Heat a large structure

Heating, insulation, ventilation, VMC, cooling ... short thermal comfort. Insulation, wood energy, heat pumps but also electricity, gas or oil, VMC ... Help in choosing and implementation, problem solving, optimization, tips and tricks ...
dedeleco
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by dedeleco » 14/04/10, 14:00

The sites presented are very interesting, a partially buried building constitutes an effective Canadian well in the Mediterranean region for 19 to 20 ° C, and it is also enough with a little Canadian well in the basement (more or less like a simplified crawl space) for the structure with 15 ° C in the Paris region in my opinion.

Solar well and solar duct are used to heat the ground deep.

Finally, vacuum insulation requires a relatively good vacuum and therefore for large volumes of vacuum, good pumps, which are inexpensive (some 1000 € max) compared to the rest of the project and which rarely work. It is the technology in much smaller LNG ships with less explosive risk !!!
The stainless steel walls are however not given.
Even under vacuum and super-insulation (large thermos bottle) autonomy does not reach 6 months in my opinion. The heat stored increases as the volume and the losses as the surface and therefore the autonomy increases as the typical dimension (radius of the optimum sphere or length of the thermos cylinder) and therefore passing from 10cm (thermos camping with a day) we pass with 10m (very large and expensive) to 100 times longer, i.e. 100 days, a bit tight but possible. With 1m we go to 10 days, too short !!
This rough assessment fixes the ideas.
Can be industrialized, the price with flat vacuum panels cheaper than stainless steel, aluminum, iron, etc. could be used in homes without increasing the thickness of the walls. The pumps are not expensive, by comparison.
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dedeleco
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by dedeleco » 14/04/10, 14:10

Note :
For this problem, we can think of not insulating the balloons, but the central room in which we put them. So we can use standard stainless steel balloons.

We can reduce the thickness with layers of cheap insulation (polystyrene) and wet earth (or cement, plaster) alternating so that the overall diffusivity is greatly reduced (low conductivity with high specific heat) and therefore the thickness necessary reduced by 10 and more !!
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bernardd
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by bernardd » 14/04/10, 18:11

dedeleco wrote:Finally, vacuum insulation requires a relatively good vacuum and therefore for large volumes of vacuum, good pumps, which are inexpensive (some 1000 € max) compared to the rest of the project and which rarely work. It is the technology in much smaller LNG ships with less explosive risk !!!
The stainless steel walls are however not given.


Absolutely. Precisely Christophe found plastic with mirror: it would be ideal for this application!
https://www.econologie.com/forums/post165899.html#165899
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dedeleco
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by dedeleco » 14/04/10, 20:13

Plastic is too porous in general for vacuum, it degasses too much compared to metals or glass, the polish is not enough and the vacuum must reach a minimum to cut conduction by air molecules.
Vacuum is more difficult than mirror polish, especially to maintain a good vacuum long enough.
The vacuum will not work miracles as evidenced by the autonomy of a thermos bottle, better but not eternal.
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bernardd
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by bernardd » 14/04/10, 21:32

dedeleco wrote:Plastic is too porous in general for vacuum, it degasses too much compared to metals or glass, the polish is not enough and the vacuum must reach a minimum to cut conduction by air molecules.


I've always heard this, but never seen a figure on the conductivity / thermal resistance as a function of pressure, especially with pressures below atmospheric pressure?
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by dedeleco » 14/04/10, 22:35

I'm going to look for the precise answer with figures, but as long as the average free path of the molecules remains less than the distance from the walls, nothing changes for thermal conductivity and viscosity, practically, from memory (a feather remains braked just as much), and only when the vacuum is good, with few molecules, it becomes insulating.
The decrease in the number of molecules is compensated for by their greater mean free path so that the conductivity changes little and only with a fairly good vacuum a decrease in the conductivity is obtained (but primary vacuum of good pump without degassing). We use this conductivity to measure the vacuum below the Torr !! Above a sensitive pressure gauge is more sensitive than by conductivity.
Random examples thanks google!
http://www.linternaute.com/science/tech ... vide.shtml
http://fr.wikipedia.org/wiki/Vide
http://en.wikipedia.org/wiki/Vacuum
typical thermos bottle with free medium path 1cm to 1m larger than the distance from the walls, therefore little thermal conductivity (proportional to the number of molecules remaining) and vacuum of 10-2 to 10-4 torr, primary vacuum without excessive degassing, small container without plastic after a few minutes of pumping. !!!
The degassing degrades the thermos bottles still in glass over time, despite absorbent or getter products to trap the degassing.
They hardly get better than polystyrene for liquid nitrogen for example !!
Plastic degasses more (long messy molecules with residual paths for small molecules, like balls of messy twine!), Is porous (strainer with residual hydrogen from air and porous with air slowly).
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bernardd
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by bernardd » 15/04/10, 17:35

Thanks for the links!

