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exploring solar wetland structures
7 apr 1996
were jesus alive on easter sunday, 5 years ago, would he have condoned the
gulf war, or might he have said that blood, like oil, should not be wasted?

would he have worried with donald hodel, a former doe secretary, that we are
'sleepwalking into a disaster'? hodel predicts a major oil crisis within a
few years, while irwin stelzer of the american enterprise institute, says
that the next oil shock "will make those of the 1970's seem trivial by
comparison," and shell oil's planners believe that "renewables may make up
a third of the supply of new electricity within three decades even if
electricity from fossil fuels continues to decline in cost. (from "mideast
oil forever?" by romm and curtis, atlantic monthly, april 1996, boston,
(617) 536-9500).

these are moot questions, if "faith without works is dead."

israelis hardly need solar heat, but they use it in the rankine cycle power
plant in the dead sea, making 5 megawatts of electricity with 170 f brine
drawn from the bottoms of 62 acres of solar ponds, unlike our largest 0.83
acre solar pond in el paso, texas, which makes heat for a food canning plant.

when last we met, gentile readers :-) i miscalculated the average winter
water temperature in a greenhouse heating system:

>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

oops, i should have written ta = 43 + 246k/1536 = 43 + 160 = 203 f...

this would be a hot sauna, almost boiling. we could fix this "problem" by
filling the sunspace with bricks, as the psic priesthood recommend, or we
could just let it be, as a heretical high-performance, inexpensive sunspace.

>the day, and the water temperature would be tw = (160 - 60)/0.714 = 140 f.

this should have been                        tw = (203 - 84)/0.714 = 167 f. 

close to that dead sea solar pond power plant temperature. hmmm...

here is a better diagram of the greenhouse site, as improved:  
 .                   .
 .                   . 
 .   chicken coop    . 12'                              n
 .                   .                          |       16'     |
 .                   .   .   .   .   .   .   .  ................. ---
 .....................        <- 30' ->         .       .       .
          20'                                   .       .       .
 .                                              .       .       .
                                                .       .       .
 .                                              .       .       .
                                                .       .       .
 . |      (does this picture suggest            .       .       .
          enclosing the 30 x 66' space?)     w  .       .       . 32' e
 .30'                                           .       .       .
                                                .       .       .
 . |                                            .       .       .
                                                .       .       .
                                                .       .       .
 .                             ..................       .       . 
                               .    straw wall  .       .       .  
 .  .  .  .  .  .  .  .  .  . ---
                            4' .         solar wall   203 f     .
                      .        |               32'              |         .
                      .                                                   .
                      .                   reflecting pool                 .
                   16'.          made from a single roll of 20' wide      .
                      .              epdm rubber roofing material         .
                      .                                                   .
                      .                   <--  50' -->                    .

                            .                   s
         .      (with an air-inflated roof,      .
                      like a tennis court?)              .
 .....................                          36 f    .     .   ---
 .                   .         .....................        ..... ~4'
 .                   .         .                .               . ---     
 .   chicken coop    .         .                . c           c .        
 .                   .      8' .   solar wall   . t c       c   . ~5'   
 .                   .         .                b i p       p i b      
 .                   ..        .                b w p       p w b         .

  my favorite structure on this property is the 12' x 20' dilapdated 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. 

how can we solar heat the chicken coop?

perhaps surround it with strawbales? a strawbale and mortar wall all around
it, and strawbales on the roof, with a layer of epdm rubber over the straw?
the chicken coop is interesting inside, mostly wood, but the outside is
covered with ugly warped green asphalt shingles, many about to fall off.

we might want to hire a team of professional structural engineers to certify
beyond all reasonable doubt with elaborate studies and tests, ie their own
high-priced chicken manure, that this chicken coop would not collapse in a
tragic heap of straw and feathers, but assuming that were not a problem...

