Subject: Yet another solar heated swimming pool

From: [email-masked] (Nick Pine)
Date: 24 Mar 1996 13:53:02 -0500

So, I'm looking at the $450 15' diameter x 42" deep swimming pool in the 1993
Spring/Summer J C Penny catalog, which would contain about 619 cubic feet of
water, ie about 5,000 gallons or 40,000 pounds of water...

Solar pool heating is easy, compared to other kinds of solar heating, since
temperatures are low, and there's a built-in thermal mass. What would it take
to keep this pool warm all winter, or to make it into a huge outdoor hot tub,
in the 5,500 F degree day climate where I live, near Philadelphia?
 
The biggest problem with solar pool heating is the cover. "Solar pool covers"
have low R-values, usually not more than R1, and they are tinted blue, which
keeps the sun out :-) Suppose we somehow cover the pool with a fairly rigid
R-20 cover, and keep it closed all the time, except when the pool is in use. 

The cover might be a 20' x 20' x 6" thick panel made with 2x6s on 4' centers,
with 6" of fiberglass insulation inside, and 1/4" flakeboard or galvanized
sheet metal on top, with some plastic-coated shiny aluminum foil on the bottom.
Innovative Insulation in Arlington, Texas, sells some shiny tough foil for
mobile home roofs in 4' wide rolls for about 50 cents/ft^2. The pool cover
might be hinged with a pipe along the north edge, with a counterweight, so
it could easily be tilted up to the south at about a 45 degree angle. This
would reflect some winter sun into the pool and reduce the wind when the cover
is open, but the main pool heating would come from a 4' high x 20' long piece
of vertical plastic glazing along the south edge, with an air gap behind that
and some dark-colored straw bales behind that, to act as a solar air heater. 

Suppose the pool were surrounded by straw bales, each 16" x 16" x 36", costing
$2 each, eg 40 straw bales under the pool, and another 70 bales around all
the sides of the pool, to make a 4' high wall around the pool, 6" taller than
the pool, with an average R-value of 20 (?) The bales around the perimeter
could be stacked up and mortared together like giant concrete blocks to make
a deck for the pool with a wide flat edge, and the soft underside of the cover
might deform and sit on the deck to make a good seal when it is closed.

The straw bales under the pool itself could be spread apart to make 8 air
ducts, each 6" wide x 16" deep, running from north to south along the ground,
to allow solar-warmed air from the south glazing to travel across the top
of the pool, under the cover, down the north back wall of the pool, between
the pool and the bales, and under the pool from north to south. This might
work with passive plastic film gravity backdraft air dampers in holes at the
top and bottom of the straw bales behind the glazing, but a PV-powered (-:)
fan would make it work better. The whole ground area should be covered with
some sort of vapor barrier, eg poly film, to keep moisture out of the straw,
and the tops of the ducts below the pool should have some bridging, eg 1/8"
of newspaper with 3 layers of chicken wire on top of that, and an inch of sand
mix cement over that. Let's ignore the heat loss from the ducts to the ground. 

A shallow reflecting pool made from a single piece of 10' wide EPDM rubber
roofing material over a 6" earth berm around the edge would augment the sun
when frozen and keep weeds from growing up in front of the glazing.

It might look like this, not to scale:     ^  (open)      
                                           |
   cccccccccccccccccccc              ccccccccccccccccccccHcccccCW 
    gggggggggggggggggg                g..................P _
    gggggggggggggggggg         <-- S  g  ppppppppppppppp P
    gggggggggggggggggg                g  pppp water pppp P 4'
    gDDDDDDDDDDDDDDDDg   r ~~~~~~~~~~ g  ppppppppppppppp P
.........................rprrpprprprp ...................P.............
                         |<--- 8' --->|  |<--- 15' --->| P
                                      |<------ 18' ----->|

    key:                 rprprprp----ccccccccccccccccccccHcccccCW 
                         p           cgsssssssssssssssssss      .
    p is the pool        r           cgs       ppp       s      .
    s is straw           p  EPDM     cgs  p-c- (D) -c-p  HcccccCW
    g is glazing         . rubber    cgs       (D)       s      .
    c is the cover       .           cgsp  -c- (D) -c-  pHcccccCW
    D are hot air ducts  .     20'   cgsp      (D)      ps      .
    rp is a reflecting pool          cgsp  -c- (D) -c-  pHcccccCW
    H is a hinge         .           cgs       (D)       s      .
    P is a post          .           cgs  p-c- (D) -c-p  HcccccCW
    CW is a counterweight.           cgs       ppp       s      .
                         .           cgsssssssssssssssssss      .
                         rprprprp----ccccccccccccccccccccHcccccCW

                         |<--  8' -->|<---     20'   --->|

Suppose that in winter, 1,000 Btu ft^2/day makes it into the single layer
flat polycarbonate glazing, augmented 50% by the frozen reflecting pool
on an average day in December. Then the solar energy input would be 

Ein = 1,000 x 1.5 x 4' x 20' = 120K Btu/day, 

Suppose that during 6 hours of sun on an average day in December,
the outdoor temp is 36 F, and the pool has some sort of R0 cover that
just prevents evaporation, and the air heater air and the pool water
have the same temperature T. 

Then the energy that flows out of the pool during an average December day is

Eout = 6 hours (T-36)  80 ft^2/R1   from the south glazing during the day
    + 18 hours (T-36)  80 ft^2/R20  from the south glazing at night
    + 24 hours (T-36) 400 ft^2/R20  up through the cover all day
    + 24 hours (T-36) 240 ft^2/R20  out through the ENW sides all day

     = (6x80/1 + 18x80/20 + 24x400/20 + 24x240/20) (T-36)

     = (480    +    72    +    480    +    288   ) (T-36)

     = 1320 (T-36), so if Ein = Eout for an average day, we have

1320 (T-36) = 120K or
  
          T = 36 + 120K/1320 = 126.9 degrees F.

Looks good... :-) Suppose we keep the pool at 106 F, eg by allowing some of
the average 30 gallons of rain per day that fall on the pool cover in PA to
flow through the pool and into the reflecting pool, which would keep the pool
cleaner with fewer chemicals. 

How much will the pool cool on a average December day with no sun?

Eout = 24 hours (106-36)  80 ft^2/R20  from the south glazing all day
     + 24 hours (106-36) 400 ft^2/R20  up through the cover all day
     + 24 hours (106-36) 240 ft^2/R20  out through the ENW sides all day

     = (24x70x80/20 + 24x70x400/20 + 24x70x240/20)

     = 6,720 + 33,600 + 20,160 = 60,480 Btu.

Since 1 pound of water loses 1 Btu when it cools 1 degree F, the pool temp
after a day without sun would be roughly 106 - 60,480/40,000 = 104.5 F.

How many 36 F days without sun would it take for the pool to reach 70 F?

The pool has a thermal resistance of R = R20/720 ft^2, so its RC time constant
is RC = R20/720x40,000 Btu/F = 1,111 hours or 46 days.

With no sun at all, and the cover closed, the pool temperature should be about

  T = 36 + (106 - 36) exp(-t/46), over many days, so if T = 70, 

 34 = 60 exp(-t/46), or 0.57 = exp(-t/46), or t = - 46 ln(0.57) = 26 days.

If it were 10 below zero outside for t days without any sun, the pool
might just begin to form a thin layer of ice on top when

 32 = -10 + (106-(-10)) exp(-t/46) ==> t = -46 ln(42/116) = 16.7 days.

Freezing it solid from top to bottom would take another 144 Btu/lb x 40K lb
= 5.76 million Btu, as the pool loses 36K Btu/day, not counting the thermal
resistance of the ice layer, ie at least another 159 days with no sun, at
minus 10 degrees F.

Nick

It's a snap to save energy in this country. As soon as more people become
involved in the basic math of heat transfer and get a gut-level, as well as
intellectual, grasp on how a house works, solution after solution will appear.

                                          Tom Smith

---

From: [email-masked] (Nick Pine)
Date: 25 Mar 1996 07:25:14 -0500

Seems like I have to go back and fix even the simplest things, even while they
are still on paper, which is a good time to do that. Perhaps this post should
have stewed for a couple of days before posting, but you know how it is when
you get excited about an idea, especially after a dozen espressos :-)

So here's the problem, in the following paragraph--not a large enough ratio of
thermal mass surface area to glazing surface area, in this solar closet. That
ratio should be 10:1 or so, to keep the temperature difference between the
air heater air and the thermal mass low, in this system with thermal mass
indirectly heated by warm air, and make the solar collection efficient, but
the glazing here is 80 is ft^2, and it's really equivalent to 120 ft^2 of
solar heat, with the reflecting pool, and the round pool sides are only 165
ft^2 and the pool bottom only has about 32 ft^2 of surface exposed to the sun-
warmed air, so that ratio is only 2.5:1, as posted. Nobody noticed this, altho
I did get some email from someone who said he "didn't do math," but we should
cover the top with PV and water heating panels and use antifreeze and pumps
and pipes buried in a radiant concrete floor slab under the pool :-)

Please change the following paragraph:

Here's a fix: forget the newspaper, make the ducts a foot wide, use 30 instead
of 40 straw bales under the pool and put in 100 concrete blocks with holes in
them, standing on end between the ground and the chicken wire ferrocement, to
act as fins to conduct heat up to the pool bottom. Each concrete block has a
surface area of about 6 ft^2 exposed to the warm air, so this way, we have 80
ft^2 of glazing and about 200 + 6*100 ft^2 of thermal mass surface area to
gather heat from the warm air, ie a nice 10:1 ratio. 

Nick

---

From: [email-masked] (Nick Pine)
Date: 25 Mar 1996 13:32:52 -0500

Nick Pine <[email-masked]> wrote:
 
>ducts, each 6" wide x 16" deep, running from north to south...

>act as fins to conduct heat up to the pool bottom...

Better yet, forget the ducts and cement blocks and ferrocement, just build
the platform from strawbales and use a tiny pump and a 20' x 4' length of
SolaRoll, the EPDM rubber swimming pool tube-mat collector, under the south
edge vertical glazing.

Better yet, forget the store-bought pool and put a treated wood 2x4 frame
around the straw bales and line an 18' x 12' x 4' inner cavity with a single
piece of 20' wide EPDM rubber, folded up like a Chinese takeout box, so it
has no seams.

Better yet, make the solar collector out of another single piece of EPDM
rubber folded once into a 4' high x 12' long x 1" thick vertical V, with the
sides and top sealed in a 2x4 sandwich with silicone caulk between the rubber
sheets, a la Sven Tjernagel's 470 Pennsylvania site-built solar collectors,
(some still going strong), but vertical vs slanted, and filled with water,
vs a trickle down system, with some chicken wire under the north sides of
the 2x4 to limit the rubber bulging, and some glazing screwed to the south
sides of the 2x4s. 

Better yet, make an 20' wide x 16' long x 16' tall cube with 4 of these
20 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 16' long x 8' wide x 6' deep, to store 24,576
gallons of water at 130 F, ie 10 million Btu of solar heat for a nearby house
for a whole cloudy winter. This might be nice under a 16' x 16' walkout deck,
on the west side of a house... If one were going to drink the water, the inner
tank liners might better be pond liner material, not EPDM, and the collectors
might contain propylene glycol "edible antifreeze," with a heat exchanger
inside the tanks. 

The pumps could be PV-powered of course. ("Bad move," says Sven Tjernagel :-)

Nick

                                          Tom Smith

---

From: "Bruno, Thomas A." <[email-masked]>
Date: Tue, 26 Mar 1996 05:13:41 +0000

> Spring/Summer J C Penny catalog, which would contain about 619 cubic feet of

[much deleted]

What about the straw rotting?  What about rodents? What about 
compression of the straw under the pool?
-- 
Thomas A. Bruno;  Champaign/Urbana, Illinois
Voice:217.328.6000; Fax 217.328.6765
Visit my web page at http://www.shout.net/~tombruno/

---

From: [email-masked] (Nick Pine)
Date: 27 Mar 1996 05:39:16 -0500

Bruno, Thomas A. <[email-masked]> wrote:

>What about the straw rotting? 

Not a problem if it's kept dry. 

>What about rodents?

Unlikely, if the perimeter wall is put up with mortar. 

>What about compression of the straw under the pool?

One of my neighbors has a circular on-ground pool with a "stretchable liner,"
that makes it 18" deeper than it appears to be from the side... 

Nick

---

From: [email-masked] (William Bahnfleth)
Date: Wed, 27 Mar 1996 17:05:10 GMT

>that makes it 18" deeper than it appears to be from the side... 

Straw bale construction is not really a new idea, although a straw bale 
swimming pool might be...

