Heating the Air:

The solar collector converts the energy from sunlight into heat. The heat from the collector causes the air above the collector to also increase in temperature. The purpose of the solar collector is to heat the air so that it will rise, creating a wind. The energy in the wind is converted to electricity using wind turbines.

The increase in temperature of the solar collector is what drives the entire system. The higher the temperature of the collector, the higher the temperature of the air, and the more wind power the system can generate.

How hot will the collector get? In small scale tests, an area of 36 square feet was covered with black ceramic. The ceramic in direct sunlight increased in temperature as much as 40 degrees Centigrade (72 degrees Farenheight) above ambient temperature (26 degrees Centigrade; 80 degrees Farenheight). The test took place at 42 degrees latitude in August. Estimated solar radiation for that place and time is 5 to 6 kWh/m2 per day. The amount of solar radiation would be significantly higher in the southwestern U.S., where daily solar radiation values reach 7 to 8 kWh/m2 per day. At such locations, the temperature of the black ceramic solar collector would also be significantly higher.

A 100 Megawatt solar chimney power plant is projected to have an increase in air temperature of 35.7 degrees Centigrade (The Solar Chimney, Jorg Schlaich, Edition Axel Menges, p. 37). This comparison suggests that the small scale tests showed a high enough increase in temperature to drive a Wind From The Sun power plant.

A large-scale collector should give an even greater increase in temperature than 40 degrees Centigrade. A small collector loses some heat to its surrounding perimeter. A large collector has less perimeter per unit area and so loses less heat. The collector will be hotter towards its center.

In other words, an area of ceramic heated by the sun gets hotter if it is surrounded by more hot ceramic. The surrounding hot ceramic keeps the ceramic within from losing much of its heat. A very large area of land covered with black ceramic gravel should theoretically increase in temperature to a much greater extent than a small area of land. (The effect would be somewhat like a solar oven, where the temperature increases as long as the sun's light adds more energy than is escaping from the oven's sides.)

The center of the collector should therefore reach a higher temperature than the outer edges of the collector. This effect is important because it causes the air flow over the collector to move horizontally, inwards from the edges of the collector, then increasingly upwards as the air moves towards the center. The air flows over the collector, causing the air to increase in temperature, resulting in a faster updraft wind speed and more wind power in the system overall. See Hot Air Rises for more details on this point.

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