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double glazing with convection damping

an array of 'whiskers' within double glazing
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Double glazing could be made a better insulator if the convection of the gas within the double glazing could be suppressed.

The obvious way to suppress convection is to break the cavity into lots of smaller cells using baffles or walls. The problem is, sticking anything inside the double glazing cavity is going to detract from the uniform transparency of the window.

My solution is to use an array of fibres (whiskers) extending between the panes of glass.

So I hear you immediately ask two questions: 1. why are whiskers going to suppress convection?, 2. won't the whiskers increase the opacity of the window?

The answer to these questions is that the whiskers are very thin and fairly densely arrayed. I hypothesise that if the diameter of the whiskers is less than the wavelength of light (e.g. 200nm), the fibres will be essentially invisible in an analogous way that 'moth-eye' anti-reflective coating is invisible. If the whiskers are packed densely (e.g. 1µm) the convection will be almost completely suppressed.

The whiskers could be formed by, for example, starting with an array of liquid polymer droplets between adjacent panes of glass and drawing the panes apart.

xaviergisz, Jan 03 2013

Aerogel Windows
Similar idea. However aerogel tends to be translucent rather than transparent. [xaviergisz, Jan 03 2013]

Moth-eye anti-reflective coating http://en.wikipedia...ve_coating#Moth_eye
[xaviergisz, Jan 03 2013]

Advanced glazing and transparent insulation http://www.cenerg.e...dvanced_glazing.pdf
See in particular page 18 onwards. [xaviergisz, Jan 03 2013]

Development of windows based on highly insulating aerogel glazings http://www.research...ng_aerogel_glazings
[xaviergisz, Jan 04 2013]

patent WO2011/068426 http://www.google.c...s/EP2507440A1?cl=en
similar, but with nanofibers parallel with the window panes (my idea has the fibers extending orthogonally to the window panes). [xaviergisz, Jan 08 2013]

[link]






       Won't the whiskers conduct heat from one pane to the other?
pocmloc, Jan 03 2013
  

       //I hypothesise that if the diameter of the whiskers is less than the wavelength of light (e.g. 200nm), the fibres will be essentially invisible//   

       And I hypothesize that every single one of these fibers would shatter the moment the window was moved, to say nothing of the effects of thermal loading.   

       Second, if the panes are properly spaced, convection is a minimal issue, since there isn't sufficient air space for the movement to occur.
MechE, Jan 03 2013
  

       If the whiskers were hexagonal in cross-section and perfectly stacked, I would't worry about breakage, but wouldn't it be so much (relatively) simpler to just seal a vacuum between the panes? That way you'd have zero thermal transmission (except through the material of the frame) and zero opacity.
Alterother, Jan 03 2013
  

       Vacuum "filled" double pane windows do exist, but require regular pumping out (in-gassing) and can lose the seal. Given that it is never a perfect vacuum, you do have some conduction losses, but they are minimized.   

       In fact window design is usually a trade off between increasing the spacing to minimize conduction and limiting it to get rid of convection. This idea would allow larger spacing without convection, but since it would produce thermal bridges between the panes, the advantage would be lost.   

       Trust me, however, when I say that 200 nm threads of glass, 3-6 mm long, will break under their own weight in normal handling, so you would end up with a typical double pane window, with a large amount of dangerously fine glass dust in the bottom if it ever shattered.   

       Also, it would be far cheaper to simply upgrade to triple glazed windows (which do exist) than to use nano-structured glass.
MechE, Jan 03 2013
  

       14.7 PSI just isn't that high, and glass is fairly tough. They aren't perfectly evacuated, but they do exist. The real problem is that with to great a thermal gradient the seal cracks.
MechE, Jan 03 2013
  

       Since triple-pane windows were mentioned, suspended film windows should also be mentioned. Those often have two layers of film between the outer glass panes, breaking the cavity into 3 separate chambers to reduce convection.
scad mientist, Jan 03 2013
  

       Yes, but as I said, glass is fairly tough. It's stronger than human skin, and that can survive true vacuum, and has a lot larger surface area.
MechE, Jan 03 2013
  

       //human skin, and that can survive true vacuum//   

       That's somewhat misleading. Humans in a vacuum do not experience a pressure *differential* across their skin.   

       Any attempt to create such a differential (for example, by holding one's breath before being put under vacuum) is rather balloonishly unsuccessful.
MaxwellBuchanan, Jan 03 2013
  

       //Second, if the panes are properly spaced, convection is a minimal issue, since there isn't sufficient air space for the movement to occur.//   

       I disagree. Convection appears to be a significant factor (see link).   

