strawbale/epdm solar heat storage SOLAR.KnowledgePublications.com

strawbale/epdm solar heat storage
28 mar 1996
>...better yet, make a 16' wide x 16' long x 16' tall cube with 4 of these
>16 x 8 x 8' boxes, using 2 16' long x 8' tall site-built epdm collectors on
>the south face, 2 6 gpm pumps each having an 8' head capability, and 4
>internal water cavities, each about 13' long x 6' wide x 6' deep, to store
>15,000 gallons of water at 130 f, ie 6 million btu of usable solar heat for
>a nearby solar house for a cloudy month. this might be nice under a 16' x 16'
>walkout deck, on the west side of a house, or as a house foundation... 

still thinking about how to build this, and how well it would work. suppose
the bale walls were r40, and the average temp in december were 36 f, and the
average sun on a south wall were 1,000 btu ft^2/day, augmented 50% by ground
reflection. then our 16' cube with water at temp t inside might receive about 

ein =  16' x 16' x 1,000 x 1.5 = 384k btu/day, and lose, over an average day, 

eout = 6 hr (t-36) 256 ft^2/r1  from the south wall, daytime 
    + 18 hr (t-36) 256 ft^2/r40 from the south wall, at night
    + 24 hr (t-36) 256 ft^2 x 4/r40 from enw walls and roof, all day

     = (1536 + 115.2 + 614.4) (t-36) = 2265.6 (t-36), so if ein = eout,

t = 36 + 384k/2265.6 = 205.5 f. this looks good, but it probably won't get
that warm without a selective surface in the solar collector. on the
other hand, iron oxide is a somewhat selective surface...

we might start out with a straw bale box. my neighbor ray lehman sells rye
straw bales about 16" wide x 16" deep x 3' long for $1.75 each, delivered,
and rodents are not fond of rye straw, from what i hear. he also has some
4' tall and 4' wide and 8' long bales... each of the 4 cube modules would be
16' wide x 8' deep (or 8" deeper, since the internal wall of the completed
cube would only be 1 bale wide) x 8' tall, and one bale thick all round. the
bottom would be about 5 bales long x 6 bales deep, ie 30 bales, the south wall
would be 5 bales long x 5 bales high, another 25 bales, and the east and west
walls would each be about 3 bales long x 5 bales high, 85 bales total, at a
cost of about $150. now we need an epdm rubber liner, something like this:

                                      |             30'             | 
epdm comes in 20' wide rolls.   ---     .     .             .     .
the liner might cost 28 cents         2'.  6' .     14'     .  6' . 2'
per square foot, ie another $150      ..u.....l.............l.....u..
or so. it would be folded up            .     .             .     . 
like a chinese takeout box      19'     .     .           7'.     .
inside the straw box, so it             .     .             .     .
would have no seams. the points       ..u.....l.............l.....u..
marked l here would be the              .     .           6'.     .
bottom 4 inside corners of the  ---     .     .             .     .
straw box. the rubber would 
fold over along a nw crease at points u to lay flat on the upper edge of
the bale sides... the ew upper edges of the rubber would be clamped in a
2x4 sandwich along the top edge of the nw walls. the liner might want to be
a vinyl swimming pool liner, not epdm, if this is potable, eg rainwater.
(btw, this ascii sketch makes more sense if seen in a non-proportional font,
eg courier on a mac.) 

this sides of the box need some reinforcement to hold back the water pressure.
the sideways water pressure at the bottom of the box would be about 60 pounds
per square foot, diminishing to 0 psf at the top, so an 8' vertical wall stud
on 4' centers would have a total load of 960 pounds, distributed towards the
bottom of the stud. using simple uniform load beam formulas, we might design
the studs with

l = 8' span,
f = 1000 psi, max fiber stress in bending,
w = 30 psf uniform load,
oc = 4' on center spacing,
w = w x oc x l = 960 pounds total load,
m = w x l x 12/8 = 11520 in-lb bending moment,
s = m/f = 11520/1000 = 11.5 in^3 section modulus,
b = 1.5" beam width, and 
d = sqrt(6s/b) = 2.76" beam depth, so a 2 x 4 may work...

