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Solar house heating in Austin

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Posted by nicksanspam on November 10, 2006, 10:30 am
Ian plans...

NREL says 940 Btu/ft^2 of sun falls on the ground and 1200 falls on
a south wall on an average 48.8 F day with a 58.9 F max in January
in Austin. An 84.5 F average July day with a 73.9 min has 640 Btu
on a south wall, with humidity ratio w = 0.0156 pounds of water
per pound of dry air (humid.)

With a 30.3 north latitude and a max beam sun elevation
J = 90-30.3+23.5 = 83.2 degrees at noon on 6/21 and
D = 90-30.3-23.5 = 36.2 degrees on 12/21, a p foot deep horizontal
overhang would completely shade an h foot tall window with an s foot
separation above the glass if p = h/(tan(J)-tan(D)) and s = ptan(D).

So an h = 6' tall window in Austin would need a p = 0.784' overhang
s = 0.574' above the top edge of the glass. NREL uses a different
criterion with J = 108-lat and D = 71-lat, which provides no shading
for south-facing windows from 11/17 to 1/25 and complete shading
at solar noon from 5/12 to 8/2, but:

   Stations in southern states can benefit from more summertime shading;
   threfore, the shading geometry was modified to accomodate a longer
   shading period. Their monthly heating degree day (base 65 F)
   requirements were examined to find the first fall month with a value
   greater than zero. If this month was October [17 DD in Austin] or
   later, then J = 92-lat [= 61.7] and D = 66.5-lat [= 36.2]...
which makes p = 6/(tan(61.7)-tan(36.2)) = 5.33' and s = 3.90' in Austin,
a lot more, but as you say:    

Wow. NREL says the average daily max is 95.5 in August, with a 30-year
104.0 max. An adjustable overhang might extend less in November (average
temp 60.9) than April (average temp 69.6.) You might stack a few boards
on top of each other on some horizontal joists extending to the south or
lower a curtain in November and spread the boards back out or raise
the curtain in April. Steve Baer talks about "pruning day," eg a day in
November when you look at vegetation near south walls and cut back plants
that cast shadows on windows.    

For the best of both worlds, you might make it a sunspace with $50 7'x8'
R2 sliding glass doors with 80% solar transmission as the south wall, with
a curtain for summer shading but no overhang. Leave the doors open with
screens in summertime and close them from November through March, with
an 80% black shadecloth curtain hanging a few inches inside the glass for
more comfort and solar collection efficiency, with 70 F air filling the
sunspace and space south of the curtain and warmer air rising behind it.  
A 48'x48' R24 6" SIP house with an 8'x48' sunspace and no windows nor
air leaks nor electrical energy use would have a thermal conductance of
1408ft^2/R24 = 59 Btu/h-F for the walls + 1920/24 = 80 for the ceiling,
totaling 139, so it would need about 24h(65-48.8)139 = 54K Btu of heat
on an average January day in Austin.
If 1 ft^2 of sunspace glazing gains 0.8x1200 = 960 Btu and loses
6h(70-54)1FT^2/R2 = 48 Btu, 54K/(960-48) = 60 ft^2 of glazing might keep
the house warm for 6 hours per day, and we might store overnight heat in
a shiny ceiling mass C that warms to 120 F by dusk and cools to 70 by dawn,
where C = 6h/24hx54K/(120-70) = 810 Btu/F, eg 15 4"x10' thinwall PVC pipes
full of water, with a slow ceiling fan and a room temp thermostat and
an occupancy sensor to bring down heat as needed.
We might store heat for 5 cloudy days in 5x54K/(120-80) = 6750 pounds
(108 ft^3) of water that cools from 120 to 80 F in a well-insulated
stratified tank on the ground, and pump some cold tank water up through
the pipes to keep the tank hot on an average day and pump or allow some
hot tank water to thermosyphon up through the pipes on a cloudy day.

The pipes and tank could also be a cool store with night ventilation or
a roofpond in July, with a small AC for dehumidification to 60% RH.


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