As part of a "sustainable" technology program, a group of Architectural 
Engineering/Architecture/Landscape Architecture students here at PSU have been
designing and plan to build a straw bale building that will serve as a meeting 
center.  The walls will be covered with stucco.  The R-value of such walls is 
quite high and, as Nick notes, it doesn't rot if you keep it dry.  I've 
occasionally wondered whether it would be cost effective to shrink-wrap the 
bales before installing them to make sure there's no moisture migration, but 
people who know say that it shouldn't be necessary.

Bill Bahnfleth

--
William P. Bahnfleth, Ph.D., P.E.
Department of Architectural Engineering
Penn State-University Park
[email-masked]

---

From: [email-masked] (Nick Pine)
Date: 28 Mar 1996 16:44:01 -0500

>...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
>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

---

From: [email-masked] (Nick Pine)
Date: 30 Mar 1996 08:18:57 -0500

A PV friend says: 

>I've been dreaming of suspending an array in between two trees
>at about the 90' level...

With ropes going east and west? That seems interesting, 3/4 up those tall
trees 300' to the south of your house, with some nylon ropes running north
and south to the ground from the bottom? Seems like this would usually lead
to shading by other trees, if not falling branches, etc. I would like to
build a treehouse, a large one for living in, even. I traded some letters 
with serious treehouse people years ago, one a prof in North Carolina.

Suppose the PV structure had some internal volume and a floor, with a rope
ladder for courageous children attached to safety ropes. We might insulate that
volume and put some thermal mass inside, eg a few batteries surrounded by an
air gap and insulation, to capture some of the other 90% of the sun's heat...

For two axis tracking, suppose it were supported from above at three points,
with a fixed rope at the top on the north side going up and north to a tree,
another at the north bottom center going to the ground, and two more going up
SE and SW from the EW upper south corners, said ropes being attached to two
small DC reversible winches or garage door openers inside the treehouse. A
rope ladder hanging from the south side deck would make this more stable. To
point east, crank in the west rope and let out the east rope, and vice versa
to point west. To tilt the array upwards, crank in both ropes. To tilt it down
towards the south horizon, let out both ropes. Picture a 4' x 4' x 8' shed,
with a reflective front porch and a 1:2 roof sloping up to the south and a
reflective overhang. It might look like this:

         from the east                from the south front

     \         8'                \             12'           /
       .                           .........................
       .   .   ^                   . .                   . .
       .       .                   .   .................   .
       .       .   .               .   .       8'      .   . 6'
       .       .       .           .   .             4'.   .
       .       .     PV. 2'        .   .      P V      .   .
       .................           .........................
           4'      4'    \                     l
       l                                       4'
                                           .........   
       l                                  .    PV   .
                                         .           .
       l                                .             .
                                       .       .       .    8'
       l                              .                 .
                                     .                   .
       l                            .                     .
                                   .........................
       l                         /                           \
                                          from the top

This would have a PV area of 8 ft^2 and a frontal area of 72 ft^2 with
a concentration ratio of 9, so the peak sun power would be 6.5 kW. How
would we cool it? Perhaps make the PVs the back of an thin aquarium tank,
with the sun shining through a little water, and a radiator above.

How big a radiator? Say the PVs collect 650 watts peak, with a solar energy
input of 5.5 kWh/m^2/day in July in Seattle, ie 36.8 kWh of solar energy in,
and 3.68 kWh/day of electrical energy out. The difference is 33.1 kWh/day.
That is the heat we need to deliberately waste. The average temperature in
Seattle in July is 18.4 C. Suppose we want to keep the average PV temp
at 28.4 C. The average windspeed is 3.5 m/s, or 7.8 mph. So assuming we
have enough water to store the heat for 24 hours, we have to waste 33.1 kWh
of heat, about the same as the heat in a gallon of oil, in 24 hours, with a
10 C temperature rise, or 138 watts/degree C, or 261 Btu/hr/degree F. A smooth
174 ft^2 surface or rough 130 Ft^2 surface would do that with no wind, as
would 44 ft^2 of rough surface exposed to the wind, or the wind from an
exhaust fan to the north on a calm day. Or we might try to evaporate about
0.005 gallons (0.7 oz.) per minute of water by slowly pumping it up to the
ridge peak and letting it trickle back down into the tank, or let some
rainwater flow thru the tank. 

How much water would we need to store that heat? 33.1 kWh will heat 6720
pounds of water 10 degrees C. Perhaps we should try to get rid of most of
the heat in real time, and cover the backs of the PVs with fins and such. 

It would be interesting if we could turn this structure around to the north,
swing the PVs out of the way with a linear actuator, say, and scoop up some
wind to flow through the exhaust fan at night, or on a cloudy day, to make
more electrical power.

With a couple of batteries inside and an inverter, we could run 110 or
240VAC wires up the north rope and back to the house. 

The rope ladder might not touch the ground. It might just end in a large
weight, say 3' above the ground, to make a large and interesting pendulum
with a period of about 5 seconds. If the rope ladder slanted up to the
treehouse from the south, we might add two more ropes to the ground to the
NW and NE and enclose the EW sides of the tetrahedron between the treehouse
and the ground, eg with dacron sailcloth, or burlap/cement, and put a wind
turbine on the ground at the south lee corner, just north of the base of
the rope ladder, to make a big north-facing windscoop (and snowscoop--we
might make the EW sides transparent, or enclose the north side too, and make
another house, barn, or solar closet and sunspace underneath, parking a car
or a tractor under the PV shed, or storing some hay or animals... 

A PV shed on top of a flat roof in a city might be suspended from the point
of a tetrahedral frame made with 6 poles, with some sandbags at the corners.

Suspending a real solar house from a 3-cornered manmade arch this way would be
interesting. It wouldn't have to be very high off the ground to let the house
track the sun, altho a 3-cornered St Louis-type arch would be nice :-), and
that might also serve as the house foundation... Electric and water might
enter the house with flexible lines from the top, with the house rotating no
more than 360 degrees, perhaps facing north on summer days. 
 
It might be less expensive to suspend the house from 3 telephone poles,
like this, seen from the top:
                                          s
                                          .
                                          .
                                          .
                                          p
                                          .
                                          .
                                          .
                                          H
                                        .   .
                                      .       .
                                    .           .
                                  p               p
                                .                   .
                              .                       . 
                            s                           s

and like this from the side:      p       p       p
                                . p   .   p   .   p .
                              .   p       H       p   .
                   .........s.....p.......p.......p.....s............
                            s                           s

The poles might each hold up about 30K lbs from the weight of the house
and another 30K pounds or so from the guy wires, so each pole would need
a 30 ft^2 footing, perhaps, and at 50 K psi, the steel cables would need
to be about 1" diameter. Big nylon or dacron ropes might work instead of
the steel cables. The wood of the poles would probably hold the weight in
compression parallel to the grain, but the poles should probably be cemented
on the outside, over wire reinforcement, to make them more durable and
increase their polar moment of inertia, so they aren't so inclined to buckle.
Tilting the poles out a bit would make the guy wires smaller. Tilting them in
so they touch would help too, but that doesn't leave much room for the house.

Althernatively, the house might be held up by a geodesic telephone pole
structure, 20 poles, each 30' long (connected by what, ropes thru holes in
the poles? Bolts, and steel plates. Lots of chicken wire and cement?) This
would be about 150' in circumference, about 50' in diameter (if the poles 
poles were curved and pi were 3) and about 40' tall at the central point.
Adding 5 more poles on the ground would solve the footing problem, and make
a nice berm for the EPDM rubber reflecting pond and wastewater treatment
facultative lagoon, with a small fountain. Enclosing the whole thing in
polyethylene film might be nice. Un gout special.

What would the building inspector say? Perhaps not much about safety,
if the house were only 6" above the ground.

What would the architect say ("THAT doesn't look like a house!" :-)

What would the health department say?

What would the neighbors say?

What would the bank say?

Or... the house could be floated on a shallow pool. A 30 x 30' two story
house might weigh 100 psf, requiring a 2' deep pool, taking some or all of
the load off the telephone poles. 

We could try this out with a prefab 8 x 12' shed, with PVs mounted on the
south wall, a reflective south deck and an overhang, and reflective wings,
sitting in a pool made with a 20' x 20' piece of EPDM rubber over a 2' tall
perimeter earth berm. The pond corners might point to compass directions,
and the shed might have a 12' rope from the center of the south wall to the
south pond corner. We might mount two winches or garage door openers at the
EW corners of the north wall of the shed, high up, connected to ropes to the
EW pond corners, for treehouse-type two axis tracking, tilting the shed back
to the north to point it up into the sky. The shed might have a waterproof
skirt coming 2' up the walls on the outside, made from a single piece of
EPDM rubber. So might a modular home... With all that water around it,
PV cooling should be easy. One might even want to solar heat the house.

>The sun hits the tops of the trees at 6 am or so, and doesn't hit the PVs
>I have now for several hours after that. It just becomes a question of how
>much it costs in money and aesthetics to put them up there, and how much
>more vulnerable they become.

A small PV tracking treehouse, 90' up, could be delightful. It might have
a few mirrors or a bit of stained glass to glint onto your house, 300' to
the north. Some wind chimes, perhaps, or some propellors and generators on
the guy wires. With the right combination of strong and elastic ropes and
mass and damping, vulnerability should not be a problem. It wouldn't be very
expensive either. Small DC winches cost less than $200. Some people use
$150 Sears radio-controlled garage door openers to haul out boats. 

Nick

---

From: [email-masked] (Nick Pine)
Date: 1 Apr 1996 15:07:23 -0500

For 2-axis tracking, suppose it were supported from above at three points,

     \         8'                \             12'           /
       .       .                   .   .               .   .
       .       .   .               .     .     4'    .     . 6'
       .       .       .           .      ..........       .

How big a radiator? Say the PVs collect 650 watts peak, with a solar energy
input of 5.5 kWh/m^2/day in July in Seattle, ie 36.8 kWh of solar energy in,
and 3.68 kWh/day of electrical energy out. The difference is 33.1 kWh/day.
That is the heat we need to deliberately waste. The average temperature in
Seattle in July is 18.4 C. Suppose we want to keep the average PV temp
at 28.4 C. The average windspeed is 3.5 m/s, or 7.8 mph. So assuming we
have enough water to store the heat for 24 hours, we have to waste 33.1 kWh
of heat, about the same as the heat in a gallon of oil, in 24 hours, with a
10 C temperature rise, or 138 watts/degree C, or 261 Btu/hr/degree F. A smooth
174 ft^2 surface or a rough 130 ft^2 would do that with no wind, as would
44 ft^2 of rough surface exposed to the wind, or the wind from an exhaust fan
to the north on a calm day.

Or we might let some rainwater flow through the tank, or try to evaporate
0.005 gallons (0.7 oz.) per minute of water by slowly pumping it up to the
ridge peak and letting it trickle back down into the tank, over the 64+ ft^2
roof surface. The partial pressure of water in air at 18 C is 15.3 mmHg,
which becomes 10.1 mmHg in Seattle's 66% average July humidity, I guess,
and at 28 C, the partial pressure of water in air is 28 mmHg. Duffie and
Beckman's _Solar Engineering of Thermal Processes_, 2nd edition, has this
formula 18.5.2 on page 664:

   Qc/Qe = 0.46(Tp-Ta)/(Pwp-Pa),

where Qc is the heat loss by convection from a surface in W/m^2-C and
      Qe is the heat loss by evaporation, and Tp and Ta are in deg C, and 
      Pwp and Pa are the partial pressures of warm pond water and cooler
      water vapor in ambient air. This hardly changes with wind speed.

Cranking through all this, I found that we might waste all this heat by
evaporating a little PV cooling water at just under 20 C, not 28 C. At 28 C,
about 4 times more heat is lost by water evaporation than by convection.

Suspending a real solar house from a 3-cornered manmade arch this way would be
interesting. It wouldn't have to be very high off the ground to let the house
track the sun, altho a 3-cornered St Louis-type arch would be nice :-), and
that might also serve as the house foundation, and deter termites and mice...
Electric and water might enter the house with flexible lines from the top,
with the house rotating no more than 360 degrees, perhaps facing north on
summer days. 

What would the bank say?

We could try this out with a prefab 8 x 12' shed, with PVs mounted on the
south wall, a reflective hinged floating south dock and an overhang, and
reflective wings, sitting in a pool made with a 20' x 20' piece of EPDM
rubber over a 2' tall perimeter earth berm.

The pond corners might point to compass directions, and the shed might have
an 10' rope or conduit from the center of the south wall to the south pond
corner. We might mount 2 winches at the EW corners of the north wall of the
shed, high up, connected by ropes to stakes in the ground 12' NE of the NE
pond edge and 12' SE of the SE pond edge, for treehouse-type 2-axis tracking,
tilting the shed back to the north to point it up into the sky. The shed might
have a waterproof skirt coming 2' up the walls on the outside, made from a 
single piece of EPDM rubber. So might a modular home... Only one winch, eg
with a continuous rope wrapped round the drum would be needed for adjustable
tilt, 1-axis tracking. With all that water, PV cooling should be easy.
We might even heat the house with the sun.