...we'd have a box with r-40ish walls, about 12' deep by 20' wide (ew) by 8'
tall, with about 64 ft^2 of r2 (?) doors and windows. the building envelope
would have a total area of about 2(12x20+12x8+20x8)=864 ft^2, with a strawbale
thermal conductance of 800ft^2/r40 = 20 and a window, etc thermal conductance
of 64ft^2/r2 = 32, ie a total thermal conductance of 52 btu/hr per degree f.
note that the windows and doors are less than a tenth of the wall area, but
they account for more than half of the thermal loss of the structure.

to keep the chicken coop at 76 f inside for 24 hours a day, on an average
36 f december day in philadelphia, we would need 24 hr (76f-36f) 52 = 50k btu.
about a half-gallon of oil, or 75 ft^2 of solar air heater area. suppose we
add a low-thermal-mass lean-to sunspace to the south wall, eg 20' x 9' of
south-facing polycarbonate glazing, with the bottom edge buried in the
ground 4' in front of the chicken coop, with a 4' reflective/shading overhang
at the top of the sunspace, at a steeper slope than the roof. that might
look something like this from the east:

                           .          straw
                        g  du          .           
                           .                       .
			 s .                       . s
                      g  t .                       . t
			 r .          ~8'          . r
                         a .                       . a
                    g    w .                       . w
                           dl                      .
                  g        |           12'         |

the single layer polycarbonate might be dynaglas corrugated sheets, about 4'
wide x 12' long, distributed by sps at (408) 997-6100, and sold by greenhouse
suppliers like e c geiger at (800) 4 geiger, or, with
web site, or d & l growers at (800) 732-3509, or
stuppy greenhouse manufacturing, inc., at (800) 877-5025.

dynaglas and similar products made by replex plastics and ge cost just over
a dollar per square foot, require support on 4' centers, and have a 10 year
guarantee against loss of light transmission and an expected mechanical
lifetime of over 25 years. ge's application note on page i-18 of the stuppy
catalog says "for added strength and rodent control, sheets at ground level
may be buried 4" to 6"." some landscaping timbers staked to the ground with
4' of #4 rebar might be a suitable foundation, and the glazing wall studs
might be 2x4s on 4' centers north of a mid-height 2x4 purlin. 

how much thermal mass would would we need to keep this building at 76 f
inside, 24 hours a day, during a very cloudy december, eg for 20 days in
a row, at 36 f outside, with no sun? 

about 20 x 50k = 1 million btu.

suppose our thermal mass were water inside a solar closet, and after a long
string of average december, with some sun, the water had a temperature of
130 f. then a 55 gallon drum full of water would store about

55 gal x 8 lb/gal x (130f-80f) = 25,000 btu of usable heat.

how many drums do we need in this solar closet?

1 million/25k = 40 drums.

how much space would they require? each drum is just under 3' high x 2' in
diameter, so we might line up 6 of them in a row from north to south along
the east side of the coop. if the drums were stacked two-high, each 2' x 12'
slice of the building could store about 12 drums, and there would be an
insulated wall running ns between the solar closet and the plant propagation
area. the drums would fill up the 8' x 12' area where the chickens lived.
this is 40% of the floor space in this building, but that fraction might
decrease to less than 2% of the floor space of a new 2,000 ft^2 building
with, say, 5 days, vs 20 days of thermal storage. large buildings with small
surface to volume ratios are easy to solar heat. 

what will the water temperature in the solar closet be, after a string of
average december days, eg days with 6 hours of sun at an average daytime
outdoor temperature of 43 f, and an average outdoor 24 hour temperature of
36 f, and an average 1000 btu/ft^2 of sun that falls on the sunspace?

this isn't hard to estimate, so why spoil the fun? as tom smith said in 1980,
"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."

here are some hints: the solar closet might have its own air heater, with an
inner glazing as a part of the house wall between the sunspace glazing and
the inside of the house. in this case, that inner glazing might be 8' tall
x 8' wide, so that approximately 64k btu/day enter the solar closet. one way
to calculate how warm the water would be after a long string of average days
is to figure out how much net energy the sunspace collects on an average day,
and how much is needed to keep the coop warm, and how much is leftover.
the leftover energy can go into heating the water inside the solar closet,
which can be very well-sealed with insulation all round during the night,
and never used for house heating on an average day, if the house contains
sufficient thermal mass of its own... 

at this point we might have a solar heated greenhouse with an 8' solar wall
running 16' west to help keep it warm, and a solar heated chicken coop in
the other corner of that 30 x 50' rectangle. they would both lose heat to
the outdoor air through two sides of their structures. we might extend the
solar wall and build more straw walls and put a lightweight air-supported
roof over the 30 x 58' rectangle between the sw corner of the chicken coop
and the ridge of the greenhouse, like this:

