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sizing a sunspace and solar closet to match a house
17 oct 1996
>here are some numbers from the manual for columbia, sc:

              dec  jan     dec  jan
>columbia     46.9 43.8 f  1160 1140 btu/ft^2/day.

>i'm not clear on what the btu per square foot figures mean and how it
>compares to say us average etc. i'm assuming that this is just a measurement
>for the energy of the sun that is striking the surface during the time period

yes, and that varies depending on the orientation of the surface. these
averages are for a south-facing wall, because the sun arrives at an angle
closer to horizontal than vertical in the middle of winter (elevation at noon
on 12/21 = 90-23.5-lat, in degrees.)

a btu is about the same as the energy in a kitchen match; 1 btu can heat 1
pound of water 1 degree f, and when the water cools 1 f, it gives back 1 btu.

              air temp     amount of sun
              dec  jan     dec  jan
>columbia     46.9 43.8    1160 1140 btu/ft^2/day.

january looks like the more challenging month for solar heating here, since
it has less sun and it's cooler than december. you have 1140/(70-43.8)
= 43.5 btu/degree-day in january. phila has 26.6. it's 50 in some places
in new mexico and texas, eg abilene, where certain people really should
solar heat their houses. places in ny and canada have 12. barrow, alaska
has an average daily temperature of -13.4 f in jan, with 0 btu/ft^2/day
of sun, making this index 0. (the average outdoor temp in july in barrow 
is 39.3 f.) the higher this number, the easier it is to solar heat a house.

>how we harness that energy efficiently is another matter! any ideas.

sure, a sunspace and solar closet! see our solar closet paper at:

>i'd appreciate your feedback, and advice on the above, and would like to know
>more about your solar closet and sunspace house.

check out our web paper: your house might have some sort of attached but
thermally isolated low-thermal-mass sunspace, without a lot of bricks, etc
inside it, nor many water-vapor-producing plants, with warm air flowing from
the sunspace into the house on an average winter day, and a $100 2 watt
honeywell motorized damper (or a $12 window fan with a $15 thermostat) that
stops the airflow at night and lets the sunspace get cold while the house
stays warm. airflow also stops if the house gets too warm. meanwhile, there's
a small room in the house completely surrounded by insulation, with some
sealed containers of water inside, which provides warm air for the house on
a cloudy day when the sunspace is cool, and can also easily provide close
to 100% solar hot water, with some fin tube near the ceiling and an ordinary
water heater upstairs, which seldom turns on, like a big fin system in a
sunspace, but working 24 hours a day.

here are some basic ss/sc sizing steps for 100% solar house heating:

1. look up the local average temp and amount of sun that falls on a south
wall, in the worst-case solar heating month, 43.8 f and 1140 btu/ft^2/day.

2. find out how much heat the house needs on that average day. say it's
32x32x16' tall, with average r20 walls (including the windows, making the
r20 average harder to achieve) the walls would have a thermal conductivity
of 2,000 ft^2/r20 = 100 and the ceiling might have 1000 ft^2/r20 = 50, for
a total of 150. so on an average day, the house might need 24 hr(70-43.8)150
= 94k btu. about the same as the energy in a gallon of oil, inefficiently
burned. call this 100k btu/day, in round numbers.

3. find how much south glazing a low-thermal-mass sunspace needs, to supply
that heat on an average day. suppose 1 square foot of r1 single pane glazing
gains 1140 btu/day, and loses 6hr(70-43.8) = 157 btu, for a net gain of 
982 btu/ft^2/day. call this 1000. then we need 100k/1000 = 100 ft^2 of
sunspace glazing, if the sunspace is shallow, or a bit more if deeper, with
larger endwall losses. this "sunspace" could be a $5000 clear redwood and
glass structure, 12' wide x 8' deep x 16' tall, or a $500 commercial plastic
film greenhouse, 12' wide x 8' deep x 16' tall, with curved galvanized pipes
leaning up against the south side of the house, or an 8' tall x 8' deep x 16'
long quarter cylinder with 5 curved spaced beams made from $20 worth of 12'
1x3s, or an 8'x 16' patch of transparent "solar siding" instead of some of
the south siding on the house, at about the same cost, or a transparent
polycarbonate roof, at a lower cost than the usual roof, with no shingles or
tarpaper or sheathing, and less labor, and lots of light in the attic, or a
walk out basement with 2 or 3 8x8' sliding glass doors on the south wall. 

4. find how much thermal mass you need in the solar closet, to keep the
house warm for 5 cloudy days in a row. 5x100k = 500k btu, right? a 55
gallon drum cooling from 130f to 80f releases about 25 k btu, so you could
use 500k/25k = 20 drums, say 2 layers of 10 drums in a 4' deep x 10' wide
x 8' tall solar closet, with 10' x 8' of single pane glazing and an air gap
over the insulated south side, ie in the wall between house and sunspace. 

powerful natural forces, smart low-power controls, and minimal elegant
beautiful structures, like sailboats, but less expensive... 


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