For degassing on a large installation, a holding pump must be provided. At 1 mbar, this remains simple with a diaphragm pump, which seems to be the beginning of the vacuum range of thermos.

Still to be tested :-) One more item on my list ...
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more complex




by dedeleco » 16/04/10, 01:16

At 1 mbar, this remains simple with a diaphragm pump, which seems to be the beginning of the vacuum range of the thermos.

But in fact :
Thermos bottle pressure 1 to 0.01 Pa [1] 10−2 to 10−4 Torr 1 cm to 1 m of mean free path 1014 to 1012 molecules per cm3

1mbar equals 100Pa and therefore 1mbar the pressure is 100 times too strong, and the thermal conductivity is almost unchanged !!
100 Pascal = 000 Torr = 760Bar
http://www.vacuum-guide.com/francais/te ... ession.htm
http://www.convertworld.com/fr/pression/Torr.html
http://www.vide.org/pagestechniques/pconvunites.html

3-stage diaphragm pump: 1.5 mbar limit pressure !!!
http://www.adixen.fr/pompes_primaires/s ... 5-1-p.html
Very very insufficient !!!!!

Not too expensive silent vane pump (relative uh for house) which are suitable for thermos and dewars bottles:
http://www.adixen.fr/pompes_primaires/s ... 1-1-p.html
limit pressure 210-3 mbar
http://www.adixen.fr/all/dyn/download/d ... prod81.pdf

http://www.vide.org/pagestechniques/
http://www.vide.org/pagestechniques/pvideenceinte.html
http://www.vide.org/pagestechniques/pregecoul.html
http://www.vide.org/pagestechniques/ppermeation.html
all gases diffuse significantly through plastics and elastomers (example: helium and elastomeric seals).


The thermal conductivity becomes very low but not zero if the pressure is at the limit of a good quality vane pump (not diaphragm !!) pumping over a volume not too large for its flow rate.
Once vacuum with residual gas trap the vacuum is kept and we rarely pump to restore the vacuum. It is necessary to measure the vacuum by conductivity (Pirani gauge) and restore them if poorly insulated !!
The average free path must be large in relation to the dimensions
http://www.vide.org/pagestechniques/
Vacuum measurement methods:
http://en.wikipedia.org/wiki/Vacuum
http://paillard.claude.free.fr/lampes/vide/vide.html
http://www.belljar.net/tcgauge.htm
The thermal conductivity becomes zero when the Pirani gauge using this conductivity no longer measures 10-3 to 10-4 Torr !!!
Finally the conductivity by radiation is to be stopped with a well-worked superisolation (reflective multilayers), in fact with errors to be avoided.
See NASA which kept cryogenic fluids nitrogen and liquid helium on satellites, months and months in space vacuum, from memory.

LNG carriers prefer polyurethane to vacuum !!! Autonomy 700 days large tanks !!
http://wapedia.mobi/fr/M%C3%A9thanier
For liquid nitrogen, my past experience has found that an expanded polystyrene tank is not dramatically worse than a commercial glass thermos bottle, which have good radiation losses without superinsulation !!

Insulating a room with vacuum walls currently seems expensive (stainless steel, or other cheaper metal, even with modular vacuum plates) !!
http://www.alma.nrao.edu/memos/html-mem ... emo554.pdf
According to this paper you have to pump as much as possible !!! by putting a portion with liquid nitrogen !!!!

An overview to compare the vacuum to other solutions without vacuum !!!
http://www.ecbcs.org/docs/Ann39_2001_1_ ... roc%20.pdf

http://www.freepatentsonline.com/4444821.html
http://www.freepatentsonline.com/4444821.pdf
http://www.informaworld.com/smpp/conten ... a713948040
http://jen.sagepub.com/cgi/content/abstract/28/4/319
Scientists Create Material More Insulating than the Vacuum
http://www.physorg.com/news179672831.html

http://www.glacierbaytechnology.com/cat ... nsulation/
high price !!!!

Info on this subject aerogels which requires little vacuum

http://eetd.lbl.gov/ecs/aerogels/sa-thermal.html
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