Solar ponds:

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The concept of a solar pond was first proposed in 1902. The solar pond is simple and consists of water with a very high saline content. The heated salty water can boil gases such as freon or propane. Afterwards, the heated gas can turn the blades of a turbine next to the solar pond and the turbine can then drive a generator to produce electricity. Before the electricity generation process, the following set-up is required:

• There must be a large body of water. This can be an inland sea, a lake or a big dam. If a dam is built for the purpose of electricity generation, the bottom of the dam is usually lined with black plastic. This black plastic lining will act as an efficient absorber of solar radiation.
  
  
• The water must be two to three metres deep over its entire area.
  
  
• The depth must be constant for most of the year.
  
  
• There must be a minimum of movement in the water. This means that there must be a limited movement of wind across the surface and minimal currents within the body of water.
  
  
• The water in the solar pond must be very salty, at least three times saltier than the sea. If a dam is built especially for a solar pond project, then salt must be brought to the dam to make the water salty enough. This adds to the cost of the solar pond. In Australia, salt for this purpose can be obtained from the waste products of solar salt production.
  
  
• Water in the solar pond is saltier at the bottom than at the top. The heavier, dense salt water sinks while the fresh water floats to the top. If the body of water is still, with no movement within the water and little or no wind blowing over the surface, there will be very little mixing. This makes sure that the water will stay saltier at the bottom. The bottom layers will be hotter while the surface layer remains cool.
  
  
• Concentrated salt water can reach temperatures of 80 degrees to 100 degrees Celsius, using only the sun as the heat source. In some places in the world, temperatures close to boiling can be found at the bottom of natural salt water lakes and ponds. Such places with natural salt water lakes include Central Australia, California, Israel, the Soviet Union and Africa.

There are two gradients that are important in understanding the solar pond:

• First, a salt pond gets saltier as the water deepens but it is almost fresh at the surface – a salt gradient.
  
  
• Second, the salt water is hotter at the bottom, where the salt concentration is greatest, and cooler at the top – a thermal (heat) gradient.

The pond is filled by mixing salt water in batches next to the site. The bottom layer (which has the heaviest salt concentration) is made up first and pumped onto the dam floor. Then the next layer is mixed and carefully poured onto the layer already in the dam. Slowly, the rest of the layers are mixed and poured until the pond is filled. The result is a giant, salty layer cake made of water.

Now, the solar pond must be heated by the sun and this could take months. Evaporation has the effect of keeping the surface water cool. And convection (the movement of water from one heat zone to another heat zone) brings warmer water to the surface, where it cools. The heating of the pond depends on wind speed, humidity and temperature. To some extent, these influences can be controlled and the solar pond will reach a high-enough temperature in the bottom layers.

Then the engineers get to work to produce electricity from the solar pond. This is known as a Rankine cycle. The generating plant that does this work on a solar pond has three cycles:

• There is a hot water cycle. The hot water is drawn from the hottest layer of the salt water at the bottom of the solar pond. This water goes into a boiler. Remember it has been made hot by the sun, so it does not need any other fuel to make it hotter.
  
  
• There is an organic working cycle, where the gas is boiled by contact with the hot water. Freon or propane gas is generally used because it will boil at a much lower temperature than the temperature of boiling water (which is 100 degrees Celsius). The gas then evaporates quickly and it is sent to a turbine. The pressure of the gas turns the blades of the turbine. The turbine is connected to a generator, which produces electricity.
  
  
• There is a cold water cycle. Water is drawn from the surface of the pond where it is cool. This cool water is pumped into a condenser, where it is used to cool the hot gas that has been used to turn the turbine. By cooling the gas, it is being restored to its original state. It is then ready to be used again. The water in the condenser is then returned to the surface of the solar pond.

These three cycles are in continuous motion. The hot water, gas and cool water never come into direct contact, but circulate in pipes. They do not need to be replaced. Once they have started working together, their motion is constant and there is no need for any other fuel. The heating “fuel” is free, renewable sunlight.

It takes several months of steady sunlight to make solar pond water hot enough to boil the gas, freon. Once the pond has reached its maximum temperature, the system will continue working for up to two weeks without any sunlight at all. But this depends on the thickness of the hot salt water layer in the pond. Even under cloudy conditions, the solar pond continues to absorb some energy (heat) from the sun.

It is necessary to restrict the flow of wind across the surface of the solar pond. The wind can create waves that set up water currents in the pond. These currents can carry cold water down to warmer levels, cooling the hottest water on the bottom. This would reduce the efficiency of the solar pond system. Chain mesh wire fences around the pond area, and nets on the surface, are usually used to prevent wind disturbance. When there is no disturbance, then the heaviness of the salt layers on the bottom prevents rapid movements along the heat and salt gradients or variations.

There are other problems of control. The salt and heat gradients must be monitored to prevent the salt layers from mixing together. If the salt layers mix together, then the solar pond becomes just another pond and it is highly unlikely that electricity can be generated.

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