something also needs to keep the box from becoming a circle, as we look at it
from above, eg some some perimeter sill plates, perhaps with a dacron rope
crossing the box ns in the middle at ground level on top of the vapor barrier,
connecting the plates.

now, how do we make the vertical epdm rubber collectors? perhaps fold a 16'
long piece of 20' epdm rubber in half, to make a u with a 16' crease at the
bottom, leaving 2' of rubber sticking out on one long side, to overlap the
north top edge of the bale wall. then assemble the wall, screwing 2 horizontal
16+' long pieces of 4' wide, 26 gauge sheet metal to the insides of the studs,
leaving a 3" bulge in each 4' cavity, and attach the outer top edge of the
rubber to the top edge of the 16' long x 8' high stud wall, and tilt it up.
then caulk the ew vertical edges of the rubber bladder on the inside, attach
a pipe to the top and bottom edges, and squeeze them together in a wood
sandwich, somehow attaching the sheet metal firmly as well, so that when the
wall is tilted up, and the top edges of the bladder are also sealed in a
sandwich, the rubber bladder will contain an inch of water from south to north
when filled, and attach a single layer of polyethylene plastic to the north
sides of the 2x4s.

the bottom edge of the tank might look something like this:

                     glazing     |metal epdm
                           ^     ^      ^
                           g     |me    e         
                           g     |meeeeee           
                           g-----|m-----            straw
                           | 2x4 |m 2x4 |
                          -|     |m     |-
                        =| | b o |m l t | | 
			  -      |m     |-
                           |     |m     |
........................... -----  ----- ...............................


how thick would the metal sheet have to be? a 1" strip near the bottom would
have to support a side load of about 8' * .43 psi/ft x 48", or 160 pounds,
over a 4' distance. if it were made of 40k psi steel, with an intentional 3"
bulge in the middle, the left and right sides of the strip would each have a
tension of about 24/3" x 160/2 = 640 pounds, so the metal would have to be
about 640/40k = 0.016" thick, ie 1/64th of an inch thick. it would have to be
carefully attached to the 2 x 4 sandwich, with lots of screws and washers...

the inner horizontal layer of this tank might have a large piece of epdm
rubber covering the whole straw structure, from outside edge to outside edge
in both directions, with the rubber drooping about a foot down into the tank
to rest on top of the water. one might then put top support floor joists
on the tank walls on 2' centers, on top of that rubber, the kind that look
like this:

                            |             ~8'              |
                      ---   ................................
                      ~1'       .                      .
                      ---        ......................

		                |         ~7'          |,

with some plywood or osb on top of that to hold up the tank above. the second
story walls would take about 55 more bales. the top tank roof might have the
space around these floor joists filled with polystyrene beads.

yes there are a few more details here :-) if this box were also used for
sewage treatment, it might have only a single layer of rubber covering the
north side of the straw, with a small aerobic wastewaterfall leaking over the
14' north side of the tank, to trickle down the rubber, which would take a
little more pump power. we could also use more details about some good way
to heat the water directly with sun-warmed air, with some sort of fins under
the water tank, to remove the heat from the air and conduct it to the water
via some sort of thermally-conductive surface under the rubber, eg ferrocement
over some chicken wire and straw, or a corrugated iron layer with a layer of
concrete poured on top, so we don't have to build this epdm solar collector.

nick

     when we play tennis or walk down stairs, we are actually solving
     whole pages of differential equations, quickly, easily, and
     without thinking about it, using the analogue computer which we
     keep in our minds. what we find difficult about mathematics is
     the formal, symbolic presentation of the subject by pedagogues
     with a taste for dogma, sadism and incomprehensible squiggles.

		  from _structures: or why things don't fall down_
			by j. e. gordon, a da capo press paperback