Solar Pond

Solar Pond Tutorial

The concept of solar pond was derived from the natural lakes where the temperature rises towards the bottom. It happens due to natural salt gradient in these lakes where water at the bottom is denser. In salt concentration lakes, convection does not occur and heat loss from the water takes place only by conduction. This technique is utilized for collecting and storing solar energy.

An artificially constructed pond filled with salty water in which significant temperature rises are caused to occur in the regions by preventing convection is called solar pond. The usual method adopted to prevent convection is to dissolve a salt in the water and to maintain a concentration gradient. Such ponds are also called as salt gradient solar pond.

The first solar ponds were constructed in Israel in the early sixties by Tabor and his co-workers. A maximum temperature of about 100 degree celsius was attained at the bottom of the pond. One of the largest solar ponds built so far was the 250000 m² pond at Beit Ha’aravah in Israel. The heat collected in this pond was used to generate electrical power using an organic fluid Rankine cycle. In India the first solar pond with an area of 1200 m² was built at the Central Salt Research Institute, Bhavnagar in 1973. Since then several ponds have been built and are in operation.  The largest pond in India built so far was located at Bhuj, Gujarat constructed by the Tata Energy Research Institute (TERI), and was used to supply process heat to a nearby dairy.

Working Principle

The concept of a solar pond is derived from the observation that in some naturally occurring lakes, a significant temperature rise of about 40 degree Celsius to 50 degree Celsius. This is because of the fact that there is a natural salt concentration gradient in these lakes, whereby the water at the bottom remains denser even when it is hotter than the water at the top. Thus convection does not occur and heat is lost from the hot water only by conduction. The salt concentration gradient in such lakes is maintained naturally because of the presence of salt deposits at the bottom of the lakes, which approximates to saturation concentrations because of fresh water streams which flow across the top.

The schematic diagram of solar pond is shown in above figure. The top layer remains at ambient temperature while the bottom layer attains a maximum steady state temperature of about 60 degree celsius to 85 degree celsius. As stated earlier it combines the functions of heat collection with long term storage and can provide sufficient heat for the entire year. Typically it is about two or three meters deep with a thick liner that is durable and made of plastic placed at the bottom. Materials used for the liner include low density polyethylene (LDPE), high density polyethylene (HDPE), woven polyster yarn (XB-5). Salts are dissolved in the water, the concentration varying from 20 to 30 percent at the bottom to almost zero at the top.  Left to itself the concentration gradient will disappear over a period of time because of upward diffusion of the salt. Fresh water is added at the top of the pond in order to maintain the concentration gradient, while slightly saline water is run off. Simultaneously, concentrated brine is added at the bottom of the pond. The amount of salt required for this purpose is about 50 g/m² – day, which is large quantity when considered on an annual basis. Hence the normal practice is to recycle the salt by evaporating the saline water runoff from the surface in an adjacent evaporation pond.

In order to extract the energy stored, hot water is removed continuously from the bottom passed through a heat exchanger and returned to the bottom. Alternatively heat is extracted by water flowing through a heat exchanger coil submerged at the bottom. Because of movement and mixing of the fluid both at the top and the bottom, the solar pond is characterized by 3 zones such as a surface convective zone, a non convective concentration gradient zone and a lower convective zone. The upper convective zone or surface convective zone (SCZ) usually has a small thickness around 10 to 20 cm. It has a low uniform concentration, which is close to zero, as well as a fairly uniform temperature which is close to the ambient air temperature.Both temperature and concentration increases with the depth in this zone. It serves principally as an insulating layer and reduces heat losses in the upward direction. Some of the heat collection also takes place in this zone and it serves also as part of the thermal storage. The lower convective zone (LCZ) is comparable in thickness to the non convective zone. It serves as the main heat collection as well as thermal storage medium. The lower convective zone is often referred to as storage zone or as the bottom layer.

Typically the temperature in the lower convective zone of a well designed large pond operating in India might fluctuate cyclically between a maximum value of 85 degree celsius to 95 degree celsius in summer and a minimum of 50 degree celsius to 60 degree celsius in winter. The annual collection efficiency generally ranges between 15 and 25 per cent. These values are lower than those obtained for a flat plate collector. Nevertheless solar ponds are more cost effective since their cost per square meter is much less than that of a liquid flat plate collector system. This is particularly true when the area is of the order of 1000 m² or more.

