6 ms·

From the FAQ. His answer is: > Air inside the chimney is not affected by adiabatic cooling. Unlike freely rising parcel of air, the air in the chimney is restr

From the FAQ. His answer is: > Air inside the chimney is not affected by adiabatic cooling. Unlike freely rising parcel of air, the air in the chimney is restricted in its horizontal expansion and thus, it is not free rising. When air rises in the chimney, it also expands but only into upper direction. It compresses the layer of the air above it, heats it up and loses its own heat. At the same time the air below does the same thing. And that how it goes all the way until the chimney exit: layers of air are being pushed and push themselves. That results in maintaining the same amount of heat in every layer of air, and that is why the chimney works.

mikeash 9y ago
Sounds like complete bullshit to me. Expanding upwards will accelerate the air above, not compress it.
- deleted 9y ago
  [deleted]
- erikpukinskis 9y ago
  Why not both? And why can't you state your analysis without calling other analyses offered in good faith bullshit?
  - mikeash 9y ago
    This site is pretty much on the same level as sites advertising perpetual motion machines. I don't need every little detail to observe that it's bullshit. Unless you confine a parcel of air in all directions, it will match the pressure of the surrounding air. Consider an arbitrary cubic meter of air at 15°C at sea level at the base of the tube. Now raise it 1,000m inside the tube. That air now occupies roughly 1.12 cubic meters and is at a temperature of roughly 5°C, the same as if it had risen 1,000m outside the tube. The tube will work sometimes, but only when the atmosphere is unstable. Since the chimney is 5km tall, that means thunderstorm conditions. You can only extract energy from a tube like this if there's some energy potential between the two ends. The atmosphere is constantly erasing differences in energy potential, so large-scale differences are ephemeral.
    - elago 9y ago
      >You can only extract energy from a tube like this if there's some energy potential between the two ends. The difference in temperature between the air at the bottom of the tube and the top of the tube is that potential.
      - mikeash 9y ago
        It's not, because it's canceled out by the pressure difference. The net energy change of raising a parcel of air in the atmosphere is roughly zero. (Depending on the exact temperature profile of the atmosphere at any given moment, of course.)
        deleted 9y ago
        [deleted]
        elago 9y ago
        Consider either a thermoelectric generator[1] or sterling engine[2]. Either of these can produce power given a hot and cold reservoir. Of course neither device is appropriate for this specific application (the temperature differential between 'surface' air and air in the high atmosphere), but they demonstrate the concept that any temperature gradient represents a form of potential energy. Whether or not this specific design is an effective way to capture usable energy, I am not sure about. But, having something hot on one end, and cold on the other is a form of potential. [1]https://en.wikipedia.org/wiki/Thermoelectric_generator https://en.wikipedia.org/wiki/Thermoelectric_generator [2]https://en.wikipedia.org/wiki/Stirling_engine#Theory https://en.wikipedia.org/wiki/Stirling_engine#Theory
        wahern 9y ago
        Both of those involve effectively closed systems where heat transfer can only occur at a point where you can extract work. But what makes ambient air at the ground preferentially rise through the chimney? Real solar chimneys, as posted elsethread, have a greenhouse at the bottom. Air at the bottom of the chimney, heated by the sun, preferentially rises up the chimney because it's constrained within the greenhouse. If it were possible to "bootstrap" the system with an initial kick so that it became self-perpetuating with solar energy alone, notwithstanding changes in weather patterns, then I would think we'd also see standing hurricanes and tornados. But we don't because, I assume, these phenomena develop precisely because they're highly efficient at dissipating energy; and they dissipate it faster than a stable system can setup which preserves the initial constraints which developed (e.g. large scale climatic pressure differentials, boundary layers, etc).
gus_massa 9y ago
That explanation doesn't make any sense. If you put that in a Physics exam midterm the T.A. give you an F-. One classic error is to try to analyze each part separately using handwaving to estimate how strong is each effect and get the result you wish. It's always better to use conservation rules to analyze the global effects altogether, in this case the Bernoulli Equation and the adiabatic process laws. The pressure in the chimney is not constant, because it's very high. Air is actually a good insulator, so you can assume that there is not heat transfer between the layers of air. All the heating and cooling is due to the work in the adiabatic decompression. So the air will not be at a constant temperature.
- labster 9y ago
  As a former atmospheric science T.A., I'd give that a D for at least knowing the word adiabatic. I'd expect adiabatic cooling from air expanding as it rises up the tube. Expansion is in the vertical direction of course. Nothing magic is going to affect atm pressure inside the tube; it's the weight of the column of air above it, just like everywhere else. I'm not so sure that the benefits of removing mixing would be significant compared to slowing from having a boundary layer all the way up. Does it work because insolation on the chimney itself heats the air inside warmer than its environment? Maybe I should just look at the paper.
  - mannykannot 9y ago
    > I'm not so sure that the benefits of removing mixing would be significant compared to slowing from having a boundary layer all the way up. For practical purposes, the expansion of a rising thermal can be treated as adiabatic, because the mixing is not large in comparison to the volume. One consequence of the flawed argument presented on the website is that this system will not run continuously, but only when the lapse rate is that of adiabatic expansion (dry or wet, depending on the relative humidity) - i.e. the same condition as for natural convection (and if there is condensation in the tube, that complicates the matter.) On the other hand, I suppose, if the proposal has some validity, that pumping air into the tube to raise it through an inversion might allow convection to start where it has not done so naturally, or to trigger conditional convection (where the air is buoyant only once saturated, on account of the release of latent heat as it rises further.)) I am also wondering about the Venturi effect, and the assumption that the tube will support itself against outside pressure with a 300mph wind blowing through it - though that figure comes from what appears to be a fatally flawed calculation.