A small PV tracking treehouse, 90' up, could be delightful. It might have
a few mirrors or some of stained glass glinting onto your house, 300' to
the north. Some wind chimes, perhaps, or some propellors and generators on
the guy wires. With the right combination of strong and elastic ropes and
mass and damping, vulnerability should not be a problem. It wouldn't be very
expensive either. Small DC winches cost less than $200. Some people use $150
Sears radio-controlled garage door openers to haul boats out of the water. 

Nick

A word from Lewis Carroll, on this fine day of fools:

(1) Every idea of mine, that cannot be expressed as a syllogism,
    is really ridiculous.
(2) None of my ideas about bath-buns are worth writing down.
(3) No idea of mine, that fails to come true, can be expressed as a syllogism.
(4) I never have any really ridiculous idea, that I do not at once
    refer to my solicitor.
(5) My dreams are all about bath-buns.
(6) I never refer any idea of mine to my solicitor,
    unless it is worth writing down.

There is a simple conclusion to be derived from these statements...

---

From: [email-masked] (Nick Pine)
Date: 2 Apr 1996 10:37:40 -0500

It is hard to think of any other system that supplies so much heat
   (to an existing house) at such low cost...

   One could shorten the warm-up time of the enclosure and increase
   the amount of heat delivered to the rooms by making the enclosure
   virtually massless--by greatly reducing its dynamic thermal capacity.
   This can be done by spreading a 2-inch-thick layer of lightweight
   insulation on the floor and north wall of the enclosure and then
   installing a thin black sheet over the insulation. Then, practically
   no heat is delivered to the massive components of floor or wall;
   practically all of the heat is promptly transferred to the air.
   And since the thermal capacity of the 100 or 200 lb. of air in
   the room is equal to that of one fourth as great a mass of water
   (about 25 to 50 lb. of water), the air will heat up very rapidly.
   I estimate that its temperature will rise about 40 F. degrees in about
   two minutes, after the sun comes out from behind a heavy cloud cover.
   At the end of the day, little heat will be "left on base" in the 
   collector floor or north wall and, accordingly, the enclosure will
   cool off very rapidly.

     New Inventions in Low Cost Solar Heating--
     100 Daring Schemes Tried and Untried
     by William A. Shurcliff, PhD
     Brick House Publishing, 1979, 293 pages, $12

   A sunspace has extensive south-facing glass, so sufficient thermal mass
   is important. Without it, the sunspace is liable to be uncomfortably hot
   during the day, and too cold for plants or people at night.

   However, the temperature in the sunspace can vary more than in the
   house itself, so about three square feet of four inch thick thermal
   mass for each square foot of sunspace glazing should be adequate...

   The sunspace floor is a good location for thermal mass. The mass floors
   should be dark in color. No more than 15-25% of the floor slab should be
   covered with rugs or plants... Another good location for thermal mass
   is the common wall (the wall separating the sunspace from the rest of
   the house)... Water in various types of containers is another form of
   energy storage often used in sunspaces.

	     Passive Solar Design Guidelines--
	     Guidelines for Homebuilders
	     for Philadelphia, Pennsylvania
	     Passive Solar Industries Council
	     National Renewable Energy Laboratory
	     Charles Eley Associates 
	     Current edition, 88 pages, $50 

So, which is the most energy-efficient sunspace in a partly cloudy climate
like Philadelphia?

Shurcliff's plastic film sunspace, wearing the green uniform in this contest,
might cost about $2/ft^2 and begin an average December day at 36 F, and like
the PSIC sunspace, it would receive about 1000 Btu/ft^2/day over the average
day. Let's assume that both sunspaces have a perfectly insulated wall between
them and the house, to avoid the thermal disaster of a poorly insulated Trombe
wall in a partly cloudy climate. (Trombe walls can't even get to the starting
line in this race.) And that there is no air infiltration from the outside in
either case. Shurcliff's sunspace air would be circulated through the house
with some dampers or fans, keeping the sunspace at 80 F, say, while the house
remains at 70 F. With single glazing, about 900 Btu/ft^2 of sun might enter
the sunspace during the day, and the amount of heat lost through a square foot
of Shurcliff's sunspace over a typical 6 hour December day would be about
6 hr (80-36)/R1 = 264 Btu/ft^2/day, for a net gain of 636 Btu/ft^2, ie his
$2/ft^2 sunspace would be about 64% efficient, as a solar collector. As
an auxiliary living space, it could be heated up instantly on some starry
night for a party, by moving some warm air from the house into the sunspace.

A PSIC sunspace, wearing the brown uniform, would perform better with double
glazing. He might cost $10/ft^2. Say his thermal mass is poured concrete,
4" thick, with an official PSIC heat capacity of 8.8 Btu/ft^2, and say it
absorbs 100% of the sun that falls on it, vs the official PSIC solar
absorptance of bare concrete of 0.65 (table K, page 57.) Then about 800
Btu/ft^2/day of sun will enter the double glazing and be absorbed by the
concrete, and the concrete surface will warm up the sunspace air, and that
warm air can be used to heat the house when the sunspace temperature is more
than 80 F. Suppose the concrete loses no heat at all to the soil below (I'm
giving quite a few handicaps to the PSIC sunspace in this efficiency race.)
Suppose the PSIC sunspace starts out the day at temperature T, and the
concrete charges up in the sun to a maximum temperature of T + dT, and
returns to temperature T at dawn, on an average day in December. How can we
calculate T and dT? We have an equivalent electrical circuit that looks
something like this:

                             Ts sunspace temperature
                             |
                      R2     |            S 
       36 F ---------wwww-----------------|-------------------- 70 F
       outdoors      glazing |            open switch to heat   house 
                             |
                             w
                             w  R0.5 concrete - sunspace air resistance 
               800 Btu/ft^2  w
                   per day   |
            |      ---       |
          | | ----|-->|------|--Tc concrete temperature
            |      ---       |
               sun current   w
                   source    w  R0.4 concrete bulk thermal resistance
                             w
                             |
                          ------- 8.8 Btu/F thermal mass of concrete
			  -------           
                             |
                             |
                            ---
                             -

Let's simplify this, by assuming the thermal mass of the concrete is infinite,
vs 8.8 Btu/ft^2/F. Lots of concrete, or a water wall, or something that has
so much thermal mass that the temperature inside the sunspace never changes
at all from day to night or day to day over a long string of average December
days, with some sun. This is an optimal sunspace with more than "adequate" or
"sufficient" thermal mass by official PSIC standard guidelines. Let's also
assume that the two small resistors have a value of zero, ie let's assume the
the R0.4 bulk thermal resistance of the concrete, that makes the surface heat
up more than the inside, while the sun is warming it up, and makes it harder
to get heat out of the inside of the concrete and into the sunspace air, and
the R0.5 concrete-sunspace air resistance, are both R0 conductors. What will
Tc be in that simplified case?

The sun shines into the sunspace during the day and adds 800 Btu to our
concrete capacitor, and over 24 hours, 24(Tc-36)1ft^2/R2 = 12 Tc - 432 Btu
flow out of the capacitor. If Ein = Eout (providing no heat for the attached
house), then Tc = (800+432)/12 = 103 F. Pretty nice, but this sunspace is
not providing any heat for the house, just keeping itself warm on an average
day, and losing lots of heat on a cloudy day. Suppose we allow some heat to
flow from the sunspace into the house, ie close the switch, ie turn on the
fan or open the damper between the sunspace and the house often enough to
limit the maximum sunspace temp to 80 F instead of 103 F. Then the heat loss
to the outside world over the course of a day is 24(80-36)1 ft^2/R2 = 528 Btu,
and the rest of the heat that enters the double glazing, ie 800 - 528 = 272
Btu/ft^2/day goes into heating the house, so the solar collection efficiency
of this $10/ft^2 sunspace in terms of useful heat provided for the attached
house is 27%. As an auxiliary living space, the temperature of this sunspace
is largely out of our control. It takes a long time and a lot of house heat to
warm it up on an evening or cloudy day, and after we leave the space, it stays
warm for a long time, giving up precious house heat to the outside world.

How curious that by carefully following the current official guidelines of
the Passive Solar Industries Council, we can reduce the performance of
Shurcliff's low-thermal-mass sunspace from 64% to 27%, while increasing the
price from $2/ft^2 to $10/ft^2, unimproving the cost-effectiveness of the
sunspace by a factor of 12, even with all these PSIC-slanted assumptions... 

Here's a quote from the Acknowlegements section of the PSIC guidelines:  

   _Passive Solar Design Strategies: Guidelines for Home Builders_ represents
   over three years of effort by a unique group of organizations and
   individuals. The challenge of creating an effective design tool that
   could be customized for the specific needs of builders in cities and towns
   all over the U. S. called for the talents and experience of specialists
   in many different areas of expertise.

   _Passive Solar Design Strategies_ is based on research sponsored by the
   United States Department of Energy (DOE) Solar Buildings Program, and
   carried out by the Los Alamos National Laboratory, the National Renewable
   Energy Laboratory (NREL)... and the Florida Solar Energy Center (FSEC.) 

   The National Association of Home Builders (NAHB) Standing Committee on
   Energy has provided invaluable advice and assistance during the
   development of the Guidelines.

   Valuable information was drawn from the 14 country International Energy
   Agency (IEA) Solar Heating and Cooling program, Task VII on Passive and
   Hybrid Solar Low Energy Buildings...

   Although all the members of PSIC, especially the Technical Committee,
   contributed to the financial and technical support of the Guidelines,
   several contributed far beyond the call of duty. Stephen Szoke, Director
   of National Accounts, National Concrete Masonry Association, Chairman of
   PSIC's Board of Directors during the development of the Guildlines; and
   James Tann, Brick Institute of America, Region 4, Chairman of PSIC's
   Technical Committee during the development of these guidelines... 
   gave unstintingly of their time, their expertise, and their enthusiasm. 

Nick

---

From: [email-masked] (Nick Pine)
Date: 6 Apr 1996 08:16:01 -0500

There has a plan to create a new wetland as well as restore an old greenhouse,
perhaps, at 121 Wartman Road owned by the Perkiomen Valley School District
in SE Pennsylvania, that looks approximately like this:

                                              N
                                      |       16'     |
The greenhouse sits on a cinder       ................. ---
block foundation that rises 2-3'      .       .       .
above the ground. The greenhouse      .       .       .
itself is made of metal angles and    .       .       .
glazing bars to fit overlapping       .       .       .
single panes of glass approximately   .       .       .
2' x 1.5' each. Above the block wall  .       .       .
is about 2' of vertical glass, then   .       .       .
another 4' or so of glass sloped at  W.       .       . 32' E
slightly less than a 1:1 pitch.       .       .       .
More than half of the panes of glass  .       .       .
are broken. There are no ventilation  .       .       . (These ASCII
fans, but the top sashes can be       .       .       .  pictures make
cranked open. The greenhouse has no   .       .       .  more sense in
insulation. Inside the walls there    .       .       .  a non-kerning
are two block troughs about 4' wide   .       .       .  font like courier.)
and 3' deep with some fin-tube pipe   ................. ___
beneath bench supports. To the 
north of the greenhouse is a small            S
equipment shed which used to contain
an oil burner (?) to heat the fin-tube pipes, with an expansion tank
in the ceiling. A tree is growing up through the broken glass of the
greenhouse at the north end. The top of the tree is about 6' above the
9' peak of the greenhouse. 
                                              .         ---
                                          .       .     ~4'
How might we solar heat this          .               . ---
greenhouse? This is a                 .               .
challenging project because the       .               . ~5'
greenhouse faces the wrong way        B   B       B   B
for that. Solar greenhouses usually   B   B       B   B 
run east and west, ideally with   ...............................
a reflective and insulating 
north wall, in the northern hemisphere.

It seems to me that the way to start this project is to remove all the
glass from the roof of the greenhouse and replace the broken glass on the
sides and endwalls with some of the unbroken panes. Then stretch two layers
of polyethylene film over the greenhouse itself, using one large 32' long
roll, and inflate a poly duct tube running north and south near the peak
of the roof to stretch the two films on cold or windy days with a small
(50 watt) blower, more or less following standard commercial plastic film
greenhouse practice. On warm calm sunny days the poly duct would be flat,
letting the two poly films form one surface, which would decrease the
R-value and increase the solar transmission, and save electricity. (The
cost of running a 50 watt blower 24 hours per day is about $50 per year.)

Next it seems like a good idea to remove the two tilting interior block walls
of the troughs and replace them with OSB or plywood walls with pressure
treated 2 x 4 bracing, to make a box that will be lined with some foamboard
and single pieces of EPDM rubber to make two water troughs about 32' long
x 4' wide x 2' deep, below the benches, to add about 512 ft^3 x 64 = 33K Btu/F
of thermal mass to the greenhouse. 

