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another swamp thang
26 apr 1996
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




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