[Greenbuilding] Solar house heating in Bend, OR

[Nick Pine]

>I am designing an off-grid house in the Bend area of Oregon...

NREL says 850 Btu/ft^2 of sun falls on a south wall on an average 31.8 F 
December day with a 41.7 daily max in Redmond.

>We know that the sun will provide about 95% of the heat required to 
>maintain comfort, as long as we provide a sufficient amount of solar 
>glazing, internal thermal mass, and well insulated envelope.  The basic 
>heating strategy is: solar heat gain through large south facing 
>glass... large surfaces of 4" thick thermal mass surfaces exposed to 
>the indoor air, and a tight well-insulated envelope.

A 40'x60'x8'-tall direct gain house with A ft^2 of R4 south windows with 
50% solar transmission and a thermal conductance of G Btu/h-F could be 
70 F on an average December day if 0.5x850A > 24h(70-31.8)G, eg A = 
2.16G. With  0.1 ACH (31 cfm) of air leaks and Rv walls and ceiling, G = 
31+A/4+(4000ft^2-A)/Rv, so Rv = (4000-A)/(0.213A-31). With an A = 60'x8' 
south glass wall, Rv = R49, eg 10" of R5/inch Styrofoam, with G = 223 
Btu/h-F.

If cloudy days are like coin flips, 95% requires storing heat 
for -ln(1-0.95)/ln(2) = 4.3 days. A direct gain house with no internal 
heat gains (eg indoor electrical use) that cools from 70 to 60 F over 
4.3 cloudy days would have a time constant RC 
= -4.3/ln((60-31.8)/(70-31.8)) = 14.2 days, ie 340 hours, with C = RCxG 
= 75.8K Btu/F, eg 75.8K/25 = 3031 ft^3 (112 yd^3) of concrete, eg 9092 
ft^2 of 4" walls, eg 9092/(40'x8') = 28 40'x8' north-south walls inside 
the house, which would make it something like a dog kennel :-)

A low-mass isolated sunspace or air heater vs a south glass wall would 
lower the cloudy-day conductance to 113 Btu/h-F and lower the concrete 
requirement to 340x113/25 = 1532 ft^3, eg 14 north south walls.

A high-temp store could have a lot less mass with good room temp 
control. With R40 walls and ceiling and a 31+4000ft^2/R40 = 131 Btu/h-F 
conductance, the house needs 4.3dx24(65-31.8)131 = 449K Btu, eg 
449K/(140-70) = 6400 Btu/F of mass cooling from 140 to 70 F, eg 256 ft^3 
of concrete or 103 ft^3 of water, eg a 4'x8'x3.2'-tall plywood tank with 
a folded EPDM rubber liner. If 1 ft^2 of R2 sunspace glazing with 80% 
solar transmission (eg 2 $1/ft^2 layers of GE HP92W Lexan) gains 0.8x850 
= 680 Btu and loses 6h(140-37)1ft^2/R2 = 309, for a net gain of 371 
Btu/ft^2 and the house needs 104.4K Btu on an average day, it needs 
104.4K/371 = 281 ft^2 of sunspace glazing.

Nick