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Solar heat in Montana

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Posted by nicksanspam on March 14, 2007, 6:09 pm
John M writes:

That's a difficult solar climate. NREL says 310 Btu/ft^2 falls
on the ground and 490 falls on a south wall on an average 22.7 F
December day with a 29.9 max. The average yearly (deep ground
and water) temp is 44.9 F. The WBAN No. 24146 TMY2 hourly weather
data file is useful for simple Kalispell solar house simulations.

A square foot of  R2 window with 80% solar transmission on an isolated
low-mass sunspace might gain 392 and lose 6h(70-26)1ft^2/R2 = 132 Btu/day,
for a net gain of 260. A more expensive R4 window with 50% transmission
on a living space might gain 245 and lose 24h(65-17.9)1ft^2/R4 = 283, for
a 38 net loss, aka "passive solar gain" :-)

An evacuated tube system could work all year.

Sand is a poor heat conductor. Adding water helps.

They are likely to lose vs gain heat on an average December day.

How about a 32' cube with 1 story below grade? :-) With US R-32 walls
and ceiling (eg 8" SIPs), it would have a thermal conductance of about
110 Btu/h-F and need about 24h(65-17.9)110 = 124K Btu/day of heat.
If 68K comes from 600 kWh/mo of indoor electrical use, it needs
an additional 56K.

On an average day, the sunspace could make 56K with T (F) air if 56K
= 768(392-6h(T-26)0.5), ie T = 132 F. Bare collectors in the sunspace
could keep a basement water tank hot, with one or two $0 300'x1"
13-gallon pressurized PE pipe coils in the tank for DHW.

A tank to store heat for 5 cloudy days in a row might have 5x56K
= (130-80)62.33 = 90 ft^3. It might be a 4'x8'x3' tall plywood box
lined with a single folded 10'x14' piece of EPDM rubber.

That requires a much larger tank. November only has 580 Btu of sun
on a south wall on a 31 F average day. October is OK, with 1050 Btu
on a 41.9 F day.


Posted by Michelle P on March 14, 2007, 6:34 pm
Check with Sunelco they are in Victor, MT.
They will give you an honest answer.

nicksanspam@ece.villanova.edu wrote:

Posted by Jeff on March 14, 2007, 6:38 pm
 nicksanspam@ece.villanova.edu wrote:

   I don't know what happened to the threading on this, or if this is
just originated by Nick.

   Doesn't Gary live in Montana? Certainly a visit to
<URL: http://builditsolar.com  />, Gary's site, would be in order. In
other words, he's been there and done that!


Posted by Solar Flare on March 15, 2007, 3:31 am
 How does an R4 lose twice as many BTU per day as an R2 window? You
used 24 hours for one and 6 hours for the other.

Not sleeping well Nick?...LOL

Posted by nicksanspam on March 15, 2007, 11:08 am


US DOE passive solar guidelines suggest using windows with
a min 60% solar transmission and max U0.35. How well would
they work on a Kalispell sunspace and living space?


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