The site selected for the construction of a solar pond should have the following features:

• It should be close to the point where thermal energy from the pond is to be utilized.
• It should be close to the source of water for flushing the surface mixed-layer of the pond.
• The thermal conductivity of the soil where is pond is constructed should not be too high.
• The water table should not be too close to the surface.

Advantages of Solar Pond

• Saves money as the cost associated with it is only its initial cost. Very large area collectors can be constructed just with the cost of the clay or plastic pond liner.
• This technology is attractive for rural areas in developing countries.
• The accumulated salt crystals which is a by-product from solar pond can be used further.
• Separate collector is not needed for this thermal storage system.
• Environmental friendly.
• Flexibility of location as they can be located anywhere regardless of the distance to the nearest power outlet provided that there is access to direct sunlight near the pond site.

Disadvantages of Solar Pond

• The accumulated salt crystals have to be removed periodically and this adds maintenance expense.
• Solar ponds require a significant amount of land area to function properly.
• Can only operate in sunny days and if the site is shaded by tall trees or building, it may not work properly.
• Proper algae and dust removal is needed as the thermal efficiency of a solar pond sturdily dependents on the clarity of the pond.

Alternative types of Solar ponds

A number of other concepts have also been proposed for building a solar pond. Some of these are discussed below.

Solar Gel Pond

The concept of any solar pond centered on the presence of a non convective zone to trap the solar energy collected in the lower convective zone. In a solar gel pond, a thick layer of polymer get floats on the lower convective and acts as a non convective zone. The gel has good optical and thermal insulating properties. A small salt gradient is necessary in order that the gel would float on the top. A gel pond constructed in New Mexico has a surface area of 400 m² and was 5 meter deep. The lower convective zone was made up of 2 to 7 per cent salt solution and was 3.5 meter deep. The composition of the gel was 98.3 per cent water and 1.7 per cent polyacrylamid. It was kept in thin transparent plastic bags made from Tedlar and floated on the salt solution. The thickness of the gel layer was 0.6 m. The pond was designed to supply a minimum of 1 GJ of energy per day at a temperature of 70 degree Celsius. However the temperature actually obtained was 60 degree Celsius. In comparison with salt gradient solar pond, gel pond has the following disadvantages:

• Evaporation losses from the surface are eliminated, while heat losses are smaller in magnitude.
• Since salt water is used only to float the gel, a salt concentration gradient is not required to be maintained. Thus maintenance requirements are reduced.
• The environmental hazards associated with handling salt are eliminated.

The main disadvantage of this pond is its high cost due to the chemicals required for making the gel.

Equilibrium Solar Pond

It is special type of salt gradient pond. It uses a salt whose solubility in water increases strongly with temperature. In this pond thermal diffusion of salt occurs from lower to higher temperature zones in the fluid. This is called negative ‘Soret effect’. In this solar pond, the direction of this movement is downward and opposite to that of the mass diffusion occurring in the upward direction due to the concentration gradient. In the equilibrium pond the two mass fluxes are made to balance each other so that the net salt flux is zero. Thus the need for having regular operational procedures for maintaining the salt concentration gradient as in a salt gradient pond is eliminated. The concept has been demonstrated experimentally with potassium nitrate salt solution.

Partitioned Salt-stabilized Pond  

In his pond a transparent membrane or partition is used to separate the lower convective zone from the non convective top zone. Thus the non convective zone instabilities are suppressed. Other features are similar to the solar gel pond.

Applications of Solar Pond

• Power production: A solar pond can be used to generate electricity by driving a thermo electric device or a Rankine cycle engine. The organic working fluid produces mechanical power in a Rankine cycle, which in turn generates electrical power using an alternator.
• Industrial process heat.
• Heating animal housing on farms: In farms which have enough land for solar ponds, the low grade heat from them can be used for heating animal housing, drying crops etc. 
• Salt production: Salt is obtained as the byproduct from solar pond. Solar pond can be used for purification of salt that is for the production of ‘vacuum quality’ salt.
• Aquaculture: Using the saline water from the solar pond fishes like brine shrimp can be grown.
• Heating and Cooling of Buildings.
• Desalination: The low cost thermal energy can used to purify water for drinking or irrigation.