re: deployable doubt dispellers
2 mar 2006
>> >> imo, there's a tremendous amount of doubt that an inexpensive house
>> >> can be close to 100% solar-heated. ask a a few builders, architects,
>> >> or ordinary people.
>> >define inexpensive.
>> no panels on the roof, for starters...
>alrighty, i asked for your definition of inexpensive and you designed
>the house. now just go build the thing.
i'm not a builder, but here's a more detailed example: say you wanted to
solar heat a 20' cube in rochester, ny, where 540 btu/ft^2 (not much) falls
on a south wall on an average 29 f december day with a 32 f daytime temp...
first, use enough insulation and south sunspace glazing to make a suitable
sunspace air temp (high enough to provide all the heat the cube needs on
an average day with reasonable airflow and low enough to minimize loss
from the glazing to the outdoors). a south sunspace with 20'x20' of u0.58
thermaglas plus twinwall polycarbonate (from palram americas at 800 994-5626,
about $85 for a 4'x12' sheet) with 80% solar transmission would collect
0.8x400x540 = 172,800 btu on an average day. with average r20 cube walls
(eg r32 walls and r4 windows, counting air leaks and indoor electrical use),
it might lose 6h(ts-32)400x0.58 [the daytime glazing loss to the outdoors]
+ 18h(65-29)400/r22 [= 11782 btu night loss from the south wall]
+ 24h(65-29)4x400/20 [= 69120 btu from the rest of the walls all day],
which makes the average sunspace air temp ts = 98 f.
if the "sunspace" is deeper than a few inches, adding a dark mesh curtain
a foot or so from the glazing can make it more comfy for people and lower
the glazing loss by filling the space between the curtain and glazing with
70 f room air.
if 98 f sunspace air heats the cube for 24-hours on an average dec day (with
a night setback and lots of thermal mass surface inside--this is difficult)
with upper and lower a ft^2 vents and a 20' vent height diff, (11782+69120)/6h
= 13484 btu/h = 16.6asqrt(20')(98-70)^1.5 makes vent area a = 1.2 ft^2, with
16.6x1.2xsqrt(20x28) = 482 cfm of natural thermosyphoning airflow.
next, size the air heater glazing inside the sunspace glazing and find the
air heater temp on an average day. with 160 ft^2 of r1 air heater glazing
with 90% transmission, the sunspace (not the air heater) might collect
0.8x540(400-160) = 103680 btu/day and lose 6h(98-32)400x0.58 [= 91872 btu
from the glazing] + 18h(65-29)400/r22 [= 11782 btu from the south wall at
night] + 24h(65-29)4x400/20 [= 69120 btu from the rest of the walls all day]
- 6h(tc-98)160/r1 [the sunspace gain from the air heater glazing], which
makes the average air heater temp tc = 170 f.
with no heat loss from the cloudy day tank on an average day (a first
approximation) the tank water temp on an average day will be 170 f. now
find the min water temp needed to keep the cube 70 f. on a cloudy day, the
cube's thermal conductance is 5x20x20/20 = 100 btu/h-f, so it needs about
(70-29)100 = 4100 btu/h to stay 70 f during the day (and (60-29)100 = 3100
at night.) if the air heater contains 64' of fin-tube with 5x64 = 320 btu/h-f
of conductance (behind foamboard insulation with a flow organizer at the top),
the min water temp is about 70+4100/320 = 83 f.
over 5 cloudy days, the cube needs about 5x24h(65-29)100 = 432k btu from
432k/(170-83) = 4966 pounds or 620 gallons or 80 ft^3 of water cooling from
170 to 83 f in a 2'x4'x10' epdm-lined plywood box or a 620 gallon w163 $1300
insulated 4' deep x 6' diameter circular tank from stss at (717) 761-5838,
with one of their $400 hex wt66-113 120' 3/4" copper pipe heat exchange
coils to preheat water for showers, with the help of two pe pipe coils in 2
55 gallon drums as greywater heat exchangers.
this system could be tweaked more. it's just a bubble diagram of a heating
system, vs a whole house. there are lots of other architectural decisions to
make about shapes and colors and materials, where to put the kitchen sink,
and so on.