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
The 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,
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
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,
the bottom temperature can be well
Problems with Solar
Despite 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
that penetrate the surface water of the pond.
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
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
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.
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
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
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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