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re: insulation theory [was re: is solar viable for our new house?]
17 feb 2001
harvey taylor wrote:
>...why are there thermos bottles all over the place using vacuums...
i'd guess the vacuum insulation is cost-effective, once you have decided
to make a glass bottle. some thermos bottles use foam insulation instead,
around a plastic vs glass "bottle." they are cheaper and harder to clean
and don't last as long.
> as i understand it:
> -radiation loss is a function of temperature kelvin.
it's proportional to t1^4-t2^4, using absolute temperatures. and emissivity
(which depends on wavelength, for a selective surface) and the "view factor"
(often 1 for solar stuff.)
> -convection loss is complicated, depending upon the gases, airflow
> geometry and various gaseous conduction rates.
it's simpler, depending only on the temperature difference. r-values are
usually measured with tests that account for all those mechanisms. you
might roughly figure that a dead air space (eg a storm window) as us r1,
and r2/3 for heat flow from an object to still air.
> -contact (aka conduction) loss is a function of the 'heat transfer
> rates' (what is the word? (thermal conductance?)) of the two
> materials and the temperature difference.
thermal conductance (u) is the term. like electrical conductance (g),
it's proportional to the area a that heat flows through and inversely
proportional to the distance d the heat travels. the proportionality
constant is called the conductivity k. u = ka/d... k = 0.033 w/mc for
styrofoam, so the conductance of a 1 m x 2m x 4 m styrofoam shoebox is
u = 0.033(1x2)/4 = 0.0165 w/c, with heat flowing the long way. if the
temperature difference between the 1x2 faces is 10 c, 0.165 w of heat
flows from one to the other.
> to bring this back to what i recall of the original context, if one
> has two solar heat systems with identical collectors, and some high
> temperature capable fluid being held in two styles of storage device.
>
> one is a variation on a thermos bottle using a vacuum, made of
> material x, to keep as many factors as possible common; while
> the other is a bottle of material x, insulated with with material
> y with a given r factor.
>
> which would hold heat most effectively?
it depends on the temperatures and x and y and r and the shapes and
sizes of the collectors and how much fluid they hold and its heat
capacity. these questions and answers need numbers, as well as words.
nick
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