Cooling things: Difference between revisions
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Glass is by nature emissive. | Glass is by nature emissive (0.8, 0.9), though multiple-pane can reduce that drastically. | ||
Double glazing tries to avoid connecting the inside temperature to the outside pane to be emitted | |||
(because there will be very little convection going on compared to a single sheet of glass), | |||
triple pane more so. | |||
Because heat loss (measured by U-value) and solar energy heat gain (may be seen measured in 'solar factor') are entirely separate concerns | |||
that apply to different climates (and, in theory, seasons), windows that care about heat may do any of: | |||
: be double or triple pane to lower convection-based loss | |||
: have a coating that that lowers trasmission - by reflecting sunlight's heat before it enters, but lets most visible light through | |||
:: this also exists in foil form, though those are often not quite as good at the job, and may peel off | |||
: have a coating that that lowers the emissivity | |||
Because low-emissivity ("Low-E") windows are a combination of all, | |||
some people incorrectly and confusing use the term to refer to windows that do only some of that. | |||
In theory you could also put reflective coating inside to keep heat in, but that's barely worth it because most heat is lost through convection, not from radiation (...in most situations. Fireplaces are not so common). | |||
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https://homeadvisor.com/r/low-e-windows/ | https://homeadvisor.com/r/low-e-windows/ | ||
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Revision as of 15:49, 16 August 2023
Physical mechanics of cooling
Passive cooling
Passive cooling tends to mean 'what happens with no moving parts'.
...so whatever amount of conduction, radiation, or convection would happen anyway.
Sometimes includes adding a fan.
You're stirring the air better than just convection would, so heat transfer goes a little faster than if warm air just sits around - but the difference is rarely much (if you're in gravity, convection will happen).
And you could argue that's technically active cooling (because you're adding work, so using energy), but intuitively it feels like it hardly qualifies.
On the technical side
This tends to mean
- conduction - a good conductor spreading heat throughout
- if any cooling happens, conduction's spreading brings the whole down
- radiation - thermal radiation means movement of charges in materials (anything above 0 K) is radiated as EM at the surface
- (black-body radiation can be seen as a "thermal radiation's real-world math becomes easier if we make some assumptions like that it's not really interacting in other ways")
- convection - fluid flow, in this context often
- air,
- flow caused by heat changing temperatures and densities
- that flow assisting better heat interchange with that fluid, because warmer air moving up tends to draws in colder air from the sides (which technically is an effect that needs gravity)
In practice there's more than one of these happening, but often one that counts for most exchange.