h a l f b a k e r yA riddle wrapped in a mystery inside a rich, flaky crust
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Air conditioners use energy to move heat from one piece of air to another, venting the hotter air and keeping the colder. This does not make the air fresher and in fact admitting fresh outside (hot) air costs more energy as opposed to further cooling already conditioned inside air. Air conditioners
can be inefficient as well.
Consider instead liquid air. This could be mass produced at a central location, trucked to buildings, and gradually released into the building over the day. The liquid air would necessarily be water free to avoid corrosion of the tanks - this would also serve to reduce humidity in the building. One could also purge the liquid air of CO2 or take other measures to improve the perception of "freshness". No energy would be used on site except to bring in the new tank and take out the old. The added air would also be free of pollen, radiation or other pollutants which might be problematic in the area. One would need to vent air from the building to prevent pressure from building up.
Such a system could be used in deep mines, Venusian colonies or other places where air supply is an issue as well as heat.
AC using ice slabs
AC_20using_20ice_20slabs/addnote#addnote
My inspiration! But I think ice is too heavy and heat transfer is less than ideal. [bungston, Jun 08 2011]
//places where air supply is an issue//
http://upload.wikim...of_love_excerpt.ogg
[spidermother, Jun 09 2011]
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can you compare the cooling effect of an ice slab with releasing say a similar volume of 250bar compressed air ? |
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Latent heat of fusion of water is 334 kJ/kg. Specific heat of ice is 2.11 kJ/(kg.K). Specific heat of liquid water is 4.1813 kJ/(kg.K). Latent heat of evaporation of nitrogen is 200 kJ/kg. Specific heat of nitrogen gas is 1.040 kJ/(kg.K). |
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Latent heat of fusion of water is 334 kJ/kg. Specific heat of ice is 2.11 kJ/(kg.K). Specific heat of liquid water is 4.1813 kJ/(kg.K). Latent heat of evaporation of nitrogen is 200 kJ/kg. Specific heat of nitrogen gas is 1.040 kJ/(kg.K). |
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The relative cooling ability depends on the temperature of the ice, but ice at the temperature of boiling liquid nitrogen is clearly less heavy than liquid nitrogen with equivalent cooling potential (liquid air has similar properties, but I can't find them. You could just find the pro rata average of nitrogen and oxygen, but I can't be bothered.) |
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As you point out, though, liquid air neatly replaces the stale air, and would provide perfect heat transfer, and no waste product. |
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It's always going to be thermodynamically inefficient, though, as you are removing heat at 79K. Maybe you should allow the air to boil inside a freezer room, and allow the rest of the building to be cooled by the overflow from that. Just to make better use of the low temperature. |
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//250bar compressed air// has considerably less cooling potential than the same volume of liquid air, and thus less than ice. I think the exact amount of cooling depends on how it's expanded - how much work it does on its surroundings. Pressure-change cooling is also less efficient overall than phase-change cooling, because it happens over a range of temperatures, whereas phase change is constant temperature. |
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The freezer room would be in the attic, with the cold air streaming down and displacing the hottest air back up thru the roof. |
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it takes a lot more energy to produce liquid oxygen than it does to cool air to the inlet temperature for ac, 15ish C? |
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seems like this would be more expensive. LOX is not cheap. Also you wouldn't want to increase the 02 content in a house it would ruin all sorts of things, not to mention your gas burner or sparks would be catastrophic |
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this is baked, many industrial sites use liquid oxygen for industrial processes. However the specific heat of oxygen is not great. |
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[metarinka] He's talking about liquid air, not liquid oxygen. Also, //this is baked// would mean that delivering liquid air for the purpose of air-conditioning a building has been done. |
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I mentioned the inefficiency and expense. To reword my 5th paragraph above, if a building has another use for liquid nitrogen or liquid air, such as cryogenics, then it might make sense to use the cold exhaust gas to cool the rest of the building, but it's not going to be economical to use liquid air exclusively for air conditioning. |
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//if a building has another use for liquid nitrogen or
liquid air//
Most hospitals have LOX tanks receiving regular
deliveries, and those tanks have heat exchangers
that are covered in frost and
ice, in all weather. Most hospitals also have air
conditioning plants. And yet the two are never
combined. Seems like there's some efficiency gain
to be had there. |
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totally dry air is not good: causes static, bad for skin. |
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Then combine with //AC_using_ice_slabs// as
already linked. |
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