We might add another layer of plastic film inside the vertical glazing along
the walls, and a diagonal transparent roll-up R2 cover over the benches as
shown below from the south. This might be made from a 4' wide roll of bubble
pack insulation or some aluminized greenhouse fabric. 

    36 F   .         ---
       .       .     ~4'
   .               . ---        c is the transparent roll-up cover 
   . c           c .            p is the new plywood wall
   . T c       c   . ~5'        i is insulation under the plant benches
   B i p       p i B            w is water 
   B w p       p w B            T is the temperature of the plant benches
..............................

Suppose we keep the plants at T=66 F, 24 hours a day, when the roll-up
covers are closed. How much energy will that take, on an average winter day?

The plants will lose heat to the outside through the side glazing and plastic
films, which altogether have an R-value of about two. Let's be conservative
and assume the greenhouse itself is also 36 F inside, like the outdoors on
an average December day in Philadelphia. Then the amount of heat that leaks
out from the plant benches on an average day, Eout, will be about

Eout = 24 hr (66F-36F) 6'x 32'/R2 x 2 benches = 138K Btu/day.

Since a south wall in Philadelphia receives an average of about 1000 Btu/ft^2
of solar heat on an average day in December, of which 600 might be captured,
this means we need a solar collection area of about 138/0.6 = 230 ft^2, eg
a south wall 8' tall and 29' long. Adding a shallow frozen reflecting pool
(skating rink?) along the wall with a pond liner draped over a perimeter earth
berm would reduce the required size of the wall by about 50%, and prevent
vegetation from blocking the sun.

How warm must the water in the troughs be to provide stored heat for, say,
5 days without sun? About 66 F + 5 days x 138K/day/33K = 87 F.

The solar collecting wall could have a foundation made from the cinder blocks
recycled from the inner trough walls. It might look like this in cross section,
seen from the west:

          |  1 to 4'  |
      --- ............g     S -->
        iii        f  g
        iii    --> f  g     f might be the 4 sections of 32' fin-tube pipe
        iii--fan?--f  g       removed from the troughs
        iii   ^    f  g
        iii   |   s   g     s might be two layers of greenhouse shadecloth
        iii       s   g
        iii      s    g     g might be a single layer of flat Replex
      8'~~~           ~       polycarbonate glazing, or perhaps polyethylene
        iii   s       g
        iii   s       g     i might be fiberglass insulation in a stud wall
	iii  s <--    g       or a more interesting wall made of strawbales 
	iii  s        g       and mortar. Ray Lehman on 8th Avenue, Trappe,
	iii           g       sells 16" x 16" x 36" R30 rye strawbales for 
	iii s         g       $1.75 each. Rodents do not like rye straw.
 .......iii...........g.....  Ray also has some interesting strawbales that 
        cbcbcbcbcbcbcbcb      are 4' wide x 4' tall x 8' long...
     3' cbcbcbcbcbcbcbcb
    --- cbcbcbcbcbcbcbcb

The sun would shine in through the glazing and heat up the greenhouse
shadecloth here, which would heat up the air in the wall as it moves from
south to north, and the warm air would rise up north of the shadecloth and
flow through the fin tubes at the top, heating some antifreeze solution
that circulates through the fin tubes with a tiny pump when the sun is
shining and the trough water needs heat. How well would this work?

Each foot of fin-tube passes about 5 Btu/hr for each degree F of air-water
temperature difference in still air, so the overall thermal conductance of
128 linear feet of fin-tube is about 640 Btu/degree F. Or higher, if a
photovoltaic fan is used to whirl some air around through this large fin-tube
radiator at the top of the wall. The overall thermal conductance of 8' x 32'
of wall glazing is about 256 Btu/F, which seems promising, since heat tends
to take the path of least resistance, ie greater conductance. So the solar
heat that enters the wall will be happier going into the fin-tubes than back
out the glazing. What would the water and air temperatures Tw and Ta be on
an average 43 F (daytime) day in December using this system, with no fan? 

Here's the equivalent electrical circuit:   (sun current source)
                                                  ---------
                                                 | 256K Btu|       |
                      Tw -------wwww---------<---| over 6  |-------| |
                                 Rf      |       | hours   |       |
                                         |        ---------
                                         |
                                         |---------wwww---------- 43 F
                                         Ta         Rg

Rf is 1/640 and Rg is 1/256, and the sun power in is about 43K Btu/hour.

This is equivalent to the following (Thevenin) circuit:
 
                                                  ---------
                                                 | 43K Btu |       |
                      Tt -------wwww---------<---|   per   |-------| |
                                 Rt      |       |   hour  |       |
                                         Ta       ---------
                                          
in which Rt = 1/(640+256) = 1/896  and Tt = 0.714 Tw + 12.3, so

Ta = Tt + 43K/896 = Tt + 48 = 0.714 Tw + 12.3 + 48 = 0.714 Tw + 60.3 F.

So if Tw = 87 F, the sunspace air temperature will be about 122 F. Not bad.

If the energy that flows into the greenhouse during a day, Ein, is equal
to the energy that flow out of the greenhouse during a day, Eout, then
we have this approximate equation: 

Ein = 1000 (256) = 256K Btu/day sun energy 
    = 6hr (Ta-43) 256 solar wall loss (day)
    +18hr (28-28) 256 solar wall loss (ie no loss at night)
    +138K             greenhouse loss (24 hr) = Eout.

Solving this equation, we find Ta = 43 + 118K/1536 = 119.8 F, which is
pretty close to 122 F. This means the water will be almost 87 F. Good.
But it looks like we will need the frozen reflecting pond here, or a bigger
wall, or more fin tubes, or a fan or two, or two layers of glazing, or
a bench curtain with a higher R-value.

With the skating rink, we might have Ein = 1000 (256) 1.5 = 384K/day, so
the sunspace air temperature would be Ta = 43 + 246K/1536 = 160 F during
the day, and the water temperature would be Tw = (160 - 60)/.714 = 140 F.
Now we're cookin':-) There is lots of extra solar heat this way, so
the greenhouse interior can be a lot warmer than 43 F during the day.
This is all done with mirrors, of course :-)

So, here is one possible solution: 
                                              N
                                      |       16'     |
                                      ................. ---
 .....................                .       .       .
 .                   .                .       .       .
 .   chicken coop    .                .       .       .
 .                   .                .       .       .
 .                   .                .       .       .
 .                   .                .       .       .
 .....................                .       .       .
                                    W .       .       . 32' E
                                      .       .       .
                                      .       .       .
                                      .       .       .
                                      .       .       .
                                      .       .       .
                     ..................       .       . 
                     .    straw wall  .       .       .  
                     .......................glass...... ___
                  4' .         solar wall             .
            .....................................................
            .        |               32'              |         .
            .                                                   .
            .                   reflecting pool                 .
         16'.          made from a single roll of 20' wide      .
            .              EPDM rubber roofing material         .
            .                                                   .
            .                      <--  50' -->                 .
            .....................................................

                                     S

                                       36 F   .         ---
                     .....................        ..... ~4'
                     .                .               . ---     
                     .                . c           c .        
                  8' .   solar wall   . T c       c   . ~5'   
                     .                B i p       p i B      
            .        .                B w p       p w B         .
.........................................................................

Happy Easter, everyone.

Nick

    My favorite structure on this property is a 12' x 20' dilapidated chicken
    coop, with the long dimension running EW, the right way to orient a solar
    structure. It has a roof that slopes up 2' from the north to the south
    along the 12' dimension. Part of the building was probably used for plant
    propagation, since there is a bench on the south side behind some windows,
    with an old green 4" x 6" box on the wall marked "Mist-a-Matic," made by
    E C Geiger of North Wales, PA. More about that chicken coop later...

---

From: [email-masked] (Nick Pine)
Date: 14 Apr 1996 11:47:11 -0400

paul milligan <[email-masked]> wrote:
>   [email-masked] (Nick Pine) wrote:
>~~>A quote from _The Unbelievable Bubble Book_, by John Cassidy...
>~~>
>~~>  Once the World's Leading Fizzicist, Dr. Aristid V. Grosse, for many years
>~~>  the director of the Research Institute at Temple University, spent his
>~~>  youth in the study of high-energy particle physics. He worked on the
>~~>  Manhattan Project... published numerous scientific papers and to all
>~~>  appearances, pursued a dignified... distinguished, scientific career...

>	Nick, I think you should take a sabbatical, and do a long involved
>study that features Dr. Grosse standing in front of one of your collectors.

Hey, I don't work at Villanova... I'm always on sabbatical :-)
And I don't do "collectors."

OK, OK, OK, that bubble roof might not be very practical for the location
near the high school, because some students might find it amusing to pop
the roof with their knives in fits of pique. 
 
So maybe we should cover it with some burlap and chicken wire and 1/2" of
cement, and replace the telephone pole posts and beams underneath with 2 x 4s 
to hold up the plastic film ceiling over the shadecloth. But that lets out
daylighting thru the ceiling.... Hmmm. Let's take another more fundamental
look. We don't really want sun shining thru the ceiling in summertime, but
we do want light and heat coming in thru the south wall in the winter, and
we would still like to try to take advantage of the spirit of section 1610.2 
of the 1993 BOCA code, which defines a "continuously heated greenhouse" as

  a production or retail greenhouse with a constantly maintained interior
  temperature of 50 F or more during winter months... the greenhouse roof
  material shall have a thermal resistance (R) less than 2.0...

Which we'd interpret to mean that one can turn off the bubble pump and reduce
the R-value when it is snowing, thus melting the snow with stored solar heat,
which is similar to the practice of local commercial growers, who deflate
their poly film greenhouse pillows when it snows to reduce their R-value from
1.2 to 0.8. 

Section 1610.4 of the code goes on to say "The flat-roof snow load on
continuously heated greenhouses shall be calculated using the following
formula..." which comes out to be 12 psf where I live...
  
And it further drones on:

  Retail greenhouse: A greenhouse occupied for growing large numbers
  of flowers and plants and having general public access for the purposes
  of viewing and purchasing the various products. Included in this category
  are greenhouses occupied for educational purposes.

So how about looking again at something like that tensile structure with
the profile shown below, from the east:

                       .
                       .     .                               
                     . .             . ferrocement             
		    .  .                       .           
		   .16'. poly film bubble ceiling        .   ...........
		  .-24'.                           .         .         .
		 .     .                                     . chicken .
         pond   .      .<-              38'     .          ->.   coop  .
    ............................................p ~~~~~~~~~ p................
                         thermal storage pond->  ppppppppppp
  
This might look like a 16' x 66' transparent wall from the south. The roof
might be made with 4' wide chicken wire strips running north and south, wired
together and joined with pressure-treated 2x4s that sandwich the chicken wire,
like this:
                       .........
                       . NS 2x4.
                       .........  -- 1/2" cement --
                  ccccccccc cccccccccc  chicken wire
                  bbbbbbbbb bbbbbbbbbbbb  burlap
                  pppppppppppppppppppppppp  poly film or EPDM
                       .........                                             
                       . NS 2x4.
                       .........
..............................................................
                2 x 4 on edge, running EW

..............................................................
                       . spacer.
		       .       .
		       .       .
		       .       .
		       .........
                         steel 
                         cable                 

Let's see. How big does the steel cable have to be? If the roof has a
nice slump in the middle, and the distance from top to cable is, say, 6',
we need roughly 20 psf x 32 x 4'/2 = T 4'/32', where T is the cable tension,
so T = 10240 pounds. A 10K/50K diameter steel cable every 4'? That's not bad,
but there will be a lot of force trying to collapse the north and south walls
inwards... This would work better with a few posts in the middle, as before.
Then we can reduce the height of the south roof peak to 16', vs 24'. We don't
need THAT much solar heat. Recall that we only needed a roof R-value of 1.7
with bubbles in place to make this work.

So the roof might look like this in slightly more detail, with different
dimensions:

                    48'                             
         r.r.r.r.r.r.r.r.r.r.r.r.r           .      32'
         P  ppp  P  ppp  P  ppp  P           P       .
     16' PpppppppPpppppppPpppppppP           PpppppppPppppppp.
         P       P       P       P           P       P       P 8'
    .....P.......P.......P.......P...     ...P.......P.......P......

                                             PsssssssPsssssss.
         p is the poly film ceiling here,    .       .       .
           made in 3 16' x 32' pillows,      .       .       .
           sloped slightly upwards from      .       .       .
           north to south, with the bubble   PsssssssPsssssss.
           pipes along the north edge.       .       .       .
                                             .       .       .
         The poly film would attach along    .       .       .
           struts s, which would be in       PsssssssPsssssss.
           compression to keep the roof      .       .       .
           from collapsing.                  .       .       .
                                             .       .       .
                                             PsssssssPsssssss.

