"The inside and outside air will be rising up. However, the air outside will be cooling adiabatically, so its temperature will be dropping. The air inside will be not affected by adiabatic cooling and will maintain its energy, so it will be warmer and less dense than outside air."
Is this a joke, or a crazy person? Air in the tube will expand and cool just like air outside the tube does.
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.
> 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.
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.
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.
> 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.
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.
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.)
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.
Sure, you can potentially exploit the differential with other sorts of devices. But you can't do it using the same fluid that already contains the temperature difference, and attempting to use that temperature difference to drive motion in the fluid.
A generator that runs on heat differentials works by transferring heat from hot to cold and arranging it so that transfer does something useful. In a stable atmosphere, air rising through an open tube doesn't transfer heat, because it cools as it rises at the same rate that the surrounding atmosphere cools.
Why do you keep denying the principle when people have already pointed out that solar updraft towers (which exploit the exact same principle) exist and work? https://en.m.wikipedia.org/wiki/Solar_updraft_tower
> The chimney had a height of 195 metres (640 ft) and a diameter of 10 metres (33 ft) with a collection area (greenhouse) of 46 hectares (110 acres) and a diameter of 244 metres (801 ft), obtaining a maximum power output of about 50 kW.
That's not a super chimney. It's a normal chimney. People believe in normal chimneys! You have a temperature difference between the bottom and the top of the chimney, but it is so short tat you can assume that the pressure of the surrounding air is constant, and that the pressure of the air in the chimney is constant.
When the chimney is very high, the pressure inside and outside the chimney changes with the height, so the temperature inside and outside changes with the height. So to compute the difference in the temperature you have to adjust it. They are implicitly using that the temperature changes outside and ignoring that it will change inside too.
Also, the design/experiments in Wikipedia use a large glass bell or something similar to trap the solar heated air and send it to the chimney. So they are replacing a good fire of firewood with solar power. But they ignore the HUGE glass bell in the project and in the video. They expect that the hot air will go spontaneously into the chimney instead of trying to go up in another path.
Trying to harvest some of the energy and also using the air column to keep the column up will make the hot air in the surface prefer to go in another path and ignore the chimney. Unless you put a huge glass bell to force the air path.
By the way, the design/experiments in Wikipedia use self porting towers, they don't use the same air to keep the structure up. I think that the idea of using the flow to keep the structure up is very fishy, but my handwaving is not powerful enough to be sure it's wrong, I'd like to see some calculations. But you surely need the glass bell too.
"They are implicitly using that the temperature changes outside and ignoring that it will change inside too."
I just want to point out that they are quite explicit about this. That's what prompted my original comment, and I quoted it there. That's why I can't figure out why it's not obvious to everyone that it's full of crap. They all but come right out and say, "This idea is full of crap because it's based on a terrible assumption which you can see right here."
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.
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.
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.
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.
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.
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#... 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"?
That link doesn't support the article's thesis at all. It describes a system where the low-lying air has been heated up to be significantly warmer than surrounding atmospheric air. The energy being extracted is coming from solar power, not from the temperature difference at different levels of the atmosphere.
Because "the principle" involves operating 24/7 to support itself based on the temperature difference between the top and bottom, and handwaving that the air inside will somehow magically not expand and cool like it would outside.
Obviously solar updraft towers work, but they exploit the temperature difference between air at the base of the chimney and air outside the chimney. Completely different.
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).
Waterseer is a different impossible idea: They ignore that they need to remove a lot of heat to transform water vapor into liquid vapor. And that there is not so much water in the air anyway. [Clouds are huge.]
SuperTallChemney: They ignore that in equilibrium the column of air is has different pressures and temperatures that are related by the adiabatic process laws.
Anyway, it's interesting that both project try to be people friendly and offer a lot of water for the desert. In the SuperTallChimney it's not very clear if the plan is to put it in the middle of a dessert [1] or nearby a wet place that usually has no so big temperature variations.
[1] As another user commented, desserts are very hot during the day and very cold at night. What is the plan to keep the chimney working at night?
Ummmmmmmmm.... You do know about thunderstorms and other clouds with unstable vertical develompment, right? Because that's how they work, by warm humid air violently rising into cool drier air.... Don't forget about the latent heat of condensation. This is just a thunderstorm in a sock.
.... That does however mean it would need humidity to operate... The adibiatic cooling alone would be a wash, I'm pretty sure. Perhaps differences in radiative cooling between altitudes would be a help.
But.... Even if the self powering potential is not there to maintain the chimney by itself, what's to stop us from putting 1gw of solar panels and powering the thing with fans? If the heat transfer numbers aren't wrong.... That could still be a very useful characteristic.
The system described operates continuously and supports itself with that constant airflow. Unless you live in a place where thunderstorms happen all day every day, it won't work.
Is this a joke, or a crazy person? Air in the tube will expand and cool just like air outside the tube does.