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solar house heating in portland
8 nov 1998
dan writes:

>...i though it would be best to ask you about some of the best energy
>savings concepts i can use during the construction of my new home. looking
>for things that could be incorporated pretty cheaply, but save energy.

how's your sense of aesthetics? i can make very cheap sunspaces with $5
doubled curved 1x3 beams on 4' centers bolted to used tires (minus $1.25
each) full of dirt for foundation/planters, with a layer of black plastic
film on the ground and shredded wood playground mulch or used carpeting
for a floor. all this can be yours, covered with cloudy 4-year greenhouse
polyethylene film costing 5 cents/ft^2. the materials for this could cost
$0.00 per square foot, if you use enough tires. you might want to do this
on the back side of the house, facing south. when do we start? 

compact fluorescents come to mind. and daylighting. a well-lit office has
50 footcandles of illumination (like 50 candles 1' away, a lot more than
abe lincoln used--i can read in 0.1 fc, eg 1 candle 3' away), but full sun 
is 10,000 fc, 200x brighter. so in principle, a 1 ft^2 window could light
a 200 ft^2 room, if the light were well-diffused. a few high windows or
skylights (with reflective south sunscoops above for winter concentration
and summer shading?) might do this, with a reflective light-shelf underneath
and a white ceiling and walls. you might have occupancy sensors and controls
in each room to turn off the lights when nobody's in the room or daylight
is sufficient. grainger's $8.53 6p008 photocontrol will turn off up to
1 kw of fluorescent lights when daylight exceeds an adjustable threshold
(15+ fc.) it has some hysteresis and about a 2 minute delay, so it doesn't
flicker on and off much.

the boca code requires that houses have at least 4% of their floor space
as windows, but you might use fewer heat-leaky expensive windows in the
24-hour living space, and more windows or glazing in a thermally-isolated
low-thermal-mass sunspace that provides solar heat for the house during
the day, and gets cold at night, wasting little solar heat through the
solar glazing, which has a low thermal resistance compared to an insulated
wall, eg r2 vs r20 or r30. 

>2900 square foot, 2 story...  portland oregon...

not an easy place for solar house heating. not much sun in december,
310 and 470 btu/ft^2 that fall on the ground and a south wall, when
the average outdoor temp is 40.2 f, with an average daily max of 45.6.
maybe your house should be heated by cogeneration, eg a $900 1500w
90 pound honda generator that also makes 7500 w or 26k btu/h of heat
while burning 0.23 gallons of gasoline per hour in a basement "solar
closet." might be a good item in the millenium.

at any rate, i'd go heavy on insulation and making the house airtight,
more than the usual local practice. passive solar pioneer steve baer
recently wrote: "the greatest discovery of solar heating researchers
has been that if we heavily insulate a house, it needs very little heat
from the sun or any other source."

do you have a utility program to certify electrically-heated houses,
with higher-than-usual standards for insulation and a requirement for
blower door testing? you might put in an electric resistance forced air
system and ask that the house pass that test, and get a lower electric 
rate, even though the electric resistance heating element seldom turns on,
because the house is mostly solar heated or heated by the occupants,
their pets, and their everyday electrical energy consumption.

a 40' two-story square house with r30 walls and r60 ceiling and 0.25 ach
and 60 ft^2 of r3 windows in the walls of the 24-hour living space would
have a thermal conductance of about 2500ft^2/r30 = 93 btu/h-f for the
walls, 1600/60 = 27 for the ceiling, 60/3 = 20 for the windows, and
0.25x25.6k/55 = 116 for air leaks, a total of 256 btu/h-f. keeping it at
an average of 65 f over an average december day in portland requires about
(65f-40.2f)256 = 6.3k btu/h or 150k btu/day. two people using 500 kwh/mo
of electrical energy might supply 60k btu/day of that, leaving 90k.