mikeash 9y ago

Sounds like complete bullshit to me. Expanding upwards will accelerate the air above, not compress it.

deleted 9y ago
[deleted]
erikpukinskis 9y ago
Why not both? And why can't you state your analysis without calling other analyses offered in good faith bullshit?
- mikeash 9y ago
  This site is pretty much on the same level as sites advertising perpetual motion machines. I don't need every little detail to observe that it's bullshit. Unless you confine a parcel of air in all directions, it will match the pressure of the surrounding air. Consider an arbitrary cubic meter of air at 15°C at sea level at the base of the tube. Now raise it 1,000m inside the tube. That air now occupies roughly 1.12 cubic meters and is at a temperature of roughly 5°C, the same as if it had risen 1,000m outside the tube. The tube will work sometimes, but only when the atmosphere is unstable. Since the chimney is 5km tall, that means thunderstorm conditions. You can only extract energy from a tube like this if there's some energy potential between the two ends. The atmosphere is constantly erasing differences in energy potential, so large-scale differences are ephemeral.
  - elago 9y ago
    >You can only extract energy from a tube like this if there's some energy potential between the two ends. The difference in temperature between the air at the bottom of the tube and the top of the tube is that potential.
    - mikeash 9y ago
      It's not, because it's canceled out by the pressure difference. The net energy change of raising a parcel of air in the atmosphere is roughly zero. (Depending on the exact temperature profile of the atmosphere at any given moment, of course.)
      - deleted 9y ago
        [deleted]
      - elago 9y ago
        Consider either a thermoelectric generator[1] or sterling engine[2]. Either of these can produce power given a hot and cold reservoir. Of course neither device is appropriate for this specific application (the temperature differential between 'surface' air and air in the high atmosphere), but they demonstrate the concept that any temperature gradient represents a form of potential energy. Whether or not this specific design is an effective way to capture usable energy, I am not sure about. But, having something hot on one end, and cold on the other is a form of potential. [1]https://en.wikipedia.org/wiki/Thermoelectric_generator https://en.wikipedia.org/wiki/Thermoelectric_generator [2]https://en.wikipedia.org/wiki/Stirling_engine#Theory https://en.wikipedia.org/wiki/Stirling_engine#Theory
        wahern 9y ago
        Both of those involve effectively closed systems where heat transfer can only occur at a point where you can extract work. But what makes ambient air at the ground preferentially rise through the chimney? Real solar chimneys, as posted elsethread, have a greenhouse at the bottom. Air at the bottom of the chimney, heated by the sun, preferentially rises up the chimney because it's constrained within the greenhouse. If it were possible to "bootstrap" the system with an initial kick so that it became self-perpetuating with solar energy alone, notwithstanding changes in weather patterns, then I would think we'd also see standing hurricanes and tornados. But we don't because, I assume, these phenomena develop precisely because they're highly efficient at dissipating energy; and they dissipate it faster than a stable system can setup which preserves the initial constraints which developed (e.g. large scale climatic pressure differentials, boundary layers, etc).

gus_massa 9y ago

That explanation doesn't make any sense. If you put that in a Physics exam midterm the T.A. give you an F-. One classic error is to try to analyze each part separately using handwaving to estimate how strong is each effect and get the result you wish. It's always better to use conservation rules to analyze the global effects altogether, in this case the Bernoulli Equation and the adiabatic process laws. The pressure in the chimney is not constant, because it's very high. Air is actually a good insulator, so you can assume that there is not heat transfer between the layers of air. All the heating and cooling is due to the work in the adiabatic decompression. So the air will not be at a constant temperature.

labster 9y ago
As a former atmospheric science T.A., I'd give that a D for at least knowing the word adiabatic. I'd expect adiabatic cooling from air expanding as it rises up the tube. Expansion is in the vertical direction of course. Nothing magic is going to affect atm pressure inside the tube; it's the weight of the column of air above it, just like everywhere else. I'm not so sure that the benefits of removing mixing would be significant compared to slowing from having a boundary layer all the way up. Does it work because insolation on the chimney itself heats the air inside warmer than its environment? Maybe I should just look at the paper.
- mannykannot 9y ago
  > I'm not so sure that the benefits of removing mixing would be significant compared to slowing from having a boundary layer all the way up. For practical purposes, the expansion of a rising thermal can be treated as adiabatic, because the mixing is not large in comparison to the volume. One consequence of the flawed argument presented on the website is that this system will not run continuously, but only when the lapse rate is that of adiabatic expansion (dry or wet, depending on the relative humidity) - i.e. the same condition as for natural convection (and if there is condensation in the tube, that complicates the matter.) On the other hand, I suppose, if the proposal has some validity, that pumping air into the tube to raise it through an inversion might allow convection to start where it has not done so naturally, or to trigger conditional convection (where the air is buoyant only once saturated, on account of the release of latent heat as it rises further.)) I am also wondering about the Venturi effect, and the assumption that the tube will support itself against outside pressure with a 300mph wind blowing through it - though that figure comes from what appears to be a fatally flawed calculation.