There would be a layer of greenhouse shadecloth under the poly film and ropes
on 4' centers under that. Struts s might look like this from the south:

               poly film      poly film
               poly film      poly film
                         ww ww aluminum extrusion clamps
               30% greenhouse shadecloth 
                       .........
                       . NS 2x4. (on 16' centers)
                       .........  
                         . 2 .
              <--EW rope . x . EW rope -->  
...NS rope...            . 4 .             ... NS rope...
			 .....
                           |          4'          |

There, is that better, Paul?

Nick

---

From: [email-masked] (Nick Pine)
Date: 15 Apr 1996 07:29:34 -0400

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...
 
 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.

Nobody seems to have figured this out yet. Here's a hint:

                       32 F        132 F
       70 F ----wwww----|----wwww----|----wwww----70F
                R10/A   (R2 or R12)/4     R10/A

                ---->       <----         ---->
                 Q1           Q2           Q3

Nick

---

From: [email-masked] (Nick Pine)
Date: 15 Apr 1996 09:19:37 -0400

Nick Pine <[email-masked]> wrote:
>>~>A quote from _The Unbelievable Bubble Book_, by John Cassidy...
>>~>
>>~>  Once the World's Leading Fizzicist, Dr. Aristid V. Grosse, for many years
>>~>  the director of the Research Institute at Temple University, spent his
>>~>  youth in the study of high-energy particle physics...

>>study that features Dr. Grosse standing in front of one of your collectors.

Well, maybe you don't like all that chicken wire. How about this?

Still trying to take advantage of the spirit of section 1610.2 of the 1993
BOCA code, which defines a "continuously heated greenhouse" as
 
   a production or retail greenhouse with a constantly maintained interior
   temperature of 50 F or more during winter months... the greenhouse roof
   material shall have a thermal resistance (R) less than 2.0...
                   . 
                       .
                       .     .                               
                     . .             . <-- plywood or OSB under EPDM rubber
 		    .  .                       .           
 		   .20'. poly film bubble ceiling        .   ...........
 		  .    .                           .         .         .
 		 .     .                                     . chicken .
          pond  .      .<-              38'     .          ->.   coop  .
     ............................................p ~~~~~~~~~ p................
                          thermal storage pond->  ppppppppppp
   
In more detail, with different dimensions:
 
                     48'                  .          
          r.r.r.r.r.r.r.r.r.r.r.r.r           .      32'
          P  ppp  P  ppp  P  ppp  P           P       .
      20' PpppppppPpppppppPpppppppP     <--S  PpppppppPppppppp.
          P       P       P       P           P       P       P 8'
     .....P.......P.......P.......P...   pond.P.......P.......P......
 
                                              PsssssssPsssssssP
          p is the poly film ceiling here,    .   E   E   E   .
            made in 3 16' x 32' pillows,      .   W   W   W   .
            sloped slightly upwards from      .   B   B   B   .
            north to south, with the bubble   PsssssssPsssssssP
            pipes along the north edge.       .   E   E   E   .
                                              .   W   W   W   .
          The poly film would attach along    .   B   B   B   .
            struts s...                       PsssssssPsssssssP
                                              .   E   E   E   .
          EWB are the East West Beams.        .   W   W   W   .
                                              .   B   B   B   .
          r might be 2x4 rafters.             PsssssssPsssssssP
 
The roof might weigh about 15 psf, with 12 lb of snow load and 3 lb of
dead load. If the plywood or Oriented Strand Board roof has rafters on 2'
centers with 8' spans we might have f=1200 psi, W=8x2x15=240 lb, M=WL/8
=240x8x12/8=2880 in-lb, S=M/f=2.4 in^3, b=1.5" and d=sqr(6S/b)=3.09",
so we might use 2x4s as rafters.

The EWBs need to hold up 32x48x15x15/5/3=1536 pounds over their 16' span,
so if M=1536x16x12/8=36864 in-lb, S=30.72 in^3, b=3" ==> d=7.8", so we
might use 2 2x10s for EWBs, or a couple of 2x6s with a downwards king post
and a wire.

Suppose we use diagonal tension supports like this, as seen from the east:

  20'P
     P  \          EWB
     P              K              EWB
     P      t       K          /    P    \          EWB
     P        \     K       t       P       t        K          /   P
     P              K    /          P           \    K     t        P
   8'P..............K...............P................K/.............P
     P                              P                               P
     P                              P                               P
     P                              P                               P
pond.P..............................P...............................P...
     P                              P                               P
     |                             32'                              |

If the kingposts K were 6' long, the diagonals t might have an approximate
tension T such that 1536/2 = 6T/8 ==> T=1024 lb. Some twangy wires with
good attachments...

Each post P would have a load of about 32x48x15/12=1920 lb. Suppose they are
Eastern Spruce 4x4s with Cp=750 psi and E=1,200,000 psi. Then L/d=8'x12"/3.5"
=27<50, C=1920/(3.5"x3.5")=157 psi<Cp and 0.3E/((L/d)^2) =479<Cp, so perhaps
we can use 4x4 posts with a little side-bracing above.

               poly film      poly film
               poly film      poly film

The struts would only be holding up poly film pillows filled with 1-2' of
bubbles. The pillows would act as solar collectors for some of the
960x1.3x20x66'= 1.65 million Btu or 483 kWh/day average sun that would fall
on the south wall of the Perkiomen Valley wetlands structure in December.
The peak solar power would be about 160 kW. Some of that heat would be stored
in the thermal pond, with a little warm water pumped back up through the
collapsed pillows infrequently, to melt the snow off the roof. The roof
could be painted white underneath, or covered with 4' wide builder's foil
under the rafters, with an operable vent door to the rafter cavity, and
another to the outside.

Nick

---

From: Laurie Neverman <[email-masked]>
Date: Tue, 16 Apr 1996 13:40:32 -0700

Bruce Hoglund wrote:
> 
> In article <[email-masked]>, [email-masked]
> (Nick Pine) wrote:
> 
> [snip]
> 
> > and at night the glazing fills up with bubbles, keeping the heat in.
> 
> [snip]
> 
> Hi Nick,
> 
> Your bubble wall moveable insulation wall is a clever idea, which reminded
> me of a TV (Discovery Channel?) show I saw fairly recently (~ <1 year)
> about an artist who makes "sculptures" of double panes of glass with
> bubbles between.  It was quite beautiful, and he got surprising amounts of
> money for his art, of course art prices always surprise someone like me
> who has an extensive collection of Black Velvet Elvis & Dogs Playing Poker
> "art"  ;-)
> 
> Unfortunately, I don't remember much about the TV article.  Perhaps
> someone else does?
> 
> Bruce Hoglund
> 
> ____________________________________________________________
> 
> NOTE: The thoughts & comments above are mine alone!

It was on the Discovery Channel, and his sculptures were displayed
in malls and airports and other places.  I think he had some patents
on bubble making apparatus.

-- 
********************************************************************
Laurie Neverman              Public Energy Systems
email:  [email-masked]       URL:  http://www.netnet.net/energy/
voice:  414-465-6563         fax:  414-4659894
********************************************************************
Energy Alternatives and More -- Focusing on Applied Solar Technology

---

From: [email-masked] (george)
Date: 16 Apr 1996 22:28:00 GMT

:> > 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.

hmmm...the thermal conductivity of soap film isn't in my heat transfer
text.  I'm guessing its a reasonable conductor - it your "wall" is
thin (like a double paned window) you wouldn't gain anything filling
it with bubbles.  If you had a thicker wall you might gain something
assuming you could get the bubbles to stay put.

Of course if your aim is to impress the neighbors go for it :-)

-- 
george 	
[email-masked]

---

From: [email-masked] (Nick Pine)
Date: 16 Apr 1996 20:36:17 -0400

george <[email-masked]> wrote:

>...the thermal conductivity of soap film isn't in my heat transfer text.

I wonder what heat transfer text you are using.

I guess it's like a very thin film of water, or anything else, eg plastic
film, as far as stopping convection, and good enough to stop some of the
IR radiation, ie the bubble wall would ideally be thick enough or opaque
enough slow down IR losses considerably.

>I'm guessing its a reasonable conductor -

The wall itself, yes, but the air inside the bubble is less conductive.

>it your "wall" is thin (like a double paned window) you wouldn't gain
>anything filling it with bubbles.

Can you define "anything"? :-)

>If you had a thicker wall you might gain something

That's the idea...

Nick

---

From: Albrecht Kadlec <[email-masked]>
Date: 17 Apr 1996 17:56:51 +0200

Hi Nick,

> enough slow down IR losses considerably.

I always care about radiation losses last: 
conduction and convection losses come first.

So from my point of view, it could be a problem that the thing is NOT
SOLID: 
The trick in using bubble plastic is, that the walls are too thin for
heatloss via conduction.
And the air in the bubbles is trapped in such small bubbles, that no 
convection can take place in the bubbles.
The only losses left are radiation losses.

With soap bubbles, the water around the bubble can move:

So a water molecule could absorb energy, and while the bubbles are
going up, I'd expect warmer bubbles to go up faster. So on each
sloping or curved construction they tend to reach the outside pretty
soon, losing their heat energy via conduction to the PE film /
outside.

The bubbles would be great if they were stationary. (not moving)
I can't believe they're of great value, when they're moving up,
especially, if the air is exhausted to the outside.

I'd expect it to be equivalent to have a 50 Watt blower blow air
between the two PE sheets which is exhausted through various gaps to
the outside.

Maybe even worse, since the air would absorb some moisture thus also
absorbing heat from the surroundings. (evaporative cooling!!!!)
just imagine the HUGE water surface the bubbles create.

Evaporative cooling would be eliminated, if the PE sheets were leak
free and the water AND air would be cyclead over & over (sucking air
>from  the top to create the new bubbles).

So what is left is the cycling of the water & air from the inside
sheet to the outside sheet:
There would be a significant amount of water running down the inside
sheet (it's a curved/sloped construction, and gravity does it's job),
absorbing pretty much heat from the inside.
Some of this water is then contacting the outside sheet as it's
re-bubbled, losing its energy to the outside.

What we would need to gain any profit over the blower-alone approach
would be an air-tight construction of the two sheets and a liquid
that'd produce very "sturdy" bubbles, so that the air-movement rate
could be lowered.
As long as we're moving much air, we'll be seeing these
convection/conduction losses. 

just my thoughts....

cheers,
albrecht

---

From: [email-masked] (george)
Date: 17 Apr 1996 18:28:34 GMT

:>it your "wall" is thin (like a double paned window) you wouldn't gain
:>anything filling it with bubbles.
:
:Can you define "anything"? :-)

my premise is that if the gap is thin enough then convection is not
a big issue.  So the effect of the bubbles is merely to ad an (admittedly
small) conduction path.

:
:>If you had a thicker wall you might gain something
:>assuming you could get the bubbles to stay put.

I was imagining that you fill the chamber with bubbles and they somehow
just stay there. Or are they continuously moving up and breaking
with the soapy water draining back to be recycled. (might be pretty
anyhow )

---

From: [email-masked] (george)
Date: 17 Apr 1996 18:31:43 GMT

:What we would need to gain any profit over the blower-alone approach

I'm not getting the blower approach at all. You blow cold outside
air through the pane? You blow warm air from inside in and vent it to
the outside?  Sorry if this is old hat, but you know you posted
this to a bunch of groups where we aren't all up on the latest
enviro-tech..

---

From: [email-masked] (Nick Pine)
Date: 18 Apr 1996 08:07:13 -0400

Albrecht Kadlec  <[email-masked]> wrote:
>> enough to slow down IR losses considerably.
>

Sounds like a good idea, until you get up to some temperature. Hmmm. A black
body plate with good insulation on the back and no glazing absorbing 300
Btu/hour/ft^2 of sun in still air on a (average Phila December) 36 F day
with a US R value of 2/3 should have a temp of 236 F. If it had only radiation
loss, ie 0.174x10^-8((T+460)^4-496^4) Btu/hr/ft^2, the temp T might be
234.7 F... It might actually get up to a temperature of 130 F or so.

>So from my point of view, it could be a problem that the thing is NOT SOLID: 

I guess you mean "advantage" when you say "problem"? :-)

>The trick in using bubble plastic is, that the walls are too thin for

The thin bubble walls have little conductive thermal resistance for heat
flowing perpendicularly thru the wall, but I guess you are talking about
heat flowing parallel to the walls, lengthwise, say, somehow making it
>from  one side of a large wall full of bubbles to another, by flowing along
the thin bubble walls over a torturous random path from one side of the
large wall to the other by conduction. If the bubble walls were very thick,
like Swiss cheese, that could happen more easily. 

>convection can take place in the bubbles.

Not as much as if they were big bubbles. 

>The only losses left are radiation losses.

