If you can bootstrap the thing at all, the air in the tube can only expand and cool by going up the tube. If it expands at the base, it runs into air at the same pressure. If it didn't already have upward momentum, that might be the end of it, but if it's already moving upward into a region where the outside temp/pressure is lower, if it tries to expand, it can only do so upwards. Since air inflow at the bottom has to equal air out the top in equilibrium, that expansion can only push air through the tube faster, which helps the concept's viability. Granted, this is a slightly different theory of operation, but it uses the same basic logic of exploiting the pressure difference by limiting the ability of the air to expand freely.
He does say a lot of things that are goofy (you can't just assume the outside air is rising, still air is a thing that happens), and the theory I'm putting forward is a bit different than his, but I'm just saying I'm not sure you can dismiss the whole concept on thermodynamic grounds. IANAPhysicist, so I'm open to clarification on any point.
"if it tries to expand, it can only do so upwards."
There also can be an inversion inside the tube. For example, air might cool more on the shadow side of the tower, and start going down there.
I think it is 'obvious' that that will happen if you make the chimney wide enough (as a thought experiment, make it 2000 km wide, treat the outside of the chimney as the inside of a chimney with diameter of close to the circumference of the earth, or consider the case of a kilometer high wall on the equator with air intakes at the bottom. You argument that air must go up on both sides of it)
Question is whether the proposed chimney is wide enough for that to happen, and to what extent.
Air will accelerate as it moves up the tube, but that doesn't do anything useful for us. If the pressure drops in half then your speed will double, so a 1m/s inflow produces a 2m/s outflow, but you can't exploit that to produce energy. Or rather you can, but only by slowing the flow. You'll either slow it to a stop, or you need a source of heat to keep it going.
Part of the idea is that the tower supports itself on the venting air, which means it has to be running non-stop. Even at night, when there's no sunlit Earth to drive it.
Yes, there are many days where a structure like this could operate for several hours off of solar heating. But that's not what's being described here.
If you do some mountaineering, you'll notice that the elevation temperature difference is persistent day and night. It is always cooler higher up at the desert latitudes described. If the sun stopped shining for a few days, then yes, the surface temperature differential would produce less and less potential.
This idea requires some creativity, but it's among the most interesting I've heard. And the science that you question is definitely sound -- the only real issue I see is finding a material to handle the stresses.
If you do some glider flying, you'll notice that atmospheric convection almost always stops well before sunset.
Yes, it's almost always cooler at higher altitudes. That's because air pressure is lower, making the air less dense. When air expands, it cools.
Because of this, merely having warm air below cold air isn't enough to make the warm air buoyant. The temperature difference needs to be big enough that it will still be warmer when it has risen to the altitude of the cold air and expanded to match the pressure there.
To state it with some jargon, the temperature difference must exceed the altitude difference multiplied by the adiabatic lapse rate, otherwise the warm air doesn't go anywhere.
So: warm air at the bottom of the chimney will rise in the chimney if and only if the temperature at the top of the chimney is a lot colder. For a 5km chimney with dry air (moisture complicates the numbers but doesn't change the principles at work), the temperature at the top needs to be 50°C lower than the temperature at the bottom just to be in equilibrium. In order for air at the bottom to experience any force upwards, it will need to be a fair bit more than 50°C warmer than the air at the top.
The web site here says that this is not an issue because the air within the chimney does not experience adiabatic cooling as the air outside the chimney does. Which is complete nonsense.
> The web site here says that this is not an issue because the air within the chimney does not experience adiabatic cooling as the air outside the chimney does. Which is complete nonsense.
Even if the temperature differential needs to be a bit higher to make this consistently effective, there are certainly ways that can be done. Such as channeling heat via thermal conductors and radiative materials at night, or consuming waste heat from industrial processes that would be happening regardless. I think it's worth exploring.
I can't find much information about those caves, but I bet the wind isn't constant.
It's worth exploring chimneys, and indeed people are. It's not worth exploring chimneys which generate airflow 24/7 without a heat source because they magically suppress adiabatic cooling of the air within.
He does say a lot of things that are goofy (you can't just assume the outside air is rising, still air is a thing that happens), and the theory I'm putting forward is a bit different than his, but I'm just saying I'm not sure you can dismiss the whole concept on thermodynamic grounds. IANAPhysicist, so I'm open to clarification on any point.