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mobile homes inside commercial plastic film greenhouses
24 jun 1996
suppose we put a 10' x 30' x 7' tall mobile home up on blocks, and put 75
55 gallon plastic drums full of water underneath to make a solar closet
3' tall x 30' long x 10' deep, with a single wall south polycarbonate glazing
area of 90 ft^2, and a glazing area/thermal mass area ratio of about 25x75/90
= 21:1 :-) we could cover the south wall with another layer of polyethylene
or single wall polycarbonate (about $1/ft^2 from replex at (800) 726-5151)
that extended up to the roof of the mobile home, ie another 30' x 10' of
glazing to make a low-thermal-mass sunspace...

or we might put it the mobile home inside a 30' wide x 70' long x 13' tall
commercial plastic film greenhouse to act as a sunspace. the usual covering
is 2 layers of 5 cent/ft^2 polyethylene film, with a 3 year guarantee,
statically-inflated with a 50 watt blower. a better cover for a residence
might be 1 layer of very clear mylar, ie polyester film, which might cost
10 cents/ft^2 and last 10 years, with a layer of greenhouse shadecloth over
the outside in summertime. wind fatigue might be prevented by making a partial
vacuum inside with an exhaust fan when the wind is blowing, using a wind
switch, or making some sort of venturi ridge vent. putting the mobile home
inside this $1500 greenhouse would allow more daytime living space, storage
of not very valuable items, rainwater collection, food growing, sewage
treatment, methane production for cooking gas (?), and water recycling,
as well as a place to work during the day.

where i live in pa, with an average 4"/month of rainfall, this 3000 ft^2
greenhouse roof would collect an average of 240 gallons of water a day.
a reliable water supply might require a cistern/pond volume of 10k gallons,
eg a pond 4' deep x 12' wide x 26' long, made from a single piece of 20' wide
epdm rubber. less volume would be needed if used water were recycled by
condensing it between the poly film greenhouse pillows. 

using aluminum extrusion clamps (from e c geiger at (800) 4geiger, geigerintl
@hortnet.com, http://www.hortnet.com.geigerintl/) 33-sl16 unilock, $12.80/16',
also available from stuppy at (800) 877-5025) for the poly film makes changing
and recycling it every 3 years not much more work than changing a bedsheet,
on a calm day.

composting would help with food growing too. i wonder how much co2 a ft^3 of
good compost makes each day. my nraes 33 book (page 120) says plants need
about 0.003 ft^3 co2/hr-ft^2 of greenhouse floor area, and give off co2 
(how much?) at night, and a greenhouse might have about 0.5 ach, and the
earth's atmosphere is about 0.03% co2. the more co2 the better, i guess.
are high levels harmful to humans? helpful to dinosaurs?

putting the solar closet under the mobile home means we get some heat through
the floor, which would otherwise be wasted if the closet were adjacent, and
if we open a motorized damper under a floor return, we might easily get warm
air to flow up into the mobile home by natural convection. the drums need a
vapor barrier underneath. 

suppose we run this "polytunnel" ew and capture all the sun that falls on
its south "wall," about 13' high and 70' long, ie about 1000 btu/ft^2/day
x 13' x 64' x 0.92^2 (2 layers of poly film) = 704k btu/day on an average
day in december in philadelphia, when the average outdoor temp is 36 f.
the tunnel ridge might be raised to 15' with longer ground posts.

adding a strip of white poly film on the ground or a white deck along the
south side of the mobile home, or adding a shallow frozen reflecting pool
along the south side of the greenhouse, outside, might increase the solar
input by about 1/3. the polytunnel would be a half cylinder with a roof
length along the curve of 47', so the roof area would be 47x70 = 3300 ft^2,
and the endwalls will make this 4000 ft^2 total. if this is all us r1.2, we
have 704k btu/day going in over 6 hours, and 6(t-40)4000/1.2 = 20k(t-40)
leaving, so the average december daytime temp inside the greenhouse would be
36 + 704k/20k = 71 f. we would want a slow ceiling fan or two to bring down
some warm air from the peak. 

recall the mobile home is 10 x 30 x 7' tall. suppose the r-values of the
walls and roof are us r11, and the windows are us r2, double glazed, with
64 ft^2 of window area on the south and 64 ft^2 on the other sides. 

this makes a thermal conductance of about 2(10+30)7/11 + 10x30/11 + 128/2
= 50 + 27 + 64 = 141 btu/hr-f, so keeping the mobile home warm for 24 hours
on a cloudy day might take 24x(68-36)141 = 108k btu/day. less on a day with
an average amount of sun, since the sunspace side or polytunnel will be
warmer during 6 hours. the sunspace, eg the glazed south side of the mobile
home might collect about 300x1000 = 300k btu/day, and lose 1/4 of that to
the outside via the sunspace glazing, which seems fine on an average december
day, and the 75 drums full of water underneath would store (120-80)75x500
= 1500k btu of usable space heat at 120 f (we might want some insulation on top
of the drums), enough for about 14 cloudy days in a row. sounds like too many
drums. will the steady-state december temp of the drums be at least 120 f?

say the mobile home has an r11 floor, and the skirt around the drums is
insulated with r30 strawbales. then we have about 90 ft^2 x 1.33 (with
a white surface in front) x 1000 btu/ft^2/day x 0.92^2 = 101k btu going
into the solar closet each day, and about 

   6 hours (t-71) 90 ft^2/r1        south wall, day
+ 18 hours (t-36) 90 ft^2/r30           "       night
+ 24 hours (t-68) 300 ft^2/r11      mobile home floor, 24 hours
+ 24 hours (t-36) 150 ft^2/r30          "       enw skirt, 24 hours,
--------------------------------
= 540t - 38340
+  54t -  1944
+ 654t - 44509
+ 120t -  4320
---------------
= 1368t - 89113 = 104k,

so the steady-state december temperature of the drums would be 

(104k + 89k)/1368 = 141 f. looks good... 

we might want a window fan or two turning on with a thermostat set to 130 f
in a small darkened glazed box exposed to the sun, to help move warm air
through the solar closet when the sun is shining... 3' is probably not enough
height to make plastic film backdraft dampers work well. 

and we'd want about 20' of fin-tube pipe under the floor, in a thermosyphon 
loop with the water heater in the mobile home, so the electric heating element
seldom turns on. that might require a bit more solar closet glazing, ie we
might put the mobile home 4' off the ground, so the solar closet glazing is
4' tall instead of 3', since replex and dynaglas plastics come in rolls and
sheets a bit over 49" wide.

nick



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