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re: new home passive solar help
18 jun 2000
dan berry  wrote:

>greetings!

greetings...

>we have picked out a place on our land to build our retirement house.
>we picked the south side of the hill/mountain to build on with the
>basement mostly buried.  we are building in the mountains near guffey
>co, at about 8500 feet elevation.

january looks like the worst-case month for solar house heating in alamosa
(7536 feet up, nearby?), with a 24-hour average temperature of 14.7 f and
average daily min and max of -3.9 and 33.3 and 1720 btu/ft^2 (5.4 kwh/m^2)
of sun energy falling on south walls on an average day, according to nrel.
a dry climate temperature swing (vs 65.6/72.9/80.1 for cloudier honolulu
in january :-) alamosa's yearly average deep ground temperature is 41.1 f. 
july has 47.8/64.9/82.0 min/avg/max temps and an average humidity ratio
w = 0.085, corresponding to a 53 f dew point, so it's a good place for
natural cooling.

>the house plan we designed would please us if it had a few windows on
>the south side of the house or a lot of windows.  we would like to take
>advantage of as much passive solar (free) heating as possible without
>roasting ourselves in the summer.  i was considering a larger overhang
>for the roof, but this is not a prow house... 

direct gain solar houses with windows between the outdoors and the
living space have an optimal south window area. too many windows, and
the house loses lots of heat on a cloudy day. too few, and the house
doesn't gain much on a sunny day. you can get around this dilemma by
putting as many south windows as you like in a low-thermal-mass sunspace
with an insulated wall between it and the 24 hour living space and
circulating sunspace-warmed air through the house on a sunny day, and 
letting the sunspace get cool at night and on cloudy days.

you can shade the sunspace in summertime with plants or greenhouse
shadecloth or stacked exterior overhangs, ie a bris-soleil that lets
in low-angle winter sun and excludes summer sun, like what you see
on the flattish rear windows of sports cars. venting helps too.
 
>the south facing area of the house is about 1200 square feet.
>are there some formulas that will tell me how much glass would be
>optimum and how much overhang for the roof would be optimum?

sure. old-fashioned mass and glass. weather and ohm's law for heatflow.
more heat storage (eg concrete floors) allows more windows without
overheating. save 30% on your fuel bill. boring and low-performance,
compared to tossing the woodstoves, heaters, and propane tanks. 

>the interior square footage including the basement is 3400 square feet.

consider a 32' cube with 6" r25 sip walls and ceiling and 4% of the
floorspace as r4 windows and 0.2 house volumes per hour of air leaks
with a dramatic 4-story sunspace over the southern 8' and an insulated
wall between it and the living space. the living space to outdoor
conductance is 124ft^2/r4 = 31 btu/h-f for windows plus 3460ft^2/r25
= 138 for walls plus 31 for the ceiling. air leaks of 24x32x32x0.2 
= 2458 ft^3/h or 82 cfm make the total conductance about 31+138+31+82
= 282 btu/h-f. keeping this house 70 f over an average january day
requires about 24h(70f-14.7f)282btu/h-f = 374k btu.

we might ignore internal heat gain, if you are electrically frugal.
a single layer of clear r1 polycarbonate plastic sunspace glazing
with 90% solar transmission would collect 1720btu/ft^2x0.9x1024ft^2 
= 1585k btu (465 kwh) of sun on an average january day, 1211k more
than the house needs. we might keep the sunspace t degrees f over,
say, 6 hours on an average day, with an average 24 f outdoor temp,
where 6h(t-24f)1024ft^2/r1 = 1211k btu, ie t = 221 f, with this linear
model. we measured a 157 f winter sunspace temp in pennsylvania :-) 

let's make it 130 f for comfort, and store enough of the heat to keep
the house warm for 5 cloudy days in a row. let's try to store it in
bricks, for starters, inside the house. a 5 pound brick stores about
1 btu/f, like a pound of water. the house needs 5x374k = 1870k btu
over 5 cloudy days. if we let the room temperature droop from 70 to
60 f over that time, we need 187,000 bricks indoors, about 468 tons
of them, eg a 32x32x8' basement full, with air spaces between them. 

perhaps there's a better way. let's use water, with 2-3 times the
specific heat by volume, and raise the maximum storage temperature 
to store more heat and eliminate cloudy day temperature droop. we
may also be able to make hot water for showers this way, and water
may be cheaper and more useful than bricks. we can store 1870k btu
in 1870k/(130-70) = 31.2k pounds or 16 tons or 3896 gallons of water
cooling from 130 to 70 f. we might put 72 55 gallon plastic water
drums in an 8'x10'x20' insulated box in the sunspace and circulate
sunspace air through the box during the day to heat them, or eliminate
the sunspace and collect hot water with a solar attic concentrator
and store it in 2 or 3 of tractor supply's 87" diameter x 65" tall
$420 1500 gallon plastic agricultural tanks in the house and heat
the house with a hydronic slab.

>we plan to build a timberframe home with stress-skin panels
>to cut down on our power bill.

good choice. sips have less air leaks and thermal bridging than studs. 

nick




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