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Yes, I know there are already existing Solar Power Towers. This idea has some similarities, but is different enough for posting here, as you will see.
This is a tall tower in a desert, hundreds of meters high or more, which is both tubular and slightly funnel shaped (wider at the open top), but
at least a few meters wide at the bottom, which is also open. Some sort of remote-controlled lid may be appropriate for the top of the tower on rainy days. The entire tube is transparent with anti-reflection coating; a black turbine shaft occupies the center of this tube, with attached black windmill/turbine blades. It may be appropriate to include fixed blades (also black) on the interior of the tube, as is true of steam turbines in general.
A large field of sun-tracking mirrors surrounds this tower. The entire height of the tower is the intended target of the reflected light, so a little less precision is needed, than for other Solar Power Towers (where all light is focussed at a single hot spot). In this case, all the black fittings inside the tubular tower absorb solar energy and heat the air inside the tower. Hot air rises and expands; so the gentle funnel shape is required. The rising air also turns the turbine blades, of course, and eventually escapes at the top. Replacement air constantly enters at the bottom.
Sure, this thing only generates power in the daytime. But electrical loads for which power is needed also goes up in the daytime. I firmly believe that solar power facilities just naturally need to be built as "topping cycle" or "peak load" plants, and not as round-the-clock power sources. I will only change that opinion after the power grid becomes so capacious and global that solar energy shining upon one side of Planet Earth can power the other side. (It may happen, but certainly not soon.)
Australia approves world's largest solar thermal power station
http://www.cbc.ca/s...21/aus_power_020821
"The plant will consist of two parts: a massive, greenhouse-like solar collector, and a giant tower that will produce power by moving hot air through turbines." [phoenix, Oct 04 2004, last modified Oct 21 2004]
[link]
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It's called "solar thermal" and it's Baked. The linked one is bigger on the bottom than on top to provide as much air as possible. Note that the linked version provides a heat sink to allow the unit to operate overnight. |
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I don't understand why you wouldn't want to heat the air near the bottom rather than along the entire length as you suggest. Also, by allowing the air to expand in your model you may be allowing energy to escape unused. |
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phoenix, I agree that a large part of this idea is more baked than I originally thought. I shall delete it depending on later annotations. |
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However, before then, I suggest you ask yourself why steam turbines are designed to allow for expansion of steam. The answer is that MORE thermal energy can be extracted that way. Well, thermal energy is thermal energy, so if mere hot air is used instead of steam, expansion should still be allowed. Q.E.D. |
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Next, the Australian design has an APPARENT advantage in that the roofed area at the base of the tower collects solar energy without any active mirrors. But I submit that because hot air expands, much of the heated air will be lost at the edges of the covered area, while sure, a fair amount will enter the tower. Gas expansion is a three-dimensional thing, remember. And if you seal the edges at the base, to force all hot air to enter the tower, then how do you let fresh air replace the rising air, without energy-wasting forced ventilation? |
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Meanwhile, in a funnel-tower, cool air entering at the base gets heated, and that heated air expands and rises because it is easier than pushing back through the denser cooler air at the bottom -- AND, of course, there is more room up there. Since in this case the expanding air pushes against turbine blades and loses some energy, its tendency to expand is reduced (meaning a gentle funnel is the correct design). However, it gets continually re-heated by reflected sunlight! So it has every chance to rise some more, expand some more, and turn some more turbine blades, all the way to the top of the tower. |
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Fair enough. Widening the tower near the top (but after the turbines) would allow the used air to expand and evacuate faster, possibly removing the need for the next paragraph. |
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I don't know how the Aussies do it, but I would put one-way air intakes on the rim to ensure as much air went "up" as possible. This would also ensure the whole system remained as efficient as possible - no leaks. |
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I was hoping that at some point a paying customer would jump into to the middle of it, release a parachute, and be projected out the top by the rising air currents—making it a solar powered parachutist gun. And the turbine would store power so that the gun could run day and night. |
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Thanks phoenix. I thought of the one-way flaps, too, after I wrote the other annotation. I suspect there may be a bit of a flaw in such a thing, though, which is why I didn't edit it. Consider: Initially cool air at the base of the tower is heated; its pressure goes up, only to be relieved by flow toward and through the power tower. Remember that the total power produced will depend on the total MASS of air moving past the turbine blades, as well as its velocity. This will be maximal at first, but will diminish as the pressure in the base area is relieved. Only after ENOUGH pressure is relieved can more air come in through the one-way flaps. But, because of the pressure situation, only relatively small amounts of air-mass can enter, to replace the much larger mass that originally went up through the power tower. Thus the one-way flaps constitute a bottleneck to power production. Using forced ventilation to overcome that pressure will require some power, but it will be more-than-made-up by the expansion of a thereby-maintained greater mass of air. Still, I think that such trade-offs are best avoided by simply allowing the rising air to expand in a gently funnel-shaped tower. |
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I saw plans for a giant tower such as you suggest but surrounded with a field of dark material covered in water with a layer of glass sealing the water in. Above this they had another layer of glass with air in between. Air is allowed to enter on all edges of the field and leave through the tower turbine. If the Field Tower system is large, the average temperature in between the glass and inside the tower will be great enough to spin your turbines even during the night. |
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