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An improved solar attic - Page 6

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Posted by daestrom on December 20, 2005, 11:40 pm
 


I don't think it is all that hard to replicate.  Nick could have just
*added* alt.solar.thermal to his reply.  Change the 'subject' line, and in
some groups it looks like a reply to some previous message with the subject
line changed, but in a.s.t it looks like a new thread.

Perhaps Nick felt the subject was applicable to a.s.t ??  That's the only
group in this reply that I read, but I left the others in, in case someone
is reading it from another.

daestrom



Posted by nicksanspam on December 29, 2005, 8:38 am
 
http://www.cibse.org/pdfs/8cimbabi.pdf  has an equation for the dynamic
metric U-value of a breathing wall, as corrected:

Ud = VRhoaCa/(e^(VRhoaCaRs)-1) W/m^2K, where

     V is the air velocity in meters per second,
     Rhoa is air density, 1.2 kg/m^3,
     Ca is the air's specific heat, 1000 J/(kg-K), and
     Rs is the wall's static thermal resistance in m^2-K/W.

Using V = 1/3600 (1 meter per HOUR :-), and Rs = 5.7 m^2K/W (a US R32 wall),
Ud = 0.058 W/m^2, like a US R98 wall. A more typical V = 10 meters per hour
makes Ud = 1.7x10^-8 W/m^2K, like a US wall with an R-value of 334 million :-)

But less air saves more energy, if outdoor air flows in through the wall with
no heat exchanger: with 0 C outdoors and 20 indoors and a square meter of R5.7
wall losing 20Ud = 24000V/(e^(6840V)-1) watts and V m^3/s flowing in through
the wall requiring 1200V = 20Ud watts of warming, V = ln(21)/6840 = 0.000445
m/s, ie 1.6 m/h is optimal, and the wall loses 0.534 W, with a 1m^2x20K/0.534W
= R37 effective metric R-value, ie US R213 :-)

How does this compare with ASHRAE's 15 cfm per occupant fresh air standard?

Nick


Posted by nicksanspam on December 29, 2005, 2:49 pm
 http://www.cibse.org/pdfs/8cimbabi.pdf  has an equation for the dynamic
metric U-value of a breathing wall, as corrected:

Ud = VRhoaCa/(e^(VRhoaCaRs)-1) W/m^2K, where

    V is the air velocity in meters per second,
     Rhoa is air density, 1.2 kg/m^3,
     Ca is the air's specific heat, 1000 J/(kg-K), and
     Rs is the wall's static thermal resistance in m^2-K/W.

Using V = 1/3600 (1 meter per HOUR :-), and Rs = 5.7 m^2K/W (a US R32 wall),
Ud = 0.058 W/m^2, like a US R98 wall. A more typical V = 10 meters per hour
makes Ud = 1.7x10^-8 W/m^2K, like a US wall with an R-value of 334 million :-)

But counting air heating energy, the total is 1200VdT(1+1/e^(1200VRs)-1),
which increases with airflow. If 30 cfm of 0 C outdoor air flows through
4000 ft^2 of metric R7 (US R40) exterior surface in a typical 40'x60'x8' US
house with no heat exchanger and warms to 20 indoors, V = 3.8x10^-5 m/s, with
0.914 W/m^2 of air heating. The walls and ceiling (eg 8" fiberglass with 9"
TGI joists and studs) would lose 0.914/(e^1200V7-1) = 2.4 W/m^2h, for a total
of 3.31 W/m^2, ie metric R6 or US R34, including fresh air heating as needed,
with (68-32)4000ft^2/R34 = 4235 Btu/h at 30 cfm. This would only work well
in an extremely airtight house.

In which case, maybe it's better to take the R40 with simpler non-breathing
walls losing (68-32)4000ft^2/R40 = 3600 Btu/h and add an 80% air-air heat
exchanger (eg some bidirectional breathing walls) losing about 0.2x30(68-32)
= 376, for a total of 3976 Btu/h.

Nick


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