A very large diameter pipe or air channel connects the area of low air pressure over the solar collector to an area of higher air pressure away from the heat of the collector. Air moves from high pressure to low pressure, creating a wind within the pipe.
The amount of wind power generated by this system depends on the air pressure difference from the center of the collector to the surrounding land. An air pressure difference of about 400 Pascals should generate a wind speed of 15 meters per second (m/s). An air pressure difference of about 700 Pascals should generate a wind speed of 20 m/s.
More than one pipe can feed air into a large collector. The diagram below shows a 4-pipe power plant. The optimum number of pipes has not been determined and will depend on the size of the collector, the amount of heat generated by the collector, the length and diameter of the pipe, and the time of year. The sun is higher in the sky and gives more sunlight in the summer than in the winter. It may be that the plant will operate more pipes in summer and fewer in winter.
The diagram below shows 4 pipes, each of which is 2000 meters in length and 175 meters in diameter. The solar collector has a diameter of 4000 meters. The pipes extend 1500 meters into the collector because the inner portion of the collector has the lowest air pressure. The pipes extend only 500 meters away from the collector. Cool air is constantly moving in towards the collector, so perhaps the pipes could extend an even shorter distance away from the collector.
The pipes shown in the diagram above are circular in cross-section. However, they could be rectangular in cross-section (see below). This shape may be less expensive and easier to build. It should also take better advantage of the heat from the collector, since it will be closer to ground level. To obtain the same cross-sectional area, these rectangular air channels (instead of pipes) would need to be about 60 meters high by 400 meters wide. Pipes of the same cross-sectional area would have a diameter of about 175 meters.
Air flows into the air channel (blue rectangles below) from the perimeter of the collector. Air flows out of the air channels near the center of the collector and rises upward. The air rises for two reasons. First, a strong updraft occurs in the center of the collector. The updraft results from the solar collector heating the large volume of air which does not travel through the air channels. Second, the air which does travel through the air channels is heated by the collector as it travels through the air channels and after it exits the air channels.
The figures above show an example of 4 pipes per collector and an example of 2 air channels per collector. However, the optimum number has not been determined yet. Perhaps such a power plant would only need one air channel to produce sufficient power. A test plant would have to be built and measurements taken in order to have sufficient data to make such a determination.
What drives the air through the pipes?
The hot air rising from the center of the collector creates and maintains a low pressure area in the center of the collector. The pipes have a constant difference in air pressure from one end to the other. The air pressure is higher away from the collector and lower towards its center.
This difference in air pressure moves air through the pipes.
Each pipe has pressure-staged wind turbines which present a kind of obstacle to the air movement. The wind turbines remove mechanical energy from the wind in the pipes and convert it to electricity. Even so, air continues to move through the pipes because the rising hot air in the center of the collector maintains a significantly lower air pressure at one end of the pipe.
The wind speed can be controlled by increasing or decreasing the size of the collector. Such a change can even be made after the power plant has been built, by covering or uncovering part of the collector's surface with a white material. In summer, the collector will tend to reach a higher temperature and produce a higher wind speed. It is possible to reduce the wind speed by covering part of the collector, thus reducing the effective size of the collector. In winter, the collector will not reach as high a temperature and so the collector can be completely uncovered to obtain maximum wind power.
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