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a bubble wall?
12 apr 1996
certain standard works like the psic guidelines for passive solar house
design teach inefficient passive solar house design. but to me the principles
of efficient passive solar house design aren't hard to learn. unlearning is
often the hard part. a couple of days ago, i explained most of them in
2 hours to a smart 13 year old, working thru 7 pages of simple equations
and examples together. i had asked her if she had studied algebra in school
yet (she was reading a physics text as i walked up to her at a science fair),
and she said "no, but i know all that" :-) and she did.

there was also a 12 year old doing bubble experiments. she discovered that
if she doped her standard bubble mix (1/2 cup green dawn, 3 tbs glycerin,
and 2 quarts of water) by adding 1/4 tsp of lemon juice, the bubbles lasted
111 seconds at 80 f and 506 sec at 45 f, vs 309 sec and 185 sec for the
standard solution. 1/4 tsp of maple syrup changed this to 431 and 345 sec.
she also tried adding jello, perfume, and another dozen substances... her
parting thought was quite serious: "dust is the enemy of bubbles."

so, led by this little child, let's all buy some glycerin, and try to invent
a bubble wall, eg two pieces of single-layer polycarbonate plastic with butyl
tape over a plastic 1x3" frame that can be filled between with bubbles at
night that emerge from a pvc pipe with a few holes immersed in a soapy
solution at the bottom, connected to a small aquarium air pump with a timer?
this could be very useful in a passive solar house, like beadwall. simple
movable insulation. during the day, the sun shines in on some thermal mass,
and at night the glazing fills up with bubbles, keeping the heat in.
commercial greenhouses use two huge layers of uv-treated polyethylene film
inflated to form an air pillow 4" thick. a tiny 50 watt blower can inflate
a 1 acre greenhouse. would bubblewalls work with poly film pillows?

bubbles tend to last longer in cold humid condtions. a layer of frozen
bubbles inside an outside glazing might be very good insulation on a very
cold night. perhaps there is an optimal bubble size for insulation. too
small, and convection losses might be small, but conduction losses and air
pump power and water transport thermal losses might be large. too big, and
convection losses go up. steve baer tried making solar collectors with
bubblewalls years ago, but he let the sun shine though the bubbles during
the day, vs leaving the bubblewall cavity empty. he found that the walls 
of his bubbles were too thin to block ir re-radiation, altho he read later
that others had had more success. a bubblewall that is empty during the day
could be filled at night with bubbles with thicker or more opaque walls,
made with some viscous opaque solution, perhaps containing some green dye.
i guess we have to let a little air leak out of the top of the glazing 
cavity, but we would want to break the bubbles at the top and let the water
run back down through the cavity.

ohm's law for heatflow is a good start: the amount of heat q in btu/hour
that flows through a wall with area a ft^2 and r-value r and temps ti (f) on
one side and to on the other, is q = (ti-to)a/r. 

here's one bubble wall test setup, a 2' cube divided in half by a bubble wall
that might have an r-value of 2 when empty and 12 when full. we might make
the cube out of 2" styrofoam with an r-value of 10. one side could be kept
at 32 f with some melting ice at the top, with some foam on top and around
the ice tray, and the other side could be kept at 132 f with a thermostat
and a light bulb, with a piece of aluminum foil to shade the bubblewall from
the bulb. how much ice water might we collect in an hour with the bubblewall
empty and full of bubbles? what might we get for readings in each case if
we hook up the light bulb to a kwh meter? it's nice to have two ways to
check the heatflow through the bubble wall.

                       .........................      70 f room
                    .   ice     ..          .  .
                 .........................     .
                 .           ..          .     .
                 .    to     ..  ti      . r10 .  2'
              2' .    32 f   ..  132 f   .     .
                 .           ..  light   .     .
                 .           ..  bulb    .  .
                 .........................    2'
                       1'         1' 
                             ^
                             |___ bubble wall

it takes 144 btu to melt a pound of ice, and there are 3410 btu in a kwh.

nick




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