The principle of the solar updraft tower is as simple as the fact that heat rises. Even the basic configuration of a solar updraft "chimney" is simple.
The tower stands at the center of huge plastic sheet that's supported several feet off the ground on posts. As the sun shines down on the plastic -- the "greenhouse" -- the air beneath it heats up rapidly and looks for a place to escape. The only outlet is through the narrow tower in the center. The air rises into turbines that drive generators, producing electricity but no greenhouse gases.
One particular charm of the updraft tower is that it even works at night, as well as on cloudy days and periods of rain, making it the most efficient of available wind-power sources. This efficiency depends on the difference between the temperature under the material and the temperature at the top of the tower – the greater the differential, the faster the air will rise. During the day, the heat beneath the material reaches about 50 degrees C, warming the ground as well as the air. At night, the air is cooler at the top of the tower than it was during the day, but the ground, still radiating stored heat, keeps the air warm enough to create a temperature differential that's more than sufficient to send warm air upward and drive the turbines. The amount of energy that the tower produces -- day or night -- is proportional to the area of material and the height of the tower – the larger the greenhouse and the taller the tower, the greater the energy will be.
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