4 ms·

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

by 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).
mikeash 9y ago
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.
- mrb 9y ago
  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 https://en.m.wikipedia.org/wiki/Solar_updraft_tower
  - fenomas 9y ago
    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.
  - gus_massa 9y ago
    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.

by elago 9y ago

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).

mikeash 9y ago

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.

mrb 9y ago
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 https://en.m.wikipedia.org/wiki/Solar_updraft_tower
- fenomas 9y ago
  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.
- gus_massa 9y ago
  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.