Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: arumalpra on 22/12/2015 03:19:50
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Guys!
Would this be possible?
Cool a warm city by pumping cold air from hill country? In my country the coastal area town reaches 32 C in day time but the temperature of the hill side is about 18 C, just 50km away.
Don't know how far the south pole from Australia but is it technically feasible pumping air from the south pole to Australia? This will greatly reduce the energy cost for cooling.
Regards,
Mal
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Heat-pumps are used to heat & cool buildings ... https://en.wikipedia.org/wiki/Heat_pump
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There are quite a few systems where heating is pumped around towns from a communal source and a few from geothermal sources eg Iceland.
I haven't come across cooling.
I think Antarctica is too far away for a practical system.
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You can use a heat pump to transfer heat from air to water. I heat a building in the UK by this method (we rarely need cooling!). Since most occupied buildings use hot water, you can recycle surplus heat from the air inside the building, into the hot water tank. Domestic hot water needs to be at least 50 deg C to prevent algae or other bugs breeding, so you can calculate the size of tank you would need to cool a house (say 1000 cubic meters of air) by any given amount (assuming the water starts at the same temperature as the air). This is a lot more efficient than pumping air across oceans, which the sun does anyway!
Little point in attempting to cool an entire city. What matters is the perceived outdoor temperature, and in hot places this is as much due to direct radiant heating from the sun as it is to convection heating from the ambient air.
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The town of Coober Pedy in South Australia is located in the desert, with an average daily maximum temperature of 37C (98F) in January.
Many people there live underground (http://www.outback-australia-travel-secrets.com/coober-pedy-underground-homes.html), with the soil providing a layer of insulation from the daytime temperature, and some thermal inertia (the average night temperature in January is a more tolerable 22C/72F).
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you guys have a point. But lets imagine we pump cool air from arctic or hill side or mile up in the sky. we wouldn't need air conditioners (Homes, offices, vehicles) and fans anymore. there may be millions of air conditioners and fans in a large city. we don't need to manufacture them again. we don't need to operate them round the clock. Not sure how much air we need to pump and how much does it cost though. We virtually cut off unnecessary warming by the Sun. We wouldn't need much air to be pump. It might has chain effect. The air pump today would have effect for tomorrow. so less air is required.
Other use would be the cultivating and living in deserts..
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The town of Coober Pedy in South Australia is located in the desert, with an average daily maximum temperature of 37C (98F) in January.
Many people there live underground (http://www.outback-australia-travel-secrets.com/coober-pedy-underground-homes.html), with the soil providing a layer of insulation from the daytime temperature, and some thermal inertia (the average night temperature in January is a more tolerable 22C/72F).
Wow - I never knew that existed! In the old opal workings too.
How many people live like that there? And what proportion of the population? Surely aircon has made that approach somewhat redundant?
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The problem with trying to cool by pumping cold air from a mountaintop into the valley is the reason why it is cold to begin with: its altitude. As the atmosphere circulates, air rises and falls. As it rises, it expands and cools; and falling, it heats. If you pump cold air from a high altitude to a lower altitude, the greater atmospheric pressure at the lower altitude will cause it to compress and heat, thereby nullifying the cooling. An alternative that might work is to transfer the temperature of the high altitude to a less-compressible medium and pump that instead. Thus, a heat exchanger on the mountain would cool water, which would then be pumped down into the valley. Undergoing very little compression, it would remain at essentially the same temperature. Then the cold water could be used to cool building, be pumped back up the mountain, discharge the heat up there, and so on. This would work, but the problem is to be able to do it at less cost than is required for other cooling methods. That might be easier said than done. If you want to keep pumping costs low, a large diameter pipe and capacious heat exchanger are needed so that little energy is lost due to viscosity. Also, the pipe needs to be well insulated, and the slower the water moves, the better the insulation must be. I don't know how the economics of all this would work out, but it is clear that a substantial infrastructure must be installed, the cost of which may make it infeasible.
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You pump 15c montain to 25c city. The air will not reach 25c as pressure is only a one factor. What we are trying to avoid is the temperature increased by the sun light in the city.
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The problem with trying to cool by pumping cold air from a mountaintop into the valley is the reason why it is cold to begin with: its altitude. As the atmosphere circulates, air rises and falls. As it rises, it expands and cools; and falling, it heats. If you pump cold air from a high altitude to a lower altitude, the greater atmospheric pressure at the lower altitude will cause it to compress and heat, thereby nullifying the cooling. An alternative that might work is to transfer the temperature of the high altitude to a less-compressible medium and pump that instead. Thus, a heat exchanger on the mountain would cool water, which would then be pumped down into the valley. Undergoing very little compression, it would remain at essentially the same temperature. Then the cold water could be used to cool building, be pumped back up the mountain, discharge the heat up there, and so on. This would work, but the problem is to be able to do it at less cost than is required for other cooling methods. That might be easier said than done. If you want to keep pumping costs low, a large diameter pipe and capacious heat exchanger are needed so that little energy is lost due to viscosity. Also, the pipe needs to be well insulated, and the slower the water moves, the better the insulation must be. I don't know how the economics of all this would work out, but it is clear that a substantial infrastructure must be installed, the cost of which may make it infeasible.