       //Trust me, however, when I say that 200 nm threads of glass, 3-6 mm long, will break under their own weight in normal handling//   

       OK, are polymer nanofibers any better?
xaviergisz, Jan 03 2013
  

       That article doesn't suggest that convection is a major player, although it does suggest it is a factor, something I never denied. You'll note that it indicates that for a properly spaced window, the frame material is far more of a factor than convection. This factor would be greatly increased by the sort of bridging fibers you suggest.   

       You'll also note it indicates an optimum distance where heat transfer is minimized which is the point where you get the minimum conduction without inducing significant convection (fig. 9). At distances narrower than that point, still air conduction dominates, which your threads would not help. Only at distances larger than that point does convection become a factor.   

       Thus I don't deny that being able to increase the spacing further without inducing convection would be desirable, but given that the optimum point is already pushing the limit of thickness for most residential windows, there is limited benefit to this. Even where there is a benefit it is better served by the addition of an additional planar layer (either triple pane, or [scad]'s film which I will admit I had not encountered before). These retain optical clarity and sub-convective air layers without the cost or complexity of nanoscale materials, and without adding thermal bridges.   

       Page 18 and onward are primarily (with the exception of the monolithic aerogel section) are primarily focused on translucent rather than transparent applications, and in doing so they are looking mostly at radiative rather than convective or conductive heat transfer.
MechE, Jan 03 2013
  

       Well leave it o MechE to ruin it for the rest of us. Man that is one smart dude. If the goal is to see outside, with out letting heat in/out then a workaround might be a 50" LED screen powered by solar panels outside, fed images from a camera. Screen only turns on when you are in the room and the Blinds are open. Plus you could get Facebook updates while spying on your neighbors.
Brian the Painter, Jan 03 2013
  

       //You'll also note it indicates an optimum distance where heat transfer is minimized which is the point where you get the minimum conduction without inducing significant convection (fig. 9). At distances narrower than that point, still air conduction dominates, which your threads would not help. Only at distances larger than that point does convection become a factor.//   

       Even taking into account the thermal conduction of the whiskers themselves, I still think they are going to provide an insulative advantage at any spacing of double glazing.   

       Figure 9 shows a minimum heat transfer coefficient of the gas within the double glazing to be 2 W/m²K for air or 1.2 W/m²K for krypton. Aerogel (which, because of it's air-trapping qualities, is an analogous composition to the whiskers) has a heat transfer coefficient of less than 0.7 W/m²K (see most recent link).
xaviergisz, Jan 04 2013
  

       Once again, that's based on sufficiently thick window spacing that natural convection takes over as the dominant energy transmission. Again, I don't argue that reducing convection helps in those ranges. Nor do I argue that if you can reduce convection, increasing the over-all spacing is beneficial. I do argue that the increase in thermal transmission from glass bridging that gap is more severe than the benefits from increased spacing provide. I argue this especially since adding an additional pane of material (film or glass) will achieve the same affect without any bridging, and with a much lower cost than a nano- structured material. You'll note in section 3.1.2.6.3 of your earlier link, it mentions a U value around 0.5 W/m²K for a film spaced window, an improvement over even aerogel.   

       And the material structure of Aerogel is not functionally identical to straight whiskers, because Aerogel has a very convoluted material path, reducing, but not eliminating, the bridging effect as shown by the lower U value of the film windows.
MechE, Jan 04 2013
  

       if evacuated double glazing exists, does each glass pane not deform tremendously? Wouldn't the window end up being a weird biconcave lens?
bs0u0155, Jan 04 2013
  

       //Once again, that's based on sufficiently thick window spacing that natural convection takes over as the dominant energy transmission//   

       No, I was basing that on the optimal spacing such that convection is minimised (the minimum point on the curves in Figure 9).   

       //I argue this especially since adding an additional pane of material (film or glass) will achieve the same affect without any bridging, and with a much lower cost than a nano- structured material. You'll note in section 3.1.2.6.3 of your earlier link, it mentions a U value around 0.5 W/m²K for a film spaced window, an improvement over even aerogel.//   

       But that is for a double glazing that is more than twice as thick as the aerogel comparison. It is also using layers of a 'heat mirror' (thus I'm not sure of the transparency) and uses Xenon gas filling (which is expensive and requires good gas sealing).   

