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re: freezing water supply
28 mar 1997
>"judith r. sarber" wrote:
>>need some suggestions for keeping my water supply from freezing.
>>i collect rainwater off roof and store in cistern...
can this be done passively? start with a rectangular 1500 gallon septic tank
that sits on the ground to the east or west of the house? where i live, these
6' wide x 5' tall x 12' long x 4" thick concrete tanks cost $600, delivered,
with a lid. (or make it of concrete blocks, or make it larger and circular,
out of ferrocement, following s. b. watt. or make it a monolithic dome...)
we might place the tank on 3" of styrofoam on the ground, with the long way
running ew. we might add some insulation on all 4 sides and earth berms on
all but the south side, and attach a 4' x 12' strip of clear plastic along the
south side to collect some sun, with an 8" air gap and some black aluminum
window screen between the plastic and the insulation, and let the warm air
in the gap rise up through some holes between the tank and the lid and float
along the underside of the lid, while the lid and its support conduct heat
down into the concrete walls of the tank and the water.
the lid itself has 60 ft^2 of surface, which is not enough for the warm air
from the 48 ft^2 of glazing to heat it very efficiently, with a still air film
thermal conductivity of about 1.5 btu/hr-f-ft^2. hot air, cold lid...
so we might put 3 courses of hollow concrete blocks around the top edge of the
tank, about 75 blocks, to raise the lid to about 8' off the ground, and make
the 3 holes in each block horizontal, with small gaps between them, to add
about 150 ft^2 to the 60 ft^2 of the tank top. that makes the thermal mass
area/glazing area ratio about 4:1--larger would be nicer, eg 10:1, since full
sun is about 300 btu/ft^2/hr, making an air-lid temperature difference of
about 20 f with a 10:1 mass/glazing area ratio, and no other losses.
so let's hang another 250 ft^2 of surface inside the upper part of the tank,
ie about 50 hollow concrete blocks hung from 1/4" dacron ropes attached to the
north and south sides of the tank, say 12 ropes with 4 vertical blocks per
rope, with their bottom thirds in the water, to make this a better air-water
heat exchanger.
>>...cold in this area is zero to 25-degrees, but cold weather means
>>sunny weather. i have "good" solar available for only 2-3 hrs during
>>deepest winter. does anyone have any suggestions?
the coldest place in alaska with this kind of winter sun seems to have an
average air temp in the coldest month of about 10 f, so let's use that number.
(we can use average monthly temperatures because of the large rc time constant
and time to freeze, for all this water surrounded by insulation.)
suppose we glue 17 8' vertical 2x6s on 2' centers on edge around the tank with
some polyurethane foam from a can, and staple on 2' wide x 5.5" r20 foil-faced
fiberglass insulation, with some sort of siding over that. the roof might be
covered with a layer of epdm rubber. the south side insulation needs to cover
the upper 4' of the south side to make an igloo-type heat trap, so little heat
is lost at night through the glazing.
what will the average water temperature be (assuming the solar collector
doesn't completely fill up with ice from condensing water vapor)?
energy in = 48ft^2x0.9x500btu/ft^2/day = 22k btu/ft^2/day?
= 3hr(t-10)48ft^2/r1 "daytime" glazing loss
+21hr(t-10)48ft^2/r20 nightime glazing loss
+24hr(t-10)48ft^2/r20 24 hour upper south wall loss
+24hr(t-10)6x12/r20 24 hour roof loss
+24hr(t-10)8x2x(5+12)/r20 24 hour wall loss
+24hr(t-10)6x12/r30 24 hour floor loss
= (t-10)(144+50+58+86+326+58) = 722(t-10) = energy out?
so t = 10 + 22k/722 = 40 f. looks like it's above freezing...
to freeze the water completely solid, we need 1500x8x144 = 722(32-(-25))d,
so d = 42 days at -25 f with no sun...
nick
this might make a nice solar closet, with a house around it and sunspace in
front to keep it warmer. perhaps a few tanks side-by-side or stacked on top
of each other (except in earthquake country) to store more water and heat...
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