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Solar pond technology full report
Post: #1

Solar pond technology



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Introduction
Solar energy is an abundant and renewable energy source. The annual solar energy
incident at the ground in India is about 20,000 times the current electrical energy
consumption. The use of solar energy in India has been very limited. This is because
solar energy is a dilute energy source (average daily solar energy incident in India is
5 kWh/m 2 day) and hence energy must be collected over large areas resulting in high
initial capital investment; it is also an intermittent energy source. Hence solar energy
systems must incorporate storage in order to take care of energy needs during nights
and on cloudy days. This results in further increase in the capital cost of such systems.
One way to overcome these problems is to use a large body of water for the collection
and storage of solar energy. This concept is called a solar pond.

Principle of a solar pond

In a clear natural pond about 30~ solar radiation reaches a depth of 2 metres. This
solar radiation is absorbed at the bottom of the pond. The hotter water at the bottom
becomes lighter and hence rises to the surface. Here it loses heat to the ambient air
and, hence, a natural pond does not attain temperatures much above the ambient.
If some mechanism can be devised to prevent the mixing between the upper and
lower layers of a pond, then the temperatures of the lower layers will be higher than
of the upper layers. This can be achieved in several ways. The simplest method is to
make the lower layer denser than the upper layer by adding salt in the lower layers.
The salt used is generally sodium chloride or magnesium chloride because of their
low cost. Ponds using salts to stabilize the lower layers are called 'salinity gradient

ponds'. There are other ways to prevent mixing between the upper and lower layers.
One of them is the use of a transparent honeycomb structure which traps stagnant
air and hence provides good transparency to solar radiation while cutting down heat
loss from the pond. The honeycomb structure is made of transparent plastic material.
Ortabasi & Dyksterhuis (1985) have discussed in detail the performance of a
honeycomb-stabilized pond. One can also use a transparent polymer gel as a means
of allowing solar radiation to enter the pond but cutting down the losses from the
pond to the ambient. Wilkins & Lee (1987) have discussed the performance of a gel
(cross-linked polyacrylamide) pond.
In this review we discuss salinity gradient solar ponds as this technology has made
tremendous progress in the last fifteen years. Typical temperature and density profiles
in a large salinity gradient solar pond are shown in figure 1. We find that there are
three distinct zones in a solar pond. The lower mixed zone has the highest temperature
and density and is the region where solar radiation is absorbed and stored. The upper
mixed zone has the lowest temperature and density. This zone is mixed by surface
winds, evaporation and nocturnal cooling. The intermediate zone is called the nonconvective
zone (or the gradient zone) because no convection occurs here. Temperature
and density decrease from the bottom to the top in this layer, and it acts as a
transparent insulator. It permits solar radiation to pass through but reduces the heat
loss from the hot lower convective zone to the cold upper convective zone. Heat
transfer through this zone is by conduction only. The thicknesses of the upper mixed
layer, the non-convective layer and the lower mixed layer are usually around 0"5,
1 m and 1 m, respectively.



Pond construction
The site selected for the construction of a solar pond should have the following
attributes;
(a) be close to the point where thermal energy from the pond will be utilized;
(b) be close to a source of water for flushing the surface mixed-layer of the pond;
© the thermal conductivity of the soil should not be too high;
(d) the water table should not be too close to the surface.
An estimate of the area required for a solar pond (in the tropics) can be obtained
from figure 6 (adapted from Fynn & Short 1983). To minimize heat losses and liner
costs, the pond should be circular. Since a circular pond is difficult to construct, a
square pond is normally preferred. In some cases, such as the Bangalore solar pond,
the site constraints may force one to construct a rectangular pond with large aspect
ratio. For large solar ponds (area > 10,000m2), the shape will not have a strong
influence on cost or heat losses. The depth of the solar pond must be determined
depending on the specific application. The usual thicknesses of the surface, gradient
and storage zone of the pond are 0.5, 1 and 1 m, respectively. If a particular site has
low winds, one can reduce the thickness of the surface layer to 30 cm. If the temperature
required for process heat applications is around 40°C (such as hatcheries) then the
thickness of the gradient zone can be reduced to 0"5 m. Storage zone thickness higher
than 1 m may be required to take care of long periods of cloudiness.
The excavation for a solar pond is similar to that for construction of water reservoirs.
The side slope of the pond can vary between 1 : 1 to 1: 3 depending upon the type of
soil. After the excavation and bunding is completed, and before a liner is laid, one
must ensure that the area is free of sharp objects which may damage the liner when
it is being laid.
Post: #2
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Post: #3
abstract
Solar energy is an abundant and renewable energy source. In India, the annual solar power incident on land is about 20,000 times the current electrical energy consumption. Solar energy use in India is a thin energy source (the average daily solar energy event in India is 5 kilowatt / m 2 day) and therefore large amounts of energy must be collected, which will lead to higher initial capital investment. It is also an intermittent energy source, so solar energy systems need to include storage to take care of energy requirements in the days of nights and clouds. This has increased the capital cost of such systems. One way to overcome these problems is to use a large part of the water for storage and storage of solar energy. This concept is called a solar pond.

Solar pond principle
In a clear natural pond, about 30 ~ solar radiation reaches a depth of about 2 meters. This solar radiation is absorbed under the pond. The hot water below becomes lighter and therefore increases on the surface. Here the heat in this ambient air decreases and therefore, a natural pond does not get much temperature from the surroundings. If some mechanism can be prepared to stop the mixture between the upper and lower layers of a pond, then the lower layers of the temperature will be higher than the upper layers. It can be obtained in many ways. The easiest way to make lower layer density compared to the upper layer by adding salt to the lower layers is to use salt generally, because they have low cost because they have sodium chloride or magnesium chloride. Ponds that use salts to stabilize the lower layers are called 'salinity slope'. There are other ways to prevent mixing between upper and lower layers.
 

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