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re: a strawbale doghouse
5 mar 1999
the solar heated version might be two concentric plastic film
half-cylinders with an air gap between the leaf bags and
the outer lower south film:         
                                   up
                              .  .   .  .           [use courier font]
                         .llll leaf bags llll.
                     .e.lllll.   .   .    .lllll.         ---
                  .c  v2. -->    h is a      .llll.   l are leaves
                .a  v1.  h  -->    vent        .llll.       in bags 
               .p.lll.  |f|        hole         .llll.            
    south    .s.lll.    |o|  4'  v are           .llll.    8'
            .n.lll.solar|a|        automatic      .llll.  
           .u.lll.closet|m|        foundation      .llll.  h are holes
          .s  h <--      v3 <--    vents            .tire.   
      ----------0'------~2'------------------------16'-------

the greenhouse bows on 4' centers might be doubled 1x3s bent to an
8' radius with 1x3 spacer blocks every 2' and deck screws to hold
those curved sandwiches together. the foundation might be 2 east-west
rows of old tires on 4' centers, laid flat on the ground between the
inner and outer films, with an east-west 2x4 bolted through the treads
on each side, and the bow bottoms screwed to the 2x4s. this would make
the gap between the films about 2' near the ground.

home depot sells the $11 leslie-locke afv-1b foundation vents. you can
adjust the minimum soft temperature threshold where the bimetallic spring
opens the louvers with a screwdriver, or take the spring off and reverse
it if you'd like the louvers to close (v3) vs open on temperature rise.

v1 opens with warm sunspace air to heat the closet. v3 opens when the
room is cool to heat the room from the closet. v2 opens when the sunspace
is warm to heat the room if needed. 

a long version of this structure would have a conductance of about
8pift^2/r48 = 0.5 btu/h-f per linear foot, so keeping it 70 f over a
30 f day requires 24h(70f-30f)0.5 = 480 btu/linear foot. a square foot
of sunspace that gains 900 btu/ft^2/day and loses 6h(100f-30f)1ft^2/r0.8
= 500 btu in january in phila gains 400 btu/day, net, so we need about
1.2 ft^2 of sunspace per linear foot of greenhouse.  

if we make the sunspace cavity about 4' tall, it will receive about
3,600 btu/day, and supply 480 to the structure on an average day. this
leaves 3,120 btu = 6h(t-30f)4ft^2/r0.8 and an average daytime sunspace
temperature t = 134 f. the water in the solar closet might be close to
this temperature.

keeping the structure warm for 5 30 f cloudy days in a row with water
cooling from 120 to 80 f means the closet needs 5x480/(120f-80f) = 60
pounds of water per linear foot, about 1 cubic foot of water in small
containers, or a combination of larger water containers and stone or
hollow concrete blocks.

the closet needs to supply 20 btu/h per linear foot to the structure
at night and on cloudy days. an 8x16" = 0.9 ft^2 foundation vent with
a 4' chimney might allow 16.6x0.9square_root(4'x(80-70f)) = 93 cfm of
airflow and about 93(80f-70f) = 930 btu/h of heatflow, according to
one empirical formula, so one of those v3s every 32' might work. 

the sunspace needs to supply 20 btu/h to the structure during the day,
and that foundation vent might give 16.6x0.9square_root(4'x(134f-70f))
= 236 cfm of airflow and 236(134f-70f) = 15,104 btu/h of heatflow, so
a single v2 sunspace vent might work. 

the sunspace needs to supply 360 btu to the closet over something
like a 6 hour solar collection day, ie 60 btu/h per linear foot of
structure. if we budget a sunspace-closet temperature difference of
10 f to drive convective flow, a foundation vent can move 930 btu/h,
so we need a v1 sunspace-to-closet vent about every 16 feet.

nick

ps: my apologies if this got double posted during our computer glitch.




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