 .s                  .
 .t                  . straw                             n
 .r     chicken coop . 12'                      |       16'     |
 aa                  a..........................................a
 .w                  .      straw wall          ................. ---
 a...................a...........................       a       .
 .   .    20'        |        <- 30' ->         .       .       .
 . s .                                          .       .       .
 . t .                                          .       .       .
 . r .         38' x 66' membrane cover         .       .       .
 . a .                                          .       .       .
 . w . |                                        .       .       .
 .   .                                       w  .       .       . 32' e
 . w .26'                                       .       .       .
 . a .                                          .       .       .
 . l . |                                        .       .       .
 . l .                                          .       .       .
 .   .                                          .       .       .
 .   a...........................................       .       . 
 .                  straw wall                  .       .       . ---
 .                          4'           solar wall             .
                          reflecting pool                                 .
  16'           made from a single roll of 20' wide                       .
                    epdm rubber roofing material                          .
                          <-- 100' -->                                    .

                            .                   s
         .       air-inflated cover?          .

 ....a..................................................a         ---
 .                   .                             .        ..... ~4'
 .                   .                          .               . ---     
 .   chicken coop    .                          . c           c .        
 .                   .      9' tall solar wall  . t c       c   . ~5'   
 .                   .                          b i p       p i b      
 .                   .                          b w p       p w b         .

the cover might attach along lines with corners marked a above. it could
be rectangular in shape, as seen from above, and curved as seen from the
east or south. it might be made in a north and a south section, and the
main part, the north section, perhaps 2/3 of the cover, might be opaque
and roughly parabolic in the ns profile and reflective underneath to help
keep heat inside the structure, and to bounce and concentrate winter sun
down into an indoor pond, as in howard reichmuth, pe's very successful
full-size ecotope concentrating greenhouse built 20 years ago, in cloudy
seattle, which has half the winter sun of philadelphia and a fourth of
new mexico's. we also want to avoid summer overheating with this opaque
north section, while the steep-sloping south section would be more
transparent to let in light, and let in the heat of the winter sun.

the cover might rise 18' above the wall at the highest point, with an
average south-facing height of 9' above the straw wall, to admit a daily
average total winter sun heat of 

ein = 18' x 66' x 1000 btu/ft^2/day x 1.5 = 1.8 million btu, ie 522 kwh,
with a peak sun power input of about 157 kw or 210 horsepower.

the main heat loss from this enclosure would be through the cover, with 
somewhat less heat lost through the greenhouse. ignoring the greenhouse
and walls for the moment, what would the average r-value for the 1500 ft^2
cover have to be to keep the inside 66 f for 24 hours a day in december?

ein = 1.8 million btu = 24 (66-36) 1500 ft^2/rcover ==> rcover = 1.7.

we might use an r1 clear plastic for the south part, and something like
the ludvig svensson aluminized environmental screen (r3.2?) for the north
half, described on page e-5 of the current stuppy catalog this way: "the
permeable closed-structure construction allows the transfer of humidity
to prevent condensation, while providing a light-controlled environment
for your crops." this material costs 32 cents/ft^2, with additional small
fabrication charges for sewing custom sized pieces out of standard 10-14'
widths, adding tapes and grommets and hooks, etc, as specified.

a less expensive possibility for the north section is klerk's k-white  
tri-layer greenhouse film, a "strong eva copolymer resin for excellent
durability," treated with uv inhibitors and filled with titanium dioxide
white pigment to give a 45% light transmission. this material costs
7.8 cents/ft^2, and it has a 3 year guarantee. it might last longer with
no southern exposure. it comes in rolls up to 50' wide, 6 mils thick.
i would guess it is recyclable.

in either case, it seems prudent to have a few flagpoles and wires
underneath in case the fabric rips or pressure is lost, and some ropes
attached to some points overhead to control wind flapping. 

a more permanent solution would be a monolithic dome cap (800) 608-0001,
costing about $20/ft^2, put up as a turnkey shell, with 2" of reinforced
concrete under 3" of polyurethane foam, all sprayed on from the inside
under the inflated airform. the thermal mass and conductivity of the
concrete inside the foam insulation would make this an excellent solar
structure. one might reduce the cost by leaving out the concrete or using
thin ferrocement instead, on a geodesic framework faced with wire mesh. 