Steve Baer discovered that his bubbles had little IR thermal radiation
resistance, because the walls were so thin, he thinks. But he was trying
to solve a much harder problem, because he wanted the sun to shine through
the bubbles and heat up some water in a dish to 130 F, with no glazing (as
I recall.) He wanted his bubbles to have the greenhouse effect, selectively
transmitting shortwave solar radiation and blocking longwave IR re-radiation
in the other direction. A very nice thing, if possible to arrange. In our
case, during the day there would be no bubbles, just 2 layers of glazing,
and there would be no evaporation, and the temp inside the room might be
70 F, not 130 F. These bubbles would only be inside the glazing cavity at
night, trying to stop ALL heat transfer in the longwave IR and shortwave
visible spectra.
 
Bubble walls thinner than light wavelengths look transparent or "black."
As they get thicker, they acquire reflections and colors in the visible
spectrum. We want bubbles that are at least that thick. Reflective bubbles
in the visible spectrum may still be transparent to IR. I suppose these
reflections mainly come from Fresnel losses, ie those resulting from the
impedance mismatch or difference or between the indexes of refraction of
air and water. Each air-water interface might reflect ((n-1)/(n+1))^2 of
the incident normal radiation, assuming the index of air is 1 and the index
of water is n (what is n for water? Less than glass. It's about 1.5 for glass,
which makes the air-glass reflection coefficient rho about 4% at normal
incidence. Reflection coefficients go up for non-perpendicular incidence 
angles. For glass-air with a 60 degree incidence angle, rho becomes 9.3%.
There is a complicated formula on page 218 of Duffie and Beckman's book 
showing how to calculate this, starting with Snell's law.)  

The index and thus reflection coefficient depends on the wavelength, and rho
also depends on the wall thickness somehow, and whether the water has color
or not, eg some sort of dye, in the IR spectrum... Absorbing lots of energy
in the bubble wall may be almost as good as reflecting it, if there are lots
of small bubbles so the heat has to pass from one to the other in a long
chain from indoors to outdoors. Letting the heat pass right thru the bubble
walls is not good.

>With soap bubbles, the water around the bubble can move:

You mean the bubble wall itself... sure. Slowly, one hopes. 

>going up, I'd expect warmer bubbles to go up faster.

I'd hope they wouldn't move much at all. Dr. Grosse's didn't move much. Some
of his bubbles lasted more than a year. His principles of bubble health and
long life are: 1. Dust is the enemy of bubbles. 2. Carbon dioxide poisons
bubbles (better bubbles are not blown by humans), and 3. Bubbles love cold.
They also like humidity. _The Unbelievable Bubble Book_ says (in the
"Permanent Bubbles" section)

    As it turns out, bubble death is preventable, or certainly postponable,
    a discovery most definitely made by Dr. Grosse nearly 30 years ago. Among
    Dr. Grosse's many other scientific awards and achievements exists the
    lesser-known fact that he was once the owner of the world's oldest bubble.

    Prof. Grosse's office was home to a huge collection of aging bubbles
    in various states of decay. Under Prof Grosse's meticulous care,
    bubbles did not burst. They flattened. But it was a process that often
    took many months, in some cases, years. They were created out of an
    esoteric bubble solution, and protected fromm deadly dust particles 
    by glass bell jars. 
 
    Prof. Grosse has described his peculiar collection in this way:
    "My first bubbles all died in infancy. They were blown with much
    the same kind of soap that Isaac Newton used in his classic bubble
    experiments. I turned to the literature and found that a much better
    soap was described more than a century ago by the blind Belgian physicist
    Joseph A. F. Plateau, who laid the foundations of our present knowledge
    of soap bubbles. Bubbles blown with Plateau's solution will last for
    several minutes in an ordinary room and for several hours with
    proper protection."

The book mentions that Plateau also created bubbles lasting more than a year.

>So on sloping or curved construction they tend to reach the outside pretty
>soon, losing their heat energy via conduction to the PE film / outside.

Steve said that as he recalled, this heat transfer by moving mass wasn't a
problem. Yes, if you pump air and especially liquid at a high rate thru an
empty glazing cavity, that would reduce the cavity resistance, but he said
that didn't seem to be a problem. 
 
>The bubbles would be great if they were stationary. (not moving)

That's the idea. Bubbles that are fairly stationary, with a long lifetime.
Humidity is one thing that makes bubbles burst. Dust is another. Neither
would be much of a problem inside a glazing cavity. But recall that Dr.
Grosse made bubble foams that he hung on his office walls, with no glazing...
Sprinkled with sequins, even.

>especially, if the air is exhausted to the outside.

I agree this should be a closed system, at least to avoid the buildup of dust.
Loss of water and heatloss by massflow out of the cavity would be minor, it
seems to me, even if the air were exhausted to the outside. 
 
>I'd expect it to be equivalent to have a 50 Watt blower blowing air

That doesn't sound like a great way to insulate a window :-)

>just imagine the HUGE water surface the bubbles create.

It sure is mind-boggling. But these would be clumped bubbles, not free
floating bubbles. A foam.

>from the top to create the new bubbles).

Good idea. I wonder how to pop the bubbles and separate the water and
air at the top, so the air can go thru an aquarium air pump. Something
like this? 
                        ---------------------------- 
		       |  ----------------------    |
       sand filter?--> | |             <--     |    | <--bubbles
                       | |                     |    |
                       | |   ----------        |    |
                       | |  |  -  air  | -->   |    |
                       |  --  | | pump  ----   |    |
		       |  ----  ----------  |  |    | <--water
                       |w|                | |  |wwww| <--water levle
		       | |                |  --     |
		       | |                 ----     |
		       | | water return->      |    |
                       |  ---------------------     |
                        ----------------------------

Or maybe the right kind of air pump wouldn't mind pumping a few bubbles,
reliability-wise, anymore than it would mind pumping air with 100% RH.

>sheet to the outside sheet:

Very slowly, one hopes.

>absorbing pretty much heat from the inside.

The flow rate seems like an experimental question, and a matter of
picking the right bubble solution... And as you say, geometry. The
flow rate would be different for a vertical window or a horizontal
bubble ceiling or the sloped underside of a bubble cathedral ceiling.
It would be nice if these were all minimal water flow rates. Larger
air flow rates are (1000 times) more tolerable. 

                   . 
                       .
                       .     .                               
       <-- south    .  .                       .           
    refl. pond  .      .<-              38'     .          ->.   coop  .
   
In the above case it might be nice to try to make the bubbles flow up
>from  the chicken coop along the underside of the sloped roof and over
the peak and down the south glazed wall at night. During the day, it
would be nice to have bubbles along the sloped ceiling, but not in the
south glazed wall. 

>re-bubbled, losing its energy to the outside.
 
Little, we hope.

>that'd produce very "sturdy" bubbles, so that the air-movement rate

Right.

>just my thoughts....

Thanks Albrecht. Time for less gedanken experiments?

Nick

---

From: David Davis <[email-masked]>
Date: Mon, 22 Apr 1996 11:15:28 -0700

> >assuming you could get the bubbles to stay put.

I have to agree with George. Logic tells me you have provided numerous 
small wet paths for energy transfer via the bubble walls. Before the 
bubbles transfer relied on convection in the air in the space.

David davis
[email-masked]

---

From: [email-masked]
Date: Tue, 23 Apr 1996 02:38:32 GMT

>> >...the thermal conductivity of soap film isn't in my heat transfer text.
Perhaps we are being just a little too restricted in our conceps here?
What if we developed some' lateral thinkng' It just so happens that I
have a (proven) system to which I had never considered 'air bubble
technology' Why ? it never occured to me, until now. Please have a
look at my site on <http://www.nsl.co.uk/nightsky> and let me know how
you see 'air bubble technology' fitting in with 'DIY Basement
Renovations'? or 'Green Roof Technology' for that matter?

Kind regards,

Joe O'Donoghue.

---

From: [email-masked] (Nick Pine)
Date: 24 Apr 1996 20:00:07 -0400

>bubbles transfer relied on convection in the air in the space.

A lot of little convection paths in series will have more thermal resistance
than one big one...

Nick

---

From: Andreas Haffner <[email-masked]>
Date: Thu, 25 Apr 1996 10:51:33 -0700

> A lot of little convection paths in series will have more thermal resistance

Misunderstanding here, I think:

Before you had one large convection path

With the bubbles you have many small convection pathes and additionally many
*conduction* pathes.

Andreas
____________________________________________
* This is my own opinion! ........ I think *
MATFORSK - Norwegian Food Research Institute
Phone +47 64970168          Fax +47 64970333

---

From: [email-masked] (Rick Matthews)
Date: 25 Apr 1996 12:05:16 GMT

Andreas Haffner ([email-masked]) wrote:
: 
: Misunderstanding here, I think:
: 
: Before you had one large convection path
: 
: With the bubbles you have many small convection pathes and additionally many
: *conduction* pathes.

Forgive me if I am off-target, since I missed the initial post.  There
seems to be debate over whether filling walls with some sort of bubble
structure will improve insulation.  As long as the bubble surfaces are
thin, the answer is clearly yes.  Many small convection paths within
bubbles would have much lower net heat transport than one big cavity.
The smaller the bubbles, the more effective the material.

After all, that is what styrofoam is.

Consider fiberglass.  Fiberglass itself is a relatively poor
insulator.  Fiberglass insulation (such as the Pink Panther stuff), on
the other hand, is mostly air.  The principal purpose of the
fiberglass in fiberglass insulation is to disrupt convection currents.
It breaks the big cavity up into lots of tiny ones.  Reduction in
radiative transport because of the many layers is also important.

-- 
Rick Matthews                             [email-masked]       
Department of Physics                     http://www.wfu.edu/%7Ematthews 
Wake Forest University                    910-759-5340   (Voice)
Winston-Salem, NC 27109-7507              910-759-6142   (FAX)
USA

---

From: Brad Lindsay <[email-masked]>
Date: Tue, 23 Apr 1996 11:18:24 -0700

Nick Pine wrote:
> NickI just stumbled across your thoughts on heat movement through walls.... 
"radiation is my last concern"...????  Maybe in your part of the country. 
Here in Phoenix, we have been building test houses since 1986. What we 
have found is radiation the primary source of heat movement.  This was 
proven in our last house using no insulation, only a reflective film 
(radiant barrier) in the walls.  The 2400 sq ft home consumed $56.00 in 
electricity in the month of August!  A comparable home would use $300+ 
with standard insulation products which focus on limiting conductive and 
convective heat movement.  I can be reached at: [email-masked]
Brad Lindsay

---

From: [email-masked] (Nick Pine)
Date: 25 Apr 1996 15:13:10 -0400

>Misunderstanding here, I think:

Perhaps so...

>Before you had one large convection path

Right...

>With the bubbles you have many small convection paths and additionally many
>*conduction* paths.

That may not be so bad... Water has a US R-value of about 0.25 ft^2-hr/Btu
per inch, as I recall, eg for downward heat conduction. So what would we have
as the simple aggregate conductive thermal resistance owing to the water films,
>from  one side of a wall to the other, if the 4" wall space were filled with
a regular matrix of 1/4" cubical bubbles with a 0.0001" (approx 12 micron,
on the order of an 80 F black body IR wavelength) wall thickness, ie if the
wall cross section were 2500 parts air and 1 part water in each direction? 

A 2500 x 2500 foot wall would have the equivalent of a 1 ft^2 thermally
conductive water shunt 4" thick with a US thermal resistance of 1, so the
wall would have an effective thermal conductance of 1/(2500^2 ft^2) or
an R-value of 6,250,000. Not bad :-) This is more easily investigated by
experiment. We need a few more serious 12 year old scientists... 

Nick

---

From: [email-masked] (Nick Pine)
Date: 25 Apr 1996 15:39:09 -0400

>>Nick, I just stumbled across your thoughts on heat movement through walls... 

Good...

>"radiation is my last concern"...????

That was Albrecht's thought, not mine. It looks like the >> >'s above
somehow got out of sync.

>Maybe in your part of the country. 

Definitely. We are more concerned about heating than cooling here, and
for that, R-values work pretty well up to some temperature like 200 F.

>electricity in the month of August!

Sounds nice, but why so much? How about ventilating your adobe house
at night or building a tower with a few 55 gallon drums full of water
behind your frame house, with some south glazing on top? In the summer,
allow the sun to heat up the top drums during the day and vent the whole
tower at night with cooler night air, using the top as a solar chimney
with stored heat from the day. Allow this cool night air to circulate
into the house as well. Close the outer vents during the day. Putting a
wet mesh over the lower vent would further lower the night temp on an
average August day, swamp-cooler-style. And pumping some water up from
a shallow pond in the ceiling above the cool drums to a shallow pond
on the roof would allow additional radiative cooling on clear still nights
when the roof pond could be up to 20 F cooler than the air temp, owing to
night-sky radiation. You might use one window AC for dehumidification.

And solar heat all the water drums in the winter. 

>A comparable home would use $300+ with standard insulation products which
>focus on limiting conductive and convective heat movement. 

Sounds better and cheaper :-)

Nick

Nicholson L. Pine                      System design and consulting
Pine Associates, Ltd.                                (610) 489-0545 
821 Collegeville Road                           Fax: (610) 489-7057
Collegeville, PA 19426                     Email: [email-masked]

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.