a square foot of r2 south low-thermal-mass 80 f sunspace glazing or
transparent siding or attic roof might collect 470x0.8 = 376 btu and
lose 6h(80f-43f)1ft^2/r2 = 111 btu over an average 6 hour december day
in portland, for a net gain of 265 btu/ft^2-day, so collecting 90k btu
requires about 90k/265 = 340 ft^2 of vertical south glazing. expensive,
if it's fancy house windows at $50/ft^2, but doable, if it's say, two
layers of very clear 49" wide polycarbonate plastic that costs $1.25/ft^2
with a 10-year guarantee. (you might fill the space between the layers
with soap bubbles at night :-)

you might make the sunspace 16' tall (dramatic, with a few hanging chairs
and ropes and pulleys and plants) and 12' deep and 32' wide, like the one
on the front of my house. the building inspector sold me a $150 permit
for this structure with a materials cost of about $500, and said "it's a
good thing you called this a solar air heater, nick, vs a 2-story 400 ft^2
addition to your house. that way, it's hvac space, and the permit is
three times cheaper, and your property taxes will not change."

your solar closet might be under an 8' high deck inside the sunspace. 
keeping this house warm for 5 days without sun requires about 450k btu,
which might come from 130 f water cooling to 80 f, ie 450k = (130-80)c,
so c = 9000 pounds of water, eg 20 55 gallon drums. you might make the
closet an 8' cube, with a 4x4x2-high array of 32 2' diameter x 3' tall
drums, and some hollow cement blocks underneath and a solar-heated 
hot tub on top of the deck.

a couple of years ago, i got 300 free 55 gallon drums from a local turkey
packing plant. they were happy to have me take them, since they were
paying to have someone else remove them. i asked for 200 more last week,
but they said no, for liability reasons. lawyers :-( i've heard that some
companies have been sued when drums with their labels on them turned up in
landfills with unspeakable contents (eg jimmy hoffa?) at any rate, you
should be able to find local used drums for $1-5 each, if you ask around.

you might have a reflecting pond in front of your sunspace to add about 30%
to the solar intensity, and a reflective ground surface in front of the
solar closet. with 1 layer of r1 90% transmissive glazing and r20 all round,
the 8' cubical closet would gain about 1.3x376x0.9x64ft^2 = 28k btu and
lose 6h(t-80)64ft^2/r1 during the day and 18h(t-40)64ft^2/r21 at night for
the south wall, 6h(t-80)192ft^2/r20 during the day and 18h(t-40)192ft^2/r20
at night for deck and ew walls, 24h(t-70)64ft^2/r50 for the north wall
(the house wall adds another r30), and 24h(t-55)64ft^2/r30 to 55 f ground
(which adds another r10.) if ein = eout, over an average december day,
28k = (384+57.6+57.6+172.8+30.7+51.2)t-(30.7k+2.3k+4.6k+6.9k+2.2k+2.8k),
= 753.9-49.5k, or t = (28k+49.5k)/753.9 = 102.7 f, if i did that right.

hmmm. coolish. what to do? add more insulation? add another layer of
closet glazing, add more south closet glazing or thermal mass, eg make
the closet 8' deep x 12' wide x 8' high, with 48 drums inside? add more
glazing on the west side to collect about 210x0.9^3 = 153 btu/ft^2-day
and lose about 6(102-80)1ft^2/r1 = 132? make the closet 4' deep and 12'
long, and make the deck 8' deep, overhanging the closet by 4', with a
crude fresnel foil-faced foamboard reflector underneath, and hinge the
deck along the north (house) edge, with a counterweight, and use a small
gearmotor and pulley to automatically raise the deck up to a 45 degree 
angle to concentrate (2:1, 450 btu/ft^2-day) sun down into the closet
during the day? put a few cows or a "compost furnace" in the basement?
i'm sure we can work something out, if you'd like to try this... 

after your house is 100% solar-heated, hot water may be the biggest
slice of its energy pie. that might come from the closet, with more
glazing, or from something like overhead big fins in the sunspace, ie
dark painted aluminum extrusions about 4' long and 8" wide with a
c-shaped channel on the back that clip solidly on to copper pipes
to make a bare water heating solar collector panel that can circulate
water through a conventional water heater in the attic via a warm-water
convection loop, with no pumps or antifreeze or heat exchangers.

nick




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