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Comment by elago | Hacker News Reader

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 [2]https://en.wikipedia.org/wiki/Stirling_engine#Theory

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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

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.

From that article:

> 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.

"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.

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"?

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.

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 "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).

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