       //And the material structure of Aerogel is not functionally identical to straight whiskers, because Aerogel has a very convoluted material path, reducing, but not eliminating, the bridging effect as shown by the lower U value of the film windows.//   

       I'm guessing that it is the nanoscale as much as the convoluted path that reduces the conductivity of the aerogel (since I intuit a 'convoluted material path' will not reduce the thermal conductivity of a material at macro scale).   

       One advantage of my idea is that it doesn't require a gas seal to trap the gas within the double glazing. This may increase the longevity of the window, since it is my understanding that double glazing can lose its seal integrity after about 15 years.
xaviergisz, Jan 04 2013
  

       // No, I was basing that on the optimal spacing such that convection is minimised (the minimum point on the curves in Figure 9).//   

       Sorry, the "that" in my statement was the aerogel paper. It discusses the use of aerogel in slabs thicker than optimum, which of course increases insulation properties. To increase insulation in any air filled space, you have to increase the thickness without inducing convection. Your original idea only provides benefits if the pane to pane distance is increased. Aerogel allows thicker panels without convection, and without getting excessively heavy, hence it's advantages.   

       And yes, a convoluted path will reduce thermal conductivity in the macro scale. The critical factor is that the thickness of a material affects its thermal conductivity. It's logical enough that a one meter bar of aluminum will conduct less heat from one end to the other than a 1 cm bar. What's less obvious is that the same holds true (ignoring transfer through air and a few other factors) if that 1 meter bar is bent to fit within that 1 cm space.   

       And the section I referenced does not refer to krypton fill or anything similar. It is simply a quadruple glazed window, except the center two panes are thin plastic film rather than glass. The film is optically transparent, so there are no issues there. Once again, this is simply to allow a thicker air space without convection and without excessive weight gain, exactly the same as your idea. And it isn't sealed, at all. Seals are not required for double pane windows at all, although they are often used because they limit moisture (they are required for gas filled, but again this isn't). What is required is a sufficiently minimal air exchange to avoid convection into the room, but this won't be affected by a cracked seal.
MechE, Jan 04 2013
  

       OK, point conceded on the conductivity of convoluted path materials.   

       //And the section I referenced does not refer to krypton fill or anything similar.//   

       I thought you were referring to the bottom entry on the table in chapter 3.1.2.5.4 on page 16. The 0.5 W/m²K example on page 15 is presumably thicker than 20mm so it's not an apples to apples comparison.
xaviergisz, Jan 04 2013
  

       My point on that is that there's no advantage to reducing convection unless the airspace is thicker than the optimum point, because it's essentially a non-issue.
MechE, Jan 04 2013
  

       //[the aerogel paper] ... discusses the use of aerogel in slabs thicker than optimum, which of course increases insulation properties.//   

       I still don't get what we're arguing about. If I wanted a reasonably thin (15-20mm) window with the *best* insulation properties (and perhaps with the added benefit of good longevity), a monolithic aerogel filling would be the best choice. If I also wanted excellent transparency, then a variation on aeorogel (like this idea) would be an option worth investigating.   

       I'm not saying this is definitely going to work (e.g. I'm not sure it would be more transparent than aerogel), but I don't think its a completely absurd starting point of investigation.
xaviergisz, Jan 04 2013
  

       // *best* insulation properties//   

       It would be vacuum, that's my point, although that does require maintenance. Aerogel in that thickness is not significantly better than air, and may be worse.
MechE, Jan 04 2013
  

       But (assuming my numbers quoted are correct) aerogel is three times better than air (2 W/m2K vs 0.7 W/m2K).
xaviergisz, Jan 04 2013
  

       Okay, maybe a little, but only because convection is starting to take over at that point. The same thickness with a film middle pane should be more efficient than a simple double pane. What I think I've been neglecting is that aerogel also sharply reduces brownian mixing, which, again, this idea wouldn't.   

       Regardless, it's critical that your central material not produce a bridging effect, which this idea will.
MechE, Jan 04 2013
  

       I've come up with a small variation on the idea to overcome the thermal bridging problem.   

       Instead of the whiskers extending from one pane of glass to the other, they could instead be extend only from one pane only and not quite touch the other.   

       The whiskers would need something to prevent them from drooping. If the whiskers were given a negative charge the electric fields would repel thus keeping them erect. The opposite pane of glass could be given a positive charge to also help pull the whiskers straight and taut. The charge could be provided by a battery or the whiskers could be made of an electret material.
xaviergisz, Jan 06 2013
  
      
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