how large a thermal mass would we need to keep this enclosure at 66 f for
5 days without sun when the outdoor air temperature is 36 f? suppose the
thermal pond has a temperature of 100 f, with an 16' x 40' area, and the
cover has an average r-value of 2. then the enclosure will lose about

24 (66-36) 1500/2 = 540k btu/day, or about 2.7 million btu over 5 days.

if the pond can keep the enclosure at 66 f until the water reaches 70 f,
and each cubic foot of pond water stores 62 btu when heated 1 degree f,
and the pond has a depth of d feet,

62 x 16 x 40 x d (100f-70f) = 2.7 million btu, so d = 2.3'.

it would be interesting to observe the organisms that evolve over time
in this indoor wetland, vs the one outdoors...

we would need a smaller or cooler thermal storage pond if the roof were
better insulated. one way to accomplish this is to use a tensile structure
with the profile shown below, from the east:

                       .     .                                
                     . .             .                         
		    .  .                       .           
		   .16'.                                     ...........
		  .    .                                     .         .
		 .     .                                     . chicken .
         pond   .      .                                     .   coop  .

this might look like a 16' x 66' transparent wall from the south, perhaps
with a sag in the middle. the main roof, the ie north slope, might be made
with 4' wide chicken wire strips running north and south, joined with
galvanized metal strips that sandwich the chicken wire. the wire web might
be covered with 1/2" of cement, and it might have a layer of fluffy plant
material on top, eg composted water hyacinths, with a synthetic waterproof
breathable fabric layer tied on over that. the pole supports at the se and
sw corners might be telephone poles wrapped with ferrocement, ie a layer of
chicken wire with some cement on top to strengthen and weatherproof them.

this taller south wall would benefit from a wider frozen reflecting pond in
the ns direction, since winter sun only reaches a height of 26 degrees above
the horizon in this area, and tan(26) = 0.43, ie the sun from a mirror 37'
wide in the ns direction would just strike the top of the 16' solar wall at
noon on 12/21. so we might make the reflecting pond 38' wide using 2 strips of
epdm rubber roofing material, perhaps with an ew standing seam in the middle.

we might use that pond for sewage treatment...

us epa design manual number 74, "subsurface flow constructed wetlands for
wastewater treatment," written by sherwood c. reed, pe, is available for
about $12 from the small flows clearinghouse at west virginia university
at pob 6064, morgantown, wv, 26506-6064, (304) 293-4191 or (800) 624-8301. 

here's a quote:

  the proponents of subsurface inlet manifolds claim they are necessary
  to avoid the buildup of algal slimes on the rock surfaces and resulting
  clogging adjacent to a surface manifold. the disadvanatages of a subsurface
  manifold are the inability for future adjustment and the limited access
  for maintenance. in one case, a buried manifold became clogged with turtles
  which entered the piping system from the preliminary treatment lagoon and
  had to be removed. 
there is a lot of nice, simple math in this manual, which explains how
to build a natural wastewater treatment system for a home or community.
the manual doesn't say where the frogs came from :-)

reed's book _natural systems for waste management and treatment_ describes
how to build ponds, lagoons and artificial wetlands, and predict their
performance. mcgraw hill, 1995, second edition, isbn 0-07-060982-9,
434 pages, about $55.

the back cover says: 

  here is your chance to learn about biologically-based systems for handling
  waste that are fast becoming the technology of choice in communities and
  municipalities across the united states... the new edition of this classic
  reference will introduce you to low-cost, low-energy methods of processing
  waste and wastewater naturally...

here are some quotes:

  serious interest in natural methods for waste treatment reemerged in the us
  following the passage of the clean water act of 1972... the major initial
  response was to assume that the "zero-discharge" mandate of the law could be
  obtained via a combination of mechanical treatment units capable of advanced
  wastewater treatment (awt). in theory, any specified level of water quality
  can be achieved via a combination of mechanical operations, however the
  energy requirements and high cost of this approach soon became apparent,
  and a search for alternatives was commenced... more and more systems were built... it was noticed that these natural
  systems... could usually be constructed and operated for less cost and with
  less energy... ...there were about 400 municipal land treatment systems
  using wastewater in the us in the early 70's. that number had grown to at
  least 1400 by the mid 1980's and is projected to pass 2000 by the year 2000. 

