vertical salinity gradient also known as a "halocline", in

which low-salinity water floats on top of high-salinity water. The layers of salt solutions

increase in concentration (and therefore density) with depth. Below a certain depth, the

solution has a uniformly high salt concentration.

There are 3 distinct layers of water in the pond:
The top layer, which has a low salt content.
An intermediate insulating layer with a salt gradient, which establishes a density gradient

that prevents heat exchange by natural convection.
The bottom layer, which has a high salt content.

If the water is relatively translucent, and the pond's bottom has high optical absorption,

then nearly all of the incident solar radiation (sunlight) will go into heating the bottom

layer.

When solar energy is absorbed in the water, its temperature increases, causing thermal

expansion and reduced density. If the water were fresh, the low-density warm water would

float to the surface, causing a convection current. The temperature gradient alone causes a

density gradient that decreases with depth. However the salinity gradient forms a density

gradient that increases with depth, and this counteracts the temperature gradient, thus

preventing heat in the lower layers from moving upwards by convection and leaving the pond.

This means that the temperature at the bottom of the pond will rise to over 90 °C while the

temperature at the top of the pond is usually around 30 °C. A natural example of these

effects in a saline water body is Solar Lake, Sinai, Israel.

The heat trapped in the salty bottom layer can be used for many different purposes, such as

the heating of buildings or industrial hot water or to drive an organic Rankine cycle

turbine or Stirling engine for generating electricity.

The largest operating solar pond for electricity generation was the Bet Ha-Arava pond built

in Israel and operated up until 1988. It had an area of 210,000 m² and gave an electrical

output of 5 MW.[1]

Advantages and disadvantages
The approach is particularly attractive for rural areas in developing countries. Very large

area collectors can be set up for just the cost of the clay or plastic pond liner.
The evaporated surface water needs to be constantly replenished.
The accumulating salt crystals have to be removed and can be both a valuable by-product and

a maintenance expense.

No need of a separate collector for this thermal storage system.

Efficiency

The energy obtained is in the form of low-grade heat of 70 to 80 °C compared to an assumed

20 °C ambient temperature. According to the second law of thermodynamics (see Carnot-cycle),

the maximum theoretical efficiency of a power plant's heat engine is:

1-(273+20)/(273+80)=17%. By comparison, a solar concentrator system with molten salt

delivering high-grade heat at 800 °C would have a maximum theoretical limit of 73% for

converting absorbed solar heat into useful work (and thus would be forced to divest as

little as 27% in waste heat to the cold temperature reservoir at 20 °C). The low efficiency

of solar ponds is usually justified with the argument that the 'collector', being just a

plastic-lined pond, might potentially result in a large-scale system that is of lower

overall levelised energy cost than a solar concentrating system.

Development

Further research is aimed at addressing the problems, such as the development of membrane

ponds. These use a thin permeable membrane to separate the layers without allowing salt to

pass through.


read more
http://en.wikipediawiki/Solar_pond
http://images.googleimages?hl=en&client=opera&rls=en&hs=YmF&q=solar+ponds&um=1&ie=UTF-8&ei=Lr12SpfXGs-IkQW8xZmnDA&sa=X&oi=image_result_group&ct=title&resnum=4
http://edugreen.teri.resexplore/renew/pond.htm
http://mhathwar.tripodthesis/solar/solar_ponds.html

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