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Salt - An Important Ingredient
You might wonder what salt has to do with a solar power tower, or you might want to know what the solar power tower is in the first place. A solar power tower refers to an arrangement where a huge centrally located tower is bombarded by sunlight via an arrangement of hundreds (possibly thousands) of sun reflecting mirrors so that all the available sunlight in the area gets concentrated at the top of the tower.
During your childhood you must remember distracting your friends by throwing the rays of the sun on their faces using your wristwatch or a small piece of mirror. If you have been adventurous enough to do that, you will also remember how irritated your friends used to be, which shows that they felt discomforted due to the heat and light. Just imagine the amount of energy that is concentrated, using several hundred mirrors, at a single place. It results in the temperature of that region being elevated to anywhere between 500 to 1500 degrees Celsius.
Just take a look at the picture below which shows a typical solar power tower; as you can see there is a centrally located tower and there are a large number of mirrors. Technically these reflecting mirrors are referred to as "heliostats". Another thing to be remembered is that since the sun is not stationary with respect to the earth, the heliostats need constant adjustment to ensure that they receive the sunlight and reflect it back towards the solar tower in the most efficient manner. Obviously you wouldn’t expect a team of workers adjusting these mirrors every other minute, sot this is done automatically through the use of computerized signals and attached servo motors which rotate and adjust the angle of these heliostats every few tens of seconds.
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How does it work?
You might think that I mentioned about salt in the first sentence and then completely changed topic. Actually I was building the background for talking more about it. As you know, the temperature of the top portion of the solar tower where the sunlight gets reflected reaches nearly 1000+ degrees, so there are two ways to transfer this heat to the boiler. That is through the use of water (the conventional way) plus there is another technique in which salt is used by converting it into liquid state. Salt melts at nearly 700 degrees Celsius, and it is more efficient to store heat overnight when the sun is not available or during cloudy or rainy periods. The rest of the procedure is quite traditional wherein the heat is transferred to a boiler that generates steam that powers a turbine providing motive power to electrical generators.
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Image of Solar Tower: US Department of Energy