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Solar Ponds Will Determine the Future of Energy Creation

One of the most interesting ways to generate solar thermal energy is through the use of a solar pond. A solar pond is a pond full of saltwater that is used for capturing and storing solar heat.

The effect of solar ponds also occurs in natural salt lakes, from which the technology derives. A natural solar pond, where the effect is really noticeable is at the shore of the Red Sea in Egypt, at Solar Lake. Due to the effect of solar heat the lake can get as hot as 140°F

It is a relatively inexpensive form of solar power that is especially used in rural areas for heating, refrigeration, drying, solar power generation as well as for other purposes.

How Does a Solar Pond Work?

The density of water normally goes up as the depth of the water increases, which means the temperature rises, as warm water is normally lighter than cold water. When the hot water rises to the surface, the heat from the water is quickly released to the surrounding water given that this is colder. 

solar pondsThe cooled drops are pressed back down again by the remaining hot water, and a circular flow is established, causing quick cooling. This effect can clearly be felt in hot baths. 

This effect, known as convection, is a major issue in heating bodies of water. For instance, a pool loses almost all of its heat through evaporation as any heat it absorbs from the sun stays at the surface and it quickly lost.

The above upward circulation flow comes to a halt when the hot water has a high salt content (salinity), since the density of water increases drastically with salinity. This means that by using salt water, you are able to counteract this circulation and allow hot water to stay towards the bottom of the pond. 

This allows solar ponds to hold in their heat, as it is not exposed to the surface where it would instantly be cooled down by evaporation. 

In a solar thermal pond, this salt is deposited from a store at the bottom of the pond. Instead of a circulation flow, a stable temperature stratification is formed in the water, so the heat stay in the lower depths of the water and is not subject to convection which causes the hot water to rise and eventually evaporate. 

The water in a solar thermal pond is layered based on the level of salinity. At the surface the saline water will be coldest and have the smallest salt content, and the bottom layer has the highest salt content so is therefore the best insulator. The middle layer basically creates a barrier between the warm water on the bottom and the cooler water on top. So a solar pond can end up have a surface temperature of less than 60°F, while the bottom temperature can be well over 120°F.

Problems with Solar Thermal Ponds

solar pondsDespite the reduced heat losses in salt water ponds, the water will eventually lose heat, until heat is supplied to it. This heat must then be carried to the lower levels parts of the water. This is achieved with naturally occurring solar rays that penetrate the surface water of the pond. 

Alternatively cold salt water can be removed from the surface of the solar pond and pass through external heating, i.e. a solar collector, and then supplied back. 

The stratification of the water in the pond will keep the water with a lower salinity on top and allow the sun’s rays to pass through into the lower layers. This is due to the fact that solar thermal ponds are constructed with a bottom which is designed specifically to absorb the sun’s rays. The bottom of a solar pond should be very sun absorbent, which can be achieved with a black cover on the bottom or with clay soil. 

This also means that the water in the solar thermal pond must be very clear and translucent to allow the sun’s rays to penetrate as deeply into the water as possible. Cloudy or murky water in the pond would stop the sun’s rays from penetrating into the water which would limit the amount of solar thermal heat that is generated and stored by the pond.

If water loss is due to evaporation at the surface and filtration of the surface is not continuously compensated for by a supply of fresh water, or if crystallized salt is not removed from the bottom of the pond, the pond will dry out over time or eventually become filled with salt. 

solar pondsThis is what has happened to natural salt flats and salt works. In solar ponds, this is obviously an undesirable effect. To stop this from happening, it is possible to install a cover over the surface of the pond

A cover allows the pond to withstand storms and can also help to raise temperatures in the body of water when it is properly placed over the surface of the water. The cover is more transparent to visible radiation from the sun than to infra-red radiation, and is anchored and sealed around the border of the solar pond and across the surface of the body of water. 

 
The only negative is that this type of cover reduces the pressure between the bottom of the cover and the surface of the water, which can cause issues with the pond. Therefore it is recommended to flood the cover with water periodically and drain it again to negate this effect. Despite these few problems, solar ponds are still a very efficient way of generating solar thermal energy, and their use should increase dramatically in the years to come.
 


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