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a solar cabin
10 apr 1996
a friend writes: 

>i am interested in your solar closet for home heating and water heating.


>i have a vacation cabin in east tennessee that i will eventually retire to.

well, i see from the nrel _solar radiation data manual for buildings_
( that knoxville gets 940 btu/ft^2/day of sun on a
south wall on an average day in december, the average 24 hour outdoor
temperature is 40 f, and the average daytime temp is 50 f. sounds like a
pretty good climate for solar heating... microclimate could matter a lot
near the smokies. i camped outdoors on mt laconte once, with a bear huffing
and puffing around my sleeping bag at 3 am :-) is there a lot of fog around
your cabin? are there lots of bears?

what's the south wall look like? is it shaded much by trees, etc.? do you
have a south porch you could enclose, or could you add a lean-to sunspace?
with a reflecting pond in front of the sunspace?

>it is 970 square feet and is presently heated by two 110vac in the wall
>strip heaters and a wood stove.

can you guess how much fuel it might use per year if it were occupied all
the time? a cord of wood is something like 100 gallons of oil or 3000 kwh
of electrical energy or 100 ft^2 of south glazing over a winter. 

how well is the cabin insulated? can you make it fairly airtight? a square
cabin that size might have about 1000 ft^2 of ceiling and 1000 ft^2 of walls.
with r11 insulation (fiberglass in a 2x4 wall), the thermal conductance might
be about 2000 ft^2/r11 = 180 btu/hr/degree f, so on an average 40 f day in
december it would need about 24 hours (70f - 40 f) 180 = 130 k btu/day
to keep it at 70 f inside. 

water heating for 3 showers a day, each lasting 10 minutes at 3 gpm, would
need another 3 x 10 min x 3 gpm x 8 lb/gal x (110f - 60f) = 36k btu/day, so
the house total might be 170k btu/day, not counting internal heat gain from
electrical consumption. 

where you are, a square foot of sunspace with no reflecting pond might gain
865 btu/day and lose 6 hours (80f - 50f) 1 ft^2/r1 = 180 btu/day, for a net
gain of 700 btu/ft^2/day, so to provide a total of 170k btu/day, you might
need 170k/700 = 240 ft^2 of sunspace, ie a sunspace running the length of
the south wall, 30' long x 8' tall. it might be some very clear single-layer
polycarbonate glazing that comes in rolls 4' wide, attached to 8' 2x4s or 12'
2 x 6s leaning against the cabin wall. or a steep south polycarbonate roof.

or... some very clear long lasting mylar film (eventually), starting out now
with polyethylene film, which costs about 4 cents/ft^2, and has a 3 year
guarantee and comes in very wide rolls and can be replaced like a large
bedsheet and recycled every three years, less often perhaps if the sunspace
is covered with greenhouse shadecloth in the summer. this might be a curved
sunspace, with the film stretched over galvanized pipes. it could be very
large and inexpensive, less than $1 per square foot. how does that sound?  

behind or inside that, you might store heat for 5 days without sun in a small
insulated room, ie a solar closet storing 5 days x 170k = 850k btu of heat.
a gallon of water at 130 f stores useful house heat until the water reaches
80 f or so, ie (130-80) 8 lb/gal = 400 btu of house heat. so you might have 
850k/400 = 2000 gallons of water in your solar closet, in the form of 40
55 gallon plastic drums stacked up 2 high and 2 deep in a row 20' long, ie
a solar closet that is roughly 4' deep x 20' long x 8' tall. or you might use
400 5 gallon plastic pails with lids, sitting on shelves made from cement
blocks and boards. if it turns out you don't need that much thermal mass,
you could put something else on the shelves.

for water heating, it might be good to have two sections of closet at
different temperatures, with the hotter section better-insulated and used
as a last resort for house heating, and the cooler one containing the section
of fin-tube pipe loop with the cold water input. you might have an electric
water heater in the closet with a small circulating pump, or a small fan
powered by a pv panel in the sunspace, drawing air down from the ceiling
and pushing it out the bottom of a 4' wide, 4" thick duct built onto an
inside wall, past 8' of fin-tube along the duct bottom, in a u-shape. 

a better place for an electric water heater (whose heating element almost
never turns on) might be on the floor above, so it could use a natural warm
water convection loop. i wonder if you have any sort of attic above the
south wall, or you would like to make a small sunspace up there... 

the solar closet glazing inside the sunspace glazing might have an area
of 8' x 20', a little more than half of the sunspace glazing. an ascii
picture from above:

                           <-- old | new -->
                  30'               4'
     ...........................................   .   .   .   .
     .                             .   .       .
     .                             .   . refl  .                
     .                             .sc . surf? .
     .                             .   .       .               .
     .                             .   .20'    .
     .                             .   .       .                
     .                             .   .       .30'  south -->
     .      existing cabin         .   .  ss   .               .
     .                             .   .       .
     .                             .....       .  reflective    
     .                             .           .   surface?
     .                             .           .               .
     .                             .           .
     .                             .    12'    .
     ...........................................   .   .   .   .
                           <-- old | new -->

or perhaps like this?
                           |   8'  |  8'   |      8-16'?

                              straw wall?
                    --     .................   .   .   .   .
                         s .      s.       .
                         t .      t. refl  .               .
                     |   r .  sc  r. surf? .
                    16'  a .      a.       .
                     |   w .......w.       .               .           
                         ? .      ?.   s   .       
                           . sauna .   u   .  reflective
     ...............................   n   .   surface?    .  south -->
     .              30'            .   s   .
     .                             .   p   .                       
     .                             .   a   .
     . 30'                      16'.   c   .               .
     .                             .   e   .
     .                             .       .                
     .                             .       .              
     .      existing cabin         .........   .   .   .   .
     .                             .  8'               
     .                             .           these pictures make more
     .                             .           sense when viewed in a
     .                             .           non-proportional font
     .                             .           like courier...
     .                             .        

if your existing cabin has little thermal mass, you might want to add some,
because it would be nice (but not necessary) if the cabin could get through
an average december night with no help from the solar closet. is 55f at night
ok? starting at 75 f at dusk, your cabin might need 18/24 x 130k = 100k btu
to keep warm until dawn, ie 100k btu = (75-65) m, where m is the number of
pounds of water in the house, so here, m = 5000. that's a lot of water...
10 more 55 gallon drums sprinkled around the house, or another 110 5 gallon
pails here and there :-) perhaps the solar closet should provide some
overnight heat in this case, or the cabin should have more insulation.
at any rate, you see how this works.

the sunspace and solar closet and cabin thermal mass could be smaller if
the cabin were better insulated. half the size with r22 insulation, a third
as big with r40, i guess. surround the cabin with strawbale/mortar walls?

>if you are interested in designing a solar
>closet for my cabin please let me know what you would charge.

hey, i just did it, free :-) let me know how it works! would you like me to
help you build it in may, august or september? send me a sketch if you like,
or let me know if you would like to talk about modifications or if you have
questions. i could check out a specific design with a simulation using 30
years of knoxville hourly weather data, if a certain physics student or
professor would get that data off a cd-rom. we would really like to see this
technique tried out on a larger scale than our 2x4x8' outdoor test box... 


nicholson l. pine                      system design and consulting
pine associates, ltd.                                (610) 489-0545 
821 collegeville road                           fax: (610) 489-7057
collegeville, pa 19426                     email:

microprocessor hardware, memory, asic, and computer design. telecommunication
system design. computer simulation and modeling. high performance, low cost,
residential solar heating and cogeneration system design. bsee, msee. senior
member, ieee. registered us patent agent. fluent in french.

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