  stabilization ponds have been employed for treatment of wastewater for over
  3000 years... the most common type is the facultative pond. other terms
  commonly applied are oxidation pond, sewage lagoon, and photosynthetic pond.
  anaerobic fermentation occurs in the lower layer and aerobic stabilization
  occurs in the upper layer... a continuous ice layer on a facultative pond
  will lower performance [but a partial ice layer on a cold day might make a
  very nice solar reflector--np]... the occasional high concentration of
  suspended solids (ss) in the effluent... is the major disadvantage of pond
  systems. the solids are composed primarily of algae, not wastewater solids. 

  aquatic treatment is defined as the use of aquatic plants or animals as a
  component in a wastewater treatment system. in many parts of the world,
  wastewater is used for the production of fish... the floating aquatic plants
  with the greatest potential for wastewater treatment include water hyacinths,
  duckweeds, pennywort and water ferns... hyacinths are one of the most
  productive photosynthetic plants in the world. it has been estimated that
  10 plants could produce 600,000 more during an 8 month growing season and
  completely cover 0.4 ha (1 acre) of a natural freshwater surface. the rate
  can be even higher in wastewater ponds... the dense canopy of leaves shades
  the surface and prevents algal growth... the plant can survive and grow in
  anaerobic waters, since oxygen is transmitted from the leaves to the root
  mass. the attached biological growth on the root mass is similar to...
  rotating biological contactor (rbc) slimes. bacteria, fungi, predators,
  filter feeders and detritovores have been reported in large numbers on and
  among the plant roots... an effective mosquito control method is to stock
  each basin with gambusia or other small surface feeding fish that prey on
  the mosquito larvae... [other species include goldfish, frogs, grass shrimp,
  blue tilapia and japanese koi. the hyacinths are sometimes harvested and
  processed in a biogas digestor or used for animal feed...] 

  ...duckweeds are the smallest and simplest of the flowering plants and have
  one of the fastest reproduction rates... lemna sp. grown in wastewater
  effluent (at 27 c) doubles in frond numbers, and therefore area covered,
  every 4 days. [not surprisingly, ducks like to eat duckweed, a lot--np]
  ...duckweed can grow at least twice as fast as other vascular plants. the 
  plant is essentially all metabolically active cells, with very little
  structural fiber... duckweeds are more cold-tolerant than hyacinths, and are
  found throughout the world. in 1992 there were at least 15 operational
  wastewater treatment facilities designed specifically as duckweed systems...
  mosquito larvae will not be able to penetrate a fully developed duckweed
  mat, and are therefore not a problem... duckweed, like hyacinth, contains
  about 95% water... duckweed contains at least twice as much protein, fat,
  nitrogen and phosphorous as hyacinth. several nutritional studies have
  confirmed the value of duckweed as a food source for a variety of birds
  and amimals [footnote]... the harvested plants may be used directly in the
  wet state as poultry or animal feed. composting... is also feasible. 

  pond systems in colder climates can be designed for the seasonal use
  of duckweed to significantly improve performance durign the normal algal
  growth season... duckweed plants form a "winter bud" at the onset of cold
  weather. this "winter bud" has a high specific gravity and sinks to the
  bottom of the pond, where it remains all winter [under the reflective ice
  layer--np.] in the following spring they float and repopulate the pond... 

  the aquatic animals that have been considered for use in wastewater treatment
  include daphnia, brine shrimp, and a wide variety of fish, clams, oysters
  and lobsters...  except for the predatory fish and the lobsters, the primary
  function of the other species is the removal of the suspended solids or
  algae. assuming that the animals are routinely harvested, this will in turn
  also improve nutrient removal... fish activity is highly dependent on
  temperature, and most of the species... with the exception of catfish...
  require relatively warm water... the final lightly loaded cells in
  wastewater pond systems can be used for fish culture if a market for the
  harvested fish exists. at present, federal and state health regulations
  prevent the sale of such fish for direct human consumption, even though
  microbiological studies have not detected any contamination... major markets
  for this harvested material would be bait fish, pet food or fertilizer. 


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