---

From: [email-masked] (Nick Pine)
Date: 26 Apr 1996 21:27:22 -0400

Ah'm just back from Texas. "Why do aggies think all wise men are firemen?
Because they say they have come from afar." :-)
 
Texas is two-dimensional, with more wind in the north and sun in the west.
The SE corner is more humid, with warm nights in August. The NW corner is
more desertlike, with cool summer nights. Texans are starting to realize that
they now use more energy per capita than anyone else on earth, as net energy
importers, consuming more energy than they export. These days, Texans can make
$3,000 per acre per year growing cabbages and onions, vs $2,000 per acre
wind farming or $1,600 per acre pumping oil.

One man says "Ah have mah very own backyard gas well, 30,000 cubic feet
per day at 180 pounds per square inch. It shore is nice to be warm in the
winter. Ah just use it for heat. All ah do is add stinkum. Gotta do that or
you'll blow yourself up. Not worth hookin' up to a pipeline, since they're
run by thieves." In Abilene, electricity comes from natural gas.

One frugal Abilene house uses $100/month in winter for gas heat and $100/month
in summer for air-conditioning. It has 6 window air conditioners. Can the
owners use less fossil fuel?

Perhaps describing a particular system to help this house will help others 
see how this might be done more generally, as well as in other particular
ways for different houses in different situations.

This house needs 200 Btu/hr/degree F of heating or cooling. With an indoor max
temp of 80 F on a 24 hour average 84 F July day with an average daily high of
95 F it might need 12 hr(95 F - 80 F) 200 = 36K Btu of cooling. The average
minimum nightime temperature in Abilene in July is 74 F, with a relative
humidity of 71%, ie a wet bulb temp of about 67 F. Suppose we store 5 days of
cooling load at this temp, allowing a thermal mass C to rise from 67 to 77 F:
36K x 5 = 180K Btu = 10 x C, requires C = 18K lb of water, so we might use 36
500 lb 55 gallon drums full of water for cooling. Let's use 42 cool drums, and
put another 28 on top, behind some south glazing, to make a solar chimney to
move some night air up around the cool drums. Used plastic drums cost about
$2 each in Abilene. Straw bales for walls should be about the same price, and
chickenwire/fiberglass ferrocement costs about 20 cents/square foot. 

December days in Abilene have an average 24 hour outdoor temp of 46 F and
an average daytime temp of 57 F, with 1400 Btu/ft^2 of south wall sun. On
such a day this house needs about 24 hrs(68F-46F)200=100K Btu to keep warm.
Water heating might require another 50K Btu/day. How much low-thermal-mass
sunspace glazing is needed to keep this house at 68 F on an average December
day? Let's assume that 100% of the of sun that falls on the sunspace glazing
passes into the sunspace, which has a temperature of 87 F during an average
6 hour day, when the outdoor temp is 57 F. A shallow low-thermal-mass
sunspace would lose about 6 hr (87-57) = 180 Btu/ft^2 when the sun is shining,
and lose no heat at night, for a net gain of about 1200 Btu/ft^2/day, so
keeping the house warm would require a minimum of about 100K/1.2K = 83 ft^2
of "solar siding," or an 8' tall x 12' wide x 4' deep sunspace, or an 8' tall
x 16' long x 8' deep sunspace. The sunspace glazing could be smaller if the
ground in front had a reflecting pond or a white surface. To avoid heat gain
in the summer we might use a 4' overhang, some operable vent doors, and some
greenhouse shadecloth hanging inside in the winter and outside over the
glazing in summer. 

How warm would our 70 drums full of water have to be to store enough heat
for 5 days without sun? Storing 5 x 150K Btu with a heat capacity
C = 70 x 500 = 35,000 Btu/degree F and a minimum usable drumwater temp
of about 80 F, means the fully-charged drumwater temp for this heat battery
should be at least T = 80 F + 750K Btu/35K Btu/F = 101 F.

                                                night pool
Suppose our swamp thing looks like this:        .......... ---
        (from the east)                          g D hot. vent door
                                .                g Drums.
        <--south        .                .       g.......  
                .                                . cool .  16'
   .    .       .                           vent . Drums.
        .  sun  .             house              . D  D .
  pond? . space .                           vent . D  D . vent door
......................................................................

                                                    8'
        (from the top)                           ........
                                                 . D  D .
The upper solar closet part of this structure    . D  D .
might have 8' x 16' of south glazing, with 2     . D  D .
layers of drums stacked vertically. The lower    . D  D . 16'
part might have 3 layers of vertically-stacked   . D  D .
drums with 14 drums in each layer. There might   . D  D .
be a shallow EPDM rubber pond on the bottom,     . D  D .
another pond between the upper an lower parts,   ........ 
another just under the roof, and another on top
of the roof, with straw bales underneath. The floor of this
drum structure might be the lower EPDM rubber pond, a few inches deep.

We would need to pay attention to fire ants and termites in Abilene, as
well as rodents, where the straw bales touch the ground. It seems to me that
a rubble foundation wall with strawbales laid on top, surrounded by a half-
inch of ferrocement would take care of all that. Here's a quote from page
468 of Aden B. and Marjorie P. Meinel's 1977 book, _Applied Solar Energy_:

  In our home solar heating system we used water as the thermal storage
  medium for an air-transfer unit, the water being contained in 1000
  one-gallon polyethylene bottles stacked so that air could flow between
  them. They worked satisfactorily until some desert pack rats invaded
  the storage bin, making nests of the insulation and chewing holes
  in the water bottles.

What will the drum temperature be after a string of average December days?
With an average roof reflectivity of 60% (some aluminum paint) we might have
a solar input of about 1400 x 1.6 x 128 ft^2 = 287K Btu/day. If we stored
this heat all over inside the structure, we would lose heat through the
128 ft^2 of R1 south glazing during the day, and the 128 ft^2 of R30 wall
behind the glazing and the other 768 ft^2 of R30 straw bale walls at night.

If the energy that flows into the drum structure, Ein, equals the energy that
flows out of the structure on an average day, then we have Ein = 287K, and 

Eout = 6 hr (T-46) 128 ft^2/R1        south glazing,          day
     +18 hr (T-46) 128 ft^2/R30         "      "              night
     +24 hr (T-46) 768 ft^2/R30       remainder of structure, 24 hours

     = (T-46)(768+76.8+614.4) = 1460(T-46) = 287K = Ein, so 

T = 46 + 287K/1460 = 242.7 F.

Of course the water won't really get that hot, because it will boil at 212 F,
and radiation loss will limit the temperature sooner than that. Suffice it to
say the water temp in December should be greater than 101 F, even though the
structure is only half-glazed on the south side. Moving the structure away
>from  the house so it is no longer shaded below and glazing the whole south
side would make it collect more solar heat.

The purpose of the EPDM rubber pond on the ground is to help transfer heat
downwards from the glazed area above. We need to transfer about 80 K Btu/day
>from  the upper drums to the lower ones, and we might do this by pumping some
water up from the ground level pond to the pond under the ceiling at the top
of the structure. How much water? 80K Btu/24 hours = 3.4K Btu/hour. If the
upper pond water were 10 F warmer than the lower pond water, we would need
to move 340 pounds of water per hour or 5.6 pounds of water per minute or
0.7 gallons per minute from the lower pond to the upper one, ie 16', which
we could do with a perfect 0.003 horsepower immersion pump in the lower pond,
requiring 2 watts of electrical power if it were 100% efficient.  

The ceiling pond might also be used to extract winter heat for the house,
especially if the 55 gallon drums below had a thermal conductor like sand
between their tops and bottoms. If the ceiling pond water were 10 F cooler
than the drumwater, the maximum heat transfer rate via the ceiling pond might
be something like 10 F (70 drums x 25 ft^2/drum) x 1.5 Btu/hr-ft^2-F = 26K
Btu/hour, corresponding to an outdoor temp of 68 - 26K/200 = -63 F for this
house. A fan below the ceiling pond would increase the heat transfer rate.
The house might use a fan coil unit or auto radiator to transfer the heat
>from  the ceiling pond water to the house air.

Domestic hot water could be supplied via a simple concentric pipe heat
exchanger located beneath the existing water heater in the house, upstream
of the fan coil unit in the same circulation loop.

The heat transfer rate from the house to the lower pond in the summer would
also depend on the lower pond-drumwater temperature difference. If the house
were 80 F, and the floor pond water were 77 F, and the outdoor temperature
95 F, the house would need 3K Btu/hour of cooling, which might come from a
few $139 all-copper 2' x 2' SHW 2347 duct heat exchangers made by Magicaire,
which transfer 45K Btu/hour between 125 F water and 68 F air at 1400 cfm,
with a 0.1" H20 pressure drop, attached to the suction side of a few $11
window fans. How many of these fan coil units would we need to cool the
house, at 95 F? At a temperature difference of 3 F, each one should transfer
45K x 3/(125-68) = 2368 Btu/hour, so one or two should be enough, especially
if the house still has one window air conditioner for dehumidification. There
are still a few details to work out here. Direct heat transfer from the drums
to the house air via vents would be more efficient, but that might involve
Legionnaire's disease or swampy smells.

In the summer, we need to remove 36K Btu/day from the lower drums on an
average night, by letting some 67 F moist air flow over them, drawn in by
a fan, or a vacuum created by the solar chimney above (perhaps with help from
a windscoop using the average 10.3 mph wind in Abilene in July.) A solar
chimney with height H feet between unobstructed top and bottom vent openings
each having area Av square feet, with a temperature difference of DT degrees
>from  top to bottom will have an approximate airflow Q in cfm of 

     Q = 16.6 Av square root (H DT).

For instance, if the vents have an area of 8 ft^2 and the height is 8' and
the temperature at the top of the chimney is 100 F and the temperature at
the bottom is 80 F, the chimney will have an airflow of about 

     Q = 16.6 (8 ft^2) square root (8' (100 F - 80 F)) = 1680 cfm.

Suppose we make our vent doors 8 square feet, eg 8' long x 1' wide.
The warm part of our chimney is 8' tall. What does the temperature
difference DT between the outside air and the solar closet have to be
to remove 36K Btu from the drums overnight, assuming the drums below
are a perfect heat exchanger? 

An airstream of Q cfm with a temperature difference DT moves about DT Q  
Btu/hour of heat, so overnight we need DT Q cfm x 60 m/hr = 36K Btu, ie 

     DT (16.6) (8) square root (8 DT) (60) = 36K Btu, or 
                             DT^(3/2) 1358 = 36K, or
                                        DT = (36K/1358)^(2/3) = 8.9 F.

This shouldn't be too hard to manage, since with the roof reflector
the summer solar input should be about the same as the winter input,
which made for almost a 200 F temperature rise above ambient...

The roof pond might be used for further summer cooling, by pumping some water
up from the pond above the cool drums through the roof pond on a clear night
with no wind. Baruch Gavoni's _Passive and Low-Energy Cooling of Buildings_
book says that under these circumstances, rooftop temperatures can be up to
10 C below outdoor air temperatures, owing to night sky radiation, even with
no evaporation of water. 

Nick

---

From: [email-masked] (Raul Almquist)
Date: 28 Apr 1996 00:08:10 GMT

Nick Pine ([email-masked]) wrote:
: Texas is two-dimensional, with more wind in the north and sun in the west.
: The SE corner is more humid, with warm nights in August. The NW corner is
: more desertlike, with cool summer nights. Texans are starting to realize that
: they now use more energy per capita than anyone else on earth, as net energy
: importers, consuming more energy than they export. These days, Texans can make
: $3,000 per acre per year growing cabbages and onions, vs $2,000 per acre
: wind farming or $1,600 per acre pumping oil.

  You forgot one!!!

  OR, you can wind farm AND grow cabbages & onions while making $5K per acre!

---

From: [email-masked] (Nick Pine)
Date: 28 Apr 1996 14:47:13 -0400

>  OR, you can wind farm AND grow cabbages & onions while making $5K per acre!

Good idea :-) For some reason, several speakers at the Abilene Renewables 
conference implied that with windfarming, no other land use was possible.
But one of them, Michael Osborne, mentioned that a wind farm only uses up
about 2% of the space under it, for foundations, roads, etc.

Nick

---

From: [email-masked] (Nick Pine)
Date: 29 Apr 1996 11:03:39 -0400

Still thinking about my 12 year old friend who added 1/4 tsp of lemon juice
to her standard bubble mix (1/2 cup green Dawn, 3 Tbs glycerin, 2 quarts H2O)
to make the bubbles last 111 seconds at 80 F and 506 sec at 45 F, vs 309 sec 
and 185 sec; 1/4 tsp of maple syrup instead made this 431 and 345 sec. What
is a good bubble solution for blocking longwave IR radiation from a house
window? What is that IR wavelength?

