Elenco Solar Energy User Manual

Page 8

Experiment no. 9 The solar pond
At the southern tip of Israel, near the town of Eilat, is a small lake. At first glance

nothing seems remarkable about it. The water is warm and salty and not particularly

clean. However, this lake attracts scientists from all over the world because it is a

“solar pond.” If current research proves successful, this small lake may make a major

contribution towards solving the world’s energy shortage.
If the sun shines on a pool of water, the top layer becomes slightly hotter since hot

water usually rises to the top. But hot water evaporates faster than cold, and when

water evaporates, it takes heat energy with it.
You can easily test this by wetting your forearm and blowing on it. When you blow on

your wet arm, you speed the process of evaporation. You arm feels cold during this

process because heat is lost from your arm while the water evaporates. The water has

used the heat from your arm as a part of the evaporation process.
In an ordinary pool the sun shines on the water, adding heat. Then the water

evaporates, losing heat. Eventually, a balance is reached where any extra heat causes

more evaporation and the pool reaches a steady temperature.
A solar pool is different. Underground there is a salt-water well so the water at the

bottom is much more salty than the layer at the top. The top layer acts as a magnifying

glass, focusing the sun’s heat onto the lower layer. The salty water is heavier than

ordinary water, even when hot. Therefore, unlike ordinary water, the heated salty

water sinks, instead of rising. It is now strictly prohibited to bathe in this pool. Before

the scientific facts relating to this solar pond were known, swimmers who dived into the

bottom were badly scalded. Today, scientists are investigating the possibility of building

artificial solar pools in order to exploit this solar energy trapped in the salt-water layer.

You too can try out this idea. Take a coloured plastic

bowl or glass (not white.) Fill it to one-third of its

depth with water and put in as much salt as will

dissolve. If you have some ink or dark food dye,

colour this solution.
Now, carefully and slowly pour into the bowl or glass

some fresh water. Pour it slowly down one side,

using a spoon to direct the flow. It is important to

prevent the two liquids from mixing. The ink in the

salt water will tell you if you have been successful.

Let it stand in the sun for some time and then measure

the temperature at the top, the middle and the bottom.

concentrated
salt water
and ink

fresh water