Solar Pond Technology
Published on Aug 15, 2016
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 be 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 gradientponds'. 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.
The steady-state analysis of a solar pond is useful in the sizing of the pond for a specific application. There will, however, be strong seasonal variation in the performance of the pond on account of seasonal variations in solar insolation, wind and temperature. Srinivasan (1990) has proposed a simple two-zone model for the simulation ofthe storage zone temperature of the pond. How does this simple two-zone model predict the observed features of the seasonal variation of storage zone temperature in the Bangalore solar pond? The observed values of solar radiation, heat extraction and gradient zone thickness in the Bangalore solar pond were used in the simulation.
The predictions of the storage zone temperature are compared with the observations . The predicted storage zone temperatures are in good agreement with observation. Predictions based on climatological variation of solar radiation show higher deviation. This is because solar radiation in September 1986 was much lower than predicted by climatology.
In the salt-gradient ponds , dissolved salt is used to create layers of water with different densities-the more salt, the denser water .Thus a solar pond has three zones with the following salinity with depth :
1) surface convective zone (0.3-0.5m),with salinity<5% concentration
2)Non-convective zone(1-1.5m , salinity inc with depth
3)storage zone (1.5-2m , salt=20%) The Non-convective zone is much thicker and occupies more than half the depth of the pond. Both the concentration and the temp increase with the depth in it. It mainly serve as a insulating layer and reduces the heat transfer.
A some part of this zone also acts as thermal storage. The lower zone is the storage zone. Both the concentration and temp are constant in this zone. It is the main thermal storage medium. The deeper the zone. The more is the heat stored. The lowest zone traps heat energy for longer periods. The capacity to store heat for long periods is the chief advantage of solar ponds. Even in the cloudy days and in the ice covered regions the energy can be stored , since the salt water near the bottom heats up and expands. However it cannot rise to the because it is denser than the less salty water above. Hence a non convective solar pond is best utilized for storing the solar energy at a reasonable cost.
The site is selected for the construction of 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;
(c)the thermal conductivity of the soil should not be to high;
(d)The water table should not be too close to the surface.
For constructing a non convective solar ponds an insulated and double-glazed covered salt gradient solar pond having a surface area 1.6 m × 1.6 m and a depth of 1 m has been fabricated. The LCZ IS 0.5 m high from the bottom of the pond with highdensity brine. Approximately 0.1 m of fresh water on the top makes the UCZ containing light density brine , above which there is gradually decreasing density brine. This region performs the insulation , that is , keeps the stored energy in the bottom zone ,called NCZ. This zone has a height of 0.3 m . the total thickness of the side walls in which a glass wool is used for insulation is 8 c.m.The energy obtained from this system can be stored below the boiling point of brine . under the bottom of the pond a concrete with blocking with filling gravel of 0.12 thickness is fixed . The inner surface which is made by Al, is blackened to absorb and store the maximum sun’s energy .
The bottom surface area is 1.2 m× 1.2m and expanded to an exposed surface area to collect the heat energy in a large amount .The side walls of both the models were exposed at an angle of 110 from the bottom surface .The total thickness of top glazing is 1.4 cm with gap space of 1 cm.The source of water supplying in solar pond is a tank of 300-liter capacity placed at height of 5m from the bottom surface level of the pond. A galvanized iron pipe (DIA – 2M ,L=6M) is directly connected to the bottom of the pond. Below different types of salts ,and the extracted thermal and electrical power are given. Salts like sodium chloride ,magnesium chloride and sodium carbonate are considered and the power extracted from the theses sources are encompassed in the below table. Also the amount of flow rate maintained with different pond area are explicated
An estimate of the area required for solar pond 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 constrains may force to construct a rectangular pond with large aspect ratio. For large solar ponds the shape will not have a strong influence on cost or heat losses. The depth of the solar pond must be determined depending on specific application. The usual difference of the surface gradient and storage zone of the pond are 0.5,1 and 1m respectively. If a particular site has low winds , one can reduce the thickness of surface layer to 30 cm. If the temperature required for the process for the heat applications is around 40°c then the thickness of the gradient zone can be reduced to 0.5m.Storage zone thickness higher than 1m may be required to take care of long periods of cloudiness.
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