Black bodies radiate heat at many wavelengths. Planck's law describes
a curve with a peak for this, ie at wavelength lambda, a blackbody with
temperature T (Kelvin) radiates with an intensity

     E(lambda) = C1/(lambda^5[exp(C2/(lambda T))-1]), where

            C1 = 3.74 x 10^-16 m^2W and 
            C2 = 0.0144 mK.

Wein's displacement law says where the peak is: the wavelength corresponding
to the maximum intensity of blackbody radiation is Lmax = 2898/T microns.
80 F is about 27 C or 300 K, so the wavelength with max intensity at 80 F is
about 10 microns or 0.0004 inches, ie just less than half a thousandth of an
inch. How can we measure a bubble thickness like that? What makes bubble
walls thick?

Compared to the intensity at this wavelength, this blackbody radiates
at some other wavelength L with a relative intensity of approximately

          Irel = (Lmax/L)^5 x exp(C2/(Lmax T)-1)/exp(C2/(L T)-1).

L: 1 micron       10^5        120.5             / 7 x 10^20 = 1.7 x 10^-14
   5 microns      32          120.5             / 14765     = 0.26
   10 microns     1           120.5             / 120.5     = 1
   20 microns     0.031       120.5             / 10        = 0.38
   50 microns     0.00032     120.5             / 1.61      = 0.024

It looks like thicker bubble walls are better, up to the point where the
bubble wall itself begins to be a sideways path between the two glazings.
If the bubble wall were 10 times thicker than the longest interesting
wavelength, (as people think about transmission lines), it might be 500
microns or 0.020" thick, which seems very thick for a bubble. A wall 4" thick
filled with 0.2" bubbles like that would have a 10% water cross section, so
we might think of such a 10' x 10' wall as having a 1' x 1' thermal shunt 
made of water with a thermal conductance of 1 Btu/hr. If that were the only
thermal path, that wall would have an R-value of 100... Perhaps a bubble
with a given wall thickness is a better insulator at higher temperatures
with shorter wavelengths.

Water has an index of refraction of about 4/3, so each air-water interface 
with a sufficiently thick layer of water compared to a wavelength would have
a Fresnel loss of about ((4/3-1)/(4/3+1))^2 = 1/7^2 = 2%. A 4" thick wall
with 20 clear 0.2" bubbles in series might have an attenuation of 1.02^20
= 1.5 for IR radiation. Not much... We may need smaller bubbles than that,
perhaps with thinner walls and some dye. Let's see, an 80 F room loses about
(80-30)/R2 = 25 Btu/hr/ft^2 of heat on a 30 F day through an ordinary double
glazed window. Two layers of poly film with little greenhouse effect (IR
blocking) have an R-value of about 1.2... And a 1 ft^2 80 F blackbody with
no glazing radiates 0.174E-8(460+80)^4 = 148 Btu/hr, while the 30 F world
radiates 100 Btu/hr back at it, so it loses 48 Btu/hr by radiation, ie the
R-value owing to radiation is about 1 (50/48.) 

An attenuation of 10, ie 1.02^N=10 makes N=ln(10)/ln(1.02)=114 0.035" diam.
bubbles across a 4" wall, at 28 bubbles per inch... How big are the bubbles
that come out of an airstone in an aquarium? What happens if we remove the
fish and fill the aquarium with some interesting soaps and IR dyes (?) and
warm water, and glue some foamboard on the sides (or start with a styrofoam
cooler) and put a glass lid on the top and measure how fast the water cools
with two thermometers and a clock... Suppose our cooler is 1' tall x 1' wide
x 2' long with a 1' x 2' glass lid, and the sides are 1" thick R4 white
beadboard, coffeecup foam, with a thermal conductance of Coth = 8 ft^2/R4
= 2 Btu/hr-F. This situation might be like a bubblewall ceiling under a
clear polycarbonate roof, with natural daylighting and vertical heatflow. 

Without the bubbles, we might have a thermal conductance of Coth + 2 ft^2/R1
= 4 Btu/hr-degree F. With bubbles, the conductance might decrease to
Coth + 2/Rx, close to 2 if Rx is large. So if we filled the cooler it with 
8" of 130 F water, ie 80 pounds of hot water, we might expect the box with
no bubbles to lose (130-70)4 = 240 Btu to a 70 F room in an hour, reducing
the water temp to 130-240/80 = 127 F. The box might be 124 F after 2 hours
or 70+(130-70)exp(-4/20) = 119 F after 4 hours, since the RC time constant
of the box would be about 1/4*80 = 20 hours with no bubbles. (After the
first hour, the water is cooler, so it loses less heat in the next hour,
so the temperature drops less in the next hour, etc.)

If we measure a water temperature of Tb (F) after 20 hours, with bubbles,
and Tnb without bubbles, what is Rx? T = 70+(130-70)exp(-20/RC), so
RC = -20/ln((T-70)/60) = R x 80, so R = -1/(4 ln((T-70)/60). Suppose we
measured Tnb = 92 F and Tb = 106 F. Then Rnb = -1/(4 ln((920-70)/60) =
0.24918 F-hr/Btu and Rb = 0.48940, so 1/Rnb-1/Rb = (Coth+2/R1)-(Coth+2/Rx)
= 2 - 2/Rx, so Rx = 2/(2+1/Rb-1/Rnb) = 2/(2+2.0433-4.0132) = R33.

Dr. Aristid Grosse made bubbles that lasted more than a year, as did Belgian
physicist Joseph Plateau in the 1800s. Dr. Grosse's principles of bubble
health and long life are: 1. Dust is the enemy of bubbles. 2. Carbon dioxide
poisons bubbles (better bubbles are blown by non-humans), and 3. Bubbles
love cold. They also like humidity. Glycerin helps bubbles fight humidity.
Maybe we don't need it inside a glazing cavity with 100% humidity.  

Perhaps this should be a closed system, to avoid building up dust.
 
Something like this? 

                       |w|                | |  |wwww| <--water level

Let's all try to invent an insulating bubble wall, perhaps starting with
two pieces of glass or single-layer polycarbonate plastic with butyl tape or
caulk over a plastic 1x3" or foamboard frame that can be filled between with
bubbles at night emerging from an aquarium airstone immersed in a soapy
solution at the bottom. This could really be useful in a passive solar
house or a greenhouse, like Beadwall. Simple, elegant, 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 conditions. 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. 

Ohm's law for heatflow is a good starting point here: 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 another 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 20 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 window screen 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? This might be a more accurate way
to measure the R-value than the aquarium way, especially since house windows
are usually vertical, with lower heat losses. 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.

Here is the following electrical circuit analog:

                    Iir    32 F       132 F   Ier
                      -->   |   <--    |    -->
             70 F ---wwww---*---wwww---*---wwww--- 70 F
                    R10/12  |   Rx/4   |  R10/12
                    = 0.83  |          |  = 0.83
                            |          |
                           ---     ^  ---
                        Ui ---     |  ---
                            |      Ue  |
                           ---        ---
                            -          -

Where Ui and Ue are flows of ice and electrical energy (Ui flows down,
in this picture.) 

What would Ui and Ue be if Rx were 2 without bubbles and 20 with bubbles?

In either case, Iir = (70-32)x12 ft^2/R10 ft^2 = 45.6 Btu/hr flows into
the ice from the room, and Ier = (132-70)x12 ft^2/R10 = 74.4 Btu/hr flows
out of the light bulb into the room. If Rx = 2, an additional
(132-32)x 4 ft^2/R2 = 200 Btu/hr flows out of the light bulb into the ice,
and if Rx = 20, only 20 Btu/hr more flows that way. The total ice and
electrical energy flows would be  

     Rx = 2 (no bubbles)                  Rx = 20 (bubbles)

Ui   245.6 Btu/hr (27.3 oz ice water/hr)  65.6 (7.3 oz ice water/hr)

Ue   274.4 Btu/hr (80.5 watts)            94.4 Btu/hr (27.7 watts)

Nick

---

From: [email-masked] (Nick Pine)
Date: 4 May 1996 09:11:26 -0400

>Something that has been bugging me for the last week, and I can't believe
>I stumbled upon it on the Net - how exactly do you calculate
>[exp(C2/(lambda T))]?

I use the "e-to-the-X" button on my $20 Casio fx-991H calculator, which takes
the base of natural logs, e = 2.718... to the Xth power. For instance, exp(1)
is 2.71828..., exp(2) is 7.389... If T is 300 Kelvin and lambda is 10 microns,
ie 10 x 10^-6 meters,

exp(C2/(lambda T)) = exp(0.0144/(10^-5 x 300)) = exp(4.8) = 121.5104...

>I've been attempting to learn about solar radiation from first principles
>from my copy of 'Solar Engineering of Thermal Processes' by Duffie and
>Beckman

That is not an easy thing to do. I hope you are using the second edition,
1991, I often have trouble following that good book, with BS and MS degrees
in electrical engineering.

>Many thanks if you can help the mathamatically challenged :-(

Glad to try, altho it seems to me that we only need arithmetic to do solar
space heating, with at most high-school algebra. 

Another common and more easily-understood application of exponentials
is in calculating how fast a house or some other thermal mass with
insulation around it cools... A thermal mass C surrounded by a thermal
resistance R has a natural RC "time constant" with a dimension of time,
eg in hours. If we heat the thermal mass up to some temperature DT above the
surrounding temperature Ta, RC is the time it will take to the temperature
difference DT to decrease to 1/e th, ie about 1/3 of its original value.

For instance, an 8' cube of water surrounded by (US) R-20 insulation has
C = 62 x 8^3 = 31K pounds of water with a thermal mass of 31K Btu/degree F,
and a thermal resistance R = 20/(8x8x6) = 0.052 F-hr/Btu, so it has an RC
time constant of RC = 32K x 0.052 = 1653 hours or 68.9 days. So if we heat
the water up to an initial temperature T(0) = 100 F and sit the cube in a
Ta = 70 F room, the water temperature T(D) after D days will be 

T(D) = Ta + (T(0)-Ta) exp(-D/(RC),

so after 10 days, the water temperature would be 

T(10) = 70 + (100-70) exp(-10/68.9) = 70 + 30 exp(-0.145) = 95.9 F.

After 68.9 days, the temperature would be 

T(68.9) = 70 + 30 exp(-1) = 70 + 11 = 81 F.

After a year, the temperature would be 

T(365) = 70 + 30 exp(-5.3) = 70.15 F.

If we made this a 16' cube with R40 insulation, we would have 

RC = 62 x 16^3 x R40/(16^2 x 6) = 6613 hours or 276 days, so after a year
the water temperature would be  

T(365) = 70 + 30 exp(-365/276) = 78 F.

If this cube were outside in 30 F air for a year, the water temp would be 

T(365) = 30 + (100-30) exp(-365/276) = 38 F.

If we added an solar air heater with R1 glazing over one insulated side of
our 8' cube, with some simple plastic flap dampers to let the warm air into
the cube during the day and keep out the cold air at night, and collected
8' x 8' x 1000 Btu/ft^2 of heat over 6 hours every day (an average December
day in Philadelphia), and waited a year, ie 5 time constants, the water would
forget its original temperature, and if the average outdoor temperature over
the year were 55 F, the water would have a more or less constant temperature
T such that the energy that flowed into the cube every day would equal the
energy that flowed out of the cube:

64K Btu/day = 6(T-55)64 ft^2/R1         air heater side, day
           + 18(T-55)64 ft^2/R20        air heater side, night
           + 24(T-55)64x6 ft^2/R20      other sides, 24 hours a day,

ie 64 K = (T-55)(384+57.6+460.8) = 902.4(T-55), so T = 55 + 71 = 126 F.

This might be an interesting outdoor exhibit at a science museum or some
unusual outdoor art at a college, a monolith moving even more slowly than
a very large Foucault pendulum, reminding us that solar energy really can
work in partly cloudy climates, if well-applied. 

We might fill up this 8' cube with 18 55-gallon plastic drums full of water,
each 3' tall and 2' in diameter, in two 3x3 drum layers, with about 300
2-liter plastic soda bottles tucked around in the spaces among them, inside
a few 2x4s and $100 worth of 6 1/2" fiberglass insulation, or strawbale/mortar
walls, with a "truth window" exposing a large thermometer inside.

I have the drums...

Nick

---

From: [email-masked] (Michael Spangler)
Date: Sat, 04 May 1996 08:58:07 -0700

>owners use less fossil fuel?
(much snipped)

It doesn't sound very frugal to me. My (former) house in Winnemucca, NV
cost half that to heat in winter, and one quarter that to cool, although
the cooling was with a swamp cooler, which may not be an option on your
area. The heating was with gas. construction was standard for manufactured
housing, with 6" walls and extra insulation in the ceiling. But it was all
ordered right out the catalog.  

The rest of the post was well done. Good energy balance.

-- 
Mike Spangler 
University of Idaho
[email-masked]