a strawbale doghouse (was: re: poly water tanks' outdoor longevity?)
28 feb 1999
r bishop wrote:
> email@example.com (lehunger) wrote:
>>the thought has crossed my mind... any instructions available?
sure. how about a big solar version, say 16' square x 8' high? :-)
it might look like this from above:
8' 8' [use courier font]
| s| straw |
| poly s|------------- | s is a
| - - - - - - so _ | | movable
| s|| | | |s | shutter
| film s|| |f| room |t | o is an
south | - - - - - - s||ww|o| 12' |r | 16' opening
| s|| |a| |a | ww is a
| house- s|| |m| |w | water
| - - - - - - s|| | | | | wall
| s|------------ |
| warmer s| straw |
and like this from the east: epdm roofpond
. s 6" fiberglass | h is a
. s-------------- | vent
s|| | h |s | hole
south . house- s|| |f| |t | 8' v is an
warmer s||ww|o| room |r | automatic
s|| |a| |a | foundation
. ( o p e n ) s|| |mv |w | vent
the shutter would be hinged at the base, and automatically fold down
flat to the south during the day as in steve baer's drumwall house
to allow sun to shine on the water wall. the shutter would return to
the vertical closed position at night. the housewarmer protects the
shutter from the weather and collects some additional solar heat.
sealing the shutter against air leaks around the edges when closed
isn't easy. it might work best if made in 4 independently hinged
4x8' sections with an axle along the top edge of the room to wind up
4 ropes that raise the sections. a garage door opener might help.
they are powerful, inexpensive and safe, with built in limit switches.
or we might use an old windshield wiper motor or a few small 12v
12 w gearmotors like grainger's 41 in-lb 9 rpm 2l007. the shutter
does not have to move quickly.
each section might have 3/4" foam weatherstripping mounted on the
12x8' waterwall and the 4x8' opening between the room and the
housewarmer on the west end. the sections might be made by gluing
2 pieces of 2" styrofoam to each side of a thin piece of plywood,
with another thin piece of plywood on the north side, painted white
to reflect sun onto the waterwall.
the walls might be 72 strawbales protected from the weather with
some greenhouse poly film on the outside, with the film extending
out over the ground at the bottom, and some stone over that. the
epdm rubber roof would overlap the film on all sides at the top.
it might make a shallow roofpond, with a 4x4 under the perimeter.
this could supply an average of 10 gallons a day of potable rainwater
in pa, if it had a poly film greenhouse on top to keep it cleaner
and warmer. at the same time, it might also act as a solar still
for wastewater treatment and reuse.
the wall might be compressed with some ropes connecting the roof
platform and some 2x4 deadmen under the strawbales and over black
plastic film on the ground. the floor of the room might carpeted,
and the housewarmer floor might be 6" of shredded wood playground
mulch with a few concrete blocks to support the shutter when it's
unfolded, so it can be walked on. the waterwall would keep the
structure from blowing away in the wind, and the south edge of the
housewarmer could be bolted to some old tires filled with soil.
the room might have a thermal conductance of about 6 btu/h-f for
the ceiling, 5 for the strawbale walls, and 1achx8x8x16/55 = 19 for
air leaks, a total of 30 btu/h-f, excluding the waterwall. on a 30 f
day, it needs about (70f-30f)30btu/f = 1200 btu/h or 29k btu. it might
be heated with 4 100 watt light bulbs or ashrae-standard 50 pound dogs.
the shutter's thermal conductance is 8'x12'/r20 = 4.8 btu/f, so the
waterwall would lose about 24h(100f-30f)4.8 = 8k btu/day, for a total
of 37k btu/day for the whole structure, or about 185k btu for 5 30f
cloudy days in a row with the shutter closed. if the waterwall cools
from 120 to 80 f over this time, we need 185kbtu/(120f-40f) = 4625 btu/f
of capacitance, ie 4625 pounds of water, so the waterwall needs to be
4625lb/(12'x8'x64lb/ft^3) = 0.75 feet or 9" thick, with say, 2'x4'x9"
wide compartments lined with 38"x60" 55 gallon plastic film drum liners
inside some wire fencing, with some 2x4 framing, or some drum liners
inside homemade wire fence cylinders, like tomato cages.
the housewarmer cover might be a 16' long x 12' wide $10 piece of r0.8
4-year greenhouse polyethylene film with a solar transmission of 90%.
the waterwall cover might be a single layer of r1 polycarbonate with
90% solar transmission. the housewarmer might collect 8'x16'x0.9x1620
= 187k btu of sun on an average january day near philadelphia (more
if it had a frozen pond to the south.)
with sun passing through 2 layers of glazing and some reflection from
the shutter, the waterwall gains about 0.9x0.9x1.3x1000 = 1050 btu of
sun per square foot or 101k btu per day. let's say the housewarmer has
an average temperature th over an average 6 hour solar collection day
in january, and it's about 30 f at night, and the waterwall temp tw
doesn't change much over an average day.
if the housewarmer keeps the room warm for 6 hours on an average day,
it supplies about 7k btu/day of heat, and the waterwall supplies the
other 22k that the room needs overnight, and loses some heat of its
own through the shutter...
so, if the solar energy that flows in to the waterwall over an average
day equals the heat energy that flows out, we have roughly
101k = 22k overnight heat for the room,
+ 6h(tw-th)8'x12'/r1 heat loss to housewarmer, daytime,
+ 18h(tw-30)8'x12'/r21 heat loss to housewarmer, nighttime, so
81k = 658tw - 576th (1).
the housewarmer gains 187k btu during an average day, and about 110k of
that shines on the waterwall, and 7k keeps the house warm for 6 hours,
which leaves 70k, so
70k = 6h(th-30)12'x16'/r0.8 heat loss to the outdoors
- 6h(tw-th)8'x12'/r1 heat gain from waterwall, and
113k = 2016th - 576tw, so th = 56.2 + 0.286tw (2).
substituting (2) into (1) gives
81k = 658tw - 32k - 165tw, so tw = 229 f :-) th = 122 f, from (2).
but reradiation loss will limit the water temperature to something
closer to 130 f and make the housewarmer closer to 100 f.
during the day, the room needs 1200 btu/h of heat, which might come
from warm air that circulates through the open 4x8' doorway between
the room and the housewarmer, with a temperature difference dt, where
1200 = dt cfm = 16.6x16ft^2xsquare_root(8')dt^1.5, using an empirical
chimney formula, so dt = 1.4 f. it looks like we don't need a fan here.
at night we might let cool room air flow under the north waterwall
insulation through an $11 automatic foundation vent with an adjustable
bimetallic spring that opens some louvers to regulate air temperature
at the bottom of each section, and let warmer air flow out through
a slot at the top. with an 8"x16" vent and 400 btu/h per section,
dt = 4.5 f. so it looks like we don't need a fan there either.
three 12 w grainger gearmotors with 2" pulleys might open and close
the shutters. they would run about 43 seconds per day, consuming
3x12x43/3600 = 0.43 wh or 0.00043 kwh of electrical energy at a
cost of less than 2 cents per year. the system cop, ie the ratio of
useful heat energy moved to electrical energy consumed would be
29k/3.41/0.43 = 19,778, vs 3 for a typical heat pump. we might raise
the cop by counterweighting the shutters, and the electrical energy
might be photovoltaic...