Bravo! A near perfect response with solid physics.
Your engineering is sound, the idea doesn't come apart until economics are factored in.
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Here in South Africa we et a very hot wind which starts off as cold mountain air descending down the mountain. This is heated by compression so it eventually reaches the coast as a wind of around 35C, but it started up as a sub zero air mass.
Much better would be to instead paint all the roof surfaces white or silver, so as to reflect light and heat away, and to cover parking areas with a roof structure that is all solar panels, using this power to run regular air conditioners. Less energy hitting the ground from the reflected light, and the panels absorb light that otherwise would hit bare concrete or tar, reducing local heating. Then your rejected heat from the coolers would both be rejected at a lower ambient, and would be lower due to less solar load on the buildings. First part is very cheap ( and does work, dropping rooftop temperature by around 40c from regular, and the indoor hot spots by around 20C) and easy to do during regular maintenance, and the second is also doable in stages, along with reducing the input energy in electricity to the building.
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Hello guys, maybe I can shed some light on this idea,
Can we pump cold air from one place to another ? certainly
Could a town be cooled by pumping in cold air?
No it could not by the rules of thermodynamics,
Things have an entropy , entropy is basically a system as a whole and the amount of energy a system contains or can contain.
So imagine your town inside a box, and the atmosphere is warm and you want to cool down the atmosphere by air being cooled by your intake fans, Your fans/pumps, also inside your box generate heat, this adds to the entropy of the energy already inside the box, creating more heat, so one cancels one out as such.
Now you could say put the pumps at the north pole end, but then this would affect the entropy of the north pole and in eventually you would then be blowing in warm air.
You would need to do the opposite and extract the heat with extractor fans having the extractors outside the box. In simple terms when it is hot in a room you open the window.
Hope this helps.
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Much better would be to instead paint all the roof surfaces white or silver, so as to reflect light and heat away, and to cover parking areas with a roof structure that is all solar panels, using this power to run regular air conditioners. Less energy hitting the ground from the reflected light, and the panels absorb light that otherwise would hit bare concrete or tar, reducing local heating.
Pleased to report that here in Arizona this stuff is already happening. White roofs have been preferred here for a long time, and solar panels are popping up on roofs and also over parking lots, although this particular technology is not yet by any means universal, probably because of the cost. However, government policy here encourages it through electric utility regulation.
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You know, a variant of this idea worth considering, inasmuch as not all localities needing cooling have ready access to a nearby mountaintop, but some of them do have ready access to a body of water, is to check the temperature in the water, and if it is substantially less than that of the ambient air, this water could be used in cooling radiators inside buildings. Of course, someone spoke of using such water as a heat reservoir in connection with heat pumps, which is a proven technology; but if the water were cold enough, the heat pump would be unnecessary.
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Hope this helps.
I think you may have misunderstood what the plan is. The plan is not just to blow air around inside an enclosed box, nor even to just blow air from the outside in one end and out the other. That would admittedly be of limited value. Instead, the idea was to bring already much colder air in from a higher altitude (which is thermodynamically problematic), or bring in the cold from that altitude using a fluid that would then be run through cooling fins inside the building as in a conventional air conditioner.
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Thanks to natural convection, warm air rises and is displaced by cold air flowing in from the poles, or, more locally, mountains. It's called "weather".
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Can we pump cold air from one place to another ? certainly
Could a town be cooled by pumping in cold air?
No it could not by the rules of thermodynamics,
Things have an entropy , entropy is basically a system as a whole and the amount of energy a system contains or can contain.
Yes, I was going to say something like this. This is very similar to what I call the "air conditioner problem." It's basically a feedback loop. Climate change makes the temperature go up, so we turn on the air conditioner. Stand outdoors next to that air conditioner, and you can feel it making the temperature outside a little hotter. That means you need to use the air conditioner more, which makes it hotter outside, so you need to use the air conditioner more, etc. A million or a billion air conditioners has a far greater cumulative effect.
The same principle would work moving cool air from one place to another. That requires energy. Most of the energy sources we have rely on some sort of combustion, and even when they don't, heat still shows up as a byproduct of that energy. My power in San Francisco is hydroelectric, but my outlets still get warm when I plug stuff in and use it, light bulbs still get hot, fan motors still get hot, my refrigerator still radiates heat, etc.
Wind power doesn't produce any heat. Maybe we could move air with wind power. Sounds silly, yes, but maybe it's as simple as deploying a bunch of giant sails to divert air currents. Furthermore, white sails would reflect sunlight back out into space, and they would cast a lot of shadows.
Some other ideas: Bring to the city what makes it cooler in the country. Put gardens and parks on rooftops, put parking garages underground, Chicago is starting to take some steps like this. Or, you can simply use white shingles for your roof. That helps keeps your house cool more than most people might imagine.