4 ms·

Imagine a normal 200m solar updraft chimney at elevation 0km; it sustains enough updraft to work, and spans 0 to 0.2km. Now imagine another 200m chimney at elev

by mrb 9y ago

Imagine a normal 200m solar updraft chimney at elevation 0km; it sustains enough updraft to work, and spans 0 to 0.2km. Now imagine another 200m chimney at elevation 0.2km; it too is self-sustaining and spans 0.2km to 0.4km. If we stacked these 2 chimneys on top of each other, it would be a self-sustaining 400m chimney. Repeat this for 5km, and it's a self-sustaining super chimney. But according to you this physical mechanism somehow breaks between 200m and 5km... The glass bell may be necessary for a 5km chimney but I don't see why this would invalidate the physical principle behind it.

mikeash 9y ago
It's not self sustaining, it's sustained by the sun. When the sun goes down, the chimney quits. For a 200m chimney that supports itself physically, that's fine. For a 5km chimney that relies on continuous airflow to hold itself up, that's substantially less fine.
- mrb 9y ago
  "it's sustained by the sun." That's what I meant. But you fail to explain why, if the principle works for a series of 200m chimneys at different elevations, it would somehow stop working if these chimneys were connected/stacked on top of each other. I'm not arguing about the engineering difficulty of building a self-supporting chimney made of fabric, or whether or not it works at night. Just the thermodynamic aspect of it even working during the day.
  - mikeash 9y ago
    Because it doesn't stop working. A tall chimney still works. It works during the day just fine, as long as the atmosphere is sufficiently unstable, or you can concentrate sunlight to get excessive heating at the base of the chimney. But the web site linked here claims that their super-chimney can work forever by exploiting the temperature difference between the bottom and top of the chimney. This is nonsense. It justifies this by saying that the air in the chimney somehow doesn't expand and cool as it rises the way that air outside the chimney does. This is bullshit.
    - mrb 9y ago
      Your logic seems to make sense but how would you explain the purported chimney effect that has been observed on https://en.wikipedia.org/wiki/P%C4%B1narg%C3%B6z%C3%BC_Cave#Wind https://en.wikipedia.org/wiki/P%C4%B1narg%C3%B6z%C3%BC_Cave#... which is a cave with openings at low and high elevations and the difference in temperature is claimed to create "constant wind of up to 166 km/h"?
      - mikeash 9y ago
        I'd need more information to say, but I bet it's not actually constant. I had trouble digging up any concrete info on the nature of this cave's wind.
  - gus_massa 9y ago
    In a chimney there is a difference between the pressure at the bottom and at the top. If you move some "isolated" block of air from the bottom to the top, the reduction of pressure makes it cooler, because you can assume that there is very few heat interchanged and it's an adiabatic process. (It doesn't matter too much if the air is in the real chimney, or in an imaginary chimney nearby, or in a very soft balloon, ...) In a normal chimney the difference of pressure is small, and the difference in temperature is small. So you can usually just ignore them. If you have hot air at the bottom, you can simply subtract the temperature of the surrounding air that is essentially equal at the top or at the bottom and be happy with that result. If you put a lot of this chimneys together, the difference between the temperature of the air that enter at the bottom and the air that exits at the top is big enough that it can't be ignored. I don't have hard numbers now, but assume that in a 200 chimney you have a ~1% difference that you can safely ignore. With 25 chimneys (5000m/200m) you have a ~25% difference in the temperature that you can't ignore. So you can't compare the temperature of the air that enters the chimney at the bottom, with the temperature of the air that is surrounding the top of the chimney. The chimney can work if the corrected temperature at the top after the expansion in the chimney is bigger than the temperature of the air around the top of the chimney. [You probably need more difference, 0.000001°C will not be enough.] The physics doesn't change, but if the chimney is short you can ignore the correction.

by mrb 9y ago

mikeash 9y ago

It's not self sustaining, it's sustained by the sun. When the sun goes down, the chimney quits. For a 200m chimney that supports itself physically, that's fine. For a 5km chimney that relies on continuous airflow to hold itself up, that's substantially less fine.

mrb 9y ago
"it's sustained by the sun." That's what I meant. But you fail to explain why, if the principle works for a series of 200m chimneys at different elevations, it would somehow stop working if these chimneys were connected/stacked on top of each other. I'm not arguing about the engineering difficulty of building a self-supporting chimney made of fabric, or whether or not it works at night. Just the thermodynamic aspect of it even working during the day.
- mikeash 9y ago
  Because it doesn't stop working. A tall chimney still works. It works during the day just fine, as long as the atmosphere is sufficiently unstable, or you can concentrate sunlight to get excessive heating at the base of the chimney. But the web site linked here claims that their super-chimney can work forever by exploiting the temperature difference between the bottom and top of the chimney. This is nonsense. It justifies this by saying that the air in the chimney somehow doesn't expand and cool as it rises the way that air outside the chimney does. This is bullshit.
  - mrb 9y ago
    Your logic seems to make sense but how would you explain the purported chimney effect that has been observed on https://en.wikipedia.org/wiki/P%C4%B1narg%C3%B6z%C3%BC_Cave#Wind https://en.wikipedia.org/wiki/P%C4%B1narg%C3%B6z%C3%BC_Cave#... which is a cave with openings at low and high elevations and the difference in temperature is claimed to create "constant wind of up to 166 km/h"?
    - mikeash 9y ago
      I'd need more information to say, but I bet it's not actually constant. I had trouble digging up any concrete info on the nature of this cave's wind.
- gus_massa 9y ago
  In a chimney there is a difference between the pressure at the bottom and at the top. If you move some "isolated" block of air from the bottom to the top, the reduction of pressure makes it cooler, because you can assume that there is very few heat interchanged and it's an adiabatic process. (It doesn't matter too much if the air is in the real chimney, or in an imaginary chimney nearby, or in a very soft balloon, ...) In a normal chimney the difference of pressure is small, and the difference in temperature is small. So you can usually just ignore them. If you have hot air at the bottom, you can simply subtract the temperature of the surrounding air that is essentially equal at the top or at the bottom and be happy with that result. If you put a lot of this chimneys together, the difference between the temperature of the air that enter at the bottom and the air that exits at the top is big enough that it can't be ignored. I don't have hard numbers now, but assume that in a 200 chimney you have a ~1% difference that you can safely ignore. With 25 chimneys (5000m/200m) you have a ~25% difference in the temperature that you can't ignore. So you can't compare the temperature of the air that enters the chimney at the bottom, with the temperature of the air that is surrounding the top of the chimney. The chimney can work if the corrected temperature at the top after the expansion in the chimney is bigger than the temperature of the air around the top of the chimney. [You probably need more difference, 0.000001°C will not be enough.] The physics doesn't change, but if the chimney is short you can ignore the correction.